Constructing a Stable 2D/2D Heterojunction of Oxygen-Cluster-Modified Ti3AlC2 MAX Cocatalyst with Proton-Rich C3N4 for Highly Efficient Photocatalytic CO2 Methanation

Unveiling the potential of MXene-fabricated catalysts: an effective approach for H2 generation from water splitting

Hydrogen has enough potential and can be successfully used as an alternative to the conventional fuel. It can be successfully produced from water that is not only a sustainable source but exists everywhere on earth. Additionally, its combustion releases water that is quite safe and environment friendly. The current project was designed to generate hydrogen from catalytic water splitting on TiO2@Ti3C2T x catalysts. To obtain the required catalytic characteristics, titania was engineered on Ti3C2T x surfaces in situ using an ethanol-assisted solvothermal approach. After careful recovery, the catalysts were characterized and assessed for the photoreaction. All photoreactions were performed in a quartz reactor (150 mL), where hydrogen evolution activities were monitored on GC-TCD (Shimadzu-JP). The comparative activities indicated that TiO2@C and TiO2@Ti3C2T x catalysts deliver 9.37 and 18.57 mmol g-1 h-1 of hydrogen, respectively. The higher activities of TiO2@Ti3C2T x were attributed to the existence of higher active sites (charge trapping centres) on the multilayer MXene that progressively promote and facilitate redox reactions. Reason is that existence of titania on MXene interfaces develops heterojunctions that rectify the charge transfer; hence reduce the charge recombination (i.e., back reaction). On the basis of encouraging activities, it has been concluded that the aforementioned approach holds promise to replace the costly and conventional hydrogen generation technologies.

Synergistic Effect of Nickel Nanoparticles Dispersed on MOF-Derived Defective Co3O4 In Situ Grown over TiO2 Nanowires toward UV and Visible Light Driven Photothermal CO2 Methanation

Catalytic CO2 hydrogenation is an effective approach to producing clean fuels, but this process is expensive, in addition to the low efficiency of catalysts. Thus, photothermal CO2 hydrogenation can effectively utilize solar energy for CH4 production. Metal-organic framework (MOF) derived materials with a controlled structure and morphology are promising to give a high number of active sites and photostability in thermal catalytic reactions. For the first time, a novel heterostructure catalyst was synthesized using a facile approach to in situ grow MOF-derived 0D Co3O4 over 1D TiO2 nanowires (NWs). The original 3D dodecahedral structure of the MOF is engineered into novel 0D Co3O4 nanospheres, which were uniformly embedded over Ni-dispersed 1D TiO2 NWs. In situ prepared 10Ni-7Co3O4@TiO2 NWs-I achieved an excellent photothermal CH4 evolution rate of 8.28 mmol/h at 250 °C under low-intensity visible light, whereas UV light treatment further increased activity by 1.2-fold. UV irradiations promoted high CH4 production while improving the susceptibility of the catalyst to visible light irradiation. The photothermal effect is prominent at lower temperatures, due to the harmonization of both solar and thermal energy. By paralleling with mechanically assembled 10Ni-7Co3O4/TiO2 NWs-M, the catalytic performance of the in situ approach is far superior, attributing to the morphological transformation of 0D Co3O4, which induced intimate interfacial interactions, formation of oxygen vacancies and boosted photo-to-thermal effects. The co-existence of metallic/metal oxide Ni-Co provided beneficial synergies, enhanced photo-to-thermal effects, and improved charge transfer kinetics of the composite. This work uncovers a facile approach to engineering the morphology of MOF derivatives for efficient photothermal CO2 methanation.

Recent progress of cocatalysts loaded on carbon nitride for selective photoreduction of CO2 to CH4

A photocatalytic system driven by solar light is one of the promising strategies for converting CO₂ into valuable energy. The reduction of CO₂ to CH₄ is widely studied since CH₄ has a high energy density as the main component of nonrenewable natural gas. Therefore, it is necessary to develop semiconductor materials with high photocatalytic activity and CH₄ selectivity. Graphitic carbon nitride (g-C₃N₄/CN) has attracted widespread attention for photocatalytic CO₂ reduction due to its excellent redox ability and visible light response. A hybrid system constructed by loading cocatalysts on g-C₃N₄ can significantly improve the yield of target products, and serve as a general platform to explore the mechanism of the CO₂ reduction reaction. Herein, we briefly introduce the theory of selective CO₂ photoreduction and the basic properties of cocatalysts. Then, several typical configurations and modification strategies of cocatalyst/CN systems for promoting CH₄ selective production are presented in detail. In particular, we systematically summarize the application of cocatalyst/CN composite photocatalysts in the selective reduction of CO₂ to methane, according to the classification of cocatalysts (monometal, bimetal, metal-based compound, and nanocarbon materials). Finally, the challenges and perspectives for developing cocatalyst/g-C₃N₄ systems with high CH₄ selectivity are presented to guide the rational design of catalysts with high performance in the future.

Interface Engineering in 2D/2D Heterogeneous Photocatalysts

Assembling different 2D nanomaterials into heterostructures with strong interfacial interactions presents a promising approach for novel artificial photocatalytic materials. Chemically implementing the 2D nanomaterials' construction/stacking modes to regulate different interfaces can extend their functionalities and achieve good performance. Herein, based on different fundamental principles and photochemical processes, multiple construction modes (e.g., face-to-face, edge-to-face, interface-to-face, edge-to-edge) are overviewed systematically with emphasis on the relationships between their interfacial characteristics (e.g., point, linear, planar), synthetic strategies (e.g., in situ growth, ex situ assembly), and enhanced applications to achieve precise regulation. Meanwhile, recent efforts for enhancing photocatalytic performances of 2D/2D heterostructures are summarized from the critical factors of enhancing visible light absorption, accelerating charge transfer/separation, and introducing novel active sites. Notably, the crucial roles of surface defects, cocatalysts, and surface modification for photocatalytic performance optimization of 2D/2D heterostructures are also discussed based on the synergistic effect of optimization engineering and heterogeneous interfaces. Finally, perspectives and challenges are proposed to emphasize future opportunities for expanding 2D/2D heterostructures for photocatalysis.

