Unlocking solar-derived CO2 conversion to hydrocarbons over plasmonic Cu-Fe tandem catalysts under industrially relevant conditions

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Abstract Summary

This article describes a photo-thermal CO₂ hydrogenation system based on a potassium-promoted Cu–Fe tandem catalyst, not a molecular photocatalytic CO₂ reduction system. The reported chemistry converts CO₂ and H₂ into CO and hydrocarbons under continuous-flow illumination and external heating. The best-performing material, 2K Cu₂Fe₁, gave high selectivity toward C₂₊ hydrocarbons under elevated pressure, with the article attributing the performance to tandem catalysis in which Cu promotes reverse water-gas shift chemistry and in situ formed χ-Fe₅C₂ promotes Fischer–Tropsch-type carbon–carbon coupling. Mechanistic studies support a major role for light-induced non-thermal effects together with localized heating and accelerated iron carburization.

Advances and Special Progress

The article presents several advances within photo-thermal CO₂ hydrogenation chemistry.

First, it reports strong hydrocarbon formation under conditions described as industrially relevant by the article itself: continuous flow, elevated pressure, and moderate external heating combined with illumination. Under 250 °C and 20 bar, the catalyst reached 77.4% selectivity to C₂₊ hydrocarbons and a C₂-C₅ hydrocarbon yield of 5.04 mmol g⁻¹ h⁻¹.

Second, the catalyst uses earth-abundant transition metals, Cu and Fe, rather than noble-metal-centered molecular photocatalytic designs. The article emphasizes a tandem architecture in which the two metals play distinct catalytic roles.

Third, potassium promotion improved CO₂ adsorption, CO adsorption, and chain growth probability relative to the unpromoted material, giving an optimized 2 wt% K loading.

Fourth, the study provides unusually detailed mechanistic evidence for a photo-thermal hydrocarbon-forming system. Direct versus indirect illumination experiments, in situ DRIFTS, in situ XRD, CO-TPR, CO₂-TPSR, transient photocurrent, TRPL, and wavelength-dependent catalysis were used to distinguish thermal from non-thermal contributions and to connect Cu plasmon excitation with faster formation of χ-Fe₅C₂.

Fifth, the catalyst showed stable performance during repeated light on/off cycling after activation, supporting structural robustness under the tested conditions.

Additional Remarks

This article is chemically important, but it does not describe a molecular photocatalytic CO₂ reduction system. Instead, it reports a heterogeneous plasmonic photo-thermal catalytic process that couples light absorption, heat generation, reverse water-gas shift reactivity, and Fischer–Tropsch-type hydrocarbon synthesis.

That distinction matters for interpretation. In molecular photocatalysis, one usually analyzes discrete photosensitizers, sacrificial electron donors, quenching pathways, and molecular catalyst redox states in solution. None of that framework is the primary one here. The present system operates in the gas phase with H₂ as reductant, elevated pressure, external heating, and a solid catalyst bed under Xe-lamp irradiation.

A strength of the work is that it addresses carbon–carbon bond formation rather than only C₁ products. Another strength is the mechanistic focus on active phase evolution, especially the conversion of Fe to χ-Fe₅C₂. A limitation for a reader specifically seeking molecular photocatalytic CO₂ reduction data is that common molecular-photocatalysis descriptors such as photosensitizer identity, sacrificial donor concentration, quenching constants, and product TON values are not part of the reported system.

Content of the Published Article in Detail

The article studies the direct conversion of CO₂ with H₂ to hydrocarbons using a solid Cu–Fe catalyst under combined illumination and heating. The catalyst family was prepared by co-precipitation, calcination, reduction, potassium impregnation, and passivation. Different Cu/Fe ratios and potassium loadings were screened, and 2K Cu₂Fe₁ was identified as the best composition.

The reaction setup was a continuous-flow reactor equipped with a quartz window and irradiated with a 300 W Xe lamp. The catalyst mass was 50 mg. A thermocouple was placed about 1 mm below the catalyst surface. The feed gas had H₂/CO₂ = 4:1, with N₂ as an internal standard. Most key tests discussed in the main text used 250 °C external heating, 4 W cm⁻² light intensity, a gas hourly space velocity of 12000 mL g⁻¹ h⁻¹, and pressures between 10 and 20 bar. Additional tests used lower space velocity, lower temperature, indirect illumination, or different light intensities.

