Anthony Garron

Károly Lázár

Florence Epron
email: florence.epron.cognet@univ-poitiers.fr

Dr Florence

Effect of the Support on Tin Distribution in Pd-Sn/Al 2 O 3 and Pd-Sn/SiO 2 Catalysts for Application in Water Denitration

Keywords: Pd-Sn catalysts, catalytic reduction in water, bimetallic catalyst, 119 Sn Mössbauer spectroscopy, support effect

Effect of the support on tin distribution in

Pd-Sn/Al2O3 and Pd-Sn/SiO2 catalysts for application in water denitration

INTRODUCTION

Ground and surface water pollution by harmful nitrogen-containing compounds is a growing environmental problem. In surface it contributes to river and sea eutrophication. In ground water they are potentially harmful compounds for human health (blue baby syndrome, cancer…). Nitrate is one of the most problematic and widespread among the potential groundwater contaminants. Sources of nitrate in groundwater originate from four categories:

(i) natural sources (ii) waste materials (iii) row crop agriculture and (iv) irrigated agriculture [START_REF] Canter | Nitrates in Groundwater[END_REF].

Biological and physicochemical treatments allow an effective removal of nitrates but have several economical and ecological disadvantages. Catalytic reactions constitute also promising approaches for the destruction of pollutants in water. Since the first paper of Tacke and Vorlop [START_REF] Tacke | Dechema Biotechnology Conferences[END_REF] on the use of palladium-copper bimetallic catalysts for nitrate reduction, numerous studies have been aimed to the development of suitable catalysts for the selective reduction of nitrate into nitrogen gas [START_REF] Tacke | Dechema Biotechnology Conferences[END_REF][START_REF] Hörold | [END_REF](4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20). Their application in various reactor types such as suspended-bed reactor, hollow fibers (5), batch recycle reactor (11) and membrane reactors (13) has been studied. Nitrate reduction for drinking water treatment demands very active catalysts because the reaction has to be performed at the temperature of the groundwater. Moreover, a high selectivity is necessary to avoid the production of ammonium ions by over-reduction of intermediates of the stepwise nitrate hydrogenation. This selectivity is directly controlled by the hydrogen coverage of the precious metal. The global reaction is the following: 2 NO3 -+ 5 H2 -catalyst N2 + 2 OH -+ 4 H2O or NO3 -+ 4 H2 -catalyst NH4 + + 2 OH -+ H2O

The reduction of nitrate leads to the formation of hydroxide ion, which causes an increase of the pH value up to 10, which could be avoided by using a buffering system as controlled addition of hydrochloric acid or carbon dioxide (4,7,15). Palladium catalysts have proved to be active and the most selective for nitrite reduction. To reduce nitrate, it is necessary to activate the noble metal by addition of a second metal of group 11, 12, 13 or 14. Mostly, copper was used as a promoting second metal [START_REF] Tacke | Dechema Biotechnology Conferences[END_REF][START_REF] Hörold | [END_REF](4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)17,20), but other suitable promoters, such as tin, indium or zinc (4)(5)11,(14)(15)(16)(17)(18)(19) have demonstrated interesting performances. Whereas the addition of a promoter to the noble metal is necessary to reduce nitrates, it leads to contrasting results for the activity and the selectivity. Indeed, depending to the operating condition and to the preparation method, tin or copper leads to the best selectivity toward ammonium ion (4,9,11).

Moreover, in the case of Pt-Cu/-Al2O3 bimetallic catalyst, it was demonstrated that the preparation methods involved different behaviors in nitrate abatement. Precisely, the preparation method by catalytic reduction led to the most active catalysts compared to those resulting from classical methods such as co-impregnation or successive impregnation (20).

Then, the highest activity and selectivity for nitrate removal under controlled addition of carbon dioxide should be observed with Pd-Sn bimetallic catalysts prepared by catalytic reduction. In order to correlate the catalytic performances of this type of catalyst with the metal-metal interaction, it is of major importance to localize tin species. But the characterization of tin is a challenge, due to its high oxidability in air. Some studies on Pt-Sn/Al2O3 and Pt-Sn/SiO2 catalysts (21,22) gave interesting results on the different phases of tin present in the catalyst and their influence on the activity and the selectivity of the catalyst for n-hexane rearrangement reactions or for the 1,2-dichloroethane dechlorination. A study (23) on Pd-Sn/SiO2 evidenced by XPS and CO adsorption a modification of electronic configuration of Pd due to the effective interaction between Pd and Sn. Some authors (19) used 119 Sn Mössbauer spectroscopy to characterize tin based catalysts supported on alumina for nitrate reduction in water. Other studies (24,25) proved that the types of tin species are very dependent on the support and on the preparation method. The Pt-Sn catalyst is the most studied tin based catalyst in the literature, but for catalytic nitrate reduction the Pd-Sn catalyst is more attractive.

The aim of the present study is to characterize mono-and bi-metallic Pd-Sn catalysts prepared by controlled surface reaction, favoring the deposition of tin on palladium particles, and deposited on alumina or silica with 119 Sn Mössbauer spectroscopy TEM, EDS, XRD and O2/H2/O2 titration. The characteristics of these materials are compared with their catalytic performances in nitrate and nitrite reduction, chosen as test reaction since it is very sensitive to the metallic distribution.

EXPERIMENTAL

Catalyst preparation

Monometallic catalysts

A powdered -alumina from Procatalyse (surface area (BET method): 216 m 2 .g -1 ; pore volume: 0.55 m 3 .g -1 ; isoelectric point: 7) and a silica from Degussa (surface area (BET method): 175 m².g -1 ; pore volume 0.47 m 3 .g -1 ; isoelectric point : 3) were used as support.

First, they were ground and then sieved to retain particles with sizes between 0.04 and 0.08 mm.

Monometallic catalysts were prepared by impregnation of the support with the precursor salt Pd(NH3)4(NO3)2 (Johnson Mattey, 99.9%) in an aqueous solution brought to pH 11 by addition of ammonia. This pH value was chosen to ensure the cationic exchange between the precursor salt and the support surface (26). The mixture was stirred for 12h and then ammonia and water were evaporated by heating on a sandbath. After drying overnight at 120°C, catalysts were calcined in flowing air (80mL.min -1 ) at 400°C (6°C.min -1 ) for 4 h, and cooled to room temperature under N2. Finally the catalysts were reduced by flowing hydrogen (60mL.min -1 ) at 400°C (6°C.min -1 ) for 6 h. Monometallic catalysts Pd/-Al2O3 and Pd/SiO2 have a metal loading of 5 wt. %. In the following, these catalysts are identified by their respective notation, Pd5A and Pd5S.

