Concentration Hysteresis in the Oxidation of Methane over Pt/γ-Al₂O₃: X-ray Absorption Spectroscopy and Kinetic Study

Abstract

It is found that CH4 oxidation over Pt/Al2O3 catalyst at 330–460 °C can stably proceed via two distinctly different regimes at identical feed gas composition: “low activity regime” and “high activity regime”. Switching between the regimes depends on the O2 concentration and the direction of its change. For the reaction mixture with constant methane concentration of 1%, the incremental decrease in O2 concentration from 2.5% (O2/CH4 ∼ 2.5, lean conditions) to ∼1% (O2/CH4 ∼ 1, stoichiometric or rich conditions) results in the catalyst activation and its switching to the high activity regime. The catalyst remains active upon further incremental increase in O2 concentration until reaching O2/CH4 ∼ 2, when switching to low activity regime takes place. Such responses of the catalyst activity to the changes in the feed gas composition result in the appearance of the pronounced concentration hysteresis. The evident correlation between in situ X-ray absorption spectroscopy and catalytic data strongly suggests that the switching of the catalyst between low activity regime and high activity regime, which causes the concentration hysteresis, stems from the change in the electronic state of Pt particles. According to in situ XANES data, the electron density on the Pt particles decreases in the low activity regime, as evidenced by an increased Pt-L3 white line intensity. In contrast, switching the catalyst to high activity regime is accompanied by a decrease in the white line intensity, indicating the electron density increase. In turn, the changes in the Pt electronic state are attributable to changes in the O/Pt surface ratio or formation/decomposition of a two-dimensional layer of surface Pt oxide triggered by variation of oxygen concentration in the reaction mixture. The addition of highly reactive CO or H2 to the reaction mixture shifts the hysteresis loop toward higher oxygen concentration as a result of consuming oxygen in H2 or CO oxidation, which decreases the effective oxygen concentration. In contrast to this, the increase in the methane concentration widens the hysteresis window, presumably because of the different stoichiometry of CH4 oxidation upon catalyst activation (when partial CH4 oxidation prevails) and deactivation (when total oxidation of CH4 predominates).