(461b) Effects of Liquid Water on Selective C-O Hydrogenolysis Catalyzed By Ru-TiO2 interfacial Sites

Several groups have recently shown the importance of metal-TiO₂ interfacial sites for selective activation of oxygen in a variety of reactions, including CO oxidation [1] and hydrodeoxygenation of phenolics [2,3]. We have previously shown that TiO₂-supported catalysts containing small Ru nanoparticles are highly active for the conversion of phenol to benzene [3], which is consistent with the results from Crossley and coworkers, who suggest the reaction occurs at the Ru-TiO₂ interface [2]. Both observations can be explained by a mechanism wherein surface hydroxyls present on the TiO₂ surface in the immediate vicinity of the Ru nanoparticles are protonated during heterolytic H₂ dissociation, leading to acidic Ti-OH₂ surface species. Phenol then adsorbs across the Ru-TiO₂ interface, and the acidic proton from Ti-OH₂ facilitates C-O scission, regenerating the Ti-OH species and leaving phenyl to be hydrogenated on the Ru surface [3]. This mechanism is predicted to be favorable in the presence of water, and reports from the literature indicate an enhancement of between 30-200% depending on the fugacity of water [2,3].

In this work, we seek to clarify the role of water in C-O hydrogenolysis catalyzed by Ru/TiO₂ materials. We measure reaction orders for phenol hydrogenolysis over Ru nanoparticles supported on three different TiO­₂ materials: pure anatase TiO₂, pure rutile TiO₂ and pyrogenic titania (i.e., the P25 material sold by Evonik, which is approx. 85% anatase and 15% rutile). The reaction is positive-order with respect to water for Ru/rutile, negative-order for Ru/anatase, and weakly positive-order for Ru/P25. These observations correlate with differential heats of water adsorption on TiO₂ powders [4] and suggest that water adlayers at the TiO₂ surface lead to blocking of the Ru-TiO₂ interface sites. The seemingly inhibitory effect of water for some catalysts is explained in the context of reaction orders with respect to phenol and dihydrogen at several water fugacities.

References

[1] Saavedra, J.; Pursell, C. J.; Chandler, B. D. CO Oxidation Kinetics over Au/TiO2 and Au/Al2O3 Catalysts: Evidence for a Common Water-assisted Mechanism. JACS, 2018, 140, 3712-3723.

[2] Omotoso, T.O.; Baek, B.; Grabow, L. C.; Crossley, S. P. Experimental and First-Principles Evidence for Interfacial Activity of Ru/TiO2 for the Direct Conversion of m-Cresol to Toluene. ChemCatChem, 2017, 9, 2642-2651.

[3] Nelson, R. C.; Baek, B.; Ruiz, P.; Goundie, B.; Brooks, A.; Wheeler, M. C.; Frederick, B. G.; Grabow, L.C; Austin, R. N. Experimental and theoretical insights into the hydrogen-efficient direct hydrodeoxygenation mechanism of phenol over Ru/TiO2. ACS Catal., 2015, 5, 6509-6523.

[4] Fubini, B.; Bolis, V.; Bailes, M.; Stone, F. S. Solid State Ionics, 1989, 32/33, 258-272.