Bimetallic catalysts

Bimetallic catalysts were prepared by a controlled surface reaction. In order to favor metalmetal interactions, the second metal was deposited by a redox reaction occurring between the hydrogen chemisorbed on the pre-reduced noble metal and the oxidized modifier [START_REF] Barbier | Advances in Catalysis preparation[END_REF]28). A palladium monometallic catalyst was modified by a surface reaction between chemisorbed hydrogen and the second metal, according to the overall reaction Sn 4+ + xHads-Pds  Sn (4-x) ads-Pds + x H + A known amount of a monometallic catalyst was placed in an atmospheric glass reactor. The reactor was flushed with a counter current nitrogen flow for 15 min at room temperature.

Afterwards the catalyst was reduced by a hydrogen stream at a temperature rising to 400°C and then cooled to room temperature. At the same time, the desired amount of SnCl4 (Aldrich, 99%) modifier, chosen in order to obtain 1.5 wt.% of tin in the bimetallic catalyst, was dissolved in hydrochloric acid solution and degassed by bubbling with N2. The solution was then added to the monometallic catalyst and maintained under stirring by hydrogen flow for 2h. The pH value of the resulting mixture was equal to 0.7. The bimetallic catalyst was separated from the aqueous solution via a filter at the bottom of the reactor, and washed.

Afterwards, the resulting material was dried overnight under a nitrogen stream at 80°C.

In this study, each bimetallic catalyst is identified by its specific support A for alumina and S for silica. The catalyst is noted PdSnA or PdSnS.

The experimental nominal composition of each catalyst was verified by elementary analysis.

Generally, the experimental error was within 5 %.

Characterization methods

2.2.1. O2/H2/O2 titration O2/H2/O2 titration was performed in order to estimate the palladium accessibility in the mono and bimetallic catalysts (29,30) and the amount of reducible tin in the bimetallic catalyst (29).

After reduction in H2 at 400°C, the catalyst was flushed for 30 min with argon at the same temperature to remove adsorbed hydrogen, and then cooled to 80°C. The experiment was conducted at 80°C to avoid the formation of -PdH (31)(32)(33)(34). Before O2/H2/O2 titration, the sample was saturated by hydrogen and then purged by flushing argon for 10 minutes. The same flushing under argon was performed between each titration cycle.

After pretreatment, pulses of oxygen were introduced until the full saturation of the catalyst was achieved to determine the oxygen uptake OT1. The chemisorbed oxygen was then titrated by hydrogen to determine HT. Afterwards a second oxygen titration (OT2) was carried to confirm the HT value. On the mometallic Pd catalyst, the reactions involved during the titration are the following:

(1) Titration of chemisorbed hydrogen by oxygen (OT1)

x PdS-H

+ 3x/4 O2  x PdS-O + x/2 H2Oads
where x is the number of palladium surface atoms Pds

(2) Titration of chemisorbed oxygen by hydrogen (HT)

x PdS-O + 3x/2 H2  x PdS-H + x H2Oads

(3) Titration of chemisorbed hydrogen by oxygen (OT2)

x PdS-H

+ 3x/4 O2  x PdS-O + x/2 H2Oads
The palladium dispersion in the monometallic catalyst was calculated from the value of hydrogen uptake assuming that the surface stoichiometry of the irreversibly chemisorbed hydrogen is unity. The average particle size of palladium was calculated from the dispersion values, assuming cubic shapes for the palladium particles.

TEM measurements

TEM measurements were carried out with a Philips CM120 electron microscope operating at 120kV with a resolution of 0.35nm. The powder was ultrasonically dispersed in ethanol, and the suspension was deposited on an aluminum grid coated with a porous carbon film. The particle size distribution was obtained from TEM pictures calculating the surface average particle diameter from dp = nidi 3 /nidi 2 .

X-ray energy dispersive spectrometry (EDS) was performed on the metal, on the support and on the vicinity of the metal for each sample using the STEM mode of the microscope using a Si-Li Super UTW detector.

Powder X-Ray Diffraction

Powder-XRD was used for crystallites identification. The diffractograms were obtained at room temperature with a Siemens D500 diffractometer equipped with a copper target. All the catalysts were pre-reduced before measurement. The spectra were acquired with the software DIFFRAC and evaluated with the software EVA. The databank used comes from the International Centre for Diffraction Data (ICDD).

119 Sn Mössbauer spectroscopy

119 Sn Mössbauer spectra were recorded at 77 and 300K in an cell (35) built to allow treatments in various gas atmospheres, with a pellet supported by a thin Be plate. The use of two different temperatures can give important information about the nature of the tin species environment (36). The pellet was made from a powdered catalyst sample (ca. 0.5 g) by pressing (100 MPa). Spectra were obtained for as-prepared samples. Each sample has been tested as received to identify the various oxidation states of tin present in the sample, and thereafter reduction in a H2 flow at 400°C (2 h, 10 mL.min -1 , pH2 = 1).

The sample was isolated in an in situ cell, and was kept in the atmosphere of treatment during collection of spectra. After treatments, spectra were obtained at room temperature first, then at 77K.

Mössbauer spectra were recorded with a constant acceleration KFKI spectrometer equipped with Ba 119 SnO3 source (300 MBq). For fitting purposes, components of the Lorentzian line shape were assumed, and no isomer shift parameters were initially constrained. Successive iterations were applied to obtain a better fit to the experimentally generated data. The estimated accuracy of positional parameters is  0.03 mm.s -1 . The isomer shift values are referred to SnO2 (room temperature).

fA (recoilless fraction) values were also approximated by calculating A = -d ln( A300/ A77)/dT data, where A300 and A77 are the actual absorption areas of components in the spectra recorded at 300 and 77K, respectively. fA is connected to the probability of the Mössbauer effect. It provides a rough estimation of the bonding strength of particular species, the smaller the temperature dependence, i.e. A, the stronger the bond.

Nitrate and nitrite reduction

Nitrate and nitrite reduction reactions were performed in a semi-batch reactor, at atmospheric pressure and 25°C. In a typical experiment, 64 mg of the powdered catalyst was used. The catalyst is at first reduced under hydrogen (180°C; 0.5 h). During this time 90 mL of ultra pure water (18.2 M) is placed in a vial purged with nitrogen. When the catalyst is stabilized in temperature the ultra-pure water is added over the reduced catalyst. Then, the catalyst is maintained in water under bubbling hydrogen for 30 min before introduction of the reactant, in order to be aged. The catalyst was mixed to water by the gas bubbling. This treatment is sufficient to avoid any metal dispersion modification during the reaction (20).

The catalytic test was carried out under CO2 in order to buffer the pH solution at a value of 5.

The buffering began 10 minutes before the reaction to stabilize the pH. Afterwards, 10 mL of a solution (16 mmol.L -1 ) of nitrate (KNO3) or nitrite (KNO2) were introduced in the reactor to start the reaction. The catalyst dispersion in the aqueous medium was achieved by the gas flowing (pH 2 = 0.5 bar, pCO2 = 0.5 bar, flow rate = 250 mL.min -1 ) through a porous glass located at the bottom of the reactor. It was checked that the hydrogen flow and the resulting stirring were sufficient to ensure that the reaction was not rate-limited by reactant diffusion.

