Cocatalysts in Semiconductor-based Photocatalytic CO2 Reduction: Achievements, Challenges, and Opportunities.

HI-Light: A Glass-Waveguide-Based "Shell-and-Tube" Photothermal Reactor Platform for Converting CO2 to Fuels.

Electrocatalytic reduction of CO2 has the potential to convert CO2 into carbon-based fuels by using renewable energy. While a wide range of electrocatalytic materials have been investigated, the process is still limited by poor reaction selectivity or large overpotentials. Hence many have studied the effects of surface oxides, crystal facets, and nanoparticles on the activity and selectivity of CO2 reduction. Others have also shown that the reaction is sensitive to conditions such as temperature, CO2 pressure, electrolyte buffer concentration and electrode porosity. These effects mean that it can be challenging to determine the underlying cause for differences in intrinsic catalytic behaviour [1,2], and thus it is important to understand the effect of experimental conditions on CO2reduction. Firstly, we will review the literature and our own experimental work which clearly highlight the importance of interfacial pH on the reaction [2-4]. It is well known that in weak buffers such as KHCO3, the pH at the surface of the cathode is significantly higher than the bulk due to the hydrogen evolution and CO2 reduction reactions. By conducting experiments over a range of KHCO3 concentrations (and thus different interfacial pH values), it is clear that this alters the selectivity and activity of CO2 reduction. As the hydrodynamics at the cathode surface will also alter interfacial pH, we have examined the effect of hydrodynamics on the CO2 reduction reaction using a Cu rotating cylinder electrode [5]. Given that the enhanced mass transport will also increase the CO2 concentration at the electrode surface [6], it seems clear that mass transfer effects should influence the reaction selectivity. We confirm that this is indeed an important factor and suggest that increasing mass transport not only decreases the interfacial pH (closer to bulk values) but also decreases the surface coverage of CO on the cathode (a key immediate during CO2 reduction), which ultimately lowers the current going to the CO2reduction reaction. As these experiments revealed the importance of both KHCO3 concentration and mass transport, we developed a numerical model to predict how the bulk electrolyte composition changes during long term electrolysis experiments. This model was validated against experiment data and shows that changes in the bulk electrolyte (ionic resistance, pH, CO2-electrolyte equilibria) can occur over the course of long-term electrolysis experiments. These changes can complicate the interpretation of long-term electrode behaviour as well as the control of the electrochemical process. From these findings, we suggest a range of strategies to improve the experimental aspects of electrochemical CO2reduction investigations. [1] A.S. Hall, Y. Yoon, A. Wuttig, Y. Surendranath, Mesostructure-Induced Selectivity in CO2Reduction Catalysis, Journal of the American Chemical Society 137 (2015) 14834-14837. [2] R. Kas, R. Kortlever, H. Yılmaz, M.T.M. Koper, G. Mul, Manipulating the Hydrocarbon Selectivity of Copper Nanoparticles in CO2Electroreduction by Process Conditions, ChemElectroChem 2 (2015) 354-358. [3] K.J.P. Schouten, E. Pérez Gallent, M.T.M. Koper, The influence of pH on the reduction of CO and to hydrocarbons on copper electrodes, Journal of Electroanalytical Chemistry 716 (2014) 53-57. [4] A.S. Varela, M. Kroschel, T. Reier, P. Strasser, Controlling the selectivity of CO2electroreduction on copper: The effect of the electrolyte concentration and the importance of the local pH, Catalysis Today 260 (2016) 8-13. [5] C.F.C. Lim, D.A. Harrington, A.T. Marshall, Effects of mass transfer on the electrocatalytic CO2 reduction on Cu, Electrochimica Acta (2017) in press. [6] N. Gupta, M. Gattrell, B. MacDougall, Calculation for the cathode surface concentrations in the electrochemical reduction of CO2 in KHCO3solutions, Journal of Applied Electrochemistry 36 (2006) 161-172. [7] K. Hara, A. Tsuneto, A. Kudo, T. Sakata, Electrochemical Reduction of CO2 on a Cu Electrode under High Pressure: Factors that Determine the Product Selectivity, Journal of The Electrochemical Society 141 (1994) 2097-2103.

