Development of Time-Resolved XAFS Instrumentation for Quick EXAFS and Energy-Dispersive EXAFS Measurements on Catalyst Systems

From its very earliest days, at which time measuring a spectrum took at least 1 h, XAFS has been used for catalysis studies. In more recent times technological advances have improved the time resolution to minutes and seconds with quick EXAFS, and subsecond to millisecond with energy-dispersive EXAFS. This paper describes the instrumentation necessary and elucidates with specific examples where this has been used. In particular the Daresbury QEXAFS monochromator will be shown and several examples of combined XAFS and diffraction, a powerful technique for in situ studies. The development of energy dispersive EXAFS at Daresbury will be described. Recent results on in situ Rh catalysis combining EDE and mass spec measurements at the ESRF are highlighted, and finally the details of the latest microstrip detector that will enable continuous measurements at 10 s time intervals is presented.     

Combined EXAFS and XRD for the in situ structural elucidation of solid catalysts under operating conditions

Two distinct kinds of experimental arrangement permit X-ray absorption and X-ray diffraction data to be recorded in a combined manner under in situ conditions. Results are shown for the conversion of a copper-zinc hydroxycarbonate precursor to a live water-gas shift catalyst and for a zinc oxide and a zeolite model catalyst heated to ca. 1000 ° C. The short-range and long-range structural changes accompanying the conversion of the zeolite into cordierite are charted at high temperature. EXAFS and XANES data can be recordable both in fluorescence or transmission mode with one of the arrangements, which also permits successive measurements (around more than one absorption edge) within a short space of time.     

Synthesis, characterization and evaluation of a novel resid FCC catalyst based on in situ synthesis on kaolin microspheres

A new way was provided for in situ synthesized FCC catalyst. Characterization and evaluation results indicated the catalyst is more suitable for cracking resid feed because of its unique manufacturing process which makes this catalyst has appropriate pore structures in which large resid molecules are more accessible to the active sites. The high zeolite content (above 40%) keeps the catalyst having higher activity and selectivity. These advantages ensure the in situ catalyst developed more adaptable for cracking resid and resid-containing feedstocks, particularly the feed containing large amounts of contaminant metals.     

Influence of the Oxidation State of Rhodium in Three-Way Catalysts on Their Catalytic Performances: An in situ FTIR and Catalytic Study

Simultaneous IR spectroscopic and catalytic measurements have been performed in order to investigate the nature of adsorbed species involved in the formation of N₂O on Rh/Al₂O₃ in the course of the CO + NO reaction. Only nitrosyl species have been isolated that could be involved in the formation of N₂ and N₂O in accordance with previous kinetic investigations [Granger et al. J. Catal. 175(1998) 194]. Sequential and simultaneous NO and CO exposures lead to the observation of different nitrosyl species that could act as intermediates in the formation of N₂ and N₂O. Correlations between the appearance/disappearance of Rh(NO) ⁺ species and an extra formation of N₂O have been established.     

Investigation of Ammonia Oxidation over Copper with In Situ NEXAFS in the Soft X-Ray Range: Influence of Pressure on the Catalyst Performance

The oxidation of ammonia over polycrystalline copper was investigated by means of in situ NEXAFS (near-edge X-ray absorption fine structure) spectroscopy in the soft X-ray range. The reaction, carried out in a 1:12 excess of oxygen, was observed by mass spectrometry. The simultaneous detection of the surface electronic structure and its catalytic performance allows correlation of different reaction products to the current surface structure of the catalyst. It is shown that a change in total pressure from 0.4 to 1.2 mbar severely affects the reaction path. Copper(I) nitride was identified as poison and a copper oxide was found to be the active phase for the selective oxidation of ammonia to nitrogen.     

Voltammetric and Morphological Characterization of Heteropolyanions and Copper Co-Electrodeposition by In Situ AFM

The electrodeposition of three different heteropolyoxometalates:(HPOM)(NH₄)₃[RhMo₆O₁₈(OH)₆]· 7 H₂O, HPOM (1), Cu(II)(NH₄)[RhMo₆O₁₈(OH)₆]· 7 H₂O, HPOM (2) and Cu(II)NH₄[CoMo₆O₁₈(OH)₆]· 7 H₂O HPOM (3) was studied by in situ atomic force microscopy (AFM) and cyclic voltammetry. It was found that the voltammetric response of compounds 2 and 3 show the deposition of the copper countercation as well as the simultaneous reduction of the corresponding heteropolyanion (HPA). AFM was used to monitor the in situ film formation of the electroreduced species on the working electrode surface. The AFM images show important differences in the film texture when copper is present in the complex.     

