# Enhanced CO evolution for photocatalytic conversion of CO2 by H2O over Ca modified Ga2O3

## Abstract

Artificial photosynthesis is a desirable critical technology for the conversion of CO2 and H2O, which are abundant raw materials, into fuels and chemical feedstocks. Similar to plant photosynthesis, artificial photosynthesis can produce CO, CH3OH, CH4, and preferably higher hydrocarbons from CO2 using H2O as an electron donor and solar light. At present, only insufficient amounts of CO2-reduction products such as CO, CH3OH, and CH4 have been obtained using such a photocatalytic and photoelectrochemical conversion process. Here, we demonstrate that photocatalytic CO2 conversion with a Ag@Cr-decorated mixture of CaGa4O7-loaded Ga2O3 and the CaO photocatalyst leads to a satisfactory CO formation rate (>835 µmol h−1) and excellent selectivity toward CO evolution (95%), with O2 as the stoichiometric oxidation product of H2O. Our photocatalytic system can convert CO2 gas into CO at >1% CO2 conversion (>11531 ppm CO) at ambient temperatures and pressures.

## Introduction

Carbon dioxide (CO2) concentrations in the atmosphere have increased drastically over the past few centuries owing to the combustion of carbon-rich fossil fuels such as coal, oil, and natural gas. As a major anthropogenic greenhouse gas, these ever-increasing CO2 emissions are detrimental to the environment and will affect both ecosystems and the global climate1. Therefore, there is a critical requirement of mitigating CO2 emissions to achieve sustainable development. Since the pioneering work on the photocatalytic conversion of CO2 into formic acid (HCOOH) and methyl alcohol (CH3OH) over semiconductors reported by Halmann and Inoue et al.2,3, the photocatalytic conversion of CO2 into other valuable feedstocks at ambient temperatures and pressures has attracted considerable attention from the scientific community as a feasible strategy for CO2 storage and conversion4,5,6,7,8.

In general, the photocatalytic conversion of CO2 over an excited semiconductor-based catalyst involves three main steps. First, CO2 molecules are adsorbed on the photocatalyst surface9,10,11. Second, the photogenerated electrons react with the adsorbed CO2 species and protons (H+) to yield products such as carbon monoxide (CO), methane (CH4), CH3OH, and HCOOH. Among these possible reduction products, CO is one of the most useful because it is widely combined with H2 to provide synthetic gas for use in many chemical processes, such as methanol synthesis12,13 and the industrial Fischer–Tropsch process that produce various chemicals and synthetic fuels14,15. Third, the products are desorbed from the photocatalyst surface. However, as the H/H2 redox potential (−0.41 V vs. NHE at pH 7) is more positive than that for CO2/CO (−0.52 V vs. NHE at pH 7), the generation of H2 from H+ is preferable for the photocatalytic conversion of CO2 into CO, where H2O acts as the electron donor16,17,18. Moreover, because of the high thermodynamic stability of the linear CO2 molecule, the fixation and activation of CO2 are also immense challenges in the photocatalytic conversion of CO2 by H2O4,19. Thus, although various heterogeneous photocatalysts have been reported for the photocatalytic conversion of CO2 into CO with H2O as the electron donor20,21,22,23,24, the photocatalytic activity for CO evolution remains limited to a few micromoles, while the photocatalytic conversion rate of CO2 into CO is <0.15%.

Based on the processes involved in the photocatalytic conversion of CO2 described previously, we deduce that the photocatalytic activity of the photocatalyst for CO2 conversion can be improved by increasing CO2 adsorption, charge separation, and product desorption. Due to the fact that CO2 acts as a Lewis acid that bonds easily with Lewis bases25, many studies have focused on improving CO2 adsorption by modifying the photocatalyst surface with a CO2 adsorbent, such as NaOH26, amino groups27, and rare earth species28, to increase the photocatalytic activity and selectivity for CO2 conversion by H2O. Our group reported that modifying the photocatalyst surface with alkaline earth metals (e.g., Ca, Sr, and Ba) enhanced the conversion of CO2 and the selectivity toward CO evolution29. Moreover, we found that a Ag@Cr core/shell cocatalyst suppresses the backward reaction from CO and O2 to CO2, and enhances the adsorption of CO2, resulting in a highly selective photocatalytic CO2 conversion30,31.

