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Abstract
Due to the increasing attention focused on global warming, many studies on reducing CO2 emissions and developing sustainable energy strategies have recently been performed. One of the approaches is CO2 methanation, transforming CO2 into methane. Such transformation (CO2 + 4H2 → CH4 + 2H2O) provides advantages of carbon liquification, storage, etc. In this study, we investigated CO2 methanation on nickel–magnesium–alumina catalysts both experimentally and computationally. We synthesized the catalysts using a precipitation method, and performed X-ray diffraction, temperature-programmed reduction, and N2 adsorption–desorption tests to characterize their physical and chemical properties. NiAl2O4 and MgAl2O4 phases were clearly observed in the catalysts. In addition, we conducted CO2 hydrogenation experiments by varying with temperatures to understand the reaction. Our results showed that CO2 conversion increases with Ni concentration and that MgAl2O4 exhibits high selectivity for CO. Density functional theory calculations explained the origin of this selectivity. Simulations predicted that adsorbed CO on MgAl2O4(100) weakly binds to the surface and prefers to desorb from the surface than undergoing further hydrogenation. Electronic structure analysis showed that the absence of a d orbital in MgAl2O4(100) is responsible for the weak binding of CO to MgAl2O4. We believe that this finding regarding the origin of the CO selectivity of MgAl2O4 provides fundamental insight for the design methanation catalysts.
Highlights
The amount of fossil fuel used worldwide continues to increase, and the resulting greenhouse gas emissions are known to represent a major global challenge
When MgO is added to nickel–alumina catalysts, three possibly active structures for CO2 hydrogenation may be formed, viz., nickel, nickel aluminate, and magnesium aluminate
We explored the use of nickel and magnesium aluminates catalysts for CO2 hydrogenation, experimentally and computationally
Summary
The amount of fossil fuel used worldwide continues to increase, and the resulting greenhouse gas emissions are known to represent a major global challenge. Group 8 to 10 metals such as Ni, Ru, Rh, Co, and Fe are catalytically active with respect to CO2 hydrogenation [8,9] Among these metals, nickel is preferred as an active catalyst metal because it is cheaper than Ru or Rh. The particle size of nickel strongly determines its selectivity for CO2 hydrogenation products produced by methanation or RWGS [10]. In addition to the direct dissociation of CO2 on nickel, the dissociation of H2 and the resulting atomic hydrogen can facilitate the dissociation of C–O bonds This hydrogen-assisted mechanism has been reported by observing the formation of formates and carbon hydroxyl species on a nickel-based catalyst [7]. When MgO is added to nickel–alumina catalysts, three possibly active structures for CO2 hydrogenation may be formed, viz., nickel, nickel aluminate, and magnesium aluminate. The following questions are addressed in this article: (1) What are properties of the nickel–magnesium–alumina catalysts synthesized by co-precipitation?; (2) Are nickel and magnesium aluminates active CO2 hydrogenation catalysts?; (3) What is the reaction mechanism responsible for CO2 hydrogenation on nickel and magnesium aluminate?
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