Mechanistic Studies of the Water-Gas Shift Reaction over Pt/ Ce x Zr 1-x O 2 Catalysts: The Effect of Pt Particle Size and Zr Dopant C. M. Kalamaras, † D. D. Dionysiou, ‡ and A. M. Efstathiou* ,† † Department of Chemistry, Heterogeneous Catalysis Laboratory, University of Cyprus, University Campus, CY 1678, Nicosia, Cyprus ‡ Department of Civil, Environmental Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0071, United States * S Supporting Information ABSTRACT: A series of y wt % Pt/Ce x Zr 1-x O 2 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 CeO 2 doping with Zr 4+ and of Pt particle size (Pt/Ce 0.5 Zr 0.5 O 2 ) 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 active 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 D 2 O and 13 CO 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 x Zr 1-x O 2 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 x Zr 1-x O 2 (x = 0.3, 0.5, and 0.7) catalysts in comparison with Pt/CeO 2 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 Zr 4+ 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. KEYWORDS: WGS reaction mechanism, ceria-zirconia, SSITKA-DRIFTS, SSITKA-MS, supported Pt, transient isotopic techniques 1. INTRODUCTION The concomitant development of a fuel processor in which carbonaceous fuels are converted into hydrogen (H 2 ) is in increasing demand. 1 The heterogeneously catalyzed water-gas shift (WGS) reaction (CO + H 2 O ↔ CO 2 +H 2 , ΔH o = -41.2 kJ/mol) is a key step in a fuel processor and in a number of chemical processes, including steam reforming of hydrocarbons, sugars, alcohols, and bio-oil, which can increase the H 2 concentration in the product gas and at the same time reduce the CO concentration. 2-6 The low-temperature (180-300 °C) operating industrial Cu/ZnO/Al 2 O 3 WGS reaction catalyst is pyrophoric and deactivates if exposed to air and condensed water, and it is characterized by low thermal stability. 7,8 Attempts have recently been focused toward the development of low-loading robust noble metal-based catalysts with high activity at low temper- atures and that are nonpyrophoric to reduce catalyst volume and cost. 5 Pt metal supported on reducible metal oxide carriers appears to be a promising candidate for replacing current industrial low-temperature WGS catalysts. 6 To increase the intrinsic catalytic activity of supported Pt at low temperatures, various supports, including CeO 2 , 5,9-17 ZrO 2 , 11-13,17-19 and Ce x Zr 1-x O 2 13,19-24 have been examined. However, the stability of these catalysts under practical conditions is still problematic and depends on the synthesis method employed. 25 An important aspect in the design of such catalytic systems is the development of a metal oxide support with small grain size, high surface area, controlled porosity, and tailor-designed pore size distribution in an effort to enhance catalytic activity and stability with time on-stream. Fundamental understanding of the WGS reaction at the molecular level is certainly an important tool toward the design of suitably functional catalytic materials for activity, selectivity, and stability optimization under industrial WGS reaction conditions. To achieve this goal, mechanistic studies of the WGS reaction employing in situ coupled spectroscopic and kinetic measurements under reaction conditions (operando studies) become important. 26,27 Received: September 20, 2012 Revised: November 4, 2012 Published: November 5, 2012 Research Article pubs.acs.org/acscatalysis © 2012 American Chemical Society 2729 dx.doi.org/10.1021/cs3006204 | ACS Catal. 2012, 2, 2729-2742