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Sn surface-enriched Pt–Sn bimetallic nanoparticles as a selective and stable catalyst for propane dehydrogenation
... As for the reason, it is noteworthy that PtSnCe/SiO 2 showed a slightly higher content of Pt 0 than PtCe/SiO 2 (Table 2), which may originate from the improved interaction between SnO 2 and Pt due to the presence of CeO 2 . This coincides well with the presence of Sn 0 species, indicating the possible formation of Pt-Sn bimetallic nanoparticles [28,43]. ...
... As for the reason, it is noteworthy that PtSnCe/SiO2 showed a slightly higher content of Pt 0 than PtCe/SiO2 (Table 2), which may originate from the improved interaction between SnO2 and Pt due to the presence of CeO2. This coincides well with the presence of Sn 0 species, indicating the possible formation of Pt-Sn bimetallic nanoparticles [28,43]. ...
The oxidative dehydrogenation of propane with CO2 (CO2-ODP) has been extensively investigated as a promising green technology for the efficient production of propylene, but the lack of a high-performance catalyst is still one of the main challenges for its industrial application. In this work, an efficient catalyst for CO2-ODP was developed by adding CeO2 to PtSn/SiO2 as a promoter via the simple impregnation method. Reaction results indicate that the addition of CeO2 significantly improved the catalytic activity and propylene selectivity of the PtSn/SiO2 catalyst, and the highest space-time yield of 1.75 g(C3H6)·g(catalyst)−1·h−1 was achieved over PtSn/SiO2 with a Ce loading of 6 wt%. The correlation of the reaction results with the characterization data reveals that the introduction of CeO2 into PtSn/SiO2 not only improved the Pt dispersion but also regulated the interaction between Pt and Sn species. Thus, the essential reason for the promotional effect of CeO2 on CO2-ODP performance was rationally ascribed to the enhanced adsorption of propane and CO2 originating from the rich oxygen defects of CeO2. These important understandings are applicable in further screening of promoters for the development of a high-performance Pt-based catalyst for CO2-ODP.
... One of the important takeaways from the XAS data analysis is that not all the Pt and Sn participate in formation of the alloy when prepared on a higher surface area support, and it takes multiple redox cycles to force more of the Sn present in the sample to enter into the alloy phase. A possible way to circumvent this would be to prepare the sample by different methods such as colloidal synthesis [69,70], by incorporating the Sn into a hydrotalcite type support [71], or by means of a surface organometallic chemistry method to derive a surface enriched Pt-Sn nanoparticle [72]. ...
... This can be ascribed to the XAS observations for the Pt:Sn/LSA sample, indicating that most of the Pt and Sn are participating in alloy formation, enabling high selectivity and conversion. This conclusion is in line with the activity results presented by Haibo Zhu et al. [72]. This also points towards lower carbon formation for an alloyed Pt-Sn catalyst than for a monometallic Pt catalyst, when compared at similar conversions, owing to the better selectivity in the case of the Pt-Sn. ...
CO2-assisted propane dehydrogenation has been studied on Pt-Sn/MgAl2O4 catalysts with support surface area of ∼127m²/g or ∼5m²/g, 3wt% Pt and a Pt/Sn molar ratio of 3/1. In situ XAS was employed to track the dynamic changes occurring to the catalyst in presence of a reductive (H2) or oxidative (CO2) atmosphere. Reduction leads to the formation of a Pt-Sn alloy, the active compound for propane dehydrogenation. Oxidation by CO2 led to the loss of the Pt-Sn alloy due to firstly oxidation of Sn to SnO and subsequent oxidation of SnO to SnO2. The electronic and structural properties of the catalyst were determined by modelling of the EXAFS data. The Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) method was used in conjunction with the XAS data to determine the amount of Sn present in the Pt-Sn alloy phase and the phase of the alloy itself: after a single step reduction 42% of all Sn goes into Pt3Sn alloy, participating in the reaction, with the remainder being SnOx. The percentage of Sn going into the Pt3Sn alloy increased after 10 H2/O2 redox cycles to 72%. A combination of in-situ XAS with CO2-PDH activity data covering CO2:C3H8 ratios from 0.25:1 to 1:1 allowed to show that CO2 helps to improve conversion of propane by means of the reverse water gas shift reaction, wherein the product H2 generated from PDH reacts with feed CO2 to shift the equilibrium towards products. The reaction performed better at lower ratios of CO2:C3H8. Increasing ratios of CO2:C3H8 induced faster deactivation of the catalyst by the oxidation of Sn to SnOx, leading to loss of Pt-Sn alloy. Suppression of carbon accumulation occurred by means of the reverse Boudouard reaction with the carbon formed during PDH. As possible reaction network for the entire CO2-PDH reaction, a combination of Langmuir-Hinshelwood (L-H) and the Mars-van Krevelen mechanism (MvK) was proposed. The MvK steps were the oxidation of Sn to SnO by CO2, which would then subsequently react with the H2 and C generated from PDH that takes place on Pt sites by the L-H mechanism, to go back to the Pt3Sn alloy.
... Catalytic dehydrogenation through C-H activation is the most important step in the production of ne chemicals such as olen from light alkenes. [79][80][81] Pt-based catalysts are the most frequently used catalysts for C-H activation, but it is easy to break the C-C bond, leading to carbon deposition and deactivation of the catalyst, and the high price of Pt metal prohibits its widespread use. 82,83 Thus, to solve this problem, Sykes proposed the strategy of using the PtCu SAA catalyst to facilitate C-H bond activation and avoid the formation of coke observed on clusters or particles. ...
Single-atom alloys (SAAs) are a different type of alloy where a guest metal, usually a noble metal (e.g., Pt, Pd, and Ru), is atomically dispersed on a relatively more inert (e.g., Ag and Cu) host metal. As a type of atomic-scale catalyst, single-atom alloy catalysts have broad application prospects in the field of heterogeneous catalysis for hydrogenation, dehydrogenation, oxidation, and other reactions. Numerous experimental and characterization results and theoretical calculations have confirmed that the resultant electronic structure caused by charge transfer between the host metal and guest metal and the special geometric structure of the guest metal are responsible for the high selectivity and catalytic activity of SAA catalysts. In this review, the common methods for the preparation of single-atom alloys in recent years are introduced, including initial wet impregnation, physical vapor deposition, and laser ablation in liquid technique. Afterwards, the applications of single-atom alloy catalysts in selective hydrogenation, dehydrogenation, oxidation reactions, and hydrogenolysis reactions are emphatically reviewed. Finally, several challenges for the future development of SAA catalysts are proposed.
... Considering this reaction mechanism, the development of catalysts that can enhance the selectivity of dehydrogenation and suppress sequential reactions is required. Several metals, including Pt, Ga and Cr, have been studied as active species in PDH [8][9][10][11][12][13][14][15] . Pt is the most commonly used PDH catalyst; however, it is a precious metal and expensive to use. ...
Catalytic propane dehydrogenation (PDH) is an attractive process that can meet the growing demand for propylene. Among the extensively studied PDH catalysts, Co-based catalysts are considered especially promising because of...
... 24,25 In that context, Surface Organometallic Chemistry (SOMC), in combination with thermolytic molecular precursors, has emerged as a powerful tool for interrogating the active structure of supported heterogeneous catalysts by generating tailored materials containing small, size-homogeneous (bimetallic) NPs with control of interfaces and composition, in the absence of bulk metal/oxide. [26][27][28][29][30][31][32] This approach has been successfully applied to various reactions, particularly CO2 hydrogenation to methanol, [33][34][35] propane dehydrogenation, 36,37 and reforming. 38 For CO2 hydrogenation catalysts, the most notable examples are silica-supported CuGa, CuZn, and PdGa NPs, which display a distinct reactivity compared to their monometallic counterparts. ...
Silica-supported PdGa nanoparticles (NPs) prepared via Surface Organometallic Chemistry are selective catalysts for the hydrogenation of CO2 to methanol. However, despite their notable catalytic performances, that exceed the corresponding Cu-based systems, little is understood regarding the local structure of the PdGa NPs, their adsorption properties, and their behaviour under CO2 hydrogenation reaction conditions, making the development of structure–activity relationships challenging. Here, we use ab-initio Molecular Dynamics and Metadynamics at the density-functional theory level combined with in situ X-ray absorption spectroscopy to explore the structures and the dynamics of the alloyed PdGa NPs under various conditions. We look in particular at the impact of the SiO2 surface and adsorbates (H*, CO*, O*), expected under CO2 hydrogenation conditions, onto the structure of the NPs. Overall, addition of Ga to Pd generates alloyed PdGa NP with isolated Pd sites at the surface. This structural change decreases the amount of adsorbed hydrogen or CO on the NPs and changes the dominant binding mode of the adsorbates to the metal, from mainly bridging to terminal CO and from mainly internal hydrides to terminal and μ2-bridging hydrides. Under more oxidizing conditions, akin to CO2 hydrogenation for PdGa NPs, Ga is partially oxidised, forming a GaOx layer on the surface of the NP, with a partially dealloyed PdGa core, that retains some isolated Pd sites at the surface. Overall, these bimetallic NPs show high structural dynamics and a variable extent of alloying in the presence of different adsorbates relevant for CO2 hydrogenation.
... Some metal promotors, such as Sn, Cu, Tl, Ge, and Pb, were used to modify Pt/ZSM-5 and the aromatics selectivity could be improved obviously. Introducing Sn or Pb could enhance the electronic cloud density of Pt, which decreased the hydrogenolysis ability of Pt and inhibited the formation of methane and ethane [74][75][76][77]. Beside the unsatisfying aromatics selectivity, the stability of Pt/ZSM-5 catalyst was also relatively poor. ...
Chapter 5 surveys the recent advances and forthcoming challenges in aromatics (mainly BTX) preparation from light hydrocarbons. The chapter first summarizes the development history of commercial processes and emerging methods, and introduces the brief principle of light hydrocarbon dehydroaromatization. Then, according to the carbon number of raw materials, the chapter reviews the reaction mechanism and catalyst system of BTX preparation from various light hydrocarbons. Two special sections are devoted to a brief comparison between Zn/HZSM‐5 and Ga/HZSM‐5 and the deactivation of zeolite and metal–zeolite catalysts used for aromatization. The chapter is concluded with a summary of the research status and a general outlook of challenges in BTX preparation from light hydrocarbons.
