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

Abstract

Kinetic and isotopic data and H2 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 H2–O2 reactions on Pt surfaces. Turnover rates are proportional to O2 pressure and decrease and then reach constant values as H2 pressures increase, leading to apparent first-order rate parameters (rO2/PO2) that reflect reactive collision probabilities of O2 with surfaces and which depend solely on the H2 pressure at all temperatures (540–680 K). H2–D2 isotopic exchange rates during reactions with O2 are much faster than water formation rates, consistent with quasi-equilibrated H2 dissociation steps and with prevalent H* coverages that can be determined independently from H2 uptakes; such measurements enable the decoupling of non-Langmuirian adsorption parameters from kinetic parameters in rate equations. These data indicate that H2–O2 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 O2 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 O2 precursors. Kinetic isotope effects are near unity for both routes because H* is not involved in direct O2 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.