A review on heterogeneous photocatalysis for environmental remediation: From semiconductors to modification strategies

Photocatalytic production of solar fuels from CO2 is a promising strategy for addressing global environmental problems and securing future energy supplies. Although extensive research has been conducted to date, numerous impediments to realizing efficient, selective, and stable CO2 reduction have yet to be overcome. This comprehensive review highlights the recent advances in CO2 photoreduction, including critical challenges such as light-harvesting, charge separation, and the activation of CO2 molecules. We present promising strategies for enhancing the photocatalytic activities and discuss theoretical insights and equations for quantifying photocatalytic performance, which are expected to afford a fundamental understanding of CO2 photoreduction. We then provide a thorough overview of both traditional photocatalysts such as metal oxides and state-of-the-art catalysts such as metal–organic frameworks and 2D materials, followed by a discussion of the origin of carbon in CO2 photoreduction as a means to further understand the reaction mechanism. Finally, we discuss the economic viability of photocatalytic CO2 reduction before concluding the review with proposed future research directions.

Solar fuels: research and development strategies to accelerate photocatalytic CO2 conversion into hydrocarbon fuels

Photocatalytic CO 2 conversion is vital technology to realize global carbon neutrality and generate future energy supplies. This review proposes fundamentals, challenges, strategies, and prospects for photocatalytic CO 2 conversion research. 

Solar fuels: research and development strategies to accelerate photocatalytic CO<sub>2</sub> conversion into hydrocarbon fuels

Sunlight-driven water splitting to produce hydrogen fuel has stimulated intensive scientific interest, as this technology has the potential to revolutionize fossil fuel-based energy systems in modern society. The oxygen evolution reaction (OER) determines the performance of overall water splitting owing to its sluggish kinetics with multielectron transfer processing. Polymeric photocatalysts have recently been developed for the OER, and substantial progress has been realized in this emerging research field. In this Review, the focus is on the photocatalytic technologies and materials of polymeric photocatalysts for the OER. Two practical systems, namely, particle suspension systems and film-based photoelectrochemical systems, form two main sections. The concept is reviewed in terms of thermodynamics and kinetics, and polymeric photocatalysts are discussed based on three key characteristics, namely, light absorption, charge separation and transfer, and surface oxidation reactions. A satisfactory OER performance by polymeric photocatalysts will eventually offer a platform to achieve overall water splitting and other advanced applications in a cost-effective, sustainable, and renewable manner using solar energy.

Semiconducting Polymers for Oxygen Evolution Reaction under Light Illumination.

Electrochemical CO2 reduction to value-added chemicals and fuels is a promising approach to mitigate the greenhouse effect arising from anthropogenic CO2 emission and energy shortage caused by the ...

Architectural Design for Enhanced C2 Product Selectivity in Electrochemical CO2 Reduction Using Cu-Based Catalysts: A Review.

To realize the evolution of C2+ hydrocarbons like C2H4 from CO2 reduction in photocatalytic systems remains a great challenge, owing to the gap between the relatively lower efficiency of multielectron transfer in photocatalysis and the sluggish kinetics of C-C coupling Herein, with Cu-doped zeolitic imidazolate framework-8 (ZIF-8) as a precursor, a hybrid photocatalyst (CuOX@p-ZnO) with CuOX uniformly dispersed among polycrystalline ZnO was synthesized Upon illumination, the catalyst exhibited the ability to reduce CO2 to C2H4 with a 329% selectivity, and the evolution rate was 27 μmol·g-1·h-1 with water as a hole scavenger and as high as 223 μmol·g-1·h-1 in the presence of triethylamine as a sacrificial agent, all of which have rarely been achieved in photocatalytic systems The X-ray absorption fine structure spectra coupled with in situ FT-IR studies reveal that, in the original catalyst, Cu mainly existed in the form of CuO, while a unique Cu+ surface layer upon the CuO matrix was formed during the photocatalytic reaction, and this surface Cu+ site is the active site to anchor the in situ generated CO and further perform C-C coupling to form C2H4 The C-C coupling intermediate *OC-COH was experimentally identified by in situ FT-IR studies for the first time during photocatalytic CO2 reduction Moreover, theoretical calculations further showed the critical role of such Cu+ sites in strengthening the binding of *CO and stabilizing the C-C coupling intermediate This work uncovers a new paradigm to achieve the reduction of CO2 to C2+ hydrocarbons in a photocatalytic system

Photocatalytic C-C Coupling from Carbon Dioxide Reduction on Copper Oxide with Mixed-Valence Copper(I)/Copper(II).