In this study, we exploited the above techniques and successfully fabricated a Ag@Cr-decorated mixture of CaGa4O7-loaded Ga2O3 and CaO photocatalyst, which exhibits a high CO formation rate (>835 µmol h−1) per 0.5 g of catalyst, in addition to high selectivity toward CO evolution (>95%) with the stoichiometric production of O2 as the oxidation product of H2O during the photocatalytic conversion of CO2 by H2O. Approximately 1.2% of the CO2 in the gas phase was transformed into CO (11531 ppm) as a product. The results reported in this study represent almost an order of magnitude higher than most previously published results, as summarized in Supplementary Table 1.

## Results and discussion

### Photocatalytic reduction of CO2 by H2O

Table 1 shows the formation rates of CO, H2, and O2, selectivity toward CO evolution, and the balance between consumed electrons and holes over the bare Ga2O3, Ag-modified Ga2O3 (Ag/Ga2O3), Ag@Cr-modified Ga2O3 (Ag@Cr/Ga2O3), and Ag@Cr-modified Ca-loaded Ga2O3 (Ag@Cr/Ga2O3_Ca) photocatalysts during the photocatalytic conversion of CO2 by H2O. No liquid products were detected in the reaction solutions in these photocatalytic systems, and H2, O2, and CO were detected as gaseous products. As no reduction products other than H2 and CO were generated, the selectivity toward CO evolution and the balance between the consumed electrons and holes were calculated as follows:

$${\mathrm{Selectivity}}\,{\mathrm{toward}}\,{\mathrm{CO}}\,{\mathrm{evolution}}\,\left( \% \right) = 2R_{{\mathrm{CO}}}/(2R_{{\mathrm{CO}}} + 2R_{{\mathrm{H}}2}) \times 100,$$
(1)
$${\mathrm{Consumed}}\,e^ - /h^ + = (2R_{{\mathrm{CO}}} + 2R_{{\mathrm{H}}2})/4R_{{\mathrm{O}}2},$$
(2)

where RCO and RH2 represent the formation rates of CO and H2, respectively. If H2O acts as an electron donor, the value of e/h+ should be equal to 1.

We obtained stoichiometric amounts of H2 and CO as reduction products in addition to O2 as the oxidation product, indicating that H2O serves as the electron donor. Bare Ga2O3 exhibited a particularly low selectivity toward CO evolution (4%) as the electrons generated by charge transfer were not consumed in the reduction of CO2, but rather in the production of H2 from H+. Modifying Ga2O3 with a Ag cocatalyst enhanced the selectivity toward CO evolution (29%); however, this was not sufficient to obtain a selectivity >50%. In contrast, we succeeded in the selective photocatalytic conversion of CO2 by H2O over Ag@Cr/Ga2O3. A relatively high CO formation rate (499.6 µmol h−1) was achieved with 77% selectivity toward CO evolution. The photocatalytic reaction for the conversion of CO2 by H2O over Ag@Cr/Ga2O3 and Ag@Cr/Ga2O3_Ca was carried out for at least four times, and errors in the product formation rates (H2, O2, and CO) were smaller than 5%. Controlling both, the bulk and surface of the photocatalysts, is highly important for achieving a considerably high CO formation rate and selectivity toward CO evolution. We found that the amount of Ca species significantly affected the H2 and CO formation rates (for the product formation rates and selectivity over various Ag@Cr/Ga2O3_Ca photocatalysts see Supplementary Fig. 1). The formation rate of CO increased first and then decreased as the Ca content increased (Supplementary Fig. 1a–g). In contrast, the formation rate of H2 over the Ag–Cr/Ga2O3_Ca_x samples increased monotonically with increasing amount of Ca species. The Ag–Cr/CaGa4O7 photocatalyst was only active for H2 evolution derived from water splitting (Supplementary Fig. 1h). The Ag@Cr/Ga2O3_Ca photocatalyst exhibited the highest CO formation rate (794.2 µmol h−1), and the selectivity toward CO evolution was approximately 82%. Additionally, CO production from the photocatalytic conversion of CO2 after photoirradiation for 15 h over Ag@Cr/Ga2O3_Ca was more stable than that over Ag@Cr/Ga2O3 (for the product formation rates for 15 h see Supplementary Fig. 2), which indicates that the presence of Ca species is not only beneficial for improving the photocatalytic activity and selectivity, but also for improving stability during the photocatalytic conversion of CO2 to CO.

Various control experiments were carried out to confirm the source of CO during the photocatalytic conversion of CO2 by H2O, the results of which are shown in Supplementary Fig. 3. We did not detect any appreciable amounts of products under dark conditions or in the absence of a photocatalyst. In addition, H2 was the main product formed when Ar gas was used instead of CO2 or in the absence of NaHCO3. The control experiments confirmed that the evolved CO originated from the CO2 gas introduced into the samples and not from carbon contaminants.

