Ni-MgO/Al2O3 Dual Functional Material for CO2 Capture and Conversion
Résumé
Introduction The integrated CO2 capture and utilization (ICCU) process consists in the adsorption of effluent CO2 and its conversion into renewable energy fuels such as methane.1 A dual functional material (DFM) is needed for the direct adsorption and catalysis in the process. DFMs consist of basic oxides with high surface area (possibly promoted by alkali or alkaline earth metals such as Na, K, Ca, and Mg) loaded with catalytic group VIII metals such as Ni, Ru, and Rh.2-4 Here, we focus on the prototypical Ni-MgO/γ-Al2O3 (NiMgAl) system, which has been elaborated by two methods in order to boost the ICCU performance and identify critical structural parameters affecting it. Materials and Methods NiMgAl DFM preparation was based on aqueous wet impregnation of Mg(NO3)2 and Ni(NO3)2 precursors over γ-Al2O3. Co-impregnation (CI) mixes both metal precursors in the same solution, followed by drying, calcination in air flow at 500 °C, and reduction in H2 flow at 600 °C. In contrast, sequential impregnation (SI) involves first the Mg salt impregnation, drying and calcination, and then the Ni salt impregnation followed again by drying, calcination and reduction. The materials were characterized by ICP-OES, BET, XRD, STEM-EDS, and CO2-TPD. CO2 hydrogenation (10% CO2 + 40% H2 + N2) and ICCU tests were carried out at atmospheric pressure in a flow-fixed bed reactor equipped with automated valves and a fast IR gas detector. One ICCU cycle consisted of 3 min exposure to 7.5% CO2/N2 flow, 1.5 min purging with N2 flow, 3 min exposure to 25% H2/N2 flow, and 1 min purging with N2. Results and Discussion NiMgAl properties in Table 1 show little difference in the metal loadings, surface area, and crystallite size. Fig. 1a depicts the presence of Ni NPs (3.1 ± 0.9 and 4.4 ± 1.4 nm in average size for SI and CI, respectively). According to XRD, some Mg is inserted the alumina lattice. The CO2-TPD curves of pristine Al2O3, MgO/Al2O3 (MgAl), Ni/Al2O3 (NiAl), and both NiMgAl samples (Fig. 1b) suggest that Mg species strengthen the surface basicity, i.e. CO2 is desorbed at higher temperatures than from (Ni/)alumina. In addition, the introduction of Ni with Mg further increases the CO2 uptake, which is superior in the case of the CI sample. CO2 hydrogenation results (Fig. 1c) show that Mg increases the activity but decreases the selectivity to methane, especially at low temperature. However, no CO byproduct is detected during the ICCU cycles (see Fig. 1d for NiMgAl_CI), i.e. this process is 100% selective to methane over the DFMs. In addition, there is no deactivation after ca. 20 ICCU cycles, which indicates excellent stability properties. Average CH4 production values are 54 and 26 mmol CH4 per mol CO2 per gram of DFM for CI and SI samples, respectively. Therefore, compared to the sequential protocol, the one-pot impregnation of Ni and Mg nitrates on alumina not only enhances the CO2 adsorption capacity of the DFM but also accelerates its conversion activity. In the SI sample, Ni may hamper CO2 adsorption on basic Mg sites, which would be determining for the ICCU process. Ongoing experiments aim at identifying fine structural differences between the two DFMs as well as tuning the ICCU process parameters to maximize the capture & conversion efficiency. Significance A simple and cost-effective DFM has been elaborated for CO2 capture and methanation. The DFM preparation details critically affect its ICCU performance. This work is relevant to green energy production since CO2 greenhouse gas can be converted, through the use of renewable hydrogen, into a valuable fuel utilizable within a “power-to-gas” process. The French ANR is acknowledged for funding through the DuCaCO2 program (ANR-21-CE05-0023) References 1. S. Omodolor et al., Ind. Eng. Chem. Res. 2020, 59, 17612 2. K. Chai, L. Leong, D.Wong, D.Tsai, S.Sethupathi, J. Chin. Chem. Soc., 2020, 67, 998 3. A. Bermejo-Lopez et al., Appl. Catal. B 2019, 256, 117845 4. X. Wang, D. Hu, Y.Hao, L. Zhang, N.Sun, W. Wei, Sep. Purif. Technol., 2023, 322, 124295