Surface Electron Localization in Cu-MOF-Bonded Double-Heterojunction Cu2O Induces Highly Efficient Photocatalytic CO2 Reduction

Truncated octahedron Cu₂O (TOC) has attracted more attention for its suitable band gap and high carrier separation efficiency due to introduction of the facet heterojunction, but its practical drawback is still the instability caused by the irreversible disproportionation reaction (Cu₂O → Cu + CuO). Here, we design and fabricate the TOC/Cu-MOF (MOF: metal-organic framework) double-heterojunction structures with different Cu-MOF loadings. The introduced heterojunction between TOC and Cu-MOF not only produces a stable interface Cu^x+ bonding structure with the electronic states localized within the average collisional diameter of electrons 1.72 nm for TOC/2.1 wt %Cu-MOF as the active sites, but also promotes the surface energy level difference between the (100) and (111) facet heterojunctions. Meanwhile, the bonded Cu-MOF with a narrow bandgap effectively consumes holes by recombination with the photoexcited electrons from Cu-MOF itself. In our experiments, the TOC/Cu-MOF double heterostructure with a loading amount of 2.1 wt % Cu-MOF shows an optimal photocatalytic CO₂ reduction performance. The CO evolution rate reaches 23.01 μmol g^-1 h^-1, which is about 2.01 and 4.47 times larger than those of octahedral and hexahedral Cu₂O/Cu-MOF, respectively, and an excellent photostability is shown for four cycles with each cycle lasting for 4 h. Such a double heterostructure provides insight into highly efficient electron transfer and photostability in Cu₂O-related composite materials.

Recent advances and challenges in 2D/2D heterojunction photocatalysts for solar fuels applications

Although photocatalytic technology has emerged as an effective means of alleviating the projected future fuel crisis by converting sunlight directly into chemical energy, no visible-light-driven, low-cost, and highly stable photocatalyst has been developed to date. Due to considerably higher interfacial contact with numerous reactive sites, effective charge transmission and separation ability, and strong redox potentials, the focus has now shifted to 2D/2D heterojunction systems, which have exhibited effective photocatalytic performance. The fundamentals of 2D/2D photocatalysis for different applications and the classification of 2D/2D materials are first explained in this paper, followed by strategies to improve the photocatalytic performance of various 2D/2D heterojunction systems. Following that, current breakthroughs in 2D/2D metal-based and metal-free heterojunction photocatalysts, as well as their applications for H₂ evolution via water splitting, CO₂ reduction, and N₂ fixation, are discussed. Finally, a brief overview of current constraints and predicted results for 2D/2D heterojunction systems is also presented. This paper lays out a strategy for developing efficient 2D/2D heterojunction photocatalysts and sophisticated technology for solar fuel applications in order to address the energy issue.

Research Progress and Reaction Mechanism of CO2 Methanation over Ni-Based Catalysts at Low Temperature: A Review

The combustion of fossil fuels has led to a large amount of carbon dioxide emissions and increased greenhouse effect. Methanation of carbon dioxide can not only mitigate the greenhouse effect, but also utilize the hydrogen generated by renewable electricity such as wind, solar, tidal energy, and others, which could ameliorate the energy crisis to some extent. Highly efficient catalysts and processes are important to make CO2 methanation practical. Although noble metal catalysts exhibit higher catalytic activity and CH4 selectivity at low temperature, their large-scale industrial applications are limited by the high costs. Ni-based catalysts have attracted extensive attention due to their high activity, low cost, and abundance. At the same time, it is of great importance to study the mechanism of CO2 methanation on Ni-based catalysts in designing high-activity and stability catalysts. Herein, the present review focused on the recent progress of CO2 methanation and the key parameters of catalysts including the essential nature of nickel active sites, supports, promoters, and preparation methods, and elucidated the reaction mechanism on Ni-based catalysts. The design and preparation of catalysts with high activity and stability at low temperature as well as the investigation of the reaction mechanism are important areas that deserve further study.

Hybrid Plasmonic Nanomaterials for Hydrogen Generation and Carbon Dioxide Reduction

The successful development of artificial photosynthesis requires finding new materials able to efficiently harvest sunlight and catalyze hydrogen generation and carbon dioxide reduction reactions. Plasmonic nanoparticles are promising candidates for these tasks, due to their ability to confine solar energy into molecular regions. Here, we review recent developments in hybrid plasmonic photocatalysis, including the combination of plasmonic nanomaterials with catalytic metals, semiconductors, perovskites, 2D materials, metal-organic frameworks, and electrochemical cells. We perform a quantitative comparison of the demonstrated activity and selectivity of these materials for solar fuel generation in the liquid phase. In this way, we critically assess the state-of-the-art of hybrid plasmonic photocatalysts for solar fuel production, allowing its benchmarking against other existing heterogeneous catalysts. Our analysis allows the identification of the best performing plasmonic systems, useful to design a new generation of plasmonic catalysts.

Photocatalytic CO2 reduction to CH4 on iron porphyrin supported on atomically thin defective titanium dioxide

The synergistic effect of OVs and FeTPP on 2D TiO2 improves the efficiency and selectivity of CO2 photoreduction to CH4.