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Investigation of Pt-Ga/CeO 2 -ZrO 2 -Al 2 O 3 bifunctional catalyst for the catalytic conversion of n-butane into olefins
Abstract
The molecular structure of n-butane is relatively stable with high CC bond energy, which makes it difficult to be effectively utilized, and most of it is currently burned as a low-value fuel. The study of converting n-butane into high-value-added light olefins through dehydrogenation, cracking, and other reactions is also of great scientific significance. In this study, Pt and Ga were sequentially impregnated onto CeO 2-ZrO 2-Al 2 O 3 (CZA) support using a stepwise method to prepare the PtGa/CZA catalyst. Pt-Ga 2 O 3 was introduced as a dehydrogenation catalyst bifunctional catalyst for the catalytic conversion of n-butane into olefins.
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Nonoxidative dehydrogenation of light alkanes has seen a renewed interest in recent years. While PtGa systems appear among the most efficient catalyst for this reaction and are now implemented in production plants, the origin of the high catalytic performance in terms of activity, selectivity, and stability in PtGa-based catalysts is largely unknown. Here we use molecular modeling at the DFT level on three different models: (i) periodic surfaces, (ii) clusters using static calculations, and (iii) realistic size silica-supported nanoparticles (1 nm) using molecular dynamics and metadynamics. The combination of the models with experimental data (XAS, TEM) allowed the refinement of the structure of silica-supported PtGa nanoparticles synthesized via surface organometallic chemistry and provided a structure–activity relationship at the molecular level. Using this approach, the key interaction between Pt and Ga was evidenced and analyzed: the presence of Ga increases (i) the interaction between the oxide surface and the nanoparticles, which reduces sintering, (ii) the Pt site isolation, and (iii) the mobility of surface atoms which promotes the high activity, selectivity, and stability of this catalyst. Considering the complete system for modeling that includes the silica support as well as the dynamics of the PtGa nanoparticle is essential to understand the catalytic performances.
Metal nanoparticle (NP), cluster and isolated metal atom (or single atom, SA) exhibit different catalytic performance in heterogeneous catalysis originating from their distinct nanostructures. To maximize atom efficiency and boost activity for catalysis, the construction of structure–performance relationship provides an effective way at the atomic level. Here, we successfully fabricate fully exposed Pt3 clusters on the defective nanodiamond@graphene (ND@G) by the assistance of atomically dispersed Sn promoters, and correlated the n-butane direct dehydrogenation (DDH) activity with the average coordination number (CN) of Pt-Pt bond in Pt NP, Pt3 cluster and Pt SA for fundamentally understanding structure (especially the sub-nano structure) effects on n-butane DDH reaction at the atomic level. The as-prepared fully exposed Pt3 cluster catalyst shows higher conversion (35.4%) and remarkable alkene selectivity (99.0%) for n-butane direct DDH reaction at 450 °C, compared to typical Pt NP and Pt SA catalysts supported on ND@G. Density functional theory calculation (DFT) reveal that the fully exposed Pt3 clusters possess favorable dehydrogenation activation barrier of n-butane and reasonable desorption barrier of butene in the DDH reaction. Direct dehydrogenation has been a thematic research area resulting from the energy shortage and petroleum gas upgrading scenario. Here, the authors fabricate fully exposed Pt3 clusters and correlate dehydrogenation activity with average coordination number of Pt-Pt bond.
Ni-based catalysts (Ni-γ-Al2O3, Ni-HTASO5 and Ni-CeZrOx) were prepared by impregnation method and characterized by BET, AAS, XRD, H2-TPR, CO-TPD, NH3-TPD, XPS, TG-DSC-MS and Raman spectroscopies. Using CeZrOx-modified Al2O3 (HTASO5) as support, the catalyst exhibited good catalytic performance (TOFCH4 = 8.0 × 10−2 s−1, TOFH2 = 10.5 × 10−2 s−1) and carbon resistance for steam-methane reforming (SMR) reaction. Moreover, CeZrOx was able to enhance water-gas shift (WGS) reaction for more hydrogen production. It was found that the addition of CeZrOx could increase the content of active nickel precursor on the surface of the catalyst, which was beneficial to the decomposition of water and methane on Ni-HTASO5. Furthermore, Ni-HTASO5 could decrease the strong acid sites of the catalyst, which would not only contribute to the formation of low graphited carbon, but also decrease the amount of carbon deposition.
Homogeneous 1 wt% Pt/CeO2–ZrO2–Al2O3 cryogels with Ce/Zr = 1 and Ce/Al = 0–1/2 were synthesized by one-pot sol–gel processing and subsequent freeze drying. Catalysis for CO and CH4 oxidation was investigated after heating in air at 800 °C for 5 h. It was shown that Ce/Al = 1/3 was the most desirable to obtain the highest activity. When the ratio was below 1/3, sufficient catalytic activity was not obtained, owing to the lack of ceria which played an important role in improving Pt dispersion and oxygen storage capacity (OSC). On the other hand, when the ratio was above 1/3, the activity decreased, owing to the lack of alumina as a diffusion barrier for ceria–zirconia solid solution, resulting principally in encapsulation of platinum inside the solid solution. It was revealed that catalytic activity correlated positively with Pt dispersion and OSC of the cryogel, implying that the dispersion and OSC were the essential factors for controlling the activity. However, it was also revealed that the dispersion and OSC did not correlate positively with BET surface area and pore volume of the cryogel, which suggested that maintenance of as large surface area and pore volume as possible was not always important to obtain high dispersion and OSC. Since Pt dispersion and OSC increased basically with increasing the Ce/Al ratio until Ce/Al = 1/3 and BET surface area and pore volume decreased, it was suggested that chemical interaction between platinum and cerium oxide was important for improving the dispersion and OSC.
