Recent progress of photothermal catalysts for carbon dioxide conversion
0 Abstract
Photothermal catalysis has emerged as a promising strategy for converting carbon dioxide (CO2) into value-added chemicals and fuels, offering a dual-energy approach that combines light and thermal energy to drive reactions under mild conditions. Photothermal effects are usually demonstrated by using plasmonic nanoparticles, which generate hot carriers and localized heating through light absorption. These effects facilitate chemical reactions by lowering activation barriers and increasing reaction rates. The synergy between hot carrier-induced redox reactions and thermocatalytic processes driven by localized heating allows for the activation of challenging reactions with reduced energy inputs. The balance between these pathways can be optimized through rational design of photothermal catalysts. In this review, we highlight recent advancements in catalyst materials, especially emphasizing the importance of photothermal effects to achieve higher efficiencies in CO2 conversion reactions such as CO2 hydrogenation and dry reforming of methane, both of which are vital for reducing greenhouse gases and producing clean fuels. Finally, the current challenges, outlook, and new strategies for catalyst optimization will be discussed to realize the full potential of photothermal catalysis in creating a sustainable and low-carbon energy future.
Keywords
INTRODUCTION
Carbon dioxide (CO2) conversion into value-added chemicals and fuels is an essential strategy to transition towards a sustainable energy future[1-3]. The increasing levels of atmospheric CO2 concentration require innovative approaches for carbon capture and utilization. Among the various techniques developed for CO2 conversion, photothermal catalysis has emerged as a particularly promising method[4-8]. By harnessing both light and thermal energy, photothermal catalysts can drive chemical reactions under mild conditions, offering a synergistic effect that reduces operating temperature and enhances efficiency. This dual-energy approach enables reactions that are often more challenging or less efficient when using conventional thermal or photocatalytic methods independently.
One of the most studied reactions in photothermal CO2 conversion is CO2 hydrogenation, a process that reduces CO2 to produce valuable hydrocarbons and alcohols, such as carbon monoxide (CO), methane (CH4), and methanol[9-17]. Photothermal CO2 hydrogenation benefits from the combined effects of light absorption and localized heating, which facilitates the activation of CO2 molecules and their subsequent hydrogenation reaction with gaseous hydrogen (H2). This reaction not only holds promise for producing clean fuels but also offers a pathway for integrating renewable solar energy into chemical processes. Recent advancements in catalyst design have significantly improved the efficiency and selectivity of photothermal CO2 hydrogenation.
Another important photothermal CO2 conversion catalysis is the dry reforming of methane (DRM). This reaction simultaneously converts two major greenhouse gases (CO2 and CH4) into syngas (a mixture of H2 and CO), which serve as a precursor for various industrial chemicals and fuels[18-24]. In conventional thermocatalytic DRM, the reaction typically requires high temperatures of 700-900 °C, which are necessary to overcome the thermodynamic barriers. However, these harsh conditions pose several challenges, including the high energy input and the carbon coking on the catalyst surface, which leads to rapid catalyst deactivation. Photothermal catalysis offers a promising solution to these challenges by enabling DRM to proceed at significantly lower temperatures while maintaining high conversion efficiencies. Recent studies have shown that photothermal catalysts can mitigate carbon coking due to the photo-excited charge carriers and enhanced reaction kinetics.
This review will provide an in-depth examination of recent progress in photothermal catalysis, with a focus on two key CO2 conversion reactions: CO2 hydrogenation and DRM. We will introduce the underlying mechanisms that drive the enhanced catalytic activity observed in photothermal processes. This review also explores the latest advancements in catalyst materials, including the design of nanostructured and hybrid systems that have demonstrated superior performance. Finally, we will address the current challenges and perspectives, such as the need for a deeper understanding of the photothermal effects and the development of durable catalysts. Through this comprehensive review, we aim to provide future directions and opportunities for research in photothermal catalysis, emphasizing its potential to contribute to a low-carbon and sustainable energy future.
FUNDAMENTALS OF PHOTOTHERMAL CATALYSIS
Photothermal effects
Photothermal catalysis is an emerging field that combines light and thermal energy to drive chemical reactions with improved efficiency and selectivity. A key aspect of this approach is the photothermal effects, which involve converting absorbed photons into heat while simultaneously generating high-energy charge carriers, often called hot carriers. These photothermal effects arise from several interconnected processes, including plasmonic light absorption, hot carrier generation, photon-to-phonon conversion, local heating, and heat transfer/dissipation [Figure 1]. Understanding these mechanisms is significant for designing effective photothermal catalysts for CO2 conversion reactions.
Figure 1. Key processes in the photothermal effect. (A) Light absorption at specific resonant wavelengths occurs through plasmonic excitation within metal nanoparticles, generating high-energy charge carriers. (B) Hot electrons (e-) can directly participate in CO2 reduction reactions when the metal nanoparticle forms a junction with a p-type semiconductor, while in n-type semiconductors, hot holes (h+) can facilitate oxidation reactions. (C) Photon-to-phonon conversion within the crystal lattice of metal nanoparticles leads to localized heating. (D) The accumulated heat is subsequently transferred or dissipated to the surrounding environment or support material, contributing to a temperature gradient.
Plasmonic light absorption
Plasmonic light absorption is the first and the most important step for photothermal catalysis, primarily occurring in metal nanoparticles (NPs) with free electrons [Figure 1A]. These NPs possess unique optical properties due to the collective oscillation of free electrons when exposed to light at specific wavelengths[25]. This resonance results in the strong absorption of light and the concentration of electromagnetic fields at the surface. The plasmonic resonance wavelength can be finely tuned by modifying the size, shape, and composition of the NPs, allowing for precise control over the light absorption spectrum due to factors such as surface scattering and radiation damping[26-28]. For instance, when the mean free path of conduction electrons becomes comparable to the particle size, there is increased electron-surface collisions and a blue shift in the surface plasmon resonance peak[29]. In contrast, when NPs get larger, they exhibit redshifts due to enhanced radiative losses and dynamic depolarization effects. The shape of NPs also plays a crucial role in determining plasmonic properties. For example, nanorods exhibit two surface plasmon resonance modes: a transverse mode and a longitudinal mode that shifts to longer wavelengths as the aspect ratio increases. Upon light absorption, plasmonic NPs transfer energy to free electrons, generating a non-equilibrium distribution of energetic electrons and holes, known as hot carriers.
Hot carrier generation and redox reactions
The absorption of light by plasmonic metal NPs leads to the generation of hot carriers (electrons and holes) which have energies significantly higher than the thermal equilibrium state. This process begins with the excitation of electrons to higher energy states within the conduction band, leaving behind corresponding holes in the valence band [Figure 1B]. Hot electrons generated through plasmonic excitation have energies that exceed the Fermi level of the metal, enabling them to overcome activation barriers for specific reactions[30,31]. For instance, in CO2 reduction reactions, hot electrons can reduce CO2 molecules adsorbed on the surface, while the corresponding hot holes can oxidize other reactants, completing the catalytic cycle. The direct involvement of hot carriers in redox reactions resembles conventional photocatalysis, where photo-excited electrons and holes in the semiconductors participate in redox reactions[32,33]. The lifetime and energy distribution of hot carriers are influenced by various factors, such as the size and shape of the plasmonic NPs and the properties of the surrounding medium. Hybrid materials that combine plasmonic metals with semiconductors are particularly effective because they facilitate charge separation, prolonging the lifetime of hot carriers and enhancing the ability to participate in catalytic reactions. In these materials, hot electrons can be injected into the conduction band of the semiconductor, while hot holes remain in the metal, enabling spatially separated redox processes[34,35].
Photon-to-phonon conversion and local heating
Not all the energy absorbed by plasmonic NPs is converted into hot carriers; most of the absorbed photon energy is dissipated as heat through photon-to-phonon conversion[36]. When hot carriers relax back to their equilibrium state, they release energy in the form of phonons, which are quantized vibrations of the crystal lattice [Figure 1C]. This energy release results in localized heating of the NP and its surroundings, leading to a rapid increase in temperature at the nanoscale[37,38]. Local heating is a key feature of photothermal catalysis and plays a vital role in enhancing reaction kinetics. Localized heating can enable the reaction to proceed under milder conditions, reducing the overall energy input. The extent of local heating depends on several factors, including the amount of absorbed photons, the thermal conductivity of the metal NPs and support materials, and the efficiency of heat dissipation to the surrounding environment[39,40].
