Fe/Fe 3 C Boosts H 2 O 2 Utilization for Methane Conversion Overwhelming O 2 Generation

Efficient Photocatalytic CH4-to-Ethanol Conversion by Limiting Interfacial Hydroxyl Radicals Using Gold Nanoparticles

Photocatalytic CH4 oxidation to ethanol with high selectivity is attractive but substantially challenging. The activation of inert C-H bonds at ambient conditions requires highly reactive oxygen species like hydroxyl radicals (⋅OH), while the presence of those oxidative species also facilitates fast formation of C1 products, instead of the kinetically sluggish C-C coupling to produce ethanol. Herein, we developed a BiVO4 photocatalyst with surface functionalization of Au nanoparticles (BiVO4@Au), which not only enables photogeneration of ⋅OH to activate CH4 into ⋅CH3, but also in situ consumes those ⋅OH species to retard their further attack on ⋅CH3, resulting in an enhanced ⋅CH3/⋅OH ratio and facilitating C-C coupling toward ethanol. The ⋅CH3/⋅OH ratio is further improved by transporting CH4 via a gas-diffusion layer to the photocatalytic interface, leading to even higher ethanol selectivity and production rates. At ambient conditions and without photosensitizers or sacrificial agents, the BiVO4@Au photocatalyst exhibited an outstanding CH4-to-ethanol conversion performance, including a peak ethanol yield of 680 μmol ⋅ g-1 ⋅ h-1, a high selectivity of 86 %, and a stable photoconversion of >100 h, substantially exceeding most of the previous reports. Our work suggests an attractive approach of in situ generation and modulation of the ⋅OH levels for photocatalytic CH4 conversion toward multi-carbon products.

Modulating the local electron density at built-in interface iron single sites in Fe-CN/MoO3 heterostructure for enhanced CO2 reduction to CH4 and photo-Fenton reaction

The catalytic efficiency of heterogeneous photocatalytic CO2 reduction and photo-Fenton H2O2 activationisclosely related to the local electron density of reaction center atoms. However, electron-hole recombination from random charge transfer significantly restricts the targeted electron delivery to the active center. Herein, Fe-C3N4/MoO3 heterojunction with interfacial coordination of atomically dispersed Fe-N4 sites with the O interface of MoO3 was synthesized by simple hydrothermal method. Based on the experimental results and density functional theory calculation (DFT), the heterojunction structure fosters accelerated interfacial electron transfer due to directional interfacial electric field (IEF) between Fe-CN and MoO heterogeneous interfaces, and the interfacial bond between Fe-N4 sites and O at the built-in interface regulates the local electron density of Fe-N4 active center. DFT further reveals that the interfacial electron flow and concentrated electron density at Fe-N4 sites result from the coordination between Fe-N4 and MoO3 interfaces. This directs electron flow towards the Fe center, significantly enhancing CO2 adsorption and H2O2 conversion efficiency. PDOS analysis shows that the dyz and dz2 orbitals of the isolated Fe atom in Fe-CN overlap with the pz orbital of the O atom in MoO3, playing a pivotal role in CO2 adsorption. Consequently, the Fe-CN/MoO3 heterojunction demonstrated highly efficient photocatalytic CO2 reduction to CH4, coupled with benzyl alcohol oxidation and photo-Fenton tetracycline degradation. These findings offer a promising multifunctional catalyst strategy for the development of energy conversion and environmental remediation.

Defective ZnIn2S4 Nanosheets for Visible-Light and Sacrificial-Agent-Free H2O2 Photosynthesis via O2/H2O Redox

