Catalytic partial oxidation of methane over a 4% Rh/ α-Al 2O 3 catalyst Part II: Role of CO 2 reforming

Abstract

A kinetic study of the CO 2 reforming of CH 4 over a 4 wt% Rh/ α-Al 2O 3 catalyst was performed in a short contact time annular reactor. Experiments were carried out under nearly isothermal conditions, at high space velocity ( 2 × 10 6 N l / K g cat / h ), within the temperature range 300–800 °C, at varying feed composition. CO 2/CH 4 tests with excess CO 2 showed a strong similarity with previous H 2O/CH 4 tests. At CO 2/CH 4 = 1 (an experiment characterized by negligible amount of H 2O in the product mixture), the measured conversion of methane was significantly lower. Additional experiments with co-feed of O 2 or H 2 indicated that H 2O had a limiting role on the conversion of CH 4. 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 CO 2 reforming, all the observed trends could be well described as a cycle of H 2O reforming and RWGS (initiated by a trace amount of H 2O 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 CH 4 CPO data at varying reactant concentrations.