The activation and utilization of substrates mediated by Frustrated Lewis Pairs (FLPs) was initially believed to occur solely via a two-electron, cooperative mechanism. More recently, the occurrence of a single-electron transfer (SET) from the Lewis base to the Lewis acid was observed, indicating that mechanisms that proceed via one-electron-transfer processes are also feasible. As such, SET in FLP systems leads to the formation of radical ion pairs, which have recently been more frequently observed. In this review, we aim to discuss the seminal findings regarding the recently established insights into the SET processes in FLP chemistry as well as highlight examples of this radical formation process. In addition, applications of reported main group radicals will also be reviewed and discussed in the context of the understanding of SET processes in FLP systems.

Insights into Single-Electron-Transfer Processes in Frustrated Lewis Pair Chemistry and Related Donor-Acceptor Systems in Main Group Chemistry.

Heterocyclic diradicaloids with atom-precise control over open-shell nature are promising materials for organic electronics and spintronics. Herein, we disclose quinoidal π-extension of a B/N-heterocycle for generating B/N-type organic diradicaloids. Two quinoidal π-extended B/N-doped polycyclic hydrocarbons that feature fusion of the B/N-heterocycle motif with the antiaromatic s-indacene or dicyclopenta[b,g]naphthalene core were synthesized. This quinoidal π-extension and B/N-heterocycle leads to their open-shell electronic nature, which stands in contrast to the multiple-resonance effect of conventional B/N-type emitters. These B/N-type diradicaloids have modulated (anti)aromaticity and enhanced diradical characters comparing with the all-carbon analogues, as well as intriguing properties, such as magnetic activities, narrow energy gaps and highly red-shifted absorptions. This study thus opens the new space for both of B/N-doped polycyclic π-systems and heterocyclic diradicaloids.

Diradical B/N-Doped Polycyclic Hydrocarbons.

Adding biochar to anaerobic digestion (AD) system is an attractive regulation strategy to promote methane production efficiency from wastes/wastewater. The biochar has high porosity, large surface area, alkaline property, good conductivity, high ion exchange capacity, and abundant functional groups, among them, hierarchical porous structure is one key factor to affect AD. This review systematically summarized the recent progress for the roles of pore size of biochar in AD, including identification and regulation of pore size in biochar and potential improvement effects related to pore size of biochar in AD. Moreover, future perspectives and challenges have been concluded. The hierarchical pore structure and size of biochar play crucial roles in performance enhancement, environment improvement, substance transport promotion, microbial community regulation, and biogas purification. An effective synthesis strategy for achieving directed control of pore size is needed, for which, machine learning may be helpful. Then, the hierarchical porous channels of biochar can be monitored in situ while AD is in progress. It is important to understand the correlation between pore size in biochar and AD performance. Moreover, a comprehensive life cycle assessment focused on biochar preparation, AD efficiency, biochar recycling, final disposal of digestate is urgently needed to evaluate economic and environmental benefits of engineering applications. This review aims to provide some references for the latest developments in the use of hierarchical porous biochar in AD. 

Biochar regulates anaerobic digestion: Insights to the roles of pore size

Modern lean-operated internal combustion engines running on natural gas, biogas, or methane produced from wind or solar energy are highly fuel-efficient and can greatly contribute to securing energy supply, e.g. by mitigating fluctuations in the power grid. Although only comparably low emission levels form during combustion, a highly optimized emission control system is required that converts pollutants over a wide range of operation conditions. In this context, this review article pinpoints the main challenges during methane and formaldehyde oxidation as well as selective catalytic reduction of nitric oxides. The impact of catalyst formulation and operation conditions on catalytic activity and selectivity as well as the combination of several technologies for emission abatement is critically discussed. Additionally, recent experimental and theory-based progress and developments are assessed, allowing coverage of all time and length scales relevant in emission control, i.e. ranging from mechanistic and fundamental insights including atomic-level phenomena to full-scale applications. 

