A Combined Experimental and DFT Investigation of Selective Hydrodeoxygenation of Guaiacol over Bimetallic Carbides

摘要

Catalytic hydrodeoxygenation of fast pyrolysis bio-oils is a potential approach for producing green fuels. This process requires active and inexpensive catalysts with bifunctionality favoring C-O bond scission under high pressure (145-2900 psig) and temperature (350-500 degrees C) conditions. In this context, metal carbides are promising candidates for the selective deoxygenation of oxygenated compounds. The understanding of their active sites and mechanism is however limited. In the present work, the electronic interaction between Mo and W in their mixed carbide and its effect on the energetics and mechanism of guaiacol hydrodeoxygenation (HDO) were explored by using experiments and first-principles density functional theory calculations. Experimentally, CO and H-2 were used as probe molecules in pulse chemisorption to identify the different natures of active sites in metal carbides. The calculated binding energies and adsorption configurations available for CO and H atoms on the surface of the catalysts describe their different adsorption capacities. Regarding the deoxygenation reaction, our calculations showed that oxygen bonded similar to 1 eV stronger on the bimetallic carbide (MoWC) than on the monometallic molybdenum carbide surface, confirming the enhanced oxophillicity of carbides in the presence of W. The observed preferential selectivity toward deoxygenated products in the HDO of guaiacol on MoWC surfaces was further explained by the mechanistic investigation of MoWC and Mo2C surfaces. Our calculations indicated that the direct deoxygenation (DDO) pathway was kinetically favored on the bimetallic MoWC surface (leading to benzene) because of its high oxophilicity, while the hydrogenation-dehydration and DDO pathways proceeded with competitive barriers on Mo2C.