Pd/silica cluster catalysts: synthesis and reactivity with H2 and C2H4

Silica-supported Pd clusters were characterized by in situ EXAFS spectroscopy. Clusters with an average nuclearity of six atoms were derived from either an inorganic or an organometallic precursor by reduction at 100–150C. Despite the small size of the clusters, EXAFS contributions from the metal-support interface were not detected. These clusters and larger ones formed by reduction at 320C absorb hydrogen on cooling in H₂ to 30C; the resultant interstitial hydride species decompose in vacuo at 30C. Vacuum treatment at 300C removes chemisorbed H₂ yielding bare Pd clusters. In contrast to larger crystallites, the Pd clusters do not react with C₂H₄ at 150 to form interstitial carbide species.     

Metal-catalyzed gasification of carbon

The catalytic hydrogenolysis(gasification) of carbon by dihydrogen in the presence of molybdenum carbide and platinum has been studied around 900 K. As indicated by X-ray absorption, the catalytic phases during the reaction were stable molybdenum carbide or slightly electron-deficient platinum. The rates of hydrogenolysis with molybdenum carbide were only slightly less than those with platinum, one of the most active metals for the reaction. However, these two types of catalyst showed different kinetic parameters and different behavior of catalyst particles during the reaction.     

Arsenic re-mobilization in water treatment adsorbents under reducing conditions: Part II. XAS and modeling study

The mechanism of arsenic re-mobilization in spent adsorbents under reducing conditions was studied using X-ray absorption spectroscopy and surface complexation model calculations. X-ray absorption near edge structure (XANES) spectroscopy demonstrated that As(V) was partially reduced to As(III) in spent granular ferric hydroxide (GFH), titanium dioxide (TiO2), activated alumina (AA) and modified activated alumina (MAA) adsorbents after 2 years of anaerobic incubation. As(V) was completely reduced to As(III) in spent granular ferric oxide (GFO) under 2-year incubation. The extended X-ray absorption fine structure (EXAFS) spectroscopy analysis showed that As(III) formed bidentate binuclear surface complexes on GFO as evidenced by an average As(III)-O bond distance of 1.78 A and As(III)-Fe distance of 3.34 A. The release of As from the spent GFO and TiO2 was simulated using the charge distribution multi-site complexation (CD-MUSIC) model. The observed redox ranges for As release and sulfate mobility were described by model calculations.     

Flame-synthesized LaCoO3-supported Pd: 2. Catalytic behavior in the reduction of NO by H2 under lean conditions

A 0.5 wt% Pd/LaCoO₃, prepared by flame-spray pyrolysis (FP), was tested as catalyst for the low-temperature selective reduction of NO by H₂ in the presence of excess O₂. In particular, the effect of the precalcination and prereduction temperature on catalytic activity was compared with that of a similar Pd/LaCoO₃ sample prepared by impregnation with a Pd solution of FP-prepared LaCoO₃. The FP-made catalyst allowed full NO conversion at 150 °C, with 78% selectivity to N₂, thus outperforming the catalytic behavior of the corresponding sample prepared by impregnation. The higher activity of the FP-made catalyst has been attributed to the formation of segregated Co metal particles, not present in the impregnated sample, formed during the precalcination at 800 °C, followed by reduction at 300 °C. Two reaction mechanisms can be deduced from the temperature-programmed experiments. The first of these, occurring at lower temperatures, indicates cooperation between the Pd and Co metal particles, with formation of active nitrates on cobalt, successively reduced by hydrogen spillover from Pd. The second, occurring at higher temperature, allows 50% conversion of NO, with >90% selectivity to N₂, and involves N adatoms formed by dissociative NO adsorption over Pd. Prereduction at 600 °C led to a slight increase in catalytic activity, due to the formation of a PdCo alloy, which is more stable on reoxidization compared with Pd alone. Moreover, the cooperative reaction mechanism seems to be favored by the proximity of Co and Pd in metal particles.     

Structure of Co and Co oxide clusters in MCM-41

The structural properties of Co/MCM-41 with pore diameters between 2.9 and 3.6 nm prepared by direct synthesis and impregnation were investigated. For both preparation methods, the size of the metal particles decreased with the pore diameter. For Co/MCM-41 with the same pore diameter we observed that the direct synthesis method led to significantly smaller metal clusters compared to the impregnation method. For all Co/MCM-41 samples constraints of the metal cluster sizes were observed, which are speculated to result from influences of the micro structure during the formation of the catalyst precursor.     