Herein we report that hydrogenated TiO2 can act as hydride donor to realize the selective photocatalytic CO2 reduction to formic acid under UV illumination. In situ FT-IR studies indicated that the adsorption modes of CO2 were divergent on Ar-treated and H2-treated TiO2, and consequently, CO was generated on Ar-treated TiO2 whereas the exclusive reduction product on hydrogenated TiO2 was formic acid during the photocatalytic CO2 reduction. EPR and XPS spectroscopy further demonstrated that EPR-active O-vacancy was dominant on Ar-treated sample, while the hydrogenation introduced H on the surface to form EPR-silent hydride-like Ti-H-Ti site. The 2D 1H-13C solid-state NMR experiments corroborated the specific adsorption of CO2 on this H site. Accordingly, the preferential generation of formic acid should originate from the hydride transfer to the adsorbed CO2. Our results first demonstrate that hydrogenation can produce hydride-like sites and enable TiO2 to act as a hydride reagent during the photocatalytic reactions.

An unprecedent hydride transfer pathway for selective photocatalytic reduction of CO2 to formic acid on TiO2

Obtaining high-value-added products like C2+ hydrocarbons from CO2 reduction in a photocatalytic system has always remained a great challenge. Herein we fabricated a π-π stacking hybrid photocatalyst by combining two two-dimensional (2D) materials of g-C3N4 and Cu-porphyrin metal-organic framework (MOF). This hybrid photocatalyst exhibited the excellent capability to reduce CO2 into C2H6 in a selectivity of 44% and the selectivity of total hydrocarbons (C2H6 and CH4) was as high as 71%, as one of the best performances among the reported photocatalytic systems. The node sites of 2D-MOF were identified to be critical for the generation of C2H6, and a self-reconstruction during photocatalysis was clarified: the initial paddle-wheel CuII2(COO)4 node was reconstructed to the partially reduced Cu1+δ2(COO)3. Such reconstruction strengthened the trapping of in-situ generated CO and the synergistic action of the dual-Cu-site, therefore, achieved the efficient C-C coupling to form C2H6. 

Self-reconstruction of paddle-wheel copper-node to facilitate the photocatalytic CO2 reduction to ethane

In Fenton or Fenton-like reactions, ·OH obtained by the reductive activation of H2O2 is regarded as the major reactive oxygen species (ROS); however, the generation of other ROS like O2·– and 1O2 cannot be negligible. Since the degradation capability of a certain ROS would be varied to distinctive pollutants, the regulation of ROS production from H2O2 activation should be applicable for a more targeted pollutant degradation. Herein, a series of carbon nitride (C3N4)-supported Fe catalysts with the state of Fe ranging among single-atom, oxide-cluster, and nanoparticle catalysts were fabricated and their activities in photo-Fenton reactions were evaluated. It was uncovered that the single-atom catalyst favored the generation of O2·– and 1O2 via the oxidative activation of H2O2 and the selectivity of 1O2 toward ·OH dynamically increased with the proceeding of H2O2 activation, while the catalysts with an abundance of oxide clusters exhibited a significantly higher conversion of H2O2 to ·OH. In situ FT-IR studies demonstrated that during the H2O2 activation, the single-atom catalyst underwent a more significant surface hydroxylation than the oxide-cluster catalyst. Such a result was further consolidated by the theoretical calculation that the adsorption energy of surface hydroxyl was remarkably higher on the single-atom catalyst. Thereafter, distinctive from the easy desorption of hydroxyl during the reduction of H2O2 to ·OH in oxide-cluster catalysts, the in situ surface hydroxylation on the single-atom catalyst alters the adsorption mode of H2O2 to a H-bonded structure, which steers the selectivity of ROS generation to a more favored oxidative transformation of H2O2 to O2·–/1O2. This work uncovers the decisive role of surface hydroxyl in regulating ROS generation in a heterogeneous Fenton reaction. 

In Situ Hydroxylation of a Single-Atom Iron Catalyst for Preferential 1O2 Production from H2O2

It is of great significance to investigate aqueous CO2 reduction on catalysts with redox-noninnocent ligands as they effectively facilitate CO2 reduction but circumvent the H2 evolution. By developing the methodology of rapid-scan FT-IR in external reflection mode, the pathway of CO2 reduction on iron porphyrin was in-situ monitored. It’s uncovered that coupled with first-electron reduction of [FeIITAPP]0, the porphyrin nitrogen is protonated as the observation of NH+ ammonium band in IR spectra, which avoids the formation of Fe–H structure for proton reduction. Moreover, an inverse kinetic isotope effect (KIE = 0.59) in formation of *COOH indicates the protonated porphyrin-N would also act as a proton relay for the efficient inner-sphere proton transfer to the activated CO2 molecules. This work experimentally identified the proton migration pathway via the reduced porphyrin ligand as proton shuttle for the first time and should benefit the design of the catalyst of high CO2 reduction selectivity. 

Yangfan Li

Papers

Photocatalytic C-C Coupling from Carbon Dioxide Reduction on Copper Oxide with Mixed-Valence Copper(I)/Copper(II).

An unprecedent hydride transfer pathway for selective photocatalytic reduction of CO2 to formic acid on TiO2

Self-reconstruction of paddle-wheel copper-node to facilitate the photocatalytic CO2 reduction to ethane

In Situ Hydroxylation of a Single-Atom Iron Catalyst for Preferential 1O2 Production from H2O2

Spectating the proton migration on catalyst with noninnocent ligand in aqueous electrochemical CO2 reduction