Thermodynamic and Kinetic Insights into the Formation of Metallic Rhenium via Solution Combustion Synthesis

This study investigates the formation of metallic rhenium through Solution Combustion Synthesis (SCS), focusing on the thermodynamics, kinetics, and phase composition of the process. The impact of the fuel-to-oxidizer ratio (phi) on the synthesis of rhenium was evaluated, demonstrating that the stoichiometric ratio (phi = 1) leads to the highest combustion temperature and the formation of pure metallic rhenium, as confirmed by XRD. Deviation from this stoichiometric condition, either by fuel excess (phi > 1) or oxidizer excess (phi < 1), resulted in incomplete reduction and the formation of rhenium oxides (ReO2 and ReO3). Thermodynamic calculations revealed that under reducing conditions, metallic rhenium is the primary product while oxidizing conditions favor the formation of rhenium dioxide. Kinetic analysis of the thermal decomposition of ammonium perrhenate suggested that the process involves a multi-stage reaction, with the reduction of rhenium occurring in a stepwise manner. The findings provide new insights into the role of rhenium as both a metal and an oxidizer in SCS and emphasize the critical influence of the fuel/oxidizer ratio in controlling the phase composition and crystallinity of the final product.