Surface modification of layered perovskite Sr₂TiO₄ for improved CO₂ photoreduction with H₂O to CH₄

Abstract

Layered perovskite Sr₂TiO₄ photocatalyst was synthesized by using sol-gel method with citric acid. In order to increase the surface area of layered perovskite Sr₂TiO₄, and thus to improve its photocatalytic activity for CO₂ reduction, its surface was modified via hydrogen treatment or exfoliation. The physical and chemical properties of the prepared catalysts were characterized by X-ray diffraction, high-resolution transmission electron microscopy, elemental mapping analysis, energy-dispersive X-ray spectroscopy, N₂ adsorption-desorption, UV-Vis spectroscopy, X-ray photoelectron spectroscopy, photoluminescence, and electrophoretic light scattering. CO₂ photoreduction was performed in a closed reactor under 6 W/cm² UV irradiation. The gaseous products were analyzed using a gas chromatograph equipped with flame ionization and thermal conductivity detectors. The exfoliated Sr₂TiO₄ catalyst (E-Sr₂TiO₄) exhibited a narrow band gap, a large surface area, and high dispersion. Owing to these advantageous properties, E-Sr₂TiO₄ photocatalyst showed an excellent catalytic performance for CO₂ photoreduction reaction. The rate of CH₄ production from the photoreduction of CO₂ with H₂O using E-Sr₂TiO₄ was about 3431.77 μmol/g_cat after 8 h.

Introduction

As is well known, the temperature of Earth is increasing owing to greenhouse gas emissions, and CO₂ makes up the largest portion of these greenhouse gases. The United Nations Framework Convention on Climate Change (UNFCCC) has concluded that, to maintain a stable environmental, global warming since industrialization must not exceed 2 °C. However, according to their investigation, the average ground temperature since industrialization has already increased by 0.75 °C. When we consider the heat absorbed by the sea, this temperature increases by a further 0.6 °C. Consequently, we have a temperature increase of only 0.65 °C until the temperature change set by the UNFCCC is already reached. Because of the global warming, various natural disasters have been occurred¹. In order to avoid this problem, we must reduce CO₂ emissions. However, it is impossible to stop CO₂ emissions completely because of the current industrial systems are depending on the fossil fuels.

Carbon capture and storage (CCS) technology has been developed in order to reduce and/or control the CO₂ emissions^2,3. However, this CCS technology has several problems such as high cost, additional energy requirements, stability, and storage limitations. Therefore, carbon capture and utilization (CCU) technology is expected to replace it^4,5,6, and is advantageous in economic and environmental view because it can prevent CO₂ emissions, and convert them into useful materials. There are various strategies for CO₂ utilization. One is the non-conversional use of CO₂, and the others involve its conversion using chemical, biochemical, photochemical, and electrochemical methods. Of these, the photochemical method is a more promising technology in environmental terms. CO₂ photoreduction using a photocatalyst can generate useful compounds such as CH₄, HCOOH, HCHO, and CH₃OH⁷. Much research has been devoted to this field since the first report by Inoue et al.⁸ on the photoreduction of CO₂, and many different semiconductors have been developed and used as photocatalysts, such as TiO₂ ⁹, WO₃ ¹⁰, ZnO¹¹, GaP¹², CdS¹³, and SiC¹⁴. Of these, TiO₂ is one of the most well-known and widely used materials. In order to improve the optical properties of TiO₂, it has been combined with various metals to form hybridized composites such as Ag/TiO₂ ¹⁵, Pt/TiO₂ ¹⁶, Ru/TiO₂ ¹⁷, Pd/TiO₂ ¹⁸, Ni/TiO₂ ¹⁹, Cu/TiO₂ ²⁰, TiO₂/Cu-TiO₂ ²¹, CeO₂-TiO₂ ²², MgO-TiO₂ ²³, NiO-In₂O₃/TiO₂ ²⁴, CuS_x-TiO₂ ²⁵, NiS-TiO₂ ²⁶, In₂O₃/TiO₂ ²⁷, TiO₂/Fe-TiO₂ ²⁸ and multi-walled carbon nanotube (MWCNT)@TiO₂ ²⁹ in an attempt to reduce the band gap or suppress recombination of photogenerated charge carriers. Also, catalysts having a combination of an organic and metal material such as g-C₃N₄ ³⁰, Ni₁₂P₅/g-C₃N₄ ³¹, Au cluster-NP/C₃N₄ ³², AgX/g-C₃N₄ (X = Cl and Br)³³, RuRu′/Ag/NS-C₃N₄ ^34,35,36,37 and Co-ZIF^38,39,40,41 are being developed.