The kinetics of propane dehydrogenation over single‐Pt‐atom‐doped Ga2O3 catalyst has been examined by combining density functional theory calculations and microkinetic analysis. The doping of Pt not only can improve the selectivity of the Ga2O3 catalyst by hindering the deep dehydrogenation reactions but also helps to achieve a long‐term stability by improving the resistance of Ga2O3 to hydrogen reduction. Microkinetic analysis indicates that upon Pt doping the turnover frequency for propane consumption is increased by a factor of 2.8 under typical operating conditions, as compared to the data on the pristine Ga2O3 surface. The calculated results suggest that the Pt1–Ga2O3 catalyst shows a bifunctional character in this reaction where the Pt–O site brings about dehydrogenation while the Ga–O site is active for desorbing H2, which provides a beautiful explanation for the previous experimental observation that even trace amounts of Pt can dramatically improve the catalytic performance of Ga2O3.
Single atom catalysts have attracted attention because of improved atom efficiency, higher reactivity, and better selectivity. A major challenge is to achieve high surface concentrations while preventing these atoms from agglomeration at elevated temperatures. Here we investigate the formation of Pt single atoms on an industrial catalyst support. Using a combination of surface sensitive techniques such as XPS and LEIS, X-ray absorption spectroscopy, electron microscopy, as well as density functional theory, we demonstrate that cerium oxide can support Pt single atoms at high metal loading (3 wt% Pt), without forming any clusters or 3D aggregates when heated in air at 800 °C. The mechanism of trapping involves a reaction of the mobile PtO2 with under-coordinated cerium cations present at CeO2(111) step edges, allowing Pt to achieve a stable square planar configuration. The strong interaction of mobile single atom species with the support, present during catalyst sintering and regeneration, helps explain the sinter-resistance of ceria-supported metal catalysts.
Pt nanoparticles (NPs) immobilized on a core-shell hybrid carbon support can deliver efficient direct dehydrogenation of n-butane at a lower temperature. The hybrid carbon support is composed of a nanodiamond core and a reinforced ultrathin graphene shell (ND@G), which improves the stability of anchored Pt NPs against sintering under reaction condition. The as-prepared Pt/ND@G catalyst demonstrates a distinguished catalytic performance (~25% conversion with >95% selectivity toward the olefins) at 450 oC for over 10 h as a result of the strong metal-support interaction between the Pt NPs and the hybrid carbon support. The catalyst can also be fully regenerated by a post oxidative thermal treatment.
In this study, the dehydrogenation component of Ga2O3 was introduced into ZSM-5 nanocrystals to prepare Ga2O3/ZSM-5 hollow fiber-based bifunctional catalysts. The physicochemical features of as-prepared catalysts were characterized by means of XRD, BET, SEM, STEM, NH3-TPD, etc., and their performances for the catalytic conversion of n-butane to produce light olefins and aromatics were investigated. The results indicated that a very small amount of gallium can cause a marked enhancement in the catalytic activity of ZSM-5 because of the synergistic effect of the dehydrogenation and aromatization properties of Ga2O3 and the cracking function of ZSM-5. Compared with Ga2O3/ZSM-5 nanoparticles, the unique hierarchical macro-meso-microporosity of the as-prepared hollow fibers can effectively enlarge the bifunctionality by enhancing the accessibility of active sites and the diffusion. Consequently, Ga2O3/ZSM-5 hollow fibers show excellent catalytic conversion of n-butane, with the highest yield of light olefins plus aromatics at 600 °C by 87.6%, which is 56.3%, 24.6%, and 13.3% higher than that of ZSM-5, ZSM-5 zeolite fibers, and Ga2O3/ZSM-5, respectively.
This work is mainly focused on the investigation of the influence of the amount of a few CeO2 on the physicochemical and catalytic properties of CeO2-doped TiO2 catalysts for NO reduction by a CO model reaction. The obtained samples were characterized by means of XRD, N2-physisorption (BET), LRS, UV-vis DRS, XPS, (O2, CO, and NO)-TPD, H2-TPR, in situ FT-IR, and a NO + CO model reaction. These results indicate that a small quantity of CeO2 doping into the TiO2 support will cause an obvious change in the properties of the catalyst and the TC-60 : 1 (the TiO2/CeO2 molar ratio is 60 : 1) support exhibits the most extent of lattice expansion, which indicates that the band lengths of Ce-O-Ti are longer than other TC (the solid solution of TiO2 and CeO2) samples, probably contributing to larger structural distortion and disorder, more defects and oxygen vacancies. Copper oxide species supported on TC supports are much easier to be reduced than those supported on the pure TiO2 and CeO2 surface-modified TiO2 supports. Furthermore, the Cu/TC-60 : 1 catalyst shows the highest activity and selectivity due to more oxygen vacancies, higher mobility of surface and lattice oxygen at lower temperature (which contributes to the regeneration of oxygen vacancies, and the best reducing ability), the most content of Cu(+), and the strongest synergistic effect between Ti(3+), Ce(3+) and Cu(+). On the other hand, the CeO2 doping into TiO2 promotes the formation of a Cu(+)/Cu(0) redox cycle at high temperatures, which has a crucial effect on N2O reduction. Finally, in order to further understand the nature of the catalytic performances of these samples, taking the Cu/TC-60 : 1 catalyst as an example, a possible reaction mechanism is tentatively proposed.
Cerium oxide (CeO2) nanoparticles have been synthesized from (NH4)2Ce(NO3)6, ethylenediamine and hydrazine by a hydrothermal method. Ethylenediamine and hydrazine with pH management can control the particle growth and have an important role in the formation of nanoparticles. Various experimental parameters were examined, such as the reaction temperature, the reaction time, and the molar ratios of the starting reagents. The as-synthesized products were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and energy dispersive X-ray spectroscopy (EDX).
A novel catalyst material for the selective dehydrogenation of propane is presented. The catalyst consists of 1000 ppm Pt, 3 wt% Ga, and 0.25 wt% K supported on alumina. We observed a synergy between Ga and Pt, resulting in a highly active and stable catalyst. Additionally, we propose a bifunctional active phase, in which coordinately unsaturated Ga(3+) species are the active species and where Pt functions as a promoter. © 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. This is an open access article under the terms of the Creative Commons Attribution Non-Commercial NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.
