The role of catalyst in supercritical water oxidation of acetic acid

The oxidation kinetics of acetic acid in supercritical water (1.02 less than or equal to T-r less than or equal to 1.15 and 1.04 less than or equal to P-r less than or equal to 1.13) was examined in the homogeneous phase as well as in the presence of a solid catalyst consisting of supported copper, zinc, and cobalt oxides. For the conditions studied, the uncatalyzed oxidation reaction was found to be first order in acetic acid and 0.3 order in oxygen, with an activation energy of 182 kJ mol(-1). The rate of catalyzed oxidation was found to be well described by means of the power-law kinetic formulation based on non-uniform surfaces. It is postulated that oxygen is adsorbed on active sites and that a reaction between adsorbed species and organic molecules from the van der Waals sublayer forms a carbonate complex which further decomposes to carbon dioxide and water. The apparent activation energy of catalyzed oxidation is 110 kJ mol(-1). The observed products in uncatalyzed supercritical water oxidation were carbon monoxide, carbon dioxide, and water. The oxidation of acetic acid over transition metal oxides favors the production of carbon dioxide over carbon monoxide.