Recent Progress of Single-Atom Photocatalysts Applied in Energy Conversion and Environmental Protection

Photocatalysis driven by solar energy is a feasible strategy to alleviate energy crises and environmental problems. In recent years, significant progress has been made in developing advanced photocatalysts for efficient solar-to-chemical energy conversion. Single-atom catalysts have the advantages of highly dispersed active sites, maximum atomic utilization, unique coordination environment, and electronic structure, which have become a research hotspot in heterogeneous photocatalysis. This paper introduces the potential supports, preparation, and characterization methods of single-atom photocatalysts in detail. Subsequently, the fascinating effects of single-atom photocatalysts on three critical steps of photocatalysis (the absorption of incident light to produce electron-hole pairs, carrier separation and migration, and interface reactions) are analyzed. At the same time, the applications of single-atom photocatalysts in energy conversion and environmental protection (CO₂ reduction, water splitting, N₂ fixation, organic macromolecule reforming, air pollutant removal, and water pollutant degradation) are systematically summarized. Finally, the opportunities and challenges of single-atom catalysts in heterogeneous photocatalysis are discussed. It is hoped that this work can provide insights into the design, synthesis, and application of single-atom photocatalysts and promote the development of high-performance photocatalytic systems.

Converting CO2 into Value-Added Products by Cu2 O-Based Catalysts: From Photocatalysis, Electrocatalysis to Photoelectrocatalysis

Converting CO₂ into value-added products by photocatalysis, electrocatalysis, and photoelectrocatalysis is a promising method to alleviate the global environmental problems and energy crisis. Among the semiconductor materials applied in CO₂ catalytic reduction, Cu₂ O has the advantages of abundant reserves, low price and environmental friendliness. Moreover, Cu₂ O has unique adsorption and activation properties for CO₂ , which is conducive to the generation of C₂₊ products through CC coupling. This review introduces the basic principles of CO₂ reduction and summarizes the pathways for the generation of C₁ , C₂ , and C₂₊ products. The factors affecting CO₂ reduction performance are further discussed from the perspective of the reaction environment, medium, and novel reactor design. Then, the properties of Cu₂ O-based catalysts in CO₂ reduction are summarized and several optimization strategies to enhance their stability and redox capacity are discussed. Subsequently, the application of Cu₂ O-based catalysts in photocatalytic, electrocatalytic, and photoelectrocatalytic CO₂ reduction is described. Finally, the opportunities, challenges and several research directions of Cu₂ O-based catalysts in the field of CO₂ catalytic reduction are presented, which is guidance for its wide application in the energy and environmental fields is provided.

Recent progress of theoretical studies on electro- and photo-chemical conversion of CO2 with single-atom catalysts

The CO₂ reduction reaction (CO₂RR) into chemical products is a promising and efficient way to combat the global warming issue and greenhouse effect. The viability of the CO₂RR critically rests with finding highly active and selective catalysts that can accomplish the desired chemical transformation. Single-atom catalysts (SACs) are ideal in fulfilling this goal due to the well-defined active sites and support-tunable electronic structure, and exhibit enhanced activity and high selectivity for the CO₂RR. In this review, we present the recent progress of quantum-theoretical studies on electro- and photo-chemical conversion of CO₂ with SACs and frameworks. Various calculated products of CO₂RR with SACs have been discussed, including CO, acids, alcohols, hydrocarbons and other organics. Meanwhile, the critical challenges and the pathway towards improving the efficiency of the CO₂RR have also been discussed.

Electronegativity Enhanced Strong Metal-Support Interaction in Ru@F-Ni3N for Enhanced Alkaline Hydrogen Evolution

Precious metals (Pt, Ir, Ru, and so on) and related compounds usually demonstrate superb catalytic activity for electrochemical hydrogen production. However, scarcity and stability are still challenges for hydrogen evolution reaction, even for single-atomic-site electrocatalysts. Herein, a fluorine (F) doping strategy is proposed to enhance the strong metal-support interaction between the F-doped Ni₃N support and the loaded ruthenium (Ru) species. Via synergistically modulating both the Ru loading amount and F doping concentration, outstanding HER activity was achieved in Ru@F-Ni₃N with an overpotential (η) of 115 mV at 100 mA cm^-2, superior to the benchmark Pt/C (η = 201 mV). Density functional theory simulation in combination with X-ray photoelectron spectra and X-ray absorption spectroscopy characterizations convincingly demonstrate that, with the strongest electronegativity, F doping could effectively stabilize Ru atoms doped in the F-Ni₃N substrate and simultaneously reduce the H bonding strength, which accelerated the desorption of H₂. These findings provide a facile strategy to modulate both catalytic activities and stabilities of heteroatom-loaded catalytic materials.

