Low-temperature redox activity in co-precipitated catalysts: a comparison between gold and platinum-group metals

We have identified two main mechanisms for inducing low-temperature activity in precious metal catalysts: normal-support activation (NSA) and active-phase enhancement (APE). The usual roles of a precious metal and its metal oxide support are reversed in NSA, resulting in highly active sites being created on the metal oxide. This applies when metallic gold is well dispersed within a defect-forming metal oxide (such as CeO2 or even ZrO2), and leads to formation of new sites for CO, NO and alkene conversion (to CO2, N-2 and H2O). In APE, established metal oxide catalysts can be deliberately made to operate at lower temperatures, by providing precious metal sites for oxygen activation. An example is iron(III) oxide, where incorporation of palladium leads to substantial lowering of the temperatures for CO oxidation, water-gas shift and oxidative dehydrogenation. Gold does not have the same effect, except for CO oxidation at high metal loading. However, the presence of gold or palladium in a hydroxylated metal oxide can lead to another route to low-temperature oxidation, in which highly reactive peroxide-like intermediates are believed to be formed on exposed metal surfaces. (C) 2002 Elsevier Science B.V. All rights reserved.