Acid-Base Hydrothermal Pretreatments to Improve the Catalytic Activity of Low-Grade Hematite Iron Ore in Methane Cracking: Role of CaO and MgO Impurities

To remove impurities, improve methane cracking for hydrogen production, and also promote the use of natural ores as potentially low-cost alternatives to synthetic catalysts, acid and base hydrothermal pretreatments were applied to a low-grade hematite iron ore (41 wt% Fe). The calcined ore was treated with 0.5 M HNO 3 , 5 M NaOH, sequential treatments and post-calcination, and characterized by ICP-OES, XRD and H 2 -TPR. Acid treatment removed Ca- and Mg-bearing phases and markedly improved catalytic performance. At 900 °C acid-treated ores achieved a carbon yield of 2.6 g C /g cat compared with 1.3 g C /g cat for the untreated samples, whereas NaOH as the final pretreatment reduces their performance. An inverse correlation was observed between Ca/Mg content and catalytic performance, together with a higher Fe 3 C fraction in samples richer in these alkaline-earth species. XRD and TEM analyses showed predominantly graphitic carbon structures, with chain-like and bamboo-like morphologies preferentially observed at low Ca and Mg contents.