In order to analyse local catalytic activity with high spatial resolution a transient redox competition mode of scanning electrochemical microscopy (RC-SECM) has been developed and described [1]. In a bi-potentiostatic experiment the SECM tip is positioned in close proximity to the surface of sample. Both electrodes are polarised to consume oxygen. If the SECM tip is close enough to an active catalyst, it will start to compete with the sample for the very same analyte in the gap between them. This leads to a noticeable tip current decrease over active sites of the sample. In order to avoid complete O₂ depletion a potential pulse profile is used for electrochemically oxygen generation prior to the competition detection. Highly dispersed electro-deposited spots of gold and platinum on glassy carbon as catalysts for reduction of molecular oxygen were investigated and successfully visualised in chloride free phosphate buffer (pH = 7) using RC-SECM [1]. However, visualisation of local catalytic activity of catalysts for oxygen reduction in chloride containing solutions by high pH has not been reported yet. 

Figure 1. Local catalyst activity visualised by means of  RC-SECM   

In chloride containing solutions the oxygen generation pulse in RC-SECM leads to chloride oxidation to chlorine, which is very corrosive and can affect the activity of the catalysts. In this contribution, cyclic voltammetry and chronoamperometry were used for investigation of chloride oxidation process in hydrochloric acid solution. To overcome chlorine evolution during the lateral activity measurements the pulsprofile  has  to  be  adapted. With this modified  RC-SECM  the local catalytic activity  of  catalysts in a hydrochloric acid solution has been studied and successfully visualised (Fig.1). A specifically designed cell allows the investigation of powder catalysts. Development of this approach and first results are presented.

[1] K. Eckhard, X. Chen, F. Turcu, W. Schuhmann, PCCP 2006, 8, 5359-5365

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