Dependence of the oxygen reduction reaction at sol-gel derived Co-based catalysts on acidic solution pH and temperature

This work is focused on the oxygen reduction reaction (ORR) at non-precious metal catalysts formed using a modified sol-gel (SG) synthesis approach, in which carbon and nitrogen were added in the form of either ethylenediamine (en) or 1,2-phenylenediamine (pda) to a pre-formed Co oxide gel. It was confirmed that the Co-pda-derived material, when heat-treated at 900 degrees C, is a much better ORR catalyst than the analogous en-derived material, heat-treated to its optimum of 700 degrees C, giving an activity about 10-11 times higher and a lower H2O2 yield. The ORR was examined over a range of acidic solutions (all at room temperature and an oxygen pressure of 1 atmosphere). For both catalysts, the reaction rate was found to be independent of the H+ concentration at pH < 2.5, with the Co-pda and -en catalysts giving a transfer coefficient of ca. 1 and 0.5-0.7, respectively. At pH > 2.5, an unusual pH response was seen, suggestive of the development of local pH conditions inside the catalyst layer. While the catalysts were stable only to solution temperatures of ca. 55 degrees C, the ORR activation energy in the kinetic (E-act = 34 kJ/mol) and diffusion controlled (E-act = 8.8 kJ/mol) regions could still be determined. Based on all of these results, a possible mechanism for the ORR at below and above pH 2.5, for both the Co-pda and Co-en catalysts, was proposed. (C) 2012 Elsevier B.V. All rights reserved.