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

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


Schwartz, T. - Presenter, University of Maine
Frederick, B. G., University of Maine
Austin, R., Barnard College
Grabow, L., University of Houston
Stück, D., University of Maine

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.


[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.