Influence of Si/Al ratio on ammonia desorption properties of amorphous silica–alumina for efficient low-temperature release

Ammonia has attracted considerable attention as an energy carrier. For its effective utilization, materials capable of adsorbing and easily desorbing ammonia are highly desirable. In this study, we focused on amorphous silica–alumina materials and investigated the influence of the Si/Al ratio on the ammonia adsorption and desorption behavior. Silica–alumina samples were synthesized using the coprecipitation method. Characterization analyses revealed that all the synthesized samples exhibited comparable specific surface areas, regardless of the Si/Al ratio. 29 Si and 27 Al magic-angle spinning nuclear magnetic resonance spectroscopy confirmed the formation of a silica–alumina structure in all samples. The samples with lower Si/Al ratios contained a greater amount of Al species not incorporated into the silica network. Ammonia temperature-programmed desorption profiles revealed that although the total acid amount increased with decreasing Si/Al ratio, the overall ammonia adsorption–desorption performance was superior for samples with a higher Si/Al ratio. This is because at higher Si/Al ratios, ammonia adsorption is less hindered by Al species not incorporated into the silica network, and the interaction between ammonia and the silica–alumina surface is weakened, thereby facilitating ammonia desorption into the aqueous phase even at room temperature.