Acetic Acid Reduction by H2 over Supported Pt Catalysts: A DRIFTS and TPD/TPR Study

Abstract

The utilization of DRIFTS combined with TPD and TPR experiments resulted in the identification of molecular acetic acid, acyl species, and bidentate acetate species present on the titania surface plus CO adsorbed on Pt following acetic acid adsorption on Pt/TiO2 catalysts at 300 K. TPR studies in the presence of H2 provided evidence that reduction of acetic acid on TiO2-supported Pt begins via a reaction between adsorbed H atoms from the Pt surface and an acyl species on titania, and this pathway serves as the major route for production of acetaldehyde, which can be further hydrogenated to ethanol. The stability of the acetate species under the steady-state reaction conditions employed here discounts it as an active intermediate; however, at temperatures above 500 K these acetate groups may also become reactive intermediates to form acetaldehyde. The absence of both surface acyl species and stable acetate species on Pt/SiO2 accounts for the lack of reduction activity on this catalyst and explains why only decomposition products were formed. Based on this new spectroscopic information, a reaction mechanism proposed recently was modified slightly to incorporate an acyl species as the principal intermediate, rather than molecular acetic acid, and the resulting rate expression not only gives an identical fit to the kinetic data, but also possesses thermodynamically meaningful parameters, such as the heat of adsorption of H2 on Pt, that are consistent with literature values.