Basic concepts of the photocatalytic reduction of CO₂

CO₂ photoreduction is the conversion of CO₂ into hydrocarbon fuels using sunlight, which is also known as artificial photosynthesis. Because CO₂ is one of the most thermodynamically stable carbon compounds and because it cannot absorb light in the wavelengths of 200–900 nm, the CO₂ photoreduction process requires a suitable photosensitizer, which is generally a semiconductor catalyst.^[ Reference Indrakanti, Kubicki and Schobert ^{21 ,} Reference Roy, Varghese, Paulose and Grimes ^{22 ]} Figure 1 shows the basic mechanism for the photocatalytic process of CO₂ reduction. In the photocatalytic process, photons having energy greater than or equal to the band gap energy are absorbed by the semiconductor, and the electrons in the valence band (VB) are excited to the conduction band (CB). Consequently, equal numbers of electrons and holes are generated in the CB and VB, respectively. The photogenerated electron–hole pairs separate from each other and transfer to the active sites on the surface of semiconductors; then, the photogenerated electrons with strong reduction potential reduce CO₂ into hydrocarbons such as CH₄ and CO, etc., and the photogenerated holes with strong oxidation potential oxidize H₂O into O₂.

Figure 1. Schematic illustration of the photoexcitation and electron transfer process for the photocatalytic reduction of CO₂ with H₂O as a reductant.

However, not all photogenerated electron–hole pairs reaching the surface of semiconductor catalysts can participate in the redox reaction, as the energy of the photogenerated electron–hole pairs must meet a certain thermodynamic conditions.^[ Reference Li, An, Park, Khraisheh and Tang ^{5 ,} Reference Indrakanti, Kubicki and Schobert ^{21 ]} That is, the bottom of the CB of the semiconductor photocatalysts should have a more negative potential than the reduction potential of CO₂ forming HCOOH, CO, HCHO, CH₃OH and CH₄; likewise, the top of the VB should have a more positive potential than the oxidation potential of H₂O. Equations (1)–(8) show the possible reactions involved in CO₂ photoreduction and their corresponding theoretical potentials (vs. the normal hydrogen electrode in an aqueous solution at pH 7 and 25°C, assuming unit activity).^[ Reference Tu, Zhou and Zou ^{4 ,} Reference Li, An, Park, Khraisheh and Tang ^{5 ,} Reference Indrakanti, Kubicki and Schobert ^{21 ]}

(1) $${\rm C}{\rm O}_2 + {\rm e}^ - \to {\rm C}{\rm O}_{\rm 2}^{ \bullet -} ;\quad E^0 = - {\rm 1}.{\rm 9}0\;{\rm V}$$

(2) $${\rm C}{\rm O}_2 + 2{\rm H}^ + + 2{\rm e}^ - \to {\rm HCOOH};\quad E^0 = - 0.{\rm 61}\;{\rm V}$$

(3) $${\rm C}{\rm O}_2 + 2{\rm H}^ + + 2{\rm e}^ - \to {\rm CO} + {\rm H}_{\rm 2}{\rm O};\quad E^0 = - 0.{\rm 53}\;{\rm V}$$

(4) $${\rm C}{\rm O}_2 + 4{\rm H}^ + + 4{\rm e}^ - \to {\rm HCHO} + {\rm H}_{\rm 2}{\rm O};\quad E^0 = - 0.{\rm 48}\;{\rm V}$$

(5) $${\rm C}{\rm O}_2 + 6{\rm H}^ + + 6{\rm e}^ - \to {\rm C}{\rm H}_{\rm 3}{\rm OH} + {\rm H}_{\rm 2}{\rm O};\quad E^0 = {\rm }0.{\rm 38}\;{\rm V}$$

(6) $${\rm C}{\rm O}_2 + 8{\rm H}^ + + 8{\rm e}^ - \to {\rm C}{\rm H}_{\rm 4} + 2{\rm H}_{\rm 2}{\rm O};\quad E^0 = - 0.{\rm 24}\;{\rm V}$$

(7) $$2{\rm H}^ + + 2{\rm e}^ - \to {\rm H}_{\rm 2};\quad E^0 = - 0.{\rm 41}\;{\rm V}$$