Platinum deactivation: in situ EXAFS during aqueous alcohol oxidation reaction

With a new set‐up for in situ EXAFS spectroscopy the state of a carbon‐supported platinum catalyst during aqueous alcohol oxidation has been observed. The catalyst deactivation during platinum‐catalysed cyclohexanol oxidation is caused by platinum surface oxide formation. The detected Pt–O co‐ordination at 2.10 Å during exposure to nitrogen‐saturated cyclohexanol solution is different from what is observed for the pure oxidised platinum surface (2.06 Å).     

Characterization of highly dispersed Ni/Al2O3 catalysts by EXAFS analysis of higher shells

The size and morphology of Ni/Al₂O₃ catalysts in the oxidic, reduced, and passivated state were determined by EXAFS analysis of the higher shells around the Ni atoms. In the oxidic state, the Ni cations were present in small NiO_x particles with predominant (111) plane. Below 4.5 wt% Ni loading, the NiO_x particles consisted of one Ni layer, and of two or three Ni layers above 4.5 wt% Ni. A Ni–Al contribution was observed in samples with low Ni loading. The layer which is in contact with the Al₂O₃ surface is affected by the support surface and its structure is highly distorted, while the other layers were not distorted and have a structure similar to that in bulk NiO. In the reduced state, the number of Ni metal atoms in the reduced Ni particles was smaller than 100 with a narrow distribution below a loading of 15.6 wt% Ni. Above this loading, the particle size suddenly increased and the distribution became wider. The distances and Debye–Waller factors were similar to those of bulk nickel which suggested a weak interaction between the particles and the support. In the passivated state, Ni kernels with 20–40 metal atoms were covered by a one layer thick NiO skin. This revised version was published online in June 2006 with corrections to the Cover Date.     

Local and crystal structure of Mg1.9Al0.1Ni hydrogen storage alloys during hydrogen absorption–desorption cycling

The evolution of crystal structure and chemical state of Mg_1.9Al_0.1Ni alloy during hydrogen absorption–desorption cycling was examined by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). We research the hydrogen storage capacity of the Mg_1.9Al_0.1Ni by the H/D kinetic curves. The H/D kinetic curves indicate that the hydrogen storage capacity increased with the increased cycles and the samples were activated after 10 cycles have the maximum hydrogen storage capacity. The local structure of Ni atoms was studied by extended X-ray absorption fine structure (EXAFS). The EXAFS results indicate the Ni–Ni bonds distance has no obviously change with the cycles increasing, whereas the Ni–Mg bond lengths increase, and the Ni–Mg bond lengths are longer obviously than before 10 cycles whereas it has no obviously change after 10 cycles.     

EXAFS Study on Structural Change of Charcoal-supported Ruthenium Catalysts during Lignin Gasification in Supercritical Water

Lignin gasification in supercritical water over charcoal supported ruthenium trivalent salts was studied using a batch reactor at 673 K. Ruthenium (III) nitrosyl nitrate on charcoal (Ru(NO)(NO₃)₃/C) was more active than ruthenium (III) chloride on charcoal (RuCl₃/C) for the gasification reaction. EXAFS analysis revealed that ruthenium metal particles were formed in both RuCl₃/C and Ru(NO)(NO₃)₃/C catalysts during the lignin gasification and that the size of ruthenium metal in Ru(NO)(NO₃)₃/C was smaller than that in RuCl₃/C. It was concluded that well-dispersed ruthenium metal particles were active for the lignin gasification in supercritical water.     

Extended X‐Ray Absorption Fine Structure Investigation of Carbon Stabilized Expanded Austenite and Carbides in Stainless Steel AISI 316

Low temperature carburized AISI 316 stainless steel ‐ carbon expanded austenite ‐ was investigated with EXAFS and synchrotron diffraction together with synthesized carbides of the type M₃C₂, M₇C₃ and M₂₃C₆. It was found that the chemical environment of carbon expanded austenite is not associated with any of the investigated carbides, that carbon has a strong affinity for chromium, i.e. short range order, and that carbon is in solid solution.