Catalytic oxidative cracking of propane has been investigated over nanosized gold supported Ce0.5Zr0.5O2 catalysts. Gold supported catalysts offered high propane conversions and selectivity to olefins than Pt supported catalysts under the same reaction conditions. Ce0.5Zr0.5O2 showed is slightly active, but enhancement in the catalytic activity was observed when the catalyst deposited with 0.2 wt% gold. Further improvement in catalytic activity was observed with gold supported sulfated Ce0.5Zr0.5O2. Time on stream analysis revealed that gold supported sulfated Ce0.5Zr0.5O2 is stable even after 48 h of reaction and resistant to the coke formation. Increase in Brönsted and Lewis acidity of the catalyst sample was observed with incorporation of sulfate ions and gold metal. TEM and H2-TPR results indicated that gold supported on sulfated Ce0.5Zr0.5O2 catalyst possessed homogeneous dispersion of nanosized gold particles and easily reducible metal-sulfate interactive species, which are majorly responsible for high activity and stability of this catalyst.
Graphical Abstract
New carbon-supported Pt catalysts were characterized and evaluated in the direct non-oxidative dehydrogenation of n-butane to butenes. After a comparison with catalysts supported on oxidic materials, Pt catalysts supported on carbon nanotubes and Vulcan carbon showed the best activity and selectivity to olefins, and a low deactivation throughout the reaction time. This fact is attributed to the chemical properties of such carbonaceous materials, which lead to a good metal-support interaction and metallic dispersion, excellent accessibility to reactants and to the formation of a suitable metallic phase, thus favoring the dehydrogenation reaction and inhibiting collateral ones. Pt catalysts supported on Vulcan carbon and carbon nanotubes become very promising candidates for the large-scale development of catalysts applied in the direct light paraffin dehydrogenation process.
Series of Pd/CeO2-ZrO2-Al2O3 (Pd/CZA) with different pore structures and lanthanum modified Pd/La-Al2O3 (Pd/La-A) were prepared via coprecipitation or hydrothermal method. The obtained catalysts were subjected to TWC activity evaluation, thermal stability tests as well as systematic characterizations including TEM, BET, Rietveld XRD, XPS, H2-TPR, O2-TPSR, and in situ DRIFTS. Results show that the TWC catalytic performance of Pd/Al2O3 catalyst can be obviously improved by CeO2-ZrO2 modification compared to La2O3 modification, due to the strong interaction between PdOx species and CeO2-ZrO2 support. The excellent thermal stability of Pd/CZA-2 catalyst is related to its different microstructure compared to Pd/CZA-1, because the abundant dissociated Zr⁴⁺ due to phase separation in Pd/CZA-2 could hinder the θ-Al2O3 to α-Al2O3 phase transition and retain small-sized PdOx species with excellent reoxidation ability. Besides, DRIFTS studies reveal clear structure-activity correlations between the adsorbed reaction intermediates and TWC catalytic performance, i.e. The formation of NO-Pd⁰ species on fresh Pd/CZA-1 is beneficial for NO conversion, while the rapid adsorption/decomposition of HC and carbonate species on Pd/CZA also contributes to better HC/CO oxidation. Nevertheless, the active sites poisoning by HC strong adsorption along with temperature-stable nitrate species on Pd/La-A cost its inferior catalytic behaviour regardless of thermal ageing.
1,3-Butadiene (BD) is a key raw material for the synthesis of rubber, but its efficient synthesis is a challenge. Direct dehydrogenation of n-butane in CO2 (CO2ODHB) represents an ideal way to produce BD and use the waste CO2 source. In the present work, several catalysts of CrOx supported on CeZr solid solutions were developed and applied in catalyzing the CO2ODHB to BD. Among these, a 10%CrOx/Ce0.5Zr0.5O2 could achieve the reaction with 25% of n-butane conversion and 70% of BD selectivity at 823 K. The prepared catalysts were characterized using the techniques of XRD, UV–vis-DR, XPS, Raman, N2 adsorption/desorption, FT-IR, TEM, H2-TPR, NH3-TPD, and CO2-TPD, etc. The results indicated that compared with other catalysts, the 10% CrOx/Ce0.5Zr0.5O2 catalyst possessed the highest Cr⁶⁺ species concentration and balanced acidic-basic sites, contributing to the outstanding performance in the reaction. These findings provide a strategy to build a catalyst with the stable valence of Cr⁶⁺ species, and hopefully, offer an important reference to establish other CeZr solid solution catalysts with regulable acidic-basic and textural properties.
Pt-CeO2 catalysts have been widely studied for the vehicle emission control. Designing novel CeO2 based supports with improved physical-chemical properties has become a research hotspot to further promote the catalytic performance and stability of Pt-CeO2 catalysts. In this work, through utilizing a unique, two-step incipient wetness impregnation (T-IWI) method for ceria-zirconia-alumina (CZA-T) support preparation, a Pt single site catalyst (Pt/CZA-T) with excellent thermal stability was synthesized. Higher oxidation activity and oxygen storage capacity (OSC) were achieved on activated Pt/CZA-T, comparing to Pt catalysts on regular CeO2/Al2O3 (Pt/CA) and one-step prepared CeZrOx/Al2O3 (Pt/CZA). Via the modification of hydrophilic/hydrophobic properties of γ-Al2O3 by this unique T-IWI method, finer Ce0.9Zr0.1O2 particles with higher density of surface defects were formed on CZA-T, on which a higher Pt dispersion and stronger Pt-O-Ce interaction were achieved. Upon activation, smaller, well-dispersed Pt clusters on CZA-T were generated. It was concluded that the CO oxidation performance and OSC were highly related to the size of Pt clusters on different supports that we have developed. More Pt sites located at Pt cluster-CeZrOx interfaces, which were the real active sites, were responsible for the highest OSC function and CO oxidation activity of activated Pt/CZA-T catalyst.
