(691d) Prevalence of Bimolecular Routes in the Activation of Diatomic Molecules with Strong Chemical Bonds (O2, NO, CO, N2) on Catalytic Surfaces

Low-index planes of supported metal catalysts are often incapable of dissociating the strong bonds in O₂, NO, CO, and N₂. This limitation can reflect the noble nature of the catalyst (O₂ activation on Au), the crowded state of the surface (O₂ activation on CO*-covered surfaces of Pt), and/or the intrinsic strength of the chemical bond (CO and N₂ activations on Ru). Density functional theory calculations and ultra-high vacuum studies have demonstrated that defect sites (characterized here as metal atoms with less coordination than those in low-index surfaces) can activate these strong bonds. Many of these studies, however, have examined catalysts at low coverages, often atypical of catalytically-relevant conditions where defect sites will be saturated with strongly-bound adsorbates. Here, we discuss how bimolecular routes can weaken the strong bonds in O₂, NO, CO, and N₂ prior to dissociation, thus opening up routes to activate such species on low-index planes of metal surfaces [1].

O₂ activates through H₂O-assisted pathways (forming peroxide intermediates prior to O–O cleavage) on Au surfaces during gas-phase CO oxidation [2] and aqueous-phase alcohol oxidation [3]. In contrast to Au, O₂ activation is facile on bare Pt surfaces, but such surfaces are saturated by CO* even at very low CO partial pressures, leading to bimolecular routes via formations of *OOCO* intermediates prior to O–O cleavage during CO oxidation [4]. NO bonds are significantly stronger than O₂ bonds, leading to their activation via H*-assisted routes on Pt, which form *HNOH* species prior to N–O cleavage during NO-H₂ reactions at all NO* coverages [5]. Similarly H*-assisted routes enable CO activation during CO-H₂ reactions on CO*-covered Ru [6] and Co [7] surfaces via *HCOH* intermediates which form prior to C–O cleavage. H₂O, capable of co-catalyzing O–H bond formations in O₂ (on Au [2,3] and Pt [4]), also plays this role during CO hydrogenation, where it facilitates *HCOH* formation through H*-shuttling mechanisms [8]. N₂, in contrast, has a much lower driving force for hydrogenation than CO (reaction energies to form hydrazine (N₂H₄) from N₂ are significantly lower than those to form methanol (CH₃OH) from CO). This prevents H*-assisted N₂ activations from having significantly lower free energy barriers than those for direct N₂ dissociation on Ru low-index surfaces [1], consistent with the requirement of defect sites for ammonia synthesis. This work thus includes multiple examples (and one counter-example) of strong bond activations occurring through bimolecular routes which avoid the large kinetic hurdles of direct dissociations on low-index planes by weakening bonds in a step-wise manner prior to cleavage.

References

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