Comparison of Oxidized Polycrystalline Copper Foil with Small Deposited Copper Clusters in Their Behavior in Ammonia Oxidation: An Investigation by Means of In Situ NEXAFS Spectroscopy in the Soft X-Ray Range

The oxidation of ammonia to nitrogen or nitric oxide was investigated using on the one hand a polycrystalline copper foil and on the other hand deposited copper clusters prepared with the inert gas aggregation technique. The behavior in the oxidation of ammonia of both model catalysts was studied using in situ near-edge X-ray absorption fine structure (NEXAFS) spectroscopy in the soft X-ray range and mass spectrometry. It is shown that the copper foil reacts in a similar way to the copper clusters. Differences appear only with respect to the reaction temperature required, which is lower for the cluster sample. It can be concluded that the results obtained in experiments with polycrystalline copper foil are exemplary for and can be transferred to a supported copper catalyst consisting of small copper particles.     

Molecular Engineering of Supported Vanadium Oxide Catalysts Through Support Modification

Highly dispersed, multilayered surface metal oxide catalysts (V₂O₅/MO _x /SiO₂, M = Ti(IV), Zr(IV) or Al(III)) were successfully synthesized by taking into account various factors that govern the maximum dispersion of metal oxide species on silica. The characterization results revealed that the molecular structures of the surface vanadium oxide species on the modified supports are a strong function of environmental conditions. The surface vanadium oxide species under dehydrated conditions are predominantly isolated VO₄ units, similar to the dehydrated V₂O₅/SiO₂ catalysts. Upon hydration, the surface vanadium oxide species on the modified supports consist of polymerized VO₅/VO₆ units and/or less polymerized (VO₃) _n species, which depend on the vanadia content and the specific second metal oxide loading. The surface V cations are found to preferentially interact with the surface metal (Ti, Zr or Al) oxide species on silica. The V(V) cations in the dehydrated state appear to possess both oxygenated ligands of Si(IV)–O^– and M–O^–. Consequently, the reducibility and catalytic properties of the surface vanadium oxide species are significantly altered. The turnover frequencies of the surface VO₄ species on these modified supports for methanol oxidation to redox products (predominantly formaldehyde) increase by more than an order of magnitude relative to the unmodified V₂O₅/SiO₂ catalysts. These reactivity enhancements are associated with the substitution of Si(IV)–O^– oxygenated ligands by less electronegative M–O^– ligands in the O=V(–O–support)₃ structure, which strongly suggests that the bridging V–O–support bonds play a key role in determining the reactivity of the surface vanadium oxide species on oxide supports.     

Catalytic Properties of Ag/Al2O3 Catalysts for Lean NOx Reduction Processes and Characterisation of Active Silver Species

A study of the lean NO _x reduction activity employing different reductants over Ag/Al₂O₃ samples prepared from reverse microemulsions or impregnation with EDTA-complexes is presented. A multitechnique approach is employed for characterisation of the samples and/or processes taking place in the course of the NO _x -SCR reaction with propene and propane. Results by in situ-DRIFTS reveal that, for the propene reductant, silver provides a new path for hydrocarbon activation involving generation of adsorbed acrylate species as a partially oxidised active intermediate, in line with previous proposals for other non-noble metal systems. It is shown, mainly on the basis of XAFS studies, that active silver species are related to well dispersed silver aluminate-like phases with tetrahedral local symmetry and a relatively high disorder in the oxygen first shell.     

Comparative Study of WS2 and Co(Ni)/WS2 HDS Catalysts Prepared by ex situ/in situ Activation of Ammonium Thiotungstate

A comparative study of the syntheses of unsupported WS₂ and M/WS₂ (M = Co, Ni) catalysts by ex situ/in situ decomposition of ammonium thiotungstate (ATT) is herein reported. Ex situ activation was performed under a H₂S (15% volume)/H₂ flow, whereas in situ activation consists in the direct decomposition of ATT or Co(Ni)/ATT precursors in the presence of a hydrocarbon solvent during the hydrodesulfurization (HDS) of dibenzothiophene (DBT). Precursors were characterized by thermogravimetric analysis and final catalysts by X-ray diffraction (XRD), scanning electron microscopy (SEM) and specific surface area (BET). Catalysts activated using the in situ mode of activation present higher specific surface areas with the noticeable exception of the Ni/ATT precursor. Activity measurements showed that the in situ activated WS₂ and Ni/WS₂ catalysts exhibit higher activity than the ex situ activated catalysts.     

