An attempt to improve Ag-based catalysts for allyl alcohol oxidative dehydrogenation to acrolein

Applied Catalysis A, General 549 (2018) 170–178

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Perspective

An attempt to improve Ag-based catalysts for allyl alcohol oxidative dehydrogenation to acrolein

MARK

T.T.N. Nguyena, M. Huchedea, E. Blancoa, F. Morfina, J.L. Rousseta, L. Massina, M. Aouinea, ⁎ V. Bellière-Bacab, J.M.M. Milleta, a

Institut de Recherches sur la Catalyse et l’Environnement de Lyon, IRCELYON, UMR5256 CNRS-Université Claude Bernard, Lyon I, 2 avenue A. Einstein, F-69626, Villeurbanne cedex, France b Adisseo, Antony Parc 2, 10 Place Général de Gaulle, 92160 Antony, France

A R T I C L E I N F O

A B S T R A C T

Keywords: Silver Allyl alcohol Oxidative dehydrogenation Molecular oxygen Acrolein

The oxidative dehydrogenation of allyl alcohol to acrolein over bulk alloyed and supported silver catalysts has been studied, with the aim of designing optimal Ag-based catalysts for this specific reaction. All of the tested catalysts were highly active and selective to acrolein at temperatures ranging between 340 and 380 °C. The results show that the alloying of Ag with Au or Cu leads to alloys with upper surface compositions comprising approximately between 35 and 65% silver. Although these elements improve the activity of Ag catalysts, they decrease their selectivity, leading to lower acrolein yields. Various different Ag catalyst supports were prepared (LaPO4, Carbon nanotubes CNT, TiO2, Al2O3 and SiO2) using the impregnation method, and subsequently tested. This study reveals that the catalyst with the best performance is Ag/SiO2 with a yield in acrolein of 87.2% at 360 °C. The results obtained with the tested catalysts are discussed, and the reasons of performance improvement or degradation caused by the presence of Au or Cu on the active silver sites, or by the presence of the supports, are raised.

1. Introduction Currently, acrolein is produced primarily by the gas-phase oxidation of propylene. However, the use of propylene derivatives has recently experienced rapid growth, and since this trend is likely to continue in the foreseeable future, the availability and cost of propylene is becoming a cause for global concern. One approach to the mitigation of the demands placed on this increasingly scarce and costly building block involves the development of new production processes, relying on the use of new starting materials [1]. In this respect, we have recently developed catalysts for the direct conversion of glycerol to acrolein [2,3]. However, processes based on this type of catalyst are faced by two major difficulties; the first of these is their more or less rapid deactivation; the second drawback is their selectivity, which is capped at an upper limit of less than 80%. There is thus a need for the development of novel routes for the production of acrolein. One approach would involve the conversion of allyl alcohol, which could be produced in an initial step either by fermentation, or through the use of processes such as the dehydration of 1,3-propanediol, or the dehydration/oxidation of glycerol [4,5]. With the first of these processes, allyl alcohol is produced at 325 °C, with a maximum selectivity of 98.9 mol%. In the

⁎

case of the second process, which consists in treating glycerol with an acid such as formic acid, an allyl alcohol yield of 99% can be achieved at 235 °C. Allyl alcohol reactions, produced under the action of heat and in the presence of catalysts, were first studied in 1921 [5]. The latter study showed that blue tungstic oxide (WO3-x) was the most active catalyst, with the gas effluent containing H2, CO and CO2 in variable proportions, together with unsaturated hydrocarbons. The condensed liquid comprised unchanged allyl alcohol, ethanol, acrolein, and higher aldehydes. In 1926, F. H. Constable studied the dehydrogenation and isomeric change of allyl alcohol [6], and found that the main product obtained with MnO was acrolein. It was not until 1945 that a subsequent reference was made to the catalytic oxidative dehydrogenation of allyl alcohol to acrolein [7]. In this study, allyl alcohol was reported to be oxidized to acrolein over an Ag catalyst, under the same conditions that are generally used to oxidise ethylene to oxirane. When 2 vol. % of allyl alcohol in air was passed at 15 l h−1 through a bed of Ag (100 mm deep and 20 mm diam.) at 200–400 °C, the conversion efficiency of allyl alcohol to acrolein was 76%. Later, new experiments were carried out with the same type of catalyst [8]. Mixtures of allyl alcohol and air with O2 to alcohol molar

Corresponding author. E-mail address: [email protected] (J.M.M. Millet).

http://dx.doi.org/10.1016/j.apcata.2017.09.018 Received 3 August 2017; Received in revised form 15 September 2017; Accepted 16 September 2017 Available online 22 September 2017 0926-860X/ © 2017 Elsevier B.V. All rights reserved.

