Electrochemical promotion of propylene combustion on Ag catalytic coatings
Résumé
Catalytic combustion as a process used for removal of hydrocarbons from automotive gas exhausts or for energy production has been widely implemented on supported PGM (Platinum Group Metals) based catalysts1. Since PGMs are very costly and rare, there is a strong need for an equally effective and less expensive catalyst. The electrochemical promotion of catalysis (EPOC), is a promising concept to in-operando boost catalytic processes in a reversible and controlled manner2. The aim of this study was to develop Ag-based electrochemical catalysts for low temperature propylene deep oxidation. EPOC of propylene combustion has been carried out in the literature but mainly on Pt catalytic films1,3, while this phenomenon was attributed to the modification in the propylene chemisorption3.
Nanostructured electrochemical catalysts were prepared by screen-printing and reactive Physical Vapor Deposition (PVD) method. 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 reactor1 which operated under continuous flowing conditions at atmospheric pressure. The catalytic activity was monitored in a temperature range of 100 to 400oC 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 polarization between the Ag working electrode and an Au reference electrode. Both electrodes were exposed to the same atmosphere in a single chamber configuration.
Values of Faradaic efficiencies in the range of 300 were obtained while the conversion could be tailored from 14 to 21% (Fig. 1). Negative current applications lead to the decrease of the CO2 production while positive current application corresponds to a pronounced increase of the catalytic performance. Upon positive current applications, the rate enhancement ratio increases with the intensity of the current. This indicates that the coverage of promoting ionic species (Oδ-) increases with the current, then producing more weakly bonded oxygen species coming from the gas phase4.
This study reports, for the first time, that the catalytic activity for propylene of Ag coatings deposited onto YSZ can be tailored by current applications in a non-Faradaic manner. The predominant impact of current applications is to modify the reactivity of oxygen present on the Ag surface. Positive current applications increase the propylene conversion by producing more reactive oxygen species. This beneficial effect is more pronounced in an oxidizing atmosphere, where the oxygen coverage on Ag is high. This demonstrates that EPOC can enhance catalytic properties of Ag coatings for the abatement of propylene in air.
References
1.P. Vernoux et al., J. Catal., 208 (2002) 412-421.
2.C.G. Vayenas, Electrochemical Activation of Catalysis: Promotion, Electrochemical Promotion, and Metal-Support Interactions, Springer, 2001.
3.A. Kaloyannis et al., J. Catal., 182 (1999) 37-47.
4.I. Kalaitzidou et al., Materials Today: Proceedings, 5, 27345 (2018).
Acknowledgments: This study was performed in the “EPOX” project, funded by the French National Research Agency (ANR), ANR-2015-CE07-0026.