Rational design of gold catalysts with enhanced thermal stability: post modification of Au/TiO2 by amorphous SiO2 decoration

Au/TiO₂ is highly active for CO oxidation, but it often suffers from sintering in high-temperature environments. In this work, we report on a novel design of gold catalysts, in which pre-formed Au/TiO₂ catalysts were post decorated by amorphous SiO₂ to suppress the agglomeration of gold particles. Even after being aged in O₂–He at 700 °C, the SiO₂-decorated Au/TiO₂ was still active for CO oxidation at ambient temperature.     

Hydroformylation of olefins with water-soluble rhodium catalysts in the presence ofα-cyclodextrin

Hydroformylation of hex-1-ene using a water soluble rhodium catalyst HRh(CO)[PPh₂(m-C₆H₄SO₃Na)]₃ (HRh(CO)(TPPMS)₃) (I), gives lower yields when-cyclodextrin is added to the biphasic reaction system implying an interaction between the cyclodextrin and rhodium catalyst.     

Activity, Selectivity, and Long-Term Stability of Different Metal Oxide Supported Gold Catalysts for the Preferential CO Oxidation in H2-Rich Gas

A comparative study of the catalytic performance and long-term stability of various metal oxide supported gold catalysts during preferential CO oxidation at 80°C in a H₂-containing atmosphere (PROX) reveals significant support effects. Compared to Au/-Al₂O₃, where the support is believed to behave neutrally in the reaction process, catalysts supported on reducible transition metal oxides, such as Fe₂O₃, CeO₂, or TiO₂, exhibit a CO oxidation activity of up to one magnitude higher at comparable gold particle sizes. The selectivity is also found to strongly depend on the employed metal oxide, amounting, e.g., up to 75% for Au/Co₃O₄ and down to 35% over Au/SnO₂. The deactivation, which is observed for all samples with increasing time on stream, except for Au/-Al₂O₃, is related to the build-up of surface carbonate species. The long-term stability of the investigated catalysts in simulated methanol reformate depends crucially on the ability to form such by-products, with magnesia and Co₃O₄ supported catalysts being most negatively affected. Overall, Au/CeO₂ and, in particular, Au/-Fe₂O₃ represent the best compromise under the applied reaction conditions, especially due to the superior activity and the easily reversible deactivation of the latter catalyst.     

CO oxidation over Au/TiO2 prepared from metal-organic gold complexes

A series of Au/TiO₂ catalysts has been prepared from precursors of various metal-organic gold complexes (Au _n , n = 2–4) and their catalytic activity for CO oxidation studied. The Au/TiO₂ catalyst synthesized from a tetranuclear gold complex shows the best performance for CO oxidation with transmission electron microscopy of this catalyst indicating an average gold particle size of 3.1 nm.     

The dissolution of iron from the negative material in pocket plate nickel-cadmium batteries

A common mode of failure of nickel-cadmium flooded pocket plate cells is iron poisoning of the positive plate due to transfer of iron into the active material from active materials and materials of construction. Nickel plated steel pockets are sometimes used to minimize iron dissolution, particularly on the positive electrode. Sometimes-Fe₂O₃ is used as an additive to the cadmium electrode. This paper assesses the extent of dissolution of iron from-Fe₂O₃ by using electron microscopy, X-ray crystallography, cyclic voltammetry, coulometric and atomic absorption measurements.     

Reducibility of supported gold (III) precursors: influence of the metal oxide support and consequences for CO oxidation activity

The origin of CO oxidation performance variations between three different supported Au catalysts (Au/CeO₂, Au/Al₂O₃, Au/TiO₂) was examined by in situ XAFS and DRIFTS measurements. All samples were prepared identically, by deposition-precipitation of an aqueous Au(III) complex with urea, and contained the same gold loading (~1 wt %). The as-prepared supported Au(III) precursors exhibited different reduction behaviour during exposure to the CO/O₂/He reaction mixture at 298 K. The reducibility of the Au(III) precursor was found to decrease as a function of the support material in the order: titania > ceria > alumina. The as-prepared samples were inactive catalysts, but Au/TiO₂ and Au/CeO₂ developed catalytic activity as the reduction of Au(III) to metallic Au proceeded. Au/Al₂O₃ remained inactive. The developed catalytic CO oxidation activity at 298 K varied as a function of the support as follows: titania > ceria > alumina ~ 0. The EXAFS of samples pretreated in air at 773 K and in H₂ at 573 K reveals the presence of only metallic particles for Au/TiO₂ and Au/Al₂O₃. Au(III) supported on CeO₂ remains unreduced after calcination, but reduces during the treatment with H₂. CO oxidation experiments performed at 298 K with the activated samples show that the presence of metallic gold is necessary to obtain active catalysts (Au/CeO₂ is not active after calcination) and that the reducible supports facilitate the genesis of active catalysts, while metallic gold particles on alumina are not active.     

