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 + H2 → 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 ZnCr2O4(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 (VO 2C) and reacts with H at VO 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 CH4 and CH2CO, through the following pathway: CHO → CH + O; CH + H → CH2; CH2 + CO → CH2CO; or CH2 + 2H → CH4. Alternatively, CHO could undergo hydrogenation to give CH2O and CH3O intermediates, eventually leading to the formation of CH3OH and CH4. The kinetic analyses on such a complex reaction network disclose that CH4 is the dominant product, while both CH2CO and CH3OH (i.e., two experimentally controversial intermediates) exist in minority with CH2CO being relatively more readily formed. More interestingly, the kinetic model illustrates that the selectivity for CH2CO and the formation of triggered light olefins can be significantly improved over CH4 if a reaction channel converts CH2CO to light olefins when zeolite is added, providing insight into the bifunctionality of the oxide/zeolite system. Also, we demonstrate that high VO 2C coverage is a prerequisite for high activity of CO activation, and the reason for high selectivity of CO2 and low selectivity of H2O is identified to the easier removal of the lattice oxygen by CO to generate CO2 than by H2 to generate H2O. The understanding derived from this work could lay a solid theoretical foundation for comprehending the oxides for the syngas conversion mechanism.

Cr-based catalysts have drawn attention as promising room-temperature NO oxidation catalysts. However, the intrinsic active component and reaction mechanism at the atomic level remain unclear. Here, taking the Cr2O3, one of the most stable chromium oxides, as an object, we systematically investigated NO oxidation processes on Cr2O3(001) and -(012) surfaces by virtue of DFT+U calculations, aiming to uncover the activity-limiting factors and basic structure–activity relationship of the Cr2O3 catalyst. It was revealed that NO oxidation could not proceed via a Mars–van Krevelen mechanism involving the lattice oxygen on both surfaces. For the Cr2O3(001) surface exposing the isolated three-coordinated Cr3c, the reactions are inclined to occur through the Eley–Rideal route, in which the NO couples directly with the molecular O2* or atomic O* adsorbed at the Cr3c site to form two key intermediate species (ONOO* and NO2*) following a barrierless process. Nevertheless, the overall activity is limited by the irreversible adsorption of NO2 species on the highly unsaturated Cr3c. In contrast, on the (012) termination, which exposes the five-coordinated Cr5c, the NO2* can be easily released, but the reactant O2 cannot be efficiently adsorbed and also results in a limited overall activity at room temperature. To achieve a higher activity, a thermodynamically favored interface model of monochain CrO3 supported on Cr2O3(012) was proposed, which shows an improved O2 adsorption energy of −0.99 eV and thus an enhanced activity of Cr2O3(012), possibly accounting for the experimentally high activity of Cr-based catalysts usually involving the Cr3+/Cr6+ redox. This study demonstrated the catalytic ability of Cr2O3 for NO oxidation at room temperature, and the presented systematic picture may facilitate the further design of more active Cr-based catalysts.