Application of EXAFS in Catalysis. Structure of Bimetallic Cluster Catalysts

In recent years the utility of extended x-ray absorption fine structure (EXAFS) as a probe for the study of catalysts has been clearly demonstrated [1–13]. Measurements of EXAFS are particularly valuable for very highly dispersed catalysts. Supported metal systems, in which small metal clusters or crystallites are commonly dispersed on a refractory oxide such as alumina or silica, are good examples of such catalysts. The rätio of surface atoms to total atoms in the metal clusters is generally high and may even approach unity in some cases. With such catalysts it is difficult or impossible to obtain structural information by conventional x-ray diffraction methods [14].     

Importance of palladium dispersion in Pd/Al2O3 catalysts for complete oxidation of humid low-methane–air mixtures

The catalytic oxidation is considered as an environmental benign method for utilization of various methane-poor gas mixtures, including humid post-ventilation air of coal mines. The small crystallites of palladium phase in the Pd/Al₂O₃ catalyst decrease temperatures necessary to ignite the methane oxidation reaction and to achieve complete conversion of methane. The isotopic exchange of oxygen between the catalyst and the gas phase, the temperature-programmed reduction (TPR) with methane and the X-ray photoelectron spectroscopy studies suggest that it can result from a higher number of the Pd–PdO sites present on the catalysts with small palladium crystallites. The inhibiting effect of water vapour present in the reaction mixture increases with lower dispersion of palladium phase as well as with the water concentration in the feed. The larger palladium crystallites are more significantly affected by the presence of water. It is suggested that water vapour blocks the Pd–PdO active sites. The catalysts with small crystallites (<6.6 nm) of palladium can be successfully used for mitigation of the emission of methane from coal mine post-ventilation air and, after increasing of the methane concentration to 1–2 vol.%, for its utilization for the energy production. In the case of such catalysts even a high concentration of water vapour has the least negative influence on the catalyst activity and it will not interfere with obtaining of the 100% conversion of methane below 650 °C.     

Design and Preparation of Impregnated Catalysts

Solid catalysts in common use are typically in the form of small metal crystallites dispersed on the internal surface of the porous support. Advantages of these supported-metal catalysts lie in their high surface-to-volume ratio and the high thermal stability endowed by the dispersion. Such catalysts are commonly produced by liquid-phase impregnation in which a dry or wet pellet of the porous support is impregnated with a solution of a compound of the desired catalytic constituent. During impregnation and subsequent drying, small crystallites of the catalyst precursors are deposited on the internal surface of the support material. The impregnation and drying steps involve mass and /or heat transfer processes which often do not reach equilibrium, resulting in nonuniform concentration profiles of impregnant, or “impregnation profiles” along the radius of the support pellet.     

Design and Preparation of Impregnated Catalysts

Solid catalysts in common use are typically in the form of small metal crystallites dispersed on the internal surface of the porous support. Advantages of these supported-metal catalysts lie in their high surface-to-volume ratio and the high thermal stability endowed by the dispersion. Such catalysts are commonly produced by liquid-phase impregnation in which a dry or wet pellet of the porous support is impregnated with a solution of a compound of the desired catalytic constituent. During impregnation and subsequent drying, small crystallites of the catalyst precursors are deposited on the internal surface of the support material. The impregnation and drying steps involve mass and /or heat transfer processes which often do not reach equilibrium, resulting in nonuniform concentration profiles of impregnant, or “impregnation profiles” along the radius of the support pellet.     

Heterogeneous catalysts for hydrogenation of CO2 and bicarbonates to formic acid and formates

Formic acid and formates are often produced by hydrogenation of CO₂ with hydrogen over homogeneous catalysts. The present review reports recent achievements in utilization of heterogeneous catalysts. It shows that highly dispersed supported metal catalysts are able to carry out this reaction by providing activation of hydrogen on the metal sites and activation of CO₂ or bicarbonate on the support sites. Important advances have recently been achieved through utilization of catalysts using C_xN_y materials as supports. The high activity of these catalysts could be assigned to their ability to stabilize the active metal in a state of single-metal atoms or heterogenized metal complexes, which may demonstrate a higher activity than metal atoms on the surface of metal nanoparticles.     

Investigation on the textural characteristics of supported nickel hydrogenation catalysts

Nickel catalysts supported on a range of carriers such as kieselguhr, silica, γ-alumina, silica-alumina and mordenite were investigated in order to gain an understanding of their structural properties. The surface area and pore distribution characteristics of the carriers and unreduced catalysts obtained from nitrogen adsorption isotherms gave strong indications that the formation of the nickel salt submerged the carrier. Metallic nickel produced after reduction at high temperature was perhaps present in the macropores or on the outer surface of the carrier forming a microporous network of fine crystallites. The metal surface area of the catalyst in its reduced state, calculated from hydrogen chemisorption data, was not influenced by the specific surface area of the carrier. Among the different carriers, the nickel-carrier interaction was mainly found with kieselguhr and silica. Activity of the different catalysts in the hydrogenation of unsaturated fatty acid was found to be directly related to the metal area.     

