Ethylene electrooxidation into ethylene oxide on nanostructured Ag/GDC electrocatalysts

SSCI-VIDE+CARE+PVE:ACVInternational audienceEthylene oxide (EO) is nowadays a chemical intermediate of paramount importance. As one of the highest volume petrochemicals, it serves as a precursor for the further production of added value molecules, including plastics, polyester and ethylene glycol. EO is industrially produced by the partial oxidation (epoxidation) of ethylene with air (or oxygen) on Ag/α-Al2O3 catalysts at low reaction temperatures (around 220 oC) and high pressures (10-30 bar). Silver is the only one known active heterogeneous catalyst for the epoxidation of ethylene. The epoxidation is in competition with the combustion side reactions of both ethylene and EO, leading to the production of CO2 and H2O. The mechanism of the epoxidation and, in particular the nature of the involved oxygen species are under debate. We have designed by reactive magnetron sputtering a solid oxide electrochemical cell based on nanostructured Ag/Gadolinia-Doped Ceria (a well-known O2- conductor) catalyst. We found an unprecedent linear increase of the selectivity towards EO (Fig. 1) and of the ethylene conversion. These results demonstrate that EO can be electrocatalytically produced in a solid oxide electrochemical cell between 220 °C and 300 °C on a nanostructured Ag/GDC electrocatalyst prepared by reactive magnetron sputtering. Despite low values of Faradaic efficiencies, the EO selectivity and the ethylene conversion can be increased at 220 °C with the application of small positive currents (< 1 mA) that correspond to an energy consumption of around ~ 1 mW/cm². This proof-of-concept opens the way for a new environmentally-friendly route (without utilization of chlorinated hydrocarbons) for the epoxidation of ethylene in a solid oxide cell via direct electrooxidation of ethylene. These results also underline the key role of O2- species in the mechanism of ethylene epoxidation on Ag

Electrochemical Promotion of Propylene Combustion on Ag-based nanostructured catalysts

MICROSCOPIE+CARE+IKL:TCV:ABO:LBU:FGA:ACV:PVEInternational audienceCatalytic and electrocatalytic measurements have been carried out in a solid oxide cell for propylene combustion using continuous Ag catalytic films and Ag nanoparticles dispersed on a mixed ionic electronic conductor film deposited on yttria-stabilized zirconia. Similar EPOC behaviors were observed for both samples, indicating that dispersed Ag nanoparticles in LSCF can be electropromoted in a reversible and non-Faradaic manner under lean-burn conditions

Electrochemical Promotion of Propene Combustion on Ag Catalytic Coatings

SSCI-VIDE+CARE+IKL:TCV:ABO:EBA:PVEInternational audienceThe 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 (O2- 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 O2- 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 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 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

Electrochemical Promotion of Ethylene Epoxidation over Ag-based composite electrodes

MICROSCOPIE+CARE+TCV:TTG:IKL:ABO:LBU:FGA:ACV:PVEInternational audienceElectrochemical 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)

Electrochemical promotion of propylene combustion on Ag catalytic coatings

MICROSCOPIE+CARE+IKL:TCV:ABO:LBU:FGA:LRE:EBA:PVEInternational audienceAg coating

Electrochemical Promotion of Ethylene Epoxidation over Ag-based composite electrodes

MICROSCOPIE+CARE+TCV:TTG:IKL:ABO:LBU:FGA:ACV:PVEInternational audienceElectrochemical 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)

Electrochemical promotion of propylene combustion on Ag catalytic coatings

MICROSCOPIE+CARE+IKL:TCV:ABO:LBU:FGA:LRE:ACV:PVEInternational audienceCatalytic 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

Promotion électrochimique de la combustion du propène sur des couches catalytiques d’argent

MICROSCOPIE+CARE+TCV:IKL:ABO:LBU:FGA:LRE:ACV:PVENational audienceDes tests catalytiques et electrocatalytiques ont été menés sur des membranes denses conductrices ionique pour la combustion du propène en utilisant une couche catalytique d'argent synthétysée via PVD ou sérigraphie déposée sur une membrane dense de Zircone stabilisée avec 8% d'yttrium (YSZ), un conducteur ionique de O2-. Une adsorption compétitive entre le propène et l'oxygène a été observée à 300°C. Les performances catalyques des films d'argents ont pu être modifiées via une polarisation et ce, de manière non-faradique (effet EPOC) à 300°C. L'impact de la polarisation depend principalement du taux de recouvrement du propène sur l'argent : pour des taux de recouvrement faible comme dans des conditions d'oxydation, des effets de promotions ont été observés lors de polarisation positive alors que pour un fort taux de recouvrement, obtenu dans des conditions stoechiomètrique, seulement des effets d'inhibition ont pu être observé lors de polarisations négatives

Electrochemical promotion of propylene combustion on Ag catalytic coatings

MICROSCOPIE+CARE+IKL:TCV:ABO:LBU:FGA:LRE:ACV:PVEInternational audienceCatalytic 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

Electrochemical Promotion of Propylene Combustion on Ag Catalytic Coatings

MICROSCOPIE+CARE+IKL:TCV:ABO:LBU:FGA:LRE:PVEInternational audienceCatalytic and electrocatalytic measurements have been carried out in a solid oxide cell for propylene combustion using Ag catalytic films deposited on yttria-stabilized zirconia. Competitive adsorption between oxygen and propylene has been observed while catalytic performances can be tailored by current application 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 enhance the propylene conversion by producing more reactive oxygen species. This beneficial effect is more pronounced under lean-burn conditions where the oxygen coverage on Ag is high