Direct Observation of Active Sites of Ru Catalyst in Hydrodeoxygenation of Furfural

Surface and subsurface oxygen species have been recognized as species of great significance in understanding surface reaction mechanisms in noble metals. Recent theoretical investigations in our group has shown the importance of surface oxygen vacancies in the furfuryl alcohol hydrodeoxygenation (HDO) reaction over ruthenium oxide structures.^[1] In this work, we present direct evidence of the coexistence of metal and oxide (strained Ru_xO_y) on Ru catalyst by atomic resolution high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). This structure is further confirmed by X-ray absorption spectroscopic analyses of fresh, used and in situ reduced Ru/C catalyst. The results show that the fresh Ru/C catalyst is predominately RuO₂with small quantities of metallic Ru, indicative of particles with metal-core and oxide shell structure.

In order to directly correlate the strained Ru_xO_y structure to the reaction of furfural HDO we performed in operando X-ray absorption spectroscopy at beamline X18B, NSLS, BNL. All the experiments were performed with a custom-made XAS cell, which was charged with 1wt% furfural-isopropanol and pressurized with 400 psi He, heated up to defined temperatures (140 °C, 160 °C and 180 °C). Based on both near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses of the data, we conclude that the confirmed strained Ru_xO_y structure can be directly correlated to the 2-methylfuran (MF) production from furfural via catalytic transfer hydrogenation. However, once the surface is dominated by metallic Ru, MF formation is suppressed.

[1]. Mironenko and Vlachos, J. Am. Chem. Soc, 2016, Accepted