(8) $$2{\rm H}_2{\rm O} + 4{\rm h}^ + \to {\rm O}_{\rm 2} + 4{\rm H}^ + ;\quad E^0 = {\rm} 0.{\rm 82}\;{\rm V}$$

From a thermodynamic point of view,^[ Reference Indrakanti, Kubicki and Schobert ^{21 ,} Reference Schneider, Jia, Muckerman and Fujita ^{23 ]} the reaction of one electron to form CO₂ $^{ \bullet {\rm -}} $ is highly unfavorable due to the very negative reduction potential of −1.9 V (vs. NHE) for this process [Eq. (1)]. Thus, many researchers believe that the photocatalytic reduction of CO₂ is a multi-electronic process, which requires less energy for electron transfer compared with a mono-electronic process [Eqs. (2)–(6)]. In the multi-electron CO₂ reduction pathway, the reaction requires the assistance of a corresponding number of protons [Eq. (8)]. The formation of products could be different, which is determined by the number of photogenerated electron–hole pairs participating in the redox reaction.

Although photocatalytic reduction of CO₂ has been well studied, the CO₂ conversion efficiency of this process is still very low. In addition to a suitable E _g and CB potential, many factors influence the conversion efficiency of the multi-electron CO₂ photocatalytic reduction process. These factors include excitation, transport, separation (recombination) of the photogenerated electron–hole pairs, the adsorption capacity of the gas molecules, the reduction of CO₂, the oxidation of H₂O, and so on. Furthermore, the selective formation of specific products is also a considerable problem in the photocatalytic reduction of CO₂.^[ Reference Li, Wen, Low, Fang and Yu ^{3 ,} Reference Tu, Zhou and Zou ^{4 ]} So far, many strategies, including increased visible-light excitation (e.g., impurity doping, sensitization,^[ Reference Sato, Morikawa, Saeki, Kajino and Motohiro ^{24 ]} and solid solution construction^[ Reference Zhang, Ouyang, Kako and Ye ^{25 ]}), improved transport and separation of the photogenerated electron–hole pairs (e.g., loading with cocatalysts,^[ Reference Ishitani, Inoue, Suzuki and Ibusuki ^{26 ]} synthesis of nanostructured photocatalysts,^[ Reference Liu, Torimoto, Matsumoto and Yoneyama ^{27 ]} and construction of semiconductor heterojunctions^[ Reference Xi, Ouyang and Ye ^{28 ]}), enhanced adsorption of CO₂ (e.g., increasing the surface area^[ Reference Wang, Huang, Liu, Sun and Li ^{29 ]}), and suppression off-target side reactions (e.g., the competitive reaction of H₂O reduction in Eq. (7)^[ Reference Zhai, Xie, Fan, Zhang, Wang, Deng and Wang ^{30 ]}), etc., have been designed to achieve a high efficiency and selectivity.

Key points of the experimental details

The efficiency of the photocatalytic reduction of CO₂ depends not only on the factors mentioned above, a suitable semiconductor photocatalyst (with proper E _g, CB and VB), the excitation, transport and separation (recombination) of the photogenerated electron–hole pairs, etc., but also on the experimental conditions,^[ Reference Dey ^{20 ,} Reference Yahaya, Gondal and Hameed ^{31 –} Reference Zhang, Han, Hong and Yu ^{33 ]} such as the light source, response time, reaction temperature, the ratio of H₂O and CO₂, etc.

