Density Functional Theory Study of Acetaldehyde Hydrodeoxygenation on MoO3

Periodic spin-polarized density functional theory calculations were performed to investigate acetaldehyde (CH3CHO) hydrodeoxygenation on the reduced molybdenum trioxide (MoO3) surface. The perfect O-terminated alpha-MoO3-(010) surface is reduced to generate an oxygen defect site in the presence of H-2. H-2 dissociatively adsorbs at the surface oxygen sites forming two surface hydroxyls, which can recombine into a water molecule weakly bound at the Mo site. A terminal oxygen (O-t) defect site thus forms after water desorption. CH3CHO adsorbs at the O-deficient Mo site via either the sole O-Mo band or the O-Mo and the C-O double bonds. The possible reaction pathways of the adsorbed CH3CHO with these two configurations were thoroughly examined using the dimer searching method. Our results show that the ideal deoxygenation of CH3CHO leading to ethylene (C2H4) on the reduced MoO3(010) surface is feasible. The adsorbed CH3CHO first dehydrogenate into CH2CHO by reacting with a neighboring terminal O-t. The hydroxyl (OtH) then hydrogenates CH2CHO into CH2CH2O to complete the hydrogen transfer cycle with an activation barrier of 1.39 eV. The direct hydrogen transfer from CH3CHO to CH2CH2O is unlikely due to the high barrier of 2.00 eV. The produced CH2CH2O readily decomposes into C2H4 that directly releases to the gas phase and regenerates the O-t atom on the Mo site. As a result, the reduced MoO3(010) surface is reoiddized to the perfect MoO3(010) surface after CH3CHO deoxygenation.