Heat transfer
Heat transfer is a critical aspect of photothermal catalysis, as it determines how the heat generated by photon-to-phonon conversion is distributed across the catalyst and the surrounding medium [Figure 1D]. For chemical reactions that require high temperatures, it is crucial to confine heat locally to maintain elevated temperatures. In contrast, efficient heat transfer is essential to maintain the desired reaction environment and prevent overheating of the catalyst working at low temperatures. In photothermal systems, heat can be transferred through conduction, convection, and radiation, with conduction being the primary mode at the nanoscale[41,42]. Design of photothermal catalysts must consider the thermal properties of both the active material and the support. Moreover, spatial distribution of heat within the catalyst can influence the selectivity of the reaction. In some cases, temperature gradients created by localized heating can drive specific reaction pathways, favoring the formation of desired products. To optimize heat transfer in photothermal catalysts, researchers have explored core-shell structures, where a plasmonic core is encapsulated by a shell material with tailored thermal properties[9,40,43,44]. For example, applying nanoporous oxide materials on top of the metal NPs can create a nanoscale greenhouse effect, achieving high local temperatures[9]. This design enables precise control over the thermal environment and can enhance the durability of the catalyst under operating conditions.
Interplay between photothermal effects and catalytic reactions
The integration of photothermal effects into catalytic reactions represents a promising strategy for enhancing reaction efficiency, productivity, and stability. When plasmonic NPs absorb light, they primarily generate two types of energy: hot carriers and thermal energy. These energies drive chemical reactions via two major pathways: hot carrier-induced redox reactions and thermocatalytic reactions facilitated by localized heating [Figure 2]. Understanding the interplay between these pathways is crucial for optimizing photothermal catalysis.
Figure 2. (A) Plasmonic photocatalysis driven by photo-excited hot charge carriers. (B) Plasmonic thermocatalysis driven by heat energy generated from solar light. (C) Reduction of activation energy barrier by local hot charge carriers (photocatalysis) and acceleration of thermocatalytic reaction kinetics due to Boltzmann thermal energy (KBT) supplied to the reactants surrounding the plasmonic nanoparticle.
Hot carrier-induced redox reactions
When plasmonic NPs absorb light, the energy is transferred to free electrons, generating a non-equilibrium distribution of energetic electrons and holes, commonly referred to as hot carriers. These hot carriers can participate in driving redox reactions at the NP surface similar to conventional photocatalysis [Figure 2A]. Due to their high energy, hot carriers can overcome the activation energy barrier for electron transfer processes, facilitating reactions that would otherwise be thermodynamically unfavorable. The efficiency of this pathway depends heavily on the lifetime and energy of the hot carriers.
Thermocatalytic reactions driven by local heating
The photothermal effects generate localized heating through photon-to-phonon conversion. This thermal energy can drive thermocatalytic reactions, where the increase in temperature enhances the reaction rate by providing the necessary activation energy [Figure 2B][42]. The elevated local temperature can enhance the rate of bond cleavage and formation, leading to higher catalytic activity. The thermal effect also influences the kinetics of the reaction, often making the reaction pathway similar to traditional thermocatalysis. However, the localized and transient nature of the heating can lead to unique reaction dynamics that are distinct from conventional thermal catalysis.
Synergistic effects between hot carriers and thermal energy
The most compelling aspect of photothermal catalysis is the synergy between hot carriers and thermal energy, which together provide a dual enhancement to the catalytic process. When both pathways are engaged, the reaction can overcome the activation energy barrier through two complementary mechanisms [Figure 2C]. The first mechanism is related to the thermal effect, which increases the thermal energy of the system, often expressed as KbT, where Kb is the Boltzmann constant and T is the absolute temperature. This thermal energy corresponds to the average kinetic energy of the reactant molecules. As the temperature rises, the kinetic energy of the molecules increases, leading to more frequent and energetic collisions between them. These higher-energy collisions provide sufficient energy to surpass the activation energy barrier. Secondly, the hot carriers can modify the electronic structure of the surface and induce electronic interactions that further reduce the activation barrier as photocatalytic reactions. This combined effect allows for the activation of otherwise challenging reactions, improving both the reaction rate and selectivity. The interplay between these effects creates a more favorable reaction environment, enabling reactions that require lower energy input compared to those driven solely by thermal or solar energy.
The interpretation of chemical reactions within the framework of photothermal catalysis depends on understanding the dominant energy source driving the reaction. If the absorbed light primarily generates hot carriers that directly participate in the reaction, the mechanism leans toward photocatalysis. On the other hand, if photon-to-phonon conversion predominates, leading to significant thermal energy generation, the reaction resembles traditional thermocatalysis. Identifying the major energy source for the reaction is critical for designing experiments and interpreting reaction pathways in photothermal catalysis research. Currently, most studies in the field focus on the thermal aspect, analyzing reaction mechanisms through thermocatalysis. This approach is reasonable, given the significant heat generated by plasmonic NPs during light absorption. However, the role of hot carriers cannot be disregarded, as they introduce unique reaction pathways that differ from classical thermocatalysis. The strategic design of materials and catalysts to optimize these effects is critical for advancing the field, particularly in applications such as photothermal CO2 hydrogenation and DRM.
PHOTOTHERMAL CATALYST FOR CO2 HYDROGENATION
CO2 hydrogenation refers to the reaction between carbon dioxide and hydrogen to produce C1 or C2+ products. While numerous studies have been conducted on producing C2+ compounds, these reactions remain complex and require significant energy input, thus presenting several challenges[45]. Consequently, much of the research has focused on the production of C1 products, which serve as precursors for various industrial applications. The most prominent reactions in this field include CO2 methanation, Reverse Water Gas Shift (RWGS) reaction, and methanol synthesis, which are respectively written as
In these processes, CO2 and H2 are used as feedstocks, yielding CH4, CO, and CH3OH, respectively. By utilizing CO2, a major greenhouse gas, these reactions offer significant environmental and industrial benefits by producing essential fuels and chemicals. Both photocatalytic and traditional thermal catalytic methods have been widely explored to facilitate these transformations. However, thermal catalysis requires substantial energy input and often suffers from catalyst deactivation due to coking at high temperatures[46]. Photocatalysis, on the other hand, is limited by its relatively low activity[47]. To address these challenges, there has been a growing emphasis on photothermal catalysis. Photothermal catalytic reactions demonstrate higher efficiency at lower temperatures compared to thermocatalytic processes[48] and gain attention due to their ability to harness abundant solar energy. Both precious metals and conventional transition metals have been utilized in the field of photothermal catalysts. Current research is focused on improving catalytic efficiency by optimizing light absorption and designing economically feasible catalysts. Additionally, many studies are dedicated to verifying the enhancement of photothermal effects through structural modifications of catalytic materials and further elucidating the mechanisms behind these effects.
Photothermal catalyst for CO2 methanation
CO2 methanation is a process in which CO2 reacts with H2 to produce CH4, as given in:
Recent studies for photothermal catalysts to produce methane are described below [Table 1].
Photothermal catalysts for CO2 methanation
| Product | Catalyst | mcat (mg) | Flow rates (mL/min) | GHSV (mL/g·h) | Light source | Reaction condition (Photo energy in W/m2) | Temp. (°C) | CH4 production rate (mmol/g·h) | Pressure | Ref. |
| CH4 | Ru0.88Co0.12/TiO2 | 50 | 20 (CO2/H2 = 1:4) | 24,000 | A 300 W Xe lamp (200 < λ < 1,100 nm) | Photothermal (19000) | 300 | 191 | 1 bar | [49] |
| Thermal | 300 | 129.7 | ||||||||
| Ru/HNT | 5 | 25 (CO2/H2 = 1:4) | 300,000 | a 300 W Xe lamp | Photothermal (20500) | 350 | 1,704 | 1 bar | [50] | |
| Photothermal (5800) | 250 | 4.5 | ||||||||
| Thermal | 250 | 2.4 | ||||||||
| 4%Ru/HxMoO3-y | 150 | 10 (CO2/H2 = 1:1) | 4,000 | A Xe lamp (SAN-EI ELECTRIC XEF-501S) + 450 nm cut-off filter | Photothermal (7500) | 140 | 20.8 | 1 bar | [51] | |
| Thermal | 140 | 4.4 | ||||||||
| CeNiO3 | 20 | 6 (H2:CO2 = 4:1) | 18,000 | 300 W Xe lamp (PLS-SXE-300UV) | Photothermal (20000) | 300 | 40 | 1 bar | [52] | |
| Thermal | 300 | 18 | ||||||||
| CeNiO3-x | Photothermal (20000) | 300 | 80 | |||||||
| Thermal | 300 | 58 | ||||||||
| Ru/MnOx | 150 | 50 (CO2/H2 = 1:4) | 20,000 | full-arc A 300 W UV-Xe lamp | Photothermal (25000) | 200 | 113.2 | 1 bar | [53] | |
| Thermal | 200 | 7.2 | ||||||||
| Ru/MnCo2O4-2 | 50 | 20 (CO2/H2 = 1:4) | 24,000 | A 300 W Xe lamp (CEL-HXF300H5) (420 < λ < 780 nm) | Photothermal (12500) | 230 | 66.3 | 1 bar | [54] | |
| Thermal | 230 | 30.2 | ||||||||
| Ni/BN-4 | 20 | 2 (CO2/H2 = 1:4) | 6,000 | A 300 W Xe lamp (PLSSXE300D) | Photothermal | 230 | 44.06 | 1 bar | [55] | |
| Thermal | 230 | 0.64 | ||||||||
| Ni/TiO2-25 | 10 | 25 (CO2/H2/He = 1:4:5) | 150,000 | a 300 W Xe lamp (PLS-SXE3000UV) | Photothermal (210000) | 360 | 227.7 | 1 bar | [56] | |
| Thermal | 360 | 45.2 |
Although CO2 methanation is exothermic, the structural stability of CO2 requires high temperatures of
In 2022, Ge et al. focused on the development of Ru/HxMoO3-y, a catalyst synthesized via H2-spillover to Ru/MoO3 [Figure 3A]. The results show that the catalyst gained photothermal properties through the synthesis process[51]. The researchers studied the electrochemical and structural properties for the difference between Ru/HxMoO3-y and Ru/MoO3. Ru/HxMoO3-y showed strong absorption of light with a wavelength of 500 to 800 nm, while Ru/MoO3 hardly absorbed light with a wavelength over 400 nm [Figure 3B]. The researchers attributed the structural and electronic modifications to the H2-spillover effect, in which
Figure 3. Properties and performance of Ru/HxMoO3-y and Ru/MoO3. (A) Schematic of preparing for Ru/HxMoO3-y via H2-Spillover using Ru/MoO3. (B) UV-vis-NIR spectra absorption analysis of Samples. (C) Schematic of the band structure made based on analysis such as Tauc plots, VB XPS spectra, Mott-Schottky plots, etc. (D) Photocurrent density of samples under Vis-NIR light irradiation. (E) Methane production rate and selectivity depending on different light intensities. (F) Methane production rate with and without light at various temperatures. (G) Methane production rate with and without light for different Ru loading samples. (H) XPS spectra O 1s of Fresh sample, sample after CO2 adsorption, sample reduced again by hydrogen after CO2 adsorption. Reproduced with permission from Ref.[51].
