Catalytic partial oxidation of methane over a 4% Rh/α-Al2O3 catalyst Part II: Role of CO2 reforming

A kinetic study of the CO2 reforming of CH4 over a 4 wt% Rh/α-Al2O3 catalyst was performed in a short contact time annular reactor. Experiments were carried out under nearly isothermal conditions, at high space velocity (2×106 Nl/Kgcat/h), within the temperature range 300–800 °C, at varying feed composition. CO2/CH4 tests with excess CO2 showed a strong similarity with previous H2O/CH4 tests. At CO2/CH4 = 1 (an experiment characterized by negligible amount of H2O in the product mixture), the measured conversion of methane was significantly lower. Additional experiments with co-feed of O2 or H2 indicated that H2O had a limiting role on the conversion of CH4. A quantitative analysis of data was performed by means of a 1D heterogeneous model of the reactor, by assuming that steam reforming and reverse water gas shift were uniquely active and proceeded according to kinetics that were estimated on the basis of independent data. Though neglecting the rate of CO2 reforming, all the observed trends could be well described as a cycle of H2O reforming and RWGS (initiated by a trace amount of H2O in the feed) wherein the rate determining step (either SR or RWGS) depends on the gas-phase composition. Finally, experiments confirmed that the addition of CO to the reaction mixture partly slowed down the kinetics of methane activation, which had been indirectly postulated in Part I of this work on the basis of CH4 CPO data at varying reactant concentrations.

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