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Behaviour of Rh supported on hydroxyapatite catalysts in partial oxidation and steam reforming of methane: On the role of the speciation of the Rh particles
Abstract
Rh/hydroxyapatite samples were prepared by impregnation and investigated for the partial oxidation (POM) and steam reforming (SRM) of methane. The catalysts were analysed by BET, XRD, DRS, XPS, H2-TPR, TEM, H2 chemisorption, CO2-TPD and NH3-TPD techniques. The characterisation results showed that, after calcination, Rh existed in three different forms in the samples: (i) large crystallites of Rh2O3 deposited on the surface of the catalysts, (ii) RhOx in small particles exhibiting strong interaction with the support and (iii) a phase of Rh2+ species which incorporated the hydroxyapatite framework.
Operating in the POM and SRM processes the reduced Rh(x)/HAP catalysts resulted highly active and exhibited excellent stability at 973 K (for 30 h). This behaviour was explained by their high coke-resistance. The activity of the catalyst with the optimum loading (1%), in SRM, was compared with that of a commercial Rh/Al2O3 catalyst. The conversion and H2 and CO yields values achieved on the former were all close to those exhibited by the latter. This comparable behaviour was explained by suitable properties provided by the HAP support such as reducibility, lower surface acidity and larger pore sizes (ensuring a better diffusion of the reactants and the products during the SRM reaction). These properties seemed to compensate the low dispersion of the Rh active phase induced by its low specific surface area.
... Due to their distinct properties, HAP materials have attracted considerable attention as catalyst supports for numerous applications [14][15][16][17][18][19][20][21][22][23][24]. Previous reports noted that the advantage of the use of HAP as a support lies in its capacity to retain its structural properties when it undergoes significant changes in composition [7,24]. ...
... Previous reports noted that the advantage of the use of HAP as a support lies in its capacity to retain its structural properties when it undergoes significant changes in composition [7,24]. The promising results arising from the use of HAP were found to be connected to its high thermal stability, acid-base properties, and the fact that it could provide beneficial synergistic effects with a variety of metallic active phases [14][15][16][17][18][19][20][21][22][23][24]. ...
... The spectrum of the Rh/HAP sample contains a strong absorption peaked at 340 nm accompanied by a shoulder near 440 nm. According to our previous study, the observed shape and positions indicate the main occurrence of Rh2O3 species [18]. The spectrum of the Pt/HAP sample is characterized by the presence of a much less intense visible band located at 430 nm, thus suggesting a deposition of relatively smaller amounts of the Pt cationic species. ...
The combustion of lean methane was studied over palladium, rhodium, platinum, and ruthenium catalysts supported on hydroxyapatite (HAP). The samples were prepared by wetness impregnation and thoroughly characterized by BET, XRD, UV-Vis-NIR spectroscopy, H2-TPR, OSC, CO chemisorption, and TEM techniques. It was found that the Pd/HAP and Rh/HAP catalysts exhibited a higher activity compared with Pt/HAP and Ru/HAP samples. Thus, the degree of oxidation of the supported metal under the reaction mixture notably influenced its catalytic performance. Although Pd and Rh catalysts could be easily re-oxidized, the re-oxidation of Pt and Ru samples appeared to be a slow process, resulting in small amounts of metal oxide active sites. Feeding water and CO2 was found to have a negative effect, which was more pronounced in the presence of water, on the activity of Pd and Rh catalysts. However, the inhibiting effect of CO2 and H2O decreased by increasing the reaction temperature.
... This catalyst also possessed high stability durable in the 50 h of time on stream, with final CH 4 conversion of 60% [19]. Similarly, Rh/HAP showed high activity and excellent stability in partial oxidation and stream reforming of methane, with unnoticeable change in CH 4 conversion even over 30 h of time on stream [53]. HAP normally has a large range of pore size distribution (about 3-100 nm [54,55]), because of the complexity in delicately controlling the pore size for HAP, the comprehensive effect of pore structure on Ni-based hydroxyapatite catalyst for dry reforming of methane was not clear. ...
... Effectual synthesis of nickel-containing HAP catalysts was verified by XRD. Analysis on the XRD patterns of all the reduced catalysts ( Fig. 2) [19,53]. The metallic Ni crystalline phase with main diffraction peak at 44.5° [59,60] was recorded as well and the average Ni crystalline size calculated by FWHM were 18.7, 15.6, 14.3, 14.6, 13 nm for respective Ni/HAP catalysts (Table 1). ...
... The high acidity caused by the impregnation solution containing nickel and cobalt nitrate resulted in a local surface dissolution of HAP layers, hence favoring the cationic exchange. Boukha et al. [53] also reported the similar phenomenon that the diffraction peaks for Ca 3 (PO 4 ) 2 crystalline phase were detected in the XRD pattern of Rh/HAP catalyst because of the incorporation of large amounts of Rh in the HAP framework. In our study, although no characteristic peaks of CaO or Ca 3 (PO 4 ) 2 crystalline phase were found in the XRD patterns in the calcined (Fig. S2) and reduced Ni/HAP catalysts (Fig. 2), the parameter "a" of calcined and reduced Ni/HAP catalysts slightly decreased compared to that of bare HAP (Table S1), which suggested a possible incorporation of Ni species into the framework of HAP. ...
The hierarchical supports have motivated intensive research interests of chemists in the catalytic field because of the rational combination of mass transport property and large surface area. In this work, hydroxyapatite (HAP) supported nickel catalysts were prepared and employed to promote the dry reforming of methane (DRM). The proportion of mesopore/macropore in HAP was adjusted by the heating temperature (20, 50, 80, 120 °C) during the preparation process. The pore structure played an important role in regulating the physiochemical properties and thus affecting the catalytic performance in DRM. With increasing the proportion of mesopore on HAP, Ni preferentially dispersed on mesopore channel to form small-size particles. All catalysts possessed weak interaction between Ni nanoparticles and HAP; however, the interaction enhanced gradually with decreasing the Ni particles diameter. High proportion of macropore led to large-size Ni nanoparticle, while high proportion of small-size mesopore at 2–5 nm resulted in the complete blockage of these pore structures. These two boundary situations led to relatively weak interaction between nickel particles and the support, low metal dispersion and hence low activity as well as rapid deactivation in the DRM evaluation. Ni nanoparticles on HAP-80 and HAP-120 supports possessed relatively higher dispersion, which imparted these two catalysts with the function of strong anti-sintering ability and carbon resistance, thereby exhibiting better catalytic activity and stability in DRM evaluation. The deactivation of all catalysts during the reforming reaction was mainly ascribed to the sintering as well as encapsulation of Ni nanoparticles by graphitic carbon.
... On the other hand, hydroxyapatite (HAP) material has been widely investigated and considered as a good alternative to traditional supports for a variety of transition metal catalysts [1,[3][4][5]11,[13][14][15][16][17][18][19]. For instance, to overcome the drawbacks of a conventional Cu-ZnO/Al 2 O 3 system, considered as less practical for mobile device applications, we investigated the behavior of a Cu-ZnO/HAP formulation as an alternative [1]. ...
... This observation would indicate a decrease in the strength of metal-support interactions leading to deposition of metallic Pt particles on the HAP surface. In parallel, for the charge compensation, this might induce an increase in the density of oxygen vacant sites [13,25]. In fact, it is known that the apatite framework can host oxygen vacant sites resulting from a loss of O atoms in PO 4 and/or hydroxyl groups [25]. ...
... The optical properties of the Pt species deposited on the calcined samples were investigated using DRS (Fig. 2). The HAP bare support exhibits two typical bands in the NIR domain, centered at 1300−1500 nm and 1900−2100 nm, assigned to surface hydroxyls and structural OH groups, respectively [3,13,14]. In the UV region, the spectrum shows a strong absorption presenting a maximum at a very short wavelength and a shoulder at 280 nm due to Ca 2+ -O 2− charge transfers [3,13,14]. ...
The performance of a series of Pt/La(x)/HAP catalysts was investigated in the WGS reaction. It was found that the addition of La into the Pt/HAP catalyst enhances its OSC, hinders the incorporation of Pt species into the HAP framework, promotes its reducibility, decreases the Pt particle size and markedly improves its WGS activity. Moreover, the shut-down/start-up experiments demonstrate the high resistance of the investigated samples and the possibility of their use in mobile device applications.
Kinetic data show that the reaction orders measured on Pt/La(4)/HAP agree with those corresponding to 1%Pt/Al2O3 catalyst. By contrast, the general tendency found on the rest of our samples, including the unpromoted one, resembles that shown by a Pt/CeO2 catalyst. Due to the presence of OH- vacant sites on our Ca-deficient HAP, it seems that it plays a role similar to that exhibited by the CeO2 reducible support.
... As a type of clean and high efficient energy carrier, hydrogen has attracted much attention. Methane reforming, such as steam reforming, [1] dry reforming [2] and oxidative steam reforming, [3] is attractive for larger scale hydrogen production, among which steam reforming has been successfully industrialized. Over recent years, dry reforming of methane (DRM) has aroused much interest since it can convert two strong greenhouse gases into syngas. ...
... As shown in Figure 2(C), after reduction at 700°C for 2 hours, metallic Ni phase is observed in all of the Ni/La 2 O 3 catalysts, accompanying the disappearance of LaNiO 3 perovskite phase, indicating the initial LaNiO 3 perovskite has been fully reduced in all the samples. Additionally, the initial La(OH) 3 observed in 5Ni/La 2 O 3 -PP catalysts becomes its predominant phase with nearly the disappearance of La 2 O 3 phase, possibly due to the reaction between La 2 O 3 species and H 2 O originated from the reduction of LaNiO 3 perovskite. [26] On the contrary, although La(OH) 3 is also detected in the reduced 5Ni/La 2 O 3 -TD now, La 2 O 3 is still the major phase of this catalyst. ...
... [26] On the contrary, although La(OH) 3 is also detected in the reduced 5Ni/La 2 O 3 -TD now, La 2 O 3 is still the major phase of this catalyst. Interestingly, different from PP and TD samples, the reduced 5Ni/La 2 O 3 -GNC consists nearly equal amount of both La 2 O 3 and La(OH) 3 phases. This proves that La 2 O 3 supports prepared with different methods might have varied reactivity with H 2 O. Based on the intensity of the La(OH) 3 diffraction peaks, the proportion of La (OH) 3 follows the sequence of 5Ni/La 2 O 3 -PP > 5Ni/La 2 O 3 -GNC > 5Ni/La 2 O 3 -TD in the samples. ...