Recently, the development of TiO₂ photocatalysts with the perovskite structure ABO₃, has attracted due to the unique perovskite structure, their composition can be easily changed at the A, and B sites and the metal introduced can be quantitatively substituted into the skeleton. Among the perovskite semiconductors, SrTiO₃ is widely used as a photocatalyst. Much like TiO₂, SrTiO₃ has been combined with other species to form hybrid composites such as Mn/SrTiO₃ ⁴², Cu/SrTiO₃ ⁴³, N-doped TiO₂-SrTiO₃ ⁴⁴, Fe₂O₃/SrTiO₃ ⁴⁵, SrTiO₃:Cr/Ta/F⁴⁶, SrTiO₃/HZSM-5⁴⁷, SrTiO₃/TiO₂/H-titanate nanofiber⁴⁸, SrTiO₃:Rh/Sb⁴⁹, La/Cr-doped SrTiO₃ ⁵⁰, Pt/SrTiO₃ ⁵¹, Zn/SrTiO₃ ⁵², Ag₃PO₄/Cr-SrTiO₃ ⁵³, and g-C₃N₄-SrTiO₃:Rh⁵⁴ to improve its photocatalytic performance. Studies on other perovskite catalysts, including Ca_xTi_yO₃ ⁵⁵, and basalt fiber@PbTiO₃ ⁵⁶, also have been recently reported.

Another advantage of perovskite is that it forms a layered perovskite depending on the nature and contents of the A and B ions. Figure 1a shows the structure of a Ruddlesden-Popper A_j+1B_jO_3j+1 perovskite. Here, when the j value increases, the structure tends towards an ABO₃ perovskite. In particular, A₂BO₄, for which j = 1, shows a layered structure with a large gap between each BO₆ octahedron.

Figure 1

(a) Ruddlesden-Popper phase of layered oxide and (b) the overview of catalyst surface modification.

In this study, we have attempted to improve the photocatalytic performance of Sr₂TiO₄ layered perovskite by increasing its surface area (Fig. 1b). The surface of the synthesized catalyst was hydrogen treated or exfoliated to increase its interaction with the CO₂ feed material and thus amplify its CO₂ photoreduction activity. The characteristic properties of the synthesized catalysts were measured by using a variety of techniques such as using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), N₂ adsorption-desorption isotherm analysis, UV-Vis spectroscopy, photoluminescence (PL), zeta potential analysis, and X-ray photoelectron spectroscopy (XPS). Furthermore, their photocatalytic activity for the reduction of CO₂ with H₂O under UV light was studied.

Results

X-ray diffraction (XRD) patterns

The XRD patterns for Sr₂TiO₄, H-Sr₂TiO₄, and E-Sr₂TiO₄ are given in Fig. 2. The peaks for Sr₂TiO₄ are observed at 2θ = 24.076, 28.386, 31.464, 32.656, 35.673, 42.899, 43.217, 43.852, 46.791, 55.049, 55.605, 56.319, 57.431, 65.452, and 68.390°, which are assigned to the (011), (004), (013), (110), (112), (015), (006), (114), (020), (116), (024), (017), (123), (026), and (220) planes of Sr₂TiO₄, respectively, identifying it as a layered perovskite-type tetragonal structure (JCPDS 00-039-1471)⁵⁷. The crystallite size of Sr₂TiO₄ calculated using Scherer’s equation based on the (013) plane is 12.856 Å⁵⁸. After hydrogen treatment, an anatase TiO₂ peak is observed at 2θ = 25.429°. However, the other peaks for the Sr₂TiO₄ structure are maintained, and the crystallite sizes based on the Sr₂TiO₄ (013) plane and the TiO₂ anatase (011) plane are 9.371 and 3.024 Å, respectively. Conversely, the Sr₂TiO₄ sample exfoliated with HNO₃ and TPAOH presents a different XRD pattern. The peaks for E-Sr₂TiO₄ are found at 27.141, 36.070, 41.232, 54.414, 63.069, and 69.105°, which are assigned to the (110), (011), (111), (121), (130), and (031) planes of rutile TiO₂, respectively⁵⁹. These structural changes are due to the fact that the Sr ions located between the Sr₂TiO₄ layers are removed by HNO₃ treatment, and only the TiO₆ octahedra (corresponding to BO₆) remain. The crystal structure analysis reveals that the Ti forms a rutile structure in the Sr₂TiO₄ and that the Sr is intercalated between the layers. The crystallite size of the catalysts was calculated using scherrer’s equation. In the case of Sr₂TiO₄ and H-Sr₂TiO₄, (013) plane was selected, and (110) plane was selected for E-Sr₂TiO₄. As a result, the crystallite size of Sr₂TiO₄, H-Sr₂TiO₄, and E-Sr₂TiO₄ were found to be 11.568, 11.578, 1.165 Å, respectively.

