Mechanism of the CO2 storage and in situ hydrogenation to CH4. Temperature and adsorbent loading effects over Ru-CaO/Al2O3 and Ru-Na2CO3/Al2O3 catalysts

Abstract The use of fossil fuels to satisfy the growing energy demand results in the emission of a huge amount of CO2 to the atmosphere. One alternative to overcome this environmental issue is the CO2 valorization through the storage and in situ hydrogenation to CH4. In this work, Ru-CaO/Al2O3 and Ru-Na2CO3/Al2O3 dual function materials are synthesized with different adsorbent loadings, namely 5, 10 and 15 wt.%. The prepared catalysts are characterized in terms of surface area by N2 adsorption and desorption, crystallinity by XRD, Ru dispersion by H2-chemisorption and TEM, basicity by CO2-TPD and reducibility and oxidation state of the noble metal by H2-TPR and XPS. Temperature programmed surface reaction experiments with H2 on samples with pre-adsorbed CO2 reveal that the decomposition of surface carbonates and the subsequent hydrogenation occurs at lower temperatures for catalysts containing Na2CO3 than CaO. A complete reaction scheme describing the CO2 adsorption and hydrogenation process has been proposed based on the temporal evolution of reactants and products. Oxides (CaO or Na2O) and hydrated oxides (Ca(OH)2 or NaOH) have been identified as CO2 storage sites, the former oxides being more reactive towards the CO2 adsorption. CH4, H2O and minor amounts of CO are detected during the hydrogenation step. The CO2 storage and hydrogenation to CH4 is promoted with increasing the adsorbent loading. Maximum CH4 production of 414 μmol g−1 is observed for Ru15%CaO/Al2O3 at 400 °C. High temperature is needed to efficiently decompose the highly stable carbonates formed onto CaO. On the other hand, the higher Ru dispersion along with a lower stability of carbonates in Ru10%Na2CO3/Al2O3 promotes CH4 formation (383 μmol g−1) at notably lower temperature, i.e. 310 °C. Thus, Ru10%Na2CO3/Al2O3 is regarded as a suitable catalyst for the CO2 storage and in situ hydrogenation to CH4.