Active sites of Ni2P/SiO2 catalyst for hydrodeoxygenation of guaiacol: A joint XAFS and DFT study

A Ni2P/SiO2 catalyst was prepared by temperature-programed reduction (TPR), and applied for the hydrodeoxygenation of guaiacol. The physical properties of the catalyst samples were characterized by N-2 adsorption/desorption isotherms and CO uptake chemisorption. X-ray diffraction (XRD) and extended X-ray absorption fine structure (XAFS) spectroscopy were used to obtain structural properties for the supported Ni2P catalysts. Hydrodeoxygenation (HDO) tests were performed in a continuous flow fixed-bed reactor at 523-573 K, and 1 or 8 atm, and an LHSV of 2.0 h(-1). The Ni2P/SiO2 gave an HDO conversion over 90% with two different reaction pathways being identified; at 1 atm direct deoxygenation was dominant to produce benzene, and at 8 atm prehydrogenation followed by deoxygenation was preferred to produce cyclohexane. A combined X-ray absorption fine structure spectroscopy and density functional theory analysis revealed that the active site of Ni2P catalysts is composed of threefold hollow Ni and P sites which lead to adsorption of H or OH groups. These results suggest that relative populations of H or OH groups on Ni or P sites of Ni2P surface have an impact on overall reaction pathways of the HDO. (C) 2013 Elsevier Inc. All rights reserved.