Enhanced photocatalytic CO₂ conversion over LaPO₄ by introduction of CoCl₂ as a hole mediator

Abstract

The photocatalytic CO₂ reduction over LaPO₄ was significantly enhanced by simply introducing CoCl₂ into the aqueous medium. The introduced CoCl₂ functioned as a co-catalyst accelerating photogenerated hole transfer, by the reversible redox couple Co^III–Cl/Co^II–Cl, effectively suppressing the recombination of photogenerated charges.

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Photocatalytic CO₂ conversion into valuable hydrocarbon compounds over photoexcited semiconductors mimics the photosynthesis of green plants through the reduction of CO₂ with water using solar energy.^1,2 This process offers a route to reduce the greenhouse effect by recycling carbon and is a particularly attractive approach to solar energy harvesting. Enormous efforts have been devoted to the development of effective photocatalytic materials for CO₂ reduction.^3–9 Lanthanum phosphate (LaPO₄), a functional lanthanum-based material, has attracted increasing interest as a photocatalyst for CO₂ reduction in recent years, due to its suitable conduction band edge (−1.4 V vs. NHE),¹⁰ which is more negative than the reduction potential of CO₂, and both alkaline lanthanum cation and colloids with high positive zeta potential in favor of CO₂ adsorption.^11,12 For example, our recent work has demonstrated that hexagonal LaPO₄ nanorods showed superior photocatalytic performance for CO₂ reduction with H₂O to CH₄ and H₂ under mild condition, and the deposited Pt cocatalyst improved the photocatalytic activity and the selectivity for CH₄ formation.¹³ Despite these achievements, the efficiency of photocatalytic CO₂ reduction over LaPO₄ is still poor, and the noble metals are too expensive and scarce to practical application. Therefore, an effective CO₂ activation and conversion method, especially a noble metal free method, is in an urgent need.

Efficient charge separation is one of the most important factors that determine the photocatalytic activities.¹⁴ Till now, great efforts have been made to modify catalyst itself by various means to improve charge separation efficiency. Recent evidences suggest that the modification of reaction system, such as introducing suitable redox mediators in the heterogeneous photocatalysis reaction system may also retard the unwanted charge recombination process, and thus improve photocatalytic efficiency. Redox couples, such as Fe³⁺/Fe²⁺ and IO₃⁻/I⁻, which endowed with suitable redox potential, reversibility and charge transfer ability, were commonly employed as charge mediators to shuttle photogenerated charge carriers in the Z-scheme water splitting system and dye-sensitized solar cells.^15–18 Recently, some cobalt complexes have been reported as a good candidate for redox mediators accelerating photoinduced charge separation.^19–22 Co(II) polypyridine, phenanthroline, and imidazole complexes have been reported in dye sensitized TiO₂-based photovoltaic cells with IPCEs up to 50%.²³ Kudo et al. reported that [Co(bpy)₃]^3+/2+ and [Co(phen)₃]^3+/2+ redox couples showed the function of shuttling photogenerated charge from BiVO₄ to Ru/SrTiO₃:Rh in the overall water splitting system, with a maximum AQY of 2.1% at 420 nm. Therein, holes generated in the donor level of SrTiO₃:Rh could oxidize Co²⁺ to Co³⁺, and the latter subsequently regenerated back to the original state by oxidizing water to O₂.²⁴ Most of the cobalt complexes reported consist of expensive and complex organic ligands and its exact working mechanism has not been fully understood.²⁵ The simple and cheap inorganic cobalt salts such as CoCl₂ might be desirable to use as charge transfer mediators.

Herein, we report an effective hybrid photocatalytic system for CO₂ reduction by the combination of molecular cocatalyst (CoCl₂) with semiconductor photocatalyst (LaPO₄). A significant enhancement of photocatalytic activity was observed by introducing CoCl₂ into the LaPO₄ dispersion. The influence of other soluble cobalt salts on the activity was also studied, and the inherent working mechanism of CoCl₂ as hole transfer mediator was determined. Our work provides a simple and highly effective approach to suppress recombination of photogenerated charges, and create high-efficiency photosynthesis systems.

