The non-faradaic electrochemical modification of catalytic activity (NEMCA effect) or the electrochemical promotion of catalysis (EPOC) has been investigated thoroughly for more than 120 catalytic reaction systems [1,2]. In electrochemical promotion studies, the conductive catalyst-electrode is in contact with an ionic conductor and the catalyst is electrochemically promoted by applying a current or potential between the catalyst film and a reference electrode. Numerous surface science and electrochemical techniques have shown that EPOC is due to the electrochemically controlled migration of promoting or poisoning ionic species (O^2-in case of YSZ) between the ionic conductor and the gas exposed catalytic surface [1].

Propene is one of the major unburnt hydrocarbons containing in diesel cars exhausts. The most effective materials for propene combustion are Platinum-Group Metals. In spite of their high efficiency, these catalysts cannot be considered in the medium-term due to their excessive cost, which makes necessary stages of recovery and recycling [1]. Ag-based catalysts represent a possible alternative. This study reports the electrochemical promotion of the propene combustion on Ag films deposited on 8 mol% Y₂O₃ stabilized ZrO₂ solid electrolyte, an O^2-ionic conductor.

Nanostructured electrochemical catalysts were prepared by screen-printing and reactive Physical Vapor Deposition (PVD) method. Screen-printing technique is a flexible tool to prepare few µm thick porous films at low cost whereas extremely thin coatings of Ag can be produced by PVD. Thickness and porosity of Ag coatings were modified by changing the deposition parameters (duration and pressure for PVD, nature of the ink and calcination temperature for screen-printing) to optimize the catalytic properties.

Catalytic and electrocatalytic tests have been carried out in a quartz reactor [3] which operated under continuous flowing conditions at atmospheric pressure. The catalytic activity was monitored in a temperature range of 100 to 400^oC under lean-burn conditions, as encountered in Diesel exhausts. The most active Ag films were also evaluated under closed circuit conditions (± 2V) in order to measure the effect of polarisation between the silver working electrode and an Au reference electrode. Both electrodes were exposed to the same atmosphere in a single chamber configuration.

The catalytic activity of samples is depicted in Figure 1. All Ag coatings are effective from around 200°C. The catalytic performances were correlated with the microstructure of the films. Furthermore, the propene combustion was found to be electropromoted on the most active Ag films at low temperature with Faradaic efficiencies larger than 100.

References:

[1] C.G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda and D. Tsiplakides, Electrochemical Activation of Catalysis: Promotion, Electrochemical Promotion, and Metal-Support Interactions, Kluver Academic / Plenum Plublishers, New York, 2001.

[2] A. Katsaounis, J. Applied Electrochemistry 40 (2009) 885-902.

[3] P. Vernoux, F. Gaillard, L. Bultel, E. Siebert and M. Primet, Journal of Catalysis 208 (2002) 412-421.