Figure 2

XRD patterns of Sr₂TiO₄, H-Sr₂TiO₄ and exfoliation Sr₂TiO₄ catalysts.

High-resolution transmission electron microscopy (HR-TEM), element mapping and EDX analysis

The differences in the morphologies of Sr₂TiO₄ and E-Sr₂TiO₄ were investigated using HR-TEM and selected area electron diffraction (SAED) (Fig. 3). The larger particles are observed in Sr₂TiO₄, and whereas E-Sr₂TiO₄ consists of the particles form a separate sheets or randomly folded sheets. The images show that the interplanar distance for E-Sr₂TiO₄ is larger than that for Sr₂TiO₄. The d-spacings for the Sr₂TiO₄ (013) plane and the E-Sr₂TiO₄ (110) plane are 2.84 and 3.25 Å, respectively. These results are in accordance with the values derived from XRD patterns.

Figure 3

High resolution TEM and SAED images of catalysts: (a), (c) Sr₂TiO₄ and (b), (d) exfoliation Sr₂TiO₄.

The compositions of Sr₂TiO₄ and E-Sr₂TiO₄ were analyzed by HR-TEM element mapping analysis and EDX, and the results are shown in Fig. 4 and Table 1. In Sr₂TiO₄, Sr and Ti ions are uniformly distributed throughout the particles and Sr ions are more abundant than Ti ions. The atomic percent values for Sr and Ti are 16 and 11%, respectively. However, the Sr ion content of E-Sr₂TiO₄ is much lower than that of Sr₂TiO₄. Furthermore, EDX analysis showed that the Sr ion content, which is about 1.4-times that of Ti in Sr₂TiO₄, is reduced to just 0.07% that of Ti in E-Sr₂TiO₄. Thus, these results support the assertion that the layers are separated because Sr ions are removed from the interlayers by exfoliation.

Figure 4

HR-TEM elemental mapping analysis of catalysts: (a) Sr₂TiO₄ and (b) exfoliation Sr₂TiO₄.

Table 1 Atomic ratio by HR-TEM EDX analysis.

Brunaure-Emmett-Teller (BET) surface area analysis

Figure 5 shows the N₂ adsorption-desorption isotherms at 77 K for P-25, which was used as a comparative sample, Sr₂TiO₄, H-Sr₂TiO₄, and E-Sr₂TiO₄. According to the IUPAC classification, the adsorption-desorption isotherm curves of all the catalysts belong to type III. Therefore, the synthesized catalysts are non-porous materials. However, the slight hysteresis in the curves is due to the bulk pores between the particles. The specific surface areas of Sr₂TiO₄ and H-Sr₂TiO₄ are 1.19 and 0.78 m²/g, respectively, which are very low. However, E-Sr₂TiO₄ has a specific surface area of 358.54 m²/g, which is much larger than those of Sr₂TiO₄ and H-Sr₂TiO₄. The increase in the specific surface are of E-Sr₂TiO₄ is due not only to the removal of Sr ions from between the layers, but also to the separation of the layers, as shown in the HR-TEM images. The increase in catalyst surface area leads to an increase in the number of active sites for CO₂ and H₂O to react, leading to an increase in reactivity. Therefore, E-Sr₂TiO₄ was expected to exhibit improved catalytic activity compared to those of Sr₂TiO₄ and H-Sr₂TiO₄. This specific surface area is considerably larger than that of the commercial catalyst P-25, which is 43.65 m²/g.

