Thermodynamic and Kinetic Study of the Carbothermal Reduction of SnO2 for Solar Thermochemical Fuel Generation

The carbothermal reduction of SnO2 is studied as part of a two-step thermochemical process for solar fuel production. A second law analysis was applied to validate a combined methane cracking-SnO2 carbothermal cycle, which shows 86% theoretical exergy efficiency and an energy content upgrading of 28.2%. Thermodynamics predicts a predominance of solid-solid reactions over 700 degrees C (SnO2 + C -> Sn + CO2, and SnO2 + 2C -> Sn + 2CO), while a plausible two-step solid-gas mechanism can also be speculated (SnO2 + 2CO -> Sn + 2CO(2), and C + CO2 -> 2CO). Two carbon types were selected to investigate the reaction mechanism, a high specific surface area activated charcoal, and a nanosized carbon black obtained from solar methane cracking. The activated carbon favors the solid-gas mechanism (activation energy of 267 kJ mol(-1)), while carbon black favors direct solid-solid reduction through a slower but complete reaction with a first-order reaction mechanism and a lower activation energy (204 kJ mol(-1)), opening opportunities for improved profitability of both methane cracking and redox cycles for solar fuel generation.