Origin of Efficient Catalytic Combustion of Methane over Co₃O₄(110): Active Low-Coordination Lattice Oxygen and Cooperation of Multiple Active Sites

A complete catalytic cycle for methane combustion on the Co₃O₄(110) surface was investigated and compared with that on the Co₃O₄(100) surface on the basis of first-principles calculations. It is found that the 2-fold coordinated lattice oxygen (O_2c) would be of vital importance for methane combustion over Co₃O₄ surfaces, especially for the first two C–H bond activations and the C–O bond coupling. It could explain the reason the Co₃O₄(110) surface significantly outperforms the Co₃O₄(100) surface without exposed O_2c for methane combustion. More importantly, it is found that the cooperation of homogeneous multiple sites for multiple elementary steps would be indispensable. It not only facilitates the hydrogen transfer between different sites for the swift formation of H₂O to effectively avoid the passivation of the active low-coordinated O_2c site but also stabilizes surface intermediates during the methane oxidation, optimizing the reaction channel. An understanding of this cooperation of multiple active sites not only might be beneficial in developing improved catalysts for methane combustion but also might shed light on one advantage of heterogeneous catalysts with multiple sites in comparison to single-site catalysts for catalytic activity

Insight into Room-Temperature Catalytic Oxidation of Nitric oxide by Cr₂O₃: A DFT Study

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+<i>U</i> 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 Cr_3c, 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 Cr_3c 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 Cr_3c. In contrast, on the (012) termination, which exposes the five-coordinated Cr_5c, 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

Low-Temperature Methane Combustion over Pd/H-ZSM-5: Active Pd Sites with Specific Electronic Properties Modulated by Acidic Sites of H‑ZSM‑5

Pd/H-ZSM-5 catalysts could completely catalyze CH₄ to CO₂ at as low as 320 °C, while there is no detectable catalytic activity for pure H-ZSM-5 at 320 °C and only a conversion of 40% could be obtained at 500 °C over pure H-ZSM-5. Both the theoretical and experimental results prove that surface acidic sites could facilitate the formation of active metal species as the anchoring sites, which could further modify the electronic and coordination structure of metal species. PdO_<i>x</i> interacting with the surface Brönsted acid sites of H-ZSM-5 could exhibit Lewis acidity and lower oxidation states, as proven by the XPS, XPS valence band, CO-DRIFTS, pyridine FT-IR, and NH₃-TPD data. Density functional theory calculations suggest PdO_<i>x</i> groups to be the active sites for methane combustion, in the form of [AlO₂]­Pd­(OH)-ZSM-5. The stronger Lewis acidity of coordinatively unsaturated Pd and the stronger basicity of oxygen from anchored PdO_<i>x</i> species are two key characteristics of the active sites ([AlO₂]­Pd­(OH)-ZSM-5) for methane combustion. As a result, the PdO_<i>x</i> species anchored by Brønsted acid sites of H-ZSM-5 exhibit high performance for catalytic combustion of CH₄ over Pd/H-ZSM-5 catalysts