Thermochemical gas reduction process using poly-cation oxide

A two-step thermochemical gas reduction process based on poly-cation oxides includes repeatedly cycling a thermal reduction step and a gas reduction step. In the thermal reduction the poly-cation oxide is heated to produce a reduced poly-cation oxide and oxygen. In the gas reduction step, the reduced poly-cation oxide is reacted with a gas to reduce the gas, while reoxidizing the poly-cation oxide. The poly-cation oxide has at least two distinct crystal structures at two distinct temperatures and is capable of undergoing a reversible phase transformation between the two distinct crystal structures. For example, the poly-cation oxide may be an entropy tuned mixed metal oxide, such as an entropy stabilized mixed metal oxide, where the entropy-tuning is achieved via change in crystal structure of one of more of the compounds involved. The gas reduction process may be used for water splitting, CO2 splitting, NOx reduction, and other gas reduction processes.