A series of catalysts in which platinum was supported on the crystalline pure titanium silicalite (TS-1) were prepared using two different loading methods, namely, an ethylene glycol (EG) reduction method and the conventional incipient wetness impregnation (IM) technique. Various characterization techniques were used to study the effect of the loading method on the physicochemical and morphological properties of the prepared catalysts. Also the effect of the platinum-loading method on the dehydrogenation of n-butane was investigated in a fixed-bed reactor. The results show that the EG method favors the formation of a more-concentrated Pt dispersion, which results in much better catalytic activities for the selective formation of butenes and butadiene (> 97 %). This phenomenon is interpreted by carrying out density functional theory (DFT) calculations with focus on the relationship between the coverage of n-butane on Pt surface and the activation barrier for the first CH bond cleavage.
Propane dehydrogenation (PDH) is one of the most promising techniques to produce propylene. Industrial Pt-based catalysts often suffer from short-time stability under high temperature due to serious sintering and coke deposition via undesired side reactions. Detailed reaction mechanism on the surface of Pt-based nanoparticle has been well studied, while the subsurface effect remains mostly unstudied. Herein, supported PtGa nanoparticles with different surface and subsurface composition was evidenced by extended X-ray absorption fine structure (EAXFS) spectra and energy dispersive X-ray spectroscopy (EDS). Theoretical simulation demonstrated subsurface regulation would increase the electron density of surface Pt and thus weaken propylene adsorption. Propylene selectivity on the PtGa-subsurface nanoparticles was up to 98% at 600 °C while that on the Pt-subsurface nanoparticles was only 95%. Furthermore, rational designed PtGa alloy nanoparticles were encapsulated in MFI zeolite to inhibit sintering and coke deposition for enhanced catalytic stability.
Ga-based catalysts are promising for use in propane dehydrogenation (PDH) because of the relatively superior activity, but the conventional Ga-based catalysts usually suffer from serious deactivation and unsatisfactory propene selectivity. Here, ultrafine bimetallic Ga-Pt nanocatalysts encapsulated into silicalite-1 (S-1) zeolites ([email protected]) were synthesized by a facile ligand-protected direct H2-reduction method. It is indicated that this catalyst is composed of confined ultra-small GaPt alloy nanoclusters and a part of isolated tetrahedral coordination of Ga species. The confined GaPt alloy nanoclusters are the active sites for PDH reaction, and their high electron density could boost the desorption of products, resulting in a high propene selectivity of 92.1% and propene formation rate of 20.5 mol g⁻¹Pt h⁻¹ at 600 °C. Moreover, no obvious deactivation was observed over [email protected] catalyst even after 24 h on stream at 600 °C, affording an extremely low deactivation constant of 0.0068 h⁻¹, which is much lower than that of the conventional Ga-based catalysts. Notably, the restriction of the zeolite can enhance the regeneration stability of the catalyst, and the catalytic activity kept unchanged after four consecutive cycles.
The alumina supported Pd-Pt based bimetallic catalysts were prepared by modified electroless deposition and evaluated for butane dehydrogenation. The bimetallic Pd-Pt catalysts exhibited superior performance compared to that of monometallic catalysts. The superior performance may be attributed to strong synergistic interaction between metals, stronger metal-support interaction and lower acidity. The higher activity of alloyed Pd-Pt metal resulted in higher conversion for bimetallic catalysts. The lower acidity resulted in higher butene selectivity and stability for bimetallic catalysts by minimizing cracking and secondary reactions. The co-deposited catalyst with more uniform distribution of Pd and Pt metals showed better performance than bimetallic catalysts prepared by sequential deposition. Co-deposited Pd-Pt/Al catalyst showed the highest butane conversion (50 %), butene yield (42 %) and stability. The reported yield is much higher than many reported values for Pt or Pd based catalysts. The co-deposited Pd-Pt/Al catalyst also exhibited better dehydrogenation performance in comparison to corresponding co-impregnated bimetallic catalyst.
To preserve the environment for civilization, we should remove the pollutants like toxic dyes by friendly and cost efficacious method. In this study, the effect of surfactants or mixed surfactants on physicochemical, optical and adsorption properties of ternary mixed oxide CeO2-ZrO2-Al2O3 (CZA) are investigated. The ternary mixed oxide CZA was prepared by surfactants or mixed surfactants assisted ultrasonic co-precipitation method. The physicochemical and optical properties are estimated by different techniques like XRD, TEM, EDX, FTIR, SBET and UV–Vis/DR. The CZAT and CZAC have hybrid shapes and high surface area. The adsorption properties of ternary mixed oxides adsorbents were characterized by sono - removing anionic dyes such as CR and RR. The different factors like contact time, different dye concentrations and temperatures also studied. The kinetics and isotherms applications showed that, the adsorption process was followed pseudo second order kinetics and the Freundlich isotherm model. Also, the adsorption is spontaneous and endothermic process through the thermodynamic study. Finally, the results showed that the ternary mixed oxide nano-adsorbent (CeO2-ZrO2-Al2O3) is promising and functional materials for anionic dye sweep from wastewater.
Catalytic dehydrogenation of light alkanes can effectively produce olefins and hydrogen. Even though Pt and CrOx -based catalysts are widely applied in industry, research to improve the activity and stability of these catalysts continued. This review summarizes important achievements obtained in recent years, focusing on the development of supports, promoters and preparation methods of Pt and CrOx -based catalysts, which mainly aimed to improve the dispersion of the active species and to enhance coke resistance. Furthermore, the high cost of Pt-based catalysts and environmental problems encountered with CrOx -based catalysts have spurred the development of alternative catalysts. The dehydrogenation performances and characteristics of promising alternative VOx -, modified Ni- and Sn-based catalysts are also reviewed. Comparison with the catalytic reforming process of naphtha further probes the necessity of catalyst acidity in these two different processes. The choice of the dehydrogenation reactor is discussed, and future perspectives and research directions are indicated.