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ratios of 0.2-0.9, were oxidized by flowing the gas mixture at 200–400 °C through a tube filled with Ag wires, using various flow rates [8]. The best yield (53%) was found at 350 °C, using an Ag wire catalyst (12 g placed in a porcelain reaction tube 70 cm in length and 1.5 cm in diameter). The vaporization rate of allyl alcohol was 0.34 ml min−1 and the air-flow rate was 240 ml min−1. In 1965, Sulima et al. reported that allyl alcohol could be efficiently oxidized by air, over Ag supported on Al2O3 catalysts at 200–530 °C [9]. The acrolein yield decreases as the concentration of allyl alcohol and CO2 in the air increases. The maximum acrolein yield was achieved with a stoichiometric air-to-alcohol ratio in the range from 0.3 to 0.6. Seventeen years later, Imachi et al. [10] again studied the oxidative dehydrogenation of allyl alcohol, using a silver sponge catalyst described in a different paper [11]. These authors reported that their results could not be accurately reproduced from one run to the next. They hypothesized that the reaction was sensitive to the pre-conditioning of the catalysts, or to very small differences in the feed compositions. Selectivities to acrolein of 95% (with normal allyl alcohol) and 92% (with cis-allyl alcohol) were obtained at total conversion, and at 290 °C. In all of their experiments, the total gas feed flow rate was 60 cm3 min−1 and the gas carrier was N2. The allyl alcohol and oxygen concentrations in the feeds were respectively 1.0% and 1.5% and the catalyst mass 2 g. Pd-Ag thin films (monophasic alloy) were prepared on Pyrex glass substrates using the RF sputtering method, and were relatively successful when tested for the oxidative dehydrogenation reaction. The best results were obtained on Pd7 Ag93 with 95% selectivity at 50% conversion and at 300 °C [12]. More recently, the oxidation over Ag catalysts of various alcohols including allyl alcohol was studied [13], showing that at around 300 °C the transformation of allyl alcohol could be achieved with a conversion level of 87.5% and a selectivity to acrolein of approximately 97.5%. The aim of the present study is to develop new catalysts by screening various candidates under different experimental conditions, and analysing the relationships between catalyst’s properties and performances. Although new catalysts for the oxidative dehydrogenation of biosourced alcohols were recently developed with more or less of success, we chose to focus on Ag-based catalysts in this study [14–18]. The catalysts we tested were either self-supported, or prepared with various supports. In addition to the option of using a pure Ag metallic catalyst, Au and Cu alloys of Ag were tested. In Ag-Au alloys, silver has a tendency to segregate at the surface, whereas copper segregate to the surface of Ag since Ag-Cu alloys are thermodynamically unstable and systematically lead to demixing. In order to study the effects of alloying on the catalytic properties, it was thus of interest to focus on alloys that are rich in Au and poor in Cu. For the second type of catalyst, five silver-supported samples were prepared. Three conventional supports, i.e. Al2O3, TiO2 and SiO2 were qualified, in addition to two less conventional supports, i.e. LaPO4 and carbon nanotubes (CNT). The choice of the latter was guided by its large specific surface area for the first one and good electron conductivity and chemical inertness, as well as its excellent thermal conductivity for the second. The choice of LaPO4 was also driven by the fact that it has very different acid-base properties to those of other oxides, and that it has recently been used as a support for gold-based catalysts [19]. Al2O3, TiO2 and SiO2 are very conventional supports, and have already been used to support silver in other catalytic reactions [20–25]. The challenge in using these different types of support was to obtain nanoparticle catalysts without agglomeration, in order to study the role of Ag dispersion, and the possible intrinsic activity of the selected support.

nano-powder given with purity higher than 99.5%. The nanoporous AuAg alloy and an Au reference have been synthesized using a recently developed two steps method [26]. At first, an intimate mixture of a skeletal gold or gold-silver structure with ZrO2 nanoparticles was obtained by mild oxidation of an Au0.5Zr0.5 or Ag0.025Au0.475Zr0.5 intermetallic alloy. The zirconia was then selectively dissolved in fluorhydric acid. The resulting metallic system was a micrometric powder composed of grains whose sizes were between a few μm and 200 μm. Each grain looked like a nanoporous sponge." The Ag-Cu alloy was purchased from Aldrich (ref. 576824) and used directly as a catalyst. For the study of supported Ag catalysts, five supports have been selected Al2O3, TiO2, SiO2, carbon nanotubes (CNT) and LaPO4. The Ag/Al2O3 catalyst has been purchase from Alfa-Aesar (Ref. 45034) and the four others have been prepared in the laboratory. TiO2, SiO2 and the multi-walled carbon nanotubes (have respectively been obtained from Tioxide, Evonik and Pyrograf Products Inc., whereas LaPO4 was prepared using a two steps method with first the synthesis of a lanthanum bulky gelatinous slurry and secondly its digestion in an aqueous solution of phosphoric acid [27]. The final product was filtered, washed with distilled water, dried in air at 60 °C and calcined at 550 °C for 5 h. Because they are usually contaminated with metal catalysts, amorphous carbon, and graphitic nanoparticles, the carbon nanotubes (CNT) were washed prior to Ag deposition. They have been washed successively in a HNO3 7 M solution (5 g L−1) for 5 h under reflux and stirring and in 0.01 M of NaOH and HCl 0.01 M solutions and finally in distilled water [28]. The CNT were then dried in air at room temperature before heating at 800 °C for 2 h under N2. The specific surface areas of the support are given in Table 1. Silver was deposited on the supports by adding the selected mass of support to 50 ml of an aqueous solution containing AgNO3 (4.10−3 M) maintained at 80 °C. The mass was chosen to have catalysts with equal Ag dispersion (0.14 mgAg m−2). The pH of the solution was then adjusted to 9 by dropwise addition of NaOH (0.5 M). The suspension was vigorously stirred for 2 h at 80 °C. After the deposition–precipitation procedure, the samples were centrifuged, washed with water, centrifuged and dried under vacuum for 2 h at 80 °C. In the case of Ag supported on silica, the influence of the quantity of Ag deposited on the catalytic properties has been studied by preparing the same way a sample with 0.17 mgAg m−2. 2.2. Characterization of the catalysts The metal content of the catalysts has been determined by inductively coupled plasma−optical emission spectrometry (ICP-OES, Activa Horiba Jobin Yvon) and their specific surface areas measured using the BET method with nitrogen adsorption. Powder X-ray diffraction (XRD) patterns were obtained using Cu Kα radiation X-ray source operated at 45 kV and 20 mA. 2θ diffraction angle was varied from 20 to 90∘ during the measurement. International center for diffraction data (ICDD) library was used for crystal phase identification. The unit cell of the crystallized phase was refined using Bruker Topas P program. Scanning Electron Microscopy (SEM) has been performed using a ESEM FEG New XL30 environmental microscope coupled to an energy dispersion spectrometry (EDS) equipped with a diode Si-Li (PGT) for elemental analysis. The SEM sample was coated with a thin layer of carbon to enable electron conduction during observations and analyses. Ag supported samples have been characterized by transmission electron microscopy (TEM). Examination was conducted using a FEI TITAN

