Storage Material Effects on the Performance of Ru-Based CO₂ Capture and Methanation Dual Functioning Materials

Abstract

In this study, a systematic investigation on Dual Functioning Materials (DFMs) for the capture and methanation of CO2 is carried out. The attention is focused on the nature of the CO2 adsorbent component (storage material, SM) varying between alkaline (Li, Na, K) and alkaline-earth (Mg, Ca, Ba) metal oxides in combination with Ru, both supported on an Al2O3 support. Combining gas phase reactivity analysis and FT-IR characterization, the samples are characterized in terms of CO2 storage capacity. It is found that all the SM-containing samples adsorb significant amounts of CO2 as carbonate species, with the higher amounts being adsorbed when the more thermally stable species are formed, i.e., when Ca, Ba, or K are employed as SMs. In all cases, the hydrogenation of the adsorbed carbonates to CH4 occurs at lower temperature, if compared to their thermal desorption. However, in the case of Ca- and Ba-based DFMs, resilient carbonates are present on the material surface. It was found that the SMs able to form the more thermally stable carbonates upon CO2 adsorption also showed the best performances in capture/methanation cycles at 350 °C, even if some residual carbonates were left on the DFM after the hydrogenation step. In particular, the following order of reactivity has in fact been observed in terms of CH4 production: Ru–K ≥ Ru–Ba > Ru–Ca > Ru–Na ≫ Ru–Mg ≅ Ru–Li ≅ Ru. The presence of steam and O2 during the capture step has a detrimental effect on the CO2 adsorption for all samples and, as a result, on CH4 production due to the competition of CO2 and water for the same adsorption sites. Thus, only SMs able to form strongly bound carbonates species upon CO2 exposure can retain significant CO2 storage capacity also in the presence of water in the adsorption feed.

Promotion of Ru or Ni on Alumina Catalysts with a Basic Metal for CO2 Hydrogenation: Effect of the Type of Metal (Na, K, Ba)

“…This is aligned with the superior performance of the Ba-containing catalysts in our steady-state experiments ( Figure 1 ). This is at odds with other authors who observed that Ba formed more stable carbonates than Na in TPSR experiments with Ru and different alkaline [ 14 ]. This discrepancy can be explained because there are some significant differences between our experimental methodology and that of those authors.…”

“…However, in terms of CH 4 yield, only the Ba-containing catalyst outperformed the monometallic one ( Figure 1 b). The suppression of CO 2 conversion and CH 4 selectivity when adding the alkaline agrees with some of the literature [ 14 , 33 ]. This is usually attributed to the masking of the transition metal by the alkaline.…”

“…At the maximum temperature of TPD (500 °C), the CO 2 concentration reaches almost zero, indicating that alkaline carbonates have been almost completely decomposed at temperatures below 500 °C. This suggests the absence of alkaline agglomerates, which decompose at higher temperatures [ 14 , 49 ]. The quantification of CO 2 desorption is displayed in Figure 5 c. This has been carried out by integrating the area of each peak, splitting the two peaks by a vertical line at the valley.…”

“…This behavior variability stems from the fact that the performance is very sensitive to the preparation method, alkaline loadings and reaction conditions. Some works [ 13 , 14 ] report that the CO 2 conversion and CH 4 selectivity decreased when adding the alkaline with respect to the monometallic catalyst [ 13 ]. It was found that the performance of the monometallic catalyst in CO 2 hydrogenation worsened when the alkaline metal was added.…”

“…Since the type of alkaline also affects the performance, some authors screened several alkaline or alkaline-earth metals added to Ru/Al 2 O 3 or Ni/Al 2 O 3 catalysts and compared their catalytic performance [ 14 , 24 , 33 , 34 ]. Cimino et al [ 34 ] impregnated different alkaline metals (Li, Na, K) over 1%Ru/Al 2 O 3 and found a promotion only when Li was impregnated.…”