Rising atmospheric levels of carbon dioxide and the depletion of fossil fuel reserves raise serious concerns about the ensuing effects on the global climate and future energy supply. Utilizing the abundant solar energy to convert CO2 into fuels such as methane or methanol could address both problems simultaneously as well as provide a convenient means of energy storage. In this Review, current approaches for the heterogeneous photocatalytic reduction of CO2 on TiO2 and other metal oxide, oxynitride, sulfide, and phosphide semiconductors are presented. Research in this field is focused primarily on the development of novel nanostructured photocatalytic materials and on the investigation of the mechanism of the process, from light absorption through charge separation and transport to CO2 reduction pathways. The measures used to quantify the efficiency of the process are also discussed in detail.

Photocatalytic reduction of CO2 on TiO2 and other semiconductors.

Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

Photocatalysts and Photoelectrodes James L. White,† Maor F. Baruch,† James E. Pander III,† Yuan Hu,† Ivy C. Fortmeyer,† James Eujin Park,† Tao Zhang,† Kuo Liao,† Jing Gu,‡ Yong Yan,‡ Travis W. Shaw,† Esta Abelev,† and Andrew B. Bocarsly*,† †Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States ‡Chemical and Materials Science Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States

Light-Driven Heterogeneous Reduction of Carbon Dioxide: Photocatalysts and Photoelectrodes

Photocatalytic reduction of CO2 into hydrocarbon fuels, an artificial photosynthesis, is based on the simulation of natural photosynthesis in green plants, whereby O2 and carbohydrates are produced from H2 O and CO2 using sunlight as an energy source. It couples the reductive half-reaction of CO2 fixation with a matched oxidative half-reaction such as water oxidation, to achieve a carbon neutral cycle, which is like killing two birds with one stone in terms of saving the environment and supplying future energy. The present review provides an overview and highlights recent state-of-the-art accomplishments of overcoming the drawback of low photoconversion efficiency and selectivity through the design of highly active photocatalysts from the point of adsorption of reactants, charge separation and transport, light harvesting, and CO2 activation. It specifically includes: i) band-structure engineering, ii) nanostructuralization, iii) surface oxygen vacancy engineering, iv) macro-/meso-/microporous structuralization, v) exposed facet engineering, vi) co-catalysts, vii) the development of a Z-scheme system. The challenges and prospects for future development of this field are also present.

Photocatalytic Conversion of CO 2 into Renewable Hydrocarbon Fuels: State‐of‐the‐Art Accomplishment, Challenges, and Prospects

Ever-increasing fossil-fuel combustion along with massive CO2 emissions has aroused a global energy crisis and climate change. Photocatalytic CO2 reduction represents a promising strategy for clean, cost-effective, and environmentally friendly conversion of CO2 into hydrocarbon fuels by utilizing solar energy. This strategy combines the reductive half-reaction of CO2 conversion with an oxidative half reaction, e.g., H2 O oxidation, to create a carbon-neutral cycle, presenting a viable solution to global energy and environmental problems. There are three pivotal processes in photocatalytic CO2 conversion: (i) solar-light absorption, (ii) charge separation/migration, and (iii) catalytic CO2 reduction and H2 O oxidation. While significant progress is made in optimizing the first two processes, much less research is conducted toward enhancing the efficiency of the third step, which requires the presence of cocatalysts. In general, cocatalysts play four important roles: (i) boosting charge separation/transfer, (ii) improving the activity and selectivity of CO2 reduction, (iii) enhancing the stability of photocatalysts, and (iv) suppressing side or back reactions. Herein, for the first time, all the developed CO2 -reduction cocatalysts for semiconductor-based photocatalytic CO2 conversion are summarized, and their functions and mechanisms are discussed. Finally, perspectives in this emerging area are provided.

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

Carbon dioxide (CO2) photoreduction by gaseous water over silica-pillared lamellar niobic acid, viz. HNb3O8, was studied in this work. The physicochemical characteristics of samples were examined b...

Photocatalytic Reduction of Carbon Dioxide to Methane over SiO2-Pillared HNb3O8

KNb 3 O 8 and HNb 3 O 8 nanobelts were prepared by hydrothermal synthesis. The characteristics of samples were investigated by XRD, SEM, and UV–vis diffuse reflectance spectroscopy. The KNb 3 O 8 and HNb 3 O 8 nanobelts exhibited much higher activities for CO 2 photoreduction to methane than commercial TiO 2 (Degussa P25), and the KNb 3 O 8 and HNb 3 O 8 particles prepared by conventional solid state reaction. It was also found that either the HNb 3 O 8 nanobelts or the HNb 3 O 8 particles performed better than the corresponding KNb 3 O 8 counterpart. It is proposed that the nanobelt-like morphology and the protonic acidity contribute to the higher photocatalytic activity of the HNb 3 O 8 nanobelts.

Photocatalytic reduction of CO2 to methane over HNb3O8 nanobelts

Nitrogen-doped mesoporous TiO 2 photocatalysts were developed for CO 2 photoreduction by water in gas phase. The effects of nitrogen doping and noble metal loading were investigated in detail. The characteristics of samples were investigated by techniques, such as XRD, FT-IR, TEM, XPS, nitrogen adsorption–desorption, and UV–vis diffuse reflectance spectroscopy. The loading of noble metals (i.e., Pt, Au, and Ag) generally improved the photocatalytic activity, and the efficiency follows the order: Pt > Au > Ag. It was found that the loading of Pt also promoted the transformation of catalyst associated carbon residues to methane. With unique properties, such as the mesoporous structure, light absorption, and the electron transfer character, the nitrogen-doped mesoporous TiO 2 samples showed good activity for CO 2 photoreduction to methane under visible light. The optimum loading amount of Pt was 0.2 wt.%, and the optimum doping amount of N was 0.84% on the basis of the lattice oxygen atoms. Aspects such as the origination of visible light sensitivity in terms of nitrogen doping, the effect of noble metal loading, and the reaction mechanism were also discussed.

Photocatalytic reduction of CO2 over noble metal-loaded and nitrogen-doped mesoporous TiO2

A nanocasting method using SBA-15 as a hard template was adopted to prepare nanocrystalline NaNbO3. The templated NaNbO3 consists of single crystalline nanoparticles about 8 nm. The nanocrystalline NaNbO3 sample showed remarkably enhanced photocatalytic activity for 2-propanol photodegradation in comparison to the counterpart prepared by the solid state reaction.

Hard Template Synthesis of Nanocrystalline NaNbO 3 with Enhanced Photocatalytic Performance

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Photocatalytic Reduction of Carbon Dioxide to Methane over SiO2-Pillared HNb3O8

Photocatalytic reduction of CO2 to methane over HNb3O8 nanobelts

Photocatalytic reduction of CO2 over noble metal-loaded and nitrogen-doped mesoporous TiO2

Hard Template Synthesis of Nanocrystalline NaNbO 3 with Enhanced Photocatalytic Performance