423693 First-Principles Investigation of Direct Deoxygenation of Phenolic Compounds over Ru/TiO2(110)

Wednesday, November 11, 2015: 4:35 PM
355E (Salt Palace Convention Center)
Byeongjin Baek, Chemical and Biomolecular Engineering, University of Houston, Houston, TX and Lars C. Grabow, Department of Chemical and Biomolecular Engineering, University of Houston, Houston, TX

Catalytic conversion of oxygenated aromatic compounds (OACs) into oxygen-free compounds, or bio-oil upgrading, suffers from both poor selectivity and low conversion, because the direct deoxygenation (DDO) is harder to activate than decarbonylation (DCN) or hydrogenation (HYD). According to recent studies, Ru/TiO2 has been identified as promising catalyst with good activity and selectivity towards DDO products from OACs [1, 2]. However, the nature of the active site and roles of the Ru cluster and TiO2 support remain by and large unknown.

Using density functional theory (DFT), we have explored the hydrodeoxygenation (HDO) pathways of phenol and m-cresol over the Ru(0001) surface, hydroxylated h-TiO2(110), and a 10-atom cluster model for the Ru/TiO2 interface. The comparison of the HYD vs. DDO pathways for phenol and m-cresol on Ru(0001) indicates that HYD is fast and kinetically preferred over DDO, suggesting that metallic Ru is unselective for direct C-O bond scission of oxygenated aromatic compounds. For the hydroxylated h-TiO2 surface an oxygen vacancy is required to provide an adsorption site for phenol or m-cresol, but vacancy formation is kinetically limited by TiO2’s weak interaction with gas-phase H2 [3]. For the Ru10/TiO2 interface our results suggest that the presence of Ru on TiO2 catalyzes hydrogen delivery to TiO2(110) and facilitates oxygen vacancy formation at the Ru/TiO2 interface. Phenol and m-cresol subsequently adsorb via their hydroxyl groups into the formed vacancy. The energy barriers for the following C-O bond scissions in phenol and m-cresol are 0.78 eV and 0.71 eV, respectively. The eliminated OH group heals the TiO2 vacancy, and the aromatic rings of phenol and m-cresol remain on the Ru cluster. The energy barrier for C-O bond scission on TiO2 and at the Ru/TiO2 interface are not significantly different, indicating that the oxygen vacancy site plays a similar role in catalyzing the DDO pathway of phenol and m-cresol. Our theoretical analysis is consistent with experimental evidence, and we conclude that the role of Ru is to activate hydrogen and an oxygen vacancy site near the Ru/TiO2 interface is required for selective C-O bond scission.

[1]  Omotoso, T.; Boonyasuwat, S.; Crossley, S. P. Understanding the role of TiO2 crystal structure on the enhanced activity and stability of Ru/TiO2 catalysts for the conversion of lignin-derived oxygenates. Green Chem. 2014, 16, 645–652.
[2]  Newman, C.; Zhou, X.; Goundie, B.; Ghampson, I. T.; Pollock, R. A.; Ross, Z.; Wheeler, M. C.; Meulenberg, R. W.; Austin, R. N.; Frederick, B. G. Effects of support identity and metal dispersion in supported ruthenium hydrodeoxygenation catalysts. Appl. Catal. A Gen. 2014, 477, 64–74.
[3]  Rekoske, J.; Barteau, M. Isothermal reduction kinetics of titanium dioxide-based materials. J. Phys. Chem. B 1997, 101, 1113–1124.

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