Promoting Photocatalytic Carbon Dioxide Reduction by Tuning the Properties of Cocatalysts

Abstract

Suppressing the amount of carbon dioxide in the atmosphere is an essential measure toward addressing global warming. Specifically, the photocatalytic CO₂ reduction reaction (CRR) is an effective strategy because it affords the conversion of CO₂ into useful carbon feedstocks by using sunlight and water. However, the practical application of photocatalyst-promoting CRR (CRR photocatalysts) requires significant improvement of their conversion efficiency. Accordingly, extensive research is being conducted toward improving semiconductor photocatalysts, as well as cocatalysts that are loaded as active sites on the photocatalysts. In this review, we summarize recent research and development trends in the improvement of cocatalysts, which have a significant impact on the catalytic activity and selectivity of photocatalytic CRR. We expect that the advanced knowledge provided on the improvement of cocatalysts for CRR in this review will serve as a general guideline to accelerate the development of highly efficient CRR photocatalysts.

Keywords: CO2 reduction reaction; alloying; cocatalysts; metal nanoparticles; morphology; photocatalysts.

Figure 1

Schematic illustrations showing the transformation from CO₂ in the atmosphere to useful carbon feedstocks.

Figure 2

A) Schematic illustration of photocatalytic CRR: Step 1: light absorption, Step 2: charge separation, and Step 3: surface reactions. B) Principle of CRR using semiconductor photocatalysts.

Figure 3

The sunlight spectrum. Visible light (∼400≤λ≤∼800 nm) accounts for about 43 % of sunlight.

Figure 4

Schematic illustrations of photocatalytic reactions: A) one‐step photoexcitation system for CRR and B) Z‐scheme photoexcitation system for CRR. Red. and Ox. denote reductants and oxidants, respectively.

Figure 5

Band structures of A) UV‐ and B) visible‐light‐driven photocatalysts for CRR. The band position of the photocatalyst is calculated to be at pH 7.[ ¹² , ¹⁵ ]

Figure 6

Schematic illustrations of photocatalyst functionalization by controlling the A) particle size (Section 4.1), B) chemical composition (Section 4.2), C) morphology (Section 4.3), and D) surface structure (Section 4.4) of the cocatalyst.

Figure 7

A) Transmission electron microscope (TEM) images of PtNPs/TiO₂ prepared by using different Pt deposition times: 20 (a), 30 (b), 45 (c), and 60 s (d). B) CO and CH₄ yields obtained during CRR on commercially available TiO₂ powder (P25), pristine TiO₂ columnar film (TiO₂ film), and PtNPs/TiO₂ films prepared by using different Pt deposition times (10, 20, 30, 45, and 60 s). C) Schematic diagram of CO₂‐photoreduction mechanism with PtNPs/TiO₂ nanostructured films. The magnified circle (center) shows that the photogenerated e⁻ can move rapidly within the highly oriented TiO₂ single crystals and flow to the Pt deposits, where a redox reaction occurs to convert CO₂ into CO and CH₄. The energy levels of the PtNPs/TiO₂‐CO₂ system is also shown. Reproduced with permission from ref. [38a]. Copyright: 2012, American Chemical Society.

Figure 8

A) Schematic illustration of the acid‐base‐mediated alcohol reduction method for controlling the size of PtNPs (a) and HRTEM images of PtNPs of sizes 1.8 (b), 3.4 (c), 4.3 (d), or 7.0 nm (e) loaded on HTSO. B) CH₄ yields obtained during CRR on PtNPs/HTSO with different sized PtNPs under different atmospheres. C) Correlations between the selectivity for CH₄ and surface site proportion as a function of the size of PtNPs. D) Free energy diagrams for the reduction of CO₂ to CH₄ using the thermochemical model on Pt(111) surface and Pt55. Reproduced with permission from ref. [38b]. Copyright: 2018. Springer Nature Limited.

Figure 9

A) TEM images of Pd₁Cu₁ NPs/TiO₂ (a) and Pd₇Cu₁ NPs/TiO₂ (b) and HAADF‐STEM images of Pd₁Cu₁ NPs (c) and Pd₇Cu₁ NPs (d). The insets in (c) and (d) show atomic‐resolution images taken from the regions indicated by the boxes. B) Average evolution rates of CH₄ and CO during CRR on different photocatalysts in the presence of H₂O. C) Most favorable configurations and adsorption energies of CO₂ at an isolated Cu atom (a) and two neighboring Cu atoms (b). Cu: orange, Pd: blue, C: gray, and O: red. Reproduced with permission from ref. [39a]. Copyright: 2017, American Chemical Society.

Figure 10

A) Schematic illustrating enhanced electron‐trapping ability and enhanced number of the isolated Cu sites in ordered PdCu cocatalysts prepared at various Pd/Cu molar ratios and at various annealing temperatures; inset: atomic model of the ordered PdCu intermetallic structure. B) Average evolution rates of H₂, CO, and CH₄ and CH₄ selectivity during CRR on g‐C₃N₄‐based photocatalysts under visible light irradiation. Reproduced with permission from ref. [39b]. Copyright: 2018, The Royal Society of Chemistry.