Starting from structured supports based on compact spheres coated with a thin and porous layer of ZnAl2O4, mono and bimetallic catalysts were prepared. These catalysts were applied for the n-butane dehydrogenation reaction to produce light olefins. The structured supports were synthesized by coating with: bohemite-nitrate purified method (BP) and citrate-nitrate method (C). From the characterization results, the existence of strong Pt-Sn interaction in both bimetallic catalysts with probable alloys formation was found. In the PtSn/Sp-Zn-C catalyst, where higher metallic Pt-Sn interactions were observed, most of its metallic particles have sizes between 1 and 1.5 nm, which indicates a good metallic dispersion. These properties led to a catalyst with the best catalytic behavior, thus showing high yields to butenes, and a very good stability. Moreover, the presence of a structured support improves the mass and heat transference in this reaction carried out at high temperature.
A series of Nd2O3 was introduced into CeO2–ZrO2–Al2O3(CZA) materials to improve their redox property, and then their soot oxidation activities were characterized. Although Nd2O3 incorporation obviously decreased the specific surface area, 7 wt% Nd2O3 modified CZA (fresh CZAN7) also exhibited the best activity, the temperature for 50% soot conversion (T50) over which is 25 °C lower than that over fresh CZA due to the more surface oxygen vacancies and the faster mobility of oxygen species. It indicates that redox property of CZA plays a critical role than specific surface area in determining the soot oxidation activity. Meanwhile, it is interesting that 31 °C lower of T50 is detected over the aged CZAN7 (1000 °C/4h) than its fresh counterpart (600 °C/3h). Combined with the results of Powder X-ray diffraction (XRD), High-resolution transmission electron microscope (HRTEM) and Raman, a new sight about the bifunctional roles of Nd2O3 on improving the redox property of CZA has been put forward. Nd2O3 could directly incorporate into CeO2–ZrO2 lattice to form CeO2–ZrO2-Nd2O3 solid solution. Meanwhile, it could also regulate the distribution of Zr species and promote them incorporate into CeO2–ZrO2 lattice during thermal treatment. Then CZAN7 could exhibit more surface oxygen species, increased surface oxygen vacancies and better soot oxidation activity even being thermally aged.
The hydrogenation of acetylene by Ga-doped ceria is investigated using density functional theory (DFT). Different from a previously proposed mechanism that contains an unattainable barrier, our models are based on two Ga-doped ceria surfaces with oxygen vacancies, representing respectively low and high Ga dopant concentrations. The doping of Ga is shown to promote the formation of surface oxygen vacancies, which create a template containing frustrated Lewis pairs to facilitate heterolytic H2 dissociative chemisorption. This mechanism is similar in spirit to our recently proposed mechanism for bare and Ni-doped CeO2 catalysts (J. Am. Chem. Soc., 2018, 140, 12964–12973) and calls for the formation of GaH hydride species along with OH ones on the ceria surface. The former has recently been experimentally observed, thus providing strong support for this mechanism. The rate-limiting step in doped CeO2(1 1 1) with low Ga concentrations has a barrier that is 0.18 eV for the Ga/O FLP and 0.05 eV for the Ce/O FLP higher than that of undoped CeO2(1 1 1), while that for high Ga concentrations is 0.21 eV lower, consistent with the experimental observed concentration dependence of catalytic activity. However, different from the Ni dopant, which was found to be a promoter, the Ga dopant is not only a promoter but also an active participant in the catalysis.
Supported Pt-Sn bimetallic catalysts directly reduced by H 2 (DR method) are highly active for the dehydrogenation of n-butane, while the catalysts calcined in air, followed by H 2 reduction (CR method), are totally inactive regardless of the calcination temperature and Sn/Pt ratio. The present work investigated the detailed bulk and surface structural properties of the catalysts with various characterization techniques. The results suggest that both DR and CR methods led to the formation of bulk Pt-Sn alloys (e.g. Pt 3 Sn and PtSn). However, calcination before reduction significantly changed the catalyst surface properties. The SnO 2 layer was located on the surface of the Pt-Sn alloys and/or Pt nanoparticles when the catalysts were prepared with the CR method. In contrast, the DR method induced more surface exposed Pt atoms from the Pt 3 Sn alloy, which played an important role in the superior catalytic behaviors for n-butane dehydrogenation.
Surface chemistry and the nature of the adsorbed NOx species on a Pt/CeO2-ZrO2/Al2O3 catalyst were investigated by IR spectroscopy, X-ray diffraction (XRD), H2−Temperature Programmed Reduction (H2-TPR) and NOx−Temperature Programmed Desorption (NOx-TPD). Parallel studies were also carried out with benchmark samples such as: CeO2/Al2O3, ZrO2/Al2O3, CeO2-ZrO2/Al2O3 and Pt supported versions of these materials. All samples were studied in their reduced and non-reduced forms. The use of CO as a probe molecule revealed that, during the synthesis of the mixed-metal oxide systems, deposited zirconia preferentially interacted with the alumina hydroxyls, while deposited ceria was preferentially located at the Lewis acid sites. Despite the limited extent of Zr4+ ions incorporated into the CeO2 lattice, the reduction of ceria was promoted and occurred at lower temperatures in the presence of zirconia. When deposited on ZrO2/Al2O3, platinum formed relatively big particles and existed in metallic state even in the non-reduced samples. The presence of ceria hindered platinum reduction during calcination and yielded a high platinum dispersion. Subsequent reduction with H2 led to the production of metallic Pt particles. Consequently, NO adsorption on non-reduced Pt-containing materials was negligible but was enhanced on the reduced samples due to Pt0-promoted NO disproportionation. The nature of the nitrogen-oxo species produced after NO and O2 coadsorption on different samples was similar. Despite the high thermal stability of the NOx adsorbed species on the ceria and zirconia adsorption sites, the NOx reduction in the presence of H2 was much more facile over Pt/CeO2-ZrO2/Al2O3. Thus, the main differences in the NOx reduction functionalities of the investigated materials could be related to the ability of the catalysts to activate hydrogen at relatively lower temperatures.