Figure 5

N₂ adsorption and desorption isotherms of catalysts; (a) P-25, (b) Sr₂TiO₄, (c) H-Sr₂TiO₄ and (d) E-Sr₂TiO₄.

Optical properties of photocatalysts

Figures 6 and 7 show the UV-Vis spectra and Tauc’s plots of P-25, Sr₂TiO₄, H-Sr₂TiO₄, and E-Sr₂TiO₄. The UV absorptions of Sr₂TiO₄ and H-Sr₂TiO₄ are blue-shifted compared to that of P-25 because of the influence of SrO, with its band gap of 5.71 eV⁶⁰. Conversely, the absorbance of E-Sr₂TiO₄ is shifted to a longer wavelength due to the removal of Sr ions. Most interestingly, it moved to longer wavelength than that of P-25. This is because E-Sr₂TiO₄ has a rutile TiO₂ structure (band gap: 3.0 eV), as confirmed by the XRD analysis above. Therefore, the absorbance of E-Sr₂TiO₄ is shifted to longer wavelength than that of P-25, which composed mainly of anatase TiO₂ (band gap: 3.2 eV). The band gap was calculated using the Tauc equation⁶¹:

$${\rm{\alpha }}{\rm{h}}{\rm{\nu }}={\rm{A}}{({\rm{h}}{\rm{\nu }}-{{\rm{E}}}_{{\rm{bg}}})}^{1/2}$$

(1)

where α, h, ν, A, and E _bg represent the absorption coefficient, Plank’s constant, light frequency, a constant, and band gap energy, respectively. In a plot of (αhν)² versus photon energy (hν), the intercept on the x axis gives the band gap. Using this method, the band gap of P-25, Sr₂TiO₄, H-Sr₂TiO₄ and E-Sr₂TiO₄ were calculated to be 3.16, 3.33, 3.34, and 3.03 eV, respectively. Therefore, E-Sr₂TiO₄ has the narrowest band gap, making it most suitable as a photocatalyst.

Figure 6

UV-visible spectra of catalysts.

Figure 7

Tauc plot of catalysts; (a) P-25, (b) Sr₂TiO₄, (c) H-Sr₂TiO₄ and (d) E-Sr₂TiO₄.

Photocatalysts with narrower band gaps have advantages in terms of photosensitization. However, in order to exhibit good performance in the current system, the band gap of a photocatalyst should also include the CO₂/CH₄ and H₂O/O₂ reduction potentials. Figure 8 shows the XPS valance band spectra of the catalysts. Based on the data obtained, the valance band value of the catalysts was confirmed, and the values for P-25, Sr₂TiO₄, H-Sr₂TiO₄, and E-Sr₂TiO₄ are 2.46, 2.34, 1.60, and 2.06 eV, respectively. When the vacuum level of 4.5 eV is corrected to 0 V for a standard hydrogen electrode (SHE) and the work function of the XPS instrument is taken as 4.62 eV⁶², the valance band maximum values for P-25, Sr₂TiO₄, H-Sr₂TiO₄ and E-Sr₂TiO₄ are 2.58, 2.46, 1.72 and 2.18 eV (vs. SHE), respectively. According to these valance band and band gap values, the conduction band minimum values for P-25, Sr₂TiO₄, H-Sr₂TiO₄, and E-Sr₂TiO₄ are -0.58, 0.87, -1.61, and -0.84 eV (vs. SHE), respectively. Figure 9 shows the energy diagrams obtained for the catalysts using the valance and conduction band values and the band gap. All catalysts contain the CO₂/CH₄ and H₂O/O₂ reduction potential. Therefore, the synthesized catalysts are suitable for the photoreduction of CO₂ with H₂O to CH₄.

Figure 8

XPS valance band spectra of catalysts.

Figure 9

Energy diagram of catalysts.