2. Experimental

Table 1 Specific surface area of the selected supports measured using the BET method.

2.1. Preparation of the catalysts Support

A commercial Ag reference catalyst (Aldrich Ref. 576832) has been chosen to start the testing and to calibrate the reaction analyses. It is a

2

SSA (m g

171

−1

)

Al2O3

TiO2

SiO2

LaPO4

CNT

185

110

200

124

64

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ETEM G2 80-300 KV equipped with an objective Cs aberration corrector. Elemental chemical analyses were achieved using an energydispersive X-ray (EDX) analyzer (SDD X-Max 80 mm2 from Oxford Instruments™). The samples were dispersed in ethanol using a sonicator. A drop of the suspension was dripped onto a carbon film supported on a copper grid; EDX studies were carried out using a 15 nm probe to analyze the particles. XPS measurements were performed on the self-supported catalysts with a Kratos Axis Ultra DLD spectrometer. All the data were acquired using monochromatic Al Kα radiation Ehν (Al Kα) = 1486.6 eV (10 mA, 15 kV) as the photon source. The analyzer was operated with a hybrid mode (combined electrostatic and magnetic lens) under ultra-high vacuum (5 × 10−9 torr), a 160 eV pass energy for acquisition of survey and a 20 eV pass energy for acquisition of high-resolution core-level spectra of Au4f, Ag3d, Cu2p, O1s, C1s. Charge neutralization was used to compensate charges effects on samples and the area analyzed was 700 μm × 300 μm. The binding energies were calibrated to the C-(C, H) components of the C1s band fixed at 284.6 eV. The experimental precision on the quantitative measurements was considered to be around 10%. In order to determine quantitatively the top surface of the alloys, the low-energy ion scattering (LEIS) technique was used. Experiments were conducted in the same apparatus than that used for XPS experiments. The analysis was performed with 1-keV 4He+ ions at 25 °C at a scattering angle of 135°. The primary 4He+ beam intensity was 30 nA, focused on an impact spot of about 0.5 mm diameter. The relative sensitivity factors for Cu and Ag, SCu/SAg has been taken equal to 0.23 [25,28] and for Au and Ag, SAg/SAu has been taken equal to 1.1 [29]. The Temperature-Programmed Desorption experiments (TPD) were achieved with BELCAT-M apparatus [30]. 100–200 mg of sample was heated to 350 °C under He flow (50 ml min−1) and exposed to the same oxygen flow for 30 min. The catalyst was then cooled to room temperature under oxygen flow. The tubing was flushed with He, bypassing the reactor, until no oxygen was detected in the effluent Temperatureprogrammed desorption of adsorbed oxygen was then recorded by heating the sample with heating rate of 8 °C min−1 under He flow (50 ml min−1). The oxygen desorbing from the sample was continuously monitored by gas chromatography. The output was then plotted against temperature, to determine the TPD profile. It has been checked that no CO2 or H2O were formed during the TPD experiments. The catalysts have been tested for the oxidative dehydrogenation of allyl alcohol between 200 and 400 °C under atmospheric pressure using a stainless steel plug-flow micro-reactor. The catalyst mass (100–200 mg) was chosen to be able to obtain complete or near complete allyl alcohol conversion. Prior to the reaction, the samples were pre-heated at the reaction temperature for 15 min in flowing air (75 ml min−1). Allyl alcohol, which was introduced into the reaction system using a syringe pump, was vaporized in the air and He flow to the reactor using a homemade vaporization device. Reactor was a fixed bed reactor, which was placed in a furnace. The pipes were heated to eliminate condensation of allyl alcohol and liquid products. The allyl alcohol was always fed at a rate of 0.0215 mol h−1, and the air gas flow is 20 ml min−1 giving a molar relative composition allylic alcohol/ O2 = 2); the effect of oxygen partial pressure was studied by changing the N2/O2 ratio and the effect of contact time by changing the catalyst mass. The gas products (CO and CO2, propene) were analyzed online, using a gas chromatograph. N2 was used as internal standard. Organic substrates were be trapped in ethanol first at 0 °C and then at low temperature (−23 °C) thanks to the cryostat during the reaction and analyzed off line using gas chromatographs equipped with FID detectors and a Nukol column or a ZBwax plus column. Air, high purity helium gas and allyl alcohol from Aldrich with the purity higher 99% were used for the experiments. Ag catalysts, rate of allyl alcohol conversion was calculated per m2 of alloys and m2 of Ag considering the silver surface atomic content of the alloys determined from LEIS data.

