1+1>2! When plasma meets photocatalysis

The Achilles' Heel of Photocatalysis

Common photocatalytic reactions include photocatalytic water splitting, photocatalytic carbon dioxide reduction, photocatalytic hydrogen peroxide synthesis, photocatalytic ammonia synthesis, and photocatalytic methane oxidation. The bond energies of N₂ (N≡N) are approximately 946 kJ/mol (approximately 9.8 eV), H₂O (H-O) 467 kJ/mol (approximately 4.84 eV), O₂ (O=O) 497 kJ/mol (approximately 5.15 eV), CO₂ (C=O) 806 kJ/mol (approximately 8.35 eV), and CH₄ (C-H) 415 kJ/mol (approximately 4.3 eV). The high bond energies of the reactant molecules limit the efficiency of photocatalytic conversion reactions [1-3].

Plasma: The "Energy Key" to Breaking the Bottleneck

Plasma, rich in high-energy reactive species, has been widely used in the field of catalytic reactions. For example, the electron temperature of dielectric barrier discharge plasma can reach 1~30 eV, the degree of ionization is 10¹²~10¹⁵ cm⁻³, and the gas temperature is 200~500 K[4]. At room temperature, N₂, H₂O, CO₂, and CH₄ can be activated and converted into active intermediates, which can then be converted by photogenerated electrons in the photocatalyst, potentially improving the efficiency of photocatalytic reactions.

Two synergistic modes, each with its own advantages

There are two modes of synergistic catalysis between plasma and other energy fields: integrated synergistic catalysis and segmented tandem catalysis.

• Integrated synergistic catalysis mode: The catalyst is placed directly inside the plasma discharge region. The plasma discharge and catalytic reaction occur simultaneously in the same space, and the reactants are simultaneously subjected to the dual effects of plasma and catalyst.

• Segmented tandem catalysis mode: The plasma discharge region and the catalytic reaction region are completely separated in space. The reactants are first activated by the plasma region to produce active species, and then enter the downstream independent catalytic bed for further reaction. The core advantage of the integrated design lies in the strong interaction between the plasma and the catalyst; they influence and promote each other, producing a synergistic effect of "1+1>2". In the segmented series design, there is almost no interaction between the plasma and the catalyst; energy is transferred solely through long-lived active species.

Schematic diagram of integrated synergistic catalysis and segmented tandem catalysis [5]

Comparison of key performance of integrated synergistic catalysis and segmented tandem catalysis

Performance dimension

Integrated synergistic catalysis

Segmented tandem catalysis

Synergistic effect strength

Extremely strong, with triple synergistic effect and the highest energy utilization rate

Weak, only through coupling of long-lived active species, resulting in large energy loss

Conversion rate

High, especially strong activation ability for inert molecules (CH₄, CO₂, N₂)

Medium

Selectivity

Wide adjustable range, but difficult to control and prone to generating byproducts

High, can be precisely controlled by optimizing catalyst and reaction temperature

Discharge stability

Poor, catalyst particles can cause uneven discharge and easily generate local arcs

Excellent, no foreign matter in the discharge area, uniform electric field distribution, can operate stably for a long time

Catalyst lifetime

Short, high-energy electron bombardment and ion sputtering can easily lead to catalyst sintering and deactivation

Long, the catalyst is not directly affected by plasma, and the lifetime can be extended by 3~5 High Energy Efficiency

High, low energy consumption per unit product

Low, a large amount of energy is used to generate useless, short-lived active species

perfectlight  Solutions

Top-Illuminated Plasma - Photocatalytic Plate Reactor

There are two main types of reactors using dielectric barrier discharge plasma: tubular reactors and plate reactors. Tubular reactors use a single metal electrode rod as the high-voltage electrode and a layer of metal mesh/sheet as the low-voltage electrode; the dielectric layer can be one or two layers of quartz/corundum tubing. Plate reactors use two metal sheets as the high-voltage and low-voltage electrodes; the dielectric layer can be one or two layers of quartz/corundum sheets. A schematic diagram is shown below.

The electric field of a tubular reactor radiates uniformly radially, with no obvious concentrated electric field region and no edge effects; multiple tubes can be connected in parallel, possessing the potential for engineering scale-up; it is easy to combine with a tubular furnace for plasma-thermal synergistic catalysis; however, it can only use side illumination, and factors such as severe light scattering, ineffective backlight area, and limited illumination area make it difficult to combine tubular reactors with photocatalysis.

To achieve synergistic effects between plasma and photocatalysis, PLR-MPPT-X, a plate reactor structure, features more uniform discharge, higher energy utilization, and easier catalyst loading. This maximizes the light intensity and illumination area of ​​the reaction zone, enabling multi-field synergistic catalytic reactions and accommodating catalyst synthesis/processing. A heating device is installed at the bottom of the electrodes, allowing for arbitrary combinations of light, heat, and plasma. The reactor has an effective illumination area of ​​50×50 mm, a maximum heating temperature of 300℃, and a gas discharge gap of 3 mm. It is compatible with both segmented pre-activation + catalysis and integrated synergistic catalysis reaction methods.

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Cutting-edge equipment for light and plasma synergistic catalysis research—the PLR-MPPT-X plasma-induced photocatalytic reaction activity testing platform. This is a breakthrough from single light field to multi-field synergy!

References

[1] Wang, Y.; Chen, E.; Tang, J. Insight on Reaction Pathways of Photocatalytic CO₂ Conversion. ACS Catal. 2022, 12 (12), 7300–7316. DOI: 10.1021/acscatal.2c01012

[2] Shi, R.; Zhao, Y.; Waterhouse, G. I. N.; Zhang, S.; Zhang, T. Defect Engineering in Photocatalytic Nitrogen Fixation. ACS Catal. 2019, 9 (11), 9739–9750. DOI: 10.1021/acscatal.9b03246

[3] Li, X.; Wang, C.; Tang, J. Methane transformation by photocatalysis. Nat. Rev. Mater. 2022, 7, 617–632. DOI: 10.1038/s41578-022-00422-3

[4] Puliyalil, H.; Lašič Jurković, D.; Dasireddy, V. D. B. C.; Likozar, B. A review of plasma-assisted catalytic conversion of gaseous carbon dioxide and methane into value-added platform chemicals and fuels. RSC Adv. 2018, 8, 27481–27508. DOI: 10.1039/C8RA03146K

[5] Snoeckx, R.; Bogaerts, A. Plasma technology – a novel solution for CO₂ conversion? Chem. Soc. Rev. 2017, 46, 5805–5863. DOI: 10.1039/C6CS00066E