In order to understand the recombination of excited electrons and holes, PL analysis was conducted, and the results are shown in Fig. 10. The PL spectra of the catalysts show a strong emission signal at 468.1 nm. The PL intensity of Sr₂TiO₄ is smaller than that of P-25. This is due to a decrease in the number of excited electrons because of the wide band gap of Sr₂TiO₄. The intensity for H-Sr₂TiO₄ is lower than that of Sr₂TiO₄. This is because the oxidation state of the exposed Ti on the surface is reduced to (4-δ)⁺, which is not +4, and the reduced Ti suppresses the recombination of electrons and holes by trapping the excited electrons in the conduction band. The E-Sr₂TiO₄ also exhibits a PL intensity lower than that of Sr₂TiO₄ and much lower than that of P-25. Generally, excited electrons and holes move from the bulk of a particle to its surface where they react with reactants. The recombination of excited electrons and holes takes place in the bulk or on the surface of a particle during transport. When the particles are exfoliated, the internal area of the particles decreases and the distance to the surface for the electrons and holes decreases. Therefore, recombination inside the particles is also reduced. This is the reason that E-Sr₂TiO₄ has a lower PL intensity than that of P-25. Thus, the above analysis indicates that H-Sr₂TiO₄ and E-Sr₂TiO₄ will be better photocatalysts than Sr₂TiO₄.

Figure 10

PL emission spectra of catalysts.

X-ray photoelectron spectroscopy (XPS) analysis

The XPS spectra of the photocatalysts were obtained to confirm the oxidation state of the elements according to their chemical bonding, and the results are shown in Fig. 11. For Sr₂TiO₄, the peaks located at 133.38 and 134.88 eV are assigned to Sr-3d_5/2 and Sr-3d_3/2 core levels respectively. In H-Sr₂TiO₄, the Sr 3d peaks are shifted toward a slightly lower binding energy. In E-Sr₂TiO₄, the intensity of the 3d peaks is greatly reduced. This is due to the removal of Sr ions from the interlayer spaces, as described above. The Ti 2p_3/2 and 2p_1/2 peaks of Sr₂TiO₄ are observed at 457.88 and 463.78 eV, respectively. The Ti 2p peaks for H-Sr₂TiO₄ are shifted to a lower binding energy, similarly to the Sr 3d peaks. This is because some of the Sr²⁺ and Ti⁴⁺ ions are reduced by hydrogen to Sr^(2−δ)+ and Ti^(4−δ)+, respectively. The Ti 2p peaks for E-Sr₂TiO₄ have significantly different peak intensities to Sr₂TiO₄ or H-Sr₂TiO₄, and its Ti 2p_3/2 and Ti 2p_1/2 peaks are observed at 458.18 and 464.08 eV, respectively. This binding energy is shifted slightly lower compared to that of P-25. Therefore, the Ti ions of E-Sr₂TiO₄ are slightly more reduced ions than in P-25. It is believed that this can induce vacancies in the crystal framework and facilitate the movement of electrons and holes, which can be advantageous for photocatalytic activity.

Figure 11

XPS spectra of catalysts; (a) Sr 3d spectra, (b) Ti 2p spectra, and (c) O 1s spectra.

There are two O 1 s peaks in Sr₂TiO₄ and H-Sr₂TiO₄. The peak at ~529 eV is from oxygen bound to Ti, and the peak at ~531 eV corresponds to oxygen bound to Sr. For E-Sr₂TiO₄, only the peak corresponding to oxygen bonded to Ti is observed (at 529.48 eV), because the Sr is removed by exfoliation.

Zeta potential analysis of photocatalysts

Since the catalytic reaction takes place in H₂O, it is important to study the dispersion of the catalyst particles in H₂O. The zeta potentials were measured after dispersing the catalysts in distilled water or in bubbling-CO₂ solution (i.e., the reaction conditions), and the results are shown in Table 2. Generally, a larger absolute value for the measured zeta potential means that the particles are well dispersed in a solution. The zeta potentials for Sr₂TiO₄, H-Sr₂TiO₄, E-Sr₂TiO₄, and P-25 are -11.99, -11.15, -42.39, and -15.61 mV, respectively. Therefore, the degree of colloidal dispersion in H₂O follows the order E-Sr₂TiO₄ > P-25 > Sr₂TiO₄ > H-Sr₂TiO₄. Thus, all catalysts can be adequately dispersed in H₂O. However, the zeta potential shows a different pattern after CO₂ bubbling: all the negative potential values are changed to positive values. The zeta potentials for Sr₂TiO₄, H-Sr₂TiO₄, E-Sr₂TiO₄, and P-25 are 1.16, 16.29, 26.22, and 22.00 mV, respectively. Therefore, the degree of colloidal dispersion in the solution after CO₂ bubbling follows the order E-Sr₂TiO₄ > P-25 > H-Sr₂TiO₄ > Sr₂TiO₄. Thus, E-Sr₂TiO₄, P-25, and H-Sr₂TiO₄ show good dispersion under the reaction conditions, which is considered advantageous for CO₂ photoreduction performance. However, in the case of Sr₂TiO₄, the zeta potential is low, so it is likely to exhibit poor catalytic performance owing to it being more agglomerated than the other catalysts.

