Catalytic and inhibitory effects of Pt surfaces on the oxidation of CH4/O2/N2 mixtures

The catalytic oxidation of lean CH4/O-2/N-2 mixtures was studied by injecting a stream of the mixture onto a polycrystalline platinum foil whose temperatures could be varied by resistance heating between 300 and 1800 K. Experiments were performed in the stagnation point flow configuration under steady-state conditions in order to derive the fuel conversion efficiencies and the product selectivities to CO and CO,. These were compared with a numerical model. With increasing foil temperature, the first appearance of CO in the products signalled the onset of homogeneous oxidation. The surface temperatures of the transition from catalytic to homogeneous oxidations were higher than those of the 'reversed' transition from homogeneous to catalytic, causing hysteresis in the fuel conversion and product selectivities curves. For the conditions investigated, the number of thermal cycles applied to the foils were found not to affect the fuel conversion with surface temperature in the catalytic regime. In contrast, they increased the peak of CO selectivity by a factor of two and enhanced the hysteresis behaviour by lowering the temperature of the transition from homogeneous to catalytic oxidation. Decreasing fuel concentrations in the fuel-lean mixture feed did not change appreciably the catalytic oxidation conversion curve, but reduced the temperature of the transition from catalytic to homogeneous oxidation, and eliminated the hysteresis. The peak of CO selectivity was found not to vary significantly in the range of fuel mixture strengths studied. The fuel conversion efficiency curves with surface temperature were successfully modelled over the whole catalytic oxidation regime using one global chemical reaction of methane oxidation. A combination of existing detailed heterogeneous and homogeneous chemical mechanisms of methane oxidation offered the advantage of predicting the CO and CO, products selectivities, but was found to predict larger conversions in the kinetically controlled catalytic oxidation regime. The agreement with the experimental conversions was restored in the mass transport rate limited regime and permitted a discussion of the inhibitory role of the Pt surface in the transitions between catalytic and homogeneous oxidation regimes. Both the global and detailed chemical mechanisms yielded higher fuel conversions than those measured once the homogeneous oxidation regime was established, showing the need for further validation in this regime, in particular with respect to the formation of CO. Copyright (C) 2000 John Wiley & Sons, Ltd.