### Photocatalyst characterization

The close linkage between CaGa4O7 and Ga2O3 on the Ga2O3 surface was confirmed by field-emission transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) (Fig. 2). The marked lattice spacings (0.296 and 0.255 nm) in Fig. 2b correspond to the (130) and (111) planes of CaGa4O7 and Ga2O3, respectively. The core–shell-structured Ag@Cr cocatalyst was successfully loaded onto the Ga2O3_Ca surface using the photodeposition method (Fig. 2c, d), as reported previously by us31.

### Role of the Ca species

Figure 4 shows the FTIR spectra of the adsorbed CO2 species on Ga2O3_Ca_1.1 after different durations of photoirradiation. As the photoirradiation time increased from 0 to 106 h, the bands at 1225 [δ(OH)-Ga] and 1408 cm–1 [νs(CO3)-Ca] decreased and vanished after 104 h. At the same time, new bands gradually appeared at 1581, 1388, and 1353 cm–1 (asymmetric CO2 stretching [νas(CO2)], CH deformation [δ(CH)], and symmetric CO2 stretching [νs(CO2)] assigned to formate species (HCOO–Ga/Ca), respectively)34,35,36. As the photoirradiation continued, the formate species were consumed and gaseous CO (fundamental vibration band at 2143 cm−1)42 was formed simultaneously. This result indicates that the bicarbonate species is the intermediate during the photocatalytic conversion of CO2, and the formates transform into CO with photoirradiation, which is consistent with our previous results43,44. It is worth mentioning that in addition to the presence of intermediate species on the Ga2O3 surface ([δ(OH)–Ga]), the modification by Ca species further increased the amount of intermediate on Ga2O3_Ca_1.1. As the photocatalytic conversion of H+ into H2 and the conversion of CO2 into CO are two competing processes in an aqueous solution, the high adsorption of CO2 at the base site leads to high photocatalytic activity and selectivity toward CO evolution during the photocatalytic conversion of CO2 by H2O.

Notably, although CaGa4O7 exhibited high selectivity toward H2 evolution, the H2 formation rate for CaGa4O7 was significantly lower than that for Ga2O3_Ca_3.3 (for the product formation rates over Ag-Cr/Ga2O3_Ca_3.3 see Supplementary Fig. 6). This indicates that the presence of CaGa4O7 on the Ga2O3 surface enhances the overall photocatalytic efficiency during the photocatalytic reaction, including CO2 conversion and water splitting. The Mott–Schottky plot (Supplementary Fig. 7) and the absorption spectra converted from the diffuse reflectance spectra using the Kubelka–Munk equation (Supplementary Fig. 8) enabled us to estimate the conduction band (CB) and valence band (VB) positions of Ga2O3, Ga2O3_Ca_0.62, and CaGa2O7, as shown in Supplementary Fig. 9. Since the CB and VB of Ga2O3 are both more positive than those of CaGa4O7, a heterojunction system between Ga2O3 and CaGa4O7 can be formed that greatly improves the spatial separation efficiency of the photogenerated carriers45. Therefore, CaGa4O7/Ga2O3 exhibited a much higher photocatalytic efficiency than bare Ga2O3 and CaGa4O7.

We expect that by exploiting the high CO2 adsorption of CaO and the high photocatalytic efficiency of CaGa4O7/Ga2O3, we can further improve the photocatalytic activity and selectivity of the photocatalyst to maximize the conversion of CO2 into CO by H2O. Figure 6a shows the formation rates of H2, O2, and CO during the photocatalytic conversion of CO2 by H2O for the Ga2O3_Ca_3.3 photocatalyst physically mixed with 30 mol% of CaO and Ag@Cr as the cocatalyst. As indicated, a high formation rate of CO (>835 µmol h–1) was achieved, in addition to an excellent selectivity toward CO evolution (>95%), with a stoichiometric amount of evolved O2. Both 12CO and 13CO were detected using quadrupole mass spectrometry (MS), and the peaks at m/z = 28 and m/z = 29 were located at the same positions as those detected by gas chromatography (GC) during the photocatalytic conversion of 13CO2 (for the isotopic lead experiments see Supplementary Fig. 10). Indeed, our results indicate that the detected 12CO was produced from the reduction of 12CO2 derived from the NaHCO3 additive in the solution43. As shown in Fig. 6b, with the consumption of 12CO2 derived from NaHCO3, the amount of generated 12CO gradually decreased, while the 13CO content increased under continuous bubbling of 13CO2. The total amounts of 13CO and 12CO detected by MS were consistent with the amount of CO detected by GC (Fig. 6c), which indicates that the CO was generated as the reduction product of either CO2 introduced in the gas phase or from NaHCO3, rather than from any organic contaminants on the photocatalyst surface. The converted concentration of CO based on the CO formation rate was found to be 11,531 ppm, indicating that ~1.2% of CO2 in the gas phase was transformed into CO (see Supplementary Information for the calculation details. The actual amounts of CO detected are shown in Supplementary Movie 1).