The photocatalytic property was carried out in the saturated CO₂ aqueous solution dispersing catalyst powders under mild conditions (20 °C and 1 atm CO₂) without any sacrificial agent. LaPO₄ shows superior CO₂ reduction activity and the main reduction products are CH₄ and H₂.¹³ Except for trace amount of CO, other products such as CH₃OH, HCHO and HCOOH are not detected during the reaction. The amounts of CH₄ and H₂ increase almost linearly with the irradiation time. 0.58 μmol CH₄ and 0.42 μmol H₂ are produced after 5 h reaction (Fig. S1 ESI†). With 50 μmol CoCl₂ introduced, the production rates of both CH₄ and H₂ are significantly enhanced from 0.11 and 0.08 μmol h⁻¹ to 0.39 and 0.12 μmol h⁻¹, respectively (Fig. 1a), and the apparent quantum yield (AQY) is enhanced by 5 times. On the other hand, loading Pt cocatalyst can further improve the photocatalytic activity by promoting photogenerated electrons transfer.¹³ As shown in Fig. 1a, the production rates of CH₄ and H₂ over 1.0 wt% Pt/LaPO₄ with the addition of CoCl₂ reach 1.62 and 0.93 μmol h⁻¹, respectively, up to 14.7 and 11.6 times with respect to those of bare LaPO₄. Furthermore, the enhancement of photocatalytic performance with CoCl₂ is applicable to other photocatalysts. Taking TiO₂ (Degussa P25) as an example, photocatalytic experiments confirm the enhanced CO₂ reduction activity with the introduction of CoCl₂ (Fig. S2 ESI†). No clear CH₄ or H₂ evolution is obtained when LaPO₄ catalyst is absent in the reaction system (Fig. 1a), indicating that CoCl₂ does not directly initiate the reaction.

Fig. 1 The generation rate of CH₄ and H₂ over the prepared LaPO₄ samples with or without introduction of CoCl₂ (a), and with introduction of various cobalt salts (b).

The effect of the adding amount of CoCl₂ on the photocatalytic activity for CO₂ reduction over LaPO₄ was also investigated. As shown in Fig. S3 (ESI†), both CH₄ and H₂ evolutions increase along with the increase of CoCl₂ addition amount, and an optimum activity is observed when the CoCl₂ adding amount is 50 μmol, corresponding to 1.13 μmol CH₄ and 0.55 μmol H₂ after 3 hours of reaction. However, further increasing the amount of CoCl₂, the CH₄ formation is gradually decreased, which may result from the coagulation of LaPO₄ colloid caused by more electrolyte,²⁶ as shown in Fig. S4 (ESI†).

The influence of other soluble cobalt salts, such as Co(NO₃)₂ and CoSO₄, on the photocatalytic CO₂ reduction activity was also studied to determine the role of anion. As shown in Fig. 1b, no significant changes in performance is observed when CoSO₄ replaces CoCl₂, while the activity is reduced with Co(NO₃)₂ instead, which can be attributed to the shielding of UV light activating LaPO₄ sample by NO₃⁻ (Fig. S5b ESI†). In addition, when NaCl is used instead of CoCl₂, there is hardly any impact on the reaction, excluding the role played by chloride ions alone in the reaction. The changes of the absorption intensity of the cobalt were compared before and after mixing the LaPO₄ sample and different cobalt salts solution for 5 hours in dark (Fig. S5 ESI†). The UV-vis absorption intensity of CoCl₂ at 500 nm reduces by 5% after the mixing (Fig. S5a ESI†), suggesting the adsorption of CoCl₂ on the surface of LaPO₄, which is in accordance with the electrostatic attraction between the negative complex ion [CoCl₄(H₂O)₂]²⁻ forming in CoCl₂ solution²⁷ and the positively charged LaPO₄ colloidal particles.¹³ No obvious change in the absorption intensity of both CoSO₄ and Co(NO₃)₂, Co²⁺ existing as a positive ion ([Co(H₂O)₆]²⁺),²⁷ is observed after mixing with LaPO₄ (Fig. S5b and c ESI†).

