Surface dependent effects of Mn-doping on the catalytic activity of α-Fe2O3 for ortho-para hydrogen conversion

Ortho-para hydrogen conversion is a crucial step in the large-scale production, storage, and transportation of liquid hydrogen. Iron-based oxides are widely used catalysts for ortho-para hydrogen conversion, with transition metal doping serving as the most promising method to enhance their catalytic activity. In this work, we investigated the effect of Mn doping on ortho-para hydrogen conversion over α-Fe2O3 catalysts with different morphologies. It was found that Mn-doping effect on ortho-para hydrogen conversion was morphology-dependent, i.e., Mn doping increased activity in cubic and rhombohedral samples but decreased it in nanosheet samples. Combined characterizations indicate that the activity loss in Mn-α-Fe2O3-NP arises from reduced magnetization caused by a lower surface content of Mn3+ on α-Fe2O3(001) surface, whereas the activity increases in Mn-α-Fe2O3-NC and Mn-α-Fe2O3-RH correlate with a larger proportion of high valance Mn3+ and a slight increase in magnetization over (012) and (104) facets, demonstrating that the effect of metal doping depends strongly on surface facets which governs the surface structure and magnetic properties of catalysts.

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