The mechanism of the CH4/O2 reaction on the Pd–Pt/γ-Al2O3 catalyst: A SSITKA study

Abstract

This work is mainly dedicated to the studies of the reaction mechanism of complete methane oxidation on the γ-alumina palladium–platinum catalyst by means of the steady state isotopic transient kinetic analysis (SSITKA) with 12CH4/13CH4 and 16O2/18O2 switches. Such kinetic parameters as the average surface life-time and the surface concentration of methane, oxygen, carbon dioxide and intermediates leading to its formation were determined. It was found that carbon dioxide is formed with two different rates. The average surface life-time of fast formed carbon dioxide change from 11.2s at 637K to 0.2–0.3s at higher temperatures, while the surface concentration of fast formed carbon dioxide and intermediates leading to it change from 3.1μmol/g at 637K to 1.8μmol/g at 980K. Carbon dioxide formed with a slow rate is present on the catalyst surface only above 637K. At higher temperatures the values of its average surface life-time are close to 1.7–1.8s, while the surface concentration of slow formed carbon dioxide and intermediates leading to it change from 14.8μmol/g at 755K to 16.3μmol/g at 980K. The coverage of the surface of the palladium–platinum active phase with fast formed carbon dioxide and intermediates leading to it is in the order of 0.16–0.17 in the range of 673–755K and 0.125 – at 980K. Much more higher is the surface coverage with slow formed carbon dioxide – 0.8 and 0.92 at 755K and 980K, respectively. It was also found that oxygen from the crystal lattice of the catalyst takes part in oxidation of methane, as well as that oxygen atoms are exchanged between the gas phase and the crystal lattice of the catalyst. The very long live-time of oxygen leading to formation of carbon dioxide, in the order of 80–380s, and very large formal coverage of the surface of palladium–platinum active phase with oxygen desorbed to the gas phase and oxygen leading to formation of carbon dioxide, scores of times exceeding the formal monolayer, clearly prove that a very large pool of subsurface oxygen participates in methane oxidation. It was also found that only at the lowest temperature of the reaction small amount of reversibly adsorbed methane resists with a very short life-time on the catalyst surface. It may be concluded that two different kinds of active centres (α—more active, but less numerous and β—less active, but more numerous) exist on the “working” surface of the catalyst and the process of methane oxidation proceeds simultaneously according to two different reaction mechanisms (Mars–van Krevelen and Langmuir–Hinshelwood). The extent of their participation in oxidation of methane is different and depends on the reaction temperature. Moreover, the process of methane oxidation proceeds not only simultaneously according to two different mechanism of the reaction, but also with different rates determined by the kind of active centres.