Low-temperature water-gas shift on Pt/Ce1−xLaxO2−δ: Effect of Ce/La ratio

Abstract

Pt nanoparticles (1.0–1.4nm size) supported on Ce1−xLaxO2−δ (x=0.0, 0.2, 05, 0.8 and 1.0) carriers, the latter prepared by the citrate sol–gel method, were tested toward the water-gas shift (WGS) reaction in the 200–400°C range. A deep insight into the effect of Ce/La atom ratio of support chemical composition on the catalytic performance (CO conversion vs. temperature and stability) and kinetic rates of Pt-loaded catalysts was realized after employing HAADF/STEM, in situ Raman and DRIFT spectroscopies under different gas atmospheres, temperature-programmed surface reaction (TPSR) in He and O2/He gas atmospheres following WGS reaction, CO-TPD, in situ UV–vis/DRS, oxygen storage capacity measurements, and transient 18O-isotopic exchange studies followed by WGS reaction. It was found that doping of ceria with 20at.% La3+ has increased significantly the catalytic activity of 0.5wt% Pt/Ce0.8La0.2O2−δ solid in the 250–350°C range, whereas addition of 50–80at.% La3+ in ceria caused a negative effect on the CO conversion with respect to pure ceria. It was found that the Ce/La atom ratio in Ce1−xLaxO2−δ influences the catalytic site reactivity (k) along the Pt-support interface. The optimum La3+-dopant concentration of 20at.% (Ce/La=4/1) used in Pt/Ce0.8La0.2O2 compared to the worst one of 80at.% (Pt/Ce0.2La0.8O2−δ, Ce/La=1/4) correlates with (i) the higher specific kinetic rate per length of Pt-support interface (μmol COcm−1s−1), (ii) the higher concentration of oxygen vacant sites, (iii) the lower amount (μmol/g−1) of “carbon” accumulated during WGS and best stability with time on stream, (iv) the lower apparent activation energy (kcalmol−1) of WGS reaction, (v) the lower degree toward Pt oxidation (largest Pt2+/Pt4+ ratio), (vi) the lower Ce1−xLaxO2−δ support energy band gap, and (vii) the lower mobility of surface lattice oxygen.