To understand the structure‐reactivity relationship of Ni/La2O3, and eventually get more applicable catalysts for DRM, glycine nitrate combustion (GNC), precipitation (PP) and thermal decomposition (TD) methods have been used to prepare La2O3 supports. Although all the supports possess a hexagonal La2O3 phase, their bulk and surface properties are significantly changed. By using them as supports, the interactions between NiO/Ni and La2O3 are varied, thus achieving Ni/La2O3 catalysts with different activity, stability and anti‐coking ability, which follow the order of 5Ni/La2O3‐GNC>5Ni/La2O3‐PP>5Ni/La2O3‐TD. On La2O3 having a higher surface area, a catalyst with a higher active metallic Ni surface area can be achieved. Therefore, the interfaces between Ni and La2O2CO3 can be enlarged, which effectively facilitates the reaction between carbon deposits and the La2O2CO3 formed during the DRM, thus preventing the accumulation of both and keeping the catalyst surface clean, active and stable. In addition, the amount of surface alkaline and active oxygen sites of the reduced catalysts obey the order of 5Ni/La2O3‐GNC>5Ni/La2O3‐PP>5Ni/La2O3‐TD, which is well consistent with the reaction performance. Therefore, these two factors are also believed to be critical to decide the reaction performance. It is concluded that Ni/La2O3 catalysts with high activity, stability and potent anti‐coking ability for DRM can be achieved by preparing catalysts with high Ni dispersion.
... Nickel is preferred due to its high activity, selectivity, and low cost; additionally, its deactivation, due to carbon or sulfide formation, can be reduced using CeO 2 , ZrO 2 , Y 2 O 3 /ZrO 2 , TiO 2 /ZrO 2 , NiO/MgO/ZrO 2 , and Nb 2 O 5 /ZrO 2 (Boukha et al., 2018). CPOX can use gaseous and liquid hydrocarbons; its H 2 /CO ratios are close to 2, whereas the production efficiencies and selectivities are in the order of 70-80% (Silva et al., 2009;Warren and Scheffe, 2018). ...
... The operational conditions of CPOX allow it to be coupled to fuel cells (solid oxide fuel cells, SOFCs) for electricity generation (Arku et al., 2018;Boukha et al., 2018;Cheekatamarla and Finnerty, 2008;Milner-Elkharouf et al., 2020). The SOFCs result advantageous because they operate at high temperatures (800-1000 • C), are not affected by the presence of CH 4 and CO, and can be used for the production of electricity at small and large scales. ...
Hydrogen (H2), more than a promising solution, has been considered one of the pillars to reduce CO2 equivalent
emissions (CO2-eqv) in hard-to-abate sectors like transportation, petrochemical, steel, and cement. The feedstocks,
the greenness of the energy consumed, the synthesis pathway, and the mitigation strategies used during
its synthesis will be determinant to reduce its environmental footprint. In 2021, the production of H2 reached
900 × 105 t; 60% of this production was synthetized from natural gas through reforming processes, which
consumed 205 × 109 m3 of natural gas and 110 × 107 m3 of H2O, and generated 556 × 106 t CO2 and profits for
118 billion US dollars. In 2050, the Net Zero Emissions (NZE) scenario projected by the International Energy
Agency (IEA) for the energy sector will demand 530 × 106 t H2; 60% of this demand will be satisfied by electrolytic
processes launched by green energy sources (wind, solar, hydro, and biomass) and the rest by reforming
processes that use environmental mitigation technologies, like carbon capture utilization and storage (CCUS).
The latter suggests that the non-renewable methane (CH4), the biomethane (bio-CH4), and biomass gasification
processes will be key to lead the gradual decarbonizing of the economy. This work depicts the current status of
mature and emerging reforming technologies to produce H2 and focuses on the environmental implications of H2
production from feedstocks like gas and biogas.
... Additionally, it is thermally stable even above 1000 °C; the surface acidity and basicity can be tuned by controlling the molar ratio of Ca/P [1,50,51]. It can form synergistic interactions with several transition metals due to the high ion-exchange capability [52]. These attributes gave it a comparative advantage [45] and a wide variety of applications as a catalyst in several organic synthesis reactions. ...
... In the case of biodiesel, a detailed review of waste shells-derived solid oxides catalysts for biodiesel production through transesterification reaction between vegetable oils and methanol has been reported [6,97]. Rh/HA and Rh/Al 2 O 3 catalysts were evaluated for partial oxidation and steam reforming of methane; it was found that the conversion and yields of H 2 and CO were comparable [52]. This performance confirms the suitability and properties provided by the HA as a catalyst support. ...
The future of catalyst will be controlled by sustainable environment and renewability. Presently, most of the catalytic materials are sourced from finite geological deposits. Most of the resources rely up on for catalyst manufacture are dwindling and the accumulation of waste and spent catalyst poses a serious environmental problem and economic sabotage. By recovering catalyst from waste and reclamation of active metals from spent catalyst, the dependence on natural resources that are unsustainable will be reduced, likewise the quantity of materials going to landfill. This work introduces circular economic thinking into catalysis and waste and resource management. The utilisation of industrial waste, waste shells (e.g. seashells and eggshells) and animal bones as a source of catalyst not only provides an economic and environmental protection benefits, but also promotes proper waste management strategy and circular economy. This review stimulates and provides a roadmap for the transition of catalytic science and manufacturing into a circular economy. The major contribution of this work is in the design of green and renewable catalyst from waste materials, and reviews their applications in organic synthesis, hydrogen production from waster-gas-shift reaction and gasification of biomass, biodiesel production, oxidation reactions, selective hydrogenation and pollutant degradation.
... First of all, we are talking about platinum-group metals [83][84][85][90][91][92][93], among which rhodium should be noted [77,94,95]. Rhodium alloys with various transition metals are used to improve methane conversion and productivity [76,[96][97][98][99]. At the same time, considerable attention is paid to transition metals, predominantly nickel [100]. ...
Transitioning to energy-saving and renewable energy sources is impossible without accelerated development of hydrogen energy and hydrogen technologies. This review summarizes the state-of-the-art and recent advances of various hydrogen production processes, including but not limited to thermochemical and electrolytic processes. Their opportunities and limitations, operating conditions, and catalysts are discussed. Nowadays, most hydrogen is still produced by steam reforming of methane, its partial oxidation, or coal gasification. Considerable attention is also paid to natural gas pyrolysis. However, hydrogen produced using these technologies has a lot of impurities and needs additional purification. A series of technologies for hydrogen purification, including its filtration through palladium alloy membranes, and membrane catalysis, allowing hydrogen production and purification in one stage, are discussed. The main way to produce carbon-free hydrogen is water electrolysis using low-cost energy from nuclear or renewable sources. Both conventional and novel methods of hydrogen storage and transportation, which are an important part of the hydrogen economy, are reviewed. Biohydrogen production technologies are also discussed. Finally, prospects for further work in this field are provided. This review will be useful to researchers and manufacturers working in this field.
... 44 Furthermore, the appearance of low-density adsorption peaks at higher temperatures, between 650 and 800°C , is typical for strong base sites. These sites, according to previous reports, 21,45 mainly consist of surface OH− species that interact strongly with CO molecules. Basicity increases the CO 2 adsorption capacity, but a strong base makes CO 2 absorption difficult CO 2 that is strongly adsorbed CO 2 can inhibit CH 4 formation. ...
Hydroxyapatite (HA) derived from salmon bone byproducts is used as a green support for the nanostructured nickel catalysts applied in the methanation of carbon dioxide (CO2). Undoped nickel catalysts and various ceria-doped nickel supported on hydroxyapatite (HA) were prepared by coimpregnation. Characteristics of the as-prepared catalysts were investigated by the various techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET), hydrogen temperature-programmed reduction (H2-TPR), carbon dioxide temperature-programmed desorption (CO2-TPD), and energy-dispersive X-ray spectroscopy (EDX). The catalyst activity was assessed throughout CO2 methanation in the low-temperature range of 225-350 °C with the molar ratio of H2/CO2 = 4/1. The function of HA and ceria provided a high dispersity of nickel particles over the catalyst surface with the size range of 24.5-25.8 nm, leading to improvement in the reduction and CO2 adsorption capacity of the catalysts as well as enhancing the catalytic efficiency in CO2 methanation. The 10Ni/HA catalyst reduced at suitable conditions of 400 °C for 2 h showed the highest catalytic performance among the tested catalysts. CO2 conversion and CH4 selectivity reached 76.6 and 100% at a reaction temperature of 350 °C, respectively. The results show that the Ni/HA sample doped with 6.0 wt % ceria was the best, with the CO2 conversion and the CH4 selectivity reaching 92.5% and 100%, respectively, at a reaction temperature of 325 °C.
... On the other hand, the physicochemical characteristics of HA are favorable for POM; hence, it is being investigated. In light of this, Boukha et al. [69] experimented the Rh(x)/HA catalysts (with x = 0.5, 1, and 2 wt%) for POM and compared their performance with that of a commercial 1 wt% Rh/Al 2 O 3 catalyst. Nonstoichiometric HA was used to prepare the supported catalysts by impregnation and observed highest coke resistance among the assessed catalysts. ...
Hydroxyapatite (HA) has been extensively investigated and used in the field of biomaterials, while its application is more recently extended to the catalysis. Large number of catalytic processes are currently exploring the unique structural and chemical features of HA. The existence of diverse pair set of Ca 2+ ions results in exclusive catalytic features and provides scope for metal replacement and enables characteristic tuning according to the targeted catalytic application. This chapter explains the kinetics and mechanisms of selected reactions on HA‐based catalysts. Oxidative coupling of methane, partial oxidation of methane, acetone to methyl isobutyl ketone, and ethanol coupling reaction are the major reactions considered for this chapter.
... As is known from the literature, to produce synthesis gas mainly catalysts based on the noble metals Rh [7], Ru [8], and Pt [9] are used. A Pt-Ru/2 %Ce/(q+a)-Al 2 O 3 nanosized catalyst was developed for POM into synthesis gas. ...
Syngas is an important product of methane processing of natural gas, from which a wide range of chemical products can be obtained in the future. The paper presents the results of a study of the developed Ni multicomponent catalysts in the partial oxidation of CH4 to syngas. A series of samples with different percentages of active components in the Ni – Cr – Al – Mg – glycine catalyst was prepared by the solution combustion synthesis method giving synthesis gas with a high yield. It was found that the highest yields of H2 (90.5%) and CO (58.4%) were obtained at 900°C, space velocity of 2500 h–1 in the reaction mixture: 34% CH4 : 17% O2 : 49% Ar over 25% Ni – 5% Cr – 10% Al – 10% Mg – 50% glycine catalyst. Methane conversion was 97.9% at H2 : CO = 1.55.
