Mechanistic Studies of the Water–Gas Shift Reaction over Pt/Ce_<i>x</i>Zr_1–<i>x</i>O₂ Catalysts: The Effect of Pt Particle Size and Zr Dopant

A series of <i>y</i> wt % Pt/Ce_<i>x</i>Zr_1–<i>x</i>O₂ catalysts (<i>y</i> = 0.1, 0.5, and 1.0; <i>x</i> = 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/Ce_0.5Zr_0.5O₂) on important mechanistic and kinetic aspects of the water–gas shift (WGS) reaction. These included the concentration (μmol·g^–1 or θ (surface coverage based on Pt_s)) and chemical structure of <i>active</i> reaction intermediates present in the “carbon path” and “hydrogen path” of the WGS reaction in the 200–300 °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 ¹³CO 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/Ce_<i>x</i>Zr_1–<i>x</i>O₂ 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 °C. The superior WGS activity exhibited by Pt/Ce_<i>x</i>Zr_1–<i>x</i>O₂ (<i>x</i> = 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 <i>active</i> 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