To monitor the progress of the reaction, representative aqueous samples were periodically taken out and immediately separated by filtration and then analyzed as described in the following section.

"Tested catalysts" are fresh catalysts after one catalytic nitrate reduction test. "Blank catalysts" are catalysts submitted to the same treatment as the catalytic test (same pretreatment, same time, same medium and same gases) but without nitrate or nitrite. The tested catalysts are either characterized or next activated according to a procedure similar to the one used for fresh catalysts, and a new catalytic test was performed.

Analysis

The reaction components were analyzed by High Performance Liquid Chromatography (HPLC). Nitrate and nitrite concentrations were determined after separation on a C18 grafted phase column using an UV detector at =210 nm. Ammonium ions were quantified using an Alltech Universal Cation column coupled with a conductivity detector. The acidic mobile phase (oxalic acid) used provided the complete conversion of the ammonia basic form into ammonium ions. The gas phase was analyzed by on-line gas chromatography using a GIRA GC10C equipped with an automated gas sample valve, a TCD detector and a Porapack Q column.

Activities of catalysts, corresponding to the disappearance rate of nitrate and nitrite, determined at 75% conversion of the nitrite or the nitrate.

The initial concentration of 1.6.10 -3 mol.L -1 of nitrate corresponding to a concentration of 100 mg.L -1 . 75% of conversion allows to obtain water corresponding to the European guideline for nitrates concentration in drinkable water (25 mg.L -1 ).

The selectivity of the reaction, evaluated at 75% of nitrite and nitrate reduction, corresponds to the ratio between the amount of ammonium ion formed and the amount of converted reactant.

3-RESULTS AND DISCUSSION

The composition of the catalysts was determined by the central service of analysis of the CNRS (French national center of scientific research). The catalyst was initially dissolved in a solution of aqua regia and analyzed by ICP/AES (Perkin Elmer Optima 2000 DV). In the remaining part of this publication, the identifications mentioned in the Table 1 will be used.

Monometallic catalysts

The O2/H2/O2 titration was performed on the monometallic catalysts, Pd5A and Pd5S, immediately after preparation (fresh catalysts Pd5AF and Pd5SF) and also after the catalytic test (Pd5AT and Pd5ST), in order to evaluate the modifications induced by the catalytic test.

Results are summarized in Table 2.

The dispersion of palladium and the particle sizes determined from the hydrogen uptake are also reported in Table 2. The mean particle size of the fresh catalysts estimated from TEM pictures, not reported here, are identical to those determined by O2/H2/O2 titration.

For both types of catalysts, a decrease of the palladium dispersion between the fresh and the tested catalysts, due to a sintering of palladium particles, is observed. Indeed the dispersion values are 12.2% for the fresh and 3.4% for the tested alumina supported catalyst and respectively 5.1% and 2.1% for the silica supported one. The same results are observed in absence of nitrite (blank monometallic catalyst) that means that (i) the sintering is favored by the presence of both water and hydrogen, (ii) the catalytic nitrite reduction may not contribute to the sintering. This sintering phenomenon occurring in the presence of hydrogen and water has already been observed for Pt/Al2O3 catalysts (20,37,38) and for Pd/Al2O3 (39). Then the modification of the palladium accessibility occurs in the reaction media before the introduction of nitrite, when the catalyst, water and hydrogen are in contact. So, when the fresh catalyst is put in water under hydrogen bubbling before the catalytic test, the palladium dispersion at the beginning of the reaction is similar to the one of the tested catalyst in the same conditions. These results also demonstrate that the low dispersion values are observed on both supports, which could be explained by the high metal loading (5 wt %).

The catalytic performances for nitrite reduction were determined for the fresh and tested catalysts. Catalytic properties of the monometallic catalysts are summarized in Table 3. The activities of the fresh and tested catalysts are similar. As the palladium dispersions in the two catalysts are similar before the introduction of nitrites that means that the TOF is identical.

The silica supported catalysts, fresh or already used, are the most active and selective. The higher selectivity towards nitrogen for the silica supported catalysts, than the one obtained for alumina, could be linked to the difference of the particles size, the formation of N-N binding being favored on large particles (40). Then, in accordance with the results presented in (4), silica is the most suitable catalyst for nitrite reduction.

Bimetallic catalysts

Characterization

The Pd-Sn catalyst supported on alumina and silica were characterized by O2/H2/O2 titration, TEM coupled with EDS, XRD and 119 Sn Mössbauer spectroscopy.

Characterization by O2/H2/O2 titration

The O2/H2/O2 titration was performed on the bimetallic catalysts, PdSnA and PdSnS, immediately after preparation and also after catalytic test in order to localize the tin on the palladium particle surface. Indeed, if tin is deposited on the noble metal, a decrease of the hydrogen uptake, as compared to the one obtained in the monometallic catalyst, should be observed, since the presence of tin on palladium ensembles will decrease the number of palladium sites accessible to hydrogen.

The different results of oxygen and hydrogen uptakes are summarized in Table 4 for the fresh (F) and the tested (T) catalysts. As the presence of tin in interaction with palladium could modify the chemisorption stoichiometry of hydrogen on palladium sites and as the pretreatment under hydrogen could modify the tin distribution on palladium particles (41), the palladium dispersion was not calculated for the bimetallic catalysts.

The number of oxygen involved (OT1) corresponds to the amount of oxygen chemisorbed on accessible palladium surface and also on metallic tin atoms, especially those in interaction with palladium atoms. Indeed, it is well known that oxygen can be chemisorbed on metallic tin but not on ionic tin such as Sn 2+ and Sn 4+ , and only tin species in interaction with palladium are likely to be reduced to Sn° during the pretreatment (42,430). Then, during the first oxygen titration (OT1), the oxygen chemisorption on reduced tin atoms could occur according to the following reaction, assuming an approximate stoichiometry of one oxygen atom per tin atom (29):

y Sn + y/2 O2  y Sn-O
where y is the number of reduced tin atoms in interaction with palladium.

Then for the titration reactions, the first oxygen uptake would correspond to the formation of SnO (1) and to the titration of the hydrogen chemisorbed on palladium (2):

OT1 y Pd(SnS) + y/2 O2  y Pd(SnS-O) (1)
x PdS-H

+ 3x/4 O2  x PdS-O + x/2 H2Oads (2)
During the introduction of hydrogen, after outgassing, there is a titration of the oxygen chemisorbed on palladium (3) but not of the one chemisorbed on tin.

HT x PdS-O + 3x/2 H2  x PdS-H + x H2Oads (3)
Finally, after outgassing, the introduction of oxygen allows the titration of hydrogen atoms chemisorbed on palladium (4) .