An Extrinsic Faradaic Layer on CuSn for High-Performance Electrocatalytic CO ₂ Reduction

Despite global initiatives to address climate change, the year 2023 was the hottest year ever recorded in history. The need for technologies to lower carbon emissions and to remove already emitted greenhouse gases is immense. There have been massive efforts to electrochemically drive CO2 conversion to value-added chemicals such as ethylene, ethanol, and acetone using renewable energy.(1) Copper (Cu) is notable for its ability to form high-value C-C bond compounds; however, selectivity and activity challenges remain. Efforts to tackle these issues have focused on catalyst, system and micro-environment developments,(2, 3) and among the critical factors, temperature plays an important role. Of reports that study temperature effects on CO2 reduction (CO2R), usually the entire system is controlled at the same temperature, including the working electrode, electrolyte, and reference electrode. Further, the range of accessible temperatures is usually limited to a range of 10 – 70°C due to the aqueous electrolyte freezing or evaporating.(4, 5)This study focuses on the development and demonstration of an electrochemical system for decoupling the temperatures of the CO2R cathode and the electrolyte. This allows independent optimization of thermodynamic, mass transport, and kinetic, to improve both activity and selectivity of CO2 reduction. The temperatures are separately controlled from 0 – 140 °C for the working electrode, and from 0 – 70°C for the electrolyte. These temperature gradients result in clearly different activity/selectivity trends compared to when a single temperature controls the entire system. While maintaining electrolyte temperature and current density, a lower cathode temperature favored methane formation, whereas a higher cathode temperature favored CO formation. The Faradaic efficiency of ethylene was maintained at 20% despite the changes in the temperature of the cathode. Furthermore, reducing the working electrode temperature effectively restricted H2 formation down to 15%. We attributed this selectivity and activity change to the control of adsorption and desorption energies, thermal and chemical processes of CO2 conversion, and changopanoles in the CO2 solubility in the electrolyte through density function theory and computational fluid dynamics simulations.(6, 7) The system and method developed in this study can be translated to understanding the effects of mass transport and kinetics in other emerging electrochemical conversions for accelerating clean energy generation and achieving global sustainability goalsReferences K. P. Kuhl, E. R. Cave, D. N. Abram, T. F. Jaramillo, New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces. Energy Environ. Sci. 5, 7050–7059 (2012).D. Higgins, C. Hahn, C. Xiang, T. F. Jaramillo, A. Z. Weber, Gas-Diffusion Electrodes for Carbon Dioxide Reduction: A New Paradigm. ACS Energy Lett. 4, 317–324 (2019).Y. Hori, K. Kikuchi, S. Suzuki, Production of CO and CH4 in Electrochemical Reduction of CO2 at Metal Electrodes in Aqueos Hydrogencarbonate Solution. Chemistry Letters 14, 1695–1698 (1985).R. E. Vos, K. E. Kolmeijer, T. S. Jacobs, W. van der Stam, B. M. Weckhuysen, M. T. M. Koper, How Temperature Affects the Selectivity of the Electrochemical CO2 Reduction on Copper. ACS Catal. 13, 8080–8091 (2023).S. T. Ahn, I. Abu-Baker, G. T. R. Palmore, Electroreduction of CO2 on polycrystalline copper: Effect of temperature on product selectivity. Catalysis Today 288, 24–29 (2017).Y. Zong, P. Chakthranont, J. Suntivich, Temperature Effect of CO2 Reduction Electrocatalysis on Copper: Potential Dependency of Activation Energy. Journal of Electrochemical Energy Conversion and Storage 17, 041007 (2020).K. P. Kuhl, T. Hatsukade, E. R. Cave, D. N. Abram, J. Kibsgaard, T. F. Jaramillo, Electrocatalytic Conversion of Carbon Dioxide to Methane and Methanol on Transition Metal Surfaces. J. Am. Chem. Soc. 136, 14107–14113 (2014). Figure 1

With increasing global energy demand and growing concerns over climate change, methods for catalytic reduction of CO 2 have been extensively studied, among which graphitic carbon nitride (g-C 3 N 4 ) attracts remarkable attention due to its easily available raw materials and outstanding chemical stability. However, its wide bandgap and low photon usage efficiency limit its application in photocatalysis. Doping g-C 3 N 4 to introduce active sites can enhance its catalytic performance. Herein, asymmetric phosphorus–cobalt dual sites were introduced onto g-C 3 N 4 via hydrothermal treatment and thermal polymerization. P (phosphorus) could enhance CO 2 adsorption, while Co (cobalt) functions as a metallic site to boost the separation rate of photogenerated carriers. A photocatalytic CO 2 reduction to CO with a rate of 93.2 μmol/(g·h) was achieved, which was two times that of g-C 3 N 4 . During stability testing, ethylene with a formation rate of approximately 2 μmol/(g·h) was observed, as well as trace quantities of methanol and acetic acid in the liquid products. This work shows a promising strategy by the introduction of asymmetric phosphorous–cobalt dual sites for efficient photocatalytic conversion of CO 2 to CO and C2 products. Graphical Abstract

Fabrication of inverse opal TiO2-supported Au@CdS core–shell nanoparticles for efficient photocatalytic CO2 conversion

