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Surface chemistry of catalysis by gold

“…In previous IRAS studies on different catalysts systems like gold supported on zeolites (373), Au/ZrO2 (374), Au/TiO2 (374-376) and Au/Fe2O3 (377,378) similar CO stretching bands frequencies have been also detected and assigned to CO adsorbed on Au + species or Au interacting with hydroxyl groups on the surface (see Table 2). However, in our experiment, this feature appears after the main peak observed at ~2 108 cm -1 and therefore implies that CO adsorbs on such species more weakly relative to the metal clusters as has been seen by Fan et al (379), although this is in apparent contradiction with the view of some investigators that charged gold adsorbs CO more strongly (373,378,(380)(381)(382). Interestingly, the single peak observed in the annealed samples did not shift and simply grew monotonically at increasing Au coverage, supporting our finding from TPD experiments indicating no apparent change in the possible adsorption states for CO on monolayer islands as compared to large particles.…”

“…Others have attributed the presence (403,432,506) or absence (507,508,512,513) of water as a contributing factor to deactivation. Minico et al ascribed deactivation of Au/Fe2O3 to an irreversible reduction of the gold from the 1+ state to metallic gold (378). For the selective oxidation of propylene, Au/TiO2 catalysts also may experience problems with coking (527) as a result of propoxy decomposition at the acid sites of the titanosilicate support (523).…”

“…Based on IR data which showed CO adsorption 2040 cm -1 , considered to be on sites other than metallic gold, and considering the lack of activity for unsupported bulk gold, the authors proposed that these adsorption sites may be related to gold that is electronically modified by the support due to charge transfer. More recently, infrared spectroscopy of CO adsorption on gold catalysts has revealed features between 2150-2160 cm -1 (as seen in Table 2) which has led some researchers to invoke a mechanism which involves Au + (378,382,445). XPS and FTIR experiments by Galvagno and co-workers have shown a loss of Au + species over time when Au/Fe2O3 catalysts were exposed to reactive mixtures (378,446).…”

“…More recently, infrared spectroscopy of CO adsorption on gold catalysts has revealed features between 2150-2160 cm -1 (as seen in Table 2) which has led some researchers to invoke a mechanism which involves Au + (378,382,445). XPS and FTIR experiments by Galvagno and co-workers have shown a loss of Au + species over time when Au/Fe2O3 catalysts were exposed to reactive mixtures (378,446). The authors concluded that the deactivation of the catalyst over time was directly related to the loss of Au + and the growth of Au 0 .…”

Gold supported on iron oxy-hydroxides: a versatile tool for the synthesis of fine chemicals

“…However, gold in a higher oxidation state has been also shown to chemisorb carbon monoxide [37]. 197 Au Mossbauer spectroscopy [25] and XPS spectroscopy [26], have revealed that when the catalysts are not calcined at high temperature (as in the case of the catalysts reported in this paper) gold is mainly present as Au(III).…”