CO OXIDATION OVER AU/ZRO2 CATALYSTS - ACTIVITY, DEACTIVATION BEHAVIOR, AND REACTION-MECHANISM

Gold catalysts supported on amorphous zirconia have been prepared by coprecipitation. These catalysts exhibit a high initial CO oxidation activity, which is due to a synergy between the zirconia and the supported gold particles. The activity decreases significantly over a time scale of ≈20 h. The deactivation behavior is only weakly influenced by pretreatments in O2, CO, or CO2. However, the long-time stability is found to depend strongly on the O2/CO feed ratio. With stoichiometric reaction mixtures (O2/CO = 0.5), the activity stays nearly constant over the time of observation. In the presence of excess oxygen (O2/CO = 1 or 40, respectively), rapid deactivation is observed. From a careful monitoring of the carbon balance, and from thermal desorption/mass spectrometry experiments, a progressive covering of the surface by oxygenated carbon species is deduced. Diffuse reflectance FTIR experiments indicate the presence of carbonates, formate, and several types of differently bound CO molecules on the surface. Temperature-dependent experiments reveal that these species are involved in a series of interconnected equilibria, which are coupled to the relevant adsorption/de sorption processes. Oxidized and reduced states of the catalyst can be distinguished. The deactivated state in excess oxygen is characterized by a high concentration of surface formate. On the deactivated catalyst, oxydation of formate and of CO appears to be slow, possibly as a consequence of site blocking. © 1992.