In Situ IR Study of Transient CO₂ Reforming of CH₄ over Rh/Al₂O₃

Abstract

The CO 2 −CH 4 reaction on Rh/Al 2 O 3 was studied by in situ infrared spectroscopy coupled with pulse and step transient techniques. Steady-state isotopic 13 CO transient studies at 773 K and 0.1 MPa show that the formation of gaseous 13 CO 2 closely follows that of linear 13 CO, indicating that linear CO is an active adsorbate. Pulsing CH 4 into CO 2 flow and step switching from He to CO 2 /CH 4 flow showed that the formation of H 2 led that of CO, revealing that the first step of the reaction sequence is the decomposition of CH 4 into *CH x species and hydrogen. Hydrogen activated adsorbed CO 2 to produce linear CO. The linear CO was found to be the major species on Rh/Al 2 O 3 during the reaction by in situ infrared spectroscopy. The accumulation of the linear CO on Rh 0 sites revealed that the surface of Rh crystallites on Al 2 O 3 remained in a reduced state throughout the study. The O 2 pulse into CO 2 /CH 4 resulted in (i) a total oxidation of CH 4 to CO 2 and H 2 O and then (ii) a net increase in the formation of the desired products, CO/H 2, at a ratio of 1:1, revealing the promotion of the CO 2 −CH 4 reaction. These are the first reported results on the enhancement of the CO and H 2 formation rate via the pulse addition of oxygen into the CO 2 −CH 4 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 H 2 formation from the O 2 pulse further suggests that combining mixed reforming of CH 4 with CO 2 /O 2 and selective poisoning of the total oxidation sites would enhance the selectivity and rate of CO/H 2 formation.