Study of Gold-Activated Carbon Electrodes for the Production of Hydrogen Peroxide
STUDY OF GOLD-ACTIVATED CARBON ELECTRODES FOR THE PRODUCTION OF HYDROGEN PEROXIDE
Project Autek, Mintek, Private Bag X3015, Randburg 2125, South Africa
The largest use of hydrogen peroxide is the bleaching in the pulp and paper industry. However, oxidation with hydrogen peroxide has also been investigated in conjunction with the treatment of wastewater and is reported to be especially suitable for cyanide containing wastes and those containing organic impurities. For such applications producing hydrogen peroxide on-site is of increasing interest because of the costs and hazards associated with the transport and handling of concentrated hydrogen peroxide. Producing hydrogen peroxide on-site also eliminates the problems of autodegradation during storage.
This paper focus on the use of gold in the production of hydrogen peroxide by the cathodic reduction of dissolved oxygen. Dissolved oxygen can be reduced directly to water with a 4-electron transfer reaction. It can also be first reduced to hydrogen peroxide through a 2-electron transfer, which is then followed by further reduction of hydrogen peroxide to water or by the transport of the formed hydrogen peroxide to the bulk solution. It is known that on carbon materials and gold the electroreduction of molecular oxygen occurs mainly through the intermediate formation of hydrogen peroxide(1), which is a necessary condition for its production.
Polycrystalline gold, glassy carbon and gold-activated glassy carbon electrodes were compared in this investigation. The test electrodes were polytetrafluoroethylene cylinders with a gold or glassy carbon disc centred on their bottom face. The test electrode was connected to a Tacussel (Radiometer Copenhagen) rotating disc electrode (RDE) driven by a Tacussel speed control unit. A BAS (Bioanalytical Systems) electrochemical workstation was used to control the potential of the test electrode.
Two methods were used to deposit gold on the glassy carbon electrodes: a) gold sputtering with a Polaron sputter coater, b) electrodeposition by short potential pulses from a diluted KAuCl4 solution. Changing the sputter time, the electrodeposition pulse width and overpotential results in different gold particle sizes and different gold coverages. The gold particles size was determined by examination under a high-resolution scanning electron microscope. The gold coverage on glassy carbon was determined by oxidizing the surface with an anodic potential step in 0.5M H2SO4 and performing subsequently a cathodic linear sweep voltammogram and integrating the charge of the reduction peak(2).
Cathodic linear sweep voltammograms were performed on these electrodes in 0.5M H2SO4 and 0.5M KOH at various rotating speeds. The shape of the voltammograms indicates that the oxygen reduction reaction is not purely diffusion controlled but controlled both by charge-transfer and mass-transfer. Such behaviour is described by the Koutecky-Levich equation(3). Using this equation, and measuring the current value at various rotating speeds, it is possible to determine the number of electrons consumed in the oxygen electroreduction reaction.
In acid solutions two current waves are observed on the glassy carbon electrode but only one wave on the polycrystalline gold and gold-activated glassy carbon electrodes before the outset of hydrogen evolution. In each case the first wave corresponds to a 2-electron transfer reaction, indicating the electroreduction of oxygen to hydrogen peroxide. The O2 reduction to H2O2, however, occurs at lower (up to 300 mV) overpotentials on the polycrystalline gold and gold-activated glassy carbon electrodes than on glassy carbon. It also appears that this shift in potential is dependent on the amount of gold coverage on the glassy carbon (Figure 1). The second wave observed on the glassy carbon electrode corresponds also to a 2-electron transfer indicating the further reduction of hydrogen peroxide to water.
The behaviour of the glassy carbon electrode is similar in an alkaline solution; two current waves are observed, corresponding to the reduction of oxygen to hydrogen peroxide and the further reduction of hydrogen peroxide to water. The situation is, however, different in the case of the polycrystalline gold and the gold-activated glassy carbon electrodes. Due to the change in pH and, hence, the shift in the standard electrode potential of the oxygen reduction reaction two waves are now also observed with these electrodes. The determination of the number of electrons consumed for each wave indicates that in the first wave part of the produced hydrogen peroxide is further reduced to water, while in the second wave dissolved oxygen is directly reduced to water in a 4-electron transfer reaction. However, in the alkaline solution the difference in overpotential between the glassy carbon and gold electrodes is not so pronounced.
Experiments are still under way to determine the optimum gold sputtering and plating conditions and a laboratory-scale electrolysis cell will be used to assess the feasibility and efficiency of hydrogen peroxide production with these electrodes.
(1) M.R. Tarasewich, A. Sadkowski, E. Yeager, in “Comprehensive Treatise of Electrochemistry”, Vol. 7, p.361, B.E. Conway, J.O’M. Bockris, E. Yeager, S.U.N. Khan, R.E. White Eds., Plenum Press, New York (1983)
(2) M.O. Finot, G.D. Braybrook, M.T. McDermott, J. Electroanal. Chem., 466, (1999), p.234.
(3) A.J. Bard, L.R. Faulkner, “Electochemical Methods, Fundamentals and Applications”, John Wiley & Sons, (1980), p.291.
Electrocatalysis, Gold, Hydrogen peroxide, Glassy carbon