Bandgap widening through doping for improving the photocatalytic oxidation ability of narrow-bandgap semiconductors.

The trade-off relationship between narrowing the bandgap and achieving sufficient redox potentials accounts for the hindrance to the development of an efficient photocatalyst. Most of the previous researchers attempt to narrow the bandgap of semiconductors by impurity doping to achieve visible-light sensitivity, but this approach causes the losses of their oxidation and/or reduction ability. Conversely, this study presents a bandgap widening strategy by doping to improve the redox potential of photogenerated carriers. Employing first-principles simulations, we propose the lanthanum-doped bismuth vanadate (La-BiVO4) photocatalyst as a wider-bandgap semiconductor exhibiting stronger oxidation ability compared to pristine BiVO4, and the results revealed that the bismuth orbital in the valence band (VB) was diluted by lanthanum-ion doping, while the VB shifted to a higher potential (positively shifted). Thereafter, a La-BiVO4 powder was synthesized via a solid-state reaction, after which its activity was evaluated in the photocatalytic oxidation of 2-propanol (IPA). La-BiVO4 exhibited bandgap widening; thus, the number of absorbed photons under visible-light irradiation was lower than that of pristine BiVO4. However, the quantum efficiency (QE) of La-BiVO4 for the oxidation of IPA was higher than that of the pristine BiVO4. Consequently, the photocatalytic reaction rate of La-BiVO4 was superior to that of pristine BiVO4 under the same visible-light irradiation conditions. Although the bandgap of La-BiVO4 is widened, it is still sensitive to the cyan-light region, which is the strongest in the sunlight spectrum. These results demonstrate that the orbital dilution strategy by impurity elemental doping is effective for bandgap widening and contributes to improving the oxidation and/or reduction ability of the photogenerated charge carriers. This study elucidates the possibility of boosting photocatalytic performances via bandgap widening.

Highly Dispersed Ni Nanoclusters Spontaneously Formed on Hydrogen Boride Sheets.

Hydrogen boride (HB) sheets are two-dimensional materials comprising a negatively charged hexagonal boron network and positively charged hydrogen atoms with a stoichiometric ratio of 1:1. Herein, we report the spontaneous formation of highly dispersed Ni nanoclusters on HB sheets. The spontaneous reduction reaction of Ni ions by the HB sheets was monitored by in-situ measurements with an ultraviolet-visible spectrometer. Acetonitrile solutions of Ni complexes and acetonitrile dispersions of the HB sheets were mixed in several molar ratios (the HB:Ni molar ratio was varied from 100:0.5 to 100:20), and the changes in the absorbance were measured over time. In all cases, the results suggest that Ni metal clusters grow on the HB sheets, considering the increase in absorbance with time. The absorbance peak position shifts to the higher wavelength as the Ni ion concentration increases. Transmission electron microscopy images of the post-reaction products indicate the formation of Ni nanoclusters, with sizes of a few nanometers, on the HB sheets, regardless of the preparation conditions. These highly dispersed Ni nanoclusters supported on HB sheets will be used for catalytic and plasmonic applications and as hydrogen storage materials.

Metal Carbide as A Light-Harvesting and Anticoking Catalysis Support for Dry Reforming of Methane.

Dry reforming of methane (DRM) is one of the most attractive chemical reactions, since it converts global-warming gases into valuable syngas including hydrogen and carbon monoxide. Numerous previous studies used metal oxides catalysis supports, such as Al2O3, but their operating temperature was very high and severe coking occurred and deteriorated their catalytic activities. The present study reports that a metal carbide like tantalum carbide (TaC) acts as a multifunctional catalyst support for the DRM reaction, including light-harvesting properties for saving energy operation as well as an anticoking property for long-term stability. Nickel nanoparticles loaded on tantalum carbide (Ni/TaC) are prepared by impregnation and reductive hydrogen treatment. TaC particles act as a light-harvesting support to promote the DRM reaction by photon irradiation through plasmonic photothermal energy conversion in TaC. Furthermore, Ni/TaC exhibits an excellent long-term anticoking property, as compared to Ni loaded on conventional metal oxide supports such as Al2O3 or Ta2O5. According to the sole gas condition's experiment, and secondary ion mass spectroscopy, the oxy-carbide layer near the interface between TaC and Ni plays an essential role in imparting the efficient anticoking property of Ni/TaC.

Visible-light-driven dry reforming of methane using a semiconductor-supported catalyst.

Dry reforming of methane (DRM) is an attractive reaction that consumes two major greenhouse gases while producing the industrially important components of syngas. In this study, various semiconductors were examined as light-harvesting support materials to promote catalytic DRM reaction under mild conditions. Among the metal-loaded catalysts, rhodium-loaded tantalum oxynitride (Rh/TaON) drove the DRM reaction even under visible light irradiation (>400 nm), and its activity exceeded the thermal catalyst limit. According to our spectroscopic analysis and the surface temperature measurement, the bandgap excitation of TaON dominantly promotes the DRM reaction in addition to its photo-thermal effect.

CO2 oxidative coupling of methane using an earth-abundant CaO-based catalyst.

CO2 oxidative coupling of methane has been achieved by using CO2 as the oxidant. We explored various catalysts with the capability of producing C2,3 hydrocarbons and found that the use of a CaO-based oxide with sodium (Na) and chloride (Cl) allowed for remarkable direct methane conversion with a C2,3 yield of 6.6% at 950 °C. Microstructural characterisations showed that the optimal sample contained sodium carbonate (Na2CO3) covered with fine calcium oxide particles with chloride doping. Interestingly, sodium carbonate acted as a molten salt catalyst in this scenario. The synthesised active components are earth-abundant and can increase the possibility of achieving higher yields of hydrocarbons.

Nanoporous Nickel Composite Catalyst for the Dry Reforming of Methane.

The development of efficient catalysts with high activities and durabilities for use in the dry reforming of methane (DRM) is desirable but challenging. We report the development of a nanoporous nickel composite (nanoporous Ni/Y2O3) via a facile one-step dealloying technique, for use in the DRM. Focusing on the low-temperature DRM, our composite possessed remarkable activity and durability against coking compared with conventional particle-based Ni catalysts. This was attributed to the aluminum oxides present on the Ni surface, which suppress pore coarsening. In addition, the inert bundled Y2O3 nanowires are suitable for use as substrates for nanoporous Ni.