Dioxygenases (DO) add both O-atoms in O
2 to the same molecule, albeit at rates much slower than monooxygenases (MO). Neither biological nor surface catalysis is known to transfer both O-atom to two different co-reactant molecules, with ethene conversion to ethylene oxide (EO) on promoted Ag as the most practical example in surface catalysis. MO enzymes activate O
2 and use one O-atom in an insertion event but discard the other O-atom via reactions with sacrificial reductants (e.g., NADH). Ag nanoparticles expose ensembles of surface atoms (*) that mediate MO-type channels in C
2H
4-O
2 reactions, with C
2H
4 acting as the substrate and the sacrificial reductant. These channels involve the binding of O
2 at * to form O
2* moieties that transfer one O-atom to C
2H
4 to form ethylene oxide (EO) and strand the O* fragment as a nucleophile that abstracts H-atoms from C
2H
4 (or even EO) in an oxidation cascade that ultimately forms CO
2 and H
2O. When C
2H
4 acts as the only reductant, the products formed (EO:CO
2:H
2O = 3:1:1 molar) correspond to a maximum EO selectivity (85.7% C-basis) defined here as the MO limit.
EO synthesis processes in current practice give selectivities above this MO limit (~90%) through the use of organochlorides (e.g., C2H5Cl) and of alkali and early transition metals additives (e.g., Cs and Re) to Ag-based porous catalysts. These moderators and promoters must therefore enable contributions from DO channels. The optimization of promoter packages, catalyst treatments, and process conditions has led to incremental (but meaningful) selectivity improvements, even without unequivocal interpretations of the nature and coverages of the electrophilic and nucleophilic O-species or the mechanism of their formation. Specifically, the emergence of supra-MO selectivities has led to cost, atom efficiency, and energy benefits, but their mechanistic origins lack plausible proposals. This study addresses how ...
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