Figure 11

A) CO and CH₄ yields during CRR over Pt _x Cu _y NPs/TiO₂. B) CO₂ in‐situ FTIR spectra of Pt_0.4Cu_0.6/TiO₂. C) CO in‐situ FTIR spectra of Pt_0.4Cu_0.6 NPs/TiO₂. D) Free energy diagrams of CO₂ reduction on PtCuNPs. An asterisk denotes the adsorbed intermediate on the substrate. Reproduced with permission from Ref. [39c]. Copyright: 2022, The Royal Society of Chemistry.

Figure 12

A) TEM and B) HRTEM images of cube‐PdNPs (a) and tetra‐PdNPs (b). C) Optimized configurations of CO₂ adsorbed onto Pd(100) (a) or Pd(111) (b) facets; the adsorption energy E_a is also noted. Pd: green and blue, C: gray, and O: red. Reproduced with permission from ref. [40a]. Copyright: 2014, The Royal Society of Chemistry.

Figure 13

A) Evolution yields of CH₄ and CH₃OH during CRR on different photocatalysts – CN: g‐C₃N₄ only, CN‐1: photocatalyst subjected to the same treatment but without PdNPs, cube: cube‐PdNPs/g‐C₃N₄, and tetra: tetra‐PdNPs/g‐C₃N₄. B) In‐situ FTIR spectra of tetra‐PdNPs/g‐C₃N₄ subjected to different CRR conditions. C) Optimized geometrical structures of CO₂ adsorption on Pd(100) facets or Pd(111) facets at the bridge position (a), Top1 position (b), and Top2 position (c), and optimized geometrical structures oof CH₃OH adsorption on Pd(100) facets or Pd(111) facets (d). Pd: blue and orange, C: green, O: red, and H: white. Reproduced with permission from ref. [40b]. Copyright: 2017, Elsevier B.V.

Figure 14

A) TEM (top) and HRTEM (bottom) images of cube‐PtCu (left) and concave cube‐PtCu (right) NPs. B) Photocatalytic H₂, CO, and CH₄ evolution rates during CRR on photocatalysts g‐C₃N₄, cube‐PtCuNPs/g‐C₃N₄, or concave cube‐PtCuNPs/g‐C₃N₄. C) Most stable configurations of CO₂ adsorbed on PtCu(100) or PtCu(730) facets and associated adsorption energies. Pd: blue, Cu: pink, C: gray, and O: red. Reproduced with permission from ref. [39d]. Copyright: 2017, The Royal Society of Chemistry.

Figure 15

A) HRTEM images of Cu₂O/PtNPs/TiO₂‐1 h (a), Cu₂O/PtNPs/TiO₂‐2 h (b), Cu₂O/PtNPs/TiO₂‐5 h (c), and Cu₂O/PtNPs/TiO₂‐10 h (d). B) HS‐LEIS spectra of Cu/TiO₂ (a), Pt/TiO₂ (b), Pt−Cu/TiO₂ (c), and Cu₂O/PtNPs/TiO₂‐5 h (d) photocatalysts recorded at 5 keV ²⁰Ne⁺. C) Dependence of the photocatalytic behavior on the Cu content in the Cu₂O/PtNPs/TiO₂‐5 h catalysts for CRR in the presence of H₂O. Reproduced with permission from ref. [41a] Copyright: 2013, Wiley‐VCH.

Figure 16

A) Evolution rates of H₂ (▴), O₂ (▪), CO (•), and CO₂ (⧫) of the reverse reaction for the photocatalytic conversion of CO₂ in H₂O over AgNPs/Ga₂O₃ (a) and Cr(OH)₃ ⋅ xH₂O/AgNPs/Ga₂O₃ (b). Photocatalyst powder: 0.5 g; reaction solution: 1.0 L H₂O; flowing rates of gases: 20 mL min⁻¹ CO/Ar mixture gas (5.0 %), 0.64 mL min⁻¹ O₂, and 9.4 mL min⁻¹ Ar; solution: 1.0 L H₂O; Ag loading amount: 1.0 mol%; Cr loading amount: 1.0 mol%; light source: 400 W high‐pressure Hg lamp. B) TEM (a) and HRTEM (b) images of Cr(OH)₃ ⋅ xH₂O/AgNPs/Ga₂O₃. Reproduced with permission from ref. [41b]. Copyright: 2018, The Royal Society of Chemistry.

Figure 17

A) Cr K‐edge X‐ray absorption near‐edge structure spectra of Cr(OH)₃ ⋅ xH₂O (black), Cr(OH) _x (CO₃) _y (red), as‐prepared 1.0 mol% Cr(OH)₃ ⋅ xH₂O/AgNPs/Ga₂O₃ (blue), and Cr(OH)₃ ⋅ xH₂O/AgNPs/Ga₂O₃ after photoirradiation for 5 h (pink). B) Dependence of the evolution rates of CO (•) and H₂ (▴) on the thickness of the Cr(OH)₃ ⋅ xH₂O shell with various loading amounts of Ag and Cr. Dotted lines represent the data‐fitted curves. C) Schematic illustration of the mechanism of the photocatalytic conversion of CO₂ into CO on Cr/Ag/Ga₂O₃. Reproduced with permission from ref. [41c]. Copyright: 2019, American Chemical Society.