ON THE ADSORBATES FORMED DURING THE PLATINUM-CATALYZED (ELECTRO)OXIDATION OF ETHANOL, 1,2-ETHANEDIOL AND METHYL-ALPHA-D-GLUCOPYRANOSIDE AT HIGH PH

The irreversible adsorbates of ethanol, 1,2-ethanediol and methyl-alph a-D-glucopyranoside (MGP) have been studied with FTIRS and cyclic volt ammetry. Both ethanol and 1,2-ethanediol display full C-C(OH) dissocia tive adsorption and dehydrogenation. In the case of ethanol adsorbed C O and C are formed of which the latter partially oxidizes further to a dsorbed CO. In the case of 1,2-ethanediol CO is formed as the only ads orbate. The adsorption of MGP occurs similarly to the small alcohols; it decarbonylates to form adsorbed CO and a small fraction of C adatom s. It is shown that the catalytic alcohol oxidation can be regarded as an electrochemical process that consists of two independently acting half-reactions that determine the open circuit potential (o.c.p.). The roughness of the surface greatly affects the o.c.p. measured during c atalytic alcohol oxidation; smooth platinum leads to high o.c.p. value s and platinized platinum leads to low o.c.p. values. These low and hi gh open circuit potentials correspond respectively to a diffusion limi ted regime where diffusion of oxygen is rate limiting and a kinetic re gime. The reaction rate is considerably lower in the kinetic regime th an in the diffusion limited regime. The surface is highly covered with adsorbed oxygen or hydroxyl during oxidation of ethanol and MGP in th e kinetic regime, whereas the surface is devoid of adsorbed oxygen in the diffusion limited regime and is instead covered with a high steady state amount of CO and C species. The deactivation of the catalyst is found to occur both in the diffusion limited and in the kinetic regim e of the MGP oxidation. Whereas in the diffusion limited regime, the d eactivation is caused by a slow accumulation of carbonaceous residue, and in the kinetic regime, changes in the properties of adsorbed oxyge n cause deactivation. (C) 1997 Elsevier Science B.V.