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

The carbothermal reduction of SnO 2 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–SnO 2 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 °C (SnO 2 + C → Sn + CO 2, and SnO 2 + 2C → Sn + 2CO), while a plausible two-step solid–gas mechanism can also be speculated (SnO 2 + 2CO → Sn + 2CO 2, and C + CO 2 → 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.