Electrochemical promotion of catalysis on nanometric sputter-deposited films

P. VERNOUX; A. BILLARD; L. LIZARRAGA; V. ROCHE; S. SOUENTIE

Las Vegas

Encuentro; 218th Electrochemical Society Meeting; 2010

Electrochemical Society

Electrochemical promotion of catalysis (EPOC) or NEMCA (Non-Faradaic Electrochemical Modification of Catalytic Activity) effect is a promising concept for boosting actual catalytic processes and advancing the frontiers of catalysis. This innovative field aims to operando modify both the activity and the selectivity of catalysts, in a reversible and controlled manner. EPOC utilizes solid electrolytes materials as catalytic carriers. Ions containing inside these electrolytes are electrochemically supplied to the catalyst surface and act as promoting agents to modify the catalyst electronic property in order to achieve optimal catalytic performances. EPOC can be considered as an electrically-controlled catalyst support interactions in which promoting ionic agents are accurately supplied onto the catalytic surface by the polarization control. The main advantage of EPOC is that the electrochemical activation magnitude is much higher than that predicted by the Faraday?s law. Therefore, EPOC requires low currents or potentials. This presentation will review recent developments [1-4] on the utilization of nanometric sputtered films of metallic catalysts deposited on O2- ionic conductors. Our double objective is to achieve electrochemical activation at low temperatures and for extremely low noble metal loadings. The main targeted applications are the catalytic deep oxidation of Volatile Organic Compounds (VOC) or hydrocarbons and the automotive catalytic post-treatment (NOx, CO, unburned hydrocarbons). Physical vapour deposition allows to minimize the noble metal loading and to increase the metallic dispersion. The metallic loadings were in the range 20-100 μg.cm-2. In addition, we propose a new design of electrochemical catalysts in which the electronic conductivity is no more ensured by the catalytic active phase but by an oxide interlayer. In that configuration, the Pt content is about 3 orders of magnitude lower than that commonly used in EPOC studies. Catalytic performances of sputtered electrochemical catalysts, based on Pt and Pd films interfaced with Yttria- Stabilized-Zirconia (YSZ) or Gadolinia Doped Ceria (GDC) substrate, will be presented for hydrocarbons (methane, propane, propene) deep oxidation. The results will be discussed considering two aspects: - the future industrial applications of EPOC, - and the understanding of the physicochemical origins of EPOC. Electrochemical catalysts based on Pt and Pd sputtered films over YSZ were successfully used in the catalysis of propane and methane oxidation, respectively. For instance, combustion of propane was strongly improved when anodic currents were applied, due to the NEMCA effect. The sputtered-Pt electrochemical catalysts presented better characteristics for the electrochemical promotion of catalysis than Pt-paste electrochemical catalysts, due to their greater dispersion value and higher polarizability. In addition, a new design of electrochemical catalysts was prepared by adding a sputtered interlayer of strontium-doped lanthanum manganite between a GDC dense support and an ultrathin coating of Pt (2 μg/cm² of Pt). We have shown that the catalytic activity of this ultra-thin layer of Pt deposited on the conductive interlayer of perovskite can be electropromoted with high Faradaic efficiencies. An example is given on figure 1 for the propene deep oxidation