The CO2−CH4 reaction on Rh/Al2O3 was studied by in situ infrared spectroscopy coupled with pulse and step transient techniques. Steady-state isotopic 13CO transient studies at 773 K and 0.1 MPa show that the formation of gaseous 13CO2 closely follows that of linear 13CO, indicating that linear CO is an active adsorbate. Pulsing CH4 into CO2 flow and step switching from He to CO2/CH4 flow showed that the formation of H2 led that of CO, revealing that the first step of the reaction sequence is the decomposition of CH4 into *CHx species and hydrogen. Hydrogen activated adsorbed CO2 to produce linear CO. The linear CO was found to be the major species on Rh/Al2O3 during the reaction by in situ infrared spectroscopy. The accumulation of the linear CO on Rh0 sites revealed that the surface of Rh crystallites on Al2O3 remained in a reduced state throughout the study. The O2 pulse into CO2/CH4 resulted in (i) a total oxidation of CH4 to CO2 and H2O and then (ii) a net increase in the formation of the desired products, CO/H2, at a ratio of 1:1, revealing the promotion of the CO2−CH4 reaction. These are the first reported results on the enhancement of the CO and H2 formation rate via the pulse addition of oxygen into the CO2−CH4 reaction. The difference in product formation can be explained by two different types of adsorbed oxygen: one responsible for total oxidation and the other responsible for partial oxidation. The net increase in CO and H2 formation from the O2 pulse further suggests that combining mixed reforming of CH4 with CO2/O2 and selective poisoning of the total oxidation sites would enhance the selectivity and rate of CO/H2 formation.
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