Eco-Friendly Photocatalytic Transformation of Greenhouse Gas CO₂ into Precious CH₄ Fuel via Cu-Deposited Black TiO₂ under Simulated Sunlight Irradiation

1. Introduction

With the ever-increasing combustion of hydrocarbon-containing fuels, the greenhouse gas concentration in the atmosphere is gradually elevating, which leads to substantial problems about the detrimental impacts on the climate and energy shortage crisis (Lewis, 2019; Liao et al., 2019; Dong et al., 2020; Li et al., 2021; Zhang et al., 2021; Tang et al., 2023; Majumdar et al., 2024). This crucial environmental issue prompts the urgent development of the effective techniques for the mitigation of CO₂ emission into the atmosphere. Typically, much efforts have been given to explore the thermocatalytic techniques for CO₂ transformation into precious chemicals (Bermejo-López et al., 2019; Chen et al., 2020; Xu et al., 2020). However, in the conventional thermocatalytic CO₂ transformation method, hydrogen gas is employed as reducing material to produce hydrogen atoms for the formation of reactive electrons, which causes explosive risks and high costs (Bermejo-López et al., 2019; Chen et al., 2020; Xu et al., 2020). Fortunately, artificially-mimicked photosynthesis using the solar light-activated photocatalytic process has been recently suggested as an optimistic approach to address the energy shortage as well as atmospheric CO₂ level mitigation by transforming CO₂ into precious fuels such as CH₄ and other hydrocarbons (Chen et al., 2019; Choi et al., 2019; Li et al., 2019; Pan et al., 2019; Xie et al., 2019; Zhang et al., 2021; Cheng et al., 2024). In addition, CO₂ is potentially be converted into valuable chemicals that are utilized in various industrial processes (Pan et al., 2019; Xie et al., 2019; Zhang et al., 2021). Semiconductor–based photocatalysts have been most widely utilized for CO₂ transformation to precious materials due to their prominent photodurabilty and appropriate electron structures for CO₂ reduction reaction (Jiang et al., 2018; Wu et al., 2019; Guo et al., 2020; Xiong et al., 2020; Zeng et al., 2021; Zhao et al., 2021). In spite of tremendous efforts, the photocatalytic CO₂ transformation technique is yet challenging due to its thermodynamically and kinetically unfavorable reaction pathways (Li et al., 2021; Zhang et al., 2021). Thus, high-performance photocatalytic techniques with thermodynamically and kinetically favorable CO₂ transformation pathway are necessary to overcome the aforementioned problems.

To accomplish the thermodynamically-approving CO₂ transformation pathway into a precious fuel CH₄, the conduction band of a photocatalyst must be negative than the CO₂/ CH₄ reduction potential of −0.24 eV (Fang and Wang, 2018; Zhou et al., 2018; Wang et al., 2021). Among the popularly employed photocatalysts, TiO₂ has received tremendous interests in its photocatalytic application to CO₂ transformation because of its prominent photostability and higher conduction band value (−0.61 eV) than the CO₂/CH₄ reduction potential (Wang et al., 2019a; Wang et al., 2019b; Xiong et al., 2020; Zhang et al., 2021). Nonetheless, pristine TiO₂ shows an unacceptable photocatalytic capability and low quantum yields under visible-light irradiation due to its fast carrier recombination rate and wide bandgap, respectively, significantly inhibiting its actual utilization to the photocatalytic CO₂ transformation under visible-light irradiation (Shi et al., 2019; Zhang et al., 2021). This limitation may be overcome by modifying TiO₂ into black TiO₂, which can increase optical efficiencies of TiO₂ due to the disordered structure, transformed electron properties, and a range of defect groups (Ullattil et al., 2018; Rajaraman et al., 2020).

To achieve kinetically-satisfactory CO₂ transformation pathway into CH₄, the carrier separation efficiency of the photocatalyst should be considered (Fang and Wang, 2018; Zhou et al., 2018; Wang et al., 2021). The surface alteration of photocatalysts by depositing a pertinent metal cocatalyst can promote its separation efficiency of carriers due to the high carrier transportability at interfaces of photocatalyst‒cocatalyst (Shen et al., 2019; Yi et al., 2019; Ishii et al., 2020). The non-precious metal copper (Cu) can be utilized as a potential cocatalyst because of its prominent charge acceptance nature, which promotes the carrier separation rates (Chen et al., 2018; Maldonado et al., 2018; Liu et al., 2019; Ge et al., 2020). Markedly, compared to conventional aggregated metal cocatalysts, metal cocatalysts show greater probability for providing more reactive spots and cost moderation, prompting their utilization in the advancement of potential metal-deposited photocatalysts (Wei et al., 2019; Gao et al., 2020; Li et al., 2020).

