RESUMEN
Photosynthesis of H2O2 from earth-abundant O2 and H2O molecules offers an eco-friendly route for solar-to-chemical conversion. The persistent challenge is to tune the photo-/thermo- dynamics of a photocatalyst toward efficient electron-hole separation while maintaining an effective driving force for charge transfer. Such a case is achieved here by way of a synergetic strategy of sub-band-assisted Z-Scheme for effective H2O2 photosynthesis via direct O2 reduction and H2O oxidation without a sacrificial agent. The optimized SnS2/g-C3N4 heterojunction shows a high reactivity of 623.0 µmol g-1 h-1 for H2O2 production under visible-light irradiation (λ > 400 nm) in pure water, ≈6 times higher than pristine g-C3N4 (100.5 µmol g-1 h-1). Photodynamic characterizations and theoretical calculations reveal that the enhanced photoactivity is due to a markedly promoted lifetime of trapped active electrons (204.9 ps in the sub-band and >2.0 ns in a shallow band) and highly improved O2 activation, as a result of the formation of a suitable sub-band and catalytic sites along with a low Gibbs-free energy for charge transfer. Moreover, the Z-Scheme heterojunction creates and sustains a large driving force for O2 and H2O conversion to high value-added H2O2.
RESUMEN
Valence tuning of transition metal oxides is an effective approach to design high-performance catalysts, particularly for the oxygen evolution reaction (OER) that underpins solar/electric water splitting and metal-air batteries. Recently, high-valence oxides (HVOs) are reported to show superior OER performance, in association with the fundamental dynamics of charge transfer and the evolution of the intermediates. Particularly considered are the adsorbate evolution mechanism (AEM) and the lattice oxygen-mediated mechanism (LOM). High-valence states enhance the OER performance mainly by optimizing the eg -orbital filling, promoting the charge transfer between the metal d band and oxygen p band. Moreover, HVOs usually show an elevated O 2p band, which triggers the lattice oxygen as the redox center and enacts the efficient LOM pathway to break the "scaling" limitation of AEM. In addition, oxygen vacancies, induced by the overall charge-neutrality, also promote the direct oxygen coupling in LOM. However, the synthesis of HVOs suffers from relatively large thermodynamic barrier, which makes their preparation difficult. Hence, the synthesis strategies of the HVOs are discussed to guide further design of the HVO electrocatalysts. Finally, further challenges and perspectives are outlined for potential applications in energy conversion and storage.
RESUMEN
Nanostructures of the multimetallic catalysts offer great scope for fine tuning of heterogeneous catalysis, but clear understanding of the surface chemistry and structures is important to enhance their selectivity and efficiency. Focussing on a typical Pt-Pd-Ni trimetallic system, we comparatively examined the Ni/C, Pt/Ni/C, Pd/Ni/C and Pt-Pd/Ni/C catalysts synthesized by impregnation and galvanic replacement reaction. To clarify surface chemical/structural effect, the Pt-Pd/Ni/C catalyst was thermally treated at X=200, 400 or 600 °C in a H2 reducing atmosphere, respectively termed as Pt-Pd/Ni/C-X. The as-prepared catalysts were characterized complementarily by XRD, XPS, TEM, HRTEM, HS-LEIS and STEM-EDS elemental mapping and line-scanning. All the catalysts were comparatively evaluated for benzaldehyde and styrene hydrogenation. It is shown that the "PtPd alloy nanoclusters on Ni nanoparticles" (PtPd/Ni) and the synergistic effect of the trimetallic Pt-Pd-Ni, lead to much improved catalytic performance, compared with the mono- or bi- metallic counterparts. However, with the increase of the treatment temperature of the Pt-Pd/Ni/C, the catalytic performance was gradually degraded, which was likely due to that the favourable nanostructure of fine "PtPd/Ni" was gradually transformed to relatively large "PtPdNi alloy on Ni" (PtPdNi/Ni) particles, thus decreasing the number of noble metal (Pt and Pd) active sites on the surface of the catalyst. The optimum trimetallic structure is thus the as synthesised Pt-Pd/Ni/C. This work provides a novel strategy for the design and development of highly efficient and low-cost multimetallic catalysts, e. g. for hydrogenation reactions.
RESUMEN
LiBH4 is a high-capacity hydrogen storage material; however, it suffers from high dehydrogenation temperature and poor reversibility. To tackle those issues, we introduce a new LiBH4-based system with in situ formed superfine and well-dispersed Li3BO3 and NbH as co-reactants. Those are synthesized by the addition of niobium ethoxide [Nb(OEt)5] to LiBH4, heat treatment of the mixture, and then hydrogenation, where Li3BO3 and NbH are generated from the reaction of Nb(OEt)5 and LiBH4. After optimization, the system with a normalized composition of LiBH4-0.04(Li3BO3 + NbH) in molar fraction shows superior hydrogen storage reversibility and kinetics. The initial and main dehydrogenation temperatures of the system are 200 and 90 °C lower than those of the pristine LiBH4, respectively, and 8.2 wt % H2 is released upon heating to 400 °C. A capacity of 7.2 wt % H2, corresponding to a capacity retention of 91%, is sustained after 30 cycles in an isothermal cyclic regime of dwelling at 400 °C for 60 min for dehydrogenation and dwelling at 500 °C and 50 bar H2 pressure for 20 min for hydrogenation. Such a high cyclic stability for a LiBH4-based system has never been reported to date. The in situ introduced Li3BO3 and NbH have a synergistic catalysis effect on the improvement of the hydrogen storage performance of LiBH4, showing highly effective bidirectional action on both dehydrogenation and hydrogenation.
RESUMEN
Porous carbon can be tailored to great effect for electrochemical energy storage. In this study, we propose a novel structured spherical carbon with a macrohollow core and a microporous shell derived from a sustainable biomass, amylose, by a multistep pyrolysis route without chemical etching. This hierarchically porous carbon shows a particle distribution of 2-10 µm and a surface area of 672 m2 g-1. The structure is an effective host of sulfur for lithium-sulfur battery cathodes, which reduces the dissolution of polysulfides in the electrolyte and offers high electrical conductivity during discharge/charge cycling. The hierarchically porous carbon can hold 48 wt % sulfur in its porous structure. The S@C hybrid shows an initial capacity of 1490 mAh g-1 and retains a capacity of 798 mAh g-1 after 200 cycles at a discharge/charge rate of 0.1 C. A capacity of 487 mAh g-1 is obtained at a rate of 3 C. Both a one-step pyrolysis and a chemical-reagent-assisted pyrolysis are also assessed to obtain porous carbon from amylose, but the obtained carbon shows structures inferior for sulfur cathodes. The multistep pyrolysis and the resulting hierarchically porous carbon offer an effective approach to the engineering of biomass for energy storage. The micrometer-sized spherical S@C hybrid with different sizes is also favorable for high-tap density and hence the volumetric density of the batteries, opening up a wide scope for practical applications.
RESUMEN
The introduction of fluorinated aryls at zinc in Zn(4)N(8)-type (and to a lesser extent Zn(4)N(6)O) cages led to enhanced H(2) uptake. Lithium alkoxides have been shown to link such cages (non-fluorinated), but showed no substantial improvement in uptake.