Formaldehyde Oxidation over Ag Catalysts

Abstract

H2CO oxidation was studied over Ag powder, high surface area (HSA) α-alumina, and Ag dispersed on both (HSA) α-Al2O3 and SiO2. Ag powder was active above 473 K with reaction orders of unity for O2 and zero for H2CO. All the supported Ag catalysts were active, even below 473 K, but significant deactivation occurred initially at the lower temperatures. Above 473 K the reaction orders for the supported Ag catalysts were near 0.3 for both O2 and H2CO. Turnover frequencies were determined for these Ag catalysts for the first time. Unlike silica, the (HSA) α-aluminas exhibited significant activity above 473 K. In addition to CO2, CO was also a product above 493 K in the presence of alumina, probably due to the decomposition of formate species on the alumina surfaces. The IR spectra for H2CO adsorption on alumina showed that formate groups dominated while significant amounts of dioxymethylene also existed at 303 K. At higher temperatures, dioxymethylene decomposed or oxidized to form formate groups. The spectra for H2CO adsorption on a (HSA) α-alumina-supported Ag catalyst were similar to those for alumina, but a decrease in the formate species was observed after introducing oxygen at 493 K. Formate species were observed on the silica-supported Ag catalyst and were associated with H2CO adsorption on oxygen-covered Ag; introduction of O2 at 493 K after dosing with H2CO resulted in the formation of gas-phase CO2. A reaction model that assumes O2 adsorption and formate decomposition to CO2 to be the two slow steps provided excellent fits for the data for all catalysts.