In this study, the photocatalytic CO₂ transformation capability of Cu-deposited black TiO₂ (Cu/BTiO₂) was evaluated to examine whether this photocatalyst follows the thermodynamically-and kinetically-satisfactory CO₂ transformation into CH₄. For comparison, selected reference samples (i.e., pristine TiO₂, black TiO₂ (BTiO₂), and conventional aggregated Cu-deposited BTiO₂ (Cu/BTiO₂)) were evaluated for the photocatalytic CO₂ transformation into CH₄. Further, probable mechanistic insight into the photocatalytic CO₂ transformation into CH₄ via Cu/BTiO₂ was investigated, and the photostability of this photocatalyst was assessed through a recycling test.

2. Methods

3. Results and discussion

3.1. Material characteristics

The crystalline characteristics of pristine TiO₂, BTiO₂, Cu/BTiO₂, and Cu/BTiO₂-1 are presented in their XRD spectra (Fig. 1). All TiO₂-containing materials presented revealed exclusive signs indexed to the anatase and rutile TiO₂ crystals at ~25.3^o (101) and ~27.3^o (110), respectively (Naldoni et al., 2019; Rajaraman et al., 2020). Noticeably, the XRD patterns of the TiO₂-containing materials showed similar intensities at their matched 2θ number, implying that their crystal properties are not substantially altered during their fabrication. Nonetheless, the Cu-related signs did not appear in the XRD spectra of the Cu/BTiO₂-1.6, probably owing to the unmeasurable weight of Cu impregnated onto the BTiO₂.

Fig. 1.

X-ray diffraction patterns of pristine TiO2, BTiO2, Cu/BTiO2, and Cu/BTiO2-1.6.

The hetero-structural and surface properties of as-constructed Cu/BTiO₂-1.6 inspected via TEM and HRTEM are presented in Figs. 2 and 3. Especially, Fig. 2 represents the TEM result and size distribution of Cu/BTiO₂-1.6. The TEM image of the material illustrates nanoparticles with diameter range of 10‒35, which is comparable to that of P25 TiO₂ diameter (10‒30 nm), which are consistent with the results of previous studies Wang et al., 2018 ; Naldoni et al., 2019; She et al., 2020). Besides, the HRTEM mapping figure presents three images of Ti, O, and Cu in Cu/BTiO₂-1.6 and their corresponding EDS peaks (Fig. 3), demonstrating the successful incorporation of Cu and BTiO₂.

Fig. 2.

Transmission electron microscopy image and size distribution of Cu/BTiO2-1.6.

Fig. 3.

Elemental mapping results and energy-dispersive spectroscopy image of Cu/BTiO2-1.6.

The optical characteristics of pristine TiO₂, BTiO₂, Cu/BTiO₂, and Cu/BTiO₂-1.6 inspected via UV‒vis absorption spectroscopy are shown in Fig. 4. The three modified materials (BTiO₂, Cu/BTiO₂, and Cu/BTiO₂-1.6) presented a red-shift in light harvest relative to pristine TiO₂, which results from the elevated visible-light absorption of the former materials. Besides, the visible-light absorption rates of Cu or Cu-incorporated materials (Cu/BTiO₂ and Cu/BTiO₂-1.6) were greater than that of BTiO₂, ascribing to the eminent charge acceptance nature of Cu and Cu, which promotes the carrier separation rates (Liu et al., 2019; Gao et al., 2020; Ge et al., 2020; Li et al., 2020). On the basis of the converted Tauc plots of the UV-vis absorption spectra, the bad edge of pristine TiO₂, BTiO₂, Cu/BTiO₂, and Cu/BTiO₂-1.6 were 3.19, 3.05, 2.87, and 2.91 eV, respectively.

Fig. 4.

UV-visible absorption spectra of pristine TiO2, BTiO2, Cu/BTiO2, and Cu/BTiO2-1.6.

3.2. Photocatalytic CO₂ transformation into CH₄

The photocatalytic CO₂ transformation rates over as-fabricated catalysts were investigated under sunlight exposure. Prior to conducting the main photocatalytic CO₂ transformation experiments, three control experiments were carried out to demonstrate the scientific hypothesis that precious gas CH₄ is solely generated by the photocatalytic transformation of CO₂. Table 1 exhibits the control experiment results for photocatalytic CO₂ transformation into CH₄. First, CH₄ was unobserved in the presence of light but without the addition of a photocatalyst. Second, CH₄ was unobserved in the presence of a photocatalyst but without light. Lastly, CH₄ was yet unobserved in the presence of light and a photocatalyst under CO₂-free conditions. As such, the control experiments demonstrate the abovementioned scientific hypothesis.

Table 1.