In 2023, Guo et al. explored the use of heated substrates to enhance catalytic performance, beyond the widely studied LSPR effect of metal NPs[54]. They proposed that the heat generated through the non-radiative recombination of electrons could play a significant role in the reaction kinetics. In their study, Ru/MnCo2O4 catalysts were fabricated and evaluated based on the structural, redox, photothermal, and photoelectronic properties. The Ru/MnCo2O4 catalysts were compared with Ru/Al2O3 and Ru/TiO2 catalysts, all sharing the same metal loading but differing in support materials. All three catalysts were synthesized using the impregnation method. The increase in surface temperature under light irradiation was measured for each catalyst over time. The result showed that Ru/Al2O3 and Ru/TiO2 achieved a maximum temperature of approximately 42 °C, whereas Ru/MnCo2O4 reached 85 °C and MnCo2O4 alone achieved 80 °C [Figure 4A]. This temperature increase was notably higher than that of MnO and Co2O3, confirming that MnCo2O4 can act as an efficient photothermal support, effectively converting light energy into thermal energy. Additionally, finite element simulations using COMSOL Multiphysics software demonstrated that a stronger electric field was generated at the Ru/MnCo2O4 interface compared to Ru/TiO2. This enhanced electromagnetic field amplified the photothermal effect, resulting in notable differences in CO2 methanation performance [Figure 4B]. Under conditions of 12,500 W m-2, light with wavelengths between 420 and 780 nm, and 200 °C, other catalysts exhibited methane production rates of 5-10 mmol g-1 h-1, while Ru/MnCo2O4 achieved nearly 70 mmol g-1 h-1 [Figure 4C]. These results demonstrate that the photothermal support significantly increases methane production by reducing the activation energy under light irradiation [Figure 4D]. Performance evaluation based on metal loading revealed that excessive metal loading led to a decrease in activity due to NP agglomeration, reducing the number of available active sites [Figure 4E]. Thus, this study highlights that not only do metals with LSPR effects play a crucial role in photothermal catalysis, but the photothermal support itself can enhance the overall catalytic efficiency by intensifying the photothermal effect.
Figure 4. Properties and performance of Ru/MnCo2O4-X, Ru/TiO2 and Ru/Al2O3, etc. (A) Temperature changes of
In 2024, Zhai et al. revealed that Ru combined with MnOx plays a crucial role in photothermal CO2 methanation[53]. MnO2 was synthesized using a hydrothermal method, and Ru was anchored via photo-deposition. The catalyst exhibited increased CO2 adsorption capacity and surface area, with Ru existing in a metallic state and reduced under light irradiation [Figure 5A]. The Ru/MnOx catalyst, containing Mn4+, Mn3+, and Mn2+ [Figure 5B], achieved a CH4 production rate of 113.2 mmol g-1 h-1 at 200 °C under a light intensity of 25,000 W cm-2. The result was significantly more effective than the 7.2 mmol g-1 h-1 observed under dark conditions at the same temperature [Figure 5C]. Ru enhances the H2-spillover effect, resulting in structural changes and strong absorption from ultraviolet (UV) to NIR wavelengths, making the photothermal methanation reaction more efficient [Figure 5D]. The H2-spillover effect of Ru was further demonstrated using WO3 support which readily reacts with hydrogen and darkens upon reaction. Although WO3 alone showed little color change, the catalyst mixed with Ru/MnOx and WO3 darkened significantly under thermal conditions and became even darker under photothermal conditions. This observation demonstrates that H2-spillover effect of Ru is more effective under light irradiation. Additionally, when Ru was added to MnOx, the catalyst displayed broad and strong light absorption from UV to NIR wavelengths [Figure 5E]. The surface temperature under light irradiation was approximately 22 °C higher than in samples without Ru [Figure 5F]. This clearly demonstrates that Ru not only induces structural changes but also plays a crucial role in the photothermal reaction. Notably, the Ru/MnOx catalyst promotes the formation of COOH intermediates in the CO2 methanation reaction. The COOH intermediate is a key step in the production of CH4, and the photothermal catalyst significantly reduces the activation energy, thus enhancing reactivity [Figure 5G]. This catalyst also exhibits high efficiency in both CO2 adsorption and H2 dissociation, achieving a CH4 selectivity of up to 99.5%. In addition, Ru/MnOx maintained high activity and stability during long-term reactions under photothermal conditions. This performance was particularly efficient in CH4 production, yielding better results under photothermal conditions compared to thermal conditions. This study highlights the crucial role of Ru in various aspects of photothermal reaction for CO2 methanation.
Figure 5. Preparation diagram and properties of Ru/MnOX and MnOX (A) Schematic of the synthesis procedure of Ru/MnOX. (B)
In summary, various metals (Ru, Ni, Co, etc.) have been used as catalysts for photothermal CO2 methanation. Recently, Ru has been regarded as the optimal metal for achieving catalytic activity at low temperatures, despite its high cost. The studies demonstrate that Ru effectively activates hydrogen at low temperatures. Researchers also focus on whether the photothermal effect can be enhanced using different supports. The synergy between the metal and support has been shown to enhance the photothermal effect, which positively impacts the CO2 methanation reaction. These results suggest that the synergy between metal catalysts with appropriate supporting materials could further enhance the potential for developing cost-effective catalysts.
Photothermal catalyst for reverse water gas shift (RWGS) reaction
The RWGS reaction involves the conversion of CO2 and H2 into CO and water (H2O), as given in:
The RWGS reaction is one of the most prominent processes for producing CO, which serves as a crucial feedstock for the synthesis of various chemical products. Transition metal catalysts such as Cu[58], Ni[56], Fe[59], and Co[60], along with precious metals such as Pt[61], and Rh[62], are commonly utilized in this reaction, with support materials such as ceria (CeO2)[48] and titania (TiO2)[63] enhancing catalytic performance. Since the RWGS reaction is inherently endothermic, it requires high reaction temperatures to proceed efficiently, typically above 400 °C[64]. However, such high temperatures often lead to catalyst deactivation primarily due to the formation of carbon deposits (coking) on the catalyst surface, which significantly diminishes catalytic activity. Efforts to address these challenges have led to extensive research into optimizing catalyst compositions, support materials, and reaction conditions. Despite these studies, the development of catalysts capable of maintaining high activity and stability under high-temperature conditions remains a critical challenge. As a result, photothermal catalysts have garnered significant attention for their ability to utilize both light and heat, enabling high efficiency at lower temperatures while also mitigating coking. Current research on photothermal catalysts in the RWGS reaction focuses on key areas such as low-temperature activation, enhancing the photothermal synergy effect, understanding catalyst-support interactions, suppressing coking, and elucidating the underlying reaction mechanisms. These advancements aim to achieve a more efficient and economically viable CO2 conversion process. Recent studies for RWGS reaction are being conducted for use in industry. A summary of these recent studies is provided below [Table 2].