OT2 x PdS-H + 3x/4 O2  x PdS-O + x/2 H2Oads (4)
Then, it can be inferred from these reactions that (i) the amount of oxygen uptake during the first titration by oxygen OT1 is not equal to the amount determined during the second titration by oxygen OT2 and (ii) that OT2 is equal to half the amount of hydrogen uptake HT.

This is in accordance with the values of OT1, HT and OT2 reported in Table 4. Then, the amount of reducible tin, y, in interaction with palladium is simply given by y = OT1 -OT2.

The values of oxygen and hydrogen uptakes are only used for comparison. For both types of support, the HT value of the fresh catalyst is almost identical to the one of the tested catalyst.

Moreover the HT value before and after catalytic test is higher in the bimetallic catalyst than in the corresponding blank or tested monometallic one whereas it has been demonstrated in paragraph 3.1 that the catalytic test leads to a sintering of palladium particles with an important decrease of the hydrogen uptake after catalytic test. Then, it can be inferred that the deposition of tin restricts the sintering of palladium particles which occurs when the monometallic catalyst is immersed in an aqueous solution under hydrogen atmosphere. The presence of chlorine could also account for this result. The same behaviour has already been observed after introduction of tin on Pt/Al2O3 (44). In addition, for the silica supported catalyst, the HT value is higher in the bimetallic than in the fresh monometallic catalyst. This larger hydrogen uptake in the bimetallic as compared to the monometallic Pd5SF can be explained by a redispersion of palladium favoured by the presence of tin. Indeed, a change in the hydrogen chemisorption stoichiometry on palladium, due to the presence of tin, can not explain the difference obtained, the hydrogen uptake being 1.6 times higher in the fresh bimetallic than in the fresh monometallic catalyst. The same result has already been obtained

for Pt-Sn/Al2O3 catalysts (45) with low tin loadings (10% and 50% of tin in the bimetallic phase). It was proved by transmission electron microscopy that platinum particles were two times smaller in the bimetallic than in the corresponding monometallic catalyst. However in that case the bimetallic was prepared by coimpregnation (45). It can also be considered that the chemisorption properties of Pd-Sn alloys could explain the higher hydrogen uptake of the bimetallic catalysts.

From the estimated value of reducible tin reported in table 4, which contributes to the oxygen uptake OT1, it seems that the catalytic test leads to a decrease of the amount of accessible tin in interaction with palladium on the alumina supported catalyst, although these data are only a rough estimation and should be taken with precaution. This could be explained by a migration of tin from palladium on the alumina support during the catalytic test. Indeed, tin stabilized on the support is not reducible and then cannot chemisorb O2. On silica supported catalysts, the amount of oxygen chemisorbed on metallic tin is lower than on alumina catalysts, and is not affected by the catalytic test.

To conclude this part, the results on O2/H2/O2 titration do not prove that tin is deposited on palladium particles since the hydrogen uptake of the bimetallic catalysts is identical or larger as compared to the corresponding monometallic catalyst, which could be linked to a redispersion of palladium during tin deposition. On the contrary, the values of the oxygen uptake should indicate that metallic tin is present on the bimetallic catalyst after reduction, which could be an indication of the presence of bimetallic palladium-tin entities. In addition, it seems that the catalytic test induces some modifications in the tin distribution on the catalyst surface, especially for the alumina supported catalyst.

TEM, EDS and XRD

TEM coupled with EDX was carried out to collect more information on metallic particles in terms of particle size and composition. The TEM pictures of the two bimetallic catalysts are presented in Figure 1 for the fresh catalysts and in Figure 2 for the tested ones, and the distribution of the particle size estimated from the TEM pictures are reported in Figure 3.

If one refers to the particle size in the fresh and tested catalysts (Figure 3) one can see that these values are lower than those determined in the corresponding blank or tested monometallic catalyst. This confirms that the presence of tin would limit the sintering of palladium particles, in accordance with the hydrogen uptake values obtained during O2/H2/O2 titration.

As far as the composition of bimetallic particles is concerned, the EDS results evidenced an heterogeneity in the tin distribution, with tin rich and tin poor particles. The EDS study showed also that for PdSnS, fresh or tested, tin is always associated with palladium, whereas PdSnA presents a lot of isolated tin particles on the support.

The XRD technique was used on pre-reduced catalysts to determine the composition of crystallized compounds. Figures 4a and4b show the X-Ray diffraction patterns of PdSnSF and PdSnAF respectively. For the silica based catalyst the diffraction peaks correspond to two principal definite structures, metallic palladium (Pd) and Paolovite (Pd2Sn), and also to a palladium silicon tin, structure such as Pd2,5Si4,5Sn4,5.

For the alumina based catalyst, peaks corresponding to Pd-Sn and Pd-Sn-Al species are also observed but it was impossible to correlate the peaks with a particular composition referenced in the JSPER base.

119 Sn Mössbauer spectroscopy

The Mössbauer spectra of the reduced PdSnAF and PdSnSF at 77 and 300K are presented on Figure 5. The corresponding Mössbauer parameters and the different chemical species identified are presented in Table 5. The Table 6 summarizes the composition and the relative intensity of each measured tin compound.

a) Alumina supported catalyst

It was demonstrated by Mössbauer spectroscopy (19,45) that on Pd-Sn/Al2O3 catalysts, a large amount of tin, between 41 and 80%, is deposited on the support whatever the preparation method. Then, to favor the characterization of all tin species (metallic and oxide) by this technique, the tin loading chosen was 6% for Pd-Sn/Al2O3 catalyst. It was verified, by catalytic test, that this catalyst was representative of the samples PdSnAF and PdSnAT used in this study, the same activity being obtained for nitrate reduction.

Fresh catalyst

For all alumina based catalysts before reduction the tin phase is characterized by a doublet with an Isomer Shift (IS) close to 0 and a Quadrupole Splitting (QS) around 0.6 mm.s -1 , that is only characteristic of a Sn 4+ species such as SnO2 (45). This result proves that all tin species are oxidized in Sn 4+ during the storage of the catalyst.

After reduction of the catalyst the best fitting of the spectra is obtained for three different species. One corresponds to oxidized tin in Sn 2+ form (20% of the relative intensity), the two others are attributed to metallic tin associated with palladium, one tin poor phase Pd-Sn(a)

and one tin rich phase Pd-Sn(b). Contrary to the results reported by Sales and al (24), no Sn 4+ species such SnO2 or Sn 4+ species interacting with the alumina support are identified on the reduced samples.

The peak corresponding to the Sn 2+ state has an IS close to 4 and a QS close to 0.3, that is characteristic of Sn 2+ with chloride ligands. This phase is likely to be SnCl2 [START_REF] Greenwood | Mössbauer Spectroscopy[END_REF] or an oxochlorinated species of tin. This compound resulting from the precursor salt SnCl4 used for the preparation is known to be very stable on alumina and then irreducible at low temperature.

