Photo-thermal Catalytic CO2 Methanation by RuOx@MIL-101(Cr) with 9.2% Apparent Quantum Yield under Visible Light Irradiation

Abstract

Solar-assisted gaseous CO₂ hydrogenation to CH₄ is a potential strategy for favoring the transition to net zero emissions. Here, we report the development of a series of efficient metal-organic frameworks with MIL-101(Cr or Fe) topology decorated with RuO_x nanoparticles (ca. 0.2-2 wt %) as heterogeneous photocatalysts for the selective methanation of CO₂ by H₂ under simulated sunlight irradiation. The activity of RuO_x(1 wt %)@MIL-101(Cr) is between 3 and 50 times higher than related MOF-based photocatalysts under similar reaction conditions. Among the different photocatalysts, the optimized RuO_x(2 wt %)@MIL-101(Cr) photocatalyst showed 98.1% CO₂ conversion with 98.8% CH₄ selectivity reaching a production rate of 7.85 mmol g^-1 h^-1 with 720 mW cm^-2 at 200 °C. Further, this photocatalyst exhibited a record apparent quantum yield of 9.2% at 600 nm and 200 °C after subtracting thermal activity contribution compared to any previous MOF- or other heterogeneous-based photocatalyst reported so far. The photocatalyst retained its activity and integrity upon reuse for about 110 h. Transient photocurrent, electrochemical impedance, photoluminescence, and laser flash photolysis spectroscopies together with additional photocatalytic experiments suggest the occurrance of dual photochemical and photothermal reaction pathways. The photocatalytic CO₂ methanation reaction mechanism was further investigated using operando Fourier transform infrared spectroscopy.

Keywords: CO2 methanation; MIL-101(Cr); RuOx nanoparticles; metal−organic frameworks; photo-thermal catalysis.

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(a) PXRD of (a1) simulated MIL-101, (a2) MIL-101­(Fe), (a3) RuO _x (2 wt %)@MIL-101­(Fe), (a4) MIL-101­(Cr), (a5) RuO _x (0.2 wt %)@MIL-101­(Cr), (a6) RuO _x (0.5 wt %)@MIL-101­(Cr), (a7) RuO _x (1 wt %)@MIL-101­(Cr), and (a8) RuO _x (2 wt %)@MIL-101­(Cr). (b) SEM of MIL-101­(Cr). (c) TEM image of RuO _x (2 wt %)@MIL-101­(Cr) and (d) TEM image of RuO _x NPs facets in RuO _x (2 wt %)@MIL-101­(Cr).

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(a) XPS survey, (b) C 1s, (c) O 1s, (d) Cr 2p, and (e) Ru 3p of MIL-101­(Cr) with different RuO _x loadings. Legend panels: 1, 2, 3, 4, and 5 represent loadings of 0, 0.2, 0.5, 1, and 2 wt % of RuO _x over MIL-101­(Cr), respectively.

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(a) UV–vis DRS of RuO _x @MIL-101­(Cr) solids and (b) estimated energy band level diagrams of the photocatalysts under study. Legend panels: (a1), (a2), (a3), (a4), and (a5) represent loadings of 0, 0.2, 0.5, 1, and 2 wt % of RuO _x NPs over MIL-101­(Cr), respectively.

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(a) Photoassisted catalytic CO₂ methanation using MIL-101­(Cr) solid loaded with RuO _x NPs at (a1) 2, (a2) 1, (a3) 0.5, and (a4) 0.2 wt %. (b) Photocatalytic CO₂ methanation using RuO _x NPs (2 wt %) supported on (b1) Cr₂O₃, (b2) γ-Fe₂O₃, or (b3) MIL-101­(Fe). Reaction conditions: Photocatalyst (15 mg), P _H2 = 1.2 bar, P _CO2 = 0.3 bar, and simulated concentrated sunlight irradiation (420 mW cm^–2), 200 °C.

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(a) Influence of reaction temperature on the photoassisted catalytic CH₄ generation using RuO _x (2 wt %)@MIL-101­(Cr) under concentrated simulated sunlight irradiation at 420 mW cm^–2 and (b) 720 mW cm^–2 and (c) under dark conditions. Reaction conditions: RuO _x (2 wt %)@MIL-101­(Cr) (15 mg), P _H2 = 1.2 bar, P _CO2 = 0.3 bar, simulated concentrated sunlight irradiation, and temperature as indicated.

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(a) Activity and selectivity of RuO _x (2 wt %)@MIL-101­(Cr) as a function of temperature in dark and under visible light irradiation under flow conditions. The inset shows gas chromatograms of the reaction gas phase under the same conditions. (b) MS evolution of CH₄ and H₂O during the CO₂ photomethanation reaction at 200 °C with RuO _x (2 wt %)@MIL-101­(Cr) under flow conditions. Reaction conditions: RuO _x (2 wt %)@MIL-101­(Cr) (20 mg), total pressure (1 bar), CO₂:H₂ (2:8 cm³ min^–1) gas mixture, and visible light irradiation (>390 nm; 70 mW cm^–2).

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(a) Comparison of photocatalytic CH₄ production from gaseous CO₂ hydrogenation as a function of simulated sunlight irradiance at 200 °C using RuO _x (1 wt %)@MIL-101­(Cr) (★, green color, this work) and previously reported MOF-based photocatalysts. Legend: RuO _x (1 wt %)@UiO-66­(Zr/Ti)-NO₂ (⧫, orange color), RuO _x (1 wt %)@UiO-66­(Zr/Ce/Ti) (■, pink color; ●, violet color; ▼, black color), RuO _x (1 wt %)@UiO-66­(Zr/Ti) (▼, blue color), RuO _x (1 wt %)@UiO-66­(Zr/Ce) (▼, red color), RuO _x (1 wt %)@UiO-66­(Ce) (▼, light blue color), RuO _x (1 wt %)@UiO-66­(Zr) (▼, brown color), RuO _x (1 wt %)@MIL-125­(Ti)-NH₂ (▲, light brown color). (b) AQY results of RuO _x (2 wt %)@MIL-101­(Cr). Legend: as a function of the wavelengths and light intensities at 200 °C. Orange, green, and blue balls refer to experiments carried out at 10, 30, and 40 mW cm^–2, respectively.

