Master of Science in Chemical Engineering (MSChE)
Dry reforming of methane (DRM) has been widely investigated, with most studies showing rapid deactivation due to carbon deposition. This suggests a need to develop catalysts that limit carbon formation while avoiding structural changes at the elevated temperatures typical of this reaction. Here, we report CO2 reforming of methane on four pyrochlore catalysts. First, Rh was partially substituted for Zr in lanthanum zirconate (La2Zr2O7) to give La2Zr2-xRhxO7.5 (LRZ, x=0.112, 2 wt% Rh) pyrochlore. A second pyrochlore catalyst was synthesized in which Ca was further substituted into the La-site to give La1.95Ca0.05Zr2-xRhxO7 (LCRZ, x=0.055, 1 wt% Rh). A third catalyst was synthesized where Ni was substituted in the Zr-site to give La2Zr2-xNixO7.5 (LNZ, x=0.112, 1 wt% Ni). A fourth catalyst, containing no catalytically active metal, La1.97Sr0.03Zr2O7.5 (LSZ), was synthesized to provide a direct comparison to the substituted pyrochlores. Effects of substitution and kinetic measurements were examined for dry reforming of methane in a fixed-bed reactor. Results from XRD prior to reaction showed that all the pyrochlore catalysts had a cubic unit-cell lattice. Temperature programmed reduction (TPR) of the catalysts suggested the presence of two reducible Rh species in LCRZ and LRZ, and four reducible Ni species in LNZ. Textural measurements revealed that among the active catalysts, LCRZ had the highest BET surface area (10.0 m2/g) and pore volume (0.10 cc/g). Temperature programmed surface reaction (TPSR) tests indicated different light-off curves for different catalysts, with LCRZ being the most active by this measure. Steady state tests at 750°C using an equimolar reactant feed for 450 min showed that the Ni-substituted pyrochlore (LNZ) deactivated rapidly. The catalysts LCRZ and LRZ showed similar activity, however, LCRZ showed lower carbon build-up. XRD of the spent catalysts showed that the pyrochlore structure was unchanged during reaction for all catalysts. Carbon deposited on catalyst surface during reaction was characterized by TPO. The Ni based pyrochlore (LNZ) showed higher carbon deposition (1.4 g/gcat.) than both LCRZ (0.26 g/gcat.) and LRZ (0.44 g/gcat.). These results suggested that Rh substituted into the pyrochlore was more active and selective for synthesis gas compared to a directly comparable atomic loading of Ni. In addition, the doping of Ca and Rh on La and Zr sites may create oxygen vacancies in the crystal lattice resulting in higher oxygen ion mobility resulting in the oxidation of carbonaceous species deposited on the active sites on the catalyst during the reaction. Substitution of metals into the crystal lattice might have also lowered the bond-energy of La-O and Zr-O lattice bonds resulting in the release of oxygen from the lattice, which probably oxidizes surface carbon thus slowing the carbon build-up process on the catalyst during the reaction.
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Gaur, Sarthak, "Dry reforming of methane on rhodium and nickel substituted pyrochlore catalysts" (2011). LSU Master's Theses. 1341.