... The properties of HAp such as high biocompatibility, bioactivity and osteoconductive, non-immunogenic and non-toxic making it more valuable bioceramic for orthodontic and orthopedic applications [23][24]. Several biomedical applications of nHAp have been reported like bone tissue engineering, bone filler substitute, bone graft, catalysts, periodontal intrabody osseous defects and drug delivery [25][26][27][28][29][30][31][32]. Besides these biological 3 applications, nHAp and its composites are frequently being used for several other applications such as chemical gas sensor [33], alcohol sensor [34], fire alarm wallpaper for thermosensitive sensor [35], biosensor and humidity sensor [36][37][38][39] due to its highly porous and hygroscopic nature [40]. ...
The main objective of present work was to synthesize sodium doped polycrystalline hydroxyapatite NaHAp (NaOH-HAp) powder by employing wet chemical precipitation method and its composites with Al 2 O 3 (0, 10, 40, 70 wt%) using a scalable solid-state reaction method. Detailed investigations of NaHAp and its composites using X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy, Raman spectroscopy, UV-visible spectroscopy, scanning electron microscopy followed by energy-dispersive X-ray spectroscopy and sensing behaviour is carefully described. XRD results exhibited major and minor phase of HAp, Ca 2 P 2 O 7 and NaOH for NaHAp sample while for all fabricated composites of HAp-NaOH-Al 2 O 3 and revealed the major phase of hibonite CaO(Al 2 O 3) 6 along with secondary phases of Ca 2 P 2 O 7 , Na 4 [Al(PO4) 2 (OH)], Na 3 Al(OH)(HPO 4)(PO 4), NaAl 3 (PO 4) 2 (OH) 4 , NaOH. The crystallite size of NaHAp based composites was also determined and lies in the range of 200-2800 nm, which is larger than that of Al 2 O 3. FTIR and Raman spectroscopic studies reveal the bonding formation of P-O, O-P-O, and Al-O due to intramolecular interaction of Na 4 (Al(PO4) 2 (OH)), Na 3 Al(OH)(HPO 4)(PO 4), and NaAl 3 (PO 4) 2 (OH) 4 in the HAp-NaOH-Al 2 O 3 composite, while bonding formation of Al-O-H recognized to intermolecular interaction in between Al with H atoms of Na 4 (Al(PO4) 2 (OH)), Na 3 Al(OH)(HPO 4)(PO 4), and NaAl 3 (PO 4) 2 (OH) 4. The SEM and EDS analysis revealed the presence of all constituent elements of used chemicals which also validate the purity of used materials. It is concluded that the fabricated sensor (60NaHAp-40Al 2 O 3) shows lowest response and recovery time, 4 s and 3 s for the 0.5 vol. % concentration of the LPG. Therefore, among all composites, this fabricated composite can be used for LPG gas sensing applications.
... The volume area mean diameter of metal nanoparticles (d va ) was calculated assuming a spherical morphology upon measuring at least 300 particles using Digimizer software. The values of Me (Rh, Ir, and Ru) dispersion and metallic surface area were calculated according to a procedure described in the literature 32,48,54 ...
... The properties of HAp such as high biocompatibility, bioactivity and osteoconductive, non-immunogenic and non-toxic making it more valuable bioceramic for orthodontic and orthopedic applications [23][24]. Several biomedical applications of nHAp have been reported like bone tissue engineering, bone filler substitute, bone graft, catalysts, periodontal intrabody osseous defects and drug delivery [25][26][27][28][29][30][31][32]. Besides these biological 3 applications, nHAp and its composites are frequently being used for several other applications such as chemical gas sensor [33], alcohol sensor [34], fire alarm wallpaper for thermosensitive sensor [35], biosensor and humidity sensor [36][37][38][39] due to its highly porous and hygroscopic nature [40]. ...
The main objective of present work was to synthesize sodium doped polycrystalline hydroxyapatite NaHAp (NaOH-HAp) powder by employing wet chemical precipitation method and its composites with Al 2 O 3 (0, 10, 40, 70 wt%) using a scalable solid-state reaction method. Detailed investigations of NaHAp and its composites using X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy, Raman spectroscopy, UV-visible spectroscopy, scanning electron microscopy followed by energy-dispersive X-ray spectroscopy and sensing behaviour is carefully described. XRD results exhibited major and minor phase of HAp, Ca 2 P 2 O 7 and NaOH for NaHAp sample while for all fabricated composites of HAp-NaOH-Al 2 O 3 and revealed the major phase of hibonite CaO(Al 2 O 3) 6 along with secondary phases of Ca 2 P 2 O 7 , Na 4 [Al(PO4) 2 (OH)], Na 3 Al(OH)(HPO 4)(PO 4), NaAl 3 (PO 4) 2 (OH) 4 , NaOH. The crystallite size of NaHAp based composites was also determined and lies in the range of 200-2800 nm, which is larger than that of Al 2 O 3. FTIR and Raman spectroscopic studies reveal the bonding formation of P-O, O-P-O, and Al-O due to intramolecular interaction of Na 4 (Al(PO4) 2 (OH)), Na 3 Al(OH)(HPO 4)(PO 4), and NaAl 3 (PO 4) 2 (OH) 4 in the HAp-NaOH-Al 2 O 3 composite, while bonding formation of Al-O-H recognized to intermolecular interaction in between Al with H atoms of Na 4 (Al(PO4) 2 (OH)), Na 3 Al(OH)(HPO 4)(PO 4), and NaAl 3 (PO 4) 2 (OH) 4. The SEM and EDS analysis revealed the presence of all constituent elements of used chemicals which also validate the purity of used materials. It is concluded that the fabricated sensor (60NaHAp-40Al 2 O 3) shows lowest response and recovery time, 4 s and 3 s for the 0.5 vol. % concentration of the LPG. Therefore, among all composites, this fabricated composite can be used for LPG gas sensing applications.
... Ruckenstein et al. [66] observed in the reduction profiles of Rh/La 2 O 3 , Rh/MgO, Rh/Y 2 O 3 and Rh/Ta 2 O 5 a high temperature peak associated with the reduction of LaRhO 3 , MgRh 2 O 4 , YRhO 3 and RhTaO 4 species, respectively. Boukha et al. [69]attributed the higher reduction signal to the incorporation of Rh in the hydroxyapatite structure of the support. In this work the peak at high temperature would indicate a significant interaction between rhodium oxide and lanthanum [70], which is in excess at the surface in the case of LaAlO 3 as it was detected by XPS results. ...
Rh/perovskite catalysts were prepared and evaluated in ethanol steam reforming reaction. The samples were characterized by SBET, XRD, XPS, TPR, TG and SEM techniques. It could be possible to obtain perovskites supports with high purity, stability and suitable specific surface areas. The partial substitution of La with Ca and Ce in a LaAlO3 based perovskite structure was reached and changes in its porosity and surface features were detected. The inclusion of Ca into the structure increased specific surface area of support and Ce inclusion activated oxygen mobility on catalyst surface. The supports influenced the degree of reduction of Rh species at the surface of catalysts. All catalyst showed complete ethanol conversion on 6 h on stream. The best catalytic performance was achieved with Rh/LaAlO3 0.3% catalyst which presented the highest mean value of H2 product distribution, low production of byproducts (CH4 and CO) and presented an excellent long term experience performance during 24 h on stream. Rh/LaCeAlO3 0.3% presented the lowest carbon accumulation during 6 h on stream but it suffered deactivation after 14 h on stream. The deactivation was attributed to the partial oxidation of Rh° species to Rh3+ which are less active favoring a reaction pathway that produce carbonaceous deposits.
... 5a. It is observed that the sole HAP has no obvious reduction signal at low temperature region, but an additional less intense peak can be observed at higher reduction region around 550-700 °C, being probably owing to the consumption of surface oxygen anions species in HAP support[44][45][46][47]. With respect to the Pd/HAP-x catalysts, there are two obvious reduction peaks in the temperature region of 50-800 °C. ...
Selective oxidation of glycerol provides a feasible route towards the sustainable synthesis of high value-added chemicals. Herein, the hydroxyapatite (HAP) supported palladium (Pd) species were fabricated by impregnation and subsequent calcination. The as-obtained heterogeneous Pd catalyst afforded not only excellent selectivity to glyceric acid (GLA) up to 90% with 59% conversion of glycerol but also good recyclability by using molecular oxygen as an oxidant under mild conditions. The characterization of catalysts indicated that both the surface basicity and Pd sites on the catalyst played a crucial role in promoting glycerol oxidation. Notably, it demonstrated that the presence of the vicinal hydroxyl group of glycerol molecule can assist the oxidation reaction via forming a coordination between the vicinal hydroxyl group and Ca²⁺ sites on HAP-derived catalysts. In this catalytic process, the secondary hydroxyl of glycerol kept untouched and the primary hydroxyl of glycerol was converted into carboxyl group, while the Pd species acted as active centers for cooperatively promoting the subsequent oxidation to generate GLA. Additionally, this catalytic system can be extended widely for the oxidative conversion of other vicinal diol into the corresponding α-hydroxycarboxylic acids selectively. Isotope labeling experiment using H2¹⁸O confirmed that H2O not only acted as solvent but also was involved in the catalytic cycles. On the basis of the results, a possible reaction mechanism has been proposed. The HAP-supported Pd catalytic system has been shown to serve as an effective approach for the upgrading of bio-derived vicinal diols to high value-added chemicals.
... This indicates increased CH 4 dissociation over Rh 0 , which was detected by XPS. [79] Analogous to Rh, Ru-based catalysts on different supports have been studied such as Ru/La 2 O 3 and Ru/Al 2 O 3 . [80] Carbon formation on catalyst surface reported being more uniform and reactive. ...
Partial oxidation of methane (POM) offers a promising option to produce syngas for downstream processes such as hydrogen production and Fischer-Tropsch processes. POM in fixed-bed reactors requires an oxygen separation plant with high operation cost and safety risks. On the contrary, membrane reactors can provide an improved process by integrating both oxygen separation and catalytic reaction processes. With many advantages including high purity and efficient oxygen separation from the air at the catalytic reaction conditions, mixed ionic-electronic conducting membranes (MIEC) caught great attention in the scientific research field over the past two decades. In this review, POM using different catalysts in fixed-bed reactors was firstly summarized with emphasizing on perovskite-based catalysts, and then the material screening of MIEC membrane reactors was introduced and linked to the selection of conventional and perovskite catalysts. The catalytic activity, reaction mechanisms, and emerging challenges have been analyzed. Furthermore, future research directions have been outlined by highlighting the effect of electronic properties, continuous reduction-oxidation in the presence of oxygen flux, and chemical reaction mechanism on membrane/catalyst.