... Although it has been suggested that the addition of Sn can inhibit the agglomeration of Pt species, the sintering of the Pt nanoparticles during the harsh dehydrogenation and/or regeneration processes is still an unsolved fatal issue. [12,25,[28][29][30][31] Furthermore, the rational design of the support structure is another useful way to modulate the electronic and geometric structure of active metal phases. Mironenko et al. reported that hydrothermally treated γ-alumina can alter the metal complexsupport interaction, thus changing the dispersion and electronic state of supported platinum to improve the catalytic performance. ...
Catalytic dehydrogenation of propane to propylene and by‐product hydrogen is an atom‐economical and environmentally friendly route. PtSn/Al2O3 catalysts have been industrialized in this process, but still suffer from platinum sintering and coke deposition under reaction conditions. Herein, we design a calcium‐modified PtSn/Al2O3 catalyst showing a superior propane dehydrogenation performance. The presence of calcium combined with unsaturated aluminum and tin could consist of a new local microenvironment that promotes the dispersion of the platinum species and increases the electron density of the platinum species, which improves the catalytic activity, facilitates propylene desorption and inhibits coke formation. As a result, the achieved PtSnCa/Al2O3 catalyst exhibits a higher propylene formation rate and a lower coke‐accumulation rate compared to the catalyst without Ca addition. Moreover, the size of active phase clusters (∼1 nm) remained almost unchanged after the catalytic test, indicating a superior sintering resistance.
... For example, the post-transition metal elements (e.g. Sn, Zn, Ga, In) have been explored as co-catalyst to improve the dispersion, geometric structure and electronic properties of Pt for increasing the activity, selectivity, stability and anti-coking performance [37][38][39][40][41][42]. The introduction of co-catalyst species such as Ga could regulate the strong metal-support interaction (SMSI) between metal and oxide support, and further help to maintain the dispersion of Pt species [9,29,36]. ...
Catalytic conversion of propane to propylene can be achieved by non-oxidative propane dehydrogenation (PDH) by Pt-based catalysts under 550–700 °C. In this work, we report a strategy to decrease the Pt loading amount to only 0.1 wt% for obtaining a stable catalyst via silica coating for high PDH activity at a temperature as low as of 450 °C. The as-prepared 2.5%[email protected]/Al2O3 catalyst shows outstanding catalytic activity (propylene productivity rate 0.5 mol/gcat.·h) close to its equilibrium conversion (24.5%) with much better stability (deactivation constant: 0.007 h⁻¹) at 450 °C. The interface between catalyst and silica coating changes the strong metal-support interaction (SMSI) to immobilize the Ga oxide clusters, further tightly anchoring the Pt catalytic sites for PDH process. Moreover, the electronic state of Pt is also influenced by the silica coating layers. This work demonstrates a new method to harnessing the metal-support interaction in catalysis.
Looking at modern approaches to catalysis, this volume reviews the extensive literature published on this area. Chapter highlights include Fenton chemistry, advanced manufacturing in heterogeneous catalysis, membrane reactors for light alkane dehydrogenation, and new insights and enhancement of biocatalysts for biomass conversion in the bioproducts industry.
Appealing to researchers in academia and industry, the detailed chapters bridge the gap from academic studies in the laboratory to practical applications in industry, not only for the catalysis field, but also for environmental protection. The book will be of great benefit to any researcher wanting a succinct reference on developments in this area now and looking to the future.
The pursuit of new catalysts for the aqueous transformation of biomass-derived compounds under mild conditions is an active area of research. In the present work, the selective hydrogenation of 5-hydroxymethylfurfural (HMF) to 2,5-bishydroxymethylfuran (BHMF) was efficiently accomplished in water at 25 °C and 5 bar H2 pressure (after 1 h full conversion and 100% selectivity). For this, a novel nanocatalyst based on graphene-supported Pt NPs decorated with Sn-butyl fragments (-SnBun) has been used. More specifically, Pt NPs supported on reduced graphene oxide (rGO) were functionalized with different equivalents (0.2, 0.5, 0.8 and 1 equiv.) of tributyltin hydride (Bu3SnH) following a surface organometallic chemistry (SOMC) approach. The synthesized catalysts (Pt@rGO/Snx) were fully characterized by state-of-the-art techniques, confirming the presence of Sn-butyl fragments grafted on the platinum surface. The higher the amount of surface -SnBun, the higher the activity of the catalyst, reaching a maximum conversion with Pt@rGO/Sn0.8. Indeed, the latter has proven to be one of the most active catalysts reported to date for the aqueous hydrogenation of HMF to BHMF (estimated TOF = 666.7 h-1). Furthermore, Pt@rGO/Sn0.8 has been demonstrated to be an efficient catalyst for the reduction of other biomass-derived compounds in water, such as furfural, vanillin or levoglucosenone. Here, the catalytic activity is remarkably boosted by Sn-butyl fragments located on the platinum surface, giving a catalyst several times faster than non-functionalized Pt@rGO.
The Pt‐Sn/Al2O3 catalyst has been successfully commercialized for propane dehydrogenation, but it is still highly desirable to further improve its catalytic performance for maximizing the utilization efficiency of noble metal Pt. Herein, this work demonstrates that doping Nb and Ta into Pt‐Sn/Al2O3 has great impacts on its structure, and significantly improved catalytic performances are achieved in Pt‐Sn‐X/Al2O3(X=Nb and Ta). It is found that the incorporation of Nb and Ta into Pt‐Sn/Al2O3 can promote the dispersion of Pt‐Sn particles, which results in Pt‐Sn clusters being highly dispersed at the surface of Al2O3. Moreover, the presence of Nb and Ta makes the SnOx species less reducible, and thus the Pt‐Sn particles become more stable at high temperature. Systematic exploration allows to obtain the optimized Pt‐Sn‐Nb5.0/Al2O3 and Pt‐Sn‐Ta1.0/Al2O3, and these two catalysts exhibit high initial propane conversion (above 43.8 % and 50.4 %) with propylene selectivity of above 98 %. Most importantly, they show excellent regeneration in propane dehydrogenation, and their catalytic performance can be completely restored by a simple calcination.
Propane dehydrogenation (PDH) over Pt catalyst operated in a moving-bed mode is becoming the dominant process for the on-purpose production of propylene worldwide ¹⁻³ . Sintering and coking deactivations of Pt catalysts, which greatly decrease the durability and efficiency of this scarce and costly noble metal, are the most challenging issues for the sustainable development of the PDH process ⁴ . Even after over half a century of efforts, the development of long-term durable Pt catalysts that are feasible for more economic and efficient fixed-bed operation remains as a formidable task ⁵⁻¹⁰ . Herein we report a strategy to modulate the migration-agglomeration-lockup of Pt particles in zeolite silicalite-1 (S-1) for synthesizing highly sintering- and coking-resistant (Pt-Sn 2 ) 2 @S-1 PDH catalysts. It is found that the migration of Pt-Sn 2 clusters in S-1 crystals during the PDH reaction leads to a competition between intra-crystalline agglomeration and external-surface agglomeration of Pt-Sn 2 clusters. We show that, when the b -axis length of S-1 crystals is longer than 2 μm, the intra-crystalline agglomeration becomes dominant over the external-surface agglomeration, resulting in the formation of ultra-stable (Pt-Sn 2 ) 2 @S-1 in which the (Pt-Sn 2 ) 2 dimers are securely locked in the channels of S-1 crystals. The long-term durability (4500 hours or about 6 months on stream without perceptible deactivation tendency) of (Pt-Sn 2 ) 2 @S-1 is at least three orders of magnitude higher than that of the best catalyst reported, which is expected to bring about a breakthrough in revolutionizing the moving-bed operation of the current PDH process to fixed-bed operation. We anticipate that modulating metal migration-agglomeration-lockup within zeolite can act as a general strategy for the synthesis of durable noble metal catalysts, which is beneficial to greatly maximizing the service lifetime of noble metals in industry application ¹¹⁻¹⁵ .
Millimeter-scale spherical alumina used as the support of Pt catalyst for isobutane dehydrogenation was prepared by alginate assisted sol-gel method, and then hydrothermal treated in 0.5 M HNO3 to improve its porous structure and surface properties. The effects of acid etching time on the composition, structure, surface acidity, redox property, catalytic dehydrogenation performance and carbon deposition of catalyst were investigated by ICP-OES, SEM, TEM, XRD, N2 adsorption-desorption, ²⁷Al MAS NMR spectra, NH3-TPD, H2-TPR, isobutene transient response experiment and TGA. The results show that the acid treatment can effectively remove zinc species and partial aluminum from the surface of alumina beads, which can create defect sites and increase the specific surface area of alumina, thus improving the dispersion of Pt particles. In addition, acid etching significantly increases the acidity of alumina and enhances the interaction between Pt particles and alumina through the formation of defect sites. The Pt/Al2O3-NA-3 possesses the smallest size of Pt particles, the strongest Pt-alumina interaction, maximum number of acid sites and the highest isobutene yield of 33% even after 270 min reaction.
Monometallic and bimetallic Ni and Sn catalysts were prepared in different ratios by the Solvated Metal Atom Dispersed (SMAD) method for the catalytic hydrogenation of acetophenone to 1-phenylethanol. The preparation of the catalysts was carried out by evaporation of Ni and Sn metal atoms and subsequent co-deposition at 77 K using 2- isopropanol as solvent on alumina and magnesium oxide as supports. X-ray photoelectron spectroscopy (XPS) analysis showed a high percentage of nickel atoms in zero valence, while the tin phases were founded in reduced and oxidized form. The average size of the nanoparticles measured by transmission electron microscopy (TEM) ranged from 8 to 15 nm while the metal dispersion on the surface measured by hydrogen chemisorption ranged from 0.07% for Ni1% Sn0.3%/MgO to 3.2% for Ni5%/MgO. Thermogravimetric analysis shows that γ-Al 2 O 3 catalysts exhibit higher thermal stability than MgO catalysts. The catalysis results showed that the best support is MgO reaching 66% conversion in Ni5% Sn0.5%/MgO catalyst.
Pentacoordinate Al³⁺ (Al³⁺penta) sites on alumina (Al2O3) could anchor and stabilize the active site over the catalyst surface. The paper describes the specific effect of Al³⁺penta sites on the structure and the catalytic performance of Al2O3 supported Pt catalysts by modulating the quantity of Al³⁺penta sites. The Al³⁺penta site content of Al2O3 exhibits a volcano-type profile as a function of calcination temperature due to the structural rearrangement. The loading of Pt and subsequent calcination can consume a significant portion of Al³⁺penta sites over the Al2O3 support. We further find that, when the calcination temperature of the impregnated Al2O3 is higher than the calcination temperature of Al2O3 precursor, the structural rearrangement of Al³⁺penta sites could make Pt partially buried in Al2O3. Consequently, this partially buried structure leads to relatively low conversion but high stability for propane dehydrogenation. This work further elucidates the stabilization mechanism of the Al³⁺penta site over Al2O3 support.
