High-entropy oxides for nitrate reduction: elemental synergy, catalytic performance, and future design strategies

Nitrogen is one of the elements being monitored in wastewater as its different forms can lead to imbalance between water parameters if not treated properly. A solution to this problem would be to utilize the nitrate reduction reaction that converts the nitrate into more usable forms of ammonia and diatomic nitrogen. High entropy oxide (HEO) particles can be utilized as catalysts as HEOs have proven to have high efficiency and selectivity. One of the main problems of the utilization of HEOs is the vast number of possible elements and combinations that demonstrate potential as catalysts for nitrate reduction. This paper evaluates and compares the different elements used in past studies to highlight certain elements that have the highest efficiency and synergy in HEOs to help narrow down the possible synthesis choices of further studies in HEO-catalyzed nitrate reduction. The comparison showed that cobalt and copper showed the highest efficiency and selectivity when combined with other similar transition metals. Iron, zinc, nickel, manganese, titanium, and ruthenium can be used as support due to lower properties. It is recommended to test CoCuMnFeZn, CoCuNiMnFe, CoCuNiMnTi, and CoCuNiMnRu to determine the synergy of the evaluated elements. Further optimization can be done to determine the ratios of each of the elements in the HEO combinations and maximize the efficiency of the HEOs.