Photocatalytic reduction of CO₂ is mainly carried out at room pressure and temperature and is controlled by a cooling water recirculation system. A high pressure mercury lamp and/or a 300 W xenon lamp, usually with a cutoff filter, are the most common light sources in CO₂ photoreduction. Yahaya et al. found that the traditional UV lamp had a drawback in that the light intensity would decrease with rising temperature due to the lengthy illumination time, and this decrease affects the efficiency of CO₂ photocatalytic reduction.^[ Reference Yahaya, Gondal and Hameed ^{31 ]} The selection of response time for the studies of CO₂ photoreduction is different, ranging from tens of minutes to hours. Zhao et al. studied nano-sized CoPc/TiO₂ particles as a catalyst for photoconversion of CO₂ and discussed the effect of irradiation time on the formation of photo-reduced products over a period of 6–50 h.^[ Reference Zhao, Fan, Liu and Wang ^{32 ]} At short reaction times, the product yields increased rapidly because more photoelectrons were generated with increasing time of light exposure; however, the product yields flattened out after 20 h of irradiation. This result can be attributed to saturation of the reactive surface active sites absorbed with intermediate products. In addition, the molar ratio of H₂O and CO₂ is also found to play an important role in determining the reaction rate and product selectivity for CO₂ photocatalytic reduction. Zhang et al. investigated the photocatalytic reduction of CO₂ with H₂O on a Pt-loaded TiO₂ catalyst and studied the effect of the H₂O/CO₂ molar ratio on CH₄ yield. They found that the CH₄ yield of 0.15 wt.% Pt-loaded TiO₂ nanotube increased as the mole ratio of H₂O and CO₂ increased, while the different H₂O:CO₂ molar ratios had little effect on the CH₄ yield of 0.12 wt.% Pt-loaded TiO₂ nanoparticles.^[ Reference Zhang, Han, Hong and Yu ^{33 ]} The formation of CH₄ occurred mainly due to the high concentration of the surface OH group, which had been shown previously by Ikeue et al.^[ Reference Ikeue, Nozaki, Ogawa and Anpo ^{34 ]}

The main products in a gas phase reaction system for photocatalytic reduction of CO₂ are generally CH₄, which is usually on the μmol scale, as well as a small amount of CO.^[ Reference Liu, Zhao, Andino and Li ^{8 ,} Reference Chen, Wang, Huang, Zhang, Zhang, Li and Zou ^{14 –} Reference Xie, Umezawa, Zhang, Reunchan, Zhang and Ye ^{16 ]} As we mentioned above, the photocatalytic reduction of CO₂ is a complex multi-electronic reaction process. There are two plausible pathways for CH₄ formation^[ Reference Tu, Zhou and Zou ^{4 ,} Reference Dey ^{20 ]}:

(9) $${\rm C}{\rm O}_{\rm 2} \to {\rm HCOOH} \to {\rm HCHO} \to {\rm C}{\rm H}_{\rm 3}{\rm OH} \to {\rm C}{\rm H}_{\rm 4}$$

(10) $${\rm C}{\rm O}_{\rm 2} \to {\rm CO} \to {\rm CO}_2^{^{ \bullet -}} \to {\rm C}{\rm H}_{\rm 2} \to {\rm C}{\rm H}_{\rm 4}$$

It is necessary to verify whether the carbon source for CH₄ production is actually the CO₂ for the gaseous CO₂ reduction because any organic adsorbates or carbon residues may take part in side reactions, such as the photo-Kolbe reaction, mineralization reaction, photolysis reaction or reverse disproportionation reaction.^[ Reference Izumi ^{19 ,} Reference Dey ^{20 ]} These reactions could create a false impression of the formation of hydrocarbon products.

If there are any carbon residues deposited or buried on the surface of catalyst during the preparation of the catalyst for the photocatalytic reduction of CO₂, a reverse disproportionation reaction of the impurity might occur in the presence of CO₂ in the reaction system. In this case, the generated CO would react with the water adsorbed on the surface active sites of the catalyst; then, the generated CH₃OH would ultimately generate CH₄. This pathway has been shown by Yang et al. through the combined use of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and ¹³C labeled CO₂ during CO₂ photoreduction over Cu-promoted crystalline TiO₂ catalysts. The reaction process could be described as follows^[ Reference Izumi ^{19 ,} Reference Yang, Yu, van der Linden and Wu ^{35 ]}:

(11) $${\rm C}\;({\rm impurity}\;{\rm in}\;{\rm or}\;{\rm on}\;{\rm catalyst}) + {\rm C}{\rm O}_{\rm 2} \to {\rm CO} + {\rm CO}$$

