Low-temperature water-gas shift on Pt/Ce0.8La0.2O2−δ–CNT: The effect of Ce0.8La0.2O2−δ/CNT ratio

Abstract

Hybrid materials of (100−x)wt% Ce0.8La0.2O2−δ–xwt% CNT composition (x=0, 7.5, 20.5, 32.5, 44.1 and 100) were synthesized using the urea-assisted co-precipitation method and used as supports of 0.5wt% Pt toward the low-temperature WGS (LT-WGS) reaction. The main focus of this work was to provide fundamental understanding of the effect of Ce0.8La0.2O2−δ/CNT ratio on the LT-WGS catalytic activity of such materials. It was found that the material containing 44.1wt% CNT presented the best catalytic activity (kinetic rate and CO conversion), result that is correlated with the following parameters: (i) Pt-CO bond strength (TPD-CO), (ii) extent of dispersion of the Ce0.8La0.2O2−δ phase in the hybrid support system and, thus of the Pt phase; the larger dispersion of the Ce0.8La0.2O2−δ phase had a direct impact on its reducibility ability (labile oxygen species), (iii) concentration of surface Ce3+ species (XPS), indication for an increased concentration of oxygen vacant sites, (iv) PtH bond strength (H2-TPD studies), and (v) concentration of active carbon-containing intermediates, “C-pool” formed around each Pt nanoparticle (SSITKA studies). WGS kinetic studies at 300°C revealed that the reaction order with respect to CO was 0.17 and 0.13 for the catalysts containing 20.5 and 44.1wt% CNT, respectively, while the reaction order with respect to H2O was 1.40 for the latter CNT loading. Oxidation of CNTs over the catalyst containing 44.1wt% of CNTs occurred at temperatures larger than 400°C, result of practical importance for the LT-WGS reaction. The proposed WGS reaction mechanism over the present catalytic materials is that of “redox” in parallel with the “associative with –OH group regeneration” mechanism.