Table 2 Physico-chemical characteristics of the bulk catalysts: SSA: specific surface area. Catalysts

SSABET

ICP analysis

XRD Data

(m2 g−1)

(wt%)

Cell par. (nm)

Cryst. size (nm)

Ag Ag-Au

3.0 11.9

0.40869(2) 0.40807(2)

30 16

Au Ag-Cu

6.3 3.9

Ag: 100.0 Ag: 3.14 Au: 96.15 Zr: 0.71 Au: 100.0 Zr < 1 Ag: 97.5 Cu: 2.5

0.40797(1) 0.40810(1)

20 53

3. Results 3.1. Self-supported Ag, Ag0.975Cu0.025, Au and Au0.95Ag0.05 catalysts The studied catalysts are presented together with their main characteristics in Table 2. In the case of the prepared gold-silver sample, chemical analyses showed that the silver content was slightly lower than the pre-set value for the preparation, indicating a slight loss of silver during the lixiviation process or during the acidic dissolution of the sample for analysis. The formation of a small fraction of silver precipitate was indeed observed after dissolution. Specific surface areas equal to 3.0 and 6.3 m2 g−1 were determined for the Ag and Au, respectively, whereas the specific surface areas of the alloys including Au and Cu were respectively higher and lower, at 11.9 and 2.2 m2 g−1. All of the X-ray diffraction patterns were similar. Both Ag and Au crystallized with the same face-centred cubic structure, and the Ag-Au alloy formed a solid solution over the full range of compositions. At the opposite, even if pure Ag and Cu crystallize with the same structure, they form an immiscible alloy over the entire composition range, at least at temperature lower than 200 °C [31]. Calculated unit cell parameters were close to the published values: 4.0855(1) Å for Ag [32], and 4.0796(1) Å for Au [33]). The cell parameters for the Au0.95 Ag0.05 and Ag0.975 Cu0.025 alloys were closer to those of pure Au and Ag, respectively. The size of the crystallites was estimated from the broadening of X-ray diffraction peaks, using the Scherrer method to fit the complete patterns, and was in agreement with the measured values of specific surface area (Table 2). The pure Ag and Au0.95 Ag0.05 alloy was studied using scanning electron microscopy, showing that the silver and gold silver catalysts are micrometric powders composed of grains ranging in size between a few μm and 200 μm, with a sponge-like morphology composed of a network of interconnected channels, as shown on the SEM images (Fig. 1). The surface composition of the Au0.95 Ag0.05 and Ag0.975 Cu0.025 alloys was studied by XPS and LEIS spectroscopy, in order to characterize their uppermost surface layer. In order to check for possible changes in the surface composition during catalytic testing, alloys were also characterized by LEIS after catalytic testing. The results of these tests are presented in Table 3. Quantitative analysis of the Au0.95 Ag0.05 sample revealed the absence of residual Zr and a strong enrichment in Ag. LEIS revealed that this enrichment reached 35.5% in the topmost atomic surface layer. Small amounts of impurities such as O were detected. Following catalytic testing, a slight change in surface composition, associated with a higher Ag content, was detected (38.5%). These results are in agreement with those published by T. Déronzier et al. concerning poor Ag alloys (from 2 to 6% silver) [29]. These authors accounted for the surface Ag enrichment by a lower surface free energy for silver, and a stronger interaction between silver and oxygen during preparation of the alloy [24,25]. Concerning the AgCu alloy and based on thermodynamic parameters such as surface tension or size effect, we would expect a strong silver segregation. Nevertheless, LEIS analysis revealed a strong enrichment of the Cu content (37%) in the topmost atomic surface layer. 172

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Fig. 1. SEM micrographs of bulk Ag and AgAu catalysts.

Table 3 Surface atomic content calculated from XPS and LEIS spectroscopic data. Catalysts

Surface composition (%) XPS

Ag-Au before testing Ag-Au after testing Ag-Cu before testing Ag-Cu after testing

Ag 18.5 94.0

LEIS Au 78.5

Cu

6.0

Zr 3.0

Ag 35.5 38.5 63.0 65.0

Au 64.5 62.5

Cu

Zr 0.0 0.0

37.0 35.0

Fig. 3. Influence of contact time on allyl alcohol conversion (full symbols) and selectivity : W/ to acrolein (empty symbols) on Ag catalyst: ▼: W/F = 5.50 gcat h mol−1, : W/F = 7.79 gcat h mol−1. F = 8.76 gcat h mol−1,

The Pure Ag sample has first been studied as catalyst. Conversion of allyl alcohol and selectivity to acrolein obtained at different contact times, are compared in Fig. 3. In the case of a low contact time (W/ F = 5.50 h−1), a maximum conversion of 83% was obtained. This conversion level increased at higher contact times, reaching a plateau at 87%. The temperature at which this plateau was reached was approximately 340 °C at W/F = 76 gcat h mol−1, but decreased with contact time. In parallel, a similar selectivity to acrolein was observed. This selectivity was high (99%), and decreased only slightly with increasing reaction temperature, reaching 93% at 380 °C (Fig. 3). These results are in good agreement with previously published values [13]. The main by-product of the reactions was CO2, with significantly smaller amounts (totalling < 1%) of CO, propene and propionaldehyde. The first tests were carried out under allyl alcohol-rich conditions (Allyl/O2 = 2), such that the O2 remaining in the effluent was approximately 3%. It was also of interest to test the catalysts under oxygen-rich conditions. The results show that the Ag catalyst is more active under such conditions, but also less selective (Fig. 4). The Ag, Au, Au0.95 Ag0.05 and Ag0.975 Cu0.025 catalysts were then tested and compared under the same conditions. As the Au catalyst appeared to be practically inactive, the change in catalytic properties of the other three solids is presented as a function of reaction temperature in Fig. 5. These results show that the activity of the Au0.95 Ag0.05 and Ag0.975 Cu0.025 alloys is comparable and slightly higher than that of the Ag catalyst. However, these catalysts appear to be much less selective, and the maximum acrolein yield was always obtained with pure Ag. Since Ag0.975 Cu0.025 appeared to be the most active catalyst, its properties were measured again in the presence of a higher partial pressure of oxygen (Fig. 6) showing that, similarly to the case of Ag, the conversion on Ag0.975 Cu0.025 was substantially higher. Although there