Fig. 2a shows the photoluminescence (PL) spectra of the LaPO₄ in the presence/absence of CoCl₂. Two distinct emission bands at 355 and 443 nm are observed due to the d–f transition of La³⁺ ions.²⁸ A significant decrease in the PL intensity is shown with CoCl₂ adding, indicating that the integration of CoCl₂ with LaPO₄ could effectively inhibit the recombination of photogenerated charge carriers and accelerate electron transfer/migration.²⁹ It can be further confirmed by transient photocurrent responses under illumination. As shown in Fig. 2b, about 1.3 folds enhancement of photocurrent is observed when CoCl₂ is introduced. These results strongly suggest that CoCl₂ plays the role on the enhancement of electrons and holes separation. This is in good agreement with the activity shown in Fig. 1a.

Fig. 2 (a) The photoluminescence (PL) spectra under 270 nm excitation, and (b) the transient photocurrent responses of LaPO₄ in the presence/absence of CoCl₂.

The reversible redox behaviors of CoCl₂ aqueous solution were analyzed by cyclic voltammetry (CV) as shown in Fig. 3. The well-defined reversible couple of waves centered at 0.86 V are observed for the electrolyte containing CoCl₂, which are assigned to the Co^III–Cl/Co^II–Cl couple.^24,30 In contrast, no redox waves show when CoCl₂ is not introduced into the electrolyte. The reversible current peaks during the reversed cathodic scanning indicates that the oxidized state possesses a relatively long lifetime and can be reduced back to the original state by accepting an electron during cathodic potential sweep.³¹ Given that the valence band maximum of LaPO₄ locates at 3.87 V,¹³ which is below the redox potentials of Co^II–Cl/Co^III–Cl (0.86 V), the holes in LaPO₄ interfaces thus are able to readily react with the adsorbed Co^II–Cl to yield Co^III–Cl, confirming the feasibility of CoCl₂ as a redox mediator in the reaction system.

Fig. 3 Current–potential curves of aqueous CoCl₂ solution. Working and counter electrodes: LaPO₄, Pt plate, respectively, reference: Ag/AgCl electrode, electrolyte: 0.2 mol L⁻¹ of an aqueous Na₂SO₄ solution containing CoCl₂ (10 mmol L⁻¹), scan rate: 100 mV s⁻¹.

Based on the results described above, a possible mechanism for the enhancement of photocatalytic CO₂ reduction activity by addition of CoCl₂ to the LaPO₄ colloidal dispersion was proposed (Scheme 1). Upon UV irradiation, photogenerated electrons on the conduct band of LaPO₄ reduced CO₂ and H₂O to CH₄ and H₂. On the other hand, the photogenerated holes on the valence band of LaPO₄ oxidized Co^II–Cl to Co^III–Cl. Due to high oxidation potential, Co³⁺ is rapidly reduced back to the original state accompanying the simultaneous oxidation of water into molecular oxygen.^18,24 The reversible redox behaviors of Co^III–Cl/Co^II–Cl transfer photogenerated holes, leading to the efficient charge separation and enhancement of photocatalytic activities.

Scheme 1 Proposed mechanism of the photocatalytic CO₂ reduction over LaPO₄ colloidal dispersion with CoCl₂.

In conclusion, we have reported for the first time that the photocatalytic reduction of CO₂ with H₂O to CH₄ can be significantly enhanced by introducing CoCl₂ into the LaPO₄ dispersion. CoCl₂ played the role of shuttling photogenerated holes by its reversible redox couple Co^III–Cl/Co^II–Cl, leading to the enhancement of electrons and holes separation. CoCl₂ was proposed to be a promoter for charge-carrier separation and interface interactions. The enhancement effect on CO₂ reduction by introducing CoCl₂ was universal for TiO₂ photocatalyst. Considering the low cost of CoCl₂, the present work paves a significant noble-metal-free way for CO₂ conversion reactions in artificial photosynthesis.

Acknowledgements

This work was financially supported by the NSFC (Grants No. U1305242, 21373050), the National Key Basic Research Program of China (973 Program: 2013CB632405, 2014CB239303, 2014CB260410, and 2014BAC13B03).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02958b

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