Iron, Cobalt and Nickel-Based Metal Borides as Low-Cost Nanocatalysts for Highly Efficient Hydrolysis of Sodium Borohydride

In today's world, fossil fuels, commonly known as non-renewable energy sources, provide 80 percent of the energy required. Toxic gases such as CO, SO2, and NOx are produced as a result of the combustion of these fuels. These gases have a wide range of harmful consequences for the environment and human health, particularly in terms of climate change. Environmentally friendly solutions are being researched to meet the energy needs. Hydrogen energy, in particular, has received a lot of interest in recent years. Hydrogen could be extracted from a variety of substances. In terms of high gravimetric hydrogen density, controllable reaction kinetics, non-toxicity, non-combustibility, and ease of storage in the open air, sodium borohydride (NaBH4) is a good choice. However, the activation energy of the NaBH4 hydrolysis reaction is 217 kJ/mol, which is a relatively high value. In order to reduce this activation energy value and to obtain hydrogen more effectively, the reaction should be carried out in the presence of a suitable catalyst. Although traditionally known noble metal catalysts such as platinum, iridium, rhodium, and ruthenium show excellent performance, alternatives should be developed due to their limited reserves and very high costs. Due to their high strength, high hardness, high chemical stability, magnetic qualities, and superior wear/corrosion resistance, transition metal borides can be used in a variety of applications. Transition metal borides, which are less expensive and durable, are emerging as a viable alternative catalyst. In this thesis, among transition metal borides, iron, cobalt and nickel-based metal borides (Fe–Ni–B and Co–Ni–B systems) were chosen to investigate as promising catalysts for the hydrolysis reaction of NaBH4. Catalyst powders with varied mole ratios were synthesized using a mechanochemical method (followed by a wet milling step) in the Fe–Ni–B system, while inorganic molten salt technique was used in the Co–Ni–B system. Utilized methods enabled to prepare the powders with nanoscale size and a uniform particle distribution, and pure composition. In the Fe–Ni–B system, the powder having Ni3B and FeB semi-crystalline phases, homogenous shape, and 70 nm particle size displayed a remarkable catalytic performance in this direction. The availability of active iron, nickel and boron species on the surface was contributed to the enhancement of catalytic activity. It was able to produce 758 ml H2 min-1 gcat -1 of hydrogen at room temperature and reduce the activation energy of the reaction to 40.8 kJ/mol. In the Co–Ni–B system, the powder with CoB–Ni4B3 crystalline phases, which had a homogeneous morphology, approximately 60 nm particle size and pure content, exhibited an enhanced catalytic performance with a very low activation energy of the reaction of 32.7 kJ/mol. According to the recyclability tests, nanocatalyst powders in both systems exhibited catalytic activity even when used for 5 consecutive cycles. As-prepared catalysts that can compete with noble metals can be considered as recyclable, stable and low-cost materials for highly efficient hydrolysis of sodium borohydride.