Few-layer porous carbon nitride anchoring Co and Ni with charge transfer mechanism for photocatalytic CO2 reduction

The low specific surface area and low charge transfer efficiency of conventional graphite carbon nitride (g-C₃N₄) are the main obstacles to its application in photocatalytic CO₂ reduction. In this paper, graphite carbon nitride was protonated by phosphoric acid (H₃PO₄), and a new few-layer porous carbon nitride was prepared by intercalation polymerization with doping bimetal in the cavity of g-C₃N₄. Under visible light irradiation, the CO formation rate of Co/Ni co-doped g-C₃N₄ can reach 13.55 μmol g^-1 h^-1, which was 3.9 times higher than that of g-C₃N₄ (3.49 μmol g^-1 h^-1). The density functional theory (DFT) calculations showed that the addition of Co and Ni in the cavity of g-C₃N₄ can induce bimetallic synergistic regulation of the electronic structure, thus improving the separation efficiency of charges and visible light capture ability of g-C₃N₄. Our work has great reference value for designing and synthesizing novel bimetallic co-doped g-C₃N₄ photocatalytic materials.

Challenges of modeling nanostructured materials for photocatalytic water splitting

Understanding the water splitting mechanism in photocatalysis is a rewarding goal as it will allow producing clean fuel for a sustainable life in the future. However, identifying the photocatalytic mechanisms by modeling photoactive nanoparticles requires sophisticated computational techniques based on multiscale modeling. In this review, we will survey the strengths and drawbacks of currently available theoretical methods at different length and accuracy scales. Understanding the surface-active site through Density Functional Theory (DFT) using new, more accurate exchange-correlation functionals plays a key role for surface engineering. Larger scale dynamics of the catalyst/electrolyte interface can be treated with Molecular Dynamics albeit there is a need for more generalizations of force fields. Monte Carlo and Continuum Modeling techniques are so far not the prominent path for modeling water splitting but interest is growing due to the lower computational cost and the feasibility to compare the modeling outcome directly to experimental data. The future challenges in modeling complex nano-photocatalysts involve combining different methods in a hierarchical way so that resources are spent wisely at each length scale, as well as accounting for excited states chemistry that is important for photocatalysis, a path that will bring devices closer to the theoretical limit of photocatalytic efficiency.

Hybrid density functional study on band structure engineering of ZnS(110) surface by anion–cation codoping for overall water splitting

The (Ru + C)-codoped ZnS(110) surface is predicted to be a potential candidate for solar-driven water splitting.

Dual-Single-Atom Tailoring with Bifunctional Integration for High-Performance CO2 Photoreduction

Single-atom photocatalysis has been demonstrated as a novel strategy to promote heterogeneous reactions. There is a diversity of monoatomic metal species with specific functions; however, integrating representative merits into dual-single-atoms and regulating cooperative photocatalysis remain a pressing challenge. For dual-single-atom catalysts, enhanced photocatalytic activity would be realized through integrating bifunctional properties and tuning the synergistic effect. Herein, dual-single-atoms supported on conjugated porous carbon nitride polymer are developed for effective photocatalytic CO₂ reduction, featuring the function of cobalt (Co) and ruthenium (Ru). A series of in situ characterizations and theoretical calculations are conducted for quantitative analysis of structure-performance correlation. It is concluded that the active Co sites facilitate dynamic charge transfer, while the Ru sites promote selective CO₂ surface-bound interaction during CO₂ photoreduction. The combination of atom-specific traits and the synergy between Co and Ru lead to the high photocatalytic CO₂ conversion with corresponding apparent quantum efficiency (AQE) of 2.8% at 385 nm, along with a high turnover number (TON) of more than 200 without addition of any sacrificial agent. This work presents an example of identifying the roles of different single-atom metals and regulating the synergy, where the two metals with unique properties collaborate to further boost the photocatalytic performance.