Understanding the Light-Driven Enhancement of CO2 Hydrogenation over Ru/TiO2 Catalysts

Abstract

Ru/TiO₂ catalysts are well known for their high activity in the hydrogenation of CO₂ to CH₄ (the Sabatier reaction). This activity is commonly attributed to strong metal-support interactions (SMSIs), associated with reducible oxide layers partly covering the Ru-metal particles. Moreover, isothermal rates of formation of CH₄ can be significantly enhanced by the exposure of Ru/TiO₂ to light of UV/visible wavelengths, even at relatively low intensities. In this study, we confirm the significant enhancement in the rate of formation of methane in the conversion of CO₂, e.g., at 200 °C from ~1.2 mol g_Ru^-1·h^-1 to ~1.8 mol g_Ru^-1·h^-1 by UV/Vis illumination of a hydrogen-treated Ru/TiO_x catalyst. The activation energy does not change upon illumination-the rate enhancement coincides with a temperature increase of approximately 10 °C in steady state (flow) conditions. In-situ DRIFT experiments, performed in batch mode, demonstrate that the Ru-CO absorption frequency is shifted and the intensity reduced by combined UV/Vis illumination in the temperature range of 200-350 °C, which is more significant than can be explained by temperature enhancement alone. Moreover, exposing the catalyst to either UV (predominantly exciting TiO₂) or visible illumination (exclusively exciting Ru) at small intensities leads to very similar effects on Ru-CO IR intensities, formed in situ by exposure to CO₂. This further confirms that the temperature increase is likely not the only explanation for the enhancement in the reaction rates. Rather, as corroborated by photophysical studies reported in the literature, we propose that illumination induces changes in the electron density of Ru partly covered by a thin layer of TiO_x, lowering the CO coverage, and thus enhancing the methane formation rate upon illumination.

Keywords: CO coverage; CO2 hydrogenation; DRIFT spectroscopy; Ru/TiO2; charge transfer processes; heat; photothermal catalysis.

Figure 1

TEM images of Ru/TiO₂ after: (a) H₂ pretreatment; (b) reaction at 250 °C with on-off light cycles (20 min each) for a duration of 24 h; (c) stepwise increase in temperature from 250 to 450 °C and cool down to 250 °C; (d) Ru/STO; and (e) Ru/SiO₂ after H₂ pretreatment.

Figure 2

(a) XPS analysis of as-prepared Ru particles supported on TiO₂, STO, and SiO₂. The C 1s, Ru 3d 3/2, and Ru 3d 5/2 deconvolution corresponds to the green, yellow, and blue traces, respectively. (b) UV-vis absorbance spectra of the Ru/TiO₂, Ru/STO, and TiO₂ catalysts after H₂ pretreatment at 450 °C.

Figure 3

The Ru-normalized CH₄ formation rates (left axis) from 120 °C to 220 °C (temperature indicated above graph) as determined for Ru/TiO₂, Ru/STO, and Ru/SiO₂ catalysts, respectively. Exposure to UV-vis irradiation is indicated by the pink rectangular areas—see the legend, as indicated in the Figure, for the respective catalysts. The TiO₂ support does not show any activity—even when illuminated. Ilumination was provided according to the spectrum shown in Figure S1, with an intensity of 360 mW/cm².

Figure 4

(a) The Ru-normalized CH₄ formation rates (left axis) plotted as a function of temperature with light off (black) and light on (red curve). (b) The curves can be overlapped if the light-on curve is shifted to higher temperature values by ~10 °C. By comparing the Arrhenius plots (See Figure S2), the activation energies can be estimated to be quite similar in dark conditions and upon illumination—suggesting that light enhances the number of available sites. Based on the DRIFT analysis to follow, this is proposed to be due to changes in CO coverage.

Figure 5

Mass spectrometry results (normalized ion current—y-axis) of photothermal hydrogenation of CO₂ on Ru/TiO₂ in the dark and under UV-vis irradiation, by stepwise increasing temperature from 50 to 450 °C, and decreasing from 450 to 200 °C, respectively. Temperature steps are indicated above the figure.

Figure 6

In-situ DRIFT spectra of photothermal CO₂ reduction on Ru/TiO₂ in the dark and under UV-vis irradiation. At each temperature (as indicated in the figure), three conditions are compared: exposure to CO₂; exposure to H₂/CO₂; and exposure to H₂/CO₂ and UV-vis light.

Figure 7

Charge transfer processes upon light activation of the Ru nanoparticles on TiO_x. (a) and (b) illustrate photoinduced electron or hole transfer from the Ru towards the TiO_x, depending on the position of the HOMO and LUMO energy levels of the Ru nanoclusters relative to the valence band (VB) and conduction band (CB) of the TiO_x. (c) shows photoexcitation of the Ru nanoclusters, followed by the electron promoting CO desorption and hole transfer towards partly reduced TiO_x induced by the hydrogen treatment preceding the photothermal catalysis. (d) presents photoexcitation of a hybrid bonding state formed between Ru and CO towards the antibonding state, destabilizing the Ru–CO bond. The latter is unlikely to play a significant role here, as in that case Ru/SiO₂ should have shown a higher performance.

Figure 8

Effect of charge transfer processes on Ru–CO coverage—the left shows the situation upon UV light activation of the Ru nanoparticles on TiO_x, and the right shows the situation upon green light activation of the Ru nanoparticles on TiO_x.