(12) $${\rm CO} + 2{\rm H}_{\rm 2}{\rm O} \to {\rm C}{\rm H}_3{\rm OH} + {\rm O}_2$$

Yui et al. demonstrated that organic adsorbates, especially CH₃COOH, had a significant effect on the formation of CH₄ in CO₂ photoreduction over TiO₂. The organic adsorbed on the surface of TiO₂ could participate in a so-called photo-Kolbe reaction (CH₃COOH, for example)^[ Reference Yui, Kan, Saitoh, Koike, Ibusuki and Ishitani ^{36 ]}:

(13) $${\rm C}{\rm H}_{\rm 3}{\rm COOH} + {\rm h}^ + \to {}^ \bullet {\rm C}{\rm H}_3 + {\rm C}{\rm O}_2 + {\rm H}^ + $$

(14) $$^ \bullet {\rm C}{\rm H}_{\rm 3} + {\rm C}{\rm H}_{\rm 3}{\rm COOH} \to {}^ \bullet {\rm C}{\rm H}_2{\rm COOH} + {\rm C}{\rm H}_{\rm 4}$$

In addition, if there are any alcohols and O₂ in the CO₂ photoreduction reaction system producing CH₄, CO₂ would be generated in-situ due to the mineralization of alcohols on the surface of the semiconductor photocatalyst, which could be explained through the following mechanism^[ Reference Dey ^{20 ]}:

(15) $${\rm Organic}\;{\rm radical}\;{\rm (OR)} + {\rm O}_{\rm 2} \to {\rm C}{\rm O}_2$$

In conclusion, evaluations of the products for CO₂ photocatalytic reduction should be interpreted with caution. In other words, the C source should be carefully investigated with respect to the production of CH₄ and CO from CO₂ in a gaseous CO₂ photoreduction system. Most researchers carry out a series of contrast experiments with different conditions including H₂O, CO₂, irradiation and catalyst to exclude the effects of carbon residues and organic adsorbates on the formation of hydrocarbon products^[ Reference Chen, Wang, Huang, Zhang, Zhang, Li and Zou ^{14 –} Reference Xie, Umezawa, Zhang, Reunchan, Zhang and Ye ^{16 ]}; some other studies, however, reported a more precise and intuitive method: isotopic labeling (¹³C and ¹⁸O).^[ Reference Yang, Yu, van der Linden and Wu ^{35 –} Reference Li, Zhou, Li, Xu, Meng, Wang, Xiao and Zou ^{37 ]} This approach can simultaneously evaluate the C source and the O source. To guarantee the reliability of the results of CO₂ photocatalytic reduction, carbon residues or organic adsorbates must be avoided throughout the entire reaction system. These residues and adsorbates may form during the synthesis of the catalyst if it involves the use of alkoxides and organic solvents (e.g., polyethylene glycol, PEG) as precursors or in washing steps if organic cleaning solvents are used.^[ Reference Yang, Yu, van der Linden and Wu ^{35 ,} Reference Kočí, Obalová, Matějová, Placháa, Lacnýa, Jirkovskýc and Šolcová ^{38 ,} Reference Lin and Frei ^{39 ]} These residues could be removed using particular methods, such as thermal treatment in a continuous flow of N₂ or Ar inert gas or at high temperature in air, as well as prolonged washing in a water environment.^[ Reference Izumi ^{19 ,} Reference Yang, Yu, van der Linden and Wu ^{35 ,} Reference Yui, Kan, Saitoh, Koike, Ibusuki and Ishitani ^{36 ]}