Hydrodenitrogenation of Quinoline Over a Dispersed Molybdenium Catalyst Using in situ Hydrogen

Hydrodenitrogenation (HDN) of quinoline using a Mo-based dispersed catalyst was studied in a batch reactor using H₂ generated in situ via the water gas shift reaction. In situ H₂ was found to be more active for HDN than externally supplied H₂. Both water and H₂S have an effect on HDN.     

In situ XAFS Studies on the Effects of the Hydrophobic–Hydrophilic Properties of Ti-Beta Zeolites in the Photocatalytic Reduction of CO2 with H2O

The photocatalytic reduction of CO₂ with H₂O to produce CH₄ and CH₃OH was found to proceed in the gas phase at 323 K with different reactivities and selectivities on hydrophilic Ti-Beta(OH) and hydrophobic Ti-Beta(F) zeolites prepared in the OH^– and F^– media, respectively. In situ XAFS measurements have revealed that these differences are attributed to the differences in the affinity for the adsorption of H₂O molecules on the highly dispersed tetrahedrally coordinated titanium oxide species depending on the hydrophobic–hydrophilic properties of zeolites.     

Dehydrogenation of Cyclohexane Over Ni Based Catalysts Supported on Activated Carbon using Spray-pulsed Reactor and Enhancement in Activity by Addition of a Small Amount of Pt

The polycrystalline silver after pre-treated with oxygen at high temperature was found to show excellent activity and selectivity in the process of direct dehydrogenation of methanol to anhydrous formaldehyde and the active oxygen species for this reaction was observed and proposed as the active centeres with in situ Raman scattering.     

Selective Production of Hydrogen for Fuel Cells Via Oxidative Steam Reforming of Methanol Over CuZnAl Oxide Catalysts: Effect of Substitution of Zirconium and Cerium on the Catalytic Performance

H₂ fuel, for fuel cells, is traditionally produced from methanol by the endothermic steam reforming of methanol (SRM). Partial oxidation of methanol (POM), which is highly exothermic, has also been suggested as a route to extract H₂ from methanol. In both these reactions a considerable amount of CO is produced as a byproduct, which is a poison to the Pt anode of the fuel cell. A combined steam reforming and partial oxidation of methanol, which has been termed oxidative steam reforming of methanol (OSRM), reported recently is considered to be more efficient and convenient for the selective production of H₂ at a relatively low temperature. The catalysts used in the OSRM reaction were CuZnAl mixed oxides derived from hydroxycarbonate precursors containing hydrotalcite (HT)-like layered double hydroxides (LDHs)/aurichalcite phases. Substitution of Zr for Al in the CuZnAl oxide system was found to improve the catalytic performance. In the present study, the role of added Zr was investigated in detail by employing spectroscopic methods such as X-ray diffraction (XRD), temperature-programmed reduction (TPR), electron paramagnetic resonance (EPR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray induced Auger electron spectroscopy (AES). The detailed spectroscopic studies revealed that substitution of Zr for Al improved the reducibility and dispersion of copper species due to the operation of a synergistic interaction between copper and zirconium as a consequence of the formation of a Cu²⁺-O-Zr⁴⁺-O- solid solution. The higher catalytic performance of CuZn-based catalysts containing Zr in the OSRM reaction was attributed to the ease of reducibility and enhanced dispersion of copper particles on the support. The substitution of Ce in the CuZnAl system, on the other hand, did not alter the catalytic performance greatly.     