Control experiment results for photocatalytic CO2 transformation into CH4*

The photocatalytic CO₂ transformation rates determined using pristine TiO₂, BTiO₂, Cu/BTiO₂, and Cu/BTiO₂-1.6 are shown in Fig. 5. The pristine TiO₂ and BTiO₂ catalysts revealed no significant photocatalytic CO₂ transformation abilities. Contrarily, Cu/BTiO₂ and Cu/BTiO₂-1.6 displayed noticeable photocatalytic CO₂ transformation abilities: The photocatalytic CO₂ transformation rates of Cu/BTiO₂ were 61, 139, 217, 270, and 309 μmol/g at the irradiation times of 2, 4, 6, 8, and 10 h, respectively; the photocatalytic CO₂ transformation rates of Cu/BTiO₂-1.6 were 104, 209, 272, 322, and 361 μmol/g at the irradiation times of 2, 4, 6, 8, and 10 h, respectively. The greatest photocatalytic CO₂ transformation ability of Cu/BTiO₂-1.6 is assigned to the excellent electron‒hole separation tendency. This statement is demonstrated by Fig. 6, which shows the smaller PL emission signal for Cu/BTiO₂-1.6 relative to those of three reference catalysts (pristine TiO₂, BTiO₂, and Cu/BTiO₂) since a small PL signal indicates a great electron‒hole separation tendency (Xie et al., 2020; Xiong et al., 2020).

Fig. 5.

Time-series CH4 transformation rates obtained from pristine TiO2, BTiO2, Cu/BTiO2, and Cu/BTiO2-1.6.

Fig. 6.

Photoluminescence emission spectra obtained from pristine TiO2, BTiO2, Cu/BTiO2, and Cu/BTiO2-1.6.

To ensure that the composition ratio of Cu incorporated into BTiO₂ in the Cu/BTiO₂ architectures is crucial in CH₄ generation, the CO₂ transformation activities of Cu/BTiO₂ architectures with different Cu loadings. As shown in Fig. 7, the CO₂ transformation activities of Cu/BTiO₂ architectures are ordered as follows: Cu/BTiO₂-1.6 > Cu/BTiO₂-1.8 > Cu/BTiO₂-2.0 > Cu/BTiO₂-1.4 > Cu/BTiO₂-1.2 > Cu/BTiO₂-1.0. This pattern of photocatalysts in CO₂ transformation activities are ascribed to their electron‒hole separation tendencies, which is demonstrated by the PL emission signals of the surveyed catalysts (Fig. 8). As such, these results imply the existence of an optimum Cu composition incorporated into BTiO₂ in the Cu/BTiO₂ architectures.

Fig. 7.

Time-series CH4 transformation rates obtained from Cu/BTiO2-1, Cu/BTiO2-1.2, Cu/BTiO2-1.4, Cu/BTiO2-1.6, Cu/BTiO2-1.8, and Cu/BTiO2-2.

Fig. 8.

Photoluminescence emission spectra obtained from Cu/BTiO2-1, Cu/BTiO2-1.2, Cu/BTiO2-1.4, Cu/BTiO2-1.6, Cu/BTiO2-1.8, and Cu/BTiO2-2.

To verify the durability and stability of the Cu/BTiO₂ architectures, the time program of CH₄ generation employing the Cu/BTiO₂-1.6 architecture was attained under the same experiment conditions. As shown in Fig. 9, the CH₄ generation ratio is constant under prolonged light irradiation up to 50 h. Since this CO₂ transformation activity was evaluated in a closed reaction device, a just small variation in CO₂ transformation activity was observable. To survey the photostability of the architecture, five experiment cycles were carried out. regarding each experiment cycle, the sample powder was washed and positioned in the experimental system. Fig. 9 represents that the content of CH₄ increases with the irradiation time, and no recognizable reduction in CH₄ generation after the final cycle is observable. Consequently, the as-prepared architecture kept eminent photostability during the prolonged experiment.

Fig. 9.

Recycling test of CH4 transformation obtained from Cu/BTiO2-1.6.

4. Conclusions

In view of this paper, the photocatalytic CO₂ transformation capability of Cu/BTiO₂ was evaluated to investigate if this photocatalyst proceeds the thermodynamically-and kinetically-satisfactory CO₂ transformation into CH₄. The Cu/BTiO₂ and Cu/BTiO₂ architectures revealed noticeable photocatalytic CO₂ transformation abilities, whereas the pristine TiO₂ and BTiO₂ catalysts displayed no significant photocatalytic CO₂ transformation abilities. Moreover, the Cu/BTiO₂ architecture exhibited greater photocatalytic CO₂ transformation ability compared with that of the Cu/BTiO₂ architecture, which was ascribed to the promoted electron‒hole separation tendency of the former. Another important finding is that the composition ratio of Cu incorporated into BTiO₂ in the Cu/BTiO₂ architectures is crucial in CH₄ generation. The Cu/BTiO₂ architecture also revealed eminent photodurability, which was demonstrated by the consecutive experiment cycle. Consequently, the promoted photocatalytic CO₂ transformation capability of Cu/BTiO₂ verifies its utilization as an efficient tool in photocatalytic CO₂ transformation.

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