Perovskite oxide semiconductor photocatalysts for gaseous CO₂ reduction

An ideal semiconductor photocatalyst for CO₂ photoreduction should benefit from: (1) a suitable E _g such that the semiconductor absorbs incident photons over a range of wavelengths to generate electron–hole pairs; (2) effective separation between the photogenerated electron–hole pairs that then transfer to the surface of semiconductors and subsequently take part in the oxidation-reduction reaction with CO₂ and H₂O absorbed on the surface active sites of the semiconductors; and (3) a well-tuned CB and VB such that the bottom of the CB is located at a more negative potential than the reduction potentials of CO₂ and such that the top of the VB has a more positive potential than the oxidation potential of H₂O. Based on these principles, various photocatalysts, including metal oxides (e.g., TiO₂,^[ Reference Liu, Zhao, Andino and Li ^{8 ]} ZnO,^[ Reference Mahmodi, Sharifnia, Rahimpour and Hosseini ^{40 ]} BiVO₄,^[ Reference Liu, Huang, Dai, Zhang, Qin, Jiang and Whangbo ^{41 ]} and Ga₂O₃ ^[ Reference Park, Choi, Choi, Lee and Kang ^{42 ]}), metal sulfides (e.g., CdS^[ Reference Wang and Wang ^{9 ]} and ZnS^[ Reference Fujiwara, Hosokawa, Murakoshi, Wada and Yanagida ^{43 ]}), metal phosphides (e.g., GaP^[ Reference Barton, Rampulla and Bocarsly ^{10 ]}), nitrides (e.g., C₃N₄ ^[ Reference Mao, Peng, Zhang, Li, Ye and Zan ^{44 ]}), etc., have been reported for CO₂ reduction. Among them, perovskite oxide semiconductor photocatalysts are good candidates for CO₂ reduction due to their unique physical and chemical properties, including their stability, their controllable structure and their ability to be doped in two different cation sites. In addition, perovskite structures are expected to exhibit good photocatalytic activity as has been demonstrated in many studies.^[ Reference Kato and Kudo ^{12 ,} Reference Kudo, Kato and Nakagawa ^{13 ]} Recently, a series of the perovskite type materials of the form ABO₃ (such as BaZrO₃,^[ Reference Chen, Wang, Huang, Zhang, Zhang, Li and Zou ^{14 ]} BaCeO₃,^[ Reference Wang, Huang, Chen, Zhang, Li and Zou ^{15 ]} SrTiO₃,^[ Reference Xie, Umezawa, Zhang, Reunchan, Zhang and Ye ^{16 ,} Reference Zhou, Guo, Li, Fan, Zhang and Ye ^{45 ,} Reference Kou, Gao, Li, Yu, Zhou and Zou ^{46 ]} etc.) have been studied for the photocatalytic reduction of CO₂. Table I shows the summary of perovskite oxide semiconductors used in the photocatalytic reduction of CO₂ in a gas reaction system. These semiconductors were roughly divided into five types: tantalates, niobates, titanates, zirconates and cerates.

Tantalates

Teramura et al. carried out the photoreduction of CO₂ over ATaO₃ (A = Li, Na and K) synthesized by a conventional solid-state reaction in the presence of H₂. CO was the only gaseous product over all of the samples tested under 200 W Hg-Xe lamp irradiation.^[ Reference Teramura, Okuoka, Tsuneoka, Shishido and Tanaka ^{54 ]} LiTaO₃ presented the highest activity for CO yield (0.42 µmol/g in 24 h), and the trend of the photocatalytic activities were consistent with that of the E _g (LiTaO₃ > NaTaO₃ > KTaO₃). In addition, they also found that the amount of evolved CO gas depended on the amount of chemisorbed CO₂ (as shown in Fig. 2),^[ Reference Teramura, Okuoka, Tsuneoka, Shishido and Tanaka ^{54 ]} which meant that the adsorption of CO₂ had an effect on the reduction efficiency of CO₂. H₂ was used as reductant in this photocatalytic reduction of CO₂ process, whereas other gas reaction systems usually used H₂O as reductant.

Figure 2. (a) Amount of evolved CO gas for the photocatalytic reduction of CO₂ in the presence of H₂ as a reductant over ATaO₃ (A = Li, Na, K) after 24 h of photoirradiation. (b) Amount of Chemisorbed CO₂ on ATaO₃ (A = Li, Na, K). (Copyright 2010, Applied Catalysis B: Environmental.^[ Reference Teramura, Okuoka, Tsuneoka, Shishido and Tanaka ^{54 ]}).

