Effect of Pr in CO2 Methanation Ru/CeO2 Catalysts

Abstract

CO₂ methanation has been studied with Pr-doped Ru/CeO₂ catalysts, and a dual effect of Pr has been observed. For low Pr content (i.e., 3 wt %) a positive effect in oxygen mobility prevails, while for high Pr doping (i.e., 25 wt %) a negative effect in the Ru-CeO₂ interaction is more relevant. Isotopic experiments evidenced that Pr hinders the dissociation of CO₂, which takes place at the Ru-CeO₂ interface. However, once the temperature is high enough (200 °C), Pr improves the oxygen mobility in the CeO₂ support, and this enhances CO₂ dissociation because the oxygen atoms left are delivered faster to the support sink and the dissociation sites at the interface are cleaned up faster. In situ Raman spectroscopy experiments confirmed that Pr improves the creation of oxygen vacancies on the ceria lattice but hinders their reoxidation by CO₂, and both opposite effects reach an optimum balance for 3 wt % Pr doping. In addition, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments showed that Pr doping, regardless of the amount, decreases the population of surface carbon species created on the catalysts surface upon CO₂ chemisorption under methanation reaction conditions, affecting both productive reaction intermediates (formates and carbonyls) and unproductive carbonates.

Figure 1

CO₂ methanation experiments performed in a fixed-bed reactor under steady-state conditions using Ru catalysts with different LnO_x supports (Ln = Ce and/or Pr). Reduction pretreatment at 500 °C for 1 h in 50% H₂/He. Reaction mixture: 10% CO₂, 40% H₂, and He balance. (a) CO₂ conversion for different temperatures and (b) CO₂ conversion at 270 °C for different Pr loadings.

Figure 2

N₂ adsorption–desorption isotherms of the catalyst.

Figure 3

XRD patterns of the catalysts.

Figure 4

H₂-TPR profiles of the catalysts.

Figure 5

Main signals detected upon CO₂ (¹³C¹⁸O¹⁸O; m/z 49) pulses at 250 °C to the different catalysts under CO₂ methanation conditions (10% ¹²C¹⁶O¹⁶O (m/z 44) + 40% H₂ in He). (a) Ru/CeO₂, (b) Ru/Pr3CeO_x, and (c) Ru/Pr25CeO_x. x-axis origin (0 s) corresponds to the time of the pulse.

Figure 6

Mass balance of carbon species upon CO₂ (¹³C¹⁸O¹⁸O; m/z 49) pulses at different temperatures under CO₂ methanation gas flow. (a) Ru/CeO₂, (b) Ru/Ce3PrO_x, and (c) Ru/Ce25PrO_x. Pretreatment of 50% H₂/He at 450 °C, 1 h; methanation mixture 10% ¹²C¹⁶O₂, 40% H₂, and He balance.

Figure 7

In situ Raman spectra recorded at 25 °C under methanation mixture (10% CO₂, 40% H₂, and N₂ balance) after reduction at 450 °C with H₂/N₂ for 1 h.

Figure 8

Evolution of the oxygen vacancy sites on CeO₂ during the reduction pretreatent (50% H₂/N₂) and further heating under methanation mixture (10% CO₂, 40% H₂, N₂ balance) monitored by Raman spectroscopy.

Figure 9

In situ DRIFTS spectra recorded in steady-state conditions under 10% CO₂, 40% H₂, and He balance for (a) Ru/CeO₂, (b) Ru/Ce3PrO_x, and (c) Ru/Ce25PrO_x. Pretreatment at 450 °C under 50% H₂/He.

Figure 10

Evolution of (a) carbonate, (b) ruthenium carbonyl, and (c) formate signals with temperature during in situ DRIFTS experiments under 10% CO₂, 40% H₂, and He balance (pretreatment in H₂/He at 450 °C).