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A series of x wt.% Pt/CeO2 catalysts (x = 0.1-2.0) was prepared to investigate for the first time the effect of Pt particle size on important kinetic and mechanistic aspects of the water-gas shift (WGS) reaction, namely the concentration (μmol g-1) and chemical structure of active adsorbed reaction intermediates present in the "carbon-path" and "hydrogen-path" of the WGS reaction at 300 °C. For this, steady-state isotopic transient kinetic analysis (SSITKA) coupled with in situ DRIFTS and mass spectrometry experiments was performed using D2O and 13CO. α novel transient isotopic experiment performed allowed to quantify the initial transient rates of reactions of adsorbed formate (-COOH) and CO by water, based on which it was concluded that formate should not be considered as an important intermediate. According to the present work, it is proposed that the WGS reaction on ceria-supported Pt at 300 °C occurs largely via the "redox" mechanism, and to a lesser extent via the "associative formate with -OH group regeneration" mechanism. The TOF (s-1) of WGS was found to vary only slightly with Pt particle size (1.3-8.0 nm), while the specific reaction rate based on the length of periphery of Pt-CeO2 interface (μmol cm-1 s-1) was found to significantly increase with increasing platinum loading and mean particle size in the 250-300 °C range. © 2011 Elsevier Inc. All rights reserved. 279 2 287 300 Cited By :110

Platinum nanoparticles ( d Pt = 1.0–1.2 nm) supported on single CeO 2 and La 2 O 3 metal oxides and Ce 0.8 La 0.2 O 2− δ solid solution were prepared to investigate for the first time the effect of La 3+ -doping of ceria on important mechanistic and kinetic aspects of the water-gas shift (WGS) reaction, namely: (i) the concentration and chemical structure of active adsorbed reaction intermediates present in the C-path and H-path of WGS at 250 and 300 °C, (ii) the chemical nature of inactive species formed during WGS, and (iii) the prevailing mechanistic path among “redox” and “associative” both proposed in the literature. For this, steady-state isotopic transient kinetic analysis (SSITKA) experiments coupled with in situ DRIFTS and mass spectrometry were performed to follow the H-path (use of D 2 O) and C-path (use of 13 CO) of the WGS. In addition, other transient isotopic experiments using operando methodology (use of DRFTS and mass spectrometry) were designed to follow with time on stream the reactivity toward water of the various adsorbed species formed under WGS. It is proposed that on Pt/Ce 1− x La x O 2− δ ( x = 0.0, 0.2 and 1.0) the WGS reaction follows both the “redox” and “associative” mechanisms but the extent of participation of each mechanism to the overall WGS reaction rate depends on the support chemical composition. The WGS kinetic rate (μmol CO g −1 s −1 ) increased by a factor of 2.0 and 2.8 at 300 °C on 0.5 wt% Pt supported on Ce 0.8 La 0.2 O 2− δ compared to CeO 2 and La 2 O 3 , respectively. This was explained by (i) the larger concentration of active surface intermediates formed around each Pt nanoparticle (larger extent of reactive zone) and (ii) the higher reactivity of sites ( k , s −1 ) responsible for CO 2 and H 2 formation on Pt/Ce 0.8 La 0.2 O 2− δ compared to Pt/CeO 2 and Pt/La 2 O 3 . Active OH groups is suggested to be formed on defect sites (Ce 3+ □ s ) of Ce 0.8 La 0.2 O 2− δ as a consequence of the introduction of La 3+ into the ceria lattice, the latter enhancing the concentration of labile oxygen and its surface mobility, important characteristics of the “redox” mechanism.