In this study highly effective adsorbent ternary mixed oxide CeO2-Fe2O3-Al2O3 was prepared by precipitation method. Various methods used to treat the mixed hydroxide like calcination, ultrasonic, hydrothermal and ɣ radiation with different doses to obtain the ternary mixed oxide. XRD, TEM, EDX, FTIR and SBET are used to study the physicochemical properties of nanoparticles. The CFAH and CFAɣ0.8 have the different morphologies and high surface area. Batch adsorption experiments were performed to remove anionic Remazol Red RB-133 dye. The experimental data showed that The CFAH and CFAɣ0.8 have high adsorption rate for removing of dye. The removal of dye is enhanced by ultrasonic radiation and high temperature. The adsorption process was fitted well for pseudo second order kinetics and followed the Freundlich isotherm model. In addition to, Thermodynamic results of adsorption process displayed that, the adsorption of dye on adsorbent was spontaneous, endothermic and chemisorptions process.
A series of nano-sized iron- and cobalt-doped ceria-zirconia nanocomposites was prepared using hydrothermal synthesis technique at 180 °C for 24 h, with the successful novel incorporation of both Co and Fe on ceria-zirconia, for n-hexane catalytic cracking. Effects of dopant ions on the improvement of intrinsic properties of ceria-zirconia nanocomposites were investigated using disparate characterization techniques. The synthesized ceria-zirconia nanocomposites exhibited similar x-ray diffraction (XRD) patterns, indicating full fusion of the metal ions into the ceria-zirconia lattice structure. The synthesized nanocomposite catalysts were tested for n-hexane cracking over 10 h time-on-stream, with no previous study or report for catalytic cracking of hexane via ceria-zirconia nanocomposites. Relatively high ethylene and propylene selectivity (both > 62%), was obtained over CZ, FeCoCZa and FeCoCZb over time-on-stream. Comparatively, best catalytic activity and stability was exhibited by FeCoCZa with higher n-hexane conversion. Temperature and catalyst weight per feed flowrate (W/F) variations were investigated using the best catalyst (FeCoCZa). Higher conversions were obtained at higher temperature and lower W/F but varied product selectivity and yield, over time-on-stream. In addition, the spent catalysts were successfully regenerated after catalytic testing via calcination at 600 °C for 4 h, and re-used for two additional cycles.
Pt/TiO2/ZSM-5 catalyst with dehydrogenation-cracking bifunctionality catalytic activity was prepared by modifying ZSM-5 zeolite with titanium dioxide (TiO2) via sol-gel method and then loading Pt to the titanium modified ZSM-5 zeolite by incipient impregnation method. The as-prepared catalyst was characterized by means of XRD, N2 adsorption-desorption, TEM, XPS and NH3-TPD to analyze crystal structure, pore properties, morphology, valence states of active metal and acid properties of the catalyst, and the catalytic performance for the cracking of n-butane into light olefins was investigated. The results showed that the introduction of titanium dioxide provided additional acid sites to ZSM-5 zeolite, especially increasing the strong acid centers and enhancing the activation of n-butane. In addition, after reduction in hydrogen atmosphere, the partially reduced Ti³⁺ species was generated which was catalyzed by platinum due to the strong metal-support interaction (SMSI) between Pt and TiO2. The formation of appropriate amount of Ti³⁺ species enhanced the electron density around Pt, and thus weakened the adsorption of ethene and propene on Pt atoms. After reduction by hydrogen at 450℃, n-butane conversion of 76.1% and yield of light olefins (C2=-C3=) of 50.9% were achieved at the reaction temperature of 625℃ over Pt/10TiO2/ZSM-5 catalyst, which was 16.7% and 12.6% higher than those of Pt/ZSM-5 catalyst, respectively.
This paper describes catalytic consequencesThis paper describes catalytic consequences of Pt/CeO2-Al2O3 catalysts promoted with Ga species for propane dehydrogenation. A series of PtGa/CeO2-Al2O3 catalysts were prepared by a sequential impregnation method. The as-prepared catalysts were characterized employing N2 adsorption-desorption, X-ray diffrtaction, temperature programmed reduction, O2 volumetric chemisorption, H2-O2 titration, and transmission electron microscopy. We have shown that Ga3+ cations are incorporated into the cubic fluorite structure of CeO2, enhancing both lattice oxygen storage capacity and surface oxygen mobility. The enhanced reducibility of CeO2 is indicative of higher capability to eliminate the coke deposition and thus is beneficial to the improvement of catalytic stability. Density functional theory calculations confirm that the addition of Ga is prone to improve propylene desorption and greatly suppress deep dehydrogenation and the following coke formation. The catalytic performance shows a strong dependence on the content of Ga addition. The optimal loading content of Ga is 3 wt %, which results in the maximal propylene selectivity together with the best catalytic stability against coke accumulation.
The dehydrogenation cracking of i-butane was studied over mixed catalysts comprised of Cr2O3/Al2O3 dehydrogenation catalyst and HZSM-5 catalyst in a fixed-bed reactor. The results demonstrate that conversion and product distribution vary significantly depending on different location of Cr2O3/Al 2O3 and HZSM-5 catalyst and the amount of Cr 2O3/Al2O3 catalyst in the reactor, and the reaction performance is the best when two kinds of catalysts are homogeneously mixed. On the mixed catalyst HZSM-5 with a 20% Cr 2O3/Al2O3, the conversion of reaction is 80.99%, and the yield of ethylene, propylene and butylene is 12.42%, 24.11% and 13.43%, respectively. The conversion increases by 21.22 percentage points and the selectivity of total olefins (ethylene, propylene and butylene) increases by 2.57 percentage points, compared with the pure HZSM-5. Besides, i-butane mixed with N2 enters the reactor at a fixed residence time of i-butane, the reaction results become better, and the advantage is more obvious as dilution ratio increases.
