Kinetic and mechanistic studies of the water-gas shift reaction on Pt/TiO_2 catalyst

摘要

A detailed kinetic and mechanistic study of the water-gas shift (WGS) reaction on a 0.5 wt% Pt/TiO_2 catalyst has been carried out. The dependence of kinetic reaction rate on the partial pressures of reactants (CO, H_2O) and products (H_2, CO_2), and the concentration and chemical structure of active and inactive reaction intermediates that are found in the "hydrogen-path" and "carbon-path" of the reaction have been investigated in the 200-270 °C range. The most likely mechanistic pathway of the WGS reaction over the present catalytic system is discussed. It has been found that the reaction rate increases with an increase in the concentration of CO or H_2O in the feed stream, while it decreases significantly with the addition of H_2 in the feed stream. On the contrary, the kinetic reaction rate was found to be practically independent on the concentration of CO_2 in the feed stream. The experimental reaction rates that were estimated were fitted to an empirical power-law rate expression from which the kinetic reaction orders with respect to CO, H_2O, CO_2, and H_2 were estimated to be 0.5, 1.0, ～0.0, and -0.7, respectively. An apparent activation energy of 10.8 kcal/mol was also estimated. The formation of formate and carbonate surface species over the TiO_2 support under WGS reaction conditions was proved via SSITKA-DRIFTS experiments. However, these reaction intermediates must be considered as inactive (spectator) species for the steady-state WGS reaction. Additional transient experiments that involved ~(18)O-isotope provided strong support for the red-ox mechanism as the prevailing one on the present Pt/TiO_2 catalyst, where labile oxygen and oxygen vacancies of TiO_2 near the metal-support interface can participate in the reaction path of the WGS reaction.