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Résumé :
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The activation barriers of elementary-like reactions pertaining to the oxidation and reforming of methane on Pt(111) and Pt(211) surfaces have been calculated using periodic density functional theory (DFT) calculations. We have investigated the adsorption of CHx(x=1–3)OH and CHx(x=1–3)O, all the O and OH-assisted dehydrogenation reactions of CHx(x=1–4), all the C–O bond coupling reactions forming C1 oxygenates, and their subsequent dehydrogenation. It has been found that (i) COH and CHO are the most stable C1 oxygenates on Pt(111) and Pt(211), repectively; (ii) In the presence of O on Pt(211), oxidative dehydrogenation of CH by O is more kinetically favorable than the pyrolytic CH dehydrogenation; (iii) CO can be generated by oxidation of C with a low reaction barrier on Pt(211); (iv) The reactions involving OH and the dehydrogenation of CHx(x=1–3)OH and CHx(x=1–3)O appear to be secondary reaction pathways on Pt. Based on the activation barriers, we conclude that the major reaction pathways on Pt(111) and Pt(211) are CH4 (g) → CH3ad → CH2ad → CHad → Cad, Cad + Oad → COad and CH4 (g) → CH3ad → CH2ad → CHad, CHad + Oad → Cad, CHad + Oad → COad, respectively. Low coordination sites, such as steps, exhibit lower barriers for pyrolytic dehydrogenation except for the reaction CH2ad + * → CHad + Had that is preferred on terraces. In addition, they are lower barrier sites in the oxidation of C and CH and, thus, are expected to play a key role in partial and total oxidation of methane. In methane steam reforming, OHad may play a role only in the last step of C oxidation and certainly in the water-gas shift reaction, and, thus, this process consists of nearly decoupled methane catalytic pyrolysis and water-gas shift reactions.
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