Comparative study of aromatization selectivity during N‐heptane reforming on sintered Pt/Al2O3 and Pt‐Re/Al2O3 catalysts

BACKGROUND: The metal dispersed over a support can be present as small crystallites with sizes less than 5 nm. The smaller crystallites favour aromatization while larger crystallites favour cracking/hydrogenolysis. Sintering results in the agglomerization of smaller metal crystallites. Correlation of size with aromatization selectivity was investigated. RESULTS: The primary products of n‐heptane reforming on fresh Pt were methane, toluene, and benzene, while on fresh Pt‐Re, the only product was methane. Both catalysts exhibited enhanced aromatization selectivity at different oxygen sintering temperatures. The reaction products ranged from only toluene at 500 °C sintering temperature to methane at a sintering temperature of 650 °C with no reaction at 800 °C for the Pt/Al₂O₃ catalyst. On Pt‐Re/Al₂O₃ catalyst, methane was the sole product at a sintering temperature of 500 °C while only toluene was produced at a sintering temperature of 800 °C. CONCLUSION: This is the first time that sintering has been used to facilitate aromatization of supported Pt and Pt‐Re catalysts. A superior selectivity behaviour associated with bi‐metallic Pt catalysts is established. It was found that no reaction occurred on Pt catalyst after sintering at 800 °C whereas sintering Pt‐Re at 800 °C promoted aromatization solely to toluene. Copyright © 2008 Society of Chemical Industry     

A Fundamental Study of High Activity Catalyst for Olefin Polymerization

Soon after the commercialization of the Ziegler-Natta catalyst for polyolefin production, efforts were begun to improve the productivity of the catalyst. It was realized that the α- and σ-forms of TiC1₃ have layered crystal structures having only chlorine atoms in the basal planes, Exposed and coordinatively unsaturated Ti sites are only found along c-axis edges of the crystallites. Therefore, if a suitable support is found and Ti atoms can be anchored to its surface with stereoelectronic characteristics resembling those in the AA-TiCl₃, then a high activity catalyst would result. This search progressed from metal oxides to metal hydroxides, to metal hydroxychlorides, and finally to metal chlorides, in particular magnesium chloride. The historical developments have been discussed in detail by Karol and Hsieh at this conference.     

Kinetics and Mechanism in the Liquid-Phase Hydrogenation of Maleic Anhydride and Intermediates

The influence of zinc oxide on the kinetics and mechanism of the liquid-phase hydrogenation of maleic anhydride (MA) and intermediates was investigated on copper-based catalysts. No influence of zinc oxide on the hydrogenation of maleic anhydride was observed in previous experiments. The discontinuous hydrogenation of succinic anhydride (SA) resulted in the formation of γ-butyrolactone (γ-BL) and 1,4-butanediol (1,4-BD) on a copper/zinc catalyst. On a zinc-free copper catalyst only γ-butyrolactone was formed while the hydrogenation of γ-butyrolactone to 1,4-butanediol was inhibited. It was observed that succinic anhydride which is adsorbed on the copper surface of the catalyst prevents the adsorption of γ-butyrolactone. On copper/zinc catalysts the reversible adsorption of succinic anhydride on the inactive zinc oxide crystallites, which led to a reversible decrease of the carbon balance, is responsible for a decrease of the succinic anhydride coverage of the copper sites. It appears that the decrease of the succinic anhydride coverage of the copper surface is proceeding by surface diffusion of succinic anhydride to the adjacent zinc oxide crystallites. On this basis two different reaction pathways via succinic anhydride adsorbed on the copper surface and via succinic anhydride adsorbed on the zinc oxide crystallites were proposed for the hydrogenation of maleic anhydride and intermediates. Kinetic modeling of the reaction pathway taking into account both reaction pathways led to good agreement of calculated and experimental results.     

Platinum crystallite size effects on oxide formation and reduction parameters—II

A detailed examination of the electrochemical formation and reduction of surface oxides on platinum crystallites shows a sensitivity for the oxidation rate at constant potential and the mechanism of the oxide reduction on the platinum crystallite size. In the crystallite regions under consideration, the percentage of Pt atoms located in the surfaces of the crystallites changes from 45% at 92 m²/g (30 Å dia.) to 4% at 10 m²/g (280 Å dia.) so that very small crystallites exhibit surface physical properties different from bulk metal surfaces. This is reflected in the changes in electrocatalytic behavior shown here. The oxide reduction mechanisms show two separate “Tafel” slopes of 30 mV and 55 mV, dependent on the oxide coverage. Transitions between the two mechanisms as a function of oxide coverage are linearly dependent upon the percentage of platinum atoms located in the surfaces of the crystallites. Electrocatalytic reaction parameters obtained at bulk platinum metal electrodes may not be applied to high surface area platinum electrocatalysts.     

