Effect of interlayer anions in layered double hydroxides on the photothermocatalytic CO2 methanation of derived Ni–Al2O3 catalysts

Abstract

The concentration of carbon dioxide (CO₂) in the atmosphere is progressively increasing due to industrial development, leading to environmental concerns such as the greenhouse effect. Consequently, it is crucial to decrease dependence on the fossil fuels and mitigate the CO₂ emissions. Photothermocatalysis technology facilitates the conversion of light energy into heat energy on the surface of catalysts, thereby driving chemical reactions. This catalytic approach effectively harnesses ample solar energy, consequently reducing non-renewable energy consumption. Solar-driven CO₂ methanation is an important route to simultaneously mitigate excessive carbon emissions and produce fuels. Layered double hydroxides (LDH) can be reduced at high temperature in a reductive atmosphere of a hydrogen/argon (H₂/Ar) mixture to prepare metal-loaded oxide (MO) catalysts, which are widely used in CO₂ hydrogenation reactions as excellent photothermal catalysts. However, there is limited study on how the interlayer anion type of LDH affects the activity of CO₂ methanation. Herein, a series of LDH precursors, intercalated with various anions, were synthesized using a co-precipitation method. The LDH precursors were reduced in a H₂/Ar atmosphere to acquire a group of nickel (Ni) loaded on alumina (Al₂O₃) catalysts, referred to as NiAl-x-MO (x = CO₃, NO₃, Cl, and SO₄, which represents carbonate, nitrate, chloride, and sulfate anions, respectively). Energy dispersive spectrometer (EDS) elemental mapping and X-ray photoelectron spectroscopy (XPS) results revealed the presence of nitrogen (N), chlorine (Cl), and sulfur (S) species on the surfaces of NiAl–NO₃-MO, NiAl–Cl-MO, and NiAl–SO₄-MO catalysts, respectively. Photothermocatalytic tests were conducted on the catalysts to assess the potential influence of the residual species on CO₂ methanation. Among them, the NiAl–CO₃-MO catalyst demonstrated a CO₂ conversion of 50.1 %, methane (CH₄) selectivity of 99.9 %, along with a CH₄ production rate of 94.4 mmol g⁻¹ h⁻¹. The performance of the NiAl–NO₃-MO catalyst was found to be comparable to that of the NiAl–CO₃-MO catalyst. In contrast, the CO₂ methanation activity of the NiAl–Cl-MO and NiAl–SO₄-MO catalysts were negligible. CO₂ temperature programmed desorption (CO₂-TPD) analysis demonstrated that the presence of N, Cl, and S species had a negligible effect on the adsorption of CO₂. H₂ temperature programmed desorption (H₂-TPD) and density functional theory (DFT) results suggested that the strong coordination bond between residual Cl or S species and metallic Ni impeded the absorption and activation of H₂, which was responsible for the low CO₂ conversion. Hence, the significant role of the interlayer anion in LDH should be preferentially considered when designing LDH-derived catalysts, especially the Ni-based catalysts for hydrogenation reactions.