In our previous work, we had found that basic oxides and hydroxides such as Cr(OH)331, SrO44, and rare earth (RE) hydrates and oxides28 function as good CO2 storage materials by generating the corresponding (hydroxy)carbonate compounds (e.g., Cr(OH)x(CO3)y and RE2(OH)2(3−x)(CO3)x), and they improve the photocatalytic activity and selectivity toward CO evolution. Now, we propose a possible mechanism for the photocatalytic conversion of CO2 by H2O over Ag@Cr/CaO/CaGa2O7/Ga2O3, as shown in Fig. 7. During the photocatalytic conversion of CO2 in an aqueous solution of NaHCO3, the Cr(OH)3·H2O and CaO species that are in close contact with Ag particles easily form (hydroxy)carbonate species (named M(OH)x(CO3)y, M=Cr or Ca)31, which greatly increase the concentration of CO2-related species around the Ag active sites, thereby improving selectivity for the photocatalytic conversion of CO2 into CO instead of water splitting. On the other hand, the Ga2O3/CaGa4O7 heterojunction improves the efficiency for spatial separation of the photogenerated carriers, which also increases the photocatalytic activity for the conversion of CO2 into CO. Moreover, while the Cr(OH)3·xH2O shell outside the Ag particle can be oxidized to Cr6+ and dissolve into the solution during the photocatalytic conversion of CO246, the presence of CaO around the Ag active site compensates for the reduced activity from the dissolution of Cr species. As a result, Ag@Cr/Ga2O3_Ca is photocatalytically much more stable than Ag@Cr/Ga2O3.

Herein, we reported the photocatalytic conversion of CO2 using a Ag@Cr/CaO/CaGa4O7/Ga2O3 photocatalyst, in which a satisfactory CO formation rate (>835 µmol h−1) and an excellent selectivity toward CO evolution (95%) were achieved with the stoichiometric production of O2 as the oxidation product of H2O. Through the use of various characterization techniques, we found that the CaO and CaGa4O7 formed on the Ga2O3 surface improved the adsorption of CO2 at basic sites in addition to enhancing the total photocatalytic efficiency. In addition, the physical mixing of CaGa4O7/Ga2O3 with CaO was a particularly simple and convenient technique for exploiting the high CO2 adsorption ability of CaO and the high photocatalytic efficiency of CaGa4O7/Ga2O3. These results are of particular interest, considering that previously, only insufficient amounts of CO2 reduction products were produced during artificial photosynthesis.

## Methods

Ca-modified Ga2O3 (Ga2O3_Ca) was prepared using the ammonia precipitation method reported by Sakata et al.47. In this method, Ga(NO3)3nH2O (12.6 g) was dissolved in 200 mL of deionized water or CaCl2 solution in ultrapure water at various concentrations. Hydroxylation was carried out by dripping an ammonium hydroxide solution until the pH level reached 9.1. The obtained hydroxides were centrifuged and dried overnight. The Ga2O3_Ca sample was obtained by calcining the precursor at 1273 K for 10 h. Ag@Cr/Ga2O3_Ca was synthesized using the photodeposition method reported in our previous work30. In this method, the as-prepared Ga2O3_Ca powder (1.0 g) was dispersed in ultrapure water (1.0 L) containing the necessary amounts of silver nitrate (AgNO3) and chromium (III) nitrate (Cr(NO3)3). The suspension was purged with Ar gas and irradiated under a 400 W high-pressure Hg lamp with Ar gas flowing for 1.0 h, followed by filtration and drying at room temperature (~298 K). The Ag/Ga and Cr/Ga molar ratios were both 1.0 mol%.