Fig. 2. Fitted XPS spectrum of Cu 2p after Shirley background substraction, showing the presence of Cu2+ and Cu0 signals at the surface of the Ag0.975 Cu0.025 alloy.

This surprising result is in agreement with previous theoretical calculations by Piccinin et al., who showed that the presence of oxygen leads to copper segregation to the surface [34]. A quantitative XPS analysis of the Ag 3d and Cu 2p peaks revealed a Cu concentration of approximately 6% versus 2.5% for the bulk value showing also an enrichment of Cu at the surface (Table 3). Moreover detailed analysis of the Cu2p peak shows that 80% of the surface copper species correspond to Cu2+ (Fig. 2). The remaining 20% correspond to Cu0, although the possible presence of Cu+ could not be discarded. Although the Auger peak of the copper could in principle have been used to differentiate between the two species, however it is superposed in this case onto the Ag3p3/2 peak. Silver was in all cases present in the form of Ag0. Finally both LEIS and XPS results are in agreement with those previously obtained by in situ XPS and XANES spectroscopy on an Ag-Cu alloy with comparable Cu loading (2.5%Cu) and particle size, showing that a thin layer of CuO was present at the surface, with no detectable signs of a surface alloy [35]. The results showed that there was no substantial change to the surface composition of both alloys after catalytic testing. 173

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Fig. 4. Influence of allyl alcohol to oxygen ratio on the allyl alcohol conversion (full symbols) and selectivity to acrolein (empty symbols) on Ag catalyst: ■: Allyl/O2 = 2.0, : Allyl/O2 = 1.27, : Allyl/O2 = 1.09. Fig. 7. Comparison of TPD profiles obtained after O2 adsorption on the catalysts for 30 min, at 350 °C: a) Ag0.975 Cu0.025, b) Ag, c) Au0.95 Ag0.05.

In an effort to understand the results obtained with these alloys, the catalysts were characterized using O2-TPD. The resulting TPR profiles of the Ag, Au0.95 Ag0.05 and Ag0.975 Cu0.025 samples, recorded after O2 adsorption at 350 °C, are shown in Fig. 7. The first peak detected with these alloys, which is attributed to desorption of weakly adsorbed molecular oxygen, occurs at a temperature below 150 °C [36–40]. The protocol used for this measurement allowed most of the adsorbed molecular oxygen to desorb, before the start of the recording. It is thus plausible that molecular oxygen is also adsorbed onto pure Ag, although considerably less strongly and in lesser quantities than in the case of the alloys. With the pure Ag sample, the two peaks observed at 300 and 380 °C are attributed to the recombinative surface desorption of atomic oxygen adsorbed onto Ag (111) and Ag(110), respectively [41,42]. Alternatively, one of these peaks may correspond to oxygen adsorbed at steps onto Ag(001) [43]. It should be noted that upon oxygen adsorption at 300 °C (data not shown), a broad peak was observed at 340 °C, without the presence of two distinct individual peaks. The TPR profiles reveal another peak at 440 °C, which has in the past been attributed to subsurface oxygen desorption [35]. The oxygen desorption curve obtained for the Ag0.975 Cu0.025 sample above 150 °C has only two peaks, corresponding to atomic surface oxygen and subsurface oxygen species, both of which occur at a lower temperature than in the case of pure Ag (290 and 400 °C). The peak observed at 290 °C was intense, and the coverage of adsorbed atomic oxygen species increased with alloying with copper. It is also possible that the formation of atomic oxygen takes place on both copper and silver surface atoms. The oxygen desorption curve obtained for the Au0.95 Ag0.05 alloy following oxygen adsorption at 350 °C was characterised by very weak peaks at 264 and 520 °C attributed to surface atomic oxygen and subsurface oxygen species respectively. The bulk composition of the Au0.95 Ag0.05 was predominantly Au, and LEIS showed that the topmost Au surface content was close to 65%. It is thus clear that Au strongly affects the adsorption/desorption behaviour of oxygen. Dissociative adsorption leading to atomic oxygen species, and requiring several adjacent Ag atomic sites, is inhibited [44,45]. On the other hand, the adsorption of molecular oxygen, which does not require this type of multi-atomic adsorption site, may even be favoured. This is the reason for which a peak corresponding to these species is observed in the TPD curve at low temperature. It has indeed been shown that by promoting the dilution of surface atoms, gold induces the formation of single Ag atomic sites, which are conducive to the adsorption of molecular oxygen [29,34]. This is also supported by the observation of lower apparent activation energy for this type of adsorption on a similar

Fig. 5. comparison of the catalytic properties of Ag (■), Au0.95Ag0.05 ( ) and Ag0.975Cu0.025 ( ) catalysts; allyl alcohol conversion (full symbols) and selectivity to acrolein (empty symbols) are measured in standard conditions.