Chem-bio interface design for rapid conversion of CO2 to bioplastics in an integrated system

AuPd/3DOM-TiO2 catalysts for photocatalytic reduction of CO2: High efficient separation of photogenerated charge carriers

Solar-driven reduction of CO2 into fuels/feedstocks is a promising strategy for addressing energy and CO2 emission issues. Despite great research efforts, it still remains a grand challenge to achieve efficient and highly selective reduction of CO2 owing to the large bond energy of CO2 and the diversity of reduction products. In addition to the control of light harvesting and charge transfer like photocatalytic water splitting, the design of catalytically active sites is highly important to promote CO2 reduction activity and selectivity (e.g., C-C coupling). In fact, we can learn a lot from conventional CO2 hydrogenation and syngas conversion in terms of active site design. In this article, we demonstrate how to design catalytically active sites for efficient and highly selective photocatalytic reduction of CO2 by sorting out the rules from the existing research on conventional COx hydrogenation, with a focus on enhancing C[double bond, length as m-dash]O activation and C-C coupling to form value-added products. This article aims to highlight the challenges in the field of photocatalytic CO2 conversion and the connection of photocatalysis with conventional catalytic systems, providing the readers the opportunities to join the research.

The reduction in human-induced emissions of CO2 from automobiles, factories, power stations, etc., over the next 15 years is currently one of the most important issues facing the planet. We should therefore attempt to develop industrial processes using CO2 as a feedstock in order to build a sustainable society in the near future. Photocatlaytic conversion of CO2 has recently drawn increased attention due to allowing direct solar-to-chemical energy conversion. If H2O does function as the electron donor, then it is important to obtain a stoichiometric ratio between the amount of O2 evolved and the amount of CO2 reduced. Moreover, the reduction of H+, released from H2O molecules, usually competes with CO2 reduction when several heterogeneous materials are used as photocatalysts for the reduction of CO2 with H2O. Generally, the production of H2 via the reduction of H+ is the dominant pathway. Therefore, to achieve high selectivity in the photocatalytic conversion of CO2 with H2O, the electrons generated through the oxidation of H2O must be controlled to selectively. Moreover, we achieved highly selective photocatalytic reduction of CO2 by H2O by simultaneously using an inhibitor of H2 production and materials capable of CO2 capture and storage, e.g., Ag-modified ZnGa2O4/Ga2O3, La2Ti2O7, SrO/Ta2O5, ZnGa2O4, ZnTa2O6, Sr2KTa5O15, SrNb2O6, LDH/Ga2O3, and Pr2O3/Ga2O3.

Improved metal-organic frameworks (MOFs) and their application in catalytic CO2 reduction: A review

Selective photocatalytic CO2 reduction to product syngas remains a great challenge. Herein, we proposed a novel strategy by integration of metallosalen (M-Salen) active sites and Zr-MOF, to promote the selective production of syngas through photocatalytic CO2 and H2O reduction. A series of M-Salen (Co, Co2, Cu, Cu2) with carboxylic groups were embedded into a Zr-MOF (NKM-908) by replacing the terminal coordinated -OH-/H2O on the Zr6 clusters. The integration of M-Salen and NKM-908 significantly accelerate the transfer dynamics of photogenerated charge and promote CO2 enrichment around the M-Salen active sites. The M-Salen sites played a crucial role in enhancing CO production. Specifically, the mononuclear Co-Salen exhibits stronger CO2 adsorption and activation capabilities than the binuclear Co2-Salen, with enhanced *CO2 coverage. In addition, the Co-based salen catalyst exhibits a lower energy barrier for *CO desorption-the rate-determining step-compared to its Cu-based counterpart. As a result, the Co-NKM-908 exhibited optimal performance in photocatalytic CO2 and H2O conversion, achieving a high CO production rate of 1.92mmol g-1h-1 and a H2 production rate of 1.52mmol g-1h-1 with a standard syngas CO/H2 ratio of 1.3/1.

