FORMALDEHYDE OXIDATION OVER AG CATALYSTS

H2CO oxidation was studied over Ag powder, high surface area (HSA) alp ha-alumina, and Ag dispersed on both (HSA) alpha-Al2O3 and SiO2. Ag po wder was active above 473 K with reaction orders of unity for O-2 and zero for H2CO. All the supported Ag catalysts were active, even below 473 K, but significant deactivation occurred initially at the lower te mperatures. Above 473 K the reaction orders for the supported Ag catal ysts were near 0.3 for both O-2 and H2CO. Turnover frequencies were de termined for these Ag catalysts for the first time. Unlike silica, the (HSA) alpha-aluminas exhibited significant activity above 473 K. In a ddition to CO2, CO was also a product above 493 K in the presence of a lumina, probably due to the decomposition of formate species on the al umina surfaces. The IR spectra for H2CO adsorption on alumina showed t hat formate groups dominated while significant amounts of dioxymethyle ne also existed at 303 K. At higher temperatures, dioxymethylene decom posed or oxidized to form formate groups. The spectra for H2CO adsorpt ion on a (HSA) alpha-alumina-supported Ag catalyst were similar to tho se for alumina, but a decrease in the formate species was observed aft er introducing oxygen at 493 K. Formate species were observed on the s ilica-supported Ag catalyst and were associated with H2CO adsorption o n oxygen-covered Ag; introduction of O-2 at 493 K after dosing with H2 CO resulted in the formation of gas-phase CO2. A reaction model that a ssumes O-2 adsorption and formate decomposition to CO2 to be the two s low steps provided excellent fits for the data for all catalysts. (C) 1995 Academic Press, Inc.