In this work, Mg-Al hydrotalcite/γ-Al2O3 balls with diameter of about 2.0 mm were prepared by the in-situ growth of Mg-Al hydrotalcite on the surface of γ-Al2O3 in the presence of...
Propane dehydrogenation is a promising way to produce propylene. Catalysts with high propane conversion, selectivity to propylene, and stability are vital to this technology. In this work, we determined that the catalytic performance of Pt/SBA-15 can be significantly enhanced by introducing an appropriate amount of indium. A too low ratio of In/Pt (1/8) or too high of a ratio (2/1) is detrimental to the activity of the catalyst. When the nominal ratio of Pt to In is 1/1, the initial propane conversion and selectivity towards propylene over 1Pt1In/SBA-15 are highest among all catalysts tested at 57.06% and 99.04%, respectively. The conversion rate decreased to 39.63% after 28 h of reaction, but selectivity was retained at greater than 99%. Very little coke was deposited on the used 1Pt1In/SBA-15 catalyst. With catalyst characterization, it was proposed that Pt1In1 alloy nanoparticles with surface-enriched In2O3 were responsible for the selective dehydrogenation of propane.
Reaction coupling between propane dehydrogenation and nitrobenzene hydrogenation over a commercial Pt/Al2O3 catalyst is presented. The effects of coupling, use of the catalyst, and reaction temperature were studied using a fixed-bed reactor. The coupling of the two reactions drastically increased propane and nitrobenzene conversions even without the use of the catalyst. The use of Pt/Al2O3 further increased the conversions and the selectivity towards propylene and nitrobenzene hydrogenation products. Higher reaction temperatures favored the selectivity towards pyrolysis products. Reaction pathways are sketched to explain our experimental findings.
Propane dehydrogenation has been a promising method for producing propylene that has the potentials to meet the increasing global demand for propylene. However, owing to the restricted equilibrium conversion caused by the high endothermicity, even the Pt‐based catalysts, which exhibit high activity and selectivity, severely suffer significantly from coke formation and/or nanoparticle sintering at realistic reaction temperatures, resulting in a short catalyst lifetime. As a result, few innovative catalysts in terms of catalytic activity, selectivity, and stability, have been produced. In this Review, we focus on the characteristics of single‐atom‐like Pt sites for PDH and attempt to provide suggestions for developing highly efficient catalysts. First, we briefly describe the fundamental strategies. Following that, the remarkable catalysis is addressed by three different distinct sorts of state‐of‐the‐art single‐atom‐like Pt catalysts are discussed. Additionally, we present other promising catalyst design approaches that are not based on single‐atom‐like Pt catalysts, as well as future research challenges in this field.
A series of PtSn/hierarchical ZSM-5 catalysts were developed for propane dehydrogenation, in which the PtSn bimetallic particles are confined in the mesopores of hierarchical ZSM-5 zeolite. The synthesis of PtSn/hierarchical ZSM-5 catalysts was achieved via the loading of Pt and Sn species onto the hierarchical ZSM-5 catalysts that are obtained through a desilication of conventional ZSM-5. The PtSn/hierarchical ZSM-5 catalysts were fully characterized by XRD, N2 adsorption, STEM, XPS, and CO-IR techniques, which reveals that highly dispersed PtSn bimetallic nanoparticles are enclosed into mesopores of hierarchical ZSM-5. The catalytic performance of PtSn/hierarchical ZSM-5 is greatly affected by the concentrations of alkali solution in the desilication process and Sn/Pt ratios in PtSn bimetallic particles. The PtSn1.00/ZSM-5(0.8) catalyst shows the highest efficiency in propane dehydrogenation, which gives an initial conversion of 46% and selectivity of 98% at 570 oC. The high efficiency in these PtSn/hierarchical ZSM-5 catalysts for propane dehydrogenation is mainly ascribed to the confinement of PtSn particles in the mesopores of hierarchical ZSM-5 zeolite.
Low-loaded Pt-based catalysts supported on γ-Al2O3 (bare or modified by K) were studied for propane dehydrogenation. The alumina support was modified by adding 0.5 and 4 wt.% of K using potassium acetate as precursor. 0.5 wt.% of K is sufficient for suppressing the acidity of the alumina. Ultradispersed Pt supported catalysts (0.1, 0.2 and 0.3 wt.%) were obtained by ion exchange on Al2O3 or Al2O3–0.5 K. During propane dehydrogenation reaction, 0.3 wt.%Pt/Al2O3 strongly deactivates, mainly due to coke deposition. For the same Pt loading, the presence of K strongly decreases this phenomenon without modifying the initial turnover frequencies (TOF). On the series supported on Al2O3–0.5 K, there is no effect of the Pt content on the TOF values showing that whatever the sample, ultradispersed Pt entities present the same activity. Finally, the selectivity towards propene formation strongly depends on propane conversion and seems to be not affected by the change in K and Pt contents.
This article describes the synthesis and catalytic properties of supported, 2-3 nm platinum phosphide (PtP2) nanoparticles (NPs). Depending on the P loading, two PtP2 structures are formed, that is, a PtP2 surface on a (metallic) Pt core ([email protected]2) and single-phase PtP2 NPs. The structures were determined using extended X-ray absorption fine structure, in situ synchrotron X-ray diffraction, and scanning transmission electron microscopy. In PtP2 NPs, Pt²⁺ ions are geometrically isolated by P2²- ions, at a Pt-Pt distance of 4.02 Å, which is much longer than 2.78 Å in (metallic) Pt NPs. The oxidation state of Pt in PtP2 NPs was determined by in situ X-ray absorption near-edge structure and in situ X-ray photoelectron spectroscopy and was found to be consistent with Pt²⁺ ions even after treatment in H2 at 550 °C. Unlike Pt NPs, which are highly active for propylene hydrogenation at room temperature, PtP2 NPs are not active below about 150 °C, suggesting the absence of metallic surface Pt. In contrast to metallic Pt, which is poorly selective for acetylene hydrogenation, PtP2 NPs display high selectivity toward ethylene. PtP2, also has high olefin selectivity for propane dehydrogenation, although the rate per g Pt is about 7 times lower than that of metallic Pt NPs of the same size. In situ resonant inelastic X-ray scattering spectroscopy shows that the energy of the filled Pt 5d valence orbitals is 1.5 eV lower than that of metallic Pt, which leads to weaker adsorbate binding consistent with its catalytic properties. A H2-stable Pt²⁺ site suggests different catalytic applications for these catalysts as compared to Pt NPs.
Propane dehydrogenation (PDH) provides an alternative route for producing propylene. Herein, we demonstrates that h-BN is a promising support of Pt-based catalysts for PDH. The Pt catalysts supported on h-BN were prepared by an impregnation method using Pt(NH3)4(NO3)2 as metal precursors. It has been found that the Pt/BN catalyst undergoing calcination and reduction is highly stable in both PDH reaction and coke-burning regeneration, together with low coke deposition and outstanding propylene selectivity (99%). Detailed characterizations reveal that the high coke resistance and high propylene selectivity of the Pt/BN catalyst are derived not only from the absence of acidity on BN support, but also from the calcination-induced and reduction-adjusted strong metal-support interaction (SMSI) between Pt and BN, which causes the partial encapsulation of Pt particles by BOx overlayers. The BOx overlayers can block the low-coordinated Pt sites and constrain Pt particles into smaller ensembles, suppressing side reactions such as cracking and deep dehydrogenation. Moreover, the BOx overlayers can effectively inhibit Pt sintering by the spatial isolation of Pt during periodic reaction-regeneration cycles. In this work, the catalyst support for PDH is expanded to nonoxide BN, and the understanding of SMSI between Pt and BN will provide rational design strategy for BN-based catalysts.
Recently, the propane dehydrogenation (PDH) to propylene has become one of the most promising and attractive techniques for the production of propylene because the success of the large‐scale exploitation of shale gas can supply an abundance of cheap propane for this process. Al2O3 is usually used as a support for PtSn catalysts to withstand the harsh reaction environment in the PDH process. However, due to the formation of coke and thermal agglomeration of metal particles, the PtSn catalyst suffers severe deactivation. Therefore, spinel supports are rationally designed with different morphologies (sphere, sheet, and rod) to stabilize the PtSn catalyst in the PDH. Specifically, the rod‐like spinel support exhibits a larger pore size and suitable interaction with PtSn nanoparticles shows an excellent C3H8 conversion rate of 43.2% and a C3H6 selectivity of 95.0% with an extremely low deactivation rate of 0.005 h−1 for the cycle test, standing as one of the best PDH catalysts reported to date and outperforming the commercial PtSn/Al2O3 catalysts. Besides, this catalyst is compatible with the industrial process to achieve scalable production. The PtSn alloy catalysts stabilize at (111) lattice face of MgAl2O4‐rod for highly efficient propane dehydrogenation. The richer mesopores and rapid reaction kinetics prevent the blockage of the pore by coke. The strong bonding with PtSn nanoparticles thus relieves the formation of coke on the catalyst and provides high stability.
Dehydrogenation of propane (PDH) technology is one of the most promising on-purpose technologies to solve supply-demand unbalance of propylene. The industrial catalysts for PDH, such as Pt- and Cr- based catalysts, still have their own limitation in expensive price and security issues. Thus, a deep understanding into the structure-performance relationship of the catalysts during PDH reaction is necessary to achieve innovation in advanced high-efficient catalysts. In this review, we focused on discussion of structure-performance relationship of catalysts in PDH. Based on analysis of reaction mechanism and nature of active sites, we detailed interaction mechanism between structure of active sites and catalytic performance in metal catalysts and oxide catalysts. The relationship between coke deposition, co-feeding gas, catalytic activity and nanostructure of the catalysts are also highlighted. With these discussions on the relationship between structure and performances, we try to provide the insights into microstructure of active sites in PDH and the rational guidance for future design and development of PDH catalysts.