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(a) Photocatalytic reusability of RuO _x (2 wt %)@MIL-101­(Cr), (b) its corresponding PXRD pattern before and after five times use, (c) SEM and (d) TEM images of five times used RuO _x (2 wt %)@MIL-101­(Cr) solid, and (e) XPS of C 1s and (f) XPS of Ru 3p. Reaction conditions: RuO _x (2 wt %)@MIL-101­(Cr) (15 mg), P _H2 = 1.2 bar, P _CO2 = 0.3 bar, simulated concentrated sunlight irradiation (420 mW cm^–2), and 200 °C.

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(a) Transient photocurrent response with WE polarized at +0.9 V for (a1) MIL-101­(Cr), (a2) RuO _x (0.2 wt %)@MIL-101­(Cr), and (a3) RuO _x (2 wt %)@MIL-101­(Cr) and (b) Nyquist plots with WE polarized at +0.2 V for (b1) MIL-101­(Cr), (b2) RuO _x (0.2 wt %)@MIL-101­(Cr), (b3) RuO _x (2 wt %)@MIL-101­(Cr), and (b4) RuO _x (2 wt %)@MIL-101­(Cr) under simulated sunlight irradiation. (c, d) Transient photocurrent employing MIL-101­(Cr) (black line) and used RuO _x (2 wt %)@MIL-101­(Cr) (green line) as WE without WE polarization in the absence (c) and presence (d) of H₂. (e) Nyquist plots of MIL-101­(Cr) as a WE and without polarization in the absence (e1) and presence (e2) of H₂. (f) Nyquist plots of used RuO _x (2 wt %)­MIL-101­(Cr) as WE and without polarization in the absence (f1) and presence (f2) of H₂.

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(a) LFP spectra 0.02 μs after the laser pulse for RuO _x (2 wt %)@MIL-101­(Cr) acetonitrile suspensions under Ar (black) and O₂ (gray) and in the presence of CH₃OH (blue). (b) LFP decay traces for RuO _x (2 wt %)@MIL-101­(Cr) at 450 nm in Ar (black) and O₂ (red) and in the presence of CH₃OH (blue). Decay traces for pristine MIL-101­(Cr) (black), RuO _x (0.2 wt %)@MIL-101­(Cr) (red), and RuO _x (2 wt %)@MIL-101­(Cr) (magenta) monitored at (c) 350 and (d) 450 nm under Ar conditions. All measurements were performed at λ_exc = 266 nm. The fitted curves in panels c and d are shown in dashed blue.

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PL spectra of (a) MIL-101­(Cr) and (b) used RuO _x (2 wt %)@MIL-101­(Cr) upon excitation from 380 to 600 nm. (c) PL spectra of MIL-101­(Cr) and used RuOx­(2 wt %)@MIL-101­(Cr). (d) Time-resolved decay of the emissions at 430 and 440 nm of MIL-101­(Cr) and used RuO _x (2 wt %)@MIL-101­(Cr) pulsed by a 343 nm laser, respectively. Inset: zoomed-in view of the first 5 ns time scale.

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(a) Variation of the position of the 1823 cm^–1 structural band of preactivated RuO _x (2 wt %)@MIL-101­(Cr) under H₂ at 200 °C for 1 h versus temperature under dark conditions. (b) Direct surface FT-IR spectra of preactivated RuO _x (2 wt %)@MIL-101­(Cr) in the 1850–1780 cm^–1 region in dark and under irradiation at 30 °C, respectively. Inset: FT-IR spectra of the same band at different temperatures in the dark. Spectra collected under continuous flow of Ar (10 cm³ min^–1). (c) Influence of simulation sunlight irradiance intensity on photocatalytic CH₄ formation. Reaction conditions: photocatalyst (15 mg), irradiation as indicated, and 200 °C. (d) Transient photocurrent intensity using RuO _x (2 wt %)@MIL-101­(Cr).

1. Simplified Illustration of Dual Photochemical and Photothermal Reaction Mechanism during Photocatalytic CO₂ Hydrogenation to CH₄ Using RuO _x @MIL-101­(Cr)

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(a)Operando FT-IR spectra of RuO _x (2 wt %)@MIL-101­(Cr) during photocatalytic CO₂ methanation versus temperature: in the 2400–2250, 2200–1800, and the 1200–1000 cm^–1 vibrational regions. (b) Corresponding FT-IR spectra of the reaction gas phase in dark and after irradiation at steady state. (c) FT-IR spectra of the catalyst with ¹²CO₂ and the corresponding subtracted spectra: ¹³CO₂ – ¹²CO₂ (intensity was multiplied by two for clarification were noted). The * corresponds to the bands formed in the ¹²CO₂ reaction resulting in a negative peak in the (¹³CO₂ - ¹²CO₂) spectrum. (d) FT-IR gas phase spectra of gaseous CH₄ produced with (d1) ¹²CO₂ and (d2) ¹³CO₂. The arrow corresponds to the shift of the FT-IR bands due to the isotopic exchange from ¹²CO₂ to ¹³CO₂. The assignments of the different IR bands are summarized in Tables S5 and S6. (e) Proposed mechanism of the photoassisted CO₂ methanation over RuO _x (2 wt %)@MIL-101­(Cr) based on the assignment of the characteristic FT-IR bands of the different species.