A kinetic and DRIFTS study of low-temperature carbon monoxide oxidation over Au-TiO2 catalysts

Titania-supported gold catalysts however, deactivation is observed Impregnated Au-TiO2 is most active after a sequential pretreatment consisting of high temperature reduction at 773 K, calcination at 673 K and low temperature reduction at 473 K (HTR/C/LTR); the activity after either only low temperature reduction or calcination is much lower. A catalyst prepared by coprecipitation had much smaller Au particles than impregnated Au-TiO2 and was active at 273 K after either an HTR/C/LTR or a calcination pretreatment. Deposition of TiOx overlayers onto an inactive Au powder produced high activity; this argues against an electronic effect in small Au particles as the major factor contributing to the activity of Au-TiO2 catalysts and argues for the formation of active sites at the Au-TiOx interface produced by the mobility of TiOx species. DRIFTS (diffuse reflectance FTIR) spectra of impregnated Au-TiO2 reveal the presence of weak reversible CO adsorption on the Au surface but not on the TiO2; however, a band for adsorbed CO is observed on the pure TiO2. Kinetic studies with a 1.0 wt.-% impregnated Au-TiO2 sample showed a near half-order rate dependence on CO and a near zero-order rate dependence on O-2 between 273 and 313 K with an activation energy near 7 kcal/mol. A two-site model, with CO adsorbing on Au and O-2 adsorbing on TiO2, is consistent with Langmuir-Hinselwood kinetics for noncompetitive adsorption, fits partial pressure data well and shows consistent enthalpies and entropies of adsorption. The formation of carbonate and carboxylate species on the titania surface was detected but it appears that these are spectator species. DRIFTS experiments under reaction conditions also show the presence of weak, reversible adsorption of CO2 (near 2340 cm(-1)) which may be competing with CD for adsorption sites.