H2O2 photosynthesis has attracted great interest in harvesting and converting solar energy to chemical energy. Nevertheless, the high-efficiency process of H2O2 photosynthesis is driven by the low H2O2 productivity due to the recombination of photogenerated electron-hole pairs, especially in the absence of a sacrificial agent. In this work, we demonstrate that ultrathin ZnIn2S4 nanosheets with S vacancies (Sv-ZIS) can serve as highly efficient catalysts for H2O2 photosynthesis via O2/H2O redox. Mechanism studies confirm that Sv in ZIS can extend the lifetimes of photogenerated carriers and suppress their recombination, which triggers the O2 reduction and H2O oxidation to H2O2 through radical initiation. Theoretical calculations suggest that the formation of Sv can strongly change the coordination structure of ZIS, modulating the adsorption abilities to intermediates and avoiding the overoxidation of H2O to O2 during O2/H2O redox, synergistically promoting 2e- O2 reduction and 2e- H2O oxidation for ultrahigh H2O2 productivity. The optimal catalyst displays a H2O2 productivity of 1706.4 μmol g-1 h-1 under visible-light irradiation without a sacrificial agent, which is ∼29 times higher than that of pristine ZIS (59.4 μmol g-1 h-1) and even much higher than those of reported photocatalysts. Impressively, the apparent quantum efficiency is up to 9.9% at 420 nm, and the solar-to-chemical conversion efficiency reaches ∼0.81%, significantly higher than the value for natural synthetic plants (∼0.10%). This work provides a facile strategy to separate the photogenerated electron-hole pairs of ZIS for H2O2 photosynthesis, which may promote fundamental research on solar energy harvest and conversion.

A polymer tethering strategy to achieve high metal loading on catalysts for Fenton reactions

The development of heterogenous catalysts based on the synthesis of 2D carbon-supported metal nanocatalysts with high metal loading and dispersion is important. However, such practices remain challenging to develop. Here, we report a self-polymerization confinement strategy to fabricate a series of ultrafine metal embedded N-doped carbon nanosheets (M@N-C) with loadings of up to 30 wt%. Systematic investigation confirms that abundant catechol groups for anchoring metal ions and entangled polymer networks with the stable coordinate environment are essential for realizing high-loading M@N-C catalysts. As a demonstration, Fe@N-C exhibits the dual high-efficiency performance in Fenton reaction with both impressive catalytic activity (0.818 min-1) and H2O2 utilization efficiency (84.1%) using sulfamethoxazole as the probe, which has not yet been achieved simultaneously. Theoretical calculations reveal that the abundant Fe nanocrystals increase the electron density of the N-doped carbon frameworks, thereby facilitating the continuous generation of long-lasting surface-bound •OH through lowering the energy barrier for H2O2 activation. This facile and universal strategy paves the way for the fabrication of diverse high-loading heterogeneous catalysts for broad applications.

Homogeneous Carbon Dot-Anchored Fe(III) Catalysts with Self-Regulated Proton Transfer for Recyclable Fenton Chemistry

Fenton chemistry has been widely studied in a broad range from geochemistry, chemical oxidation to tumor chemodynamic therapy. It was well established that Fe³⁺/H₂O₂ resulted in a sluggish initial rate or even inactivity. Herein, we report the homogeneous carbon dot-anchored Fe(III) catalysts (CD-COOFe^III) wherein CD-COOFe^III active center activates H₂O₂ to produce hydroxyl radicals (^•OH) reaching 105 times larger than that of the Fe³⁺/H₂O₂ system. The key is the ^•OH flux produced from the O-O bond reductive cleavage boosting by the high electron-transfer rate constants of CD defects and its self-regulated proton-transfer behavior probed by operando ATR-FTIR spectroscopy in D₂O and kinetic isotope effects, respectively. Organic molecules interact with CD-COOFe^III via hydrogen bonds, promoting the electron-transfer rate constants during the redox reaction of CD defects. The antibiotics removal efficiency in the CD-COOFe^III/H₂O₂ system is at least 51 times large than the Fe³⁺/H₂O₂ system under equivalent conditions. Our findings provide a new pathway for traditional Fenton chemistry.

Current Progress on Methods and Technologies for Catalytic Methane Activation at Low Temperatures

Methane (CH4 ) is an attractive energy source and important greenhouse gas. Therefore, from the economic and environmental point of view, scientists are working hard to activate and convert CH4 into various products or less harmful gas at low-temperature. Although the inert nature of CH bonds requires high dissociation energy at high temperatures, the efforts of researchers have demonstrated the feasibility of catalysts to activate CH4 at low temperatures. In this review, the efficient catalysts designed to reduce the CH4 oxidation temperature and improve conversion efficiencies are described. First, noble metals and transition metal-based catalysts are summarized for activating CH4 in temperatures ranging from 50 to 500 °C. After that, the partial oxidation of CH4 at relatively low temperatures, including thermocatalysis in the liquid phase, photocatalysis, electrocatalysis, and nonthermal plasma technologies, is briefly discussed. Finally, the challenges and perspectives are presented to provide a systematic guideline for designing and synthesizing the highly efficient catalysts in the complete/partial oxidation of CH4 at low temperatures.

