T. Tabakova

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.

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.

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.

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.

Effect of phosphorus concentration and method of preparation on the structure of the oxide form of phosphorus-nickel-tungsten/alumina hydrotreating catalysts

Two series of phosphorus containing NiW/Al2O3 catalysts were prepared by different preparation methods and varying phosphorus content from 0 to 7.6 wt.% P2O5. The influence of the phosphorus concentration and the preparation procedure on the structure and the dispersion of the compounds formed in the oxide form of the catalysts was studied. Diffuse reflectance spectroscopy (DRS), infrared spectroscopy (IR) and X-ray photoelectron spectroscopy (XPS) were used for the characterization of the catalysts. It was demonstrated that introduction of phosphorus in NiW/Al2O3 catalysts impedes the formation of NiAl2O4 and increases the amount of Ni2+(Oh) ions in the oxide form of the samples. This effect of phosphorus is better expressed when phosphorus is first introduced to alumina, followed by co-impregnation with nickel and tungsten. Surface AlPO4 is formed in the phosphorus containing samples irrespective of the preparation procedure. Phosphorus is distributed as a monolayer up to concentrations of 1.3 P at/nm2 in both series of catalysts. It was also found that the increased phosphorus content in the samples leads to an increased degree of polymerization of the tungsten species via WOW bonds. The presence of phosphorus changes the dispersion of the active components. Evaluation of the average particle size of tungsten species shows that it increases from 10Afor the phosphorus free sample up to 20–25Afor the samples with high phosphorus content. The relationship between the structure of the phosphorus promoted NiW/Al2O3 catalysts and their hydrodesulfurization activity is discussed.

Complete oxidation of benzene over Au−V2O5/TiO2 and Au−V2O5/ZrO2 Catalysts

A gold promotional effect is reported for the complete oxidation of benzene using Au/V2O5 supported on titania or zirconia catalysts.

Influence of iron (II) on the transformation of ferrihydrite into goethite in acid medium

The transformation of ferrihydrite into goethite has been observed to be considerably facilitated under the conditions of oxidative hydrolysis of iron(II) sulphate. A probable scheme of the process mechanism has been put forward to explain the transformation that involves a step consisting of the formation of soluble complexes of ferrihydrite with ammonia.

Effect of ceria structural properties on the catalytic activity of Au-CeO2 catalysts for WGS reaction.

Two gold based catalysts supported on ceria prepared by different methods (urea gelation coprecipitation, UGC, and coprecipitation, CP) have been synthesized and tested in the WGS reaction, showing quite different catalytic behaviors. Interestingly, the two catalysts have the same gold loading (3 wt% Au was inserted by deposition-precipitation) and the FTIR spectroscopy of the adsorbed CO revealed the same amount of gold exposed sites. With the aim to elucidate how the preparation method affects the properties of the support, a morphological, structural and textural characterization has been performed by HRTEM, XRD, BET and Raman analyses, as well as FTIR spectroscopy to probe both the Au and the support exposed sites. It was found that the UGC method gave rise to an enhancement of the defectivity of ceria and to an increase of the reactivity under reductive treatment. Further FTIR measurements of adsorbed acetone demonstrated the presence of two kinds of Ce(4+) sites with different coordination, (CUS) Ce(4+) A and (CUS) Ce(4+) B, on the UGC sample. Such sites can influence the catalytic activity, possibly favoring the water dissociation, making ceria prepared by UGC a better support for Au catalysts than the CP-prepared one.

Promoting effect of gold on the structure and activity of Co/kaolin catalyst for the 2,3-dihydrofuran synthesis

Abstract The promoting effect of gold and the influence of the thermal pretreatment of the cobalt/kaolin catalysts on the reducibility, catalytic activity and selectivity has been studied. The catalytic properties were tested in the cyclodehydration of 1,4-butanediol. The gold-promoted catalysts are more selective than their non-promoted analogues. The modification by gold leads to the formation of new cobalt species, being reducible at significantly lower temperatures in comparison to those of the non-promoted catalysts. This effect of gold results not only on the dispersion, but also on the structure of the metallic cobalt phases.