The adsorption of CO in the temperature range of 300–713 K on the PtSn particles of a reduced (H2, 713 K) 1.2%Pt-2.7%Sn/Al2O3 catalyst is studied by FTIR spectroscopy to reveal the geometric and electronic effects of Sn on the Pt sites. By comparison with a 1.2% Pt/Al2O3 it is shown that Sn (a) suppresses the Pt sites forming bridged CO species due to a geometric effect and (b) displaces the IR band of a linear CO species on Pt sites at 300 K from 2066 cm−1 to 2044 cm−1 due to an electronic effect. According to the AEIR method, the change in the IR band on the PtSn particles with the increase in Ta in isobaric condition (PCO = 1 kPa) provides the heats of adsorption of the linear CO species (named L1PtSn) at high (58 kJ/mol) and low (130 kJ/mol) coverages. By comparison with the values of the linear CO species on Pt particles (named LPt): 220 and 106 kJ/mol at low and high coverages, respectively, these values reveal the strong impact of the electronic effect of Sn on the heats of adsorption of CO. For Ta > 463 K there is a reconstruction of the PtSn particles due to the segregation of Sn as SnOx species associated with an enrichment of the surface in Pt. On the reconstructed PtSn particles, a new IR band is observed at 2057 cm−1 after adsorption of CO at 300 K ascribed to a new L2PtSn CO species. Its heats of adsorption are significantly higher than those of the L1PtSn species: 165 and 65 kJ/mol at low and high coverages. Moreover, successive reconstruction/H2 reduction at 713 K cycles leads to a sintering of the PtSn particles associated to the progressive increase in the heats of the L2PtSn CO species until a value at low coverage: 210 kJ/mol similar to that of the LPt species whereas at high coverage a significant difference exists between the two species (70 and 115 kJ/mol for L2PtSn and LPt, respectively). These data show that the impacts of Sn on the heat of adsorption of the linear CO species on the Pt sites are dependent on (a) the Pt/Sn surface ratio which changes with reconstruction and the particle size and (b) the coverage of the Pt sites. The comparison of the heats of adsorption of the L CO species on the fresh, reconstructed and aged PtSn particles with experimental and theoretical literature data reveals that they are consistent with some of them.
Alumina- and silica-supported platinum and platinum/tin and the unsupported alloys Pt/sub 3/Sn, PtSn, Pt/sub 2/Sn/sub 3/, and PtSn/sub 2/ were prepared by various methods. Oxygen-hydrogen titration and temperature-programed reduction confirmed that all tin reduced to the zero-valent state (except for a small amount that interacted with the alumina supports) and that it was present in bimetallic clusters or alloys. In tests on n-hexane conversion at 520/sup 0/C, 3 bar, and 2:1 hydrogen-hexane, the addition of tin increased the catalyst stability (i.e., reduced coke deposition), decreased methylcyclohexane formation, and increased benzene formation. The results were consistent with the assumptions that the tin acted mainly as diluent, i.e., decreased the number of large platinum ensembles and increased the number of small platinum ensembles, and at the test temperature, secondary gas-phase reactions were significant.
A new generation of bimetallic catalysts exhibiting unusual activities and/or selectivities can be obtained via the reaction of organometallic compounds of main-group elements, in particular group IV, with supported group VIII transition metals. The authors report that the reaction of Sn(n-C4H9)4 with Rh2O3/SiO2 results in a Rh/Sn/SiO2 catalyst which is more active and much more selective than Rh/SiO2 for the hydrogenation of ethyl acetate to ethanol. Our results indicate the ability of tin atoms to isolate rhodium atoms from their neighbors. It is shown that the concept of rhodium site isolation by tin atoms, which may account for high chemoselectivity is supported by three experimental facts which are outlined.
A strategy for the synthesis of PtRh alloy 3D porous nanostructures by controlled aggregation of nanoparticles in oleylamine is presented. The atomic ratio between the two components (Pt and Rh) is tuned by varying the concentration of precursor salts accommodating the oxidation of methanol. The morphology of PtRh alloy nanostructure is controlled by elevating the temperature of the reaction system to 240 °C. The prepared 3D porous nanostructures provide a high degree of electrochemical activity and good durability toward the methanol oxidation reaction compared to those of the commercial Pt/C (E-TEK) and PtRh nanoparticles. Therefore, the 3D alloy porous nanostructures provide a good opportunity to explore their catalytic properties for methanol oxidation.
Density functional theory calculations have been performed to investigate the effect of Sn on the catalytic activity and selectivity of Pt catalyst in propane dehydrogenation. Five models with different Sn to Pt surface molar ratios are constructed to represent the PtSn surfaces. With the increase of the Sn content, the d-band of Pt is broadened, which gives rise to a downshift in the d-band center on the PtSn surfaces. Consequently, the bonding strength of propyl and propylene on the alloyed surfaces is lowered. With the decomposition of the adsorption energy, the change in the surface deformation energy is predicted to be the dominant factor that determines the variation in the adsorption energy on the surface alloys, while on the bulk alloys the change in the binding energy makes a major contribution. The introduction of Sn lowers the energy barrier for propylene desorption and simultaneously increases the activation energy for propylene dehydrogenation, which has a positive effect on the selectivity toward propylene production. Considering the compromise between the catalytic activity and selectivity, the Pt3Sn bulk alloy is the best candidate for propane dehydrogenation.
Composition-controlled fabrication of bimetallic catalysts is of significance in electrochemical energy conversion and storage. A novel nanoporous Pt-Cu bimetallic catalyst with a Pt skin and a Pt-Cu core, fabricated by electrochemically dealloying a bulk Pt-Cu binary alloy using a potential-controlled approach, is reported. The Pt/Cu ratio of the dealloyed nanoporous catalyst can be readily adjusted in a wide composition range by only controlling dealloying potential. The electro-catalytic performance of the nanoporous Pt-Cu catalyst shows evident dependence on Pt/Cu ratio although the surfaces of all the nanoporous catalysts are characterized to be covered by pure Pt. With optimal compositions, the dealloyed nanoporous Pt-Cu catalyst possesses enhanced electrocatalytic activities toward oxygen reduction reaction and formic acid oxidation in comparison with the commercial Pt/C catalyst.
X-ray diffraction and Mössbauer spectroscopy studies indicate that the thermal pretreatment of Pt−Sn catalysts supported on ZnAl2O4 and MgAl2O4 spinels, in the reduction medium leads to the formation of coarse-disperse particles of PtSn, Pt2Sn3 and PtSn2 alloys, which deactivate the catalysts.
Kinetic, microcalorimetric, and Mössbauer spectroscopic studies were conducted to investigate the factors controlling the selectivity of isobutane conversion over silica-supported Pt and Pt/Sn catalysts. Kinetic studies show that addition of Sn to Pt enhances the selectivity for isobutylene formation at temperatures near 700 K. Mössbauer spectroscopic studies show that Sn interacts with Pt to produce a Pt/Sn alloy. Microcalorimetric investigations show that addition of Sn reduces the number of sites that strongly interact with hydrogen or carbon monoxide. The elimination of stronger adsorption sites may be caused by preferential blocking of sites by tin or weakening of sites due to ligand effects. However, strong adsorption sites are present on a Pt/Sn catalyst that exhibits high dehydrogenation selectivity. Microcalorimetric studies of ethylene adsorption suggest that addition of Sn inhibits the formation of highly dehydrogenated surface species. These results suggest that high dehydrogenation selectivity achieved by addition of Sn to Pt is caused by a decrease in the size of surface Pt ensembles.
Two series of bimetallic Pt-Sn/alumina catalysts have been prepared by impregnation of samples of industrial Pt/alumina reforming catalysts with solutions of Sn(IV) chloride in acetone. The Pt content was constant at 0.3 wt% and the tin content was varied over the range 0.3 to 5.0 wt%. The reducibility of the catalysts was determined by temperature-programmed reduction, and by reduction in flowing hydrogen, followed by reoxidation in oxygen. The results show that the Pt catalyses the reduction of the tin. However, the average oxidation state of the tin after reduction is Sn(II), and no further reduction occurs even when Pt is present, or when reduction is continued for long times. The average oxidation state of the tin is independent of the concentration of tin on the catalyst over the whole range from 0.3 to 5.0wt%. The dispersion of the Pt has been determined, as a function of tin content, by measuring the amount of hydrogen chemisorbed. The chemisorption experiments show that the amount of hydrogen adsorbed by Pt is increased when tin is present, which indicates that tin increases the dispersion of the Pt. It is concluded that tin is stabilised in the Sn(II) state by interaction with the support and, consequently, that no proper Pt-Sn alloys are formed in these catalysts. The role of tin cannot be to divide up the surface into small ensembles of Pt atoms. It is proposed that the special properties of the catalysts are due to a change in the electronic properties of small Pt particles either by interaction with Sn(II) ions on the support surface, or by incorporation into the Pt of a few percent metallic tin as a solid solution.
The structural changes in supported NiPt/C and NiPt/γ-Al2O3 catalysts were investigated using in-situ extended X-ray absorption fine structure (EXAFS) under aqueous phase reforming (APR) of ethylene glycol conditions. Reverse Monte Carlo is introduced to analyze the EXAFS data. Parallel reactor studies of APR of ethylene glycol showed that NiPt catalysts were initially more active than monometallic Pt catalysts. The enhanced activity was correlated to changes in the catalyst structure. Under APR conditions, Ni segregated to the surface of the catalysts, resembling Ni-terminated bimetallic surfaces that were predicted to be more active than Pt from theoretical and experimental studies on model surfaces.
For the first time, shape-controlled Pt3Sn, PtSn, and PtSn2 intermetallic nanocrystals were synthesized in octadecene (ODE) by a versatile hot-injection method with 1,2-hexadecanediol (HDD) as the reducing agent. Transmission electron microscopy (TEM) measurements reveal that the metal composition has an influence on the particle morphology: with the increase in the Sn content, the Pt/Sn nanoparticles obtained by the hot-injection synthesis show flower-like, irregular faceted, cubic/tetrahedral, hexagonal, and spherical/nanowire structures. A facile phase-transfer preparative procedure for the synthesis of Pt/Sn core/shell nanoparticles was also developed, in which ligand-free Pt nanoparticles were used as precursors. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements confirm a Pt-core/Sn-shell structure. The surface characteristic of the Pt/Sn core/shell nanoparticles was also investigated by IR spectroscopy of CO adsorption experiments (i.e., with a highly surface sensitive technique). These experiments reveal a few Pt atoms to be left on the surface as adsorption sites for CO. However, the intensity of the corresponding infrared (IR) bands is almost negligible. Furthermore, Pt/Sn random-alloy nanoparticles with different metal compositions and particle sizes were synthesized in this work by heating-up methods. Energy dispersive X-ray (EDX) and XRD analyses show different alloying extent of Sn with Pt.