... In recent years, methane partial oxidation (MPO, Eq. (3)) has extensively been investigated and appears as a promising candidate to replace other technologies for producing syngas with practical H 2 /CO ratio of 2. In fact, catalyst deactivation associated with carbon deposits due to CH 4 decomposition is fundamentally inevitable for methane reforming processes [16]. In MPO, regulation of O/C feedstock ratio can strategically inhibit the deposited carbonaceous species and hence, governing the robustness of MPO performance [17,18]. Additionally, MPO is reportedly the most costeffective approach for syngas production as the energy requirement and capital cost are 10e15% and 25e30% lower than other reforming techniques, respectively [19]. ...
Thermodynamic equilibrium assessment for methane partial oxidation (MPO) and concomitant parallel side reactions was conducted by employing the Gibbs free energy minimization approach in order to study the tuning of syngas H2/CO ratio appropriate for downstream Fischer–Tropsch synthesis (FTS). The influences of operating conditions including CH4/O2 ratio (5:1–1:2), pressure (1–50 bar) and temperature (200–1000 °C) on MPO performance in terms of reactant equilibrium conversion, product and side product yields and H2/CO ratio were scrutinized. The results reveal that indirect combustion-reforming pathway was possibly the main contributory factor to the syngas yield during MPO. The lower CH4/O2 ratios possessed a positive effect on syngas yield. Carbon formation was favored at low temperatures but it could be suppressed as CH4/O2 ratios reduced at elevated temperatures. Although a rising pressure was disadvantageous for MPO performance but the quantity of carbon deposit was hindered since these processes involved gas volume expansion. A temperature at least of 800 °C and CH4/O2 ratio of 2:1 or 3:2 are the preferable operating conditions for MPO reaction to achieve the carbon-free region meanwhile maximizing the syngas yield with H2/CO ratio of 2 that appropriate for FTS process.
... Deconvolution of Rh 3d region indicated that the presence of two pairs of doublets in fresh and spent Rh/Nb 2 O 5 catalysts. The more intense doublet peaks centered at 307.6 eV and 312.3 eV were ascribed to Rh 0, while the weaker doublet with higher binding energy of 1.2 eV was assigned to Rh 3+ [39,40]. Although the fresh catalyst was pre-reduced before characterized, the surface oxide or hydroxide was inevitably formed. ...
The increasing risk of fossil fuels depletion due to surging energy demand has accelerated the search of alternative renewable sources. Lignin, one of the major components of lignocellulosic biomass, is the only abundant renewable natural resource for aromatic compounds. In this work, the effects of metals and process parameters were discussed via the catalytic hydrogenolysis of β-O-4 lignin model compound, 2-phenethyl phenyl ether. The best results were obtained over Rh/Nb 2 O 5 with an overall conversion of 99.3% and exceptional selectivity of 98.9% to aromatic products when the hydrogenolysis was carried out at 260°C and 0.1 MPa H 2 for 4 h. The high efficiency is mainly attributed to the cooperation between Rh and NbOx at the perimeter sites, in which Rh particles are responsible for the dissociation of H 2 and NbOx for activating CeO bonds. With respect to depo-lymerize of ash wood lignin, the liquid products were mostly aromatics over Rh/Nb 2 O 5 under 0.1 MPa H 2. The results achieved in this work provide a promising prospect towards valorization of lignin into production of valuable aromatic compounds under low hydrogen pressure.
... Among the calcium phosphate ceramics, Hydroxyapatite (HA) with a chemical composition, Ca 10 (PO 4 ) 6 (OH) 2 , C/P ratio of 1.67, and excellent biomaterial properties has the crystalline structure similar to the mineral part of the human skeletal system [1][2][3]. In addition, HA has many uses outside orthopedics including biosensors [4,5], catalysts [6,7], and drug delivery [8,9]. However, despite the beneficial biomaterial properties, the weakness of mechanical properties such as intrinsic brittleness, low fracture toughness, and poor wear resistance still limit the applications of HA [10][11][12]. ...
In this study, we demonstrate enhanced mechanical properties of three-dimensional graphene/hydroxyapatite (3DG/HA) nanocomposites by employing the Taguchi method. The hydrothermal process time, the total pressure, the hydrothermal temperature, and the weight percent of graphene to the hydroxyapatite were considered as control parameters. The results showed that all parameters were effective in controlling the process. According to the findings, the hydrothermal temperature had the greatest effect on the results. The predicted strength fracture toughness was 2.08 ± 0.03 MPa.m0.5, which is within the 90% confidence interval. Microscopic analysis confirmed the presence of graphene sheets with folding and wrinkling in the powders and indicated that various preferential directions played a role in the growth of hydroxyapatite crystals.
... However, they found that Rh favors Ni reduction, which led to an improvement in the conversion and yield alongside 80 % conversion at 750°C (29.5 vol % CH 4 in the feed) [33]. Boukha et al. [34] studied the role of the Rh particle size in hydroxyapatite catalysts for CPOM reaction where Rh(x)/HAP catalysts showed high coke resistance from a stability test which was performed up to 30 h at 700°C. The interaction between the Rh and the TCP support had a negative impact on the performance of the catalyst during CPOM reaction. ...
Herein, this mini review investigates the multifunctional potential of a transition and noble metal catalyst supported on either a single support or combined oxide support in the catalytic partial oxidation of methane (CPOM). Also highlighted is the close interaction and interfacial area between the metal, reducible oxide and acidic support, which are crucial for the low‐temperature CPOM reaction. The effect of the catalyst component and its preparation methods were considered herein. Their impact on the catalytic performance and stability on the CPOM reaction was evaluated. The two main mechanisms of CPOM; direct partial oxidation (DPO) and the combustion and reforming reaction (CRR) mechanism was also covered along with the most recent kinetic studies. Finally, the deactivation of the CPOM catalysts was studied in term of coke and carbon deposition along with the CO poisoning.
... Also, having properties such as bioactivity and osteoconductivity make HA one of the most valuable materials for orthopedic applications [4][5][6]. The applications of this bioceramic are very broad include orthopedics, biosensors, catalysts and drug delivery [7][8][9][10][11][12]. However, the poor mechanical properties of HA such as fracture toughness, intrinsic brittleness, and poor wear resistance still limit the applications of HA despite its excellent biomaterial properties [13][14][15]. ...
In this investigation, reduced graphene oxide/hydroxyapatite (rGO/HA) hybrid powders have been synthesized using hydrogen gas injection into hydrothermal autoclave. The powders were then consolidated with spark plasma sintering. The results showed that this method of synthesis caused the obtained powders to have high crystallinity. Microscopic analysis confirmed the presence of rGO sheets with folding and wrinkling in the nanocomposite and indicated that various crystalline planes such as (002) and (300) played a role in the growth of hydroxyapatite crystals. The results of interface analysis (HA||rGO) showed that the HA is coherently connected by its (300)-planes with the surface of the rGO sheets and this coherency is accomplished in the rGO cross-section with the (002) planes of HA. The hardness and the Young's modulus of the composite samples were 5.9–6.6 GPa and 132–146 GPa respectively.
... The internal polarization of the HAP carrier promotes the separation of photogenerated electrons and holes in the TiO 2 nanoparticles, thus mitigating the recombination of charge carriers. In order to meet the requirements of different catalytic reactions and applications, different types of metal or metal-oxide supported HAP catalysts have been developed, such as Pd and Ni-doped HAP catalysts for catalytic reaction of methane oxidation [226], Rh(x)/ HAP (x = 0.5, 1 and 2 wt%) catalysts for partial oxidation of (POM) and steam reforming (SRM) of methane [227], Au nanoparticles (AuNP)/ HAP nanowires catalytic paper for continuous flow catalytic reduction of 4-nitrophenol and an organic dye [228]. ...
For the first time, a catalyst containing 5 wt. % Ni supported on a hydroxyapatite support (HAP) has been synthesized and evaluated in tri‐reforming of methane (TRM) for synthetic gas (syngas) production. Nickel nanoparticles could be easily formed on HAP surface by using the conventional incipient wetness impregnation technique. In TRM, under unfavorable reaction conditions from the thermodynamic point of view ( e. g . medium reforming temperature of 700–800 °C, low steam‐to‐carbon ratio, etc.), this catalyst showed high methane conversion (up to 90 %), however, some deactivations took place. The latter could be explained by both the thermal sintering of nickel nanoparticles and the solid carbon formation. The impact of the main operational parameters has been studied. Increasing the reaction temperature, the molar ratios of oxygen‐to‐carbon and steam‐to‐carbon are favorable for the methane conversion and mostly for the stability of the catalyst. High methane conversion of ca . 90 % with a perfect catalytic stability during 300 hours‐on‐stream could be achieved at 800 °C and 1.4 bar, using a mixture containing low ratio of oxidant‐to‐carbon (molar ratio of CH 4 /CO 2 /H 2 O/O 2 /N 2 =1.0/0.67/0.9/0.1/0). These results offer the opportunity to further design an optimal Ni/HAP catalyst by improving metal‐support interaction and downsizing nickel nanoparticles.
Permanently polarized hydroxyapatite (p-HAp) has been reported as a feasible green alternative to conventional catalysts for the selective conversion of CO 2 into highly-valuable chemical products. However, the structural control and...
Valuable 5-amino-1-pentanol (5-AP) was efficiently synthesized from biomass-derived dihydropyran over hydroxylapatite nanorod supported Ni catalysts (Ni-HAP), by coupling the in situ generation of 5-hydroxypentanal (5-HP, via the ring-opening tautomerization of 2-hydroxytetrahydropyran (2-HTHP)) and its subsequent reductive amination. The Ni-HAP catalyst with 10 wt% Ni loading exhibited the highest 5-AP yield than the other Ni-HAP catalysts with different Ni loadings and several commercial hydrogenation catalysts, including Ru/C, Pt/C and Pd/C, as well as Raney Ni. High Ni particles dispersion, high reducibility and acidic intensity were found to account for the superior catalytic performance of 10Ni-HAP. The effect of reaction parameters on the catalytic performance was investigated, and an excellent 5-AP yield of 92% was achieved under mild reaction conditions of 80 °C and 2 MPa H2. The stability of the Ni-HAP catalyst was studied using a continuous flow reactor, and the sintering of Ni nanoparticles was considered as the major reason for the decline in hydrogenation activity.
