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Hydrogenation of CO 2 is attractive to reduce CO 2 emissions and produce valueadded chemicals (e.g., methanol) with renewable energy. However, the mechanistic understanding of the role of water, a byproduct of CO 2 conversion to methanol, is still missing. Here, we identify that water directly participates in methanol formation via methoxy hydrolysis, and the enhancement on the water vapor diffusion strongly improves methanol selectivity and yield.

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Chemical-looping reforming of methane (CLRM) offers a potentially effective approach for the coproduction of syngas and pure hydrogen. Macroporous CeO2–ZrO2 solid solutions with different pore sizes were prepared as oxygen carriers for the CLRM system. The physical and chemical properties of the oxygen carriers were characterized by the techniques of scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), X-ray powder diffraction (XRD), N2 adsorption–desorption, Raman spectra, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (TPR), and temperature-programmed oxidation (TPO). The relationship among the structural features, the concentration of oxygen defect, and the oxygen mobility of the macroporous CeO2–ZrO2 solid solutions and the nonporous sample were also discussed. It is found that the specific surface area and oxygen mobility of such oxides are closed correlated to their performance for the CLRM process. Compared with the nonporous sample, the macroporous CeO2–ZrO2 solid solutions own lower specific surface area and better oxygen mobility due to the relatively high concentration of the oxygen vacancy. This allows such oxides to own both high conversion and selectivity for the oxidation of methane to syngas using a chemical-looping concept. The presence of the macroporous structure also improves the reoxidation rate and hydrogen yield (the hydrogen yield increased by 50%) in the water splitting step. This can be attributed to the abundant channels in the materials, which may improve the dynamic conditions of the gas–solid reactions, resulting in relatively high utilization rate of oxygen carriers. The pore size also affects the activity of the macroporous oxygen carriers. The increase of the pore size slightly reduces the reactivity of the oxygen carrier due to partial collapse of the macroporous structure. The macroporous Ce–Zr-100 oxygen carrier with a pore size of 100 nm exhibits excellent activity and stability in the coproduction of pure hydrogen and high-quality syngas, even after 40 times successive redox cycles.

The effects of transition metal (Fe, Co and Ni) modification (adsorption, insertion and substitution) of CeO2 surfaces on oxygen vacancy formation and CH4 activation are studied on the basis of first principles calculations. The results indicate that the hollow, O-O-bridge and Ce-O-bridge sites are the most stable sites for Fe, Co and Ni atom adsorption on the CeO2(111) surface, and the double O-bridge, O-top and double O-bridge sites are the corresponding most favorable sites for the CeO2(110) surface. Most of the configurations that are generated by the transition metal modification of CeO2(111) and (110) surfaces are accompanied by the reduction of Ce4+ to Ce3+. Based on the calculated subsurface (SS) and sublayer (SL) oxygen vacancies of the CeO2(111) surface, the results show that the substitution of transition metals on the CeO2(111) surface can promote SS oxygen vacancy formation spontaneously, whereas the most stable adsorption of transition metal Fe and Ni atoms on the CeO2(111) surface can promote SL oxygen vacancy formation spontaneously. For the CeO2(110) surface, the substitution of transition metals can facilitate plain (P) and spilt (S)-type oxygen vacancy formation spontaneously. With respect to CH4 activation, the results show that Co atom substitution on the CeO2(110) surface can greatly facilitate the first C-H bond activation step, with an energy barrier of 0.783 eV and a reaction energy of 0.229 eV. However, Co atom substitution on the CeO2(110) surface with P and S-type oxygen vacancies is not conducive to C-H activation. The obtained results could provide new insights into the structural features of transition metal-modified CeO2 at the atomistic level, leading to the more efficient design of oxygen carriers and the optimization of the activation pathways of methane over this type of catalyst.

Syngas generation via thermochemical H 2 O−CO 2 splitting relies heavily on a high-temperature decomposition of metal oxides into a reduced state. Meanwhile, typical chemical looping partial oxidation of methane to syngas suffers from the carbon deposition and the low selectivity toward syngas. To overcome these drawbacks, the partial oxidation of methane and H 2 O−CO 2 splitting are coupled to consist of an alternative chemical looping redox scheme for the generation of Fischer− Tropsch (F−T)-ready syngas. The usability of lattice oxygen in a redox catalyst is facilitated, and its redox property is also thermodynamically optimized by using H 2 O and CO 2 as soft oxidants, guaranteeing an effective generation of syngas from both redox steps. The carbon tolerance is greatly enhanced due to H 2 O or CO 2 gasification in the reoxidization step. Experimental studies confirm the redox scheme by using CeO 2 −LaFeO 3 redox catalyst, demonstrating generation of syngas with an H 2 /CO molar ratio around 2.0 in both methane partial oxidation and H 2 O−CO 2 splitting steps over 30 repeated cycles at 850 °C. A syngas selectivity of 95% in methane partial oxidation and nearly 100% conversion of CO 2 to CO can be achieved. Synergistic effect and competing reaction between H 2 O and CO 2 splittings over the reduced redox catalyst are the key factors for the control of syngas composition and the intensification of CO 2 splitting. The proposed approach can potentially be applied for production of F−T-ready syngas with an increased yield without the need for gas separation when compared to the state-of-the-art thermochemical splitting or methane chemical looping partial oxidation processes.