Distinguishing Homogeneous from Heterogeneous Water Oxidation Catalysis when Beginning with Polyoxometalates

Polyoxometalates (POMs) have been proposed to be excellent homogeneous water oxidation catalysts (WOCs) due to their oxidative stability and activity. However, recent literature indicates that even these relatively robust compounds can be transformed into heterogeneous, metal-oxide WOCs under the oxidizing reaction conditions needed to drive O2 evolution. This review covers the experimental methodology for distinguishing homogeneous and heterogeneous WOCs; it then addresses the “what is the true catalyst?” problem for POMs used as precatalysts in the oxidation of water to O2. These results are also compared to the broader WOC literature. The primary findings in this review are the following: (1) Multiple, complementary experiments are needed to determine the true catalyst, including determination of catalyst stability, speciation, and kinetics under operating conditions. (2) Controls with hypothetical heterogeneous metal-oxide catalysts are required to determine their kinetic competence in the reaction and support the conclusion of either a homogeneous or heterogeneous catalyst. (3) Although many studies observe qualitative stability of the starting POM under the reaction conditions, there is a lack of quantitative stability studies; if one does not know where the (pre)catalyst mass lies, then it is very difficult to rule out the possibility of an alternative species as the true catalyst. (4) The stability of POMs is dependent on the polyoxometalate, the metal center, and the reaction conditions. And, (5) as a result of the variable stability of POMs under different reaction conditions, those different conditions can influence the dominant catalyst identity. Overall, knowledge of which POMs (or other starting materials) tend to transform into heterogeneous WOCs, and how they do so, is therefore critical to developing the next generation of higher stability, higher activity, and truly long-lived POM and other water oxidation catalysts.