Study of the Photothermal Catalytic Mechanism of CO2 Reduction to CH4 by Ruthenium Nanoparticles Supported on Titanate Nanotubes

Abstract

The Sabatier reaction could be a key tool for the future of the renewable energy field due to the potential of this reaction to produce either fuels or to stabilize H₂ in the form of stable chemicals. For this purpose, a new composite made of ruthenium oxide nanoparticles (NPs) deposited on titanate nanotubes (TiNTs) was tested. Titanate nanotubes are a robust semiconductor with a one-dimensional (1D) morphology that results in a high contact area making this material suitable for photocatalysis. Small ruthenium nanoparticles (1.5 nm) were deposited on TiNTs at different ratios by Na⁺-to-Ru³⁺ ion exchanges followed by calcination. These samples were tested varying light power and temperature conditions to study the reaction mechanism during catalysis. Methanation of CO₂ catalyzed by Ru/TiNT composite exhibit photonic and thermic contributions, and their ratios vary with temperature and light intensity. The synthesized composite achieved a production rate of 12.4 mmol CH₄·g_cat^-1·h^-1 equivalent to 110.7 mmol of CH₄·g_Ru^-1·h^-1 under 150 mW/cm² simulated sunlight irradiation at 210 °C. It was found that photo-response derives either from Ru nanoparticle excitation in the visible (VIS) and near-infrared (NIR) region (photothermal and plasmon excitation mechanism) or from TiNT excitation in the ultraviolet (UV) region leading to electron-hole separation and photoinduced electron transfer.

Keywords: CO2 reduction; Sabatier reaction; methanation; methane; photocatalysis; photothermal catalysis; ruthenium nanoparticles; solar fuels; titanate nanotubes; titanates.

Figure 1

(i) Absorption spectra of UV–VIS–NIR acquired for Na,H/TiNT (black line), Ru/TiNT-0.5 (red line), Ru/TiNT-3.5 (green line), and Ru/TiNT-11 (blue line). (ii) X-ray powder diffraction spectra of (a) Na,H/TiNT, (b) Ru/TiNT-0.5, (c) Ru/TiNT-3.5, and (d) Ru/TiNT-11; RuO₂ (rutile) (JCPDS-40-1290). (iii) Tauc plot assuming a direct band gap for the semiconductors (a) Na,H/TiNT and (b) Ru/TiNT-11.

Figure 2

(i) Raman spectra recorded upon a 785 nm laser diode for the samples under study submitted to increasing temperatures from ambient to 240 °C in a chamber under hydrogen atmosphere. (a) Room temperature Na,H/TiNT, (b) room temperature Ru/TiNT-11, (c) Ru/TiNT-11 at 150 °C, (d) Ru/TiNT-11 at 180 °C, (e) Ru/TiNT-11 at 210 °C, and (f) Ru/TiNT-11 at 240 °C; all spectra have been shifted in intensity axis for clarity. (ii) In situ XPS spectra of sample Ru/TiNT-11: (a) recorded at room temperature under vacuum. Blue lines correspond to raw XPS spectra, pink lines denote the presence of RuO₉ 3d_5/2-9/2 species, and olive lines belong to the RuO₂ 3d_5/2-9/2 species. (b) After submitting the sample for 1 h to a 5% H₂ stream under 210 °C, in situ XPS was recorded at 210 °C under vacuum. Black lines correspond to raw XPS spectra, olive lines belong to the RuO₂ 3d_5/2-9/2 species, and cyan lines refer to metallic Ru, Ru(0).

Figure 3

HR-TEM images of (i) the Na,H/TiNT; (ii) high-resolution image of the TiNT showing an interplanar distance of 0.75 nm; (iii) Ru/TiNT-0.5; (iv) Ru/TiNT-3.5; (v) Ru/TiNT-11, that comes with a high-resolution frame of a nanotube loaded with small nanoparticles (vi). Red arrows in the figures point to some small Ru NPs for frames (iii–v).

Figure 4

(i) Methane conversion rate for Ru/TiNT-3.5 as a function of the temperature for CO₂ hydrogenation upon irradiation with 100 mW/cm² light power (■) and in the dark (□). (ii) Methane conversion as a function of time for CO₂ methanation under 1 sun light power (equivalent to 100 mW/cm²) at different temperatures (■) 150 °C, (●) 180 °C, (▲) 210 °C, and 240 °C ().

Figure 5

(i) Photothermal catalytic hydrogenation of CO₂ to methane at 180 °C and 150 mW/cm² light power in the presence of Ru/TiNT samples with different Ru content. Na,H/TiNT (×), Ru/TiNT-0.5 (■), Ru/TiNT-3.5 (●), and Ru/TiNT-11 (▲). (ii) Production rate per mg of Ru deposited on the TiNTs.

Figure 6

(i) Photothermal catalytic reduction of CO₂ with Ru/TiNT-3.5 at 180 °C in the dark (×) and under irradiation with simulated sunlight of 100 mW/cm² (■), 150 mW/cm² (●). or 230 mW/cm² (▲). (ii) Photothermal catalytic reduction of CO₂ at 180 °C utilizing Ru/TiNT-11; the production was normalized according to light intensity used in each light region. Only heat (■), only simulated sunlight (×), visible (▲), near-infrared (▼), and UV (●). (iii) Reusability experiment for Ru/TiNT-11, performed at 210 °C upon irradiation with simulated sunlight of 150 mW/cm². HR-TEM images of Ru/TiNT-11 sample: (iv) fresh prepared sample and (v) same sample after 10 cycles.

Scheme 1

Pictorial illustration of the synthesis of ruthenium/titanate nanotubes (Ru/TiNTs) starting from TiO₂ anatase. (i) Synthesis of Na,H/TiNT by hydrothermal treatment with 10 M NaOH at 150 °C for 6 h. (ii) Ru³⁺ to Na⁺ and H⁺ ion exchange. (iii) Calcination at 200 °C under air.

Scheme 2

Scheme of the photothermal catalytic mechanism. Ru/TiNT composite gets irradiated and heated during photothermal reaction conditions. Photochemical contribution to catalysis: (a) Electron–hole charge transfer from TiNT to Ru NPs. (b) Excited Ru NPs deactivate by photothermal mechanism and catalyze the reaction, transferring localized heat transfer to absorbed molecules. (c) Plasmonic nanoparticles get excited via photon absorption and subsequently catalyze the reaction through charge transfer to molecules on the surface. Thermal contribution to catalysis: (d) Mechanism of thermal reaction. Ru NPs absorb heat from the environment and then transfer the heat to an absorbed reactant via heat transfer. Symbols description: hυ UV light; hυ’: VIS + IR light; Δ: heat; e^- electrons and h⁺ holes.