Graphical Abstract
The Ni-HAP catalyst exhibited high catalytic performance to synthesis 5-AP due to the high Ni particles dispersion, reducibility and acidic intensity.
In the thermo-catalytic hydrogen production from methane, catalyst deactivation is a major issue for the industrial applications. In this review, six modification strategies to achieve a robust catalyst have been discussed in category, including preparation methods, metal-support interaction, support confinement, surface acidity and basicity, oxygen defects and alloys. In addition, the reaction mechanisms of seven methane-involved reactions and three deactivation mechanisms (poisoning, sintering and coking) are illustrated. Moreover, a critical analysis of the synthesis-structure-performance relationship is provided. Finally, conclusive remarks and prospects are proposed.
Hydroxyapatite (HA) is an emergent catalytic material able to contribute effectively to the improvement of a number of strategic applications, including energy transformation and the generation of high added‐value products processes. As a catalyst support, it is widely used because of its interesting structural and chemical properties. In this mini‐review, we report the application of HA as a catalyst support for the methane reforming reactions. A special attention will be paid on its promising results as a catalyst carrier in the dry reforming of methane (DRM), a process in which it has been mostly investigated. The key parameters that make HA‐based catalysts suitable are described with an emphasis on its competitive performance to the state‐of‐the‐art catalysts.
The use of exhaustible natural resources to make materials for catalytic applications with limited service life, which in due course end up as waste, is an economic sabotage. There exist a number of developed techniques to recover value‐added materials from waste eggshells and animal bones. The objective of this chapter is to shed light on techniques for recovering valuable materials from these wastes and highlight potential applications with special attention on heterogeneous catalysis. In this chapter, an inclusive review is presented at several scales, where eggshell and animal bones are used as raw materials to produce valuable materials such as calcium carbonate/oxide, calcium phosphate, adsorbent, filler, and hydroxyapatite (HA). Their catalytic applications in biodiesel production, gasification, carbon dioxide capture, organic synthesis, fuel cell membrane, hydrogen production via water–gas shift (WGS) reaction, methane oxidation coupling, and other prospective reactions were explored. Eggshells and animal bones are proven source of renewable and greener catalyst to substitute conventional counterparts. They have also demonstrated suitability and stability for industrial applications including adsorbent for wastewater treatment and flue gas treatment sorbent, particularly carbon dioxide capture. Additionally, the present chapter highlights potential areas that need scale‐up to permit the use of these materials as biofilter adsorbent and heterogeneous catalysts. It also identifies areas at research and development stages. This chapter discusses the properties of eggshells and animal bones that make them potential candidate for sorbent, HA material manufacture for medical applications and in heterogeneous catalysis. Finally, the various applications of eggshell and animal bones as catalyst are presented and discussed. This promotes the concept of circular economy in which waste materials are reused as a raw material for the production of new materials or recycled into more useful material.
Selective oxidation catalysis is of prime importance for the production of many key intermediates. The control of selectivity in the presence of gaseous oxygen is quite challenging and requires the development of efficient catalysts. Thanks to their large composition range, resulting from the modulation of their stoichiometry and substitution ability, hydroxyapatites can be modified by many metals, which make them original catalytic systems gathering both tunable acid–base properties and redox properties. In this chapter, we will show the interest of metal‐modified hydroxyapatite catalysts for various classes of oxidation reactions. Due to their large range of stability, they can be implemented either in liquid phase for aerobic oxidation of alcohols (Pd, Ru) or at very high temperature (>600 °C) for gas phase alkane oxidation, partial oxidation of methane (Rh, Ni), and oxidative dehydrogenation of other alkanes (V, Co). The influence of the metal incorporation method (ion exchange, impregnation in excess of solution, or coprecipitation) on the metal dispersion and catalytic performance will be discussed. Finally, the role of the basic properties of hydroxyapatites in the alkane activation will be discussed.
The rapid and severe deactivation of methane partial oxidation catalysts remains a major hindrance restraining its potential in commercialization and industrialization for large-scale syngas production. It is imperative to provide an in-depth understanding discission about the intrinsic and synergistic interactions of catalyst components toward the catalyst efficiency during reforming reactions. This review presents a contemporary evaluation of recent works on synergistic relationship among catalyst components (support, active metals and catalyst structure) during methane partial oxidation using theoretical and state-of-the-art experimental procedures. Advancements achieved through this synergistic relationship not only enhance properties of existing catalysts but also leading to discovery and development of novel catalyst systems. Thermodynamics, reaction mechanisms, catalytic performances, catalyst deactivation induced by carbonaceous deposition and reaction kinetic modeling have been successfully explored and described using information from these essential interactive factors over these decades. This viewpoint explains the roles of the interactions and their functions toward exploration of efficient catalysts systems for industrial applications.
Homogeneous catalysts exhibit several drawbacks, such as high cost, difficulty of separation, and non-recyclability, necessitating the development of heterogenous catalysts to solve these problems. Hydroxyapatite (HAP) can be used as a catalyst carrier owing to its surface acid–base adjustability and strong ion exchange properties. Oyster shell is one of the primary natural sources of hydroxyapatite. The use of cuttlefish bone as a source of hydroxyapatite can also facilitate the recycling of bone waste and reduce the consumption of scarce biological resources and limit environmental pollution. In this study, a heterogeneous catalyst was synthesized by the immobilization of Pd on hydroxyapatite obtained by the hydrothermal method using naturally abundant oyster shells as the calcium source, ammonium dibasic phosphate as the phosphorus source, and cetyltrimethylammonium bromide as the porogen. The synthesized heterogeneous catalyst was characterized using N2-physisorption (adsorption–desorption) measurements, X-ray diffraction analysis, X-ray photoelectron spectroscopy, inductively coupled plasma mass spectrometry, transmission electron microscopy, and temperature-programmed reduction and desorption measurements. The Pd/HAP catalyst shows high catalytic activity for the cross-coupling reaction between iodobenzene and olefins, resulting in yields greater than 67%. Moreover, the Pd/HAP catalyst could be recycled and reused six times with slight reduction in the yield, and thus, is highly stable and recyclable. In addition, it is suitable for use in Heck cross-coupling reactions between various aryl halides and alkenes. Thus, the synthesized catalyst should find extensive use in synthetic chemistry.Graphical abstract
Methane (CH4) is the major component of currently abundant natural gas and a prominent green-house gas. Steam reforming of methane (SRM) is an important technology for the conversion of CH4 into H2 and syngas. To improve the catalytic activity and coking resistance of SRM catalysts, great efforts (including the addition of promoters, development of advanced supports, and structural modification, etc.) have been made with considerable progress in the past decade. Meanwhile, a series of novel processes have been explored for more efficient and energy-saving SRM. In this scenario, a comprehensive review on the recent advances in SRM is necessary to provide a constructive insight into the development of SRM technology, however, is still lacking. Herein, the improvements in catalyst construction for conventional SRM and the newly developed SRM processes in the past decade are presented and analyzed. First, the critical issues of SRM catalysts are briefly introduced. Then, the recent research advances of the most popular Ni based catalysts and the catalysts based on the other non-noble metals (Co, Cu, Mo etc.) and the efficient but costly noble metals (Au, Pt, Pd, Rh, Ru etc.) are discussed. Furthermore, the development of the representative modified SRM processes, including thermo-photo hybrid SRM, sorbent enhanced SRM, oxidative SRM, chemical looping SRM, plasma and electrical-field enhanced SRM, is demonstrated, and their advantages and limits are compared. Finally, a critical perspective is provided to enlighten future work on this significant area.
A number of mixed oxide Ce0.75Zr0.25O2 supports were prepared, tested for reproducibility, and characterized by physicochemical methods. The most reproducible preparation method was adapted for depositing the mixed oxide on a FeCrAlloy mesh substrate coated by a protective alumina layer. Based on the obtained structured Ce0.75Zr0.25O2/θ-Al2O3/FeCrAl support, the Rh/Ce0.75Zr0.25O2/Al2O3/FeCrAl and Pt/Ce0.75Zr0.25O2/Al2O3/FeCrAl catalysts were prepared and tested in the reactions of partial oxidation of LPG and deep oxidation of anode gases, respectively. Rh/Ce0.75Zr0.25O2/Al2O3/FeCrAl provided complete conversion of LPG into synthesis gas of a composition close to the equilibrium one. Pt/Ce0.75Zr0.25O2/Al2O3/FeCrAl provided complete conversion of all components of the anode gases at GHSV = 20,000–40,000 h⁻¹; however, at higher GHSV values, methane conversion decreased. The studies on the effect of methane content on deep oxidation of anode off gases showed that methane conversion began to decrease at a 1.5-fold excess of methane and dropped to 50% at a 10-fold excess of methane.
La production du biogaz ne cesse d’augmenter dans le monde entier. La combustion pour produire de la chaleur et de l’électricité, de même que la production du biométhane pour l’injection au réseau de gaz de ville sont les deux applications industrielles majeures du biogaz. La recherche actuelle sur la valorisation du biogaz se focalise sur la production des produits à haute valeur ajoutée comme l’hydrogène pour la mobilité. C’est l’objectif principal du projet VABHYOGAZ3, financé par ADEME, qui vise à déployer la production d’H2 à partir du biogaz dans le Tarn. Le procédé de reformage du biogaz adopté par les partenaires industriels du projet VABHYOGAZ3 est le vaporeformage, qui est couramment utilisé dans l’industrie pour reformer le gaz naturel, et qui est un procédé fortement énergivore. Cette thèse a pour objectif de développer des catalyseurs performants pour le reformage à sec du méthane (RSB : conversion de CH4 et CO2 en syngas – mélange de CO et H2) et pour le tri-reformage du méthane (Tri-RB : conversion de CH4, CO2, H2O et O2 en syngas). Le but ultime est d’optimiser l’efficacité énergétique du procédé global de la production d’H2 via le reformage du biogaz, qui est indispensable pour rendre ce procédé économiquement viable. En fait, les catalyseurs en RSB et Tri-RB ont souvent le problème de désactivation catalytique en raison du dépôt de coke et du frittage thermique à haute température (> 700°C). L’obtention d’un catalyseur performant sous les conditions sévères de RSB et Tri-RB est crucial pour le déploiement de ces procédés à large échelle industrielle. Dans un premier temps, une étude sur la thermodynamique des procédés globaux de la production d’H2 via le reformage du biogaz a été effectuée. Les bilans de matière et d’énergie de ces procédés ont aussi été réalisés par la simulation sur Aspen Plus. Ensuite, différents catalyseurs à base de nickel supporté sur les supports d’hydroxyapatite (HAP) et d’hydroxyapatite substituée au Mg (Mg_HAP) ont été préparés et caractérisés. Les supports à base d’HAP sont considérés comme des nouveaux matériaux catalytiques qui ont des propriétés appropriées en catalyse hétérogène, en particulier pour des procédés à haute température tels que RSB et Tri-RB. Dans cette étude, les supports HAP ayant les rapports molaires de Ca/P de 1,55, 1,67 et 1,75, et Mg_HAP (substitution de 2,2, 5,8 et 8,5% de Ca par Mg) ont été synthétisés. Ces supports ont été dopés avec 5% en masse de Ni par imprégnation à sec. Ces catalyseurs ont été évalués dans les deux réactions de RSB et Tri-RB dans un réacteur à lit fixe. Une étude paramétrique sur l’influence des conditions opératoires incluant la température, la pression totale, le débit d’alimentation du biogaz, et le rapport molaire de la vapeur d’eau sur méthane (S/C) et d’oxygène sur méthane (O/C), a été effectuée. L’objectif a été de comparer et d’identifier les meilleurs catalyseurs et les meilleurs conditions opératoires. Les bilans de matières ont été établis. Les raisons de la désactivation catalytique ont été mises en évidence. Enfin, la stabilité catalytique des meilleurs catalyseurs a été étudiée pendant 150-300 h de réaction. Les résultats obtenus montrent que les catalyseurs à base de Ni supporté sur HAP ou Mg_HAP sont compétitifs par rapport aux meilleurs catalyseurs identifiés dans la littérature. Ce travail confirme également l’intérêt de l’utilisation des nouveaux supports à base d’HAP dans la catalyse hétérogène et en particulier dans les procédés à haute température.
