Elementary Steps in D2-H2O and H2-D2-H2O Exchange Reactions on Pt Nanoparticles and Consequences of Liquid Water for the Binding and Reactivity of Chemisorbed Hydrogen

The identity and kinetic relevance of elementary steps and bound species in H2O-D-2 and H-2-D-2-H2O isotopic exchange on dispersed Pt nanoparticles are assessed as probes of the effects of chemisorbed and liquid water on surface-catalyzed exchange channels. The resulting rate equations consider the kinetically relevant chemisorption of H2O and accurately describe the kinetic effects of H-2, D-2, and H2O pressures on exchange rates. H-2-D-2 exchange is mediated by the reactions of H-2 and D* (or D-2 and H*) at spaces made available by local displacements of mobile H* or D* adatoms to alternate (atop to fcc) binding sites. Rate and equilibrium constants are similar for reactions under anhydrous conditions and in the presence of chemisorbed water. The additional exchange channels responsible for H2O-D-2 exchange involve reactions between D-2 and molecularly adsorbed H2O* also via displacements of hydrogen adatoms, as in the case of H-2-D-2 exchange. The formation of an intrapore liquid water phase via capillary condensation within the voids of gamma-Al2O3, which contain the Pt nanoparticles, did not cause detectable changes in kinetic trends or in any of the parameters required to describe exchange rates. These undetectable effects of a dense liquid phase in contact with Pt nanoparticles are consistent with the absence of preferential solvation of transition states (TS) by liquids, which leads to nearly identical activity coefficients in the gas or liquid phase and considerations using TS formalisms of thermodynamically nonideal systems. The accurate interpretation of measured exchange rates required the removal of CO2 contaminants from liquid water using procedures more stringent than those typically prescribed or used when water is present at the temperatures characteristic of isotopic exchange on metals.

Automated summary

This study examines the identity and kinetic relevance of elementary steps and bound species in H2O-D-2 and H-2-D-2-H2O isotopic exchange on dispersed Pt nanoparticles. The results show that chemisorbed water affects the exchange rates, and the rates and equilibrium constants are similar under anhydrous conditions and in the presence of chemisorbed water. Additional exchange channels involving D-2 and molecularly adsorbed H2O are also observed. Furthermore, the presence of a liquid water phase on the catalyst surface does not significantly affect the exchange rates.