Exhaust gas after-treatment of lean-burn natural gas engines – from fundamentals to application

Understanding the energetics of electron transfer at the semiconductor interface is crucial for the development of solar harvesting technologies, including photovoltaics, photocatalysis, and solar fuel systems. However, modern artificial photosynthetic materials are not efficient and limited by their fast charge recombination with high binding energy of excitons. Hence, reducing the exciton binding energy can increase the generation of charge carriers, which improve the photocatalytic activities. Extensive research has been dedicated to improving the exciton dissociation efficiency through rational semiconductor design via heteroatom doping, vacancy engineering, the construction of heterostructures, and donor-π-acceptor (D-π-A) interfaces to extend the charge carrier migration, promoting the dissociation of excitons. Consequently, functionalized photocatalysts have demonstrated remarkable photocatalytic performances for solar fuel production under visible light irradiation. This review provides the fundamental aspects of excitons in semiconductor nanostructures, having a high binding energy and ultrafast exciton formation together with promising photo-redox properties for solar to fuel conversion application. In particular, this review highlights the significant role of the excitonic effect in the photocatalytic activity of newly developed functional materials and the underlying mechanistic insight for tuning the performance of nanostructured semiconductor photocatalysts for water splitting, CO2 reduction, and N2 fixation reactions.

Band-structure tunability via the modulation of excitons in semiconductor nanostructures: manifestation in photocatalytic fuel generation.

A stable cross-conjugated diradical was prepared by the reaction of a donor-acceptor-donor (D-A-D) molecule with B(C6F5)3. Its geometry and electronic structure were characterized by single crystal X-ray diffraction, EPR spectroscopy, SQUID measurement, UV/vis spectroscopy, and DFT calculation. It has an open-shell singlet ground state with a thermally excited triplet state. It can be viewed as an intramolecular radical ion pair, and the formation mechanism is proposed as an intramolecular single electron transfer that occurs from the bis(triarylamine) donor fragment to the central dioxophenyl acceptor moiety, induced by the acidic boron atom. This work provides a Lewis acid induced approach to the formation of neutral and cross-conjugated diradicals.

The Lewis Acid Induced Formation of a Stable Diradical with an Intramolecular Ion Pairing State.

Studying the dynamic evolution of the reaction process that identifies the key intermediate is important for elaborating the reaction mechanism, but it is still challenging from the point of view of the experiment. Methane oxidation over Pd-based catalysts including Pd (111) with different oxygen coverages and PdO (101) is studied by reactive force field molecular dynamic simulations in this work to explore the dynamic character of oxygen species like chemisorbed oxygen and lattice oxygen on the C–H bond activation and the C–O bond formation. The results show that methane oxidation on the Pd (111) surfaces is initiated by the direct dissociation of methane molecules (CH4 → CH3 + H) rather than the oxygen-assisted dissociation (CH4 + O → CH3 + OH), which is in agreement with the first-principle electronic structure calculation results. A comparison of methane oxidation at different oxygen coverages shows that the catalytic activity increases with the increase of oxygen pressure in general: There is no CO at low oxygen coverage and CO is formed at higher oxygen coverage up to 3/9 ML, but no CO2 is formed. This is because the C–O bond formation barrier is higher than the H–O bond formation barrier. Therefore, it is in favor of the formation of OH species instead of the CO2 formation for methane oxidation on metallic Pd. On the PdO (101) surface, we notice that the lattice oxygen species are more active compared to the chemisorbed oxygen species in both the activation of methane and the oxidation of CHx species due to its strong hydrogen affinity and high electron density, leading to high methane oxidation activity and ultimately CO2 formation. Importantly, a novel path for the CO2 formation via a dioxymethylene (H2COO) intermediate (CH2O + O → H2COO) or a hydroxymethoxy (H2COOH) intermediate (CH2O + OH → H2COOH) has been identified from our present reactive force field molecular dynamic simulations, which is generally ignored in the static electronic structure scheme like density functional theory. It is hoped that the present results would be a guide for the rational design of the methane oxidation catalysts by controlling the active phase of Pd (or Pt) or provide more essential knowledge for a better understanding of the methane oxidation or methane reformation reaction mechanism. 

Dynamic Evolution of Methane Oxidation on Pd-Based Catalysts: A Reactive Force Field Molecular Dynamics Study

Delocalized ferroelectric engineering of efficient photo-generated charge directional transfer for solar energy conversion into H2

Oxygen vacancy-engineered in ethylene glycol solvated α-MnO2 for low temperature catalytic oxidation of toluene

An integrated process for separation and recovery of tantalum from spent TaCrTi sputtering targets by hydrometallurgical methods

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