Rh-based bimetallic catalysts are promising ligand-free heterogeneous catalysts for hydroformylation reactions. It is important to understand the mechanism of this bimetallic promotion for designing highly selective and active heterogenous catalysts. In this work, the RhCo bimetallic catalyst was investigated focusing on the promotion effect of Co for the gas-phase hydroformylation of ethene. Adding Co to Rh increased both the catalytic productivity and selectivity to oxygenates. In situ diffuse reflectance infrared Fourier transform spectroscopy and CO-temperature programmed desorption were used to characterize CO adsorption. The results showed that the addition of Co to Rh changed the CO adsorption modes and strength for the Rh-based catalyst. Modulated CO adsorption strength was important to enhance selectivity. Density functional theory calculations were carried out to reveal the reaction mechanism. A reaction pathway was proposed to clarify the reason for enhanced selectivity on a RhCo bimetallic catalyst and show that the ratio between CO migration and desorption played a great role in this reaction.
The surge of methane as the major component of natural gas and a dominant greenhouse gas calls for the development of efficient strategy to convert it into valuable liquid chemicals such as methanol. However, the current industrial method for methanol synthesis, which consists of the stream reforming of methane (SRM) to syngas and the followed reaction of syngas to methanol, is rather energy-intensive. Direct conversion of methane into methanol (DMTM) is highly desirable in terms of energy efficiency and economy. DMTM with water as oxidant, i.e. SRM to methanol, is a promising solution but the development highly efficient and selective catalyst remains a critical challenge. Mo6S8-based catalysts have shown possible potential for this reaction process. In this thesis, for the first time, the feasibility of using Mo6S8 cluster and a series of single metal atom (K, Ti, Co, Fe, Ni, Cu, Rh) doped Mo6S8 as catalysts for SRM to methanol were evaluated via DFT calculation. Chapter 4 provided the structure of Mo6S8 and M-Mo6S8 (M=K, Ti, Co, Fe, Ni, Cu, Rh) clusters and confirmed that they were stable under the reaction condition of SRM. The catalytic behaviors of Mo6S8 and M-Mo6S8 clusters toward CH4 adsorption and dissociation were evaluated in chapter 5. All metal dopants, except K and Rh, showed enhanced CH4 adsorption compared to bare Mo6S8 via the ensemble effect (the direct participation of M in binding), while CH4 adsorption was weakened on K- and Rh-Mo6S8 due to the ligand effect (the modification of the electronic structure of Mδ+ and Moδ+). Meanwhile, the doping of Co, Fe, Ti, Ni, and K accelerated the first hydrogen abstraction of CH4 while all the metals suppressed the further dissociation of *CH3, suggesting their great potential for selective CH3OH synthesis and high coking resistance. In chapter 6, the catalytic behaviors of Mo6S8 and M-Mo6S8 clusters toward H2O adsorption and dissociation were evaluated. H2O adsorption was enhanced by all metal dopants except Rh. The ensemble effect was dominating in enhancing the H2O adsorption. Compared to the bare Mo6S8 cluster, the first hydrogen abstraction was enhanced by all the single metal atom dopants, whereas the second abstraction was mostly suppressed except Ti and Fe. Furthermore, since H2O possessed stronger adsorption than CH4 on the same active site, the initial step for CH3OH synthesis via SRM was considered to be the dissociation of *H2O to *OH and/or *O species. Based on the results obtained in the above chapters, in chapter 7, the reaction pathways of SRM to methanol on Mo6S8 and M-Mo6S8 were set up and the energy barrier for each elementary step was calculated.
Study of the activity of Al - Co - Mg - Mn catalysts prepared at 300–600 °C by solution combustion synthesis method was carried out in partial oxidation of methane to synthesis-gas. The developed catalysts make it possible to achieve 98% conversion of CH4, high yields of H2 − 98–99% and CO − 40–43% at 900 °C and GHSV = 2500 h⁻¹. The presence of simple and mixed oxides, metal aluminates and spinel-type structures in catalysts, the presence of which contributes to the active work of the methane oxidative conversion catalysts, has been established.
Calcium-deficient hydroxyapatite (HAP) non-porous supports were synthesized from Ca(NO3)2 and NH4H2PO4 (conventional synthesis route, HAP_N support) and from CaCO3 and H3PO4 (new synthesis route, HAP_C support) by wet chemical precipitation method. Monometallic and bimetallic supported catalysts were prepared by incipient wetness impregnation using Co(NO3)2 and Ni(NO3)2 salts. The size of Co- and Ni-based particles varied from some nm to dozens nm. Dry reforming of methane (DRM) tests at 700 °C, and 1.6 bar shows that HAP-supported cobalt catalysts were systematically more active than HAP-supported nickel catalysts, which is a new finding in DRM. Increasing the molar ratio of Ca/P from 1.43 (HAP_N) to 1.60 (HAP_C) support led to a slight decrease of the catalytic activity but an improvement of the catalytic stability. Thus, HAP_C support synthesized from CaCO3 and H3PO4 is a good candidate for designing an efficient catalyst for the DRM reaction.
Monometallic and bimetallic RhNi catalysts supported on γ-alumina and yttria-modified alumina were developed. The relation between the support kind, the structure and catalytic properties of the catalysts in dry reforming of methane was studied by using different techniques such as: N2 isotherms, UV-Vis DRS, XRD, TPD-NH3, TPR-H2, XPS and HRTEM. Variation in the electronic environment of the nickel, rhodium and yttrium atoms as a function of the catalyst pretreatment and support kind was observed. It was shown that the couple pairs of Nio/Niⁿ⁺ and/or Rho/Rhⁿ⁺ act as active sites. The difference in the catalytic behaviors of catalysts was connected with the distribution of metal particles. Y-loaded monometallic Ni catalyst showed higher activity and stability than alumina supported Ni due to the proximity of nickel to yttrium oxide species expressed by the increase of metal dispersity. The close contact between Rh and Ni leaded to better catalytic behavior of bimetallic RhNi/Al catalyst due to the better reducibility of nickel oxide species and homogeneous distribution of the particles with average diameter size of ∼ 5 nm.
In the present study, a series of four hydroxyapatite (HAP) supported palladium samples, with a Pd loading close to 0.5%, obtained through their calcination at 773, 873, 973, or 1073 K has been investigated. These samples have been characterized using a wide battery of complementary techniques. From these studies, it was found that the rise of the calcination temperature induces a progressive dehydroxylation of the support and a structure evolution of the species containing Pd²⁺, from tetrahedral (Td) to square planar geometry (D4h). Moreover, this enhances markedly the metal-support interactions. For instance, at the highest temperature (1073 K), Pd particles were found encapsulated by a thin support layer. Consequently, two distinct reducible species have been identified; one of them manifests SMSI. This increase in the Pd-HAP interaction strength seems to (i) expand the HAP lattice, (ii) change the Pd²⁺ coordination from Td to D4h geometry, (iii) promote PdO reduction and (iv) suppress CO chemisorption. These entire properties do compensate the poor textural properties and benefit the efficiency and stability of the Pd active phase in methane oxidation reaction.
Perovskite-based catalysts with a self-regeneration property, named intelligent catalysts, were developed for CH4 partial oxidation, and the effect of redox treatment on regeneration behavior of the catalytic activity was investigated. Rh-substituted catalysts such as LaM0.95Rh0.05O3 (M: Cr, Co, Fe) were prepared by the Pechini method. Among the catalysts, LaCr0.95Rh0.05O3 exhibited the highest and the most stable catalytic activity for partial oxidation of methane before and after redox treatment. The precipitation and dissolution of Rh species were confirmed by X-ray photoelectron microscopy and transmission electron microscopy, indicating that the catalyst regenerated itself via the redox process.
Carbon monoxide oxidation over oxidized Rh surfaces is known to be sensitive to both the oxygen species present as well as the surface temperature. Although CO oxidation on Rh(111) is a prototypical heterogeneously catalyzed oxidation reaction, questions remain about the reactivity of the individual oxygenaceous phases present on the surface. For example, the effects of surface temperature or the duration of CO exposure have not been previously determined on the oxygen-rich, (2 × 1)-O surface adlayer. In this paper, we present results from a study that used a combination of ultra-high vacuum surface science techniques to measure the oxidation of CO by oxygen in the (2 × 1)-O adlayer. The surface temperature during the CO exposure was varied between 100 K and 350 K, and the effect of the surface temperature on CO oxidation was determined for CO exposures between 5 L and 300 L. We observed that the surface temperature had little effect on the CO2 yield or the amount of residual oxygen for CO exposures up to 300 K, but these quantities were reduced after CO exposures above 300 K. We also found that CO oxidation was unchanged by the extent of the CO exposure at both 300 K and 350 K. Taken together, these results show that CO was oxidized over the (2 × 1)-O adlayer during CO exposures above 300 K via a different reaction pathway than the one followed by co-adsorbed O and CO in the (2 × 2)-2O+CO adlayer and that these lower-barrier reactive sites were not regenerated during the CO exposure.