Reaction Kinetics and Intermediate Determination of Solid Acid Catalysed Liquid-phase Hydrolysis Reactions: A Real-time in situ ATR FT-IR Study

The appearance of catalyst–reactant interactions observed in on-line ATR FT-IR spectra of Nafion/silica catalysed esterification and etherification reactions of 1-octanol was investigated. It was assessed by variation of catalyst and solvent that the catalyst–reactant band is a result of the reaction of silica with 1-octanol, yields Si–O–R functionalities. Based on off-line TPD-MS and TGA characterisation of the used solid catalyst powder, the formation of Si–O–R linkages on the SiO₂ surface was confirmed. This demonstrates conclusively that in specific liquid-phase reactions, on-line analysis of intermediate species adsorbed on heterogeneous catalysts is possible. Using on-line particle analyzers, it was assessed that the absorptions were not the result of severe attrition of the catalyst particles in the course of the reaction, but apparently the result of apolar interactions of the particles with the diamond crystal.     

Structural Evolution of Highly Active Ir-Based Catalysts for the Selective Reduction of NO with Reductants in Oxidizing Conditions

NO _x reduction over Ir-based catalysts in the presence of excess oxygen with hydrocarbon as a reductant was investigated in the focus on observing microstructure of Ir particle supported on various carriers and structural evolution of highly active Ir-based catalysts in the NO _x reduction. Characterization of Ir-based catalysts using SEM, TEM, XRD, CO chemisorption and XPS, and reaction studies on various Ir-based catalysts have proved that the formation of relatively large Ir metal particle with 40–60 nm of nanocrystal size carried on inert supports has been a prerequisite for the evolution of high activities in the NO _x reduction rather than the formation of Ir metal state on the catalyst. Furthermore, in inert gas conditions in a high temperature range of 850–950°C, Ir metal was easily formed by using the support such as TiO₂ and ZrO₂ that drastically decreased its specific surface area in the temperature range.     

Partial oxidation of methane with air for methanol production in a post-plasma catalytic system

Partial oxidation of methane to methanol via post-plasma catalysis using a dielectric-barrier discharge was performed under mild reaction conditions. Air was used as the oxidizing co-reactant because of its economical practicality. Three catalysts impregnated with Pt, Fe₂O₃, CeO₂ on ceramic supports located downstream of the discharge zone were examined for increased selectivity towards methanol. It was found that all three catalysts had no significant effect on the conversion of methane, but enhanced methanol selectivity, which could be explained by a two-stage reaction mechanism. The Fe₂O₃-based catalyst showed the best catalytic activity, and high stability in the reaction. The methanol selectivity of the Fe₂O₃-assisted plasma process was 36% higher than that of the non-catalytic system at a rather low catalyst temperature (150 °C). In addition, the effects of input power, discharge frequency, discharge gap distance, total flow rate, and methane/air ratio on methane conversion and methanol yield were also studied.     

Estimation of N2 fixation in Desmodium ovalifolium from the relative ureide abundance of stem solutes: Comparison with the 15N-dilution and an in situ soil core technique

Many, but not all, legumes of tropical origin, transport fixed N from the nodules to the shoot tissue in the form of ureides, and the mineral N absorbed from the soil is principally transported in the form of nitrate. The analysis of stem xylem sap, or hot-water extracts of stem tissue, for ureide and nitrate has been used successfully to quantify BNF contributions to several grain legumes and more recently to some shrub and forage legumes. The objective of this study was to investigate the application of this technique to the quantification of the contribution of BNF to the forage legume Desmodium ovalifolium by comparing the relative ureide abundance (RUA) of stem extracts of this plant with simultaneous estimates of BNF obtained using the ¹⁵N isotope dilution technique. The first experiment was performed in pots of soil, taken from a grazing study, amended with ¹⁵N-labelled organic matter at four different application rates. The ureide concentration in the stem extracts reflected the changes in BNF activity during plant growth and the RUA was closely correlated with the proportion of N derived from BNF as determined from the ¹⁵N technique (r ² = 0.86 and 0.88 for inoculated and non-inoculated plants, respectively). The use of a calibration curve derived from a previous study where the same legume was fed increasing concentrations of ¹⁵N labelled nitrate in sand/vermiculite culture, resulted in an over-estimation of the BNF contribution which may have been due to a significant uptake of ammonium from this acidic soil. The second experiment was performed in field plots and a good agreement was found between the estimates of BNF derived from using the ureide and ¹⁵N dilution techniques at two harvests six months apart. The uptake of soil N by the D. ovalifoliumand two forage grasses (Brachiaria humidicola and Panicum maximum) was estimated using an in situ soil core technique, and, while the uptake of N by the grasses was successfully estimated, this technique underestimated the N derived from the soil by the legume as determined by the ureide and ¹⁵N dilution techniques.