Les couches d’argent pur ne permettent pas d’obtenir une activité suffisante pour étudier la réaction d’époxydation de l’éthylène dans nos conditions expérimentales. Un nouveau type de couche catalytique a alors été synthétisé. Ces couches composites entre l’argent (encre d’argent A décrite dans le chapitre 3) et de la poudre de zircone stabilisée à l’oxyde d’yttrium (YSZ) permettent d’augmenter la porosité et les interactions Ag/YSZ. Les interactions entre un métal et un oxyde conducteur ionique peuvent améliorer les performances catalytiques grâce à la migration thermique des ions oxydes.
Les couches composites Ag-YSZ ont été préparées en mélangeant 25% en masse de poudre de YSZ et 75% en masse de laque d’argent, déposées sur une pastille dense de YSZ et calcinées à 600°C sous air pendant 4 heures. Nous avons étudié les performances catalytiques de ces couches composites Ag-YSZ pour l’époxydation de l’éthylène à pression atmosphérique (C2H4/O2 : 4%/1%) au cours de plateaux de température entre 260°C et 300°C. L’impact d’un prétraitement (oxydant et réducteur) à 300°C sur les performances catalytiques des électrolyseurs Ag-YSZ a également été étudié. Des essais complémentaires sur un échantillon tubulaire (couche composite Ag-YSZ déposée sur un tube de YSZ) ont permis d’investiguer l’effet de la masse d’électrolyseur sur les propriétés catalytiques à 300°C. Nous avons comparé ces résultats à ceux mesurés sur des couches composites Ag-oxyde élaborées à partir de cérine dopée au gadolinium (CGO, un autre conducteur ionique par les ions oxydes) et d’alumine alpha (le support industriel) ainsi que sur un catalyseur en poudre constitué de nanoparticules d’argent dispersées sur YSZ.
La nanostructure des couches composites après leur passage sous mélange réactionnel aux différentes températures a été analysée par microscopie électronique en transmission. De plus, une étude in situ a été réalisée sur une couche composite Ag/YSZ en microscopie électronique en transmission environnementale, sous mélange réactionnel à 300°C afin de suivre in-situ les modifications du catalyseur. Tous les résultats obtenus sont présentés dans la publication suivante.
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Self-promoted ethylene epoxidation over Ag/YSZ composite coatings
T. Cavoué1,2, A. Caravaca1*, F. Cadete Santos Aires1, L. Burel1, M. Aouine1, M. Rieu2, J. P. Viricelle2, P. Vernoux1*
1Université de Lyon, Institut de Recherches sur la Catalyse et l’Environnement de Lyon, UMR 5256, CNRS, Université Claude Bernard Lyon 1, 2 avenue A. Einstein, 69626 Villeurbanne, France
2 Mines Saint-Etienne, UnivLyon, CNRS, UMR 5307 LGF, Centre SPIN, F -42023 Saint-Etienne France
1. Introduction
Ethylene oxide (EO) is nowadays a chemical intermediate of paramount importance. Being 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 [1,2]. The most widely used catalytic process for the production of EO is the partial oxidation (epoxidation) of ethylene with air (or oxygen) on Ag/α-Al2O3 catalysts at low reaction temperatures (> 220 oC) and high pressures (10-30 bar) [3].
In the absence of any chemical promoters, the selectivity towards EO is in the range of 50 % [4]. To enhance the efficiency of the epoxidation process, the addition of chlorinated hydrocarbons (e.g. dichloroethene or vinyl chloride) is known to improve the selectivity to values ~ 90 %. The role of chlorine as a promoter is not well understood and could be attributed to both geometric and electronic effects, changing the relative populations of surface species during reaction [5]. One of the most accepted idea is that Cl promoter weakens the Ag-O interaction [1], leading to the production of weakly adsorbed electrophilic oxygen species on the Ag active sites. Even though the reaction mechanism for ethylene epoxidation is not yet completely understood, these oxygen species are generally recognized as the active and key species for this process [6,7]. However, chlorinated promoters are not environmentally friendly, and their eventual accumulation leads to a significant poisoning of the catalyst.
In this sense, previous studies carried out by the group of Prof. Vayenas [8,9] suggested to use O2- ions as electronic promoters for the ethylene epoxidation reaction. These studies where performed in the frame of the phenomenon of Electrochemical Promotion of Catalysis (EPOC) [10–12]. This phenomenon takes place on the so-called electrochemical catalysts, which are mainly composed of one catalyst layer (which also behaves as an electrode) and a non-reducible solid electrolyte (responsible for the supply of ionic promoter species).
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In their first study [8], Stoukides and Vayenas reported the promotional effect of O2- ions on a Ag catalyst film on Ag//YSZ (catalyst-electrode//solid electrolyte) electrochemical catalysts for ethylene epoxidation. It was found that pumping O2- ions towards the Ag electrode via electrical
polarization allowed to dramatically enhance the selectivity and yield of EO production at 400
oC and atmospheric pressure. It was demonstrated that the rate of EO production can exceed the rate of O2- electrochemically supplied by a factor of 400. In other words, upon polarization, the enhancement of the EO production due to the catalytic process on the Ag catalyst-electrode (C2H4 + 0.5 O2 Æ C2H4O) was much more pronounced than that for the electro-catalytic oxidation of C2H4 with the O2- ions (C2H4 + O2- Æ C2H4O + 2 e-).
