The Oxidation of Peroxide by Disordered Metal Oxides: A Measurement of Thermodynamic Stability “By Proxy”

It is often noted that disordered materials have different chemical properties to their more “ordered” cousins. Quantifying these effects in terms of thermodynamics is challenging in part because disordered materials can be difficult to characterise and are frequently relatively unstable. During the course of our experiments to understand the effects of disorder in catalysts for water oxidation we observed that many disordered manganese and cobalt oxide water oxidation catalysts directly oxidised peroxide in contrast to their more ordered analogues which catalysed its disproportionation, that is, MnO2+2 H++H2O2→Mn2++2 H2O+O2 (oxidation) versus H2O2→H2O+1/2 O2 (disproportionation). By measuring the efficiency for one reaction over the other as a function of pH, we were able to quantify the relative stability of materials in two series of metal oxides and thereby quantify their relative thermodynamic stability, “by proxy”. We found that for the series of catalysts investigated the disorder made the materials stronger chemical oxidants and worse catalysts for the disproportionation of peroxide. Catalysts with structural disorder: Many disordered manganese and cobalt oxide water oxidation catalysts directly oxidised hydrogen peroxide in contrast to their more ordered analogues which catalysed its disproportionation. These differences in reactivity were systematically quantified and explained by changes in thermodynamic stability that occur as a function of structural disorder (see figure).