Theoretical and Experimental Assessments of Elementary Steps and Bound Intermediates in Catalytic H2-O2 Reactions on Dispersed Pt Nanoparticles

Kinetic and isotopic data and H-2 chemisorption uptakes measured under reaction conditions are combined here with theoretical assessments on model catalytic surfaces at relevant hydrogen (H*) coverages to establish the identity and kinetic relevance of elementary steps and bound species in H-2-O-2 reactions on Pt surfaces. Turnover rates are proportional to O2 pressure and decrease and then reach constant values as H-2 pressures increase, leading to apparent first-order rate parameters (r(O2)/P-O2) that reflect reactive collision probabilities of O-2 with surfaces and which depend solely on the H-2 pressure at all temperatures (540-680 K). H-2-D-2 isotopic exchange rates during reactions with O-2 are much faster than water formation rates, consistent with quasiequilibrated H-2 dissociation steps and with prevalent H* coverages that can be determined independently from H-2 uptakes; such measurements enable the decoupling of non-Langmuirian adsorption parameters from kinetic parameters in rate equations. These data indicate that H-2-O-2 reactions involve two kinetically-relevant O2 dissociation routes. One channel forms two bound O atoms on bare Pt atom ensembles available within H* adlayers, which react via fast subsequent reactions with H* to form H2O. A parallel O-2 dissociation route involves reactions with a H*-H* pair to form weakly bound adsorbed hydrogen peroxide (*HOOH*), a highly reactive species that subsequently cleaves its O-O bond in steps that are not kinetically-relevant. The free-energy barriers are dominated by losses in translational entropy incurred upon the formation of the transition states from their gaseous O-2 precursors. Kinetic isotope effects are near unity for both routes because H* is not involved in direct O-2 dissociation steps, and the *HOOH* formation transition state occurs very early along the reaction coordinate (with nearly intact O-O and Pt-H bonds from reactant states), as confirmed by DFT-derived energies and isotope effects.