The tin poor phase Pd-Sn(a) has an IS close to 1.4 which could be attributed to a solid solution of tin in palladium, such as Pd(x>3)Sn [START_REF] Charlton | [END_REF]49). The major characteristics of this solid solution is the Isomer Shift lower than 1.46. It is impossible to determine the relative quantity of tin in this compound. The tin rich phase Pd-Sn(b) has an IS close to 2.2 attributed to an alloy between tin and palladium with 57 at% of tin (Pd3Sn4). If one calculate the ratio between the tin rich and the tin poor phase, one obtain a value close to 1.6, indicating that the tin rich phase is predominant. Contrary to Berndt et al. (19), the presence of significant proportions of metallic tin was not evidenced. In addition, the amount of oxidized tin species is lower in our study than the lowest value presented in (18) obtained with a Pd(5%)Sn/Al2O3 catalyst with a low tin content (0.5%) and prepared by incipient wetness method, for which 41% of oxidized tin (Sn 2+ and Sn 4+ ) was found. Then it can be inferred that the deposition of tin by catalytic reduction is more convenient to favor the formation of bimetallic Pd-Sn entities on alumina support and to avoid strong tin support interactions.

Tested catalyst

After the catalytic test, and without pre-reduction, tin is for the major part in an oxide phase.

From the calculation of A corresponding to the bond strength between tin and alumina, it can be deduced that the interactions between tin and the support are less important after catalytic test, the value being 1.2 before and 2.1 after. Therefore, it can be assumed that the tin oxide phase deposited on the palladium is aggregated.

After reduction, 20% of tin is Sn 4+ oxide species, corresponding to SnO2. Then it seems that the catalytic test, the reduction of nitrates in water, leads to an increase of the amount of tin deposited on the support. As no Sn 4+ was present on the fresh catalyst after reduction, it appears that the Sn 4+ form is more irreducible after the catalytic test, probably due to the lower quantity of tin in interaction with palladium. This evolution of the tin phase after the catalytic test may induce a deactivation of the catalyst by loss of reduced and then active tin in interaction with palladium. Considering the mechanism proposed in (20), the decrease of tin in interaction with palladium after the catalytic test is logical, since, during the reaction tin, is oxidized by NO3 -and then goes in solution. Then, a part of this tin is likely to be redeposited on alumina in form of SnO2, rather than on palladium.

The amount of chlorinated Sn 2+ species is almost not modified by the catalytic test. This result confirms the high stability of this tin species.

After reduction, two Pd-Sn phases are also observed, but the corresponding alloys or solid solution of the tested catalyst are not identical to those observed directly on the fresh reduced catalyst. The tin poor phase Pd-Sn(a) corresponds to an alloy Pd3Sn, resulting from a slight decrease of the palladium content in this phase during the catalytic test, since the palladium content in this type of phase was higher before the test (Pd(x>3)Sn). This could be explained by a segregation between palladium and tin leading to the apparition of monometallic palladium particles on the support.

For the tin rich phase Pd-Sn(b), the Isomer Shift value of 2.51-2.53 is very close to that of ß tin (2.58 mm.s -1 (48,49)), indicating that in the bimetallic particles, tin might be aggregated at the surface of palladium particles and considered as unalloyed metallic tin.

On alumina based catalyst, the most important results are that the ratio between tin rich tin poor phase is higher after catalytic test (1.62) than before (0.75), indicating that the tin distribution is modified by the test in water. It seems that tin is aggregated with itself or is deposited as large particles on free palladium. The interactions between the metallic phase and the support decrease after catalytic test. The values of A increase from 5.5 to 6.3 and from 3.4 to 4.3 respectively for the tin poor and the tin rich phases, indicating that in both cases tin is less in interaction with the palladium and could be aggregated with itself. Then, the dispersion of tin in bimetallic particles is decreased by the catalytic test, in accordance with the results obtained by O2/H2/O2 titration.

Then, the catalytic test induces an important modification of the palladium and tin distribution on the alumina surface with (i) an increase of tin oxide species in interaction with the support and (ii) a segregation between tin and palladium in bimetallic particles.

b) Silica supported catalyst

Fresh catalyst

For the catalysts characterized as prepared, the only phase detected is the oxide phase SnO2.

The calculation of A gives a value of 6.2 indicating that the tin is weakly bonded with the support compared to the alumina based catalyst. And then, in this oxide phase, tin is not in interaction with the support, but included in large particles over palladium. The value of A is characteristic of an alloy [START_REF] Greenwood | Mössbauer Spectroscopy[END_REF].

After reduction the best fit of the spectra was obtained for two different species. A tin poor Pd-Sn(a) and a tin rich Pd-Sn(-b) phase. The tin poor phase is identical to that observed on alumina, corresponding to a solid solution of tin in The tin rich phase Pd-Sn(b) is an alloy with an Isomer Shift close to 1.8, corresponding to Pd2Sn. This phase is the same than the one observed by XRD. This phase has a relative area of 77%, and the ratio between the relative quantity of the tin rich and tin poor phases, is about 3. This value is very high compared to the 1.6 observed on the alumina supported fresh catalyst.

Tested catalyst

After catalytic test and without prereduction before measurement, the Pd-Sn/SiO2 catalyst contains 25% of a metallic phase Pd3Sn and 75% of the SnO2 oxide phase. This large amount of metallic phase, after catalytic test and contact with air, demonstrates a good stability of metallic tin in this alloy

The calculation of the bond strength between tin and silica gives 5.5, indicating that the oxidized tin could be aggregated (weak metal-support interaction). For the metallic phase of Pd3Sn, the calculation of the A gives 2.5, this value is close to that of pure tin.

Once more, after reduction, no tin oxide phase is identified. This demonstrates the good stability of the bimetallic entities over silica. The tin poor phase Pd-Sn(a) is an alloy with an isomer shift close to 1.5, characteristic of the Pd3Sn phase [START_REF] Charlton | [END_REF]49). The comparison of this composition to the composition of the tin poor phase before the catalytic test (solid solution with less than 18 at% of tin), shows that the catalytic test leads to an enrichment of tin of this phase.

The tin rich phase Pd-Sn(b) is also an alloy, with an IS close to 2.2, characteristic of Pd3Sn4 [START_REF] Charlton | [END_REF]49). As in the tin poor phase, a global enrichment in tin of this metallic phase is observed.

