ABSTRACT
In this work a titration technique has been used to characterize the amphoteric surface properties of a series of chemically reduced electrolytic manganese dioxide (EMD) samples (MnO 1.97 to MnO 1.50). The surface of the EMD was found to consist of independent acidic and basic hydroxyl groups, which were able to be characterised by their respective equilibrium constants and site concentrations. For this chemically reduced series Kb varied from (1.81-8.43)x10(-10) as reduction proceeded, with the corresponding basic site concentration varying from (0.20-2.50)x10(-4) mol/m2 over the pH range considered. Ka was ranged from (1.23-9.23)x10(-6) over the reduction range considered. The increase in Kb suggested a weakening of the MnO bond via the introduction of the larger Mn3+ ions which will increase the length of this bond. Weakening the MnO bond results in a corresponding strengthening of the OH bond giving the surface hydroxyl group a basic nature which is supported by the increasing basic site concentration. For the samples with an x in MnOx value above 1.71 the total number of acidic sites decreased which supports the increase in the concentration of basic sites; however, below 1.71, the surface concentration of acidic sites increases slightly, which can be rationalised by the fact that the pyrolusite domains within the EMD (with relatively stronger MnO bonds) are accessible at this stage of the reduction. The number of surface oxide sites (Ns) and surface hydroxyl sites (Ns(OH)) were calculated crystallographically, and from the sum of the acid and basic hydroxyl groups determined by titration. Both methods produced data with the same order of magnitude, as well as indicated the expected increase in the number of surface hydroxyl groups with increasing degree of reduction. Electrochemical analysis of the samples in 9 M KOH showed the expected decrease in capacity with an increase in the degree of reduction. It also showed a decrease in the amount of charge contributed to the overall homogeneous reduction by Mn4+ ions in surface defects and within the ramsdellite domains over the entire x in MnOx range. However, the amount of charge contributed from the pyrolusite domains remained unchanged until after a x in MnOx value of 1.71.
ABSTRACT
In this work a titration technique was used to determine the amphoteric surface properties of a series of heat-treated electrolytic manganese dioxide (EMD) samples (up to 500 degrees C). The surface of each sample was found to consist of independent acidic and basic hydroxyl sites, which could be characterized by their respective equilibrium constants and site concentrations. It was found that the acidic sites could not be characterized by a single equilibrium constant, but rather by a distribution indicating the subtle differences between individual sites, while a single equilibrium constant adequately represented the basic sites. For EMD, K(a) varied between 0.1 and 6.3x10(-5), with a corresponding [MnOH((a)T)] value varying between 9.1 and 6.4x10(-6) mol m(-2) over the pH range considered. K(b) and [MnOH((b)T)] were found to be 1.81x10(-9) and 1.93x10(-5) mol m(-2), respectively. With heat treatment, K(a) increased, suggesting a strengthening of the MnO bond via the removal of defects such as Mn(3+) ions and cation vacancies. The fact that K(b) also increased was initially counterintuitive because it suggested that the MnO bond had been weakened by heat treatment. However, assuming that the acidic and basic hydroxyl groups are independent, the trends in K(b) could be rationalized in terms of oxygen ion coordination in the progressively heat-treated samples. The number of surface sites (N(s)) was determined crystallographically and from the sum [MnOH((a)T)] + [MnOH((b)T)]. The data from both methods were of the same order of magnitude but exhibited different trends due to certain inadequacies in both methods. However, the data trends did indicate that the crystal planes at the particle surface could be changing with heat treatment due to a decrease in the value of N(s) determined from the surface titrations. Electrochemical analysis of the samples in 9 M KOH indicated that their performance degraded considerably with heat treatment. In comparison with the surface titration data, it was concluded that proton insertion into the structure occurred only through basic surface sites, the decreasing number of which could limit performance.