Catalytically, monometallic Fe showed some ability to form hydrocarbons, while monometallic Cu was nearly fully selective to CO. This comparison supports a tandem interpretation. According to the article, Cu mainly promotes the reverse water-gas shift reaction, producing CO from CO₂. The Fe-containing phase then converts CO-derived intermediates into hydrocarbons through Fischer–Tropsch-type chemistry. After reaction, the active Fe phase was identified mainly as χ-Fe₅C₂, formed in situ from metallic Fe during catalyst activation and operation.

The article presents several lines of evidence for this tandem picture. Before reaction, the reduced catalyst contained metallic Cu and metallic Fe, with a thin oxidized surface layer attributed to passivation. After reaction, diffraction, Mössbauer spectroscopy, TEM, XPS, and TGA-TPH supported conversion of the Fe component to χ-Fe₅C₂. No Fe₃C was detected by the TPH analysis described in the article.

Potassium promotion was examined by CO₂-TPD and CO-TPD. These measurements showed stronger CO₂ and CO adsorption for the K-promoted catalyst. The article interprets this as beneficial for generating CO-derived intermediates and shifting selectivity toward longer hydrocarbons, partly by moderating hydrogenation behavior.

Mechanistic interpretation was further developed with direct and indirect illumination experiments. For indirect illumination, a thin Ti₂O₃ layer was placed above the catalyst bed to provide similar photo-thermal heating while blocking direct photochemical excitation of the catalyst. Under direct illumination, CO₂ conversion increased linearly with light intensity from 2.8 to 4.0 W cm⁻², which the article interprets as characteristic of dominant non-thermal contributions with a minor thermal contribution. Indirect illumination gave substantially lower conversion, supporting the importance of direct light-induced effects. At higher intensity, the non-thermal contribution appeared to plateau.

The article also tested lower temperature operation at 200 °C. With light, the catalyst still produced hydrocarbons and formed χ-Fe₅C₂. Without direct light, the product stream was mainly CO and metallic Fe remained dominant. This is an important observation because it links illumination to accelerated formation of the carbide phase needed for Fischer–Tropsch-type hydrocarbon synthesis.

In situ DRIFTS was used to follow surface intermediates during reaction. Under dark conditions, the spectra showed bicarbonate and carbonate species at lower temperature, then formate species at higher temperature, followed by adsorbed CO, and finally hydrocarbon-related bands at 250 °C. The article therefore supports the sequence: CO₂ adsorption as bicarbonate/carbonate → hydrogenation to formate → decomposition to CO → further hydrogenation and C–C coupling to hydrocarbons.

Comparable DRIFTS experiments on monometallic Cu and Fe clarified their different roles. On Cu, carbonate, formate, and adsorbed CO were seen, but no strong methane signal, consistent with Cu functioning mainly in reverse water-gas shift chemistry. On Fe, similar oxygenated intermediates were seen together with weaker hydrocarbon-related bands, consistent with Fe-containing phases supporting hydrocarbon formation.

Under illumination, the same classes of intermediates were observed in DRIFTS, but they appeared at lower temperature and with higher intensity. The article interprets this as evidence that light does not change the basic reaction pathway, but accelerates the formation and evolution of key intermediates. The comparison of peak intensities for bicarbonate, adsorbed CO, and olefin-related species further supported faster intermediate buildup under light.

In situ XRD directly tracked iron phase evolution. Under illumination, the metallic Fe diffraction peak diminished at lower temperature, and χ-Fe₅C₂ appeared earlier than under dark conditions. CO-TPR also showed lower-temperature events under illumination, including earlier CO uptake and lower-temperature Fe-phase transformation associated with carbide formation. CO₂-TPSR showed similarly lower temperatures for CO₂ consumption and product formation under light.

