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.
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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