Sulfone-Decorated Conjugated Organic Polymers Activate Oxygen for Photocatalytic Methane Conversion

Oxidizing CH₄ into liquid products with O₂ under mild conditions still mainly relies on metal catalysis. We prepared a series of sulfone-modified conjugated organic polymers and found that the catalyst with proper S^VI content (0.10) could drive O₂ →H₂ O₂ →⋅OH to oxidize CH₄ into CH₃ OH and HCOOH directly and efficiently at room temperature under light irradiation. Experimental results showed that after 4 h reaction, decomposition rate and residual amounts of H₂ O₂ were 81.21 % and 4.83 mmol g_cat ^-1 respectively, and CH₄ conversion rate was 22.81 %. Mechanism studies revealed that illumination could induce the homolytic dissociation of S=O bonds on catalyst to produce oxygen and sulfur radicals, where the ⋅O could adsorb and activate CH₄ , and the ⋅S could supply electrons for ¹ O₂ to generate H₂ O₂ and then for decomposing the H₂ O₂ into ⋅OH timely to oxidize CH₄ . This research provided a novel organic catalysis approach for oxygen activation and utilization.

Atomic-Layered Cu5 Nanoclusters on FeS2 with Dual Catalytic Sites for Efficient and Selective H2 O2 Activation

Regulating the distribution of reactive oxygen species generated from H₂ O₂ activation is the prerequisite to ensuring the efficient and safe use of H₂ O₂ in the chemistry and life science fields. Herein, we demonstrate that constructing a dual Cu-Fe site through the self-assembly of single-atomic-layered Cu₅ nanoclusters onto a FeS₂ surface achieves selective H₂ O₂ activation with high efficiency. Unlike its unitary Cu or Fe counterpart, the dual Cu-Fe sites residing at the perimeter zone of the Cu₅ /FeS₂ interface facilitate H₂ O₂ adsorption and barrierless decomposition into ⋅OH via forming a bridging Cu-O-O-Fe complex. The robust in situ formation of ⋅OH governed by this atomic-layered catalyst enables the effective oxidation of several refractory toxic pollutants across a broad pH range, including alachlor, sulfadimidine, p-nitrobenzoic acid, p-chlorophenol, p-chloronitrobenzene. This work highlights the concept of building a dual catalytic site in manipulating selective H₂ O₂ activation on the surface molecular level towards efficient environmental control and beyond.

Green Carbon Science: Efficient Carbon Resource Processing, Utilization, and Recycling Towards Carbon Neutrality

Green carbon science is defined as "Study and optimization of the transformation of carbon containing compounds and the relevant processes involved in the entire carbon cycle from carbon resource processing, carbon energy utilization, and carbon recycling to use carbon resources efficiently and minimize the net CO2 emission." [1] Green carbon science is related closely to carbon neutrality, and the relevant fields have developed quickly in the last decade. In this Minireview, we proposed the concept of carbon energy index, and the recent progresses in petroleum refining, production of liquid fuels, chemicals, and materials using coal, methane, CO2, biomass, and waste plastics are highlighted in combination with green carbon science, and an outlook for these important fields is provided in the final section.

Regulating Local Electron Density of Iron Single Sites by Introducing Nitrogen Vacancies for Efficient Photo-Fenton Process

The activity of heterogeneous photocatalytic H₂ O₂ activation in Fenton-like processes is closely related to the local electron density of reaction centre atoms. However, the recombination of electron-hole pairs arising from random charge transfer greatly restricts the oriented electron delivery to active center. Here we show a defect engineered iron single atom photocatalyst (Fe₁ -N_v /CN, single Fe atoms dispersed on carbon nitride with abundant nitrogen vacancies) for the activation of H₂ O₂ under visible light irradiation. Based on DFT calculations and transient absorption spectroscopy results, the engineered nitrogen vacancies serve as the electron trap sites, which can directionally drive the electrons to concentrate on Fe atoms. The formation of highly concentrated electrons density at Fe sites significantly improves the H₂ O₂ conversion efficiency. Therefore, the optimized single atom catalyst exhibiting a higher ciprofloxacin degradation activity, which was up to 18 times that of pristine CN.