On the adsorbates formed during the platinum catalyzed (electro)oxidation of ethanol, 1,2-ethanediol and methyl-α- d-glucopyranoside at high pH

Abstract

The irreversible adsorbates of ethanol, 1,2-ethanediol and methyl-α- d-glucopyranoside (MGP) have been studied with FTIRS and cyclic voltammetry. Both ethanol and 1,2-ethanediol display full CC(OH) dissociative adsorption and dehydrogenation. In the case of ethanol adsorbed CO and C are formed of which the latter partially oxidizes further to adsorbed CO. In the case of 1,2-ethanediol CO is formed as the only adsorbate. The adsorption of MGP occurs similarly to the small alcohols; it decarbonylates to form adsorbed CO and a small fraction of C adatoms. 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 catalytic alcohol oxidation; smooth platinum leads to high o.c.p. values and platinized platinum leads to low o.c.p. values. These low and high open circuit potentials correspond respectively to a diffusion limited regime where diffusion of oxygen is rate limiting and a kinetic regime. The reaction rate is considerably lower in the kinetic regime than in the diffusion limited regime. The surface is highly covered with adsorbed oxygen or hydroxyl during oxidation of ethanol and MGP in the 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 regime of the MGP oxidation. Whereas in the diffusion limited regime, the deactivation is caused by a slow accumulation of carbonaceous residue, and in the kinetic regime, changes in the properties of adsorbed oxygen cause deactivation.