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Energy storage solutions are a vital component of the global transition toward renewable energy sources. The power-to-gas (PtG) concept, which stores surplus renewable energy in the form of methane, has therefore become increasingly relevant in recent years. At present, supported Ni nanoparticles are preferred as industrial catalysts for CO2 methanation due to their low cost and high methane selectivity. However, commercial Ni catalysts are not active enough in CO2 methanation to reach the high CO2 conversion (>99%) required by the specifications for injection in the natural gas grid. Herein we demonstrate the promise of promotion of Ni by Mn, another low-cost base metal, for obtaining very active CO2 methanation catalysts, with results comparable to more expensive precious metal-based catalysts. The origin of this improved performance is revealed by a combined approach of nanoscale characterization, mechanistic study, and density functional theory calculations. Nanoscale characterization with scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy (STEM-EDX) and X-ray absorption spectroscopy shows that NiMn catalysts consist of metallic Ni particles decorated by oxidic Mn2+ species. A mechanistic study combining IR spectroscopy of surface adsorbates, transient kinetic analysis with isotopically labeled CO2, density functional theory calculations, and microkinetics simulations ascertains that the MnO clusters enhance CO2 adsorption and facilitate CO2 activation. A macroscale perspective was achieved by simulating the Ni and NiMn catalytic activity in a Sabatier reactor, which revealed that NiMn catalysts have the potential to meet the demanding PtG catalyst performance requirements and can largely replace the need for expensive and scarce noble metal catalysts.

Titania-supported nickel catalysts (3, 6, 12 and 20 wt% Ni/TiO2) were prepared by a sol-gel method. The results showed that the CO2 conversion increased with increasing Ni content and 20 wt% Ni/TiO2 exhibited the highest CO2 conversion and CH4 yield. The results on XRD and EXAFS revealed unit cell expansion and lattice distortion, which indicated that Ni2+ was incorporated into the TiO2 lattice. Upon increasing the Ni content, the NiO phase was observed. The oxidation states of nickel and titanium were analyzed to be +2 and +4, respectively for fresh catalysts. After pretreatment by H-2, Ni2+ was converted to Ni-0 and this electronic state remained unchanged during the course of methanation while Ti4+ was kept unaltered for fresh catalysts and those during the reaction. The addition of Ni led to the formation of oxygen vacancy and NiO phase which might act as adsorption sites for CO2 and H-2, respectively. (C) 2020 Elsevier Ltd. All rights reserved.