The effect of La(3+)-doping of CeO(2) support on the water-gas shift reaction mechanism and kinetics over Pt/Ce(1-x)La(x)O(2- sigma )

• Published in: Applied catalysis. B, Environmental Environmental, 2013-06, Vol.136-137, p.225-238
• Main Authors: Kalamaras, Christos M, Plidou, Klito C, Efstathiou, Angelos M
• Format: Article
• Language: English
• Subjects: Associative; Cerium oxide; Concentration (composition); Mass spectrometry; Nanoparticles; Platinum; Reaction kinetics; Surface chemistry
• ISSN: 0926-3373

Platinum nanoparticles (d(pt)=1.0-1.2 nm) supported on single CeO(2) and La(2)O(3) metal oxides and Pt/Ce(1-x)La(x)O(2- sigma ) 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 (WCS) 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 degree 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- sigma )(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 ( mu mol COg(-1) s(-1)) increased by a factor of 2.0 and 2.8 at 300 degree C on 0.5 wt% Pt supported on Pt/Ce(0.8)La(0.8)O(2- sigma ) 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(1-x)La(x)O(2- sigma ) compared to Pt/CeO(2) and Pt/La(2)O(3). Active-OH groups is suggested to be formed on defect sites (Ce(3+)- phi (s)) of Ce(0.8)La(0.8)O(2- sigma ) 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.