Evaluation of Spinel Ferrites MFe2O4 (M = Cu, Ni, Zn, and Co) Photocatalytic Properties in Selective Dehydrogenation of Formic Acid Towards Hydrogen Production

Abstract

Formic acid is regarded as a promising energy carrier and hydrogen storage medium for a carbon-neutral economy. This paper introduces a scalable and efficient system for the selective photocatalytic conversion of liquid formic acid into hydrogen, under visible light. Formic acid decomposition can initiate two reactions. The first, called dehydration, results in the formation of carbon monoxide and water. The second, called dehydrogenation, results in the formation of carbon dioxide and hydrogen. In the present work, spinel ferrites (MFe₂O₄: M = Cu, Ni, Zn, and Co) are used as photocatalysts to selectively drive the dehydrogenation reaction to produce hydrogen. These catalysts were synthesized via a starch mediated auto-combustion solgel method, yielding crystallite sizes between 47 and 112 nm. Their structural, morphological, and optical properties are determined, including the band gap values ranging from 1.6 to 2.2 eV. The photocatalytic dehydrogenation process was monitored in real time using Fourier transform infrared spectroscopy (FTIR). This analysis revealed a dehydrogenation selectivity across all spinel ferrite samples but with varied amounts of the released CO₂. The highest amounts of CO₂ released after 10 h, both in the dark and under illumination, were observed for NiFe₂O₄ and ZnFe₂O₄. NiFe₂O₄ exhibits superior performance under dark conditions, while ZnFe₂O₄ demonstrates a more important activity under light irradiation. The results demonstrate that NiFe₂O₄ and ZnFe₂O₄ exhibit CO₂ production rates 3 to 4 times higher than those of CoFe₂O₄ and CuFe₂O₄ under light condition, highlighting the potential of these spinel ferrites as efficient photocatalysts for the dehydrogenation of formic acid.

Graphical Abstract