A method for the preparation of NiO and Nb–NiO nanocomposites is developed, based on the slow oxidation of a nickel-rich Nb–Ni gel obtained in citric acid. The resulting materials have higher surface areas than those obtained by the classical evaporation method from nickel nitrate and ammonium niobium oxalate. These consist in NiO nanocrystallites (7–13nm) associated, at Nb contents >3at.%., with an amorphous thin layer (1–2nm) of a niobium-rich mixed oxide with a structure similar to that of NiNb2O6. Unlike bulk nickel oxides, the activity of these nanooxides for low-temperature ethane oxidative dehydrogenation (ODH) has been related to their redox properties. In addition to limiting the size of NiO crystallites, the presence of the Nb-rich phase also inhibits NiO reducibility. At Nb content >5at.%, Nb–NiO composites are thus less active for ethane ODH but more selective, indicating that the Nb-rich phase probably covers part of the unselective, non-stoichiometric, active oxygen species of NiO. This geometric effect is supported by high-resolution transmission electron microscopy observations. The close interaction between NiO and the thin Nb-rich mixed oxide layer, combined with possible restructuration of the nanocomposite under ODH conditions, leads to significant catalyst deactivation at high Nb loadings. Hence, the most efficient ODH catalysts obtained by this method are those containing 3–4at.% Nb, which combine high activity, selectivity, and stability. The impact of the preparation method on the structural and catalytic properties of Nb–NiO nanocomposites suggests that further improvement in NiO-catalyzed ethane ODH can be expected upon optimization of the catalyst.
Copper was deposited on different shapes of ceria supports (i.e., rods, cubes, and octahedra) and used as catalysts for preferential CO oxidation in excess amounts of hydrogen. When the same amount of copper was deposited, the copper content on the surface measured by X-ray photoelectron spectroscopy differed significantly, with more copper on the ceria octahedra. Copper seemed to migrate into the bulk ceria to a greater degree on the rods. The Cu/ceria-octahedra showed the highest activity of 95% at 140°C among the three shapes, whereas the Cu/ceria-rods showed higher CO conversion than the Cu/ceria-octahedra at higher temperatures. The Cu/ceria-octahedra showed no activity degradation for CO conversion at 140°C over 100h, whereas the activity decreased by 13% for Cu/ceria-rod and 32% for Cu/ceria-cube at the same temperature. The metals Au and Pt were also deposited on the different shapes of ceria, and their activity and selectivity were evaluated.
With non-calcined bimetallic Pt—Sn/Al2O3 catalysts containing 1% Pt and 0.06 – 4% Sn, previous work has shown a complete reduction of tin for a content <1%. The properties of these catalysts were investigated for the conversions of methylcyclopentane (MCP), methylcyclopentene, hexane, cyclohexene and cyclohexane. A comparison of the product distribution clearly shows that the aromatisation of MCP follows a pure metallic route at temperatures lower than 673 K. Coke and sulphur deposition have effects similar to tin addition. The experimental evidence shows that for C-C bond rupture, the main role of tin is to dilute the platinum surface, as coke and sulphur do. Aromatisation goes through a maximum as a function of tin content. At high tin contents, the main reaction is dehydrogenation to methylcyclopentenes. Sulphur or coke does not affect aromatisation and dehydrogenation markedly. The special behaviour of tin is interpreted in terms of an electronic modification of platinum in the bimetallic catalyst.
In the study of hydrocarbon reactions carried out on unsupported and supported alloys, the ‘ensemble size effect’ has been firmly established as the major factor determining the catalytically active surface. Accordingly, the selectivity pattern of a hydrocarbon transformation is determined by the participation of different types of intermediates associated with ensembles of different size. This latter, in turn, is a function of the metal-metal combinations.The effects observed with bimetallic systems with a low metal loading are not as simple as with bulk alloys. Alloy phase formation can be doubted in most cases, and in some instances formation of a ‘metal phase’ is also questionable. Nevertheless, a bimetallic system is frequently superior in its behaviour to a monometallic catalyst. The following effects may be considered to rationalize the existing data: — Formation of bimetallics but segregation occurs; — Particle size effect, dispersion; — Matrix effect, e.g. one component grafted to the support; — Change in hydrogen coverage; — Metal-support interaction (MSI); — Suppression of ageing effects, such as coke formation. These effects are discussed here for a series of examples, e.g. Ru-Cu, Ru-Au, Ru-Pt, Ru-Fe, Pt-Au, Pt-Ir, Pt-Fe bimetallic systems, considering the influence of the factors mentioned for different hydrocarbon transformations as well as for hydrocarbon synthesis.
Kinetic
measurements on preferential CO oxidation in a H2-rich atmosphere (PROX) over a bimetallic, carbon supported
PtSn catalyst reveal a high activity and selectivity already at low temperatures (0–80°C), superior to
a commercial Pt/Al2O3 system. The selectivity, though steadily decreasing with temperature, is remarkably
high, 85% at low temperatures around 0–20°C, and even at 120°C it is, at 45%, still higher than that of standard Pt catalysts.
The observation that CO desorption is not rate limiting and that the selectivity decreases with increasing
temperature, can be explained in a mechanistic model involving separation of the reactant adsorption
sites (bifunctional surface), with competing CO and hydrogen adsorption on Pt sites/areas and oxygen adsorption predominantly on Sn sites and SnOx islands on/adjacent to the active PtSn particles. The reaction takes place
in a bifunctional way at the perimeter of these islands or by invoking a spill-over process. This model
is supported by CO temperature-programmed desorption (TPD), in situ
diffuse reflectance IR Fourier transform
spectroscopy (DRIFTS), and x-ray photon spectroscopy (XPS) measurements, which indicate that under reaction
conditions the surface CO coverage on the metallic particles is high, but decreases with temperature, and that only part of the Sn is reduced, included in PtSn alloy particles, while another part is in an oxidic state, forming
SnOx islands on and presumably also beside the active particles. Its excellent performance makes PtSn an
interesting catalyst for fuel gas purification in
low temperature
polymer electrolyte membrane
fuel cell technology (PEM-FC).
SiO2-supported PtFe catalysts with a wide range of Pt/Fe ratios were prepared from individual H2PtCl6 and Fe(NO3)3 precursors and characterized by high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR), and extended X-ray absorption fine structure (EXAFS) spectroscopies. Treatment with H2 at 350°C leads to the formation of metal particles with average sizes in the order of 2.6nm, some of which are bimetallic in nature. The fraction of Pt–Fe bimetallic contributions in each sample, the nature and extent of electronic interactions between Pt and Fe, and the strength of the CO adsorption on Pt strongly depend on the Fe content. PtFe/SiO2 samples thus prepared were found to be active catalysts for various reactions taking place in both oxidative and reducing environments, including the oxidation of CO in air, the dehydrogenation of cyclohexane, and the selective hydrogenation of citral. The results indicate that the catalytic behavior of Pt is significantly affected by the presence of Fe. The enhancement of the catalytic activity observed for the oxidation of CO in air correlates with the fraction and degree of electronic Pt–Fe interactions and the strength of CO adsorption on Pt. Similarly, Fe in small concentrations promotes the activity of Pt for the dehydrogenation of cyclohexane and the selective hydrogenation of citral, which can be attributed to either an electronic effect and/or the presence of bimetallic Pt–Fe sites. The close proximity between Pt and Fe in such sites leads to the reduction of Fe, which can thus become active for cyclohexane dehydrogenation. Furthermore, it is possible that Pt–Fe adsorption sites favor the di-σCO mode of adsorption for α,β-unsaturated aldehydes, thus promoting the selective hydrogenation of the CO bond and the formation of α,β-unsaturated alcohols.
Platinum and tin deposited on γ-Al2O3, MgO, and Mg(Al)O supports were investigated by CO chemisorption and analytical electron microscopy in the scanning transmission electron microscopy with energy-dispersive X-ray spectrometry (STEM/EDX). Composition and size of individual particles in the 1-nm range are presented and results are compared with dispersion measurements obtained from volumetric chemisorption. We demonstrate that dispersion determined by chemisorption measurements can give unrealistically low values, possibly caused by metal–support interaction, while STEM/EDX reveals the correct size of metal particles. The metal–support interaction seems not to be present on γ-Al2O3 support but only on Mg(Al)O and MgO supports. The performance of the catalyst in propane dehydrogenation is related to the amount of Pt on the metal particle surface. It is shown that there is a relationship between the composition of metal particles and the activity of the catalyst. The most active is a catalyst that contains metal particles with high Pt content; however, some Sn is necessary for reduced coking and probably for increased stability.
The selective hydrogenation of cetaloxopromegestone (17α-methyl-17β-(1,2-dioxopropyl)-estra-5,9-dien-3-ketal) to the ketal precursor of Trimegestone (17α-methyl-17β-(2(S)-hydroxy-1-oxopropyl)-estra-5,9-dien-3-ketal) was carried out on various silica-supported monometallic catalysts and on bimetallic platinum–tin catalysts prepared by the interaction of Sn(CH3)4 with reduced Pt/SiO2 under H2 at room temperature. The selective hydrogenation must occur stereoselectively at the C21 ketone of cetaloxopromegestone, which possesses another ketone at C20 and two conjugated olefinic double bonds at C5–C10 and C9–C11. Of the various supported metals (Pd, Ru, Rh, Pt), the Pt/SiO2 catalyst exhibited low chemoselectivity (52%), but the diastereoselectivity at C21 reached 70%. The chemoselectivity of PtxSny/SiO2 catalysts increased from 52 to 100%. At the same time, however, the d.e. at C21 decreased from 70 to 30%. This inverse tendency of chemo- and diastereoselectivity upon the addition of tin can be explained by the fact that the multifunctional molecule can be coordinated to the surface either by its C21 carbonyl (which leads to high chemoselectivity) or simultaneously by its C=C bonds and C21 carbonyl (which leads to high diastereoselectivity). This substrate–catalyst binding, governed by the amount of tin that is added, controls the chemo- and diastereoselectivity via the coordination mode of the chiral cetaloxopromegestone.