Table 2 Summary of physical and chemical properties of catalysts.

Photocatalytic reduction of CO₂ with H₂O, property after reaction, and mechanism

The products obtained though CO₂ reduction using the catalysts synthesized in this study are CH₄, H₂, C₂H₆, C₂H₄, and CO. Figure 12 shows the accumulation of the products according to irradiation time. The main product is CH₄ and the product amounts follow the order CH₄ > C₂H₆ > H₂ > C₂H₄ > CO. Overall, the reactivity of the surface-treated H-Sr₂TiO₄ and E-Sr₂TiO₄ is better than that of Sr₂TiO₄. After 8 h reaction, the rates of CH₄ production over Sr₂TiO₄, H-Sr₂TiO₄, and E-Sr₂TiO₄ are 844.94, 1353.46, and 3431.77 μmol/g_cat, respectively. In particular, E-Sr₂TiO₄ shows excellent reactivity and produces more CH₄ than P-25 catalyst, and. Figure 12b shows the amounts of H₂ produced. H₂ is a necessary substance for reducing CO₂. Therefore, the greater the amount of H₂ generated, the easier the CO₂ reduction. The amounts of H₂ generated over Sr₂TiO₄, H-Sr₂TiO₄, and E-Sr₂TiO₄ are 267.73, 373.65, and 640.71 μmol/g_cat, respectively. Therefore, the CO₂ reduction reaction is promoted over E-Sr₂TiO₄ is higher than the other catalysts. Furthermore, the C₂H₆ production over E-Sr₂TiO₄ is higher than that over the other catalysts. When E-Sr₂TiO₄ is used, the amount of C₂H₆ produced after 8 h is 150.8 μmol/g_cat. If the C₂H₆ reacts with H₂ on the catalysts surface it can be further converted to CH₄. The amounts of C₂H₄ and CO produced over the catalysts are similar. After 8 h, the production of C₂H₄ and CO is 457.63–573.33 and 69.35–81.30 μmol/g_cat, respectively. Figure 13 shows the product distribution on the catalysts. P-25 showed the highest CH₄ selectivity, which was 80 to 90%. Next, when E-Sr₂TiO₄ was used, the CH₄ selectivity was high and its value was about 70%. Table 3 shows the quantum yield of the catalysts and the overall quantum yield was in the order of E-Sr₂TiO₄, P-25, H-Sr₂TiO₄, and Sr₂TiO₄, which were 2.83, 1.96, 2.93, and 6.20%, respectively. The quantum yield for CO₂ photoreduction to produce CH₄ of P-25, Sr₂TiO₄, H-Sr₂TiO₄ and E-Sr₂TiO₄ were 2.69, 1.28, 2.56, 5.21%, respectively. The quantum yield for other products was less than 1% for all catalysts.

Figure 12

Photocatalytic CO₂ reduction with H₂O on catalysts. (a) yield of CH₄, (b) yield of H₂ (c) yield of C₂H₆ (d) yield of C₂H₄, and (b) yield of CO.

Figure 13

Product distribution on catalysts. (a) P-25, (b) Sr₂TiO₄, (c) H-Sr₂TiO₄ and (d) E-Sr₂TiO_4.

Table 3 The apparent quantum yield of catalysts.

The XRD analysis of the catalysts after the reaction was carried out to confirm the structural stability and displayed in Fig. 14. From the XRD analysis results, it was observed that the catalysts structure was remained stable before and after the reaction. Therefore, the catalysts structure was stable during the reaction conditions.

Figure 14

XRD patterns of the Sr₂TiO₄, H-Sr₂TiO₄, and E-Sr₂TiO₄ after the reaction.