Photothermal catalysts for RWGS
| Product | Catalyst | mcat (mg) | Flow rates (mL/min) | GHSV (mL/g·h) | Light source | Reaction condition (Photo energy in W/m2) | Temp. (°C) | Syngas production rate (mmol/g·h) | Pressure | Ref. |
| Syngas | Au/CeO2 | 50 | 3 (CO2:H2:N2 = 1:4:95) | 3,600 | Xe lamp (PLS-SXE300, visible light) | Photothermal (32000) | 400 | 0.64 | 1 bar | [65] |
| Thermal | 400 | 0.05 | ||||||||
| 6.3 Pd/TiO2 | 50 | 20 (CO2:H2 = 1:4) | 24,000 | 300 W Xe lamp (CEL-HXF300-T3) + NIR light (400 < λ < 1,100 nm) | Photothermal (25000) | 250 | 7.4097 | 1 bar | [63] | |
| Thermal | 250 | 0.4128 | ||||||||
| In2O3-x/In2O3 | 20 | 1 (CO2:H2 = 1:1) | 3,000 | a 300 W Newport Xe lamp | Photothermal ( ~8 suns.) | 300 | 2.051 | 1 bar | [66] | |
| Thermal | 300 | 0.363 | ||||||||
| 10 mol% Cu-HAP | 10 | 2 (CO2:H2 = 1:1) | 12,000 | A 300 W Newport Xe lamp | Photothermal (20000) | 300 | 1.34 | 1 bar | [58] | |
| Thermal | 300 | 0.48 | ||||||||
| Co15HAP | 100 | 26 (CO2:H2 = 1:1) | 15,600 | 150 W Xe lamp | Photothermal (1080) | 400 | 62 | 1 bar | [67] | |
| Thermal | 400 | 58 | ||||||||
| Photothermal (1080) | 250 | 16 | ||||||||
| Thermal | 250 | 3 | ||||||||
| 10Cu5Ga/CeO2 | 10 | 20 (CO2:H2 = 1:1) | 120,000 | A LA-251 Xe lamp with a HA30 filter was utilized as UV-vis irradiation source A 300 W Xenon arc lamp | Photothermal (UV-vis light) | 320 | 54.8 | 1 bar | [48] | |
| Thermal | 320 | 36 | ||||||||
| Photothermal (19520) | 285 | 111.2 | ||||||||
| CuCo5BOx | 20 | 50 (CO2:H2 = 4:1) | 150,000 | A 300 W xenon lamp | Photothermal (5000) | 300 | 124.7 | 1 bar | [68] | |
| Thermal | 300 | 35 | ||||||||
| Rh/TiO2 -2 | 50 | 20 (CO2:H2 = 1:4) | 2,000 | a 300 W Xe lamp (200 < λ < 1,100 nm), | Photothermal (27000) | 250 | 20.6 | 1 bar | [62] | |
| Thermal | 250 | 1.8 |
In 2020, Guo et al. developed an effective catalyst for RWGS reaction using economical and environmentally friendly materials[58]. They used hydroxyapatite [HAP, Ca10(PO4)6(OH)2] [Figure 6A], a material known for its simplicity in synthesis and scalability and its widespread use in various fields due to its eco-friendly properties. Additionally, copper (Cu) is abundant and cost-effective while possessing strong hydrogen activation properties. They synthesized Cu-HAP (CuxCa10-x(PO4)6(OH)2) by substituting calcium in HAP with copper for RWGS reaction. Light irradiation allowed Cu-HAP to achieve higher activity compared to the dark experiment. For the photothermal reaction, a light intensity of approximately
Figure 6. Model of HAP structure and activity test for catalysts. (A) Crystal structure of hydroxyapatite. (B) Methane production rate with and without light at various temperatures and Arrhenius plot. (~2.0 W cm-2 light intensity, H2:CO2 ratio 1:1, flow rate = 2 sccm). (C) The CO production rate of Cu-HAP depended on the Cu loading amount. (D) RWGS activities of various samples in the batch reactor. (E) Duration test of 10% Cu-HAP at 300 °C under light irradiation. (F) In bulk industrial, price and quantity for synthesis of catalysts are used for RWGS reaction. Reproduced with permission from Ref.[58].
In 2021, Deng et al. synthesized a highly dispersed Ga-Cu/CeO2 catalyst using a metal-organic framework (MOF) template[48]. They demonstrated the role of Ga and Cu in promoting the RWGS reaction. The researchers synthesized a CeO2-based MOF, then prepared the Ga-Cu/CeO2 catalyst via an ion-exchange method. The key finding of this study was the significant improvement in Cu dispersion in MOF-derived catalysts compared to those made using typical wet impregnation methods. Specifically, the
Figure 7. Structure-property analysis, activity test and mechanism study for samples. (A) Light intensity-dependence of temperature for samples supported on CeO2 different loadings of Cu and Ga. (B) Light intensity-dependence of CO production rate for samples with different loadings of Cu and Ga on CeO2. (C) O 1s: XPS spectra of 10Cu/CeO2. (D) O 1s: XPS spectra of 10Cu5Ga/CeO2. (E) in situ DRIFTS spectra analysis over 10Cu/CeO2 during photothermal RWGS reaction. (F) Over 10Cu5Ga/CeO2. (G) Schematic of photothermal RWGS reaction mechanism for catalyst supported Cu and Ga. Reproduced with permission from Ref.[48].
Previous studies have emphasized the importance of dispersion. It was known well that particle size is closely related to dispersion[69]. In 2023, Yang et al. examined how the catalytic activity varies with the size of Pd metal particles[63]. In general, catalytic activity changes depending on the size of metal NPs. In this study, they synthesized catalysts with particle sizes of 2.8, 3.4, 4.7, 5.3, 6.3, and 8.1 nm [Figure 8A]. They conducted reaction experiments from 50 to 250 °C to analyze the effect of particle size on catalytic performance. The results showed that activity increased with larger particle sizes but began to decrease beyond a certain particle size [Figure 8B]. They suggested that precisely controlling the size of Pd NPs can modify SMSI (Strong Metal Support Interaction). This alteration changes the electronic properties, significantly affecting catalytic performance. To evaluate the metal-support interaction, they analyzed binding energy. The binding energy followed a similar trend to the change in activity based on particle size. In a reaction experiment under irradiation at an intensity of 2.5 W cm-2 and a temperature of 250 °C, the production rate reached 7.4097 mmol g-1 h-1, while under dark conditions, it was only 0.4128 mmol g-1 h-1, showing a considerable difference [Figure 8C]. The turnover frequency (TOF) of 6.3Pd/TiO2 was approximately
Figure 8. Particle size and activity test for catalysts. (A) TEM images of catalysts with different particle sizes. (B) Temperature-dependence of CO production rate for samples with different loadings of Pd on support. (C) CO2 conversion rate at various temperatures in photothermal and thermal reactions. (D) Turnover frequency number for CO production on catalysts in photothermal reaction. Reproduced with permission from Ref.[63].
In 2024, Wang et al. presented a study where boron-oxide (B-O) was identified as a key component in the CuCo5BOx catalyst, enhancing photothermal catalytic performance through increased CO2 adsorption capacity and improved charge transfer[68]. The CuCo5BOx catalyst exhibited an amorphous CoBOx structure with coexisting Cu0 and Cu+ crystalline phases. Boron was well incorporated into the catalyst lattice, contributing to an increased specific surface area, thereby optimizing the reaction efficiency
Figure 9. Activity test results and properties analysis of catalysts. (A) XRD graphs of Cu, CuBOX, CuCo5BOX, and CoBOX (B) The CO2 adsorption energy of samples. (C) The reaction test of various catalysts (Reaction condition: 20 mg weight, 300 °C reaction temperature, 50 W m-2 light intensity. (D) Photocurrent density of CuCo5OX and CuCo5BOX. (E) CO production rate with and without light at various temperatures. (F) CO production rate of samples treated at different conditions (A: vacuum drying for 12 h; B: dry in the air at 25 °C for 4 h G: leave at room temperature for one year). Reproduced with permission from Ref.[68].
