Metal-support interaction in silver/titania catalysts

Metal-support interaction in silver/titania catalysts has been investigated. The dispersion of Ag/TiO{dollar}sb2{dollar} was measured by hydrogen-oxygen titration at 170{dollar}spcirc{dollar}C following either an oxidation pretreatment at 400{dollar}spcirc{dollar}C or a reduction pretreatment at 500{dollar}spcirc{dollar}C. Two Ag loadings of 1 and 10wt% on TiO{dollar}sb2{dollar} were studied and these were compared to catalysts using a SiO{dollar}sb2{dollar} support. The dispersion is always substantially greater on TiO{dollar}sb2{dollar} than on SiO{dollar}sb2{dollar} for a given pretreatment but the effect of pretreatment has the opposite effect with the two supports. Oxidation results in a pronounced increase in dispersion on SiO{dollar}sb2{dollar} but has a very modest effect on TiO{dollar}sb2{dollar} supported Ag. However, even though TiO{dollar}sb2{dollar} results in a higher dispersion of Ag, this material is not very active catalytically. All catalysts supported on SiO{dollar}sb2{dollar} have turnover frequencies for CO oxidation that are equivalent to unsupported Ag but the rates are suppressed when Ag is supported on TiO{dollar}sb2{dollar} and the degree of suppression is more pronounced on the catalyst of lower loading which has a higher dispersion.; The steady-state oxygen and CO coverages at reaction temperatures have been measured using a pulsed chemisorption technique. On Ag/SiO{dollar}sb2{dollar}, oxygen coverages are higher in the temperature range 60-140{dollar}spcirc{dollar}C and they increase with temperature. On Ag/TiO{dollar}sb2{dollar}, oxygen coverages are very low and, where they can be measured, they decrease with increasing temperature. These results can be used to rationalize both the suppression of CO oxidation activity on Ag/TiO{dollar}sb2{dollar} and the fact that there is a maximum in the Arrhenius plot. This oxygen adsorption suppression is attributed to the formation of a surface compound between Ag and TiO{dollar}sb2{dollar}. The evidence for this conclusion comes from EXAFS and XANES analysis. The EXAFS of oxidized 1%Ag/TiO{dollar}sb2{dollar} is not that of Ag metal or silver oxide. (The reduced 1%Ag/TiO{dollar}sb2{dollar} is a mixture of Ag metal and the new phase.) This new phase tentatively is identified as Ag{dollar}sb2{dollar}TiO{dollar}sb3{dollar}.; The strong interaction of Ag with TiO{dollar}sb2{dollar} relative to SiO{dollar}sb2{dollar} can act as a driving force for migration of Ag from SiO{dollar}sb2{dollar} to TiO{dollar}sb2{dollar}. This has been demonstrated with a physical mixture of 1 wt%Ag/SiO{dollar}sb2{dollar} with pure TiO{dollar}sb2{dollar} under both reducing and oxidizing conditions and followed by the resulting increase in dispersion and decrease in CO oxidation activity.