Kinetic modeling of catalytic cracking (CC) of C4 alkanes (mixture of i-butane and n-butane), over Lanthanum modified HZSM-5 catalyst has been studied at 582-630 °C in a plug flow reactor. The elementary reaction steps, i.e., protolytic cracking (PC) of C-C and C-H bonds in alkane molecules, hydrogen transfer between butane and light olefins, olefin cracking, β-scission, adsorption and desorption were taken into account. The combination of theory of reaction routes and LHHW has been employed in the complicated CC network to drive explicit overall reaction rate of reactantand product molecules. Then, the parameter estimation and model discrimination were performed based on the minimization of mean residual sum of squares between experimental and theoretical rates. The intrinsic kinetic parameters were estimated by non-linear least-square regression using MATLAB optimization toolbox.
The mesoporous CexZr1-xO2 mixed oxides with different Ce/Zr ratio were prepared via a surfactant-assisted method of nanoparticle assembly, CTAB was used as surfactant. The CexZr1-xO2 mixed oxides were used as the supports for preparing Cr-V-O/ CexZr1-xO2 catalysts by the wetness impregnation method, and the catalytic activities of the results catalysts for dehydrogenation of propane to propene were studied. The prepared catalysts were characterized by XRD and N2 adsorption techniques.The Cr-V-O/ CexZr1-xO2 catalysts exhibiting high activity and selectivity for dehydrogenation of propane to propene. The molar ratio of Ce/Zr ratio showed remarkable influence on the activity of the catalysts. The optimistic catalyst was Cr-V-O/Ce0.5Zr0.5O2 the highest yield of 20.0% was obtained, the corresponding conversion of propane was 20.9% and selectivity to propene was 95.9% at 550 °C.
Oxidative cracking of n-butane to lower olefins over BiOCl catalyst has been investigated. BiOCl catalyst shows particularly high olefins selectivity and suppresses deep oxidation reactions extremely. 52 % yield of C2-C4 olefins was achieved at 625°C.
PtSn/O-Al203 catalysts with different amount of potassium (0.4, 0.7, 0.95, 1.2 and 1.45 wt.%) were prepared by an impregnation method, and their catalytic activity in n-butane dehydrogenation was investigated at 823K, an atmospheric pressure and a GHSV of 18,000 mL(gcath)-1. The compositions listed in order of n-C4= yields at 823K were as follows: K0.95(PtSn)1.5 >(PtSn)1.5 >K0.4(PtSn)1.5 >K0.7(PtSn)1.5 >Kt 2(PtSd)1.5 >Kt 45(PtSn)1.5 >K0.9(Pt)1.5. The K0.9(Pt)1.5 and K0.95( Sn)1.5 catalyst severely deactivated in n-butane dehydrogenation. The (PtSn)1.5 (without K) catalyst showed the highest n-butane conversion, while K0.95(PtSn)1.5 did the highest n-C4= yield. The small amount of potassium on bimetallic PtSn/O-Al2 03 catalyst improved n-C4= selectivity, but slightly decreased n-butane conversion, resulting in the increase of n-C4= yield. The effect of potassium was caused by blocking the acid sites of Pt catalyst. The TPR and HAADF STEM-EDS study suggested the reduction procedure of the Pt, Sn and K species. However, the higher loaded potassium (1.2 and 1.45 wt.%) doped (PtSn)1.5 catalysts were rather highly deactivated because the sizes of Pt particles were increased by weakening the interaction between Pt and Sn. The n-C4= selectivity of the (PtSn)1.5 catalyst increased with respect to the reaction, while that of the potassium doped catalysts maintained the high n-C4= selectivity from the beginning of the reaction. Also, different alkali metals (Ca, Na and Li) were tested for the comparison with K. The potassium doped catalyst showed the highest n-C4= yield among the other alkali metals for n-butane dehydrogenation. C) 2013 Elsevier B.V. All rights reserved.
The doping of CeO2 with different types of cations has been recognized as a significant factor in controlling the oxygen vacancies and improving the oxygen mobility. Thus, the catalytic properties of these materials might be determined by modifying the redox properties of ceria. A combined experimental and theoretical study of the redox properties of gallium-doped cerium dioxide is presented. Infrared spectroscopy and time-resolved X-ray diffraction were used for temperature programmed reduction (H2) and oxidation (with O2 and H2O) studies. Additionally, X-ray absorption near edge spectroscopy shows that only Ce4+ is reduced to Ce3+ in the ceria-gallia mixed oxides when annealed up to 623 K. The oxygen storage capacity (OSC) measurements show a pronounced enhancement on the reduction of ceria by gallium doping. Theoretical calculations by density functional theory (DFT) confirm the higher reducibility of gallium-doped ceria oxides and give a molecular description of the stabilization of the doped material. On the basis of infrared spectroscopic measurements, a novel mechanism is proposed for the surface reduction of Ce4+ to Ce3+ where Ga-H species are suggested to be directly involved in the process. In addition, the reoxidation by H2O was precluded in the gallium-doped ceria oxide. © 2013 American Chemical Society.
Carbon modified magnesium oxides (CMgO-600, CMgO-700 and CMgO-800) were prepared from pyrolyzing n-hexane vapors at 600, 700 and 800 °C, respectively, on the surface of the MgO. Modification of magnesium oxide (CMgO) with carbon increased the covalent character of the Mg–O bond, consequently decreasing the basicity. TEM and H2 chemisorption showed that the average metal particle size on CMgO was ca. 3.0 nm and the HRTEM images showed that the metal particles consisted of Pt–Sn alloys with different Pt/Sn composition. PtSn/CMgO catalysts showed much higher activity and selectivity than PtSn/MgO for the butane dehydrogenation, because a high metal dispersion on PtSn/CMgO resulted from preventing MgO dissolution during the impregnation step of metal salts on the support. The PtSn/CMgO-600 catalyst among the prepared catalysts gave the highest butenes yield.