Photothermal catalyst for methanol synthesis reaction
The reaction for synthesizing methanol from CO2 and H2 can be written as follows:
Methanol is an important chemical compound with a high energy density, making it highly valuable in the chemical industry as a fuel, solvent, and chemical intermediate. The synthesis of methanol from CO2 is a crucial approach for sustainable carbon utilization, with catalysts such as Cu, Ru, Ni, and Pd being commonly utilized. Among these, Cu is widely used due to its low cost and relatively high selectivity. However, Cu-based catalysts exhibit their highest efficiency under harsh conditions, typically requiring high temperatures (200-300 °C) and pressures (3-10 MPa)[70]. These severe conditions can reduce the overall energy efficiency of the process and impact the durability of the equipment in industrial applications. Additionally, since methanol synthesis is an exothermic reaction, it can promote RWGS reaction, leading to the production of unwanted CO, which in turn decreases methanol selectivity[71]. To overcome this issue, catalyst design must focus on suppressing the RWGS reaction while enhancing methanol selectivity. To address these challenges, recent research has focused on the development of catalysts that can efficiently promote methanol synthesis under ambient pressure. Photothermal catalysis has emerged as a promising strategy for methanol synthesis, utilizing both light and heat to drive the reaction under milder conditions. This approach shows great potential by enabling effective catalytic performance even at lower temperatures and ambient pressure, thereby increasing the commercial viability of CO2-to-methanol conversion. However, the low reaction rate of photothermal catalysts is a significant barrier to commercial applications. Moreover, the long-term stability of photothermal catalysts remains a concern as catalytic performance tends to degrade over time. Recent studies for photothermal catalysts to produce methanol are presented below [Table 3].
Photothermal catalysts for synthesis MeOH
| Product | Catalyst | mcat (mg) | Flow rates (mL/min) | GHSV (mL/g·h) | Light source | Reaction condition (Photo energy in W/m2) | Temp. (°C) | MeOH production rate (mmol/g·h) | Pressure | Ref. |
| MeOH | Cu/ZnO | 20 | 20 (CO2:H2 = 1:3) | 60,000 | LA-251Xe lamp (420 < λ < 800 nm) | Photothermal (5800) | 220 | 0.1278 | 1 bar | [72] |
| Thermal | 220 | 0.0828 | ||||||||
| 2Ru/In2O3 | 10 | 20 (CO2:H2 = 1:3) | 120,000 | LA-251 Xe lamp, (400 < λ < 800 nm) | Photothermal (6442) | 250 | 0.178 | 1 bar | [73] | |
| Thermal | 250 | 0.1228 | ||||||||
| CZA | 30 | 20 (CO2:H2 = 1:3.2) | 40,000 | a LX300F Xe (350 < λ < 800 nm) | Photothermal (6000) | 225 | 7.8125 | 21 bar | [74] | |
| Thermal | 225 | 5.9375 | ||||||||
| Pd/ZnO | 100 | 34 (CO2:H2 = 1:3) | 20,400 | two 500 W high-pressure mercury lamps | Photothermal | 250 | 3.8 | 12 bar | [75] | |
| Thermal | 250 | 0.8 | ||||||||
| 10% Ni-In2O3 | 100 | 20 (CO2:H2 = 1:3) | 12,000 | AM 1.5G filter and a 300W Xenon lamp | Photothermal | 265 | 1.9812 | 10 bar | [76] | |
| Thermal | 265 | 1.16875 | ||||||||
| In2O3x(OH)y-14 NR | 15 | 8 (CO2:H2 = 1:3) | 32,000 | A Newport 130 W Xe arc lamp | Photothermal | 250 | 0.0973 | 1 bar | [77] | |
| Thermal | 250 | 0.0541 | ||||||||
| HzIn2O3-x (OH)y | N/A | 4 (CO2:H2 = 1:1) | N/A | a 300 W Xe lamp (PLS-SXE300D) | Photothermal | 300 | 0.01895 | 1 bar | [78] | |
| Thermal | 300 | 0.02303 |
In 2018, Wang et al. confirmed that the previously reported In2O3-x(OH)y catalyst plays a crucial role in the RWGS reaction by facilitating the conversion of CO2 to CO. This study also explored the potential of
Figure 10. Performance test of catalysts. (A) MeOH production rate with and without light at various temperatures. (B) MeOH production rate during 20 h. (C) MeOH selectivity of the In2O3-X(OH)y, NR-14h. Reproduced with permission from Ref.[77].
In 2019, Wang et al. developed a Cu/ZnO catalyst and conducted extensive examination on how hot electrons generated by visible light influence methanol synthesis[72]. The Cu to Zn molar ratio was set at 1:2, and the catalyst was prepared using the co-precipitation method. When comparing the light absorption characteristics of ZnO and Cu/ZnO, a distinct absorption peak was observed at 584.3 nm, attributed to the Cu NPs [Figure 11A]. This led to an improvement in production rate from 1.38 to 2.13 μmol g-1 h-1 under light irradiation [Figure 11B]. They claimed that the reduction in activation energy by 40% in the presence of Cu NPs demonstrated that the reaction was promoted by hot electrons generated through Localized Surface Plasmon Resonance (LSPR) [Figure 11C]. They also extensively examined the mechanisms of both thermal and photothermal catalysis, confirming the impact of hot carriers. In thermal catalysis, *HCOO species adsorbed on ZnO, CH3O species, and *HCOO species adsorbed on Cu followed the formate pathway. The reaction pathway did not change after irradiation. Instead, the hot carriers generated from Cu prevented the accumulation of *HCOO species on the surface, increasing the reaction rate during photothermal catalysis [Figure 11D]. This research offers the first extensive examination of the mechanism behind methanol synthesis at atmospheric pressure via photothermal catalysis. It seems clear that copper and Zn oxide play a beneficial role in methanol production. However, the role and synergy of each metal and metal oxide in photothermal reactions will be discussed in the next section.
Figure 11. Properties and reaction mechanism study of CZ. (A) UV-vis spectra absorption analysis of CZ and ZnO. (B) MeOH production rate at 220 °C in visible light off and on. (C) activity energy under dark and light conditions calculated using Arrhenius equation for MeOH formation over CZ. (D) in situ DRIFTS spectra with and without light. Reproduced with permission from Ref.[72].
In 2020, Xie et al. conducted a detailed investigation into the roles of Cu and ZnO sites in the CZA (Cu/ZnO/Al2O3) catalyst[74]. The synergy between these roles maximized the activity of the CZA catalyst. Under light irradiation in the 350-800 nm range, at 21 bar and 225 °C, the CZA catalyst achieved a production rate of 7.8125 mmol g-1 h-1, compared to 5.9375 mmol g-1 h-1 in dark conditions [Figure 12A]. Upon analyzing the catalytic activity under different light ranges, the production rate significantly increased within the 350-800 nm range, while no significant change in activity was observed within the 420-800 nm, 200-500 nm ranges. This indicates that specific wavelengths do not promote photothermal catalysis [Figure 12B]. The activation wavelengths for Cu and ZnO are different, and their simultaneous activation plays a crucial role. To further clarify the roles of Cu and ZnO, the research team conducted experiments using three catalysts: CZA, ZA (Zn/Al2O3), and CA (Cu/Al2O3). This study revealed that CO2 adsorbs onto ZnO to form HCOO under visible light, which then reacts with adsorbed hydrogen (H) on Cu(I) species to produce methanol. The critical interface for this reaction is the boundary where hydrogen from Cu reacts with *HCOO adsorbed on ZnO. In the site where Cu is present alone, an increase in CO production was observed, reducing methanol selectivity [Figure 12C]. This suggests that controlling the catalyst composition is a key factor in enhancing selectivity in the CO2-to-MeOH conversion reaction. Thus, this research not only provides a clear understanding of the individual roles of Cu and ZnO but also presents a new perspective on how the structure and morphology of catalysts can promote important reaction mechanisms [Figure 12D and E]. Ultimately, the study demonstrates that controlling composition and structure in catalyst design is essential to maximize methanol production.
Figure 12. Production rate and selectivity of MeOH; Schematic of reaction mechanism for CZA. (A) Production rate and selectivity of MeOH over CZA with and without irradiation depending on temperature. (B) Methanol production rate of CZA as irradiated with different wavelengths of light. (C) The DRIFTS spectra of CZA, ZA and CA (D and E) Proposed schematic of CO2 hydrogenation for synthesis MeOH over CZA. Reproduced with permission from Ref.[74].
In 2023, Deng et al. developed a Ru/In2O3 catalyst that significantly improved methanol production rates and highlighted the critical role of photothermal effects in enhancing catalytic performance[73]. The Ru/In2O3 catalyst demonstrated not only superior production rates compared to samples using other metals (Cu, Rh, Pt, Pd) but also achieved a selectivity of approximately 35% [Figure 13A]. The concentration of oxygen vacancies increased with higher Ru loading despite similar surface areas [Figure 13B-D]. The photothermal effect involves two key characteristics: the heating substrate effect and the generation of hot carrier electrons. A comparison between 290 °C in dark conditions and 280 °C under light irradiation showed a higher production rate at 280 °C with light irradiation, indicating that the enhancement in production was primarily because of hot carriers rather than thermal substrate heating [Figure 13E]. The analysis of the reaction mechanism suggested that the CO pathway dominated the reaction rather than the formate pathway as seen in other studies. In the overall process, the interaction between ruthenium and In2O3 increased the concentration of oxygen vacancies, providing more adsorption sites for CO2. Subsequently, hydrogen spillover from Ru facilitated CO2 activation, leading to the adsorption of *COOH, which was then converted to *CO. The combined effects of thermal substrate heating and hot carrier electrons promoted the accumulation of *CO, which was hydrogenated to produce methanol [Figure 13F]. This study effectively demonstrated the superior activity of Ru/In2O3 compared to other metal-based catalysts. The overall mechanism was shown to be a synergistic result of photothermal heating substrate effect and hot carrier-induced activation caused by Ru [Figure 13G]. Research on methanol production has been less extensive so far compared to RWGS and methanation. Methanol synthesis has many limitations compared to the other two reactions due to its complex mechanism and harsh reaction conditions.
