(425h) Direct Deoxygenation Reaction of Biomass Pyrolysis Model Compounds on Ni5P4(001) Surface: Computational Study

The proven ability of nickel phosphides in catalytic hydrodeoxygenation reactions and their capability to preserve the aromaticity/unsaturation in products has indicated that these catalysts possess exceptional surface chemistry towards H, O and C species. However, how deoxygenation reaction proceeds on these catalysts is not well understood. On this context, the present work shed light on the reaction mechanism of direct deoxygenation (DDO) reaction of phenol and guaiacol on Ni₅P₄(001) surface using density functional theory (DFT) computations. Interestingly, the phenol dissociation takes place easily, because of the low activation barrier (0.15 eV). The C-O bond cleavage step overcome an activation barrier of 1.51 eV, whereas the phenyl hydrogenation step is kinetically hindered, surmounting for high energy barrier of 1.79 eV. Notably, after O-H and C–O bond scissions, the aromatic fragments of phenol remained perpendicular/tilted over the surface, thanks to more directional bonding to Ni₅P₄(001) surface. This orientation limited the activation of C=C bond of the ring and most probably would inhibit overhydrogenation pathway, thus preserving the aromaticity of the ring. In addition, micro-kinetic modeling was carried out to determine the apparent activation energy, reaction orders and rate-limiting steps as a function of temperature. The apparent activation energy exhibited a diminishing trend (292 to 240 kJ/mol) with temperature rise in the range of 500–700 K. The reaction orders in phenol increased considerably with rising temperature, accounting for almost unity at 700 K. The hydrogenation of phenyl intermediate to benzene is the rate-limiting step at temperatures below 550 K, which is rationalized by the high kinetic barrier of this step, whereas C–O cleavage step dominates the reaction rate at higher temperatures.