Energy Partitioning in the Reaction 2H₂ + O₂ -> 2H₂O on Pd(111).
(2 + 1)-Photon resonance-enhanced multiphoton ionization (REMPI) was used to detect desorbed H₂O mols. in a state-specific manner. The reaction was studied on Pd (111) at 500 and 700 K in the regime of low O coverages (< 0.05 ML) resulting from adsorption of ambient O₂. The D₂ is dosed by using a supersonic mol. beam with pulse fluxes resulting in stoichiometric coverages of D and O atoms. The D₂O mols. are desorbed with a translational energy of .apprx. 300 K with weak, if any, dependence on the surface temp. The angular distribution is nearly cosine in form. The spectroscopic data indicate that the D2O is desorbed with an av. energy in the rotational degree of freedom close to the surface temp. Expts. probing vibrationally excited states yielded neg. results setting an upper limit of their population. These data show that most of the excess energy of the final OD + D reaction step of .apprx. 100 kJ mol-1 is not channelled into any of the degrees of freedom of the desorbed mols. The data are interpreted such that the product H₂O mols. reside on the surface for a time sufficient to dissipate most of the energy.
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