Figure 13. (A) MeOH production rate of catalysts supported on In2O3 for synthesis MeOH under dark conditions: 10 mg of catalyst,
PHOTOTHERMAL CATALYSTS FOR DRY REFORMING OF METHANE WITH CARBON DIOXIDE
PGM-based catalytic materials
PGM-based catalysts, such as those containing Pt, Rh, and Ru, have been widely investigated for their superior performance in photothermal DRM [Table 4]. Tang et al. reported that Ru/SrTiO3 catalysts efficiently combine photothermal heating with electronic interactions between Ru and SrTiO3, resulting in enhanced activation of CO2 and CH4 under light irradiation[19]. Similar to this, Hu et al. developed RuSA+NP/TiO2, where the synergistic interaction between single-atom Ru sites and NPs not only improved light absorption but also enhanced catalytic stability during prolonged DRM operations[82]. Another approach by Shoji et al. highlighted the potential of Rh/SrTiO3 catalysts, which utilize LSPR effects to overcome reaction barriers and achieve high syngas yields[22]. Also, Liu et al. demonstrated that Pt/TaN catalysts benefited from the light-induced local heating effects provided by the TaN support, significantly improving CH4 conversion rates[84]. Together, these studies illustrate a diverse range of strategies to pave the way for more efficient and sustainable catalytic systems for DRM. We will now highlight a selection of key studies in this field to further explore the innovative advancements contributing to this PGM-based DRM catalyst.
Summary of photothermal catalysts for dry reforming of CH4 with CO2
| Catalyst | mcat (mg) | Flow rates (mL/min) | GHSV (mL/g·h) | Light source | Reaction condition (Photo energy in W/m2) | Temp. (oC) | CH4 rxn rate (μmol/g·s) | H2/CO ratio | Ref. |
| Cu19.8Ru0.2/MgO-Al2O3 | 1.5 | 16 (50% CH4, 50% CO2) | 640,000 | Supercontinuum laser (Fianium, WL-SC-400-8) | Photothermal (192,000) | RT | 275 | 1 | [18] |
| Thermal | 727 | 60 | 0.22 | ||||||
| Ni/mesoporous TiO2 | 200 | 48 (21% CH4, 29% CO2) | 14,400 | Xenon light (XE-300F) | Photothermal (3.1) | RT | 0.6 | 1 | [79] |
| Thermal | 500 | 4.5 | 0.58 | ||||||
| Photothermal (3.1) | 500 | 7.9 | 0.65 | ||||||
| 0.29 wt.% Ru/SrTiO3 | 150 | 45 (36% CH4, 36% CO2) | 18,000 | A 300-W xenon lamp | Thermal | 350 | 0.6 | 0.48 | [19] |
| Photothermal | 350 | 4.7 | 0.61 | ||||||
| Thermal | 500 | 11.8 | 0.7 | ||||||
| Photothermal | 500 | 26.7 | 0.8 | ||||||
| SCM-Ni/SiO2 | 25 | 118.7 (11.7% CH4, 11.5% CO2) | 285,000 | A 500 W Xe lamp (CHF-XM500) | Photothermal (343,600) | RT | 154 | 0.86 | [20] |
| (Ni/CeO2)@SiO2 | 25 | 50 (30% CH4, 30% CO2) | 120,000 | 300 W Xenon lamp (CEL-HXF300) | Thermal | 600 | 90 | 0.62 | [80] |
| Photothermal | 600 | 143 | 0.62 | ||||||
| 1.5Ni/SiO2 | 30 | 20 (48% CH4, 48% CO2) | 40,000 | 300 W Xe lamp (CEL-PE300E-3) with a concentrator | Photothermal (90000) | RT | 28 | 0.45 | [21] |
| Thermal | 700 | 80 | 0.64 | ||||||
| Photothermal (90000) | 700 | 188 | 0.86 | ||||||
| 0.7Ni-0.8Co/SiO2 | Photothermal (90000) | RT | 15.3 | 0.29 | |||||
| Thermal | 700 | 9.4 | 0.23 | ||||||
| Photothermal (90000) | 700 | 145 | 0.80 | ||||||
| 25 wt.% Ni/SiO2 | 300 | 200 (10% CH4, 12.5% CO2) | 40,000 | a 300 W Xe lamp (Cermax PE300BUV) with a focusing lens (LGQ30/40P) | Photothermal | RT | 4.6 | 0.59 | [81] |
| RuSA+NP/TiO2 | 2.5 | 20 (10% CH4, 10% CO2) | 480,000 | full-spectrum light, 300 W Xe lamp | Thermal | 400 | 26 | 0.5 | [82] |
| Photothermal (28000) | 400 | 45 | 0.63 | ||||||
| Thermal | 500 | 118 | 0.7 | ||||||
| Photothermal (28000) | 500 | 160 | 0.77 | ||||||
| 10 wt.% Rh/SrTiO3 | 5 | 10 (2% CH4, 2% CO2) | 120,000 | 150 W Hg-Xe lamp (LA-410UV-5) | Photothermal | RT | 8.7 | 1 | [22] |
| Rh/CeWO3 | 50 | 50 (20% CH4, 20% CO2) | 60,000 | A Xe lamp (CEL-HXF300 from CEAULIGHT, 300-1000 nm) | Thermal | 300 | 1.9 | 0.9 | [83] |
| Photothermal (18000) | 300 | 3.5 | 0.7 | ||||||
| Pt/TaN | 100 | 20 (50% CH4, 50% CO2) | 12,000 | LA-251Xe lamp (Hayashi) | Thermal | 500 | 3.3 | 0.88 | [84] |
| Photothermal (560) | 500 | 4.5 | 0.92 | ||||||
| Mo2C | 120 | 20 (16.7% CH4, 16.7% CO2) | 30,000 | - | Thermal | 900 | 45 | 0.5 | [23] |
| Xenon lamp | Photothermal (30,000) | RT | 0.01 | 0.3 | |||||
| A fiber optic laser system (LSF20D, Hgtech laser) | Photothermal (20 W, 20 KHz) | RT | 2200 | 0.95 | |||||
| 0.76 wt% NiSA/CeO2 | 25 | 20 (20% CH4, 20% CO2) | 48,000 | 300W Xe lamp (PLS-FX300HU) | Thermal | 470 | 5.3 | 0.5 | [85] |
| Photothermal (24,000) | 470 | 18 | 1 | ||||||
| CoNiRuRhPd /SrTiO3 | 5 | 20 (10% CH4, 10% CO2) | 240,000 | 300 W xenon lamp (PLS-XE300) | Thermal | 500 | 1.7 | 0.9 | [86] |
| Photothermal (40,000) | RT | 11 | 1 | ||||||
| Ni-CeO2-CePO4 | N/A | 10 (25% CH4, 25% CO2) | N/A | Prizmatix LEDs | Thermal | 350 | 1.5 | 0.45 | [87] |
| Photothermal (48,100) | 350 | 7.2 | 1 | ||||||
| Ru/TiO2-H2 | 5 | 50 (8% CH4, 8% CO2) | 600,000 | 300 W Xe lamp (PLS-XE3000UV) | Thermal | 400 | 0 | N/A | [88] |
| Photothermal (120,000) | 400 | 70 | 0.9 | ||||||
| Ni/Ga2O3 | 40 | 20 (20% CH4, 20% CO2) | 30,000 | CEL-HXF300 from CEAULIGHT | Thermal | 500 | 1.3 | 0.55 | [89] |
| Photothermal (30,000) | 500 | 1.6 | 0.95 | ||||||
| Rh/Al2O3 | 230 | 50 (10% CH4, 12.5% CO2) | 13,000 | 300 W Xe lamp (Cermax PE300BUV) | Photothermal (780) | RT | 0.3 | 0.65 | [90] |
| Pt/TiO2 | 20 | 20 (48% CH4, 48% CO2) | 60,000 | Xenon lamp | Thermal | 500 | 23 | 0.26 | [91] |
| Photothermal (42,000) | 500 | 107 | 0.65 | ||||||
| 1% Rh2O3 /Ga2O3 | 94 | 10 (48% CH4, 48% CO2) | 6,400 | 300 W Xe lamp | Thermal | 500 | 1.2 | 0.14 | [24] |
| Photothermal | 500 | 1.8 | 0.22 | ||||||
| Pt/TNT | 20 | 30 (5% CH4, 5% CO2) | 90,000 | 300 W Xe lamp (PLS-SXE 300+) | Photothermal (35,000) | 300 | 20 | 0.97 | [92] |