Niobates

Niobates with a perovskite structure share many characteristics with tantalates and have been used in CO₂ photocatalytic reduction systems with water as the reductant. Shi et al. studied the photocatalytic reduction activities of CO₂ into CH₄ over NaNbO₃ samples upon irradiation from a Xe arc lamp. It was discovered that the CH₄ evolution rate (653 ppm/h/g) of Pt-loaded NaNbO₃ nanowires prepared by a hydrothermal method were much higher than that of bulk NaNbO₃ particles synthesized by a solid-state reaction.^[ Reference Shi, Wang, Chen, Zhu, Ye and Zou ^{53 ]} Subsequently, they prepared alkali niobate ANbO₃ (A = K, Na) photocatalysts using a conventional solid-state reaction method and compared their photocatalytic performance for CO₂ reduction under similar conditions. They found that KNbO₃ (7.0 ppm/h) showed a higher photocatalytic activity than NaNbO₃ (2.3 ppm/h).^[ Reference Shi and Zou ^{52 ]} The perovskite NaNbO₃ studied in most research has a typical cubic structure, whereas NaNbO₃ also has another crystal structure—an orthorhombic structure. The schematic crystal structures of cubic and orthorhombic NaNbO₃ are shown in Fig. 3.^[ Reference Li, Ouyang, Xi, Kako and Ye ^{51 ]} Li et al. discussed the photocatalytic activities of CO₂ reduction over the cubic and orthorhombic NaNbO₃ in a gas phase system and found that the CO₂ reduction activity over the cubic NaNbO₃ was nearly twice that of the orthorhombic NaNbO₃. These results might be due to the unique electron structure of cubic NaNbO₃, which may support electron excitation and transfer.^[ Reference Li, Ouyang, Xi, Kako and Ye ^{51 ]} In addition, they studied the influence of the preparation temperature on the activity of NaNbO₃ photocatalyst.^[ Reference Li, Ouyang, Zhang, Kako and Ye ^{50 ]} The cubic NaNbO₃ prepared at 500°C presented the best photocatalytic performances, and the CH₄ evolution for gas phase CO₂ photoreduction over NaNbO₃ loaded with 0.5 wt.% Pt under a 300 W Xe lamp (λ > 300 nm) could reach 12.6 µmol/m²/h.

Figure 3. Schematic crystal structures of cubic and orthorhombic NaNbO₃. (Copyright 2012, The Journal of Physical Chemistry.^[ Reference Li, Ouyang, Xi, Kako and Ye ^{51 ]}).

Moreover, differing from the simple ABO₃-type perovskite semiconduction photocatalysts, lamellar perovskite niobates, such as HNb₃O₈ and KNb₃O₈, have also been applied in the gas phase photocatalytic reduction of CO₂.^[ Reference Li, Pan, Li and Zhuang ^{48 ,} Reference Li, Li, Zhuang, Zhong, Li and Wang ^{49 ]} Figure 4 shows a schematic drawing of the layered structures of HNb₃O₈ composed of 2D Nb₃O₈ ⁻ anion thin sheets of edge- and corner-shared NbO₆ octahedral with H⁺ intercalated between the layers. Li et al. found that hydrothermally synthesized HNb₃O₈ and KNb₃O₈ nanobelts exhibited much higher yields of CH₄ compared with the HNb₃O₈ and KNb₃O₈ particles prepared by conventional solid-state reactions as well as commercial TiO₂.^[ Reference Li, Pan, Li and Zhuang ^{48 ]}

Figure 4. Schematic drawing of the layered structures of HNb₃O₈.

