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The oxidation and reforming kinetics of methane by O 2 , CO 2 and H 2 O were studied on a stepped Pt(5 5 7) single crystal from 623 to 1050 K under methane rich conditions. The rate of carbon deposition was followed by ex-situ Auger electron spectroscopy under non-oxidative conditions. The apparent activation energy for methane decomposition was significantly lower than the apparent barriers measured for both total oxidation, CO 2 and H 2 O reforming. Total oxidation of methane to CO 2 and H 2 O followed by combined dry and steam reforming (combined combustion-reforming) led to CO production rates which were higher than direct CO 2 or H 2 O reforming rates. The enhanced rates are most likely due to the ability of adsorbed oxygen to prevent carbon nucleation and/or scavenge carbon enabling the reforming reaction to turnover on a larger fraction of sites. Comparable amounts of carbon were found by Auger electron spectroscopy measurements after both direct dry or steam reforming, while combined oxidation-reforming had considerable less carbon. During direct dry or steam reforming, CO 2 and H 2 O serve only to scavenge adsorbed atomic carbon, while in the presence of oxygen, carbon is removed by both combustion and reforming routes. |
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