To explain these effects, the article considers three possibilities and argues that the most plausible one is plasmonic Cu-mediated promotion of Fe carburization. UV–vis spectroscopy showed a metallic Cu plasmon band near 505 nm. Transient photocurrent and TRPL measurements indicated more efficient charge separation or transfer in the bimetallic Cu–Fe catalyst than in the monometallic materials. Additional wavelength-dependent catalysis showed the highest activity under 500 nm irradiation, consistent with the Cu plasmon response. Taken together, the data support the article’s proposal that plasmonic excitation of Cu accelerates reverse water-gas shift chemistry and promotes interfacial electronic interactions with Fe, lowering the apparent barrier to χ-Fe₅C₂ formation.

The main products were CO, CH₄, C₂-C₅ olefins, C₂-C₅ paraffins, and C₆₊ hydrocarbons. At higher pressure, CO selectivity decreased and hydrocarbon formation increased. Product distributions from C₃ to C₅ followed the Anderson–Schulz–Flory distribution according to the article. The olefin/paraffin ratio decreased at higher pressure and lower space velocity, which the authors interpret as increased hydrogenation and chain termination.

Because this is not a molecular photocatalytic system, the article does not discuss a discrete photosensitizer, reductive or oxidative quenching cycle, a sacrificial electron donor, or solution-phase reduced catalyst intermediates. Instead, light acts through the solid catalyst itself, especially through Cu-associated plasmonic excitation and photo-thermal energy delivery.

Catalyst

The catalyst is a heterogeneous potassium-promoted bimetallic Cu–Fe material, with the optimized composition designated 2K Cu₂Fe₁. It is not molecular, not immobilized molecularly, and not a discrete coordination complex.

Before reaction, the reduced catalyst contains metallic Cu and metallic Fe, with minor surface oxidation attributed to passivation. During reaction, the Fe phase is converted in situ mainly to χ-Fe₅C₂, which the article identifies as the active phase for Fischer–Tropsch-type hydrocarbon formation. Cu is assigned the main role in reverse water-gas shift chemistry, while Fe carbide is assigned the main role in C–C coupling and hydrocarbon production.

Potassium acts as a promoter that enhances CO₂ adsorption, strengthens CO adsorption, and favors chain growth toward C₂₊ hydrocarbons. A 2 wt% K loading gave the best performance among the tested values. The catalyst also showed stable composition and performance during the reported cycling experiment after activation.

Photosensitizer

No separate molecular or heterogeneous photosensitizer is reported.

Instead, the light-harvesting function is attributed to the catalyst itself, especially the metallic Cu component, which shows a plasmon absorption band at about 505 nm. The article proposes that Cu acts as the photoactive plasmonic component, generating hot carriers and localized heating under illumination. These effects are linked to enhanced reverse water-gas shift activity and to promotion of Fe carburization at the Cu–Fe interface.

Because there is no discrete photosensitizer, the article does not report excited-state quenching by a sacrificial donor, oxidative or reductive quenching cycles, or separate photosensitizer stability data.