The selective dehydrogenation of isobutane into isobutene was studied on silica-supported bimetallic Pt-Sn. Several bimetallic catalysts were carefully prepared by selective hydrogenolysis of Sn(n-C4H9)4on Pt. Previous EXAFS studies have shown that this hydrogenolysis is a stepwise transformation of a Pt-Sn(n-C4H9)3fragment into a surface alloy. It was shown that after hydrogen treatment at 550°C, tin and platinum are in reduced form (zero-valent oxidation state) and that the tin atoms are located on the surfrace of the metallic particles. The presence of tin on platinum caused a decrease in hydrogen or carbon monoxide chemisorption, but an increase of the oxygen consumption. The decrease of H2and CO chemisorption is explained by the decrease of the number of accessible platinum atoms due to the increased number of surface tin atoms. The increase in the O2chemisorption was explained by the following reaction which represents a phase segregation: PtsSnx/SiO2+1/2(y+xy′)O2→(PtOy)s(SnOy′)x/SiO2. The values of y and y′ was about 1 and 2 at respectively 25°C and 300°C. Thermodesorption of adsorbed CO on several reduced PtSn catalysts showed no shift of the ν (CO) frequency, suggesting negligible electronic effect of tin atoms on the platinum atoms when both are reduced. At 550°C under atmospheric pressure of hydrogen and isobutane, the presence of tin increases drastically, both the selectivity and the activity of the isobutane conversion into isobutene (for Sn/Pts=0.85, the selectivity is higher than 99% and the TOF, based on total platinum atoms, is greater than 6 s−1). The increase in selectivity could be explained by the “site isolation effect” and the increase in activity could be due to the inhibition of the coke formation (which poisons the active surface). A mechanism of dehydrogenation and hydrogenolysis of isobutane based on elementary steps of organometallic chemistry has been proposed which accounts both for the high selectivity and activity of the bimetallic catalysts as compared to pure Pt/SiO2.
The kinetics of the oxidation of carbon monoxide by oxygen have been studied in the temperature range 0 < theta < 80 degrees C on alumina-supported Pt/SnO2 catalysts, the respective areas of the two active compounds being varied independently of each other. Rate measurements have been performed as well as titration experiments. The results indicate that the chemisorption of CO is restricted to platinum, whereas oxygen is adsorbed on both Pt and SnO2. The rate pattern may be described by a mechanism where the migration of adsorbed species is added to the classical elementary reaction sequence of CO oxidation on platinum. Monte Carlo simulations show that the proposed mechanism can explain the experimental results. Under conditions where the rate is zero order with respect to both CO and O-2. Simulation indicates that the migration rate of oxygen could be the determining step. Oxygen spillover from SnO2 to Pt cannot be excluded but is not necessary to describe the experimental results. (C) 1997 Academic Press.
Pt/Al2O3and Pt–Sn/Al2O3catalysts, coked during propane dehydrogenation, have been studied using temperature-programmed oxidation (TPO). Time on stream, temperature, and reaction gas composition have been varied. Three different peaks were identified from the TPO profiles on the Pt–Sn catalyst and attributed to different types of coke; coke on and in the vicinity of the metal, coke on the carrier, and graphitic coke on the carrier. The amounts of these types were related to reaction conditions. The formation of the coke belonging to the first two peaks in the TPO profiles increases with temperature and partial pressure of propene. Hydrogen, on the other hand, suppresses the formation. The amount of coke that can be attributed to the third peak increases with temperature and propane partial pressure. A model is discussed where a minor part of the coke deactivates the catalyst. This coke is formed in parallel with the coke that is seen in the first two peaks in the TPO experiments. The graphitic coke formed on the carrier is not formed through this route. The experiments with different time on stream revealed that the first peak reached a constant level after about 15 h, while the second one still increased. Hydrogen was very efficient in preventing coke formation and deactivation but could not remove coke already formed on the catalyst. The hydrogenolysis and cracking mechanisms during the propane dehydrogenation are also discussed.
A series of materials that contained 1 wt% Pt and Pt: Sn atomic ratios varying from 1 : 1 to 1 : 8 were prepared. These metals were supported on alumina (100 or 250 m²/g) or silica (700 M²/g). For both alumina supports, the fraction of Pt that was present in an alloy increased with increasing tin content; the data are consistent with, but do not require, an alloy with a composition of PtSn. For these materials, zero valent tin is more easily obtained following reduction in hydrogen when silica is used as the support. Mossbauer data alone indicate that an alloy richer in Sn than PtSn is present on the silica support; however, Mossbauer data combined with XRD data indicate that a PtSn alloy is formed with additional tin being in a “metallic” state.
An electron microdiffraction technique was employed to identify crystal structures developed in two PtSnalumina catalysts. One catalyst was prepared by coprecipitating Sn and Al and then impregnating the calcined material with hexachloroplatinic acid to give a Pt: Sn = 1 : 3 atomic ratio. The second catalyst was prepared by coimpregnating Degussa Al2O3 with an acetone solution of H2PtCl6 and SnCl4 to provide a Pt: Sn ratio of 1 :3. Pt-Sn alloy was not detected by X-ray diffraction for coprecipitated catalyst although evidence for Pt-Sn alloy was found for the coimpregnated catalyst. However, electron microdiffraction studies clearly showed evidence for Pt: Sn = 1 : 1 (hcp) alloy phase in both the catalysts. Evidence for the presence of minor amounts of Pt: Sn = 1 : 2 (fee) phase was also found for the coprecipitated catalyst. EDX analysis of selected particles show that the dominant alloy is Pt: Sn = 1 : 1 for the coimpregnated catalyst. However, for the coprecipitated catalyst, most of the Pt appear as the metal, and not an alloy. Much of the tin is not detected by EDX for the coprecipitated catalyst.
Reaction of Sn(n-C4H9)4 with NiO/SiO2 occurs above 423 K according to the apparent following stoichiometry: NiO + xSn(n-C4H9)4 → NiSnx + (2x + 1)C4H8 + (2x − 1)C4H10 + H2O. Various compositions of the bimetallic phase can be achieved by changing the initial Sn/Ni ratio. The obtained catalysts were very active and selective in the hydrogenation of ethyl acetate to ethanol. Characterization of the bimetallic phase has shown that the particles are bimetallic (STEM). As a result of chemisorption, IR, and magnetic measurements, it appears that the presence of tin has four effects: (i) it decreases significantly the amount of CO and H2 adsorbed; (ii) it isolates nickel atoms from their neighbors; (iii) it increases electron density on nickel; and (iv) it suppresses the magnetic properties of nickel. Redox behavior of NiSn/SiO2 toward surface OH indicates that surface hydroxyls can oxidize Sn(o), probably to Sn(II) with evolution of H2H2, the process being reversible with H2. It is suggested that during this oxidation process, tin migrates to the periphery of the bimetallic particle with formation of (⩾Si<.z.sbnd;O)2Sn(II) surface species.
Reaction of Sn(n-C4H9)4 with Rh supported on silica results in a new bimetallic RhSn catalyst which is extremely active and selective in the reduction of ethyl acetate to ethanol. Whereas Rh/SiO2 gives rise to a selectivity for ethanol of 57%, the RhSn catalyst obtained by the organo-metallic route results in a higher activity and a selectivity to ethanol as high as 98%. Above a Sn/Rh value of 0.3, the activity varies linearly with the tin content which suggests that the enhanced catalytic activity is due to a new intermetallic phase. The catalysts have been characterized at various steps of the preparation. The starting reduced catalyst Rh/SiO2A with CO exhibits the typical infrared absorption bands of linear and bridged CO. Reaction of oxidized A with Sn(n-C4H9)4 in refluxing heptane occurs mostly between Rh2O3 and the organotin compound to give an unreduced RhIII7z.sbnd;SnRx bimetallic surface complex B, the existence of which has been suggested from mass balance, STEM, and IR spectroscopy. Reduction of B at 773 K under H2 leads to bimetallic particles with an average size of 2.2 nm and which do not contain any organic fragment (catalyst C). C chemisorbs only 0.1 H/Rht and 0.4 CO/Rht which is in sharp contrast with the values obtained with A (1.1 H/Rht and 1.1 CO/Rht). CO chemisorption on B gives only a single absorption band at 2000 cm⁻¹ corresponding to linear coordination of CO. The presence of tin has apparently three effects: (i) it decreases significantly the amount of CO and H2 adsorbed; (ii) it apparently isolates rhodium atoms from their neighbors; (iii) it increases slightly the electron density on rhodium. Redox behavior of the RhSn/SiO2 toward O2 and silanol groups of silica has also been observed. With a fully reduced catalyst C, Rh⁽⁰⁾ and Sn⁽⁰⁾ are fully oxidized by O2 to Rh2O3 and SnO2. Thermal treatment of catalyst C under flowing He results in an oxidation of tin by surface silanol (or adsorbed water) to give a partially oxidized Sn species. H2 is evolved during this oxidation process. The origin of the high activity and high selectivity (without hydrogenolysis property) of these catalysts is ascribed to the presence of a new catalytic phase in which rhodium atoms are isolated from their neighbors without any “ensemble” able to cleave the CC and CO bonds of ethyl acetate.
Carbon-supported binary PtSn catalysts with varied alloying degree were synthesized in different processes and denoted as PtSn/C-B, PtSn/C-EG and PtSnO2/C, respectively. X-ray diffraction (XRD) characterizations showed that PtSn/C-B catalyst displayed the highest alloying degree, while PtSnO2/C catalyst had the lowest one among these samples. X-ray photoelectron spectroscopy (XPS) results revealed that the non-alloyed Sn existed in an oxidized state on the surfaces of these catalysts. By evaluating the electro-catalytic activity and analyzing the final products of ethanol oxidation reaction (EOR) on these catalysts, it was found that PtSnO2/C catalyst enhanced the products yield of acetic acid products and PtSn/C-B catalyst promoted the entire activity for EOR. It was proposed that non-alloyed SnO2 species enhanced the bi-functional mechanism, whereas PtSn alloy phase strengthened the electronic effect of PtSn/C catalyst.
The Pt–Sn/Al2O3 catalysts with 0.3wt% Pt and 0.5–1.5wt% Sn loading were prepared by one-step flame spray pyrolysis (FSP). Unlike the catalysts prepared by conventional impregnation method, the FSP-derived catalysts were composed of single-crystalline γ-alumina particles with the as-prepared primary particle size of 10–18nm and contained only large pores. The FSP catalysts exhibited superior catalytic activity and better stability than the ones made by impregnation in the dehydrogenation of propane, while they did not alter the selectivity to propylene (in all cases, propylene selectivity ≥96%). The presence of large pores in the flame-made catalysts not only facilitated diffusion of the reactants and products but could also lessen the amount of carbon deposited during reactions. As revealed by CO chemisorption, transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), the metal particles appeared to be partially covered by the alumina matrix (Al–O) due to the simultaneous formation of particles during FSP synthesis. Such phenomena, however, were shown to result in the formation of active Pt–Sn ensembles for propane dehydrogenation as shown by higher turnover frequencies (TOFs).
