SELF-ACCELERATION IN THE DECOMPOSITION OF ACETIC-ACID ON RH(111) - A COMBINED TPD AND LASER-INDUCED DESORPTION STUDY

Adsorption of acetic acid at 130 K on Rh{111} produces the following phases: multilayers (130-200 K); a mixed acetate/hydrogen monolayer (220-280 K); an acetate monolayer after desorption of hydrogen (> 300 K); and decomposition of acetate into hydrogen and carbon dioxide (similar to 400 K). Room temperature saturation of the clean surface and 200 K saturation of the surface with preadsorbed oxygen and/or potassium leads to sharp H-2 and CO2 temperature-programmed-desorption (TPD) peaks, which are characteristic of self-accelerating processes. Oxygen is found to stabilize the acetate, causing it to decompose at a higher temperature; potassium destabilizes the acetate, and O/K mixtures have an intermediate effect. The results in the absence of co-adsorbed species are discussed in terms of a simple two-step empty-site-creation autocatalytic mechanism for systems in which most of the surface is initially fully saturated with adsorbate species. This mechanism is verified by laser-induced thermal decomposition and desorption. In the presence of co-adsorbates (O and/or K), mixed acetate-coadsorbate dense phases formed on the surface cause the acetate to decompose in an autocatalytic manner, and both electronic interactions as well as site blocking influence the desorption temperature.