CO2 methanation under more realistic conditions: Influence of O2 and H2O on Ni-based catalysts’ performance

Abstract

In this work, Ni-based catalysts (15 wt%) promoted with Y (15 wt%) and Fe (1 wt%) supported on an optimized USY zeolite were prepared by incipient wetness co-impregnation, characterized by XRD, TGA, N2 adsorption, H2-TPR, and TEM, and finally tested towards CO2 methanation (total flow of 274 mL/min, H2/CO2 = 4, 2.5 bar and 0.2 g of catalyst) including water or oxygen in the feed. On one side, water (∼5 vol%) shows a limited impact on catalysts’ performance despite being a product of the CO2 methanation reaction. These results were mainly attributed to the high hydrophobicity of the studied catalysts, as well as to the absence of additional/competitive reactions. On the other side, oxygen (∼4 vol%) negatively affected the activity of the tested catalysts, decreasing the CO2 conversion, particularly for the unpromoted catalysts. Two different aspects were found to be responsible for the decline in activity: on the one hand, the H2/O2 reaction, which is faster than the CO2 methanation and decreases H2 availability for Sabatier reaction; on the other hand, the partial re-oxidation of nickel active sites, evidenced by post-test characterization, which decreases the number of Ni0 active sites, and therefore, leads to a drop in CO2 conversion. Reversibility tests to assess O2 effect were also performed, indicating that its impact on the catalytic performances mainly arises from the decrease in H2 partial pressure, leading to operation under sub-stoichiometric CO2 methanation conditions (H2/CO2 ∼ 3). When compared to a commercial Al2O3–supported Ni catalyst, zeolite-based samples demonstrated superior resistance to H2O and O2.