Dehydrogenation of propane combined with selective hydrogen combustion was studied over supported Pt-Sn bimetallic catalysts. A catalytic test for normal dehydrogenation was also carried out as part of the proposed process. Pt/Al2O3 modified with Sn and Zn was found to be suitable for both of these two reactions. In the normal dehydrogenation, conversions equal to the calculated equilibrium conversion were achieved with almost complete selectivity for propylene. Optimization of catalyst composition was done by addition of various amounts of Sn to Pt/Al2O3 and Pt/Zn-Al-O. For dehydrogenation combined with hydrogen combustion, selective hydrogen combustion was achieved over Pt-Sn bimetallic catalysts. Moreover, a stable conversion higher than that of equilibrium for normal dehydrogenation was obtained using Pt-Sn/Zn-Al-O under certain reaction conditions.
The first preparation of bis (eta6-arene)titanium(0) complexes was achieved by reduction of TiCl4 with the borate [BEt3H]- in the appropriate arene solvent. In contrast the same reduction in THF leads to soluble "TiH2", which loses H-2 when dried to give a substance which can be formulated as Ti . 05 THF with various amounts of residual hydrogen. X-ray photoelectron spectroscopy and EXAFS analysis support the assumed oxidation state of zero for the titanium.
Reaction of Sn(n-CâHâ)â with NiO/SiOâ occurs above 423 K according to the apparent following stoichiometry: NiO + xSn(n-CâHâ)â â NiSnâ + (2x + 1)CâHâ + (2x - 1)CâHââ + HâO. Various compositions of the bimetallic phase can be achieved by changing the initial Sn/Ni ratio. The obtained catalysts were very active and selective in the hydrogenation of ethyl acetate to ethanol. Characterization of the bimetallic phase has shown that the particles are bimetallic (STEM). As a result of chemisorption IR, and magnetic measurements, it appears that the presence of tin has four effects: (i) it decreases significantly the amount of CO and Hâ adsorbed; (ii) it isolates nickel atoms from their neighbors; (iii) it increases electron density on nickel; and (IV) it suppresses the magnetic properties of nickel. Redox behavior of Ni-Sn/SiOâ toward surface OH indicates that surface hydroxyls can oxidize Sn{sup (0)}, probably to Sn{sup (II)} with evolution of Hâ, the process being reversible with Hâ. It is suggested that during this oxidation process, tin migrates to the periphery of the bimetallic particle with formation of (chemical bond Si-O)âSn{sup (II)} surface species.
A tetraoctylammonium-stabilized PtSn colloid with nominal composition Pt3Sn has been prepared by coreduction of the metal salts. This colloid, which is used in the manufacture of anode catalysts for low-temperature polymer membrane fuel cells, was supported on silica and the structure studied by in situ X-ray diffraction and Debye function analysis. Results indicate that the supported colloid is best described as a highly disordered bimetallic fcc cluster with a particle size of 1.3 nm. From TEM images a slightly larger size of 1.5 ± 4 nm is derived. Point-resolved EDX (energy-dispersive X-ray spectroscopy) confirms the elemental ratio of 3:1 in the PtSn particles. However, the XRD simulation indicates some deviations from uniform alloy formation. On removal of the stabilizing tetraoctylammonium ligands by heating the samples in He to 200 °C, coalescence of neighboring particles occurs. The new alloy formed consists of a majority of cubic Pt0.81Sn0.19 particles, with average size 3.1 nm, and a minority phase of stoichiometric hexagonal PtSn, 4.4 nm in size. The former phase can be considered as a metastable supersaturated solid solution of Sn in Pt. The two phases are stable even on heating to 375 °C for 1.5 h in He.
A series of bimetallic catalysts has been prepared by impregnating a commercial Pt/alumina catalyst (series B catalyst in Part I, R. Burch, J. Catal. 71, 348 (1981)) with solutions containing compounds of tin or lead in aqueous or nonaqueous solvents. The activity and selectivity of the catalysts have been determined under flow conditions at 750 K and 1 bar for the conversion of n-hexane, methylcyclopentane, cyclohexane, and at 373 K for the hydrogenation of hex-1-ene. Under these experimental conditions it has been established that the conversion of n-hexane into branched aliphatic isomers is catalysed by a bifunctional mechanism, but excess acidity gives lower selectivity due to enhanced cracking. Benzene and methylcyclopentane are formed directly from n-hexane on metal sites and do not require acidic sites. The conversion of methylcyclopentane into benzene is bifunctional. The results show that when tin is present the catalysts are much more stable, and have much higher selectivities for isomerisation and aromatisation reactions. The Pt-Sn catalysts produce more benzene from methylcyclopentane, they dehydrogenate faster, and they hydrogenate more slowly. At a given conversion the bimetallic catalysts produce much lower concentrations of cracked products. It is concluded that tin modifies the acidity of the support, resulting in higher selectivity for isomerisation and lower selectivity for cracking, and also modifies the properties of the Pt, resulting in less self-poisoning. Taking account of the fact that the tin is mainly present in these catalysts as Sn(II) (see Part I), stabilised by the alumina, it is concluded that the catalytic results cannot be rationalised on the basis of current geometrical models for bimetallic catalysts. It is proposed that the role of the tin is to modify the electronic properties of the small Pt particles. It is tentatively suggested that similar electronic effects may be important in other bimetallic catalysts.
The electronic structures of Pt3Sn(111),Pt3Sn/Pt(111) and Pt2Sn/Pt(111) surfaces are studied using the linear-muffin-tin-orbital tight-binding method in the atomic-sphere approximation. Both ideal and rumpled surface terminations are considered. The hybridization between Pt d- and Sn p-electrons, respectively, leads to a lowering of the local density of electronic states at the Fermi level and to a downward shift of the Pt local d-band, which accounts for the lower reactivity of the Pt–Sn surfaces. The effect is more pronounced for rumpled surfaces. Generally, the situation is similar to that of bimetallic transition-metal surfaces. The initial-state approximation is used to predict Pt(4f) core-level shifts. We find moderate shifts of either sign, but the d-band centre of gravity moves to higher binding energies, as compared to Pt(111), in most cases. The correlation between the surface reactivity and core-level shifts, respectively, seems to be less favourable than at bimetallic transition-metal surfaces
A complete kinetic model of propane dehydrogenation to produce propene over a Pt–Sn–K/Al2O3 catalyst was obtained. This has been investigated over the temperature range of 460–540°C at atmospheric pressure. A Langmuir–Hinshelwood mechanism provides the best fit for propane dehydrogenation, while a monolayer–multilayer mechanism is proposed for modelling the coke formation. In addition, the reaction rate of coke formation and its influence on catalyst deactivation and subsequent regeneration have been studied. Finally, a suitable mathematical model is developed for simulating the process behaviour in a two-zone fluidized bed reactor (TZFBR).
Bimetallic Pt/Rh heterogeneous catalysts based on nanoscaled colloids supported on activated charcoal have been shown to exhibit maximum activity in hydrogenation of butyronitrile at a composition of 10 atom% Pt and 90 atom% Rh in the metallic core. This synergetic effect has to be traced back to the special structure of the bimetallic nanoparticles as elucidated by X-ray spectroscopy and CO chemisorption. The surface of Pt/Rh particles was investigated by CO chemisorption combined with IR spectroscopy. It has been shown that a Pt10Rh90 colloid exhibits the maximum ability to chemisorb CO, compared with other compositions. Based on the detection of the different types of bonding of CO molecules to the metal surface and the EXAFS results, our measurements reveal surface enrichment of the Rh component. The surface structure of the particles, which varies with the elemental composition, and the presence of a Pt-dominated core influence the CO chemisorption ability as well as the hydrogenation efficiency of the Pt/Rh colloid catalysts. Copyright © 2000 John Wiley & Sons, Ltd.
Studies of the adsorption of carbon monoxide, ethene, and deuterium on platinum, Pt3Sn, PtSn, and PtSn2 have identified significant differences between the surface and the bulk composition of the alloys, in agreement with current theories. After annealing in vacuo, the surfaces are considerably enriched in tin, this enrichment occurring mainly at the expense of the subsurface zone, which thus becomes correspondingly enriched in platinum. Carbon monoxide adsorption induces a reversible surface enrichment in platinum. Temperature-programmed desorption experiments reveal that as the proportion of tin in the alloys increases, desorption of the above-mentioned gases becomes progressively easier. This suggests a loosening of the chemisorptive bond caused by the adsorbing platinum atoms becoming surrounded by tin atoms (the “ligand effect”). Adsorption of ethene on platinum leads to extensive auto-hydrogenation of the ethene and to carbon formation; Pt3Sn and PtSn, however, adsorb ethene reversibly.
Selective hydrogenolysis of Sn(n-C4H9)4 on a Pt/SiO2 catalyst has been carried out at various temperatures and coverages of the metallic surface to prepare via surface organometallic chemistry a well-defined class of bimetallic catalysts. The stoichiometry and kinetics of the reaction was followed by the careful analysis of reagents and products, including extraction of unreacted reagents, and elemental analysis of the samples. The various surface species formed were characterized by electron microscopy (CTEM and TEM EDAX) and EXAFS analysis. Possible structures of the surface organometallic fragments were considered using molecular modeling. At 50 °C, the hydrogenolysis reaction occurs selectively on the platinum surface with exclusive evolution of n-butane. There is first formation of a Sn(n-C4H9)3 fragment grafted on the platinum particle which undergoes a stepwise cleavage of two tin−carbon σ-bonds to form a stable Pt−Sn(n-C4H9) fragment. Regardless of the reaction time, surface coverage, or loading, the number of grafted butyl fragments per platinum is never greater than unity, that is to say that when Sn(n-C4H9)3 is formed the platinum coverage by tin is 0.3 whereas when Sn(n-C4H9) is formed the platinum coverage is closer to 1. It is therefore suggested that the surface composition is governed by the bulkiness of the alkyl chains which are “close packed” on the surface. At 100 °C, the reaction takes place both on the platinum and the silica surface. On the platinum surface, the same fragments (namely Sn(n-C4H9)3, Sn(n-C4H9)2, and Sn(n-C4H9)) were identified, but simultaneously on the silica surface, the well-described SiOSn(n-C4H9)3 species was also formed. Thermal treatment under hydrogen of Pts−Sn(n-C4H9) lead to alkyl-free tin atoms which are located at the periphery of the particle as evidenced by Sn K edge EXAFS (Pt−Sn distance of 2.75 Å with a coordination number of ca. 4). Even if the organotin fragments are grafted with a coverage of unity, after their complete hydrogenolysis at 300 °C, about 40% of the platinum is still accessible to H2 chemisorption. This could be explained by the increase of the particle diameter (+0.5 Å) which prevents a close packing of the tin atoms around the particle and leaves some platinum atoms still accessible to the hydrogen. After treatment of the catalyst at higher temperatures, typically 500 °C, the structure of the catalyst is slightly changed since the tin atoms migrate into the first monolayer of the particle, as evidenced by a significant increase of the tin coordination number (ca. 4.4−5.6) as determined by EXAFS. Hypothetical surface structures have been proposed on the basis of molecular modeling of platinum particles covered by various surface organotin fragments.
