Hydrodeoxygenation of phenol over Ni-based bimetallic single-atom surface alloys: mechanism, kinetics and descriptor

Selectively activating C-O bond cleavage of phenolics by catalyst design is essential to hydrodeoxygenation (HDO) of lignin-derived bio-oil for the removal of oxygen content. Herein, using density functional theory calculations combined with microkinetic modeling, we systematically investigated HDO of phenol, a model compound for phenolics, over Ni-based bimetallic single-atom surface alloys denoted as M@Ni(111) where M = Sc, Ti, V, Cr, Mn, Fe, Co, Mo, W, and Re. It is found that alloyed M atoms can modify the electronic structure of local active sites by lifting d-band centers of three nearest neighboring Ni-M-Ni atoms which enhance the OH* binding strength and accordingly, phenol HDO. Compared to the direct deoxygenation (DDO) pathway, partial hydrodeoxygenation (PHDO) contributes the most to the benzene formation. We reveal that for optimal phenolic-HDO performance, a balance of alloyed M's oxophilicity should be achieved to maximize DDO, PHDO, and H2O formation simultaneously. The predicted turnover frequency for benzene formation follows a volcano curve varying with OH* binding strength, which can serve as an effective catalytic descriptor for deoxygenation activity producing phenyl hydrocarbon products. Our study could provide theoretical guidance for designing highly active and selective HDO catalysts for upgrading bio-oil.