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 .
O2 activates through H2O-assisted pathways (forming peroxide intermediates prior to O–O cleavage) on Au surfaces during gas-phase CO oxidation  and aqueous-phase alcohol oxidation . 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 . 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 . Similarly H*-assisted routes enable CO activation during CO-H2 reactions on CO*-covered Ru  and Co  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 ), also plays this role during CO hydrogenation, where it facilitates *HCOH* formation through H*-shuttling mechanisms . 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 , 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.
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