Methane partial oxidation (POM) by a catalytic membrane reactor is a promising process by integration of oxygen separation and catalytic reaction to produce syngas, an important feedstock for downstream processes. However, high methane conversion and syngas yield require high temperature operation (>850 °C) due to dry/steam reforming involvement, which leads to high-energy consumption and poor catalyst stability. In this study, a novel asymmetric membrane reactor incorporated with a catalyst layer of enriched oxygen vacancy was designed for direct partial oxidation (DPO) of methane to syngas. A composite of Sm0.2Ce0.8O2−δ (SDC)/γ–Al2O3 supported Ni catalyst was coated on La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) membrane for the reactor. It was found that activated oxygen species (O2⁻² and O⁻²) from SDC surface favours syngas formation (86% CH4 conversion and 92.5% CO selectivity) on the catalyst layer at 750 °C. The introduction of oxygen vacancies to the catalyst layer maintains the active oxygen species in catalysis and promotes DPO of methane over CH4 combustion at reduced temperature.
This review underlines the importance of the developments in perovskite catalysts for methane combustion from the past up to the present. In this review, after a general and brief introduction to perovskites, the mechanisms of catalytic combustion of methane have been included. Moreover, current studies on perovskites have been summarized including the effects of substitutions, doped perovskites, perovskite preparation methods, and the effect of sulfur presence on perovskite catalysts. Besides, recent studies on perovskite oxides and phenomenon of oxygen (O2) deficiency, porous perovskite oxides, and nanostructured perovskites have been conducted. In addition, partial oxidation of methane (POM) has been reviewed. The loss of active component during the POM reaction can take place in the nickel catalyst, in particular. Since nickel has a lower melting point than noble metals and other active components, such as Co and Fe, in general, to deactivate nickel is easier. Compared with conventional structure, the porous structure with the unique morphology significantly enhances the catalytic activity through a much larger surface area (SA) and greater reactivity of the active sites. Furthermore, the monolithic nanoarrayed perovskite presents very good results in well‐defined faceted catalysts and takes part in porous channel hydrocarbon combustion. This review study is prepared as a guide to cover the profound knowledge of perovskite oxides catalysts, considering the methane combustion reaction mechanisms, and addresses prospective studies in this field for researchers. • In this review, we summarize the recent developments in perovskite catalyst for methane combustion. • The effects of substitutions, doped Perovskites, perovskite preparation methods, and the effect of sulfur presence on perovskite catalysts have been examined. Besides, the phenomenon of oxygen deficiency, porous perovskite oxides, and nanostructured perovskites have been issued. In addition, partial oxidation of methane (POM) has been reviewed. • Recent advances in catalytic materials and potential mechanisms will probably cause perovskite catalysts to show excellent activity in the near future.
Co and Rh+Co catalysts supported on Al2O3 and CeO2 were investigated by temperature programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The CeO2 defects, resulted from the effects of the metals, were further analyzed by Raman spectroscopy and optical absorption. Although the interaction of Co with these two supports is fairly different, it can be concluded that Rh inhibits the strong Co-support interaction. It was revealed that Co over Al2O3 forms mainly Co²⁺ compounds, only a smaller fraction of cobalt is in metallic state. After the addition of 0.1 % Rh, Co3O4 like species is dominant, the amount of metallic state increased after reduction. Over CeO2 the Co dissolution into the support was inhibited by Rh. A wide range of TPR results proved the stepwise reduction of Co, which was promoted by the addition of Rh. By Rh the entire mechanism of this process was altered that can be observed even by XPS. On the basis of the Raman and the optical measurements we concluded that the metals induce defect sites on the CeO2 surface, and these appear as similar features on the spectra of Co and Rh containing samples, thus their density depends on the metal loading and not on the metal type. CeO2 has a bandgap of 3.27 eV, which is not altered by the metals, but an electronic contact was detected between the metals and CeO2 by photovoltammetry. The increased number of metallic species is mainly responsible for the higher catalytic activity and for the enhanced hydrogen selectivity in the stream reforming of ethanol.
Graphical Abstract
The nature and effects of rhodium and antimony doping in TiO2 have been investigated using X-ray diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS), Extended X-ray Absorption Fine Structure (EXAFS) analysis, X-ray Absorption Near Edge Structure (XANES) analysis and diffuse reflectance spectroscopy. Together these techniques build up a comprehensive picture of the dopant chemistry in this system. A range of Sb/Rh ratios have been analysed, and it is shown that the Fermi level can be tuned within the band gap through control of dopant stoichiometry, offering the possibility of designing Z scheme componetns with specific band offsets. Spontaneous spatial segregation of dopants is measured using TiO2(110) single crystals. This is shown to have a direct impact on the electronic structure in the region of the semiconductior surface, creating band bending that is expected to be critical for photocatalytic activity of these materials.
Carbonate-containing hydroxyapatites with different Ca/P ratios and optionally containing Na+ cations were successfully synthesized using a precipitation method. The solids were extensively characterized by XRD, LEIS, XPS, IR, TGA and NMR. Further, their acid–base properties were determined by NH3-TPD, PEA-XPS, CO2-TPD and by pulsed liquid chromatography using benzoic acid as a probe. The so-obtained structural, textural and acid–base properties could be finely correlated to give a clear picture of the system. The acidic properties of hydroxyapatites were attributed to Ca2+, surface HPO42− and OH− vacancies and the basic properties were attributed to PO43−, OH− and CaO species. The fine-tuning of the amount, of the nature and the strength of acid–base properties derived by varying the carbonate content in hydroxyapatites can find applications in catalysis, which was illustrated by isopropanol reactivity.
The effects of reduction by H2, heat treatment in vacuum and in O2 flow on Rh particle size changes of Rh/CeO2 samples were studied by X-ray photoelectron spectroscopy (XPS), high resolution electron microscopy (HRTEM) and CO adsorption followed by diffuse reflectance infrared spectroscopy (DRIFTS). Low-temperature (373-423 K) reduction of Rh without agglomeration is demonstrated. An average particle size of 2.3 ± 1.1 nm was measured by HRTEM regardless of the metal loading (1-5%). On Rh/CeO2 significant particle size increase of the Rh particles was detected on heating (773 K). In this work, we suggest that the temperature-induced surface decrease resulting from the sintering of Rh is only favored for well-dispersed particles. XP spectra revealed that the mobile oxygens of CeO2 fundamentally determine the oxidation state of the supported metals. At elevated temperature, the oxidation of the reduced support surface as well as the metal component takes place because of the segregation of ceria-oxygens. When the aggregated particles were re-oxidized, re-dispersion of the Rh was observed, probably because of the formation of Rh-O-Ce bonds.
Fine particles of Rh doped SrTiO3 (SrTiO3:Rh) were prepared via a newly developed facile water-based process; stable aqueous precursor solutions were prepared by simple mixing of stable aqueous titania sol with Sr and Rh salts in the existence of an acrylic emulsion, then dried and finally calcined in air at 900-1050°C. The SrTiO3:Rh particles prepared at 1000°C were smaller than 50 nm in diameter and exhibit much higher efficiency for H2 evolution from methanol aqueous solution under visible light (e.g. 13.2 % of quantum yield at 420 nm) than those prepared by conventional solid state reaction (~ 5%). On the other hand, the SrTiO3:Rh particles prepared from the aqueous titania sol without the acrylic emulsion or from other aqueous titania precursors with the acrylic emulsion were found to have increased particle size up to 100 nm, and exhibited lower photocatalytic activity, indicating that the combination of the aqueous titania sol and the acrylic emulsion effectively suppress the particle growth, and consequently provide the SrTiO3:Rh particles having small particle size and high crystallinity, both of which are necessary to achieve highly efficient photocatalysis.
Ionically dispersed Rh over CeO2 in Rh/CeO2 catalysts prepared by a single step solution combustion method is shown to improve the redox property and catalytic activity. The H2/Rh ratio obtained from hydrogen uptake measurement was 5.4, 2.4, and 2.1, respectively in 0.5, 1, and 2% Rh/CeO2, indicating a significant contribution from the reduction of CeO2 in the presence of Rh. In 1% Rh/CeO2, the light-off temperature for CO oxidation is about 80 °C lower compared to Rh metal and 190 °C lower than that of Rh2O3. The enhanced redox property and CO oxidation activity of the catalyst has been correlated with the structure. The X-ray diffraction (XRD) pattern could be refined to the fluorite structure with Rh substituting in the Ce site. Transmission electron microscopy (TEM) images show only CeO2 crystallites of about 50 nm and no evidence of any metal particles up to 1 atom % Rh. X-ray photoelectron spectroscopy (XPS) studies demonstrate that Rh is dispersed in the +3 oxidation state on CeO2 with enhanced Rh ion concentration in the surface layers. An average coordination number of 2.5 at a distance of 2.05 Å in the first shell is obtained around Rh ions from extended X-ray absorption fine structure (EXAFS) spectroscopy, indicating an oxide ion vacancy around the Rh ion. The correlations at 2.72 and 3.16 Å correspond to Rh−Rh and Rh−Ce interactions, respectively. Thus, the enhanced catalytic activity of Rh/CeO2 is shown to be due to the formation of a Ce1-xRhxO2-δ type of solid solution with −□−Rh3+−O−Ce4+− kind of linkages on the surface.
The reaction of methane with surface oxygen as well as the interaction of methane/oxygen mixtures with a Rh(1 wt%)/?-Al2O3 catalyst was studied by applying the temporal-analysisof-product (TAP) reactor. The product distribution was strongly affected by the degree of surface reduction. CO2 is formed as a primary product via a redox mechanism with the participation of surface oxygen. The dehydrogenation of methane yielding carbon deposits on the surface occurs on reduced surface sites. The formation of CO proceeds with high selectivity (up to 96%)at 1013 K via fast reaction of surface carbon species with CO2.