Even though these studies clearly demonstrated the high potential of the EPOC phenomenon to enhance the selectivity for the EO production, dense Ag electrodes with low porosity, big Ag particles and therefore low metallic dispersion were used as catalysts. It leads to an overall rather low catalytic activity compared to the conventional industrial Ag/α-Al2O3 catalysts. The idea of this study is to take advantage of the main features of electrochemical catalysts (i.e., the possibility to in-situ supply O2- promoters) and conventional industrial catalysts (i.e., higher metallic dispersion and accessibility to active sites). In this sense, we have previously reported that these ionic oxygen species could thermally migrate without any electrical polarization to the metallic nanoparticles of catalysts supported on ionically conducting ceramics, such as YSZ [13,14]. One would expect that the magnitude of this driving force will depend on the support ionic conductivity, the temperature and the catalyst/oxide interface. In other words, this phenomenon could be attributed to the Metal Support Interactions (MSI). Focusing on YSZ as a catalytic support, this phenomenon has been investigated for a wide variety of catalytic systems, such as propene deep oxidation [13] and toluene oxidation [15] on Pt/YSZ (with high activity in comparison with conventional supports such as γ-Al2O3), or ethylene oxidation on Ni/YSZ [16].
To the best of our knowledge, the use of Ag/YSZ catalysts (without electrical polarization) has never been reported for ethylene epoxidation. In this study, we will demonstrate that the activity of the Ag-based sites towards the EO production could be enhanced (in the absence of Cl species) due to the promotional effect of the O2- promoter species spontaneously generated via the MSI phenomenon above discussed. The impact of the reaction temperature, the pretreatment of the catalyst and the nature of the support were studied. In addition, the dynamic evolution of Ag/YSZ catalysts was in-situ observed with high temporal and spatial resolution by environmental Transmission Electronic Microscopy (E-TEM).
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2. Experimental
2.1. Preparation of planar and tubular Agpure and “Ag/supportoxide composite” coatings on YSZ dense solid electrolytes.
YSZ dense disks (18.7 mm diameter and 1 mm thick) and bottom-closed tubes (300 mm length, 22 mm external diameter, 1 mm thick) were prepared by sintering a commercial YSZ powder (Tosoh, ZrO2-8 mol % Y2O3) at 1350 °C for 2 h (densification higher than 98%). For the preparation of the planar Agpure film supported on YSZ, commercial silver-organic ink (Metalor ® HPS-FG32) was deposited on one side of a YSZ disk (electrode area = 2.7 cm²) by screen printing method, followed by calcination at 600°C for 2 h in air (heating ramp of 2 °C min-1). The final Ag loading was ~ 20 mg, i.e. ~ 7.4 mg.cm-2.
For the preparation of planar “Ag/supportoxide coatings”, a previously reported (for Pt/YSZ) procedure was used [17]. First, an ink was prepared by thoroughly mixing Ag-organic ink and the oxide powder (e.g., YSZ) with ethanol. The ink was then deposited with a brush on the YSZ solid electrolyte disk, and calcined at 600 oC following the same procedure above described. After calcination, the final composite loading was ~10 mg (~3.7 mg.cm-2), with an Ag/supportoxide weight % ratio of 75/25. Three different oxide powder materials were used as supports in this study: YSZ (Tosoh, ZrO2-8 mol % Y2O3, particle size= 40 nm and specific surface area = 16 m² g-1), GDC (Marion Technologies, Gadolinium Doped Ceria, CeO2-10 mol % Gd2O3, GDC10, particle size = 400 nm and specific surface area = 7.5 m² g-1) and D-Al2O3
(Rhône-Poulenc, specific surface area < 1 m2 g-1).
In addition, a Ag/YSZ composite was prepared following the same procedure and deposited on the external side a YSZ dense tube. An external area of 14 cm2 was covered with ~ 51 g of the Ag/YSZ composite (~ 3.6 mg.cm-2).
For comparison, the YSZ powder was impregnated with a silver nitrate aqueous solution obtained by dissolution of AgNO3 (Sigma Aldrich, purity 99%) to achieve 1 wt% Ag loading. This material was calcined at 700°C for 4 h in flowing air.
2.2. Ex-situ and in-situ characterization
All samples were observed ex-situ by Transmission Electronic Microscopy (TEM) after the catalytic tests using a JEOL 2010 microscope. The acceleration voltage was 200 kV with a 0.2 nm resolution. The catalyst coatings supported on the dense YSZ disks were scratched and
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dispersed in dry ethanol using an ultrasound bath. One drop of solution was then deposited on a copper grid for TEM measurements.
Ex-situ Scanning Electronic Microscopy (SEM) was performed by means of an ESEM-FEG microscope (Philips). Samples were directly analyzed without any further preparation.
