Formic acid decomposition over V-Ti oxide catalyst: Mechanism and kinetics

Abstract

A detailed kinetic model for formic acid (HCOOH) decomposition on oxide vanadium-titanium (7.3%V2O5/92.7%TiO2) catalyst has been developed based on transient kinetic studies and IR spectroscopy in situ. Formic acid decomposition to CO2 (dehydrogenation) and to CO (dehydration) proceeds via different mechanistic routes. Dehydrogenation proceeds via the conventional “formate” mechanism and include reversible dissociative adsorption of formic acid on vanadyl group V=O. The oxygen of the group acts as a proton acceptor whereas a vanadium cation is a formate anion stabilizer. Formate complexes can recombinate with protons and desorb in the form of formic acid or decompose to CO2. Dehydration reaction follows a different mechanism. An interaction of the coordination unsaturated sites V+-O− with HCOOH leads to the synchronous cleavage of the COH and CH bonds. Meanwhile the two OH hydroxyl groups are formed and CO molecule is evolved into gas phase. The path is completed by water desorption.The rate constants and activation energies of elementary steps are calculated for a dehydrogenation and dehydration reactions. The rate-limited steps of the reaction are formate decomposition stage for dehydrogenation path (Ea=60kJ/mol) and water desorption stage for dehydration rout (Ea=100kJ/mol). The difference in the values of activation energies results in an increase of selectivity towards CO with reaction temperature elevation. Water inhibits formic acid decomposition. In the case the dehydrogenation, it connected with adsorption displacement of formates with water, whereas, in the case of dehydration, it is associated with an active centers blocking by adsorbed water. The developed kinetic model takes into account the water effect of both routs and satisfactorily describes the steady state reaction behavior.