Fig. 6. Comparison of the catalytic properties of Ag (■) and Ag0.975Cu0.025 ( ) catalysts at Allyl/O2 = 1.09; allyl alcohol conversion (full symbols) and selectivity to acrolein (empty symbols) are measured in standard conditions.

was an increase in the selectivity of this alloy to acrolein, the difference between the selectivities of Ag and Ag0.975 Cu0.025 remained high, whatever the temperature, and Ag continued to be the most efficient catalyst. 174

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alloy [35]. The Ag-O bond has also been shown to weaken in presence of Au, which explains the lower observed temperature at which atomic oxygen desorption takes place [45]. Finally, the fact that Au hinders the subsurface diffusion of atomic oxygen is not surprising, in view of the bulk composition of the compound, which is comprised mainly of Au. A similar observation has already been made and explained by the higher bond strength of Ag-Au compared to Ag-Ag, which decreases the activation energy for subsurface oxygen diffusion, and by the decrease in surface atomic oxygen with which the subsurface oxygen is in equilibrium [35]. In the case of the Ag0.975 Cu0.025 compound, it is likely that Cu has a similar influence with respect to the apparently enhanced adsorption of molecular oxygen, However, the copper cation should contribute to the dissociative adsorption of oxygen, which is not inhibited as in the case of Au, but rather promoted by the Cu-weakened Ag-O bonds. The bulk composition of the Ag0.975 Cu0.025 alloy is mainly Ag, and the Ag-Cu bond strength (171 kJ mol−1) is close to that of AgAg (165 kJ mol−1) [46]. This is the reason for which the subsurface oxygen adsorption is hardly affected, or is even slightly facilitated since the subsurface species desorption peak was observed at a lower temperature. This effect may be related to promotion of the surface atomic oxygen, which is in equilibrium with the subsurface oxygen. The mechanism of gas-phase oxidative dehydrogenation of alcohols over silver is well known [47,48]. Oxygen molecules from the gas phase interact initially with the silver surface. An electron transfer from the electron-rich metallic Ag to the O2 molecules leads to the formation of atomic oxygen (Oδ− basic site) and surface positively charged Ag species (Agδ+ acid site) on which alcohol molecules can adsorb dissociatively, forming adsorbed alkoxy species that undergo βH-elimination. Subsurface oxygen, which is critical for the activation and selectivity of silver catalysts in oxidation reactions at high temperature, should not be determinant in the allyl oxidative dehydrogenation reaction. The Au0.95 Ag0.05 catalyst appeared to be very active, although it presented few or none of the latter species. The active and selective species correspond to atomic oxygen species. As these species are less numerous on the Au0.95Ag0.05 compound, as shown by O2-TPD, the activity of the catalyst should be lower. However, the intrinsic rates of allyl alcohol conversion on Ag and Au0.95Ag0.05 catalysts at 235 °C were found to be comparable (Table 4). At this temperature Au is inactive, and when the conversion rates are expressed per m2 of surface silver, the Ag catalytic sites on Au0.95Ag0.05 appeared to be even more active than on the Ag catalyst. This higher intrinsic activity can be explained by the weakening of the Ag-O bond, which is revealed by a decrease in the related O2-TPD maximum. This previously observed weakening has been explained by a lower electron density in the Ag atoms, as a consequence of an electron transfer to the neighbouring Au atoms induced by the difference of electron affinity between these two metals [35,37]. Although the Au0.95Ag0.05 compound is catalytically more active, it is unfortunately less selective; this could be related to the properties of the new active sites themselves. It is known that in the processes of alcohol oxidation the catalyst selectivity strongly depends on the effective charge of the active Ag surface sites and this charge is modified by neighbouring Au atoms. It may also be related to the reaction between allyl alcohol molecules and molecular oxygen, whose adsorption is enhanced on Au-containing catalysts.

Fig. 8. X-ray diffraction patterns of the Ag supported catalysts, peaks relative the Ag phase are marked with red losanges. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The case of the Ag0.975 Cu0.025 compound is more complex. XPS and LEIS characterization of the sample revealed strong copper enrichment at the surface of the silver particle, and similarly to that of the Au0.95 Ag0.05 alloy, an Cu content equal to 37% on the topmost atomic surface layer. The surface copper species are predominantly oxidized Cu2+ and should be present in the form of patches of Cu oxide on the Ag substrate, as evidenced by the HRTEM images shown in the study of a similar alloy [33]. These Cu oxide patches may directly catalyse the oxidative dehydrogenation of allyl alcohol, as has been observed in the case of other alcohols, thus explaining the high level of observed activity [49]. In all cases, the selectivity to acrolein on Ag0.975Cu0.025 was lower than on pure Ag. There is thus a synergy between Ag and the other metals present in this catalyst, with respect to the oxidative dehydration of allyl alcohol. However, the systematic decrease in selectivity to acrolein observed with this catalyst makes it effectively less attractive for the production of acrolein. 3.2. Supported Ag catalysts The X-ray powder diffraction patterns produced with these catalysts are shown in Fig. 8. The structure of LaPO4 corresponded to Rhabdophane, that of TiO2 to Anatase, and that of Al2O3 to γ-alumina. As could be expected, the structure observed with SiO2 was amorphous. Various peaks corresponding to the Ag0 phase were visible in the patterns of the Ag/CNT and Ag/LaPO4 Ag/SiO2 samples. However, in the case of the latter catalyst the peaks were not intense, and other peaks corresponding to Ag2O3 were more visible. TEM was used for the morphological characterisation of the supported Ag catalysts, and the highresolution TEM images obtained with five of these are shown in Fig. 9. These images reveal the presence of Ag particles, with a relatively uniform distribution for all samples. The Ag particles on the LaPO4 (15–30 nm) and CNT (> 50 nm) samples were relatively large in size, whereas those observed on the conventional Al2O3, TiO2 and SiO2 supports were considerably smaller. A relatively narrow particle size distribution was observed on the Al2O3 support (8–15 nm), and on the two other oxides (5–12 nm). The electron diffraction study showed that, with the exception of SiO2, the only particles formed on the supports