The A calculation gives a value close to 7, so it can be inferred that the tin is in very large particles over palladium. As the only phases identified are Pd3Sn and Pd3Sn4 alloys after the catalytic test with no isolated tin species, and as the catalytic test induces an enrichment in tin of these bimetallic species, it can be deduced that isolated palladium particles are likely to be present on the silica support, and that the number of palladium atoms involved in this bimetallic particles is decreased after catalytic test. From the comparison of the relative area of the two Pd-Sn phases it can be noticed that there is two times more of the tin poor phase than tin rich phase. This result, correlated with the high incorporation rate of tin in the palladium particles, can be explained by a migration of the tin in rich phase to the tin poor phase. This is likely to be due to the oxidation of tin during the nitrate reduction, and to a an higher probability of re-deposition of this species in the poor phase.

c) Conclusion of the 119 Sn Mössbauer study

To conclude, spectra of silica supported bimetallic PdSn catalysts, before and after catalytic test, after contact with air or reduced, present evidence that the formation of bimetallic entities is predominant on this type of support.

Contrary to the results obtained with the silica supported catalysts, it is difficult to prevent the deposition of tin on alumina, due to its high affinity for the support, even if the preparation method and the catalytic test, where hydrogen is chemisorbed on palladium particles, favors the catalytic reduction of Sn 2+ or Sn 4+ and then the formation of bimetallic particles. Then, it could be inferred that in the fresh bimetallic catalyst, tin species present in bimetallic entities are likely to be located at the interface between the support and the palladium particles, which would favor the migration of tin from bimetallic particles to the support during the catalytic test leading to the unreducible SnO2 species observed on PdSnAT.

Catalytic performances

In order to link the tin distribution with catalytic performances, samples were evaluated in nitrite and nitrate reduction.

Nitrite reduction

The catalytic performances for nitrite reduction were determined for the two different (PdSnA and PdSnS) fresh (F) or tested catalysts. Results are reported in Table 7.

The alumina based catalysts are the most active, but the silica ones are the most selective, in accordance with (3). This could be explained by the difference of hydrogen adsorption capacity of the two types of catalysts. Indeed, the hydrogen uptake is higher on the alumina supported catalysts as compared to silica catalysts and then the accessibility of palladium sites and the probability of nitrite reduction by hydrogen on these sites is also more important on

PdSn. This could also explain the selectivity in N2, which is lower on the alumina than on the silica supported catalysts. Indeed, the more accessible palladium surface, the higher probability of meeting of four hydrogen and one nitrogen atoms, favoring the formation of NH4 + . However, the selectivity of the tested alumina supported catalyst is lower than the one of the corresponding fresh catalyst whereas its activity and its hydrogen uptake are lower.

Then, the activity and selectivity of the bimetallic catalysts can not be only rationalized in terms of palladium accessibility to hydrogen, and the presence of tin has to be taken into account. The activity in nitrite reduction has been reported as a function of the number of oxygen atoms chemisorbed on metallic tin determined from the O2/H2/O2 titration results reported in Figure 6. This figure demonstrates that the activity of the catalysts for nitrite reduction could also be related to the number of metallic tin sites in interaction with palladium. It is well known that nitrite reduction occurs principally on metallic palladium sites [START_REF] Hörold | [END_REF]12). So the influence of the amount of metallic tin in interaction with palladium on the activity could be explained in terms of electronic effect. Indeed, the donating effect of tin toward palladium would reinforce its noble character and then its activity.

Nitrate reduction

The catalytic test was carried out with fresh and tested catalysts supported on alumina and silica. The results are presented in Table 7. During nitrate reduction no nitrite appeared in solution whichever the catalyst is, that is likely to be due to the higher activity of all the catalysts for nitrite than for nitrate reduction. As for nitrite reduction, the fresh alumina supported catalyst is more active but less selective than the fresh silica one. The same difference is observed between the tested catalysts. The figure 6 shows also that the activity in nitrate reduction could be correlated to the number oxygen atoms chemisorbed on metallic tin sites which could be oxidized by nitrate.

4-CONCLUSION

Then to conclude, it has been evidenced by Mössbauer spectroscopy that on silica supported catalysts, only Pd-Sn solid solutions or alloys are present, whereas on alumina supported catalysts, tin is present at the interface between the support and palladium, or alone on the support, due to its high affinity for alumina. The presence of tin in bimetallic catalyst restricts the sintering phenomenon occurring in water on monometallic palladium catalyst supported on alumina or silica. In addition, the estimated number of oxygen atoms chemisorbed on metallic tin is very low on the silica supported catalysts whereas the amount of oxygen chemisorbed on tin is much more important in the alumina supported catalysts. It seems that the activity of the catalyst in nitrate and nitrite reduction is directly linked to the amount of reducible tin in interaction with palladium. This confirms a redox mechanism between nitrate and tin. For nitrite reduction, this could be explained by a reinforcement of the noble character of palladium.

7 a

 7 All the experiments for a given catalyst were performed with the same sample, which was treated in situ with H2 at 400°C for 2 hours. The temperatures of the measurement (Temp. Mes.) are 77 or 300 K. b Pd-Sn(a), tin poor Pd-Sn alloys ; Pd-Sn(b), tin rich Pd-Sn alloys ; Pd-Sn(c), tin phase stable in the air c Isomer Shift relative to SnO2. d Quadrupole splitting. e Full width at half maximum. f Normalized relative intensity. Because the probability of the Mössbauer effect (recoilless fraction) is different for the various Sn species (49), the RIs do not strictly correspond to concentrations. g A = -d ln (A300/A77)/dT x 10-1, where A300 and A77 are the actual absorption areas of components in the spectra recorded at 300 and 77 K, respectively.

Figure 1 :Figure 2 :Figure 3 :

 123 Figure 1 : TEM pictures of the fresh Al2O3 and SiO2 supported Pd-Sn bimetallic catalysts,

Figure 4 :Figure 5 :

 45 Figure 4 : XRD spectra of reduced PdSnSF (a) and PdSnAF (b) 25 to 85° 2θ scale

Figure 6 : 6 Isomer

 66 Figure 6 : Activity in nitrite () and nitrate () reduction as a function of the estimated

Table 1 :

 1 Catalysts identification, metal loading and chloride content (in wt%) of fresh mono and bimetallic catalysts supported on alumina (A) or silica (S)

	Catalyst	% Pd	% Sn	% Cl
	Pd5AF	4.7	-	0
	Pd5SF	4.7	-	0
	PdSnAF	4.7	1.5	0.9
	PdSnSF	4.7	1.5	0.9

Table 2 :

 2 Oxygen and hydrogen atom uptakes, dispersion and particle size in the monometallic

	fresh (F) or tested (T) palladium catalysts supported on alumina (A) or silica (S)
	Catalyst	OT1	HT	OT2	Dispersion of	Particle size
		(μmole.gcat -1 )	(μmole.gcat -1 )	(μmole.gcat -1 )	palladium	(nm)
					(%)
	Pd5AF	74	162	58	12.2	8.3
	Pd5AT	22	46	24	3.4	28.8
	Pd5SF	32	68	34	5.1	19.8
	Pd5ST	14	28	12	2.1	47.5

