Kinetic and mechanistic study of dry (CO2) reforming of methane over Rh-substituted La2Zr2O7 pyrochlores

Abstract

The active sites and kinetics of dry reforming of methane (DRM) on lanthanum zirconate (LZ) pyrochlore catalysts are studied as a function of Rh substitution, temperature, and partial pressures of CH4 and CO2. In this work, we focus specifically on determining the catalytic active sites for CH4 and CO2 activation and their role in the DRM mechanism over the two Rh-substituted pyrochlores, i.e., 2wt% Rh, designated L2RhZ and 5wt% Rh, designated L5RhZ. Kinetic rate modeling suggests dual-site mechanism, where CH4 and CO2 are activated on different sites (dual-site mechanism). Eleven different rate models were tested against the kinetic rate data obtained over these two Rh pyrochlores. Statistical analysis shows valid and similar fits for only two of eleven models: one in which activation of CH4 is rate-determining and one in which CO2 activation is rate-determining. This dual-site mechanism is studied further in detail to test the validity of the intermediate steps predicted by the kinetic model. CH4/CD4 isotope switching shows a strong deuterium kinetic isotope effect on CH4 and CO2 conversion, suggesting that CH4 dissociation is the rate-determining step. Higher apparent activation energies of CH4 (Ea, CH4) versus CO2 (Ea, CO2) on both catalysts also confirm that CH4 activation is rate-determining. Basic nature of La–O sites activates mildly acidic CO2 to form La2O2CO3 complexes as confirmed by FTIR. From different polymorphs of La2O2CO3, the spectator and reactive species were distinguished by pulsing CH4 over La2O2CO3. Alternating pulses (CH4/Ar→CO2/Ar→CH4/Ar) at 550°C showed simultaneous formation of CO and H2, suggesting that surface carbon, formed by CH4 decomposition, is oxidized by H2O; a crucial step in understanding the catalytic behavior of pyrochlores in the reaction mechanism. These experiments were used to identify the two types of sites taking part in the dual-site DRM mechanism. The work reported here helps in determining a single set of kinetically significant steps that most closely represent the mechanistic scheme of DRM over L2RhZ and L5RhZ.