print pageprinter friendly
Reports: ND553491-ND5: Dissolution of Pt Catalyst During Electroreduction of Oxygen

Michael V. Mirkin, City University of New York (Queens College)

During the second year of the grant period, we continued to investigate the mechanism of electrocatalytic oxygen reduction reaction (ORR).  The development of better catalysts for ORR and other electrocatalytic processes requires detailed knowledge of reaction pathways and intermediate species.  Despite extensive efforts, basic questions about the ORR mechanisms remain open.1  One controversial issue is the nature of the first step in the overall four electron/four proton ORR at Pt-based catalysts.  For instance, Adzic and co-workers reported a decisive proof of superoxide intermediate (O2●-) formation on polycrystalline Pt surface in alkaline solutions.2  In contrast, a detailed microkinetic model developed by the Nørskov group does not include this species as an ORR intermediate at Pt(111) surface.3  We introduced new methodology for detecting charged reactive intermediates and used it to obtain direct evidence of superoxide formation at the bare polycrystalline Pt surface during the ORR in neutral aqueous solution and its desorption.4 

A nanopipette filled with organic phase immiscible with the external aqueous solution was used as a tip in the scanning electrochemical microscope (SECM) to detect and identify the short-lived superoxide intermediate, determine the rate of its generation at the catalytic Pt substrate and its lifetime in neutral aqueous solution.  The voltammograms of the O2●- anion transfer to the organic phase across the liquid/liquid interface were obtained for the first time.  Ion transfer (IT) voltammetry can differentiate between the anionic, neutral and cationic reaction intermediates because neutral molecules (e.g., H2O2) do not contribute to the IT current, while positively and negatively charged transferred species produce the currents flowing in the opposite directions.  The half-wave potential of the O2●- transfer was determined and used for unambiguous identification of this intermediate.  This approach can be employed for the detection and identification of charged intermediates of various heterogeneous chemical and electrochemical reactions in both aqueous and non-aqueous solutions

The half-life of O2●- in aqueous solution was evaluated to be ~2 µs.  The detection of superoxide radical in solution was possible due to extremely high mass-transfer rate of the SECM.  With the attainable distance of the closest approach between the nanopipette tip and catalytic substrate of about 1 nm, this technique should be capable of detecting ionic intermediates of heterogeneous reactions with the lifetime of a few nanoseconds.

We also developed an SECM-based technique for in situ characterization of the shape of non-spherical catalytic nanoparticles.5  The nanocubes with the 13 nm edge length were attached to the diazonium-modified HOPG surface and imaged by the SECM.  An advantage of this approach is that after determining the shape and size of a specific NP, the SECM tip can probe electrocatalytic processes occurring at its surface.

1.  Xu, Y.; Shao, M.; Mavrikakis, M.; Adzic, R. R., in Fuel cell catalysis: a surface science approach; Koper, M. T. M., Ed.; Wiley: Hoboken, NJ, 2009; pp 271-315.

2.  Shao, M. H.; Liu, P.; Adzic, R. R., J. Am. Chem. Soc. 2006, 128, 7408.

3.  Hansen, H. A.; Viswanathan, V.; Norskov, J. K., J. Phys. Chem. C 2014, 118, 6706.

4.  Zhou, M.; Yu, Y.; Hu, K.; Mirkin, M. V. J. Am. Chem. Soc. 2015, 137, 6517.

5.  Blanchard, P.-Y.; Sun, T.; Wei, Z; Matsui, H.; Mirkin, M. V., ACS Nano, submitted.