Table 3 :

 3 Activity and selectivity in nitrogen, at 75 % of nitrite conversion, of palladium

	catalysts supported on alumina or silica
	Catalyst	activity	Selectivity in
		(mmol.min -1 gcata -1 )	N2 (%)
	Pd5AF	0.450	91.9
	Pd5AT	0.440	97.0
	Pd5SF	0.728	99.4
	Pd5ST	0.725	>99.7

Table 4 :

 4 Oxygen and hydrogen atom uptakes in palladium-tin catalysts supported on alumina or silica and estimated amount of metallic tin

	Catalysts	OT1	HT	OT2	Amount of Sn°
		(μmole.gcat -1 )	(μmole.gcat -1 )	(μmole.gcat -1 )	(μmole.gcat -1 )
	PdSnAF	142	146	72	70
	PdSnAT	98	140	68	30
	PdSnSF	72	112	56	16
	PdSnST	70	108	54	16

Table 5 :

 5 Parameters of Mössbauer Spectra of the Pd-Sn catalysts supported on alumina or silica

	Sample	Treatment a	Species b	IS c	QS d	FWHM e	RI f	A g
		Temp. Mes.		(mm.s -1 )	-1 )		(%)	(a.u.)
	PdSnAF	prepared
		300K	Sn 4+	0.02	0.57	0.96	100	1.2
		77K		0.07	0.63	1.13	100
		Reduced
		300K	Pd-Sn(a)	1.43	-	1.33	38.7	5.5
		77K		1.40	-	1.31	21.5
		300K	Pd-Sn(b)	2.24	-	1.33	43.3	3.4
		77K		2.21	-	1.70	56.8
		300K	Sn 2+	4.01	0.26	0.75	18.0	5
		77K		4.07	0.30	0.78	21.7
	PdSnAT	As received
		300K	Sn 4+	0.03	0.53	0.94	100	2.1
		77K		0.07	0.57	1.03
		Reduced
		300K	Sn 4+	0.05	0.54	0.80	27.2	3
		77K		0.03	0.55	0.76	17.5
		300K	Pd-Sn(a)	1.70	-	1.31	39.8	6.3
		77K		1.69	-	1.18	32.5
		300K	Pd-Sn(b)	2.51	-	1.06	20.9	4.3
		77K		2.53	-	1.23	33.2
		300K	Sn 2+	3.89	0.32	0.73	12.1	6.1
		77K		3.97	0.33	0.72	16.8
	PdSnSF	As prepared
		300K	Sn 4+	0.02	0.54	0.96	100	6.2
		77K		0.05	0.56	0.95	100
		Reduced
		300K	Pd-Sn(a)	1.34	-	1.30	17,6	5.2
		77K		1.33	-	1.08	27,9
		300K	Pd-Sn(b)	1.81	-	1.45	82.4	3.8
		77K		1.93	-	1.46	72.1
	PdSnST	As received
		300K	Sn 4+	0.01	0.53	0.94	70.9	5.5
		77K		0.04	0.55	0.90	79.0
		300K	Pd-Sn(c)	1.56	-	1.08	29.1	3.4
		77K		1.62	-	1.04	21.0
		Reduced			-
		300K	Pd-Sn(a)	1.53	-	1.28	76.9	2.9
		77K		1.50		1.17	49.0
		300K	Pd-Sn(b)	2.25	-	1.10	23.1
		77K		2.12	-	1.50	51.0

Table 6 :

 6 Tin species identified for reduced bimetallic catalysts supported on alumina or silica before (PdSnAF and PdSnSF) and after (PdSnAT and PdSnST) catalytic test with the estimated relative amount of this tin phase

	Sample	Sn 4+	Pd-Sn(a)	Pd-Sn(b)	Sn 2+
	PdSnAF	SnO2	Pd(x>3)-Sn	Pd3-Sn4	SnOxCly
		0 %	30%	50%	20%
	PdSnAT	SnO2	Pd3-Sn	ß Sn/Pd	SnOxCly
		22%	36%	27%	15%
	PdSnSF	SnO2	Pd(x>3)-Sn	Pd2-Sn	SnOxCly
		0 %	23%	77%	0 %
	PdSnST	SnO2	Pd3-Sn	Pd3-Sn4	SnOxCly
		0%	63%	37%	0%

Table 7 :

 7 Activity and selectivity towards nitrogen at 75 % of conversion for nitrite and nitrate reduction in the presence of palladium-tin catalysts supported on alumina or silica

	Catalytst	Nitrite reduction	Nitrate reduction
		Activity	Selectivity	Activity	Selectivity
		(mmol.min -1 gcata -1 )	in N2 (%)	(mmol.min -1 gcata -	in N2 %
	PdSnAF	0.576	97.1	1 ) 0.049	92.8
	PdSnAT	0.396	94.3	0.040
	PdSnSF	0.334	> 99.7	0.034	93.2
	PdSnST	0.315	> 99.7	0.031	90


Anthony Le Valant

Anthony Garron

Nicolas Bion

Florence Epron
email: florence.epron.cognet@univ-poitiers.fr

Daniel Duprez

Hydrogen production from raw bioethanol over Rh/MgAl2O4 catalyst

Keywords: Bioethanol, ethanol steam reforming, hydrogen production, rhodium catalyst. 1

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Introduction

The production of renewable energy sources with a drastic decrease of greenhouse gas emissions is key-challenge to a sustainable development for the 21th century. In this respect, PEMFC fuel cells supplied with hydrogen have attracted more and more attention. The steam reforming of different fuels is a less expensive process for the production of hydrogen on a large scale. However this process is mainly applied to the transformation by steam of fossil hydrocarbons (natural gas or naphtha) [1, 2 and 3] or oxygenated compounds [4,5,6,7]. Biomass-derived oxygenated compounds can also be used as feedstocks for the steam reforming reaction. In particular bioethanol, much less toxic than methanol is interesting for H2 production since it can be produced by biomass fermentation (corn, beet sugar, straw…) on a large scale, which makes it a renewable source of energy. Moreover, its transport, storage and distribution are relatively simple.

The main products of ethanol steam reforming (ESR) are H2, CO2, CO (Eq. 1-2) whereas the hydrogenation of ethanol is responsible for the formation of methane (Eq. 3). C2H5OH(g) + 3H2O(g)  2CO2(g) + 6H2(g)

 0 298 ΔH +174 kJ mol 1 (1)
C2H5OH(g) + H2O(g)  2CO(g) + 4H2(g)

 0 298 ΔH +256 kJ mol 1 (2)
C2H5OH(g) + 2H2(g)  2CH4(g) + H2O(g)

 0 298 ΔH 157 kJ mol 1 (3)
However, the steam reforming reactions are generally preceded by a rapid transformation of ethanol into acetaldehyde (dehydrogenation, Eq. 4), ethylene (dehydration, Eq. 5) or methane, CO and H2 (cracking, Eq. 6).

