Direct deoxygenation reaction of oxygenated model compounds by biomass pyrolysis on the Ni₅P₄(001) surface: a computational study

The proven catalytic ability of nickel phosphides in hydrodeoxygenation reactions and their capability to preserve the aromaticity of the products indicate that these catalysts possess exceptional surface affinity towards the functional groups of the probe molecules of interest. However, to date, it remains unclear how deoxygenation reactions occur on nickel phosphides. Therefore, the present work aims to shed light on the reaction mechanism for the direct deoxygenation (DDO) of phenol and guaiacol as probe molecules on the Ni₅P₄(001) surface using ab initio calculations. Interestingly, the dissociation of phenol occurs immediately due to the low activation energy of 0.15 eV, whereas a higher activation barrier of 1.92 eV was found for the dissociation of guaiacol. Notably, the hydrogenation reaction steps for both molecules have to overcome a high activation energy (>1.79 eV). Interestingly, after O-H and C_arom-O bond scission, the aromatic fragments of phenol remain perpendicular/tilted on the surface due to their more directional bonding to the Ni₅P₄(001) surface. This orientation limits the activation of the C = C bond of the ring, and probably will inhibit the over-hydrogenation pathway, thus preserving the aromaticity of the ring. In addition, microkinetic modeling was performed to determine the overall activation energy, reaction order and rate-limiting steps. The activation energy exhibits a diminishing trend (292 to 240 kJ mol⁻¹) with temperature in the range of 500-700 K. The reaction order with respect to phenol increases considerably with temperature, accounting for almost unity at 700 K. The hydrogenation of the phenyl intermediate to benzene is the reaction-limiting step at temperatures lower than 550 K, which is ascribed to the high kinetic barrier of this reaction step, whereas the C_arom-O cleavage step controls the overall reaction at temperatures greater than 550 K. Considering the DDO reaction of guaiacol, the dehydroxylation step predominantly controls the deoxygenation process at temperatures lower than 600 K; however, the rate-controlling step shifts to the scission of C_arom-O bond, which is related to the OCH₃ group, at higher temperatures.