A Solar to Chemical Strategy: Green Hydrogen as a Means, Not an End

Abstract

Green hydrogen is the key to the chemical industry achieving net zero emissions. The chemical industry is responsible for almost 2% of all CO₂ emissions, with half of it coming from the production of simple commodity chemicals, such as NH₃, H₂O₂, methanol, and aniline. Despite electrolysis driven by renewable power sources emerging as the most promising way to supply all the green hydrogen required in the production chain of these chemicals, in this review, it is worth noting that the photocatalytic route may be underestimated and can hold a bright future for this topic. In fact, the production of H₂ by photocatalysis still faces important challenges in terms of activity, engineering, and economic feasibility. However, photocatalytic systems can be tailored to directly convert sunlight and water (or other renewable proton sources) directly into chemicals, enabling a solar-to-chemical strategy. Here, a series of recent examples are presented, demonstrating that photocatalysis can be successfully employed to produce the most important commodity chemicals, especially on NH₃, H₂O₂, and chemicals produced by reduction reactions. The replacement of fossil-derived H₂ in the synthesis of these chemicals can be disruptive, essentially safeguarding the transition of the chemical industry to a low-carbon economy.

Keywords: chemical feedstock; green hydrogen; photocatalysis; solar energy; sustainability.

Figure 1

Scheme illustrating the water splitting reaction using a semiconductor‐based photocatalyst. Reproduced with permission.^{[ 9 ]} Copyright 2023, Elsevier.

Figure 2

Comparative scheme representing the ammonia production from the traditional Haber‐Bosch process and the photocatalytic approach where water is the electron donor.

Figure 3

Schematic representation of photocatalytic N₂ reduction on a photoactivated semiconductor and the respective redox potentials associated with the reduction of N₂ from 1 to 6 electrons. For comparison purposes, the redox potentials of the water are also shown. Reproduced with permission.^{[ 42 ]} Copyright 2021, Royal Society of Chemistry.

Figure 4

The industrial anthraquinone process.

Figure 5

Challenges to be addressed for developments in sustainable H₂O₂ synthesis. i) Enhanced selectivity toward 2e⁻ ORR to H₂O₂; ii) Suppression of H₂O₂ decomposition; iii) Improved mass transfer of reactants and products; iv) Alternative solutions to sacrificial electron donors.

Figure 6

a) Illustration of Au/TiO₂ and Au/BiVO₄ photocatalysts with relevant band energies and reduction potentials. b) High‐resolution transmission electron microscopy (HR‐TEM) of Au/BiVO₄. c,d) Koutecky‐Levich analysis of two ORR photocatalysts. c) Linear‐sweep voltammograms measured on a rotating disk electrode at different rotating speeds. d) Koutecky‐Levich plots at the constant potential of −0.3 V versus Ag/AgCl. Reproduced with permission.^{[ 93 ]} Copyright 2016, American Chemical Society.

Figure 7

Schematic illustration of i) a biphasic and ii) a triphasic reactor for catalytic H₂O₂ synthesis. Reproduced with permission.^{[ 98 ]} Copyright 2021, John Wiley and Sons.

Figure 8

a) Scheme of the proposed mechanism of selective 2e⁻ ORR on C₃N₄. Reproduced with permission.^{[ 81 ]} Copyright 2014, American Chemical Society; b) Scheme of the proposed mechanism of selective 2e⁻ ORR on TiO₂ using aromatic alcohols as electron donors. Reproduced with permission.^{[ 102 ]} Copyright 2020, Elsevier; c) Illustration of the edge‐on and end‐on absorption of O₂ on catalysts and suppressed 4e⁻ reduction on single atoms. Reproduced with permission.^{[ 103 ]} Copyright 2021, Springer Nature.

Figure 9

a) Schemes of proposed mechanisms for 2e⁻ WOR. Reproduced with permission.^{[ 88 ]} Copyright 2019, American Chemical Society; b) Scheme of proposed mechanism for HCO₃ ⁻‐assisted H₂O₂ electrochemical synthesis. Reproduced with permission.^{[ 104 ]} Copyright 2022, Springer Nature.

Figure 10

Comparative illustration between the industrial hydrogenation process, which relies on molecular hydrogen derived from methane reforming, and the photocatalytic process utilizing hydrogen transfer from water as an alternative source.

Figure 11

Operando ¹H‐NMR of the photocatalytic water transfer hydrogenation of styrene induced by the photocatalyst Pd₁‐mpg‐C₃N₄. Reproduced with permission.^{[ 124 ]} Copyright 2022, John Wiley and Sons.

Figure 12

Proposed reaction pathway in the selective photocatalytic transfer hydrogenation of nitroarenes into anilines by an H₂O/glucose system catalyzed by Pd/TiO₂. Reproduced with permission.^{[ 126 ]} Copyright 2016, Royal Society of Chemistry.

Figure 13

Diagram representing the photoreduction of CO₂ using water as electron donor.

Figure 14

a) Schemes illustrating the three possibilities of CO₂ activation on the catalyst surface. Reproduced with permission.^{[ 146 ]} Copyright 2020, Royal Society of Chemistry; b) Scheme representing the different pathways proposed for CO₂ reduction; c) Free energy of CO₂ reduction using dual‐site Cu‐In to manipulate the CO and CH₄ production induced by controlled sulfur structural vacancies. Reproduced with permission.^{[ 147 ]} Copyright 2019, Springer Nature.

Figure 15

a) Differential charge density diagrams and intermediates during CO₂ reduction to CO over Ni‐SA/O model. Reproduced with permission.^{[ 159 ]} Copyright 2020, John Wiley and Sons; b) Free energy diagrams for CO₂ reduction to CO on MnMo₆ and for H₂O oxidation to O₂ on TTF of TCOF‐MnMo₆. Reproduced with permission.^{[ 167 ]} Copyright 2022, American Chemical Society; c) Gibbs free energy diagrams for CO reduction to C₂H₄ over AgInP₂S₆ with sulfur vacancies. Reproduced with permission.^{[ 174 ]} Copyright 2021, Springer Nature.