Effect of the metal-support interaction in Ag/CeO2 catalysts on their activity in ethanol oxidation

The interaction of silver with the surface of CeO2 in the Ag/CeO2 catalysts prepared by coprecipitation and impregnation techniques was studied by temperature-programmed reduction, X-ray diffraction, and high-resolution transmission electron microscopy. It was shown that coprecipitation technique led to formation of strong silver–support interaction and the epitaxy of silver particles (d111 = 2.35 Å) on the surface of CeO2 (d111 = 3.1 Å). This provided incresed catalytic activity in the oxidative dehydrogenation of ethanol at relatively low temperatures (a 15% conversion of ethanol with 100% selectivity for the formation of acetaldehyde was reached at 85°C). Above 130°C, the deep oxidation of ethanol to CO2 becomes the predominant direction of a catalytic reaction, and the Ag/CеО2 catalyst obtained by impregnation technique was most active in this region as a consequence of the weaker metal–support interaction

Investigating the influence of silver state on electronic properties of Ag/Ag2O/TiO2 heterojunctions prepared by photodeposition

Two series of Ag-TiO2 photocatalysts were prepared with the use of photodeposition method with variation of the irradiation time and precursor concentration. The obtained photocatalysts were studied by XRD, Raman spectroscopy, HR TEM, UV–vis spectroscopy, and low-temperature N2 adsorption/desorption method. It has been consequently found that the optical properties of the resulting catalysts differ significantly: the positions of surface plasmon resonance peaks red-shifted with the increase in precursor concentration. At the same time, optical absorption of the samples increases with both precursor concentration and irradiation time. Photocatalytic activity for the obtained catalysts was evaluated in decolorization of Rhodamine B with the use of Xe arc lamp (250 W) with and without λ >420 nm light filter. Radical trap experiments have shown that the amount of both superoxide anions and hydroxyl radicals increased in full spectrum of the lamp, with the latter being absent in the reaction mixture during visible light photocatalysis. Comparison of different Ag-TiO2 catalysts has also been made, with 1-Ag-90 min sample being the most active in full spectrum, and 3-Ag-90 min demonstrating the highest conversion in visible light attributed to the increased generation of superoxide species on the surface of Ag clusters. Controversially, 1-Ag-45 min sample showed the lowest activity in full spectrum being surpassed even by unmodified TiO2, but reached the highest rate constant value in visible light. This effect can be related to advanced electronic interaction between Ag plasmonic nanoparticles and titania support in the presence of sensitizer compound, and the formation of Ag/Ag2O composite system on the surface of titania. Influence of the state of silver on photocatalytic activity and mechanism details is discussed with special attention to irradiation wavelengths

Synthesis of ceria nanoparticles in pores of SBA-15: Pore size effect and influence of citric acid addition

The influence of porous structure and addition of citric acid on the formation of ceria particles inside the SBA-15 pores is studied using nitrogen adsorption-desorption at −196 °C, XRD, UV-visible spectroscopy, TEM and TPR-H2 methods. The use of pores of SBA-15 as nanoreactors provides the formation of CeO2 nanoparticles with sizes below 6 nm. According to XRD and TEM results, the addition of citric acid during the impregnation allows obtaining the CeO2 particles with sizes about 3 nm, while the agglomerates of large СеО2 particles are formed on the surface of the SBA-15, when citric acid is not added. Small sizes of ceria particles (3 nm) cause an enhanced reducibility (TPR-H2), high defectiveness of the particles and red shift in the UV-vis adsorption spectra. These properties may improve the catalytic and photocatalytic activity of the CeO2/SBA-15-based catalysts in deep and selective oxidation of organic compounds

Formation of Active Sites for Selective Toluene Oxidation during Catalyst Synthesis via Solid-State Reaction of V2O5 with TiO2

Interaction of V2O5 with TiO2 during prepn. of V/Ti-oxide catalysts via solid-state reaction was studied using in situ FT-Raman spectroscopy, HRTEM and XPS. This interaction results in the formation of monomeric vanadia species with vanadium in tetrahedral coordination. The bridging O in the V-O-Ti bond is responsible for the catalytic activity during the partial oxidn. of toluene. The formation of the monomeric vanadia species correlates with the improved catalyst performance, characterized by reaction rate and selectivity to benzaldehyde and benzoic acid. Mech. activation by intensive grinding of V2O5/TiO2 mixt. via ball milling was necessary for the interaction of the oxides during calcination. The monomeric species formation was obsd. at temps. as low as 523 K. The dynamics of V2O5/TiO2 interaction strongly depends on the presence of moisture during the calcination. (c) 2000 Academic Press. [on SciFinder (R)

Room-temperature nitrophenol reduction over Ag–CeO2 catalysts: the role of catalyst preparation method

Ag–CeO2 catalysts (20 mol % Ag) were synthesized using different techniques (co-precipitation, impregnation, and impregnation of pre-reduced ceria), characterized by XRD, N2 sorption, TEM, H2-TPR methods, and probed in room-temperature p-nitrophenol reduction into p-aminophenol in aqueous solution at atmospheric pressure. The catalyst preparation method was found to determine the textural characteristics, the oxidation state and distribution of silver and, hence, the catalytic activity in the p-nitrophenol reduction. The impregnation technique was the most favorable for the formation over the ceria surface of highly dispersed silver species that are active in the p-nitrophenol reduction (the first-order rate constant k = 0.656 min−1)