The effect of water on methane oxidation over Pd/Al2O3 under lean, stoichiometric and rich conditions
In this study, the effect of oxygen concentration and the presence of water on methane oxidation were examined over a Pd/Al2O3 catalyst. The physicochemical properties of the catalyst were investigated in detail using BET, XRD, STEM, O2-TPO and CH4-TPR. Ramping experiments from 150 to 700 °C were conducted using rich, stoichiometric and lean gas mixtures in the absence and presence of water. It was found that increasing the oxygen concentration in a dry atmosphere resulted in higher methane oxidation activity, which can be connected to the facilitation of palladium oxide formation. The TPO data showed that only minor amounts of PdO up to 700 °C were decomposed; however, in the stoichiometric and rich reaction mixture, PdO was still decomposed because of the oxygen limitation. This fact resulted in a “negative activation” during cooling, with increased activity because of palladium re-oxidation. Moreover, methane steam reforming and water gas shift reactions were important reactions under rich conditions over the metallic palladium sites. A significant inhibiting effect of water on the Pd-catalyst with loss of methane activity was found. Interestingly, the inhibition effect was much greater using high oxygen concentration in the gas mixture (500 ppm CH4, 8% O2, 5% H2O) than that at lower oxygen levels (800–1200 ppm) and we propose that the hydroxyl species formation, which blocks the active sites, are facilitated by a large oxygen excess. In addition, the re-oxidation of palladium occurring during the cooling ramp in dry feed using rich and stoichiometric gas mixtures was also significantly suppressed in the presence of a large amount of water. Thus, water impedes the oxidation of palladium, which significantly deactivates the Pd catalyst.