Preparation of magnetic catalysts for hydrogen generation from borohydride compounds

In this thesis, magnetic nanocomposites based on iron(III) oxide (Fe₃O₄) were synthesized via the hydrothermal method, and various metal-supported magnetic nanocomposite catalysts, Fe₃O₄@SiO₂/Co–Mo, Fe₃O₄@SiO₂/Co–Cr, Fe₃O₄@Al₂O₃/Co–Cs, CeO₂@(Cr–Fe/Co) and Fe₃O₄@CeVO₄/(Cr–Fe/Co), were prepared and employed for hydrogen production. The catalytic performance of these catalysts was investigated through hydrogen generation processes based on the hydrolysis and methanolysis of sodium borohydride (NaBH₄) and potassium borohydride (KBH₄). In the first stage of the study, the prepared Fe₃O₄ magnetic nanoparticle exhibited high activity in the methanolysis of NaBH₄, and under the conditions of 30 °C, 1.5% NaBH₄, and 100 mg catalyst, a hydrogen generation rate (HGR) of 2721.9 mL/gkat min was obtained. The activation energy was determined as 27.18 kJ/mol, and XRD analysis showed that the Fe₃O₄ magnetic nanoparticle possessed a highly crystalline structure with an average particle size of 10.35 nm. Reusability tests indicated that the Fe₃O₄ nanoparticle maintained 99% of its catalytic activity after six cycles. The Fe₃O₄@SiO₂ magnetic nanocomposite, in which Fe₃O₄ was used as the core material, was prepared and tested in the hydrolysis and methanolysis reactions of sodium borohydride (NaBH₄) and potassium borohydride (KBH₄); however, no significant catalytic activity was observed. Therefore, various metal-doped catalysts were prepared based on this structure, and their catalytic activities were investigated. The prepared Fe₃O₄@SiO₂/Co–Mo magnetic nanocomposite catalyst exhibited high catalytic performance in both NaBH₄ and KBH₄ hydrolysis. The catalyst had an average particle size in the range of 100–200 nm, and its surface area was determined as 26.54 m²/g. In NaBH₄ hydrolysis, under the conditions of 1% NaBH₄, 7 wt% NaOH, and 100 mg catalyst, an HGR of 16.4 L/gₘₑₜₐₗ min and an activation energy of vi 32.18 kJ/mol were obtained. In KBH₄ hydrolysis, under the conditions of 1% KBH₄, 3% KOH, and 100 mg catalyst, an HGR of 14.5 L/gₘₑₜₐₗ min and an activation energy of 23.72 kJ/mol were achieved. These results demonstrated that the addition of Co–Mo significantly enhanced the catalytic activity of the Fe₃O₄@SiO₂ magnetic nanocomposite. The prepared Fe₃O₄@SiO₂/Co–Cr magnetic nanocomposite catalyst exhibited a spherical morphology with an average particle size of 200 nm, and its surface area was calculated as 46.67 m²/g. In NaBH₄ hydrolysis, carried out under 7% NaOH, 1% NaBH₄, and 100 mg catalyst, an HGR of 22.20 L/gₘₑₜₐₗ min and an activation energy of 20.75 kJ/mol were determined. The catalyst retained its catalytic performance after six consecutive cycles, indicating high stability. The Fe₃O₄@Al₂O₃/Co–Cs magnetic nanocomposite catalyst was characterized by a high surface area of 127.60 m²/g and a flower-like morphology. Under the conditions of 1% NaBH₄, 3% NaOH, and 50 mg catalyst, an HGR of 17.24 L/gₘₑₜₐₗ min and an activation energy of 26.65 kJ/mol were obtained. Reusability experiments revealed that the catalyst maintained high activity after four cycles. The prepared CeO₂@(Cr–Fe/Co) nanocomposite catalyst exhibited a spherical morphology with a surface area of 18.53 m²/g. In NaBH₄ hydrolysis, conducted under 1% NaBH₄, 1% NaOH, and 100 mg catalyst, an HGR of 13.05 L/gₘₑₜₐₗ min and an activation energy of 18.59 kJ/mol were determined. In KBH₄ hydrolysis, carried out under 1% KBH₄, 1% KOH, and 50 mg catalyst, an HGR of 9.06 L/gₘₑₜₐₗ min and an activation energy of 30.02 kJ/mol were obtained. The prepared Fe₃O₄@CeVO₄/(Cr–Fe/Co) magnetic nanocomposite catalyst exhibited a spherical morphology with an average particle size of 60–70 nm and a surface area of 46.34 m²/g. In NaBH₄ hydrolysis, performed under 3% NaOH, 1% NaBH₄, 100 mg catalyst, and 30 °C, an HGR of 19.65 L/gₘₑₜₐₗ min and an activation energy of 30.43 kJ/mol were obtained. The catalyst showed no activity loss after five cycles, demonstrating high durability.