In 2020, Zhou et al. reported the development of a Cu-Ru single-atom alloy plasmonic photocatalyst designed for the light-driven DRM reaction[18]. The conceptual catalyst structure developed here consists of Cu NPs acting as the light-absorbing antenna with single-atom ruthenium sites functioning as the active catalytic reactor sites [Figure 14A]. The term “light-absorbing antenna” refers to the role that Cu NPs play in capturing and concentrating light energy during the reaction. Single Cu NPs undergo a phenomenon called LSPR when exposed to light particularly in the visible range. This occurs when the light waves interact with the free electrons in the copper particles, causing them to oscillate. These oscillations amplify the local electromagnetic field near the surface of the NPs. When multiple Cu NPs are arranged in close proximity, the LSPR effect can be amplified due to interactions between the electromagnetic fields of neighboring particles. This interaction leads to the formation of "hot spots" where the local electromagnetic field is much stronger than in areas without such interactions. This is similar to how a traditional radio antenna amplifies a signal by adjusting its size and position relative to the incoming waves. Theoretically, by controlling the distance between Cu NPs and their relative distribution, the LSPR effect can be tuned to control light absorption characteristics of the photothermal catalyst. In this regard, several catalysts were synthesized by adjusting the ratio of Cu to Ru using co-precipitation on a MgO-Al2O3 composite support. Under focused white light illumination (19.2 W cm-2), this photocatalyst demonstrated remarkable efficiency and stability at room temperature, eliminating the need for high temperatures (700-1,000 °C) typically required for DRM. The study achieved a turnover frequency of 34 mol H2 per mol Ru per second, and the catalyst was stable for over 50 h of operation without noticeable coke formation [Figure 14B]. Previous studies on bimetallic plasmonic photocatalysts such as Au-Pd and Au-Pt have been limited by reduced photocatalytic activity due to the attenuation of plasmon resonance caused by high Pd or Pt loading. In addition, these systems have been faced with coking problems, which leads to catalyst deactivation. However, Cu-Ru single-atom alloys overcome these limitations by optimizing the catalyst structure. The researchers found that individual dispersion of ruthenium atoms on the surface of Cu NPs retained plasmonic activity and enhanced catalytic performance. The combination of hot carriers generated from Cu NPs and single-atom Ru active sites significantly reduced the energy barrier for methane decomposition which leads to a more efficient reaction with an excellent H2:CO (1:1) ratio [Figure 14C]. The paper also showed that the catalyst resists coking, which is attributed to the isolated nature of Ru atoms preventing carbon accumulation. This group also tries to explain the high H2:CO ratio with DIET (Desorption Induced by Electronic Transitions) mechanism that can quickly remove the adsorbed H atoms on surfaces, which will be further discussed later.
Figure 14. Dry reforming of methane reaction catalyzed by platinum group metal (PGM)-based photothermal catalytic materials: (A) Schematic of a Cu-single-atom Ru surface alloy catalyst with the dry reforming reactants and products. Reaction rate and long-term stability and selectivity of photocatalytic DRM over CuRu alloy catalyst. (B) Stability of photocatalytic methane dry reforming reaction on a Cu19.9Ru0.1 sample under 19.2 W·cm-2 white light illumination. (C) Schematics of enhanced selectivity and stability in photocatalysis via the DIET mechanism. Reproduced with permission from Ref.[18].
In 2022, Yang et al. reported Rh-based catalyst design for photothermal DRM that dispersed on Ce-W mixed oxide support[83]. They focused on efficiently combining photothermal and photoelectric effects. Rh NPs (NPs) are uniformly dispersed on the CexWO3 nanosheets synthesized by a solvothermal method
Figure 15. Dry reforming of methane reaction catalyzed by platinum group metal (PGM)-based photothermal catalytic materials: (A) A SEM image and high-resolution HAADF-STEM image of as-prepared CexWO3 and a unit cell of CexWO3 projected along c axis was superimposed on it. (B) Rates of H2 and CO produced using different samples in a batch reactor and cycle experiments of DRM over Rh/CexWO3 catalyst in a batch reactor. Rates of H2 and CO at 300 °C with and without irradiation in a flow reactor (gas flow rate:
In 2023, Li et al. emphasized the significance of modulating strong metal-support interaction (SMSI) to improve the light-driven DRM reaction using Ru/TiO2 catalyst [Figure 16A][88]. They explored how SMSI can affect the electron transfer between Ru NPs and TiO2 support to suppress the catalytic activity in photothermal reactions. They prepared two variants of Ru/TiO2 catalysts, one with suppressed SMSI effect (Ru/TiO2-H2) and the other with normal SMSI effect (Ru/TiO2). Here, Ru/TiO2-H2 was prepared simply by pre-reducing TiO2 under H2 before impregnating Ru on it. The Ru/TiO2-H2 catalyst exhibited significantly improved performance, with 46 times higher CO2 conversion than the typical Ru/TiO2 catalyst [Figure 16B]. The study explains that this performance difference is due to the ability of the Ru/TiO2-H2 catalyst to better facilitate electron transfer to surface oxygen vacancies, thereby promoting CO2 activation and improving methane decomposition step.
Figure 16. Dry reforming of methane reaction catalyzed by platinum group metal (PGM)-based photothermal catalytic materials: (A) Illustration of the synthesis process of Ru/TiO2-H2 and Ru/TiO2. (B) Average production rates of H2 and CO over Ru, Pt, and Ni/TiO2 with controlled SMSI under focused full-spectrum light irradiation. Reproduced with permission from Ref.[88] (C) Schematic illustration of the synthesis process for TNT and Pt/TNT samples. Reproduced with permission from Ref.[92] (D) Schematic illustration of the proposed processes of DRM, which involves the carbon exchange between reactants and the oxygen exchange between CO2 and the lattice oxygen of supports. (E) Light-driven DRM yields of H2 and CO by mono- and quinary-component metal-loaded SrTiO3 catalysts and HEA-loaded catalysts. Reproduced with permission from Ref.[86].