An Environmental Transmission Electron Microscope (ETEM) was used to in-situ observed the nanostructure of the Ag/YSZ composite under the reaction conditions typical for ethylene epoxidation. First, the sample was heated up to 300 oC in N2 (4 mbar). Then, the reaction mixture 3.8 % C2H4 /1.1 % O2 was introduced, and after 17 minutes, images were captured every 20 seconds for 1 h and 45 minutes. This latest generation ETEM 197 (Titan 80–300 kV from FEI™) was equipped with an imaging aberration corrector and an energy-dispersive X-ray (EDX) analyzer (SDD X-Max 80 mm2 198 from Oxford Instruments™) used for elemental chemical analysis. The sample was deposited on titania grids covered with a 200 silica film and placed into a Gatan™ furnace-type holder as already described elsewhere (ref). The ETEM was operated with a beam voltage at 80 and 300 kV to evaluate the effect of the electron beam energy.
O2-Temperature programmed desorption (O2-TPD) experiments were in-situ performed on the tubular configuration in order to observe the desorption temperature of adsorbed oxygen species. First, the catalyst was pretreated at 500 oC in O2 (5 %) in order to desorb (decompose) any residual species (e.g., carbonates). After cooling down the system at 200 oC in He, 5 % O2
(He carrier, 3 L h-1) was introduced for 1 hour. Then, the reactor was cooled down to room temperature under the same reaction atmosphere. After purging the system with He (15 min), the temperature programmed experiment was performed by heating up to 500 oC (5 oC min-1) in He (3 L h-1). Online analysis of O2 concentration was performed by a micro Gas Chromatograph (SRA).
2.3.Catalytic activity measurements
The catalytic tests for the disk and tubular samples were performed in an experimental set-up described elsewhere [18,19]. The samples were placed in quartz reactors under continuous flow at atmospheric pressure. All catalytic tests were performed under a reaction mixture of 3.8 % C2H4 /1.1 % O2 (He carrier, 3 L h-1). Prior to the catalytic experiments, the samples were heated up to the desired reaction temperature (260 -320 oC) at a heating rate of 1 oC min-1 in He (3 L h-1). The influence of the sample pretreatment was also studied. Oxidizing and reducing pretreatments were performed by introducing pure O2 and pure H2 gases for 1 hour at 300 oC
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(3 L h-1). Then, the system was purged with He for 15 minutes prior to introducing the reaction mixture. The reactants and products were analyzed by an on-line micro gas-chromatograph (μGC-R3000, SRA) composed of a BF PPU 8m/PPQ 1m column (CO2 analysis) and a VAR PLOT Q 8m column (Ethylene Oxide and acetaldehyde analysis). The detection limit for Ethylene oxide analysis was ~ 2 ppm. In addition, an Infrared CO2 analyzer (HORIBA VA-3000) was used for faster resolution CO2 analysis. The concentration of acetaldehyde was found to be negligible for all measurements. CO2 and EO were the only two detected products. 3. Results and discussion
3.1.Activity of Ag/YSZ composites: Influence of the reaction temperature
Figure 1 shows the SEM images of the fresh Ag-based coatings (as-prepared) on YSZ dense solid electrolyte disks. Fig. 1 a) and b) show the morphology of the Agpure catalyst layer, while Fig. 1 c) and d) show the images of the Ag/YSZ composite. As previously observed in a recent study [20], the Agpure catalyst layer exhibits low porosity with big Ag particles in the micrometer range, as expected from the direct calcination of organic inks at high reaction temperatures. However, the addition of YSZ powder to the Ag ink (leading to the Ag/YSZ composite) led to the formation of a catalyst layer with higher porosity and slightly smaller Ag agglomerates. A similar phenomenon was observed in a previous study [17] with Pt/YSZ composites. It seems therefore that the addition of YSZ powder allowed to re-disperse the Ag particles over a higher surface area (provided by the powder) during the calcination step.
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Figure 1: SEM of Ag-based coatings, surface and top view representative images of Ag (a and b) and Ag/YSZ composite (c and d).
Regarding the catalytic activity for ethylene epoxidation, the Ag catalyst layer exhibits a poor performance (not shown here), with ethylene conversions even lower than 0.01 % at 300 oC, and a null production of the product of interest (EO). This is in good agreement with the low dispersion of the Agpure catalyst, which leads to a limited amount of metallic active sites. However, the Ag/YSZ composite exhibits a drastically improved catalytic performance. Figure 2 depicts the variation of EO and CO2 production rate (together with EO selectivity) with time at different reaction temperatures (260 - 300 oC).
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Figure 2: Catalytic activity of the Ag/YSZ composite coating on YSZ solid electrolyte disk. Influence of the reaction temperature on: a) production rate of EO, b) production rate of CO2, and c) Selectivity towards EO. Reaction conditions: 3.8 % C2H4 + 1.1 % O2 (He balance, 3 L h-1). Temperature: 260 – 320oC.
At all the reaction temperatures studied, a dynamic behavior could be observed, were the activity evolved with time in an unprecedented manner. At the “steady-state”, i.e. when the activity of the system was stabilized at each temperature, the maximum overall production of