Mechanistic Studies of the Water–Gas Shift Reaction over Pt/Ce_xZr_1–xO₂ Catalysts: The Effect of Pt Particle Size and Zr Dopant

Abstract

A series of y wt % Pt/Ce x Zr1–x O2 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 (μmol·g–1 or θ (surface coverage based on Pts)) 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 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 x Zr1–x 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 x Zr1–x O2 (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.

Effect of support composition on the origin and reactivity of carbon formed during dry reforming of methane over 5wt% Ni/Ce1−xMxO2−δ (M=Zr4+, Pr3+) catalysts

“…Such a mechanism was first proposed by Verykios and his co-workers [47,48] over Ni/La 2 O 3 . One of the important kinetic parameters in this bi-functional mechanism is the concentration of catalytic sites along the Ni-support interface, fact that makes more appropriate the estimation of intrinsic kinetic rates in terms of mol cm −1 s −1 and not in terms of TOF (s −1 ) (based on the metal surface), as recently reported for the WGS reaction on Pt supported on similar Ce 1−x M x O 2−ı (M = Zr 4+ , Ti 4+ , La 3+ ) carriers [49,50].…”

Sour water–gas shift reaction over Pt/CeZrO2 catalysts

“…2 and can be ascribed to carbonated species adsorbed onto ceria surface, such as formate, carboxylate, and carbonate species [17,19,20,47]. The bands around 1600 and 1360 cm À1 are mostly likely indicative of formate species; however, it was not possible to confirm them, because there were no bands in the region of 2500-3000 cm À1 that were due to C-H stretching [17,19]. These complementary bands may be difficult to identify because the surface area of the supports is low.…”

“…In the search for more advantageous catalysts for the WGSR, several studies with noble metal catalysts have been reported [9][10][11][12][13][14][15][16][17][18][19][20][21]. The use of noble metal catalysts is related to their capability of being stable in an oxidizing atmosphere.…”

“…Inclusion of zirconia in ceria lattices improves oxygen storage capacity, due to higher oxygen mobility in the formed solid solution [31,32]. Because of these properties, CeZrO 2 supported catalysts have shown, in some conditions, higher activity than when pure CeO 2 or ZrO 2 are used as supports [17,19,33,34]. Despite the better performance of these oxides, there is a lack of publications about the performance of CeZrO 2 -supported noble metal catalysts under sour WGSR conditions.…”

Investigation on CO catalytic oxidation reaction kinetics of faceted perovskite nanostructures loaded with Pt

“…[7][8][9][10][11][12] In particular, the shape and size control of the noble metal catalysts have been proved to signicantly affect the catalytic performance. 13,14 For example, the activation energies for the electron-transfer reaction between Fe(CN) 6 3À and S 2 O 3 2À has been determined to be effectively tailored by the shape of the Pt nanoparticles from tetrahedral to cubic to spherical.…”