In the present paper, comprehensive temperature-programmed reduction and X-ray diffraction studies were conducted, with particular emphasis on the effect of calcination temperature on the interactions between rhodium oxide and support. The formation of a compound between rhodium and support oxide is strongly dependent on the nature of the support and the calcination temperature. No such compounds were formed over γ-Al2O3 and SiO2 supported Rh catalysts even after a calcination at a temperature as high as 950°C. However, LaRhO3, MgRh2O4, YRhO3, and RhTaO4 could be formed over La2O3, MgO, Y2O3, and Ta2O5 supported catalysts after their calcination at suitable temperatures. The ease of formation of these compounds decreased in the sequence MgO>La2O3⪢Y2O3≥Ta2O5. The lower reducibilities of these compounds compared to that of Rh2O3 brought evidence of increased Rh–O bond strength. In other words, the formation of these compounds after calcination is an indication of strong interactions between rhodium and support. At a reaction temperature of 750°C, La2O3 and MgO provided stable catalytic activities and selectivities during 100 h of reaction, whereas deactivation was observed over the Y2O3 and Ta2O5 supported catalysts. However, because compounds between the latter two supports and Rh2O3 could be formed at higher temperatures, reactions at higher temperatures have also been carried out. It was found that the stability of the 1 wt% Rh/Ta2O5 could be notably improved at a reaction temperature of 900°C, and that the stability of the 1 wt% Rh/Y2O3 could be improved at a reaction temperature of 850°C.
The partial oxidation of methane over MgO- and SiO2-supported rhodium catalysts was investigated, using a CH4/O2 mixture with a molar ratio of 2.1/1 as feed gas. At 750°C and atmospheric pressure, the 1% Rh/MgO catalyst exhibited a very high stability at the high gas hourly space velocity of 720,000 ml g−1 h−1, its catalytic activity and selectivity remaining constant and high after an induction time up to 100 h of reaction. In contrast, the 1% Rh/SiO2 catalyst deactivated rapidly. To explain the above results, the reaction behaviors of CH4 and CH4/O2 (2/1) over the oxidized catalysts 1% Rh(O)/MgO and 1% Rh(O)/SiO2 were studied in a pulse microreactor. No carbon deposition occurred during the reaction of CH4 with the MgO-supported rhodium catalyst, even after the catalyst was almost completely reduced. In contrast, a notable amount of surface carbon was generated during the reaction of CH4 with the SiO2-supported rhodium catalyst. During the reaction of CH4/O2 (2/1) over the oxidized catalysts, the rhodium oxide supported on SiO2 was easily reduced by CH4, while the reduction of rhodium oxide supported on MgO occurred with some difficulty. This implies much stronger interactions between rhodium oxide and MgO than between the former and SiO2. Temperature-programmed reduction experiments also revealed much stronger interactions between rhodium and magnesium oxide and that two kinds of rhodium compounds were present in the oxidized MgO-supported rhodium catalyst. X-ray powder diffraction enabled us to identify Rh2O3 and MgRh2O4 in precalcined 10% Rh(O)/SiO2 and 10% Rh(O)/MgO, respectively. It is suggested that the strong interactions between rhodium and magnesium oxide (especially the formation of MgRh2O4) are responsible for the high stability of the MgO-supported rhodium catalysts.
The annealing effects of sodium chloride single crystals doped with Rh3+ are studied by optical absorption spectroscopy. The technique of annealing is an alternative to the most common irradiation technique which has been described earlier by other workers on Rh3+ in different matrices. This is the first report of an optical absorption spectrum of a Rh2+ site. The specific d-d transitions are well defined in the spectrum.
Rhodium/alumina catalysts with Rh loadings of 0.01–0.5wt.% with substantially different thermal stability of supports with respect to phase transformation to corundum were studied. It was shown that the stability of alumina support with respect to its transformations to corundum is one of the important factors accounting for irreversible deactivation of Rh/Al2O3 catalysts. We used laser-induced luminescence (LIL) of Cr3+ impurity and Rh3+ in α-Al2O3 and XRD for studying catalysts with different calcination temperatures. Catalytic ethane hydrogenolysis was used as a test for Rh loading on the catalyst surface. This method reliably detects Rh ions on the surface starting from concentrations as low as 0.01%. The results of LIL spectroscopy indicated that in all cases Rh3+ ions were incorporated in the bulk of the newly formed α-Al2O3 phase, and quite possible can initiates the formation of the corundum phase in catalysts with high-thermal stability.
The partial oxidation of CH4over Rh/Al2O3was studied by means of temporal analysis of products (TAP). CH4was found to adsorb dissociatively on reduced metal sites. Oxygen preadsorption decreases the methane adsorption rate. The oxidation products under oxidative conditions are CO2and H2O. The selectivity towards CO and H2rises to almost 100% as the conditions become more reductive. Water adsorbed on the support acts as an oxygen source through the inverse spillover of water or hydroxyl onto the Rh particles. A reaction scheme for the partial oxidation of methane is presented and the elementary reactions that give overall steam reforming, CO2reforming, and water–gas shift are discussed.
Palladium loaded calcium-hydroxyapatite, Pd(z)/CaHAp, and calcium-fluoroapatite, Pd(z)/CaFAp, were synthesised and characterised by TEM, XRD, IR and UV–vis–NIR spectroscopies. Introduction of palladium does not change the structure of CaHAp and CaFAp. The average size of PdO particles was found to be around 4–5 nm on Pd(1)/CaHAp but larger (6–7 nm) on Pd(1)/CaFap. The acid–base properties of the supports and of the catalysts were studied using butan-2-ol conversion. On CaHAp and CaFAp, the butenes yield (dehydration reaction) is very low either in the absence or in the presence of oxygen. The methyl ethyl ketone yield (dehydrogenation reaction) is significant only in the presence of oxygen and higher over CaFAp. Conversely, the performances of Pd(z)/CaHAp are better than those of Pd(z)/CaFAp below 180 °C. Above 180 °C, buta-2-ol combustion is favoured on Pd/CaHAp but not on Pd/CaFAp.In methane oxidation, Pd(z)/CaHAp showed also a much larger activity than Pd(z)/CaFAp. On 2 wt% Pd loaded CaHAp, the methane oxidation reaches a conversion of almost 100% at 350 °C, which is comparable with the performance of conventional Pd/Al2O3 catalysts. The reducibility of PdO under methane–oxygen mixtures is lower on Pd(z)/CaHAp. For both reactions, the lower activity of Pd(z)/CaFAp is related to its higher acidity, resulting from the substitution of OH− by F−, and to the larger PdO particle size.Graphical abstractFluoration of hydroxyapatite CaHAp has a marked effect on the catalytic behaviour of supported palladium: (i) in dehydrogenation of butan-2-ol to MEK, Pd/CaHAp is more efficient than Pd/CaFAp below 180 °C and active in butan-2-ol combustion above 180 °C whereas Pd/CaFAp is not; (ii) in methane combustion, Pd/CaHAp is much more active than Pd/CaFAp. These differences are related to PdO particle size, larger on Pd/CaFAp, and to differences in surface acidity.
The partial oxidation of methane to synthesis gas over an
a\text - Al\text2 \textO\text3\alpha {\text{ - Al}}_{\text{2}} {\text{O}}_{\text{3}}
- and a
g\text - Al\text2 \textO\text3\gamma {\text{ - Al}}_{\text{2}} {\text{O}}_{\text{3}}
-supported Rh catalyst has been studied at atmospheric pressure using in situ DRIFTS between 823 and 973 K. A surface intermediate
species with IR band at 2000 cm-1, correlating with the CO formation, was observed during the partial oxidation. DRIFT spectra of adsorbed CO at 323 K were
used to study the state of Rh during the partial oxidation. The state of Rh at 973 K is proposed to be a matrix of metallic
rhodium with clusters of partially reduced oxide phase with isolated Rh+ atoms dispersed on the support. Rh oxide with Rh+ cations is the state of Rh during partial oxidation of methane at 823 K.
The literature treating mechanisms of catalyst deactivation is reviewed. Intrinsic mechanisms of catalyst deactivation are many; nevertheless, they can be classified into six distinct types: (i) poisoning, (ii) fouling, (iii) thermal degradation, (iv) vapor compound formation accompanied by transport, (v) vapor-solid and/or solid-solid reactions, and (vi) attrition/crushing. As (i), (iv), and (v) are chemical in nature and (ii) and (v) are mechanical, the causes of deactivation are basically three-fold: chemical, mechanical and thermal. Each of these six mechanisms is defined and its features are illustrated by data and examples from the literature. The status of knowledge and needs for further work are also summarized for each type of deactivation mechanism. The development during the past two decades of more sophisticated surface spectroscopies and powerful computer technologies provides opportunities for obtaining substantially better understanding of deactivation mechanisms and building this understanding into comprehensive mathematical models that will enable more effective design and optimization of processes involving deactivating catalysts.
Kinetic and isotopic tracer methods led to a simple and unifying mechanistic proposal for reactions of CH4 with CO2 and H2O, for its decomposition on Rh clusters, and for water–gas shift reactions. Kinetic rates for forward reactions were measured by correcting net rates for approach to equilibrium and by eliminating transport artifacts. These rates were proportional to CH4 pressure (5–450 kPa) and independent of CO2 or H2O pressures (5–450 kPa) on all supported Rh catalysts; the resulting first-order rate constants were identical for H2O and CO2 reforming and for CH4 decomposition. Kinetic isotope effects (kCH4/kCD4=1.54–1.60) were also independent of the concentration or identity of the co-reactant, consistent with the sole kinetic relevance of CH bond activation steps. These data indicate that co-reactant activation and its kinetic coupling with CH4 activation via scavenging of chemisorbed carbon intermediates are fast steps and lead to Rh surfaces essentially uncovered by reactive intermediates during H2O and CO2 reforming. CO oxidation rates before and after reforming reactions showed that Rh surfaces remain uncovered by unreactive species during reforming catalysis under conditions relevant to industrial practice. CH4 conversion rates for CH4/CD4/CO2 reactant mixtures are much faster than CH4−xDx formation rates, indicating that CH bond activation elementary steps are irreversible. CH4/CO2/D2 reactant mixtures led to binomial isotopomer distributions in dihydrogen and water at all reactant conversions. Their D contents were identical and corresponded to equilibration between all H atoms in reacted CH4 and all D2 in the inlet stream. Thus, recombinative desorption steps of H atoms and OH groups to form H2 or H2O are quasi-equilibrated during CH4 reforming. 12CH4/12CO2/13CO mixtures led to identical 13C contents in CO and CO2, as expected from quasi-equilibrated CO2 activation steps. The quasi-equilibrated nature of all these steps requires that water–gas shift reactions also be at equilibrium during CH4 reforming, as found experimentally. CH4 reforming turnover rates increased as the size of Rh clusters supported on Al2O3 or ZrO2 decreased, suggesting that coordinatively unsaturated Rh surface atoms prevalent in smaller clusters activate CH bonds more effectively than atoms on lower-index surfaces, as also found on single-crystal surfaces. Turnover rates do not depend on the identity of the support; any involvement of the support in the activation of co-reactants is not kinetically relevant.
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