### Characterization

The as-prepared Ga2O3_Ca samples were characterized using the following techniques: XRD (Model: Multiflex, Rigaku Corporation, Japan) with Cu Kα radiation (λ = 0.154 nm); XPS (Model: ESCA 3400, Shimadzu Corporation, Japan) with Mg Kα radiation; SEM (Model: SU-8220, Hitachi High-Technologies Corporation, Japan); TEM (Model: JEM-2100F, JEOL Ltd, Japan); and UV–Visible spectroscopy (V-650, JASCO) with an integrated sphere accessory. The BET surface areas of the photocatalyst samples were determined from their N2-adsorption isotherms at 77 K using a volumetric gas-adsorption measuring instrument (Model: BELSORP-miniII, MicrotracBEL Corp. (formerly BEL Japan, Inc.), Japan). Prior to these measurements, each sample was evacuated at 473 K for 1 h using a sample pretreatment system (Model: BELPREP-vacII, MicrotracBEL Corp. (formerly BEL Japan, Inc.), Japan). ICP-OES (Model: iCAP7400, Thermo Fisher Scientific, USA) was used to determine the actual amounts of Ca modified on the Ga2O3 surface. The FTIR spectra of the adsorbed carbon species were recorded using an FTIR spectrometer (Model: FT/IR-4700, JASCO International Co., Ltd., Japan) equipped with a mercury–cadmium–tellurium (MCT) detector and cooled with liquid N2 in the transmission mode at 303 K. Each sample (~30 mg) was pressed into a wafer (diameter: 10 mm) and introduced into the instrument in a cylindrical glass cell with calcium fluoride (CaF2) windows. The wafer was evacuated at 673 K for 30 min before being examined, followed by treatment with O2 at ~40 Torr for 30 min, after which the wafer was evacuated for 30 min and cooled to 303 K. The data for each FTIR spectrum were obtained from 128 scans with a resolution of 4 cm−1. The energy gap of the band structure and flat band potential of the Ga2O3_Ca samples were determined using the Davis–Mott and Mott–Schottky equations, respectively; the experimental details are provided in the Supplementary Information.

### Photocatalytic reaction

The photocatalytic reduction of CO2 was carried out using a flow system with an inner irradiation-type reaction vessel. The synthesized photocatalyst (0.5 g) was dispersed in ultrapure water (1.0 L) containing 0.1 M sodium bicarbonate (NaHCO3). The CO2 was bubbled into the solution at a flow rate of 30 mL min−1. The suspension was illuminated using a 400 W high-pressure Hg lamp with a quartz filter, and the assembly was connected to a water-cooling system. The amounts of evolved H2 and O2 were detected using a gas chromatography system fitted with a thermal conductivity detector (TCD-GC, Model: GC-8A, Shimadzu Corporation, Japan) and a 5A molecular sieve (MS 5A) column, and Ar was used as the carrier gas. The amount of evolved CO was analyzed using a gas chromatography system fitted with a flame ionization detector (FID-GC, Model: GC-8A, Shimadzu Corporation, Japan), a methanizer, and a ShinCarbon ST column, and N2 was used as the carrier gas. High-performance liquid chromatography (Model: LC-4000, JASCO, USA) was used to detect the presence of liquid products.

In the isotope experiment, 12CO2 was replaced by 13CO2. The formation rates of H2, O2, 12CO, and 13CO under photoirradiation were detected using a quadrupole mass spectrometer (BELMASS, Microtrac BEL) combined with a TCD-GC detector.

## Data availability

The datasets generated during and/or analysed during the current study are available in the [figshare] repository, [https://figshare.com/s/84a5d675a273e507fb55 and/or https://doi.org/10.6084/m9.figshare.12927422].

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## Acknowledgements

This study was partially supported by a Grant-in-Aid for Scientific Research on Innovative Areas, “All Nippon Artificial Photosynthesis Project for Living Earth” [grant number 2406] of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan and the Program for Element Strategy Initiative for Catalysts & Batteries (ESICB) [No. JPMXP0112101003], commissioned by the MEXT of Japan.

## Author information

Authors

### Contributions

R.P. and K.T. designed the research. R.P. prepared the photocatalyst powder and conducted XRD, BET, XPS, SEM, TEM, ICP-OES, UV–Vis, and FTIR studies, electrochemical measurements, and the photocatalytic CO2-conversion experiments. M.M. carried out the isotopic labeling experiment. R.P., K.T., M.M., H.A., S.H., and T.T. discussed the results. R.P. and K.T. wrote the manuscript with contributions from the other authors.

### Corresponding authors

Correspondence to Kentaro Teramura or Tsunehiro Tanaka.

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### Competing interests

The authors declare no competing interests.

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Pang, R., Teramura, K., Morishita, M. et al. Enhanced CO evolution for photocatalytic conversion of CO2 by H2O over Ca modified Ga2O3. Commun Chem 3, 137 (2020). https://doi.org/10.1038/s42004-020-00381-2

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