Table 4 Comparison of intrinsic rates of allyl alcohol and selectivity to acrolein on Ag and Au0.95Ag0.05 alloy; 235 °C, W/F = 5.5 h−1 allyl al.: 0.0214 mol/h; air: 20 ml min−1, alcohol/O2 = 2.0. Catalysts

Ag Au0.95Ag0.05 Ag0.975Cu0.025

Intrinsic rate of allyl alcohol conversion (10−4 mol mcat−2 s−1)

(10−4 mol mAg−2 s−1)

Acrolein selectivity (%)

1.00 1.29 0.98

1.00 3.58 –

98 94 92

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Fig. 9. TEM micrographs of different supported Ag catalysts, (a) Ag/ Al2O3, (b) Ag/TiO2, (c) Ag/CNT, (d) Ag/LaPO4,(e) Ag/SiO2.

were Ag0. However, Ag2O3 particles were systematically detected, accompanied by just a small quantity of Ag0 particles on the silica supports. The catalyst supports were tested alone, prior to testing of the four supported catalysts, showing that they have a very low conversion (< 1%) and are selective mainly to acrolein (> 80%). The catalysts were tested under the same conditions, and the results are presented in Table 5. In addition to CO and CO2, propionaldehyde and small quantities of products such as acrylic and propionic acids and 2-propanol were detected. However, the selectivity to the latter products never exceeded 0.8–1%, except in the case of Ag/CNT. The catalysts’ activity appeared to be correlated with the size of the Ag supported particles, whereas their selectivity was related to the nature of the support. This is in good agreement with previous results obtained on supported AgAu alloy [50] The high selectivity on Ag/CNT to COx could be due to the presence of surface electrophilic oxygen species on the CNT, whereas it is thought that the higher selectivity of Ag/LaPO4 to propionaldehyde is related to the Brönsted acidity of LaPO4, which leads to the protonation of the double bond of the alcohol and simultaneous dehydrogenation of the

OH group. The same reaction also took place on the acid sites of Al2O3. The best catalytic properties were obtained with the Ag/TiO2 and Ag/ SiO2 catalysts, with very weak acid sites. With increasing contact time, maximum activity (78% conversion) and selectivity (88% to acrolein) was obtained on the latter catalyst at 360 °C (Fig. 10). After testing, no change was observed in the XRD patterns. The influence of contact time and Allyl/O2 ratio was studied on the best catalysts, i.e. Ag/SiO2 (Fig. 11) The same results were obtained as in the case of bulk Ag catalysts, with selectivity to acrolein decreasing and allyl alcohol conversion increasing for decreasing ratios and increasing contact time The strong decrease in selectivity observed in the later case, confirms the strong acrolein degradation with increasing contact time. The activity of the supported catalysts appeared to be related to the Ag dispersion at the surface of the support, whereas their selectivity was influenced by the nature of the support, with the formation of oxygenated by-products that were not observed on bulk silver and

Table 5 Catalytic properties of the Ag supported catalysts in allyl alcohol dehydration; reaction temperature: 360 °C, Ag mass: 3.1 mg, allyl alcohol: 0.0214 mol h−1; air: 20 ml min−1, alcohol/O2 molar ratio: 2.0. Catalyst

Ag/Al2O3 Ag/TiO2 Ag/SiO2 Ag/LaPO4 Ag/CNT

Conv. (%)

70.4 78.1 89.2 29.1 20.6

Selectivity (%)

Yield in acrolein (%)

ACRO

COx

PAL

Other liquid products

90 92 96 83 68

5 4 3 10 22

2 1 1 3 3

3 3 0 4 7

0.63 0.72 0.86 0.24 0.14

Fig. 10. Influence of the nature of the support on allyl alcohol conversion (full symbols) : Ag/SiO2, ■:, Ag/Al2O3, : Ag/TiO2. and selectivity to acrolein (empty symbols):

Other liquid products are mainly acrylic acid, propionic acid, 2-propanol.

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Fig. 11. Influence of contact time, : W/ F = 8.76 gcat h mol−1 ●: W/F = 7.79 gcat h mol−1 (test condition: allyl alcohol: 0.0215 mol h−1, mcata = 109 mg, allyl/O2 = 1.09) (a) and allyl/O2 ratio, ■: Allyl/O2 = 2.0, : Allyl/O2 = 1.09 (test condition: allyl alcohol: 0.0215 mol h−1, mAg = 3.1 mg) (b) on Ag/SiO2 catalyst.