沸腾床渣油加氢处理催化剂失活研究

沸腾床催化剂失活主要是由于金属和焦炭沉积导致的,同时在沸腾状态下催化剂的物理和机械性质也发生了改变。使用后的催化剂向小的粒度分布方向偏移;催化剂沉积了大量的金属和焦炭,使催化剂的堆积密度增加,同时导致催化剂的孔结构、酸性质发生了变化。失活催化剂沉积的金属和焦炭在颗粒内外分布均匀,表明催化剂利用率较高。     

Effect of the amount of reducing agent on surface structures, electrochemical activity and stability of PtRu catalysts

The effect of the amount of reducing agent used in the synthesis of PtRu alloy catalysts on their surface structure was investigated, and the prepared catalysts were characterized using transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, cyclic voltammetry, and potential cycling. The alloying degree of the catalysts was essentially the same for all of the catalysts studied, despite the use of different amounts of reducing agent. Varying the amount of reducing agent resulted in changes in the surface composition of the catalysts, wherein the surfaces were found to be composed of several PtRu domains that differed in local inhomogeneity and hence showed differences in activity for CO_ad oxidation. The highest activity for methanol oxidation was obtained when there was moderate Ru enrichment of the catalyst. The electrochemical stability of the catalysts was also investigated via potential cycling in a methanol-containing electrolyte solution. The electrochemical stability under methanol oxidation was enhanced by Ru-enrichment at the catalyst surface, because the Ru-rich surface had sufficient Ru atoms near the Pt atoms to act as a bifunctional catalyst, even though the Ru atoms were leached out by potential cycling. The most Ru-rich catalyst exhibited an increase in methanol oxidation current in the middle of potential cycling whereas the other catalysts showed a monotonic decrease.     

A microscopic model for catalyst surfaces—II: Supported catalysts

A new “pebbly surface” model of supported metal catalysts is presented which can explain observed multiplicity and oscillatory behaviour of catalyst particles. This model combines the dynamic behavior of the individual metal crystallites on the surface of the support with diffusion and heat transfer processes in the porous particle. The bifurcation of oscillations is analyzed in terms of relaxation oscillations as well as by Hopf's theorem and it is shown how the bifurcation phenomena depend on the system parameters. The parameter study may be used to suggest operating conditions and catalyst design parameters such that sintering effects may be reduced. Qualitative comparisons with data for carbon monoxide oxidation at various temperatures show very good agreement; however, this present simple form of the pebbly surface model seems unable to predict the long period oscillations observed in hydrogen oxidation.     

Models of metal catalysts: beyond single crystals

Because solid catalysts are typically complex in structure and composition, researchers have often used structurally simple models in attempts to identify catalytic sites and understand reaction mechanisms. Among models of metal catalysts, single crystals are the prototypes, but, because they cannot represent support effects or the smallest metal clusters and cannot easily be used in long-term catalyst testing, they are complemented by other types of model catalysts, including metal particles on well-defined planar supports and nearly molecular clusters of a few metal atoms each on metal oxide and zeolite supports. This account is a comparison of the various types of model metal catalysts and a summary of the advantages and limitations of each. Examples of rhodium and of platinum catalysts are given to illustrate the catalyst types and connections between them. More data are needed as a basis for comparisons of the performance of the various model catalysts.     

Fischer–Tropsch Synthesis: Oxidation of a Fraction of Cobalt Crystallites in Research Catalysts at the Onset of FT at Partial Pressures Mimicking 50 % CO Conversion

Freshly H₂-reduced catalyst samples and FTS catalyst samples (i.e., freshly reduced and immediately exposed to the onset of FTS conditions corresponding to 50 % CO conversion) were prepared. Each sample was coated in situ using molten polywax and solidified so that an air-protected sample was obtained, which was stored in inert gas. XAS was utilized to investigate the oxidation state of cobalt. A fraction of cobalt crystallites in the freshly reduced research catalysts having lower-than-commercial loading and smaller crystallites undergoes a degree of oxidation to CoO at the onset of FTS conditions simulating 50 % CO conversion (i.e., the H₂O partial pressure is high enough to induce some oxidation). Therefore, by decreasing Co content with the aim of improving the dispersion of cobalt and Co efficiency, very small Co crystallites are obtained. Their reoxidation at the onset of FTS is an unintended consequence. Thus, catalysts should be designed to have an optimum narrow cluster size range—small enough to increase Co surface site densities, but large enough to avoid reoxidation, and the stability problems that arise from having unreduced Co in the working catalyst (e.g., a complex coalescence and reduction mechanism).     

