Mechanistic Studies of the Water-Gas Shift Reaction over Pt/CexZr1-xO2 Catalysts: The Effect of Pt Particle Size and Zr Dopant

A series of y wt % Pt/CexZr1-xO2 catalysts (y = 0.1, 0.5, and 1.0; x = 0.3, 0.5, and 0.7) were synthesized and characterized to investigate the effect of CeO2 doping with Zr4+ and of Pt particle size (Pt/Ce0.5Zr0.5O2) on important mechanistic and kinetic aspects of the water-gas shift (WGS) reaction. These included the concentration (mu mol.g(-1) or theta (surface coverage based on Pt-s)) and chemical structure of active reaction intermediates present in the "carbon path" and "hydrogen path" of the WGS reaction in the 200-300 degrees C range and the prevailing mechanism among "redox" and "associative formate" largely considered in the literature. Toward this goal, steady-state isotopic transient kinetic analysis coupled with in situ DRIFTS and mass spectrometry experiments were performed for the first time using D2O and (CO)-C-13 isotopic gases. A novel transient isotopic experiment allowed quantification of the initial transient rate of reaction of adsorbed formate (HCOO-) with water and that of adsorbed CO with water under steady-state WGS reaction conditions. On the basis of these results, it was concluded that formate should not be considered as an important intermediate. It was found that on Pt/CexZr1-xO2 catalysts, the WGS reaction mechanism switches from "redox" to a combination of "redox" and "associative formate with -OH group regeneration" mechanisms by increasing the reaction temperature from 200 to 300 degrees C. The superior WGS activity exhibited by Pt/CexZr1-xO2 (x = 0.3, 0.5, and 0.7) catalysts in comparison with Pt/CeO2 was explained by the fact that the site reactivity of Pt across the metal-support interface was increased as a consequence of the introduction of Zr4+ into the ceria lattice. The concentration of active reaction intermediates was found to strongly depend on reaction temperature, support composition (Ce/Zr ratio), and Pt particle size, parameters that all determine the shape of the light-off CO-conversion curve.