A kinetic and DRIFTS study of low-temperature carbon monoxide oxidation over Au—TiO 2 catalysts

Abstract

Titania-supported gold catalysts are extremely active for room temperature CO oxidation; however, deactivation is observed over long periods of time under our reaction conditions Impregnated Au TiO 2 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 TiO 2 and was active at 273 K after either an HTR/C/LTR or a calcination pretreatment. Deposition of TiO x 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 TiO 2 catalysts and argues for the formation of active sites at the Au TiO x interface produced by the mobility of TiO x species. DRIFTS (diffuse reflectance FTIR) spectra of impregnated Au TiO 2 reveal the presence of weak reversible CO adsorption on the Au surface but not on the TiO 2; however, a band for adsorbed CO is observed on the pure TiO 2. Kinetic studies with a 1.0 wt.-% impregnated Au TiO 2 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 TiO 2, 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 car☐ylate 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 CO 2 (near 2340 cm −1) which may be competing with CO for adsorption sites.