Resolving the Intricate Mechanism and Selectivity of Syngas Conversion on Reduced ZnCr 2 O x : A Quantitative Study from DFT and Microkinetic Simulations

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By combining reduced oxide and zeolite (e.g., ZnCrO x /MSAPO), bifunctional catalysts achieve outstanding olefin selectivity for syngas conversion via a relay process (CO + H 2 → IM → olefin) and are one of the hottest catalytic systems. However, the reaction mechanism on the reduced ZnCrO x as well as the intermediate (IM = ketene versus methanol) entering the zeolite are under heated debate owing to the complexity of this system. Herein, we perform systematic density functional theory (DFT) calculations and microkinetic simulations to inspect the possible elementary steps on the highly reduced ZnCr 2 O 4 (110) surface, which quantitatively unveils the favored reaction pathways for CO activation and conversion. We find that CO tends to adsorb at the two-coordinate O vacancy sites (V O 2C ) and reacts with H at V O 2C to form CHO, while the disproportionation reaction and the direct C–O cleavage are less favored. Importantly, the conversion of CHO plays a vital role in product selectivity; the dissociation of CHO is identified to proceed easily and constitutes a major route responsible for the formation of CH 4 and CH 2 CO, through the following pathway: CHO → CH + O; CH + H → CH 2; CH 2 + CO → CH 2 CO; or CH 2 + 2H → CH 4 . Alternatively, CHO could undergo hydrogenation to give CH 2 O and CH 3 O intermediates, eventually leading to the formation of CH 3 OH and CH 4 . The kinetic analyses on such a complex reaction network disclose that CH 4 is the dominant product, while both CH 2 CO and CH 3 OH ( i.e., two experimentally controversial intermediates) exist in minority with CH 2 CO being relatively more readily formed. More interestingly, the kinetic model illustrates that the selectivity for CH 2 CO and the formation of triggered light olefins can be significantly improved over CH 4 if a reaction channel converts CH 2 CO to light olefins when zeolite is added, providing insight into the bifunctionality of the oxide/zeolite system. Also, we demonstrate that high V O 2C coverage is a prerequisite for high activity of CO activation, and the reason for high selectivity of CO 2 and low selectivity of H 2 O is identified to the easier removal of the lattice oxygen by CO to generate CO 2 than by H 2 to generate H 2 O. The understanding derived from this work could lay a solid theoretical foundation for comprehending the oxides for the syngas conversion mechanism.