(246f) Beyond “Hydrogen Spillover”: The Mechanistic Origins of Bifunctional Synergies between Pt and Non-Reducible Oxide Co-Catalysts for Arene Hydrogenation

The hydrogenation reactions of arenes on nano-particles are faster when supported or mixed with metal oxides with exposed Lewis centres (e.g. Al₂O₃, MgO, and TiO₂) compared with SiO₂. These increases in rates have been attributed to “hydrogen spillover,” whereby H-adatoms formed from H₂ on metal surfaces are transported via surface diffusion to active sites at the surfaces of oxides capable of arene hydrogenation. While such H-atom migration is well established on reducible oxides (e.g. WO₃), it requires charge-separation processes that are infeasible on non-reducible oxides (e.g. Al₂O₃, MgO).^1,2

Here, kinetic measurements and reaction-transport modelling are used to elucidate the mechanistic origins of such metal-metal oxide bifunctional synergies for toluene hydrogenation on SiO₂-supported Pt (Pt/SiO₂) mixed physically with Al₂O₃. Toluene-H₂ reactions occur on Pt surfaces that are covered with diverse hydrocarbon moieties; those with gaseous counterparts with closed-valence shells (i.e., methylcyclohexenes and methylcyclohexadienes) desorb from the surface and can diffuse to and react at nearby oxide surfaces. The Al₂O₃ catalyst considered here contains acid-base pairs that mediate H₂ addition to cyclohexadiene and cyclohexene molecules. When Al₂O₃ is admixed with Pt/SiO₂, these H₂-addition reactions scavenge Pt-derived methylcyclohexadiene that would otherwise block sites at Pt surfaces. An increase in the mean inter-function distances between the Pt/SiO₂ and Al₂O₃, moreover, decreases the rate enhancements as rates of methylcyclohexadiene egress from Pt/SiO₂ domains limits their rates of scavenging. These enhancements therefore stem from the desorption and scavenging of partially-hydrogenated molecules at a second catalyst function present within distances accessible by gas phase diffusion. These findings demonstrate, more broadly, the influence of molecular shuttles on supported metal catalysts and suggests future directions for bifunctional catalyst design that incorporate co-catalysts that leverage such molecular shuttles.

References:

1. Fischer, E. Iglesia, Journal of Catalysis, 420 (2023) 68
2. Prins, Chemical Reviews, 112 (2012) 2714