Selective reaction of (Ad)GeH3 (Ad = adamantyl) with a Rh/SiO2 surface has been carried out at 50 °C. The surface reaction and the characterization of the surface organogermanium complex have been followed by infrared spectroscopy, surface microanalysis, analysis of the gases evolved during the surface reaction, thermal decomposition of the surface organometallic complex, and use of CO as a molecular probe. In the absence of metallic rhodium, (Ad)GeH3 does not react significantly with the silica surface at room temperature. It is only reversibly adsorbed, and the molecular interaction responsible for this adsorption process is a hydrogen-bonding interaction between either the C−H or Ge−H atoms and the surface silanols. In the presence of metallic Rh, a chemical reaction occurs exclusively on the metallic particles. (Ad)GeH3 initially physisorbed onto the support migrates to the Rh surface between 25 and 50 °C, where it quickly loses one molecule of hydrogen. The grafted species still contains one hydride ligand since the reaction between (Ad)GeD3 and the rhodium catalyst gives rise to a ν(Ge−D) vibration at the expected frequency. Formulation of the grafted entity as Ge(Ad)(H) (major species) is deduced from surface microanalysis and from its thermal decomposition, which produces adamantane by a reductive elimination process (concomitant disappearance of the ν(C−H) and ν(Ge−H) vibration bands). The organogermane complex is very likely grafted onto rhodium for the following reasons: (i) (Ad)GeH3 does not react with the silanols of the support. (ii) The amount of grafted germanium is close to (but lower than) the number of surface rhodium atoms. (iii) The amount of rhodium accessible to carbon monoxide drops by a factor of 80% after the grafting reaction takes place, indicating that the metallic surface has been covered by the Ge(Ad)(H) fragment, results that are in agreement with the elemental analysis (Ge/Rhs = 0.8). (iv) The infrared results indicate a strong electronic interaction between carbon monoxide adsorbed on the remaining rhodium sites and the Ge−H bond. The major species which is present on the surface after grafting is supposed to be a kind of germylene(II) surface species stabilized by coordination to a surface rhodium atom.
The Pt-Sn-based catalyst was intensified using SAPO-34 as support for direct propane clehydrogenation to propylene. The catalyst was prepared by sequential impregnation method and characterized by XRF, BET, XRD, NH(3)-IR, NH(3)-TPD, H(2)-TPR, HR-TEM and O(2)-pulse coke analysis. NH(3)-TPD, IR spectra and XRD results suggested that the doping of metals on SAPO-34 did not affect its acidic strength and structural topology of support, respectively. Propylene selectivity of 94% and total olefins selectivity greater than 97% was achieved using Pt-Sn/SAPO-34. The results were compared with Pt-Sn/ZSM-5 under identical conditions. The possible reasons for improvement were the larger surface area, shape selectivity and particular by suitable acidity of SAPO-34. (C) 2009 Elsevier B.V. All rights reserved.
We have carried out a series of 195Pt and 13C NMR spectroscopic and electrochemical experiments on commercial Pt-Ru alloy nanoparticles and compared the results with those on Pt-black samples having similar particle sizes. The Pt NMR spectrum of the alloy nanoparticles consists of a single Gaussian peak, completely different from the broad "multi-Gaussian" NMR spectra, which are generally observed for carbon-supported Pt catalysts. Spin-echo decay measurements show that the intrinsic spin-spin relaxation time (T2) is much larger in the alloy compared to Pt-black. A "slow-beat" is observed in the spin -echo decay curve of the alloy, implying that the NMR frequencies of spin-spin coupled Pt nuclei in the alloy nanoparticles are quite similar, unlike the situation found with Pt-black. These 195Pt NMR results strongly suggest that there is a surface enrichment of Pt atoms in the Pt-Ru alloy nanoparticles. The CO-stripping cyclic voltammogram (CV) of the Pt-Ru alloy nanoparticles is broader than that observed with platinum black and is shifted toward lower potential. The two-peak structure observed previously for the CO-stripping CV behavior of Pt-black containing spontaneously deposited Ru (Tong et al. J. Am. Chem. Soc.2002, 124, 468-473) is absent in the alloy sample. The 13C NMR spectrum of CO adsorbed on the Pt-Ru alloy consists of a single peak, exhibiting only a small Knight shift. An analysis of the 13C spin-lattice relaxation results indicates that Ru addition causes a reduction in the Fermi level local density of states of the clean metal surface atoms and the 2 ﷿* orbital of adsorbed CO. These NMR results suggest that alloying with Ru reduces the total density of states (DOS) at the Pt sites, in accord with conclusions drawn previously from synchrotron X-ray absorption studies of Pt-Ru electrocatalysts. This electronic alteration could be the basis for the ligand field contribution to the "Ru enhancement".
composite nanoparticles with controlled composition (nominal weight ratio of
;
in average diam) were fabricated onto multiwalled carbon nanotubes (MWCNTs) by first assembling
nanoparticles on MWCNTs with a simple method of hydrolysis-oxidation of
in water at room temperature, then loading Pt onto the obtained
/MWCNTs with a colloidal method, and finally reducing in a hydrogen atmosphere at
. The composition of the composite nanoparticles was characterized by X-ray photoelectron spectroscopy and energy dispersive
X-ray spectroscopy.The microstructure was characterized by X-ray diffractometry and transmission electron microscopy. By cyclic
voltammetry, we found that, compared with a reference sample of Pt/MWCNTs (nominal weight ratio of
;
in average diam), the
/MWCNTs nanocomposites showed approximately the same specific active surface area of Pt but twice the activity for electro-oxidation
of methanol (EOM) at room temperature. Based on the finding that integrated charge quantity corresponding to
adsorption on the
/MWCNTs is approximately twice that on the Pt/MWCNTs within the potential range from 0.6 to
(vs
), we propose that tin oxide promotes the EOM on Pt through providing extra adsorption sites for
or enhancing the affinity of Pt to
.
CO oxidation is used as a probe reaction to evaluate the redox properties of catalysts for the low temperature oxidative dehydrogenation (ODH) of ethane. Three series of Nb1−x-NixO nanocomposites with various Nb contents (x = 1, 0.95, 0.90, 0.85, 0.80) are prepared by a sol–gel route based on citrate and tested in a 16 parallel fixed-bed unit. Reductive and oxidative pre-treatments are shown to influence both the activity and the stability of the catalysts in CO oxidation. The extent of this effect depends on the Nb content of the composites, the Nb-rich samples being generally the most affected. However, the order of reactivity in CO oxidation is the same for the three series and is maintained whatever the conditions of the pre-treatment. It is the same as that observed in Nb1−x-NixO-catalyzed ethane ODH.
This Critical Review provides an overview of the recent developments in the synthesis and characterization of bimetallic nanoparticles. Initially the review follows a materials science perspective on preparing bimetallic nanoparticles with designer morphologies, after which the emphasis shifts towards recent developments in using these bimetallic particles for catalysing either oxidation or reduction. In the final part of this review we present an overview of the utilization of bimetallic catalyst systems for the transformation of bio-renewable substrates and reactions related to the realization of a bio-refinery. Because of the sheer number of examples of transformations in this area, a few key examples, namely selective oxidation, hydrogenation/hydrogenolysis and reforming of biomass derived molecules, have been chosen for this review. Reports of bimetallic catalysts being used for the aforementioned transformations are critically analysed and the potential for exploiting such bimetallic catalysts have also been highlighted. A specific objective of this review article is to motivate researchers to synthesize some of the "designer" bimetallic catalysts with specific nanostructures, inspired from recent advances in the area of materials chemistry, and to utilize them for the transformation of biomass derived materials that are very complex and pose different challenges compared to those of simple organic molecules. We consider that supported bimetallic nanoparticles have an important role to play as catalysts in our quest for a more green and sustainable society.
Density functional theory calculations were performed to investigate the adsorption of propane, propene, and C and H atoms on Pt and PtSn surfaces employing the revised Perdew–Burke–Ernzerhof (RPBE) and vdW-DF functionals. Propane adsorption was found to be mediated by van der Waals interactions without significant site preference on any of the studied surfaces. The adsorption characteristics of propene are different: On the Pt(111) and Pt3Sn(111) surfaces, propene adsorption is covalent, and the molecule prefers a di-σ site to a π site. Alloying Pt(111) with Sn leads to weaker adsorption owing to geometric and relaxation effects, whereas electronic effects are found to be small. On the PtSn2(111) surface, propene adsorption is weak and dominated by van der Waals interactions. Our calculations show that addition of Sn leads to unfavorable geometric and electronic effects on the adsorption of carbon and hydrogen atoms. The impact of alloying with Sn on the selective propane dehydrogenation to propene is discussed.
The reactivity of the well-defined surface organometallic fragment drop Si-O-Sn(n-C4H9)(3) 1 grafted on silica(200) and on silica(500) has been studied by thermal treatment of 1 at increasing temperatures in vacuo. The surface reactions have been followed by quantitative measurements of the evolved gases, infrared and Mossbauer spectroscopies, C-13 CP-MAS and Sn-119 NMR spectroscopy, XPS measurements, and electron microscopy (CTEM and STEM EDAX). On both types of silicas, the surface reactions are similar in nature, although differences are noticeable. First, there is formation of (drop Si-O)(2)(Sn(n-C4H9)(2)) 2, which undergoes a second solvolysis process by silanols leading to (drop Si-O)(3)Sn(n-C4H9) 3 and finally surface Sn(II) and Sn(IV) atoms (as determined by XPS and Mossbauer experiments). Although the well-defined surface organometallic compound (drop Si-O)(2)Sn(n-C4H9)(2) can be prepared on silica by another route, no unique surface compound can be obtained during the thermal decomposition which transforms progressively 1 into 2 and 3. A mechanism of decomposition of the various surface organometallic complexes has been deduced from a comparison of the results obtained on both solids. The alkyl groups seem to follow a beta-H elimination mechanism leading to tin hydrides and 1-butene rather than a disproportionation mechanism leading to equimolar amounts of 1-butene and butane.