Photocatalytic non-oxidative coupling of methane: Recent progress and future

Unusual facet and co-catalyst effects in TiO2-based photocatalytic coupling of methane

Photocatalytic coupling of methane to ethane and ethylene (C2 compounds) offers a promising approach to utilizing the abundant methane resource. However, the state-of-the-art photocatalysts usually suffer from very limited C2 formation rates. Here, we report our discovery that the anatase TiO2 nanocrystals mainly exposing {101} facets, which are generally considered less active in photocatalysis, demonstrate surprisingly better performances than those exposing the high-energy {001} facet. The palladium co-catalyst plays a pivotal role and the Pd2+ site on co-catalyst accounts for the selective C2 formation. We unveil that the anatase {101} facet favors the formation of hydroxyl radicals in aqueous phase near the surface, where they activate methane molecules into methyl radicals, and the Pd2+ site participates in facilitating the adsorption and coupling of methyl radicals. This work provides a strategy to design efficient nanocatalysts for selective photocatalytic methane coupling by reaction-space separation to optimize heterogeneous-homogeneous reactions at solid-liquid interfaces.

Photoelectrochemical Conversion of Methane to Ethane and Hydrogen under Visible Light Using Functionalized Tungsten Trioxide Photoanodes with Proton Exchange Membrane

Developing methane utilization technologies is desired to convert abundant and renewable carbon resources, such as natural gas and biogas, into value-added chemical products. This study provides insights into emerging photoelectrochemical (PEC) technology for the photocatalytic transformation of methane to C2H6 and H2 using visible light at room temperature. The PEC conversion of methane to oxygenates has been investigated in aqueous electrolytes. Herein, we demonstrate the gas-phase PEC methane conversion using a proton exchange membrane (PEM) as a solid polymer electrolyte and a gas-diffusion photoanode for methane oxidation. Tungsten trioxide (WO3), a semiconductor photocatalyst responsive to visible light, is utilized as the photoanode material. Ultraviolet light (∼365 nm) excitation predominantly results in CO2 production with lower C2H6 selectivity in humidified methane. In contrast, visible light (∼453 nm) effectively promotes C2H6 production over the WO3 photoanode, attributed to preferential hydroxyl radical (•OH) formation compared to UV irradiation. Photogenerated holes formed near the valence band maximum of WO3 contribute to •OH formation through a single-electron water oxidation. The photogenerated •OH activates gaseous methane molecules to methyl radicals, subsequently coupled into C2H6 at the gas-electrolyte-semiconductor boundary. H2 is concurrently formed on the cathode electrocatalyst. Improving the selectivity for the dehydrogenative coupling of methane is pivotal for enhancing the energy efficiency in the PEM-PEC system.

Single-Atom Alloys Materials for CO2 and CH4 Catalytic Conversion

The catalytic conversion of greenhouse gases CH4 and CO2 constitutes an effective approach for alleviating the greenhouse effect and generating valuable chemical products. However, the intricate molecular characteristics characterized by high symmetry and bond energies, coupled with the complexity of associated reactions, pose challenges for conventional catalysts to attain high activity, product selectivity, and enduring stability. Single-atom alloys (SAAs) materials, distinguished by their tunable composition and unique electronic structures, confer versatile physicochemical properties and modulable functionalities. In recent years, SAAs materials demonstrate pronounced advantages and expansive prospects in catalytic conversion of CH4 and CO2. This review begins by introducing the challenges entailed in catalytic conversion of CH4 and CO2 and the advantages offered by SAAs. Subsequently, the intricacies of synthesis strategies employed for SAAs are presented and characterization techniques and methodologies are introduced. The subsequent section furnishes a meticulous and inclusive overview of research endeavors concerning SAAs in CO2 catalytic conversion, CH4 conversion, and synergy CH4 and CO2 conversion. The particular emphasis is directed toward scrutinizing the intricate mechanisms underlying the influence of SAAs on reaction activity and product selectivity. Finally, insights are presented on the development and future challenges of SAAs in CH4 and CO2 conversion reactions.

Photodriven Methane Conversion on Transition Metal Oxide Catalyst: Recent Progress and Prospects

Methane as the main component in natural gas is a promising chemical raw material for synthesizing value-added chemicals, but its harsh chemical conversion process often causes severe energy and environment concerns. Photocatalysis provides an attractive path to active and convert methane into various products under mild conditions with clean and sustainable solar energy, although many challenges remain at present. In this review, recent advances in photocatalytic methane conversion are systematically summarized. As the basis of methane conversion, the activation of methane is first elucidated from the structural basis and activation path of methane molecules. The study is committed to categorizing and elucidating the research progress and the laws of the intricate methane conversion reactions according to the target products, including photocatalytic methane partial oxidation, reforming, coupling, combustion, and functionalization. Advanced photocatalytic reactor designs are also designed to enrich the options and reliability of photocatalytic methane conversion performance evaluation. The challenges and prospects of photocatalytic methane conversion are also discussed, which in turn offers guidelines for methane-conversion-related photocatalyst exploration, reaction mechanism investigation, and advanced photoreactor design.

Photocatalytic Conversion of Methane: Current State of the Art, Challenges, and Future Perspectives

With 28-34 times the greenhouse effect of CO2 over a 100-year period, methane is regarded as the second largest contributor to global warming. Reducing methane emissions is a necessary measure to limit global warming to below 1.5 °C. Photocatalytic conversion of methane is a promising approach to alleviate the atmospheric methane concentrations due to its low energy consumption and environmentally friendly characteristics. Meanwhile, this conversion process can produce valuable chemicals and liquid fuels such as CH3OH, CH3CH2OH, C2H6, and C2H4, cutting down the dependence of chemical production on crude oil. However, the development of photocatalysts with a high methane conversion efficiency and product selectivity remains challenging. In this review, we overview recent advances in semiconductor-based photocatalysts for methane conversion and present catalyst design strategies, including morphology control, heteroatom doping, facet engineering, and cocatalysts modification. To gain a comprehensive understanding of photocatalytic methane conversion, the conversion pathways and mechanisms in these systems are analyzed in detail. Moreover, the role of electron scavengers in methane conversion performance is briefly discussed. Subsequently, we summarize the anthropogenic methane emission scenarios on earth and discuss the application potential of photocatalytic methane conversion. Finally, challenges and future directions for photocatalytic methane conversion are presented.

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.

The remarkable performance of a single iridium atom supported on hematite for methane activation: a density functional theory study

The Fe2O3(110)–OV surface is the best in terms of CH4 adsorption energy and C–H bond elongation. Therfore, the Ir/α-Fe2O3(110)–OV surface could be a candidate catalyst for CH4 dehydrogenation reaction.

Methane Conversion Under Mild Conditions Using Semiconductors and Metal-Semiconductors as Heterogeneous Photocatalysts: State of the Art and Challenges

The processes currently used in the chemical industry for methane conversion into fuels and chemicals operate under extreme conditions like high temperatures and pressures. In this sense, the search for methane conversion under mild conditions remains a great challenge. This review aims to summarize the use semiconductors and metal-semiconductors as heterogeneous photocatalysts for methane conversion under mild conditions into valuable products. First, a brief presentation of photochemical conversion of methane is provided and then the focus of this review on the use of heterogeneous photocatalysts for methane conversion are described. Finally, the main challenges and opportunities are discussed.