Thermo-regulating effect of Al–Si microencapsulated phase change material-based catalyst support on ammonia decomposition

Abstract

Ammonia is important for energy diversification and security due to its ability to transport hydrogen in the form of relatively safe and high-energy-density compounds. Ammonia decomposes into hydrogen and nitrogen gases via a highly endothermic catalytic process. For an endothermic reversible reaction, an undesirable decrease in the reactor temperature shifts the operating line to a lower equilibrium conversion point, resulting in an economic loss for the plant. The objective of this study was to develop a strategy to provide alternative energy to the endothermic reaction during the decrease in temperature. Accordingly, an Al–Si-based microencapsulated phase change material (MEPCM) was used as a heat-regulating catalyst for the ammonia decomposition reaction. The MEPCM structure consists of a spherical oxide shell and metal core that can store thermal energy based on the principle of latent heat storage. Different transition metals, such as Ni, Co, and Fe, were impregnated into the as-prepared 45 μm Al–Si MEPCM catalyst support with a core melting point of 577 °C and heat storage capacity of 285 J g⁻¹. The effects of these metals on the reaction performance were then evaluated. Furthermore, various parameters such as the phase composition, morphology, and thermal properties of the samples were characterized using X-ray diffraction, Brunauer–Emmett–Teller analysis, scanning electron microscopy-energy-dispersive X-ray spectroscopy, and differential scanning calorimetry. Under controlled cooling, the pre-melted core demonstrated heat regulation ability by maintaining a higher temperature and achieving 2–14% higher conversion than the solid core state. By implementing the concept proposed herein, chemical industries can improve their technology to stabilize the reaction temperature.