Titanates

Several perovskite titanates, such as SrTiO₃, PbTiO₃ and CaTiO₃, have also been explored for gas phase CO₂ photocatalytic reduction.^[ Reference Xie, Umezawa, Zhang, Reunchan, Zhang and Ye ^{16 ,} Reference Zhou, Guo, Li, Fan, Zhang and Ye ^{45 –} Reference Kwak and Kang ^{47 ]} Among them, SrTiO₃ is the most widely studied photocatalysts. Xie et al. prepared a self-doped SrTiO_3−δ using a carbon-free one-step combustion, and they demonstrated that the chemical adsorption of CO₂ could be improved by increasing the oxygen deficiency of the SrTiO₃. This deficiency promoted the photocatalytic activity of CO₂ reduction to generate CH₄ under visible light irradiation.^[ Reference Xie, Umezawa, Zhang, Reunchan, Zhang and Ye ^{16 ]} Zhou et al. discussed a leaf-shaped 3D hierarchical artificial photosynthetic system of ATiO₃ (A = Sr, Ca and Pb) prepared by a modified sol-gel method using fresh green leaves as a structure template. They studied the effects of various cocatalysts (Au, Ag, Cu, Pt, RuO₂ and NiO _x ) on the photocatalytic activity of CO₂. As shown in Fig. 5, CH₄ and CO were the two main products, and Au as a suitable cocatalyst exhibited the best performance for the selectivity of both CH₄ and CO (using SrTiO₃ as an example).^[ Reference Zhou, Guo, Li, Fan, Zhang and Ye ^{45 ]} In another study, Kou et al. synthesized a highly active photocatalyst with response to visible light by substituting the Ti⁴⁺ in SrTiO₃ to transition metal (Co, Fe, Ni) ions for gas phase CO₂ reduction, and Pt-SrTi_0.98Co_0.02O₃ had the highest activity (the yield of CH₄ was 63.6 ppm/h).^[ Reference Kou, Gao, Li, Yu, Zhou and Zou ^{46 ]}

Figure 5. CO and CH₄ evolution on an artificial photosynthetic system of SrTiO₃ of CO₂ photoreduction loaded with different cocatalysts. (Copyright 2013, Scientific Reports.^[ Reference Zhou, Guo, Li, Fan, Zhang and Ye ^{45 ]}).

Zirconates

Apart from the above-mentioned tantalates, niobates and titanates photocatalysts, Chen et al. first investigated the photocatalytic properties of BaZrO₃ for CO₂ reduction into CH₄ in a gas reaction system.^[ Reference Chen, Wang, Huang, Zhang, Zhang, Li and Zou ^{14 ]} Barium zirconate made by the Pechini process has a bandgap of 4.8 eV, which means that BaZrO₃ only has a UV light response. They also confirmed the effects of different cocatalysts on the photocatalytic activities and optimized the loaded amount of metal nanoparticle cocatalysts. As shown in Fig. 6, Ag was a suitable cocatalyst that exhibited the best selectivity for producing CH₄ with the BaZrO₃ system. In addition, a CH₄ yield of up to 0.57 µmol/h/g was obtained when 0.3 wt.% Ag cocatalyst was loaded onto the surface of the BaZrO₃. In this case, the Ag site acted as an electron trap to effectively decrease the recombination rate of the photogenerated electron–hole pairs.

Figure 6. (a) CH₄ yields of BaZrO₃ deposited with different cocatalysts (Ru, Cu, Au, Pt, Ag) of the same apparent amount. (b) CH₄ yields of BaZrO₃ with different amounts of Ag deposition. (Copyright 2015, Catalysis Science & Techonology.^[ Reference Chen, Wang, Huang, Zhang, Zhang, Li and Zou ^{14 ]}).

Summary and outlook

The photocatalytic reduction of CO₂ into renewable hydrocarbon fuels using solar energy is a promising approach for simultaneously achieving carbon cycling and solving the shortage of sustainable energy. An ideal semiconductor photocatalyst for CO₂ photoreduction should have a suitable E _g and proper positioning of the CB and VB, which should satisfy thermodynamic conditions of CO₂ reduction. In addition, the photogenerated electron–hole pairs should separate effectively and transfer to the surface active sites of the semiconductor photocatalysts so that they can participate in the oxidation-reduction reactions with CO₂ and H₂O.

