H₂Oxidation over Supported Au Nanoparticle Catalysts: Evidence for Heterolytic H₂Activation at the Metal–Support Interface

Abstract

Water adsorbed at the metal-support interface (MSI) plays an important role in multiple reactions. Due to its importance in CO preferential oxidation (PrOx), we examined H oxidation kinetics in the presence of water over Au/TiO and Au/AlO catalysts, reaching the following mechanistic conclusions: (i) O activation follows a similar mechanism to that proposed in CO oxidation catalysis; (ii) weakly adsorbed HO is a strong reaction inhibitor; (iii) fast H activation occurs at the MSI, and (iv) H activation kinetics are inconsistent with traditional dissociative H chemisorption on metals. Density functional theory (DFT) calculations using a supported Au nanorod model suggest H activation proceeds through a heterolytic dissociation mechanism, resulting in a formal hydride residing on the Au and a proton bound to a surface TiOH group. This potential mechanism was supported by infrared spectroscopy experiments during H adsorption on a deuterated Au/TiO surface, which showed rapid H-D scrambling with surface hydroxyl groups. DFT calculations suggest that the reaction proceeds largely through proton-mediated pathways and that typical Brønsted-Evans Polanyi behavior is broken by introducing weak acid/base sites at the MSI. The kinetics data were successfully reinterpreted in the context of the heterolytic H activation mechanism, tying together the experimental and computational evidence and rationalizing the observed inhibition by physiorbed water on the support as blocking the MSI sites required for heterolytic H activation. In addition to providing evidence for this unusual H activation mechanism, these results offer additional insight into why water dramatically improves CO PrOx catalysis over Au.

On the Limited Role of Electronic Support Effects in Selective Alkyne Hydrogenation: A Kinetic Study of Au/MOx Catalysts Prepared from Oleylamine‐Capped Colloidal Nanoparticles

“…We recently published a detailed study of H 2 dissociative chemisorption using a 1‐D periodic nanorod model; readers are directed there for a full discussion of H 2 adsorption on Au . We extended this study to include homolytic dissociation of H 2 onto the Au 10 /TiO 2 (110) model (the model used above for propyne adsorption).…”

“…The Au 10 cluster likely overestimates binding energies because the individual atoms are more highly uncoordinated than in a 2–3 nm NP. Nevertheless, with the adsorption of 1‐propyne on Au being stronger than both alkyne adsorption on the support (‐34 to −44 kJ/mol, see above) and H 2 adsorption on Au (57 kJ/mol, see Table S22), our simulations suggest that alkynes are likely present on Au sites, even if no IR fingerprint for alkyne adsorption to Au was directly observed.…”

Synthesis and Properties of Au Hydride

“…Related studies have observed similar shifts, however there is no consensus on the direction of the shift, blue or red. In this study, we show how the formation of a metastable gold hydride layer hypothesized by Sil et al ., Silverwood et al ., Whittaker et al . and Ishida et al.,, following H 2 dissociation, explains the blue shift in the dielectric function of the gold‐nanoparticle surface.…”

How Do Structurally Distinct Au/α‐Fe2O3 Interfaces Determine Surface OH/H2O reactivity, Intermediate Evolution, and Product Formation in Low‐temperature Water‐gas Shift Reaction?

“…Theoretical calculations suggested that the oxygen vacancies are highly reactive toward water dissociation, [38,47] and molecular adsorption on {001} is favored at a relative humidity >34 % . The heterolytic dissociation of H 2 O generates two types of surface OH groups, the O‐containing one is bonded to the Fe atom, and the other one is formed by attaching H to a lattice oxygen . Because of the high relative concentration of coordinately unsaturated Fe sites on α‐Fe 2 O 3 ‐QC (Table ), the π* orbital of CO molecule can easily hybridize the t 2g orbital of Fe, favoring CO adsorption via π‐bonding.…”

“…[48] The heterolytic dissociation of H 2 O generates two types of surface OH groups, the Ocontaining one is bonded to the Fe atom, and the other one is formed by attaching H to a lattice oxygen. [48,49] Because of the (Table 3), the π* orbital of CO molecule can easily hybridize the t 2g orbital of Fe, favoring CO adsorption via π-bonding. This may reasonably account for preferential CO adsorption on Au/α-Fe 2 O 3 -QC-Air.…”