Influence of Different Operating Parameters on the Partial Reduction of Oxygen for Hydrogen Peroxide Production

Abstract

Hydrogen peroxide has gained increasing importance as green oxidiser in a number of chemical and other processes. This includes processes performed in smaller scale local facilities. In order to maintain a low environmental food print a local production is to be preferred. Thereby it would be particularly beneficial if the process could make use of excess renewable electricity. This is why the electrochemical production of hydrogen peroxide has recently attracted increasing interest. An energy efficient option for the electrochemical hydrogen peroxide production is by the partial reduction of oxygen. The challenge is to obtain a high hydrogen peroxide current efficiency (HPCE). Beside the possible parallel complete reduction of oxygen to water the subsequent electrochemical reduction of H2O2 to water and the heterogeneously catalysed disproportionation of H2O2 into water and oxygen are reactions which can reduce the HPCE. In this study different factors influencing the HPCE were studied. All tests were done using a rotating ring disk electrode set-up from PINE instruments. In all tests a platinum ring electrode set to a potential of 1.2 V vs. reversible hydrogen electrode (RHE) was used to detect the hydrogen peroxide leaving the disk working electrode. The reduction of potassium ferricyanide K3[Fe(CN)6] to potassium ferrocyanide K4[Fe(CN)6] at the disk electrode and the reversed oxidation of the formed K4[Fe(CN)6] at the ring electrode were used to calibrate the capture efficiency of the RRDE tip before each experiment. For a first set of experiments a polycrystalline platinum disk electrode was used as working electrode to study the effects of reaction conditions like temperature and pH on the HPCE. The effect of the rotation speed on the HPCE was used as indicator for the reduction of the HPCE by subsequent decomposition of the primary formed hydrogen peroxide via a further reduction step or disproportionation at the disk electrode. An increase of the HPCE with increasing rotation speed is expected if the subsequent decomposition of H2O2 is significant because the mass transport for the removal of formed H2O2 out of the reaction zone gets enhanced. Further tests were made by applying different supported catalyst onto a glassy carbon disk electrode to investigate the effect of the chemical composition of the electrode on the HPCE. Catalysts investigated were gold on carbon Au/C and palladium gold alloy on carbon PdAu/C. For the tests at the platinum electrode the trends known from literature could be confirmed. In an acidic environment (0.1 M HClO4) the hydrogen peroxide production could be strongly increased by reducing the cell temperature from 25 °C to 10 °C in accordance with the findings of Yamanaka (1). It also was found that the hydrogen peroxide production is strongly increased with rotating speed indicating that subsequent H2O2 decomposition proceeds fast at the platinum disk electrode in accordance with the findings of Seidel et al. (2). If the reaction is conducted in an alkaline environment (0.1 M KOH) similar trends with respect to the effect of temperature and rotating speed were observed. The major effect of the increased pH was that the onset of the H2O2 formation was observed at significant higher potentials than under acidic conditions and that high H2O2 production rates were achieved at much lower over potentials (cf. fig 1). With a carbon supported gold catalyst higher H2O2 –concentrations were achieved in the alkaline environment, though at slightly higher over potentials than for the Pt disk electrode. The H2O2 production is almost independent of the rotation speed and is higher at 25 °C than at 10 °C. This indicates that the gold catalyst does not catalyse the decomposition of H2O2. Under acidic conditions the oxygen reduction activity as well as the H2O2 formation rate are both strongly reduced and occur only at very significant over potential (cf. fig 2) By alloying the gold catalyst with palladium an enhancement of the catalytic performance in the acidic environment was achieved even at ambient temperature (cf. fig 3). As the acidic environment is more beneficial to the downstream processing of the H2O2 PdAu/C is currently the most suitable catalyst. In the contribution further tests on the effect of the kind of acid and of H2O2 stabilisation with phosphoric acid additions will be reported. The support of this work by the Fraunhofer-Gesellschaft as part of the project “Current as Raw Material” contract 007-600830 is gratefully acknowledged.