The effect of La~(3+)-doping of CeO2 support on the water-gas shift reaction mechanism and kinetics over Pt/Ce_(1-x)La_xO_(2-δ)

摘要

Platinum nanoparticles (d_(Pt) = 1.0-1.2nm) supported on single CeC>2 and La2O3 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 D2O) 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_xO_(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 CeO2 and La2O3, 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 CO2 and H2 formation on Pt/Ce_(0.8)La_(0.2)o_(2-δ) compared to Pt/CeO2 and Pt/La2O3. 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.