Electrochemical Promotion of Ethylene Epoxidation over Ag-based composite electrodes
Résumé
Electrochemical Promotion of Catalysis (EPOC) is an innovative concept for boosting catalytic processes in a reversible and controlled manner [1]. The EPOC phenomenon takes place in fuel-cell type reactors where the catalytic coating is an electrode supported on a dense ionically conducting ceramic material (solid electrolyte). Ions, such as K+, contained in these solid oxide electrolytes are electrochemically supplied to the catalyst surface, changing its local electronic density. This way, the ions supplied behave as electronic promoters, modifying the activity and the selectivity of the catalyst. We used EPOC for designing new environmentally-friendly catalysts for the epoxidation of ethylene in order to produce ethylene oxide at atmospheric pressure with high selectivity to ethylene oxide (EO) without the need for chlorinated hydrocarbons in the gas feed. We have prepared Ag-based composite electrodes as Ag is the most efficient metal for this reaction.
Composite electrodes were prepared from a mixture between a Ag commercial paste and either a pure oxygen ionic conductor, i.e. Yttria-Stabilized-Zirconia (YSZ) or a Mixed Ionic and Electronic Conductor (MIEC), i.e. LSCF (La0.6Sr0.4Co0.2Fe0.8O3). These composites electrodes were deposited either on YSZ or on K+ conducting β-Al2O3 membranes. The impact of current applications on the ethylene oxide selectivity was carried out at 300°C while the electrochemical properties of the different Ag-based composite electrodes were investigated by cyclic voltammetry. In addition, TEM (JEOL 2010) and Environmental SEM (FEI QUANTA 650 FEG) were implemented to characterize the nanostructure of the electrodes including in-situ observations at 300°C in air.
Composites electrodes were prepared by mixing 75 wt.% of a commercial Ag paste (Metalon® HPS-FG32) with 25 wt.% of a powder of YSZ (TOSOH) or LSCF prepared with the Pechini method. These mixtures were deposited on YSZ or β-Al2O3 (Ionotec) dense membranes and calcined at 600°C for 2 h. The as-deposited morphology of these layers shows large micrometric Ag agglomerates mixed with nanometric grains of YSZ or LSCF. A period of activation of around 6 h on stream (C2H4/O2: 3.8%/1.1%) at 300°C was necessary to reach a steady-state activity with an EO selectivity at around 12% for an ethylene conversion of 4%. This activation process was attributed to the transfer of Ag from large Ag agglomerates to the conducting oxide surface in the form of 5-10 nm diameter Ag nanoparticles (Figure 1) due to the high evaporation rate of AgOx at 300°C. This evaporation process was in-situ observed in air at 300°C with the ESEM. Therefore, the activation process on stream at 300°C leads to highly dispersed Ag-based composite electrodes containing Ag NPs supported on YSZ or LSCF. The impact of potential and current applications on the EO selectivity was investigated at 300°C according to the nature of the solid electrolyte (O2- and K+ conductors) and the conducting oxide in the composite (YSZ and LSCF).