D. Andreeva

Low-temperature water-gas shift reaction over Au/CeO2 catalysts

A high and stable activity for gold/ceria catalyts has been established for the water-gas shift reaction. The relationship between gold loading and catalytic activity was studied over a wide temperature range. The influence of space velocity and H2O/CO ratio at different temperatures on the catalytic activity and stability was also investigated. The reduction/oxidation processes of ceria in the presence of gold was readily followed by TPR measurements. It was shown that ceria plays the role of an active support capable of producing oxygen. The high and stable activity of gold/ceria catalysts could arise from the high and stable gold dispersion present during the catalytic operation.

FTIR study of low-temperature water-gas shift reaction on gold/ceria catalyst

Chemisorption and reactivity of the molecules involved in the water-gas shift (WGS) reaction on gold/ceria catalyst have been studied at 90 and 300 K by FTIR spectroscopy. Forward and reverse WGS reaction at 300 K and up to 573 K have been investigated, too. The FTIR results show that gold causes a strong modification of the surface properties of the support. The nanosized metallic gold particles in close contact with defective ceria play an essential role for the genesis of high catalytic activity in WGS reaction at low temperature and appear to be of crucial importance in explaining the remarkably high stability of this catalytic system. An electronic interaction between small gold metallic nanoparticles and ceria has been evidenced.

Low temperature water gas shift over gold catalysts

This review summarizes the results reported in the literature on the water gas shift (WGS) over goldcontaining catalysts. Attention is focused on the influence of the preparation method and the nature of the support on the gold dispersion and WGS activity. Special emphasis is paid to the enhanced reducibility of the metal oxide support in the presence of gold. Some aspects of the reaction mechanism of the WGS are discussed.

Low-temperature water-gas shift reaction on Auα-Fe2O3 catalyst

The water-gas shift reaction (WGSR) has been studied on Auα-Fe2O3 catalyst. The structure of the samples has been investigated by chemical and physical methods—TEM, X-ray, DTA, FTIR. A high dispersion degree of the gold particles and an increased concentration of the hydroxyl groups on Auα-Fe2O3 has been established in comparison to the pure α-Fe2O3. The results obtained can be explained on the basis of the associative mechanism of the WGSR. The essential aspects are the dissociative adsorption of water on ultrafine gold particles, followed by spillover of active hydroxyl groups onto adjacent sites of the ferric oxide. The formation and decomposition of intermediate species is accompanied by redox transfer Fe3+ Fe2+ in Fe3O4.

Influence of the microscopic properties of the support on the catalytic activity of Au/ZnO, Au/ZrO2, Au/Fe2O3, Au/Fe2O3–ZnO, Au/Fe2O3–ZrO2 catalysts for the WGS reaction

It has been established that the gold catalysts on well crystallized supports, Au/Fe2O3 and Au/ZrO2, display higher catalytic activity in the water gas shift (WGS) reaction in comparison with the samples on amorphous and not well crystallized supports — Au/ZnO, Au/ZrO2, Au/Fe2O3–ZnO and Au/Fe2O3–ZrO2. It could be concluded that the catalytic activity of the gold/metal oxide catalysts depends strongly not only on the dispersion of the gold particles but also on the state and the structure of the supports.

Gold, silver and copper catalysts supported on TiO2 for pure hydrogen production

A catalytic study of the hydrogen production by CO water gas shift reaction (WGSR) on gold, silver and copper particles supported on TiO2 has been carried out. A deep characterisation of the catalysts by TPR and FTIR has been performed. Silver catalyst exhibits no catalytic activity, copper and gold catalysts show intermediate and very high performances, respectively. These strong differences have been interpreted on the basis of FTIR data of CO adsorption at 90 K and on the effect of coadsorbed species. Gold and copper catalysts, either oxidised or reduced, are able to adsorb CO. Reduced silver catalyst does not adsorb CO at all, while oxidised silver catalyst does quite strongly.

Kinetic parameters obtained from TPR data for α-Fe2O3 and Auα-Fe2O3 systems

Kinetic parameters were evaluated by simulation of experimental temperature-programmed reduction spectra of α-Fe2O3 and Auα-Fe2O3 systems. It was established that gold influences only the first reduction step Fe2O3 → Fe3O4, the pre-exponential value for Auα-Fe2O3 being substantially higher than that for α-Fe2O3. The presence of hydroxyl coverage entails decreasing of the activation energy of this step.

Au/α-Fe2O3 catalyst for water–gas shift reaction prepared by deposition–precipitation

A modified version of the preparation methods for the synthesis of highly-active Au/α-Fe2O3 catalysts for WGS reaction has been proposed. The results obtained show that gold dispersion is highly preserved and prevents cluster formation. On the other hand, a major part of gold remains on the surface, thus being accessible to catalysis. As a result of the preparation technique, it seems that the interaction between gold and support is weaker and, likely, more favourable for catalysis. This study demonstrates that a specific approach to preparation of catalysts of optimum structure and texture is crucial for high-catalytic activity.

Nanosize gold catalysts promoted by vanadium oxide supported on titania and zirconia for complete benzene oxidation

Abstract The complete benzene oxidation reaction was used to test the reactivity of the Au-V 2 O 5 /TiO 2 and Au-V 2 O 5 /ZrO 2 catalytic systems. A strong synergistic effect between gold and vanadia was established when molecular oxygen was used as an oxidizing agent. This effect was more pronounced for titania than for the zirconia support. In the presence of gold, predominantly polyvanadate structures are formed on the surface which are more active in the reaction of complete benzene oxidation in comparison with monovanadate species and bulk V 2 O 5 . The deposition of gold leads to a relative lengthening of the VO bond and to a higher electron delocalization. The B and C parameters calculated from ESR spectra showed no differences for the fresh and spent gold-containing samples, i.e. the catalytic systems seemed to be “stabilized” under the working conditions. The effect of gold on the vanadium oxide reducibility, which could be related to the strength of the VO-support bond, results in a considerable lowering of the temperature of the V 5+ → V 3+ transition. With ozone as the oxidizing agent, an additional lowering of the reaction temperature was achieved and very close values of the catalytic activity on all investigated samples were registered. Upon oxidation by molecular oxygen the oxidant activation takes place on the nanosize gold particles while the vanadium oxide surface species are responsible for the activation of benzene. This is in agreement with synergistic effect between the finely dispersed gold and the surface vanadium structures.

Complete benzene oxidation over gold-vanadia catalysts supported on nanostructured mesoporous titania and zirconia

Abstract The new generation of gold-vanadia catalysts supported on mesoporous titania and zirconia for complete benzene oxidation were explored. The catalysts were characterized by X-ray, TEM, SEM, N2 adsorption analysis, TPR and ESR spectroscopy. The vanadia loading stabilized structure of the both mesoporous supports and this effect is stronger for zirconia comparing to titania. The presence of gold enhanced the V5+→V3+ reduction step and depending on the preparation method, differences in the reduction behavior were established. The catalytic activity of the catalysts also strongly depends on the preparation techniques. For the both series of the studied catalysts when the gold is loaded firstly the activity in complete benzene oxidation is higher than when the vanadia is deposited firstly (VAT>AVT and VAZ>AVZ). A strong synergistic effect between gold and vanadia supported on titania was observed and the catalysts on titania exhibit higher catalytic activity than the catalysts based on zirconia. Comparing the activity of the gold-vanadia/zirconia and gold/zirconia catalysts, it was established for the both catalysts high and equal activity in the reaction studied. The observed differences in the structural and catalytic properties of the both series of the studied catalysts were connected with the nature of the supports used.