Role of metal substitution in lanthanum zirconate pyrochlores (La2Zr2O7) for dry (CO2) reforming of methane (DRM)

Dry (CO2) reforming of methane (DRM) produces syngas with H-2/CO ratio of <= 1. In this work, we report the use of pyrochlores (A(2)B(2)O(7)) as catalysts for this reaction. Here, we examine lanthanum zirconate pyrochlores, LZ, with lanthanum and zirconium occupying the A and B sites, respectively. Three catalysts are tested: LZ and two pyrochlores in which the B-site has been isomorphically substituted with (a) Ru (2 wt%) (designated LRuZ) and (b) Pt (3.78 wt%) (designated LPtZ). These levels of substitution by weight correspond to identical atomic levels of substitution at the B-site. The effect of isomorphic substitution of Ru and Pt on the structure and activity of the pyrochlores in DRM is studied in this work. ICP-OES, XRD, H-2 and CH4 TPR were used to characterize the structure of the catalysts. XRD results confirmed the formation of the La2Zr2O7 phase in the bulk of the pyrochlores. H-2 TPR showed higher reduction temperatures for LPtZ compared to LRuZ, suggesting a less reducible, stable Pt in the pyrochlore structure compared to Ru. Quantitative analysis showed that LRuZ consumed 0.534 mg H-2/gcat, whereas LPtZ consumed only 0.161 mg H-2/gcat corresponding to much greater extent of reduction for LRuZ compared to LPtZ. CO formation during CH4 TPR showed that both materials have reactive lattice oxygen which helps in reducing carbon formation during DRM. Temperature programmed oxidation studies of the catalyst immediately after CH4 TPR showed greater carbon formation over LPtZ (1.67 gc/gcat) compared to LRuZ (1.17 gc/gcat) suggesting that Pt activates CH4 to a greater extent compared to Ru. Temperature programmed DRM surface reaction showed that the light off temperature for LRuZ was about 45 degrees C lower than that of LPtZ. These are novel results and constitute the first report of which we are aware for Pt and Ru substituted pyrochlores for DRM.