Graphical Abstract
The effect of Co doping on ceria-zirconia mixed oxides was investigated for Co0.1Ce0.6Zr0.3Ox sample prepared by sol-gel method. The Pd-only three-way catalyst (TWC) was obtained by incipient wetness impregnation with 0.5 wt.% Pd loading. The structural and oxygen handling properties were analyzed by X-ray diffraction (XRD), H2-temperature programmed reduction (H2-TPR) and the dynamic oxygen storage capacity (DOSC). The introduction of Co into ceria-zirconia lattice strongly modified the mobility of oxygen and enhanced the DOSC performance. Pd-only TWC based on the Co0.1Ce0.6Zr0.3Ox support exhibited superior activity for water-gas shift and steam reforming and amplified amplitude of stoichiometric window.
Cerium/zirconium mixed oxides, with different Ce/Zr ratios, have been synthesised by a co-precipitation method using two different precipitating agents (sodium carbonate and urea) and tested for the total oxidation of naphthalene. Catalysts were characterised by N2 adsorption, XRD, Raman, TPR, XPS and DRIFTS. Ceria prepared by carbonate precipitation had low activity and this is likely to be related to the high concentration of residual surface carbonate that covers catalytic sites and inhibits reaction. For carbonate precipitation, increasing the Zr content to 1% resulted in a significant increase of activity, which is related to the decrease of surface carbonate. Increasing the Zr content up to 50% resulted in catalysts more active than ceria, but activity decreased as Zr content increased. This was in spite of increasing the number of oxygen vacancies, and this effect has been related to the decrease in the number of surface hydroxyl groups, which favours naphthalene adsorption. Ceria prepared by urea precipitation was markedly more active than that produced by carbonate precipitation. The urea-derived catalyst with 1% Zr is marginally more active than pure ceria, whilst for higher Zr contents activity was marginally lower. Two factors can account for these observations; they are the increase of oxygen vacancies contributing positively to activity and the opposing negative effect of decreasing the number of surface hydroxyl groups when the zirconium content increases.
Tungstated titania, with and without platinum, was used to catalyze the conversion of n-butane and of n-pentane at atmospheric pressure and temperatures in the range of 423–573 K, both in the presence and in the
absence of H2 in the feed stream to a flow reactor. The catalysts were active for isomerization and cracking,
and the products included alkenes, in contrast to those observed in catalysis by tungstated zirconia. The catalytic activity of tungstated titania is much less than that of sulfated zirconia and similar to that of tungstated zirconia. The results characterizing tungstated titania indicate a bifunctional reaction network involving metal and acidic sites responsible for hydrogenation/dehydrogenation and carbenium ion reactions, respectively. Platinum in the catalyst and H2 in the feed strongly suppress the otherwise rapid catalyst deactivation associated with coke deposition. The catalyst was found to be stable after a period of initial deactivation, and kinetics data are reported for partially deactivated catalysts. High platinum contents lead to high selectivities for cracking in the presence of H2 and high selectivities for unsaturated products in the absence of H2. Increasing tungsten loadings raise catalyst acidity and favor isomerization. The performance of tungstated
titania containing platinum resembles that of the zirconia-based catalysts less than it resembles
the performance of platinum supported on (chlorided) alumina, a well-known naphtha reforming catalyst.
Fe- and Mn-promoted sulfated zirconia was used to catalyze the conversion of n-butane at atmospheric pressure and n-butane partial pressures in the range of 0.0025-0.01 atm. At temperatures 350°C, cracking and isomerization occurred. Catalyst deactivation, resulting at least in part from coke formation, was rapid. The primary cracking products were methane, ethane, ethylene, and propylene. The observation of these products along with an ethane/ethylene molar ratio of nearly 1 at 450°C is consistent with cracking occurring, at least in part, by the Haag-Dessau mechanism, whereby the strongly acidic catalyst protonates n-butane to give carbonium ions. The rate of methane formation from n-butane cracking catalyzed by Fe- and Mn-promoted sulfated zirconia at 450°C was about 3 × 10−8 mol/(g of catalyst · s); for comparison, the rate of cracking of n-butane catalyzed by HZSM-5 under these conditions was estimated to be 4 × 10−9) mol/(g of catalyst · s) [as determined by extrapolation of the data of H. Krannila, W. O. Haag, and B. C. Gates (J. Catal. 135, 115, 1992)]. This comparison suggests that the catalytic activity of the promoted sulfated zirconia at 450°C is about the same as that of the zeolite, although its activity for n-butane isomerization and disproportionation at temperatures
The “one-pot” circulation reactor system was used for the modification of Pt/Al2O3 catalyst using Controlled Surface Reactions (CSRs) with the involvement of tetraethyltin. At 40°C the tin anchoring reaction resulted in exclusive formation of alloy type Pt–Sn/Al2O3 catalyst, while at higher temperatures tin was also introduced onto the alumina support. The bimetallic catalysts were characterized by Temperature Programmed Reduction (TPR), H2 and CO chemisorption, XPS and test reactions of the metallic phase (cyclohexane dehydrogenation and cyclopentane hydrogenolysis). It has been demonstrated that the decomposition of surface organometallic species of Sn in the presence of oxygen leads to the formation of Lewis-acid type active sites in the close vicinity of platinum. The formation Sn–Pt alloy phase together with oxidized Sn species has been evidenced by methods of characterization applied. The presence of these species in Pt–Sn/Al2O3 catalysts favors the catalytic behavior in n-butane dehydrogenation, thus increasing the n-butane conversion and the selectivity to olefins, and decreasing the coke deposition.