Catalytic oxidation of methanol to methyl formate over silver – a new purpose of a traditional catalysis system

Catalytic reaction was performed in the unregarded temperature region over silver catalysts with long catalytic lifetime for the conversion of methanol to methyl formate. O-saturated or O-saturated silver catalysts were studied individually to identify the roles of O, O in the oxidative esterification of methanol over an unsupported polycrystalline silver catalyst. A synergic process is proposed based on the coexistence of -oxygen species and -oxygen species on the surface of polycrystalline silver at about 573 K.     

Investigations of Fe-Ru bimetallic catalysts by in situ Mössbauer and EXAFS studies

In situ Mössbauer and EXAFS investigations have shown that the reduction of iron in the monometallic Fe/SiO₂ catalyst is only partial, the reduction being mostly to a ferrous silicate phase. In the bimetallic Fe-Ru/SiO₂ catalysts, the proportion of the FeRu alloy formed on reduction increases markedly with the increase in Ru content; clearly, Ru significantly enhances the reduction of iron on SiO₂. In the Ru-rich compositions (Ru/Fe 1.0), most of the iron is present in the alloy phase and there is no segregation of -Fe. A comparative study of the different supports has shown that -Al₂O₃ and SiO₂ interact with iron strongly at low reduction temperatures while the TiO₂ support interacts at higher temperatures. The presence of traces of Fe³⁺ often found in reduced Fe-Ru catalysts is shown to arise from the oxidation of fine segregated iron particles on the support.Contribution No. 831 from the Solid State and Structural Chemistry Unit.     

Selective liquid phase oxidation using gold catalysts

Au/C and Au/oxide (Al₂O₃, TiO₂) have been compared in the liquid phase oxidation of glycols and a different trend in reactivity revealed. On the oxides the activity of supported gold increases by decreasing particle size, whereas on carbon maximum activity is achieved with gold particle mean diameter around 7–8 nm. XPS revealed that in the latter case activity depends not only on the size of the gold particle but also on its surface concentration. This revised version was published online in June 2006 with corrections to the Cover Date.     

Chemical vapor deposition of gold on Al2O3, SiO2, and TiO2 for the oxidation of CO and of H2

In order to clarify the effect of metal oxide support on the catalytic activity of gold for CO oxidation, gold has been deposited on SiO₂ with high dispersion by chemical vapor deposition (CVD) of an organo-gold complex. Comparison of Au/SiO₂ with Au/Al₂O₃ and Au/TiO₂, which were prepared by both CVD and liquid phase methods, showed that there were no appreciable differences in their catalytic activities as far as gold is deposited as nanoparticles with strong interaction. The perimeter interface around gold particles in contact with the metal oxide supports appears to be essential for the genesis of high catalytic activities at low temperatures. This revised version was published online in July 2006 with corrections to the Cover Date.     

Performance of supported Ru-Cu bimetallic catalysts prepared from nitrate precursors

Bimetallic Ru-Cu catalysts supported on SiO₂, -Al₂O₃, -Al₂O₃ have been prepared using precursors which do not contain chlorine and characterized by CO chemisorption and TPR. Catalytic activity has been tested in the propane hydrogenolysis. It has been observed that the degree of formation of bimetallic Ru-Cu aggregates depends on the support used. It is suggested that the degree of interaction between Ru and Cu is strongly influenced by the strength of the metal-support interaction.     

Study of Gold Species in Iron-Oxide-Supported Gold Catalysts Derived from Gold-Phosphine Complex Au(PPh3)(NO3) and As-Precipitated Wet Fe(OH)3*

