Low-temperature water-gas shift on Pt/Ce0.5La0.5O2 - δ: Effect of support synthesis method

Abstract

A series of 0.5 wt% Pt/Ce0.5La0.5O2 - δ (Ce:La = 1:1) catalysts, the supports of which were prepared by different methods, namely: (i) sol-gel using citrate or oxalate as complexing agent, (ii) pechini, and (iii) urea co-precipitation, were investigated for the first time towards the water-gas shift (WGS) reaction in the 250-350 °C range and 1 atm total pressure. Towards a better understanding of the effect of support synthesis method on the intrinsic kinetic rate of WGS expressed per gram of catalyst (μmol CO g-1 s-1) or per length of the perimeter of Pt-support interface (μmol CO cm-1 s-1), a suite of various characterisation methods such as: in situ Raman, temperature-programmed techniques (TPD-H2, TPD-NH3, TPD-CO2), powder XRD, and oxygen storage capacity (OSC) measurements were applied. The intrinsic kinetic rate of WGS (μmol CO g-1 s-1) was correlated with the concentration of the active "carbon-containing" (C-pool) and "hydrogen-containing" (H-pool) reaction intermediates formed within a reactive zone (Δx, Å) around each Pt nanoparticle (1.2-1.5 nm), parameters that were estimated via SSITKA and non steady-state transient isotopic and titration with water operando experiments. The urea co-precipitation method (U) resulted in the formation of a Ce1 - xLaxO2 - δ solid solution with different composition (Ce:La atom ratio) than that formed by the other synthesis methods, which may be the main reason for Pt/Ce0.5La0.5O2 - δ (U) to exhibit the highest by far CO conversion and kinetic rate towards the WGS compared to the other supported Pt catalysts. The same method (U) resulted in the formation of La2O3 as opposed to the other methods. However, this was not considered as the main reason for explaining the higher activity of Pt supported on Ce0.5La0.5O2 - δ (U) compared to the other carriers. The Ce0.5La0.5O2 - δ (U) was also found to possess the highest surface acidity and basicity compared to the other supports but lower OSC (μmol g-1) (by more than 30% in the 250-550 °C range) than Ce0.5La0.5O2 - δ prepared by the citrate sol-gel method, in harmony with the lower content of O vacancies in Ce0.5La0.5O2 - δ (U) as evidenced by Raman studies. Transient DRIFTS formate (HCOO-) decomposition kinetic experiments towards CO2 and H2 formation have illustrated the importance of the presence of Pt and support composition. © 2014 Elsevier B.V. All rights reserved.