Single-atom catalysts (SACs) exhibit intriguing catalytic performance owing to their maximized atom utilizations and unique electronic structures. However, the reported strategies for synthesizing SACs generally have special requirements for either the anchored metals or the supports. Herein, we report a universal approach of electrochemical deposition that is applicable to a wide range of metals and supports for the fabrication of SACs. The depositions were conducted on both cathode and anode, where the different redox reactions endowed the SACs with distinct electronic states. The SACs from cathodic deposition exhibited high activities towards hydrogen evolution reaction, while those from anodic deposition were highly active towards oxygen evolution reaction. When cathodically-and anodicallydeposited Ir single atoms on Co 0.8 Fe 0.2 Se 2 @Ni foam were integrated into a two-electrode cell for overall water splitting, a voltage of 1.39 V was required at 10 mA cm −2 in alkaline electrolyte.

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To improve the photocatalytic hydrogen evolution activity of palladium-assisted graphitic carbon nitride (g-C3N4), here, palladium-single-atom-coordinated cyano-group-rich g-C3N4 (Pd/DN-UCN) are synthesized, and the synthesis process includes copolymerization of urea-derived supramolecular aggregates and NH4Cl followed by wet impregnation. By combining powerful characteristic results and theoretical calculations, the formation mechanism of Pd single atoms on the ultrathin, mesoporous cyano-group-rich g-C3N4 nanosheets is proposed, highlighting that the Pd single atoms are firmly stabilized in the interlayers of g-C3N4 nanosheets caused by the combination of the physical confinement effect of ultrathin, mesoporous g-C3N4 nanosheets and coordination bonding of cyano groups with Pd atoms; additionally, Pd–N3 coordination in the Pd/DN-UCN heterojunctions is confirmed, in which one Pd atom coordinates with one N atom of the cyano group and two sp2-hybridized N atoms in the adjacent layer. The presence of cyano groups and Pd–N coordination in the Pd/DN-UCN induces a midgap state in the band structure of g-C3N4. At optimal Pd loading levels (0.16%), the synthesized 0.16%Pd/DN-UCN0.50 exhibits enhanced photocatalytic hydrogen production activity as compared to electrostatically stabilized Pd single atoms on the “sixfold cavities” of g-C3N4, and apparent quantum yield values at the stationary point of the 0.16%Pd/DN-UCN0.50 concentration (1.2 g L–1) can reach up to 14.6, 15.8, 4.69, and 3.05% under monochromatic light irradiation at 365, 400, 450, and 550 nm, respectively. The cooperation of significantly boosted transfer of photoexcited electrons to atomically dispersed Pd sites via as-built interlayer Pd–N coordination delivery channels and the maximal Pd atom utilization efficiency dominates the enhanced photocatalytic hydrogen evolution activity of Pd/DN-UCN.

We performed ab initio DFT+U calculations to explore the interaction between methane and iron oxide oxygen carriers for chemical looping reaction systems. The adsorption of CH4 and CHx (x = 0-3) radicals on α-Fe2O3(001), and the influence of oxygen vacancies at the top surface and on the subsurface on the adsorption properties of the radicals was investigated. The adsorption strength for CH4 and C radicals at the top of the α-Fe2O3(001) surface in the presence of oxygen vacancies is lower than that on the stoichiometric surface. However, for methyl (CH3), methylene (CH2) and methine (CH) radicals, it is correspondingly higher. In contrast, the oxygen vacancy formation on the subsurface not only increases the adsorption strength of CH3, CH2 and CH radicals, but also facilitates C radical adsorption. We found that oxygen vacancies significantly affect the adsorption configuration of CHx radicals, and determine the probability of finding an adsorbed species in the stoichiometric region and the defective region at the surface. With the obtained adsorption geometries and energetics of these species adsorbed on the surface, we extend the analysis to CH4 dissociation under chemical looping reforming conditions. The distribution of adsorbed CH4 and CHx (x = 0-3) radicals is calculated and analyzed which reveals the relationship between adsorbed CHx radical configuration and oxygen vacancies in iron oxide. Also, the oxygen vacancies can significantly facilitate CH4 activation by lowering the dissociation barriers of CH3, CH2 and CH radicals. However, when the oxygen vacancy concentration reaches 2.67%, increasing the oxygen vacancy concentration cannot continue to lower the CH dissociation barrier. The study provides fundamental insights into the mechanism of CH4 dissociation on iron based oxygen carriers and also provide guidance to design more efficient oxygen carriers.

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We perform ab initio DFT+U calculations and experimental studies of the partial oxidation of methane to syngas on iron oxide oxygen carriers to elucidate the role of oxygen vacancies in oxygen carrier reactivity. In particular, we explore the effect of oxygen vacancy concentration on sequential processes of methane dehydrogenation, and oxidation with lattice oxygen. We find that when CH adsorbs onto Fe atop sites without neighboring oxygen vacancies, it dehydrogenates with CH radicals remaining on the same site and evolves into COvia the complete oxidation pathway. In the presence of oxygen vacancies, on the other hand, the formed methyl (CH) prefers to migrate onto the vacancy site while the H from CH dehydrogenation remains on the original Fe atop site, and evolves into CO via the partial oxidation pathway. The oxygen vacancies created in the oxidation process can be healed by lattice oxygen diffusion from the subsurface to the surface vacancy sites, and it is found that the outward diffusion of lattice oxygen atoms is more favorable than the horizontal diffusion on the same layer. Based on the proposed mechanism and energy profile, we identify the rate-limiting steps of the partial oxidation and complete oxidation pathways. Also, we find that increasing the oxygen vacancy concentration not only lowers the barriers of CH dehydrogenation but also the cleavage energy of Fe-C bonds. However, the barrier of the rate-limiting step cannot further decrease when the oxygen vacancy concentration reaches 2.5%. The fundamental insight into the oxygen vacancy effect on CH oxidation with iron oxide oxygen carriers can help guide the design and development of more efficient oxygen carriers and CLPO processes.

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