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

Abstract

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.

Oxidant or Catalyst for Oxidation? A Study of How Structure and Disorder Change the Selectivity for Direct versus Catalytic Oxidation Mediated by Manganese(III,IV) Oxides

“…At pH 5, close to 1 mol of O 2 is produced per mole of Mn, indicating that the material directly oxidizes H 2 O 2 and that it is not a catalyst for the disproportionation of H 2 O 2 at this pH. However, as the trend across the MnO x series is studied, it can be seen that the materials can act as both an oxidant and a catalyst, with the stoichiometric ratio of O 2 to MnO x indicating how much more favorable the catalytic reaction is versus direct oxidation . It is clear that distinct differences in reactivity can be seen across the manganese oxide series.…”

“…If the manganese oxide acts as an oxidant, it will itself dissolve, so in reactions where dissolution is complete, the moles of O 2 produced indicate the ratio of disproportionation to oxidation and give an indication of the thermodynamic stability of the manganese oxide. From the observations we make on the basis of calorimetry, we expect that the less stable materials will be better direct oxidants . As protons are consumed in the reaction [MnO 2 § (s) + H 2 O 2(aq) + 2H + (aq) → Mn 2+ (aq) + O 2(g) + 2H 2 O (l) ], the reaction (as written) will become more spontaneous at low pH, and the reactions between the test set of MnO x materials and H 2 O 2 were performed at two different pH values (5 and 8).…”

“…The propensity of a manganese oxide phase to directly oxidize H 2 O 2 or catalyze H 2 O 2 disproportionation was tested by measuring the amount of gas evolved from the reaction between a known amount of manganese oxide and hydrogen peroxide as described previously. 39 In this procedure, the manganese oxide (5.057 × 10 −5 M, as a calibrated suspension) was reacted with hydrogen peroxide (H 2 O 2 , 2 mL at 30%) in either acetate buffer (CH 3 COONa, 0.1 M, 5 mL, pH 5.0) or phosphate buffer (NaH 2 PO 4 , 0.1 M, 5 mL, pH 8.0). Gas evolution was quantified using a gas buret (Figure S1).…”

Design of Multi‐Metallic‐Based Electrocatalysts for Enhanced Water Oxidation

“…Compared with highly‐crystalline materials, the disordered structure caused by multiple doping possesses unstable Gibbs free energy. In other words, it is more likely to transform into an active intermediate with exact lattice orientation toward OER (Figure ) . Therefore, the energy needed to convert the pristine material to the active intermediate phase decreases in the chaotic structure, leading to the overall energy‐saving for OER .…”

“…In other words, it is more likely to transform into an active intermediate with exact lattice orientation toward OER (Figure 9). [131,132,133,134] Therefore, the energy needed to convert the pristine material to the active intermediate phase decreases in the chaotic structure, leading to the overall energy-saving for OER. [131,132,133,134] In brief, the twisted structure from the foreign element invasion would not only lower the energy barrier (optimal MÀ O binding) during OER but also lift the energy state of the material itself, accelerating the formation of active intermediate with less needed energy input.…”