Iron-oxide-supported gold catalysts were prepared by supporting a Au phosphine complex Au(PPh₃)(NO₃) on as-precipitated wet iron hydroxide Fe(OH)₃^*, followed by temperature-programmed calcination. The Au/Fe(OH)₃^*catalysts calcined at the temperatures 573–773 K showed extremely high catalytic performance for CO oxidation at temperatures as low as 203–253 K. Interaction of the Au(PPh₃)(NO₃) gold precursor with the Fe(OH)₃^*upon supporting, transformation of the precursor during the heat treatments, and state of the gold in the catalysts were studied by FT-IR, XRD, TEM, XPS, and EXAFS. The gold precursor dissociated on the Fe(OH)₃^*surface to produce [Au(PPh₃)]⁺species which partially decomposed at 473 K and was transformed to small gold metallic particles with coordination numbers of 7.4–8.0 for Au-Au bond at calcination temperatures ≥573 K. In contrast, decomposition of the gold complex over crystalline Fe₂O₃^*resulted in large gold particles. The Au/Fe₂O₃^*sample was inactive at 203–253 K and exhibited very low activity for CO oxidation at room temperature. The efficiency of the as-precipitated wet Fe(OH)₃^*as a support is explained in terms of a higher stability of [Au(PPh₃)]⁺on the Fe(OH)₃^*as compared to the Fe₂O₃^*due to more effective interaction of the Au species with OH groups and defects of the amorphous Fe(OH)₃^*surface. The results demonstrate the importance of support–metal precursor interactions, both upon supporting and during calcination, in the formation of highly active catalysts with small Au particles for low-temperature CO oxidation.     

Selective synthesis of α-olefins on Fe-Zn Fischer-Tropsch catalysts

Fe/Zn oxides promoted with K and Cu selectively produce -olefins at typical Fischer-Tropsch synthesis conditions (2/1 H₂/CO, 1 MPa, and 270°C). The simultaneous presence of K and Cu introduces a synergistic activity enhancement while maintaining the high olefin selectivity obtained by alkali promotion. Structural and morphological differences in Fe-Zn oxides prepared from ammonium glycolate complexes or precipitated from nitrate solutions have only a small influence on catalytic properties. Catalyst behavior is strongly influenced by synergistic promoter effects (Cu, K) and by the controlled in situ conversion of iron oxide precursors to carbides.     

Preparation and characterization of Co/SiO2, Co-Mg/SiO2 and Mg-Co/SiO2 catalysts and their activity in CO hydrogenation

Co/SiO₂, Mg-Co/SiO₂ and Co-Mg/SiO2 catalysts were prepared from acetate, nitrate or carbonyl precursors. The catalysts were characterized by XRD, XPS, SIMS and TGA. The steady-state activity and product distribution of the catalysts were evaluated in synthesis gas reactions at 0.5 MPa and 235-290°C using 3 : 1 : 3 molar ratio of Ar : CO : H₂. The activity in CO hydrogenation decreased in the precursor order Co₂(CO)₈>Co(NO₃)₂> Co(CH₃COO)₂, and the probability of chain growth decreased in the precursor order Co(NO₃)₂>Co₂(CO)₈>Co(CH₃COO)₂. Alcohol yields were highest with Co₂(CO)₈, and lowest with Co(NO₃)₂, Magnesium promotion influenced the catalyst activity and decreased the CO₂ formation, but the promotion effects were less profound than those of the precursor. Surface studies on partially magnesium covered cobalt foil model catalysts suggested that magnesium promotes CO dissociation and chain growth, neither of which were, however, observed in the supported catalysts.     

FTIR study of NO, C3H6 and O2 adsorption and interaction on gold modified MCM-41 materials

This paper is devoted to the detailed FTIR study of the adsorption, co-adsorption, and interaction of all the reagents used in NO HC-SCR process addressed to lean-burn engines with the surface of new gold catalysts based on ordered mesoporous materials. Gold was introduced into silicate and niobiosilicate matrices by the impregnation (Au/MCM-41 and Au/NbMCM-41, respectively) and via co-precipitation with siliceous and niobium sources (AuNbMCM-41). The in situ FTIR study allowed the estimation of the possible chemisorption of the reagents and their interaction towards intermediates, depending on the chemical composition of the catalyst and the way of gold introduction. It has been found that propene is chemisorbed, but not, NO, on gold species at room temperature. Chemisorbed C₃H₆ interacts with NO only in the presence of oxygen excess. Oxygen oxidizes NO to NO₂, the latter interacts with chemisorbed propene towards carboxylates (1570 cm⁻¹) and NO₂ is reduced to N₂O. At higher temperatures carboxylates interact with gaseous NO to carbonate, N₂O, CO and CO₂. The presence of niobium in the NbMCM-41 matrix enhances the oxidative properties of the catalysts and as a consequence the interaction between intermediates in NO reduction with propene in the oxygen excess. The co-precipitated AuNbMCM-41 exhibits higher NOx storage properties than the impregnated one.     