several advantages, in particular their productivity and their atomeconomic and environmentally benign properties. However, their catalytic performance does not allow any improvement to be achieved in terms of selectivity to acrolein. Under O2 rich conditions (Allyl/ O2 = 1.09) bulk Ag remains the best catalyst, with 98% selectivity at 96% conversion and 359 °C, to be compared with 94% selectivity at 95% conversion and 358 °C in the case of Ag/SiO2. Under O2-poor conditions (Allyl/O2 = 2), which could be more relevant to an industrial process, the performance of the Ag/SiO2 supported catalyst is at least equivalent to that of bulk Ag, with 96% selectivity to acrolein at 89% conversion and 361 °C. It is certainly possible to improve the activity of silver, and it is indeed known that the structure, morphology and reduction strength of the supported Ag particles affecting the catalyst’s activity depend strongly on the nature of the support [53]. However, in the present case the results suggest that the support has an adverse influence on the catalyst’s selectivity to acrolein, and that it may be difficult to find a support that is more suitable than silica. New carbide- or nitride-based catalysts could also be considered, in terms of their ability to exceed the performance of silver-based catalysts. However, the bar is rather high in terms of the catalytic properties, stability and reproducibility, which need to be achieved.

which should be formed on the support upon adsorption of allyl alcohol or acrolein. In this respect, it is not surprising that higher selectivity is obtained with supports having weaker acid sites. The TEM study focused on the SiO2-supported Ag catalyst, since the nature of the formed particles was different to that of the other catalysts. A further aim of this study was to determine the reasons for its higher activity. To illustrate this result, Fig. 12 provides images of the Ag particles on SiO2 catalysts, together with their corresponding electron diffraction pattern. It is known that, like the other silver oxides (Ag2O and AgO), Ag2O3 is easily reduced at temperatures below 250 °C [51,52]. Interestingly, under the electron beam of the microscope we observed Ag2O3 reduction at room temperature. Since it occurred relatively quickly, it was possible to record this reduction process on a TEM movie (Fig. S1 in supporting information). Fig. 13 shows the sequential images extracted from this movie. The nanoparticles did not show any overall structural change, but were progressively reduced, cation by cation, with these cations then promptly departing from the surface of the particles. This process took place continuously until the particles had completely disappeared. Meanwhile, after a certain delay, the smaller silver particles nucleated randomly on the SiO2 support and the carbon film used to disperse the catalysts. In this case, the nucleated silver particles were smaller in size. This phenomenon could be the reason for which the silver on silica catalysts were more active than the titania-supported ones, although our analyses prior to testing revealed the presence of particles with approximately the same size. From an applied point of view, supported silver catalysts present

4. Conclusion The aim of this study was to design new silver-based allyl alcohol oxidative dehydrogenation catalysts with enhanced catalytic properties. The following conclusions can be drawn from the study: Fig. 12. TEM micrograph of the Ag/SiO2 sample (a) with the electron diffraction pattern recorded on the framed Ag2O3 nanoparticle.

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Fig. 13. Time sequential images of the reduction dynamics of Ag2O3 particles by the electron beam at a pressure of 10−5 Pa.

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- Alloying silver with gold or copper did not lead to better catalysts for the oxidative dehydrogenation of allyl alcohol to acrolein. The new catalysts are more active than pure silver, but less selective to acrolein. The active and selective species, which correspond to atomic oxygen species are less numerous on the AuAg alloy but have a higher intrinsic activity due to by the weakening of the Ag-O bond. Unfortunately the selectivity of the AuAg is lower than that of the pure metal. This could be related to the properties of the sites of the alloy or to the reaction with molecular oxygen, whose adsorption is enhanced. Same conclusions could be drawn for AgCu ally although the situation may be more complex because of the presence of Cu oxide patches on the alloy particles that may directly catalyse the oxidative dehydrogenation of allyl alcohol. - The use of silver on various supports allowed only a slightly improvement to be achieved in catalyst efficiency. Among the various candidates tested during this study, the Ag/SiO2 catalyst presented the highest catalytic performance, with an acrolein yield of 87.2% at approximately 360 °C. This is the only catalyst that really outperformed the bulk Ag catalysts. Detailed characterization of the catalysts revealed that their supports have a strong influence on the Ag particle size and dispersion. These were confirmed to be the main factors affecting the catalytic activity, whereas the nature of the support itself appeared to be critical in terms of selectivity to acrolein. In this respect, the selectivity to acrolein for the supported Ag catalyst appears to depend on the acidity of the support, which may thus be a key parameter. - Characterization of the supported catalysts by transmission electron microscopy revealed that SiO2 has a special behaviour since, contrary to the other supports that stabilized mainly metallic silver particles, this support is able to stabilize nano-sized silver oxide particles with an Ag2O3 structure, which were then easily converted to Ag0. However, the reduction process, including the atomic redispersion of silver and nucleation of new metallic particles, allows smaller silver particles to be formed Although we did not focus our study on the use of synthetic procedures for silver-based nanoparticles, we are of the opinion that it may be difficult to achieve further significant improvements in the performance of silver-based catalysts, unless a new inert support can be found, which still allows high, stable level of silver dispersion to be achieved. Acknowledgment The authors would like to gratefully acknowledge ADISSEO company for financial support. References [1] L. Liu, X.P. Ye, J.J. Bozell, ChemSusChem 5 (2015) 1162–1180. [2] P. Lauriol-Garbey, J.M.M. Millet, S. Loridant, V. Bellière-Baca, P. Rey, J. Catal. 28 (2011) 68–76. [3] P. Lauriol-Garbey, G. Postole, S. Loridant, A. Auroux, V. Belliere-Baca, P. Rey, J.M.M. Millet, Appl. Catal. B: Environ. 106 (2011) 94–102. [4] S. Sato, R. Takahashi, T. Sodesawa, N. Honda, H. Shimizu, Catal. Commun. 4 (2003) 77–81. [5] X. Li, Y. Zhang, ACS Catal. 6 (2016) 143–150.

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