Among various strategies toward finding new energy sources to replace conventional fossil fuels, photocatalytic conversion of CO2 into useful chemicals using water as an electron donor and a proton source has been increasingly attracting attentions because it can convert light energy to chemical energy. The photocatalytic conversion of CO2 involves photoabsorption (light harvesting), charge separation, and H2O oxidation, as well as the consumption of the photogenerated electrons for reduction. The evolution of H2 rather than CO is preferred when H2O is used as the electron donor, since the redox potential of CO2/CO (−0.51 V vs NHE, at pH 7) is more negative than that of H+/H2 (−0.41 V vs NHE, at pH 7) in an aqueous solution. Therefore, “high selectivity toward CO2 reduction” is one of the most important requirements for the photocatalytic system to achieve the CO2 conversion in water. Various mixed oxides including tantalum-based NaTaO3, Ta2O5, Sr2KTa5O15, titanium-based BaLa4Ti4O15, CaTiO3, Na2Ti3O7, K2Ti6O13, La2Ti2O7, SrTiO3, gallium-based Ga2O3, and ZnGa2O4, and SrNb2O6 were found to be excellent photocatalysts for the conversion of CO2 into CO in the presence of Ag cocatalyst. However, these materials show significant activity for CO evolution only when irradiated with deep region UV-light (λ < 300 nm). Therefore, a series of photocatalysts which can drive under the photoirradiation with UV-light at λ > 300 nm is strongly desired in the research area of the photocatalytic conversion of CO2. Recently, Domen et al. reported that Al-doped SrTiO3 (Al-STO) with suitable cocatalysts such as MoOy/RhCrOx exhibited a promising photocatalytic activity for overall water splitting under photoirradiation at λ > 300 nm; the apparent quantum efficiency of the MoOy/RhCrOx/Al-STO reached close to 69% at 365 nm. Thus, we herein describe our investigation into the photocatalytic conversion of CO2 by H2O using the Al-STO photocatalyst with Ag and AgCo cocatalysts under photoirradiation with λ > 300 nm. Al-STO photocatalyst was fabricated by a flux method using anhydrous SrCl2 as a flux reagent. A mixture of SrTiO3, Al2O3, and anhydrous SrCl2 was ground for 10 min, and then transferred into an alumina crucible and calcined at 1423 K under air for 15 h. Ag cocatalyst was employed to modify the surface of the as-prepared Al-STO photocatalyst via a chemical reduction method. Al-STO was dispersed in deionized ultrapure water, and AgNO3 and NaH2PO2 were added to the suspension, then it was maintained at 353 K for 1.5 h. Photocatalytic activity of Ag/Al-STO photocatalyst was evaluated by using an inner-irradiation type reaction vessel in a quasi-flowing batch system. The photocatalyst powder was dispersed in 1.0 L of an aqueous NaHCO3 solution, and the dissolved air in this suspension was degassed by a flow of high-purity CO2 gas. CO2 was continuously bubbled into the reaction solution at a flow rate of 30 mL/min. The suspension was irradiated using a 400 W high-pressure Hg lamp with a Pyrex filter equipped with a cooling water system (irradiation wavelength: λ > 300 nm). The gaseous products in the outlet gas, e.g., CO, H2, and O2, were analyzed by gas chromatography (FID-GC with methanizer: CO, TCD-GC: H2 and O2). The attached figure shows a formation rate of CO (red), H2 (blue), and O2 (green) in the photocatalytic conversion of CO2 with H2O using undoped STO and Al-STO in the presence of Ag cocatalyst. It should be noted that the formation rate of CO is significantly increased by the doping of Al into STO. Moreover, the addition of Co species as a second cocatalyst to Ag/Al-STO drastically improved its activity for the photocatalytic conversion of CO2 by H2O as the electron donor, with extremely high selectivity toward CO evolution (99.8%), in which Ag and Co might enable CO2 reduction and H2O oxidation on the Al-STO surface, respectively. The formation rate of CO up to 52.7 μmol/h was observed over AgCo/Al-STO when irradiated with the UV light at wavelengths above 300 nm, which is ten-times higher than that over Ag/Al-STO (4.7 μmol/h). Furthermore, isotope-experiments using 13C-labeled CO2 gas revealed that 13C-labeled CO (m/z = 29) is selectively observed by GC-MS analysis, indicating that the CO evolved over the Ag/Al-STO and AgCo/Al-STO photocatalysts should be derived from the introduced CO2 gas. Figure caption Formation rates of products and selectivity toward CO evolution in the photocatalytic conversion of CO2 using bare Ag/STO (undoped STO), Ag/ and AgCo/Al-STO. Red: CO, green: O2, blue: H2, black circle: selectivity. Loading amount of Ag: 1wt%, Ag : Co = 2 : 1 (mol). Figure 1

From fries to fuel! Reduction of CO2 from a waste combustion product into a hydrocarbon fuel using solar energy is one of the best strategies to manage global warming and energy shortage. Homogeneously hybrid TiO2/ZnO mesoporous “french fries” (MFFs) were synthesized by a facile furfural alcohol-derived polymerization–oxidation route (see scheme). The unusual hybrid mesoporous structure of the TiO2/ZnO MFFs enhances the photocatalytic activity toward conversion of CO2 into hydrocarbon fuel (CH4) in the presence of water vapor.

Chapter 13 - Recent progress in bismuth oxyhalides-based heterojunctions for CO2 photoreduction