Partial oxidation of methane to synthesis gas over α-Al2O3-supported bimetallic Pt–Co catalysts

The partial oxidation of methane to synthesis gas was studied at atmospheric pressure and in the temperature range of 550–800°C over -Al₂O₃-supported bimetallic Pt–Co, and monometallic Pt and Co catalysts, respectively. Both methane conversion and CO selectivity over a bimetallic Pt_0.5Co₁ catalyst were higher than those over monometallic Pt_0.5 and Co₁ catalysts. Furthermore, the addition of platinum in Pt–Co bimetallic catalysts effectively improved their resistance to carbon deposition with no coking occurring on Pt_0.5Co₁ during 80 h reaction. The FTIR study of CO adsorption observed only linearly bonded CO on bimetallic Pt–Co catalysts. TPR and XPS showed enhanced formation of a cobalt surface phase (CSP) in bimetallic Pt–Co catalysts. The origins of the good coking resistivity of bimetallic Pt–Co catalysts were discussed.     

Dehydrogenation of ethylbenzene over TiO2-Fe2O3 and ZrO2-Fe2O3 mixed oxide catalysts

The mixed metal oxides TiO₂-Fe₂O₃ and ZrO₂-Fe₂O₃ were examined as potential catalysts for the dehydrogenation reaction of ethylbenzene. The acidic and basic properties and surface area, pore volume and pore size distribution of these catalysts were measured. The catalytic activities can be correlated very well with the surface area and the acidity and basicity of ZrO₂-Fe₂O₃ catalysts. However, for TiO₂-Fe₂O₃ catalysts, the surface area, the amount of acidic and basic sites and TiFe₂O₅ crystallinity are all important factors affecting the catalytic activities for ethylbenzene dehydrogenation. A synergistic effect was found for the TiO₂-Fe₂O₃ and ZrO₂-Fe₂O₃ catalyst system and also for the TiO₂-Fe₂O₃-ZrO₂ system, i.e. the activities of these catalysts can be ranked in the following order: TiO₂-Fe₂O₃-ZrO₂>TiO₂-Fe₂O₃ >ZrO₂>Fe₂O₃>TiO₂. Meanwhile, all of these catalysts showed higher activities than the conventional potassium-promoted iron catalysts.     

Zeolite NaY-supported gold complexes prepared from Au(CH3)2(C5H7O2): reactivity with carbon monoxide

Mononuclear gold complexes in zeolite NaY were synthesized from initially physisorbed Au(CH₃)₂(C₅H₇O₂), and their reactions with CO in a flow system at 298 K and 760 Torr were investigated by infrared (IR) spectroscopy and mass spectral analysis of the effluent gases. CH₄ and CO₂ were formed as CO flowed through the sample either steadily or as successive pulses. The results are consistent with the inferences that (a) CO reacted with the supported gold to form gold carbonyls, (b) CH₄ formed by reaction of methyl groups on gold with traces of H₂O or hydroxyl groups on the zeolite and (c) CO on cationic gold reacted with traces of O₂ and/or H₂O to form CO₂. In samples treated in steadily flowing CO, cationic gold was reduced to zerovalent gold, but the cationic gold in samples exposed to CO pulses was not reduced to zerovalent gold, although CO₂ formed. Thus, CO adsorbed on cationic gold reacts to give CO₂ in the absence of zerovalent gold, consistent with the inference that gold catalysts for CO oxidation need not contain zerovalent gold.     

Synthesis of (all-rac)-α-tocopherol using Nafion resin/silica nanocomposite materials as catalysts

The reaction of trimethylhydrochinone and isophytol over novel Nafion resin/silica nanocomposite materials with 40 wt% ofNafion in silica produces (all-rac)--tocopherol in high yield. The in situ prepared nanocomposite materials are more stable than the dispersed Nafion catalysts on pre-formed SiO₂-supports with respect to solvent leaching.     

Effect of Preparation Conditions on the Characteristics of Fe3+-K10 Clay Catalysts

Mössbauer spectroscopy and IR techniques were employed to characterize the iron-impregnated montmorillonite K10 clay catalysts, prepared by using acetonitrile or water solutions of iron(III) chloride. Samples prepared in a non-aqueous medium consisted of comparable amounts of -Fe₂O₃ and FeOOH, whereas those prepared in an aqueous medium showed only FeOOH-type species. The higher catalytic activity obtained using a non-aqueous medium for alkylation of arenes with benzyl chloride is attributed to the additional Lewis-type acidic sites associated with iron oxide.