The photocatalytic reduction of CO₂ is a complex multi-electronic reaction process. The main products in a gas phase CO₂ reduction are generally CH₄ as well as a small amount of CO, and the yield of CH₄ is usually on the μmol scale. This yield is relatively low compared with the conversion of water splitting.^[ Reference Chen, Wang, Huang, Zhang, Zhang, Li and Zou ^{14 –} Reference Xie, Umezawa, Zhang, Reunchan, Zhang and Ye ^{16 ,} Reference Zhou, Guo, Li, Fan, Zhang and Ye ^{45 –} Reference Teramura, Okuoka, Tsuneoka, Shishido and Tanaka ^{54 ]} The conversion efficiency and products selectivity of gaseous CO₂ photoreduction not only depends on the characteristics of the photocatalyts but also depends on some experimental conditions, such as the light source, response time, reaction temperature, the ratio of H₂O and CO₂, etc. It is necessary to verify whether the formation of hydrocarbon products is actually from the photocatalytic reduction of CO₂ or is from some side reactions with organic adsorbates (e.g., acetic acid) or carbon residues. These adsorbates or residues may be formed during the photo-Kolbe reaction, mineralization reaction or reverse disproportionation reaction in photocatalytic reaction. A series of contrast experiments with different conditions (including H₂O, CO₂, irradiation and catalyst) or a more intuitive and precise method—isotopic labeling (¹³C and ¹⁸O)—is necessary to detect and/or exclude the effects of carbon residues and organic adsorbates. These contaminants can also be removed by thermal treatment in air or inert gas or by prolonged washing with water upon the formation of products.

Based on the research and the current development of related theories and practical applications, the photocatalytic reduction of CO₂ still has a long way to go. The future studies on CO₂ photoreduction should be initiated in the following areas.

Gas adsorption ability on the surface of photocatalysts

It is well known that the reduction of CO₂ and the oxidation of H₂O are the two main half-reaction steps for the photocatalytic reduction of CO₂. On one hand, effective adsorption of CO₂ is an important process that can be promoted by increasing surface area,^[ Reference Wang, Huang, Liu, Sun and Li ^{29 ]} by introducing basic sites such as MgO, or by introducing surface oxygen vacancies^[ Reference Xie, Umezawa, Zhang, Reunchan, Zhang and Ye ^{16 ,} Reference Xie, Wang, Zhang, Deng and Wang ^{63 ]}. Optimization of adsorption is important for improving the conversion efficiency for the photocatalytic reduction of CO₂. The adsorption of CO₂ molecules on stoichiometric and oxygen-deficient SrTiO₃ has been theoretically and experimentally studied by Xie et al. They demonstrated that chemical adsorption of CO₂ molecules could be improved by increasing the oxygen deficiency in SrTiO₃, thus enhancing the photocatalytic performance for CO₂ reduction.^[ Reference Xie, Umezawa, Zhang, Reunchan, Zhang and Ye ^{16 ]} On the other hand, water oxidation favors the efficient separation of the photogenerated electron–hole pairs, thus improving the photocatalytic activity for CO₂ reduction.^[ Reference Li, Wen, Low, Fang and Yu ^{3 ,} Reference Izumi ^{19 ]} In addition, deactivation of the photocatalysts may occur after long reaction times, which may be attributed to the active sites gradually becoming covered by the adsorption of the reduction intermediate products or the final products accumulation on the photocatalyst surface over time.^[ Reference Zhao, Fan, Liu and Wang ^{32 ,} Reference Xiong, Zhao, Zhang and Zheng ^{64 ]} In conclusion, it is important to refine the research on the adsorption of gaseous reactants (CO₂, H₂O) and the products on the surface of photocatalysts.

In short, the future development for the photocatalytic reduction of CO₂ remains challenging, and the comprehensive efforts (such as materials synthesis, doping, nanostructuring and heterojunction)^[ Reference Tu, Zhou and Zou ^{4 ,} Reference Zhang, Ouyang, Kako and Ye ^{25 ]} are needed to achieve a breakthrough in the conversion efficiency of CO₂ reduction into hydrocarbon fuels that may eventually lead to industrialization.

Table I. Summary of perovskite oxide semiconductors for the photocatalytic reduction of CO₂ in a gas reaction system.

^a Maximum formation rate reported for the products in μmol/h/g, unless stated otherwise.

^b In μmol/m² _cata/h.

^c In ppm/h.

^d The entire yield for the main products in μmol/g (7 h, x = y = 1).

^e In ppm/h/g.

^f The entire yield for the main products in μmol/g (24 h).