Hydrogenation of CO2 on Nickel–Iron Nanoparticles Under Sunlight Irradiation

Abstract

Nickel–iron oxide nanoparticles were prepared by a simple mixed oxalate precursor decomposition method and used as catalysts for the sunlight-promoted CO2 hydrogenation reaction. The composition of the NiyFe1−yOx materials was designed to cover the entire Ni/Fe ratio range (y = 1, 0.9, 0.75, 0.5, 0.25, 0.1, 0). Characterisation was undertaken by means of elemental analyses, X-ray diffraction and high resolution transmission electron microscopy. The pure nickel material (NiOx) contained crystalline NiO nanoparticles. Upon introducing lower proportions of iron in Ni9FeOx and Ni3FeOx, NiO was the only crystalline phase, along with increasing amounts of amorphous iron oxides. Higher iron contents resulted in the co-existence of NiO and γ-Fe2O3 domains at the nanoscale in NiFeOx, NiFe3Ox and NiFe9Ox, whereas the pure iron material (FeOx) was composed of α-Fe2O3 as the only crystalline phase and a significant fraction of amorphous iron oxides. The hydrogenation of carbon dioxide was tested on the materials under simulated sunlight irradiation, and the activities and selectivities investigated as initial CO2 conversion rates and product distributions, respectively. The introduction of iron was beneficial for the activation of CO2, due to the known ability of this metal for promoting the reverse water–gas shift (rWGS) reaction. On the other hand, it was proven that nickel and iron favoured hydrogenation and chain growth processes, respectively. Moreover, the lack of hydrogenation sites in the pure iron material results in the expected preferential generation of olefins. Results for the entire compositional range draw a clear trend towards the enhanced formation of short-chain alkanes at middle iron contents, most likely owing to the existence of junctions between nickel and iron oxides at the nanoscale, and the related interfaces providing rWGS, chain growth and hydrogenation sites in close vicinity. The resulting hydrocarbon products, presumably produced by the efficient combination of thermal and photonic effects, can be considered as solar fuel replacements for natural gas and liquefied petroleum gases.