Performance improvement of electrode for polymer electrolyte membrane fuel cell

The very high power density output available from polymer electrolyte membrane fuel cells combined with low cost has high potential for commercialization. Such high power densities are attained via better utilization of Pt crystallites in the reaction layer. This enhanced performance can be achieved by making a thin catalyst layer on the membrane surface. The robustness in the front surface catalysts is essential to minimize the coagulation of Pt particles when the fuel cells are subjected to long-term operation. This robustness of the catalyst structure depends on the manufacturing processes and also the organic solvents used to make the slurry. In this work, five different electrodes were fabricated by using different fabrication procedures, and the poison effect of CO was investigated at the anode interface.     

负载型Co基F-T合成催化剂中贵金属助剂促进作用的研究进展

在负载型Co基催化剂中添加少量贵金属助剂能显著影响催化剂活性、选择性与稳定性。贵金属助剂的添加提高了Co活性组分的还原度与分散度,增加了催化剂表面的活性位数目,改变了催化剂的几何结构和电子结构,影响CO、H₂或中间产物的吸附活化行为,抑制催化剂积炭和金属Co小粒子的再氧化。综述贵金属助剂对催化剂活性中心性质、反应物的吸附活化行为和催化剂反应稳定性的影响。     

Oscillatory CO Oxidation Over Pt/Al2O3 Catalysts Studied by In situ XAS and DRIFTS

Fresh and mildly aged Pt/Al₂O₃ model diesel oxidation catalysts with small and large noble metal particle size have been studied during CO oxidation under lean burn reaction conditions to gain more insight into the structure and oscillatory reaction behaviour. The catalytic performance, CO adsorption characteristics using in situ DRIFTS and oxidation state using in situ XAS were correlated. Stable and pronounced oscillations only occurred over the catalyst with smaller particle sizes. Characteristic for this catalyst are low-coordinated surface Pt sites (more corner and edge atoms) which seem to become oxidized at elevated temperature as evidenced by in situ DRIFTS and in situ XAS. In situ XAS further uncovered that the oxidation of the Pt surface starts from the end of the catalyst bed and the oxidation state oscillates like the catalytic activity.     

Scanning tunneling microscopy and spectroscopy of silver and platinum catalysts

Scanning tunneling microscopic studies of silver catalysts dispersed on highly oriented pyrolitic graphite (HOPG) with 2, 5 and 10 wt% metal loading prepared by wet impregnation and hydrogen reduction show spherical crystallites, with the 2 wt% catalysts having an average crystallite size of 2 nm and the 5 and 10 wt% catalysts with size distributions of 2–5 and 4–12 nm respectively. The Ag catalysts prepared by NaBH₄ reduction show a narrower size distribution. Pt/HOPG catalysts with 2 and 5 wt% metal loading prepared by wet-impregnation and hydrogen reduction show large (2–11 nm) raft-like crystallites; small crystallites ( 1 nm) could be obtained by NaBH₄ reduction. Tunneling spectroscopic measurements reveal the nonmetallic nature of crystallites on the surface of Ag(2 wt%)/HOPG as well as Pt/HOPG prepared by NaBH₄ reduction.     

Catalyst preparation and support effects for triglyceride hydrogenation over supported nickel

Nickel catalysts have been prepared by impregnation and precipitation methods on silica and charcoal. Metal loadings were in the range 1–40% w/w Ni on the support. The catalysts were investigated by temperature programmed reduction (TPR) and subsequently reduced in the temperature range 200–500°C. Metal surface areas were measured via H₂ and CO chemisorption and limited studies of the catalysts were carried out using X-ray photon spectroscopy (XPS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) to determine the chemical and physical state of the nickel after reduction. Reduced catalysts were employed for triglyceride (soya bean oil) hydrogenation (1 atmos H₂, 160°C) in a stirred reactor (1000 rpm) and reaction rates (catalyst activity) and product distributions were determined. Charcoal supported catalysts were active and zero valent nickel was detected after reduction. However, surface areas were variable, the performance of such catalysts being affected by pore effects, agglomeration and possibly nickel–charcoal interaction. Silica supported precipitated catalysts were of relatively high activity and metal surface if methanol washed before activation and inactive (irreducible) if only water washed. Although silica supported precipitated catalysts were more active per unit weight of metal than charcoal supported counterparts, they were less active per unit metal area and gave rise to greater trans-acid production. This may be due to triglyceride molecules excluding hydrogen from small nickel crystallites.