Temperature Programmed Desorption of Ethanol over TiO₂ and M/TiO₂ (M = Au, Pd and Au–Pd) Catalysts: Dehydration Versus De-carbonylation Pathways

The reactions of ethanol over TiO₂ and M/TiO₂ (M = Au, Pd and Au–Pd) catalysts are studied by temperature programmed desorption (TPD), in high vacuum conditions, in order to study the effects of metal nature on desorption profiles and products distribution. Ethanol reacted through dehydration (major) and dehydrogenation (minor) pathways on TiO₂ and Au/TiO₂ producing ethylene and acetaldehyde, with negligible hydrogen formation. On Pd/TiO₂ and Au–Pd/TiO₂, carbon monoxide and methane were the main products, through C–C bond cleavage, in addition to H₂. TPD of ethanol on Au-Pd was almost identical to that of Pd indicating the much higher activity of Pd when compared to Au on TiO₂. Ethanol desorption from all catalysts followed second order kinetics as evidenced from desorption peak shape, shift in peak temperatures with increasing initial coverages, and by data fitting of the Arrhenius plots. With increasing ethanol coverage, the activation energies calculated from the non-linear fitting of Polanyi Wigner equation as well as from leading edge analysis decreases due to repulsive lateral interactions. Both methods gave similar results (activation energy between 145 and 95 kJ/mol depending on the surface coverage), although the activation energy obtained by the leading edge method was more sensitive to surface coverage.