In 2024, He et al. reported a method to precisely control dispersed platinum species on TiO2 nanotube support[92]. They designed a series of catalysts containing both single-atom Pt sites and sub-nanoclusters of Pt on the TiO2 nanotube surface [Figure 16C]. These Pt species were carefully controlled via a spatially confined approach, which ensured highly dispersed Pt species and prevented agglomeration. The researchers obtained various compositions of Pt species by changing the calcination conditions. The most optimized catalyst, consisting of a mixture of Pt single-atom and sub-nanoclusters, exhibited a syngas production rate of 34.41 mol gPt-1 h-1 and an apparent quantum yield of 9.1% at a wavelength of 365 nm under photothermal conditions at 300 °C. The major improvement of this work over previous photothermal DRM lies in the synergistic interaction between Pt single sites and Pt sub-nanoclusters. Single-atom catalysts are known for their high atom utilization efficiency but often struggle with bimolecular reactions such as DRM due to the lack of active sites. On the other hand, Pt sub-nanoclusters provide a larger number of active sites but tend to incur undesirable side effects such as excessive CO adsorption. By combining both single atoms and sub-nanoclusters, the researchers created a balance where single atoms activate CH4 while sub-nanoclusters activate CO2, leading to a better performance in DRM reaction. In the same year,
Nonthermal effect of light on H2/CO ratio
Many researchers report the results of photothermal DRM reactions using various catalytic materials as previously mentioned, but it is difficult to directly compare the reaction rates since each reaction system and method of light irradiation is different. However, one common phenomenon is that the H2:CO ratio is observed to be much higher when light energy is present compared to dark experiments. This implies that light energy not only simply increases the reaction rate but also plays a significant role in increasing the H2 yield. In 2022, Takami et al. investigated the photothermal DRM over Rh NPs supported on Al2O3 using a combination of Vis and NIR light irradiation, with the primary objective being to understand how photoinduced heating at the nanoscale during DRM affects the catalytic activity and local temperature of Rh NPs [Figure 17A][90]. The researchers used operational dispersive X-ray absorption spectroscopy (DXAS) to capture the real-time changes in temperature and chemical state of the Rh particles. The DXAS technique can directly measure the local temperature of Rh NPs exposed to light, enabling them to precisely track the transient heating effect and its impact on the reaction. To elaborate in more detail, Debye-Waller factor (σ) is utilized for temperature determination from DXAS. Debye-Waller factor is a parameter that describes the effects of structural and vibrational disorder on extended X-ray absorption fine structure (EXAFS) signals. This factor is related to the mean-square fluctuations in interatomic distances, and is also known as the B factor, which expresses the magnitude of thermal vibrations of atoms. As the magnitude of thermal vibrations of atoms increases with temperature, the calculated Debye-Waller factor from EXAFS signal also increases. Fortunately, it has been experimentally demonstrated that the square of the Debye-Waller factor (σ2) and temperature are well fitted by linear function. This is why the Debye-Waller factor can be used as indicators to estimate the temperature of local environment of metal NPs. Interestingly, the local temperature of Rh NPs measured by operando DXAS reached values significantly higher than the bulk reactor temperature. The local temperature of Rh NPs measured by the light source was up to 630 K, while the bulk reactor temperature measured by the thermocouple was significantly lower, approximately 473 K. This discrepancy between the local and bulk temperatures demonstrates how light directly heats Rh NPs, creating local hot spots that drive the dry reforming reaction more efficiently even at relatively low bulk reactor temperatures. Overall, this local heating effect is directly related to the high CH4 and CO2 conversions achieved under photothermal conditions. In addition, they found that smaller Rh NPs exhibited faster temperature responses and higher catalytic activity, suggesting that NP size and local heat generation have correlated with each other and are important for optimizing DRM performance. Another observation was that the H2 fraction under photoirradiation was consistently higher than that of conventional thermal DRM reactions, showing a presence of strong nonthermal effect of light on the product selectivity. In 2021, Rao et al. investigated the role of nonthermal photoeffects in the photothermal DRM over Ni/Ga2O3 catalysts is investigated [Figure 17B][89]. The researchers performed both photothermal and thermal catalytic tests in the temperature range of 616-664 K to explore how light irradiation affects the H2/CO ratio during DRM. The catalysts were synthesized using a wet impregnation method with 5 wt% Ni loading. They measured the formation rates of H2 and CO, and performed in situ DRIFTS to investigate the species adsorbed on the catalyst surface under light irradiation. They suggested reasonable explanation on how the light irradiation can promote H2 formation by distinct nonthermal effects that alter the surface reaction. The suppression of RWGS allows more hydrogen to form, thus increasing the H2/CO ratio. To summarize, the work demonstrates that light irradiation not only heats the catalyst but also introduces nonthermal effects that significantly enhance H2 production and alter the reaction pathways in DRM, achieving a more favorable product distribution.
Figure 17. (A) Schematic diagram of the photothermal reactor used in operando DXAS under concentrated visible and near-infrared light to evaluate the temperature of the catalytic active sites of Rh NPs on Al2O3. Reproduced with permission from Ref.[90]. (B) HRTEM image and EDS mapping of the Ni/Ga2O3 catalyst. DRM catalytic performances over Ni/Ga2O3 catalyst at different temperatures. Schematic diagram of the photothermal catalytic mechanism of the Ni/Ga2O3 catalyst. Reproduced with permission from Ref.[89].
CONCLUSION AND OUTLOOK
In summary, the current understanding of the photothermal catalytic mechanisms was described on how light irradiation can affect catalytic reactions including plasmonic light absorption, hot carrier generation, photon to phonon conversion and local heating effect. The initial step in photothermal processes is the design of plasmonic light absorbers that can efficiently harness light. Therefore, the development of plasmonic photothermal catalysts requires a strategic focus on material selection, structural optimization, and scalable synthesis. Combining plasmonic metals with dielectric materials can enhance charge separation and heat retention. Simultaneously, tailoring NP size, shape, and hybrid structures allows precise control over light absorption and catalytic activity. Scalable synthesis methods, such as hydrothermal growth and galvanic replacement, offer practical pathways for large-scale applications.
Previous studies on photocatalysts have often overlooked the role of localized heat effects, focusing instead on light absorption, charge carriers, and catalytic performance. To develop more effective photothermal catalysts, it is crucial to distinguish and analyze the separate contributions of photocatalysis from charge carriers and thermal effects from local heating. However, monitoring nanoscale surface temperatures remains challenging due to limitations in conventional measurement techniques. Advancing non-intrusive methods and accurate computational models to estimate local temperature gradients will be key to optimizing the synergistic effects of these phenomena.
Recent photothermal catalysts are categorically summarized by focusing on CO2 hydrogenation and DRM reactions. The works on CO2 hydrogenation demonstrate significant progress and offer directions for developing efficient catalysts. One of the challenges to be addressed is the development of economically viable catalysts. In CO2 methanation reactions, there is a tendency to use Ru extensively. To overcome this, research has been conducted using Ni, but issues such as coking and sintering remain unresolved. The problems observed in thermal catalysis persist in the field of photothermal catalysis as well. In the RWGS reaction, considerable research has been done on catalyst structure design, which helps understand the photothermal effect and the impact of structural changes on reactivity. However, the challenge is the limited research on the use of real solar energy for photothermal catalysts. Therefore, future studies require additional discussion on how to efficiently utilize the actual solar energy in practical catalytic systems. In methanol production, the reaction tends to show a much slower reaction rate and lower yield compared to other processes, and there is still a lack of information on the mechanistic details. Additionally, the use of high-intensity light sources poses a limitation for commercial applications. As the understanding of mechanisms and photothermal effects for RWGS and CO2 methanation reactions has improved, future research should focus on practical applications.
Catalyst stability during prolonged reactions is also a major challenge in photothermal catalysis. Long-term exposure to high temperatures, reactive intermediates, and frequent thermal cycling can lead to deactivation through coking, sintering, and poisoning. To address these issues, future research should focus on strategies such as alloying, surface modification, and the incorporation of robust support materials to improve catalyst resistance. Also, focusing on understanding catalyst deactivation mechanisms is essential for addressing long-term operational challenges. Potential solutions include designing catalysts with self-regenerating properties, optimizing reaction conditions to prevent coke formation, and employing in situ regeneration to restore activity during operation. Scalability of photothermal systems presents another critical challenge because Lab-scale experiments often use high-intensity light sources such as lasers or concentrated solar setups, which are not feasible for industrial applications. Efficient reactor designs, uniform light distribution, and innovative optical materials will be needed to transition photothermal systems from research to industrial scales. In addition, emerging approaches such as machine learning (ML) for catalyst optimization are poised to revolutionize the design of photothermal catalysis. Nowadays, ML can analyze vast experimental and computational datasets to predict catalyst performance and accelerate the discovery of new materials. Similarly, advanced in situ techniques, such as operando spectroscopy, X-ray absorption, and ultrafast microscopy, provide insights into photothermal effects at the molecular level. These enable real-time monitoring of photothermal catalyst structure and reaction intermediates on the surface. These tools are indispensable for elucidating reaction mechanisms and tuning catalyst properties.
Lastly, tandem catalyst design is expected to be a promising approach to improve the efficiency of photothermal catalytic processes. Combining different catalysts that work optimally at different reaction stages can facilitate sequential reactions, thereby improving the overall process yield and selectivity. This strategy can effectively address the limitations associated with single-purpose catalysts, normally with narrow operating windows and limited activity. For example, integrating a photothermal catalyst that produces hydrogen in a photothermal water-splitting reaction with a subsequent catalyst for CO2 reduction can significantly enhance the production of valuable fuels and chemicals. Likewise, developing a tandem catalytic system that integrates the syngas production and Fisher-Tropsch reaction for hydrocarbon fuel production can minimize the total energy required for storage and transportation of syngas. In this way, tandem systems can maximize the synergy between photothermal reactions by reducing energy consumption and increasing process yield since the output of one reaction step becomes the input for the next catalytic reaction. Thus, future research should focus on optimizing the interaction between different photothermal catalysts in a tandem catalytic system and exploring how different types of light energy can affect each catalytic performance. Such an approach may pave the way for more sustainable and efficient catalytic processes, ultimately contributing to realizing commercial applications of photothermal catalysis.
DECLARATIONS
Authors’ contributions
Conceptualization and funding acquisition, writing-review & editing, and supervision, guidance for the review, including language checking and polishing: Song, I.; Dong, W.
Investigation, visualization, and writing-original draft: Choi, S. H.; Song, I.; Dong, W. J.
Availability of data and materials
Not applicable.
Financial support and sponsorship
This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and granted financial resources from the Ministry of Trade, Industry & Energy, Republic of Korea, 20213091010020; National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT), RS-2024-00455924.
Conflicts of interest
All authors declared that there are no conflicts of interest.
Ethical approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
Copyright
© The Author(s) 2025.
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Choi, S. H.; Song, I.; Dong, W. J. Recent progress of photothermal catalysts for carbon dioxide conversion. Energy Mater. 2025, 5, 500062. http://dx.doi.org/10.20517/energymater.2024.227
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