The photoreaction of E-Sr₂TiO₄ showing the best activity was repeated three times. The results for CH₄ production, the main product, are shown in Fig. 15. A slight amount of difference was observed in the results but similar performance was maintained without deactivation in all three times. Therefore, it was confirmed that the E-Sr₂TiO₄ was excellent in not only structural stability but also reusability during the reaction

Figure 15

Reusability test of E-Sr₂TiO₄ to convert from CO₂ to CH₄ using photoreduction.

Based on these results, a plausible reaction pathway over E-Sr₂TiO₄, which has the best performance is proposed in Fig. 16. The excited electrons on the exfoliated improved catalyst surface react with CO₂ to produce ∙CO₂ ⁻ radicals, and the holes react with H₂O to produce OH⁻ and H⁺. Hydrogen radicals (∙H) are formed by the reaction of H⁺ with excited electrons, and then CO is produced by the reaction of ∙H and ∙CO₂ ⁻ radicals. CH₄, C₂H₆, and C₂H₄ are produced finally as the CO and ∙H radicals continuously react.

Figure 16

The proposed mechanism for the photo-reduction of CO₂ with H₂O on E-Sr₂TiO₄.

Conclusion

In this study, nanosized layered perovskite Sr₂TiO₄ photocatalyst was successfully synthesized by using sol-gel technique with the assistance of citric acid. The surface of layered perovskite Sr₂TiO₄ photocatalyst was treated to improve the CO₂ photoreduction activity. The particles were treated with HNO₃ to remove the Sr ions present between the layers, and the layers were exfoliated by treatment with TPAOH. The catalyst, E-Sr₂TiO₄ showed the rutile TiO₂ structure after exfoliation because the Sr₂TiO₄ structure was collapsed. The shape of the exfoliated thin film was confirmed by TEM. In comparison to Sr₂TiO₄ and H-Sr₂TiO₄ photocatalysts, the exfoliated catalyst E-Sr₂TiO₄ showed an excellent performance in CO₂ photoreduction to CH₄, and after 8 h, 3431.77 μmol/g_cat of CH₄ was generated. The reason for the excellent performance of E-Sr₂TiO₄ can be explained by the following factors.

First, it has a narrow band gap compared to the other two catalysts, and exhibits reduced electron-hole recombination. Therefore, a relatively larger number of electrons and holes transferred to CO₂ and H₂O. Next, a large amount of CO₂ and H₂O can interact with the active sites on the surface because it has a large surface area and is well dispersed in the solution. Based on the excellent physical and photochemical properties of the exfoliated layered perovskite catalyst, it may be employed for different photocatalytic applications as well as the CO₂ photoreduction reactions.

Methods

Synthesis of photocatlysts

The synthesis of layered perovskite Sr₂TiO₄ was performed as follows: Strontium nitrate (Sr(NO₃)₂, 97.0%, Junsei Chemical, Japan) and titanium isopropoxide (Ti(OCH(CH₃)₂)₄, TTIP, 98.0%, Junsei Chemical, Japan) were used as precursors. First, 0.1 mol of Sr(NO₃)₂ was dissolved in double distilled water (100 mL) with continuous stirring. Then, 10 mL of HNO₃ (60%, OCI company Ltd., Republic of Korea) was added with stirring to prevent hydrolysis. This solution was labeled A. In a separate vessel, 0.05 mol of TTIP was dissolved in EtOH (99.9%, OCI company Ltd., Republic of Korea), and then glacial acetic acid (CH₃COOH, 99.0%, Ducsan, Republic of Korea) was added with stirring to prevent hydrolysis. This solution was labeled as B. The solutions A and B were then mixed with stirring, and citric acid monohydrate (C₆H₈O₇·H₂O, 99.5%, Daejung Chemicals and Metals Co Ltd., Republic of Korea), which is a complexing agent for the gel, was added, and the mixture was stirred until it became homogeneous. Then, the solvent was removed without the temperature exceeding 323.15 K to obtain a sol gel, which was subsequently pretreated at 493.15 K. Finally, a white powder was obtained by thermal treatment at 1323.15 K for 6 h. In some cases, hydrogen treatment was then performed at 1123.15 K for 3 h in H₂ atmosphere. In other cases, exfoliation was achieved by a three-step process, as shown in Fig. 17. Ion exchange of Sr for H cations was carried out in 1 M HNO₃ for 5 days using ultrasonication. The powder obtained was treated in tetrapropylammonium hydroxide (TPAOH, 25.0%, in water, ACROS, Belgium) for 3 weeks using ultrasonication. The final white precipitate was washed several times with distilled water and ethanol and dried at 343 K for 24 h. The hydrogen-treated and exfoliated Sr₂TiO₄ samples were labeled H-Sr₂TiO₄ and E-Sr₂TiO₄, respectively.

