Trivalent Nickel-Catalyzing Electroconversion of Alcohols to Carboxylic Acids

The comprehension of activity and selectivity origins of the electrooxidation of organics is a crucial knot for the development of a highly efficient energy conversion system that can produce value-added chemicals on both the anode and cathode. Here, we find that the potential-retaining trivalent nickel in NiOOH (Fermi level, −7.4 eV) is capable of selectively oxidizing various primary alcohols to carboxylic acids through a nucleophilic attack and nonredox electron transfer process. This nonredox trivalent nickel is highly efficient in oxidizing primary alcohols (methanol, ethanol, propanol, butanol, and benzyl alcohol) that are equipped with the appropriate highest occupied molecular orbital (HOMO) levels (−7.1 to −6.5 eV vs vacuum level) and the negative dual local softness values (Δsk, −0.50 to −0.19) of nucleophilic atoms in nucleophilic hydroxyl functional groups. However, the carboxylic acid products exhibit a deeper HOMO level (<−7.4 eV) or a positive Δsk, suggesting that they are highly stable and weakly nucleophilic on NiOOH. The combination (HOMO, Δsk) is useful in explaining the activity and selectivity origins of electrochemically oxidizing alcohols to carboxylic acid. Our findings are valuable in creating efficient energy conversions to generate value-added chemicals on dual electrodes

Back Electron Transfer at TiO₂ Nanotube Photoanodes in the Presence of a H₂O₂ Hole Scavenger

Adding charge scavengers, which usually are more unstable than water, is an effective method to quantify the quantum efficiency loss of photoelectrode during the charge separation, transfer, and injection processes of the water splitting reaction. Here, we detected, on TiO₂ nanotube photoanodes after using hydrogen peroxide (H₂O₂) as a hole scavenger, a nearly 40% saturated photocurrent decrease in alkaline electrolyte and a negligible saturated photocurrent difference in acid electrolyte. We found that the photoelectrons were trapped in the surface states of TiO₂ with nearly the same storage capacity of electrons in a wide range of pH values from 1.0 to 13.6. However, kinetics of a back reaction, H₂O₂ reduction by the photoelectrons trapped in surface states, is about 10 times higher for that in alkaline electrolyte than in acid electrolyte. As a result, the pH-dependent kinetic difference in H₂O₂ reduction induced the negative effects on the saturated photocurrent. Our results offer a new insight into understanding the effects of back electron transfer on electrochemical behaviors of surface states and charge scavengers

Solution-Chemical Route to Generalized Synthesis of Metal Germanate Nanowires with Room-Temperature, Light-Driven Hydrogenation Activity of CO₂ into Renewable Hydrocarbon Fuels

A facile solution-chemical route was developed for the generalized preparation of a family of highly uniform metal germanate nanowires on a large scale. This route is based on the use of hydrazine monohydrate/H₂O as a mixed solvent under solvothermal conditions. Hydrazine has multiple effects on the generation of the nanowires: as an alkali solvent, a coordination agent, and crystal anisotropic growth director. Different-percentage cobalt-doped Cd₂Ge₂O₆ nanowires were also successfully obtained through the addition of Co­(OAc)₂·4H₂O to the initial reaction mixture for future investigation of the magnetic properties of these nanowires. The considerably negative conduction band level of the Cd₂Ge₂O₆ nanowire offers a high driving force for photogenerated electron transfer to CO₂ under UV–vis illumination, which facilitates CO₂ photocatalytic reduction to a renewable hydrocarbon fuel in the presence of water vapor at room temperature

In-Situ Formed Hydroxide Accelerating Water Dissociation Kinetics on Co₃N for Hydrogen Production in Alkaline Solution

Sluggish water dissociation kinetics on nonprecious metal electrocatalysts limits the development of economical hydrogen production from water–alkali electrolyzers. Here, using Co₃N electrocatalyst as a prototype, we find that during water splitting in alkaline electrolyte a cobalt-containing hydroxide formed on the surface of Co₃N, which greatly decreased the activation energy of water dissociation (Volmer step, a main rate-determining step for water splitting in alkaline electrolytes). Combining the cobalt ion poisoning test and theoretical calculations, the efficient hydrogen production on Co₃N electrocatalysts would benefit from favorable water dissociation on in-situ formed cobalt-containing hydroxide and low hydrogen production barrier on the nitrogen sites of Co₃N. As a result, the Co₃N catalyst exhibits a low water-splitting activation energy (26.57 kJ mol^–1) that approaches the value of platinum electrodes (11.69 kJ mol^–1). Our findings offer new insight into understanding the catalytic mechanism of nitride electrocatalysts, thus contributing to the development of economical hydrogen production in alkaline electrolytes

Oxygen-Vacancy-Activated CO₂ Splitting over Amorphous Oxide Semiconductor Photocatalyst

Gaseous oxides generated during industrial processes, such as carbon oxides (CO_<i>x</i>) and nitrogen oxides (NO_<i>x</i>), have important effects on the Earth’s atmosphere. It is highly desired to develop a low-cost and efficient route to convert them to harmless products. Here, direct splitting of gaseous oxides was proposed on the basis of photocatalysis by an amorphous oxide semiconductor. As an example, splitting of CO₂ into carbon and oxygen was achieved over amorphous zinc germanate (α-Zn-Ge-O) semiconductor photocatalyst under 300 W Xe lamp irradiation. Electron paramagnetic resonance and ¹⁸O isotope labeling indicated that the splitting of CO₂ was achieved via photoinduced oxygen vacancies on α-Zn-Ge-O reacting and thus filling with O of CO₂, while the photogenerated electrons reduced the carbon species of CO₂ to solid carbon. Under irradiation, such a defect reaction is sustainable by continuous photogenerated hole oxidation of surface oxygen atoms on α-Zn-Ge-O to form oxygen vacancies and to release O₂. When we used H₂O or NO in place of CO₂, H₂ and O₂ or N₂ and O₂ were evolved, respectively, indicating the same mechanism can also split H₂O or NO