Investigation

	cat	cat conc [µM]	PS	PS conc [mM]	e-D	e-D conc [M]	.	.	solvent A	.	.	.	additives	.	.	.	.	λexc [nm]	.	TON CO	.	.	TON CH4	TON H2	.	TON HCOOH	.	.
1.	2K Cu2Fe1	not reported	not reported	not reported	H2O	not reported			not reported				Molecule:100493					not reported		not reported			not reported	not reported		not reported			"not reported" is not a number. "wt%" is not declared as a valid unit of measurement for this property. "H2/CO2 = 4:1" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property.
2.	2K Cu2Fe1	not reported	not reported	not reported	H2O	not reported			not reported				Molecule:100493					not reported		not reported			not reported	not reported		not reported			"not reported" is not a number. "wt%" is not declared as a valid unit of measurement for this property. "H2/CO2 = 4:1" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property.
3.	2K Cu2Fe1	not reported	not reported	not reported	H2O	not reported			not reported				Molecule:100493					not reported		not reported			not reported	not reported		not reported			"not reported" is not a number. "wt%" is not declared as a valid unit of measurement for this property. "H2/CO2 = 4:1" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property.
4.	2K Cu2Fe1	not reported	not reported	not reported	H2O	not reported			not reported				Molecule:100493					not reported		not reported			not reported	not reported		not reported			"not reported" is not a number. "wt%" is not declared as a valid unit of measurement for this property. "H2/CO2 = 4:1" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property.
5.	2K Cu2Fe1	not reported	not reported	not reported	H2O	not reported			not reported				Molecule:100493					not reported		not reported			not reported	not reported		not reported			"not reported" is not a number. "wt%" is not declared as a valid unit of measurement for this property. "H2/CO2 = 4:1" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property.
6.	2K Cu2Fe1	not reported	not reported	not reported	H2O	not reported			not reported				Molecule:100493					not reported		not reported			not reported	not reported		not reported			"not reported" is not a number. "wt%" is not declared as a valid unit of measurement for this property. "H2/CO2 = 4:1" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property.
7.	2K Cu2Fe1	not reported	not reported	not reported	H2O	not reported			not reported				Molecule:100493					not reported		not reported			not reported	not reported		not reported			"not reported" is not a number. "wt%" is not declared as a valid unit of measurement for this property. "H2/CO2 = 4:1" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property.
8.	2K Cu2Fe1 with Ti2O3 cover	not reported	not reported	not reported	H2O	not reported			not reported				K/Ti2O3					not reported		not reported			not reported	not reported		not reported			"not reported" is not a number. "wt%/notreported" is not declared as a valid unit of measurement for this property. "H2/CO2 = 4:1" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property.
9.	2K Cu2Fe1	not reported	not reported	not reported	H2O	not reported			not reported				Molecule:100493					not reported		not reported			not reported	not reported		not reported			"not reported" is not a number. "wt%" is not declared as a valid unit of measurement for this property. "H2/CO2 = 4:1" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property.
10.	2K Cu2Fe1	not reported	not reported	not reported	H2O	not reported			not reported				Molecule:100493					not reported		not reported			not reported	not reported		not reported			"not reported" is not a number. "wt%" is not declared as a valid unit of measurement for this property. "H2/CO2 = 4:1" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property.
11.	2K Cu2Fe1	not reported	not reported	not reported	H2O	not reported			not reported				Molecule:100493					not reported		not reported			not reported	not reported		not reported			"not reported" is not a number. "wt%" is not declared as a valid unit of measurement for this property. "H2/CO2 = 4:1" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property.
12.	2K Cu2Fe1	not reported	not reported	not reported	H2O	not reported			not reported				Molecule:100493					not reported		not reported			not reported	not reported		not reported			"not reported" is not a number. "wt%" is not declared as a valid unit of measurement for this property. "H2/CO2 = 4:1" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property.
13.	2K Cu2Fe1 with Ti2O3 cover	not reported	not reported	not reported	H2O	not reported			not reported				K/Ti2O3					not reported		not reported			not reported	not reported		not reported			"not reported" is not a number. "wt%/notreported" is not declared as a valid unit of measurement for this property. "H2/CO2 = 4:1" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property.
14.	2K Cu2Fe1 with Ti2O3 cover	not reported	not reported	not reported	H2O	not reported			not reported				K/Ti2O3					not reported		not reported			not reported	not reported		not reported			"not reported" is not a number. "wt%/notreported" is not declared as a valid unit of measurement for this property. "H2/CO2 = 4:1" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property.
15.	2K Cu2Fe1 with Ti2O3 cover	not reported	not reported	not reported	H2O	not reported			not reported				K/Ti2O3					not reported		not reported			not reported	not reported		not reported			"not reported" is not a number. "wt%/notreported" is not declared as a valid unit of measurement for this property. "H2/CO2 = 4:1" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property.
16.	2K Cu2Fe1 with Ti2O3 cover	not reported	not reported	not reported	H2O	not reported			not reported				K/Ti2O3					not reported		not reported			not reported	not reported		not reported			"not reported" is not a number. "wt%/notreported" is not declared as a valid unit of measurement for this property. "H2/CO2 = 4:1" is not in the list (nitrogen, argon, CO2, CO, O2, H2, none) of allowed values for the "Feedstock gas" property.

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