Figure 17

Preparation of Sr₂TiO₄, Exfoliation Sr₂TiO₄ and H-Sr₂TiO₄ catalysts.

Characterization of photocatalysts

The structures and crystallinities of the as-prepared Sr₂TiO₄, H-Sr₂TiO₄, and E-Sr₂TiO₄ samples were confirmed with XRD (model MPD from PANalytical) using nickel-filtered CuKα radiation (40.0 kV, 30.0 mA). The morphologies were investigated using HR-TEM (Tecnal G2 F20 S-TWIN, FEI, Netherlands) operated at 200 kV. The presence of different elements was confirmed using the elemental mapping and energy dispersive X-ray spectroscopy (EDS) attached to the TEM setup. The specific surface areas (S_BET) were calculated according to the Brunauer-Emmett-Teller theory using a Belsorp II mini (BEL, Japan Inc.). The UV-Vis absorption spectra were obtained using a SCINCO Neosys-2000 spectrometer fitted with a reflectance sphere. PL profiles were obtained using a SCINCO FluoroMate FS-2 at room temperature using a He-Cd laser source at a wavelength of 325 nm. XPS measurements were performed on a K-alpha (Thermo Scientific, UK) using Al Kα X-rays as the excitation source. The zeta potential of the material was determined by electrophoretic mobility using an electrophoresis measurement apparatus (ELS 8000, Otsuka Electronics, Japan) with a plate sample cell. Electrophoretic light scattering (ELS) determination was performed in reference beam mode with a 670 nm laser light source at a modular frequency of 250 Hz and a scattering angle of 15°. The standard error of the zeta potential, converted from the experimentally determined electrophoretic mobility, was typically <1.5% with 5% error. To measure the zeta potentials, the samples were dispersed in deionized water or bubbling-CO₂ water at 0.1 wt%. The final zeta potentials were obtained by averaging 2 or 3 measurements.

Photocatalytic activity measurements

The photocatalytic tests for the reduction of CO₂ with H₂O were performed in a photoreactor comprising a quartz chamber with a total volume of 150.0 cm³ (Fig. 18). To photoreduce CO₂, 0.01 g of the catalyst was placed in the reactor chamber with 50 mL of double distilled water, and the reactor was closed. A UV lamp (6 W/cm², 20 cm length × 2.0 cm diameter, Shinan, Republic of Korea) emitting light at 365 nm was used to irradiate the reaction mixture. Supercritical-fluid-grade CO₂ with a certified maximum hydrocarbon content of <1 ppm was used as the reactant. Before the reaction was initiated by illumination, the reactor was purged with CO₂ gas for 5 min. The lamp was then switched on to start the experiment. The reaction temperature and pressure were maintained at 303 K and 1 atm, respectively. The gas products were analyzed using a gas chromatograph (iGC7200, DS Science, Republic of Korea) equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). The product yield³³ and quantum yield⁶³ during reaction was calculated using following equation (2–3).

$${\rm{Product}}\,{\rm{yield}}={\rm{Total}}\,{\rm{of}}\,{\rm{product}}\,({\rm{\mu }}\mathrm{mol})/\mathrm{Amount}\,{\rm{of}}\,{\rm{photocatalyst}}\,{\rm{used}}({{\rm{g}}}_{{\rm{cat}}})$$

(2)

$${\rm{Quantum}}\,{\rm{yield}}( \% )={\rm{Number}}\,{\rm{of}}\,{\rm{reacted}}\,\mathrm{electrons}/\mathrm{Number}\,{\rm{of}}\,{\rm{incident}}\,{\rm{photons}}\times 100 \% $$

(3)

Figure 18

The Schematic diagram of experimental set up of a circulated photo reactor for CO₂ reduction.