C2H5OH(g)  CH3CHO(g) + H2(g) 

Acetaldehyde, ethylene and methane may thus be considered as important intermediates in the formation of hydrogen from ethanol [8,9]. Once formed, methane is difficult to be reformed by steam, which penalizes the H2 yield. The ethanol dehydration reaction occurs mainly on the acid sites of the support. The formation of ethylene involves a deactivation of the catalyst, the ethylene being a coke precursor [3], at the origin of the catalyst deactivation. The ethanol dehydrogenation yields acetaldehyde. This reaction depends not only on the basicity of the supports but also on the activity of supported metals in dehydrogenation. Noble metals, especially Rh and Pt [8,9,10,11,12,13,14,15,16], were investigated in ESR. It was shown that Rh was the most active metal in the ethanol steam reforming, as for the steam reforming of many hydrocarbons.

The choice of the support is a key factor for a good stability of the catalyst. It has a very strong effect on both the activity and the selectivity of the metal. The support plays a crucial role in the steam reforming reaction: (i) it may favor water splitting into OH groups and promote the migration of these reactive species toward the metal particles, where the final step of the COx and hydrogen formation occurs [2,3]; (ii) it may catalyze reaction (4) and/or (5) affect the selectivity in the final products [8,17,18]; and (iii) it may finally contribute to the stabilization of the metal particles at high temperature under steam [19,20,21]. Alumina is a support with a mobility of OH groups starting from 400°C and it is very often used for hydrocarbon steam reforming (400°C<T<500°C) [2,3]. However for reactions carried out at higher temperatures, typically higher than 600°C, one reports a phenomenon of diffusion of the metal ions in the support and even the formation of aluminates starting from 700°C [22,23,24,25]. This is not easily reversible and leads to the deactivation of the catalyst. F.Aupretre and coworkers [26] tested several catalysts, and the results obtained with the MgAl2O4 spinel support allowed a strong limitation of deactivation by the aluminate formation.

Some papers dealt with the use of crude bioethanol for hydrogen production [27, 28 , 29], but a majority of studies reported in the literature on bioethanol steam reforming deals with the use of a mixture of water and pure ethanol. However, the use of a raw bioethanol feed, limiting the purification steps, is of a major importance for a cost effective industrial application. But the presence of impurities in the feed may induce the deactivation of the catalyst. Then, the aim of this study is to show the effect of impurities present in raw bioethanol, such as esters, aldehydes, higher alcohols or acetic acid, on the stability of the 1%Rh/MgAl2O4/Al2O3 catalyst during bioethanol steam reforming for hydrogen production.

Experimental

Catalyst preparation

The starting materials used for the catalyst preparation were -alumina beads (~200 m² g -1 , 1-2 mm in diameter) provided by Axens. The support was prepared by modification of the alumina beads by impregnation of magnesium acetate Mg(CH3COO)2.4H2O (Alpha Aesar) in order to obtain 5 wt% of Mg in the support. The spinel was formed via a high-temperature solidsolid reaction between magnesium oxide (ex-acetate) and the -alumina beads. Acetate was chosen in order to obtain a better control on the acidity of the support [26]. The rhodium catalyst (~1 wt% Rh) was prepared by wet impregnation of rhodium chloride RhCl3 (Acros) for 48h at ambient temperature. This metal precursor was chosen because it favours a good dispersion of rhodium at the support surface without a significant increase in the solid acidity [26]. The impregnated support was evaporated under stirring at 45°C during 24 h and dried at 120°C for 15 h. The catalyst was finally calcined under a 30 cm 3 min -1 air flow at 700°C for 4 h (ramp 2°Cmin -1 ) to stabilize the solid before reaction. The complete experimental procedure and the characteristics of the catalyst are described in detail in [26]. The nomenclature of the catalyst is Rh(1%)/MgAl2O4/Al2O3.

Ethanol steam reforming reaction (ESR)

The experiments were performed at 2 bara, controlled by a back pressure regulator (Swagelock).

The steam reforming reaction was carried out in a flow reactor (L = 550 mm; int = 12.5 mm). The ethanol/water mixture (0.23 mL min -1 of liquid flowrate) was injected with a liquid pump (Gilson 307) and preheated at 130°C before entering the reactor. The molar ratio R between ethanol and water was chosen equal to 4. The catalyst sample (typically 250 mg) was diluted in carborundum (SiC) (2.75 g), with roughly the same particle size. The resulting weight hourly space velocity (WHSV), calculated by dividing the ethanol flowrate (g h -1 ) by the mass of catalysts (g) was equal to 19.5 h -1 . The product gases were first analyzed on line in a gas chromatograph (GC) equipped with a FID detector (Varian) after separation on an AT™-aquawax column (30 m, 0.25 mm) (Alltech). This analysis allowed us to determine the amount of organic compounds present in the reaction flow (ethanal, ethanol, ethanoic acid, ethyl ethanoate, etc.). The reaction gases were passed through a condenser at -2°C to collect water and organic compounds mentioned above. The flow of non condensable gases (H2, CO, CO2, CH4, C2H4, C2H6, C3H6, and C3H8, etc) was measured on line using a Gas Meter (Ritter) and acquired in the computer. These gases were analyzed on line by a TCD (Varian) with helium carrier (N2, O2, CO and CO2), by a GC with a FID detector (Varian)

with helium carrier (hydrocarbon compounds) and by an external GC with a TCD detector (Perichrom) with nitrogen carrier (H2). These analyses allowed us to determine every 15 min the exact composition of the product gas and of unconverted reactants (water and ethanol).

Before each catalytic test, the catalyst sample was reduced at the reaction temperature (675°C) for 14 h under H2 (9 dm 3 h -1 ) after having been heated from ambient temperature with a temperature ramp of 10°Cmin -1 and flushed under N2 (9 dm 3 h -1 ) in order to remove physisorbed hydrogen. The temperature of 675°C was chosen because at this temperature and with the chosen WHSV, the ethanol conversion is not total. It reaches 78% after 8h of time-on-stream with pure ethanol while keeping a good H2 yield. These reaction conditions allowed us to discriminate the possible effects of the compounds present in bioethanol. Various types of impurities were studied on model mixtures by adding 1% (molar percentage) of the chosen impurity in the water and ethanol mixture.

If nX in and nX out are the molar flow rates of the product X at the inlet and the outlet of the reactor, respectively, the catalyst performance is characterized by:

-the product yield (mole of X per mole of introduced EtOH) :

X Y = In Et OH Out X n n -the ethanol conversion : XEtOH = In Et OH Out Et OH In Et OH n n n  x 100 -the selectivity for CO/COx compounds: SCO/COx = out 2 CO out CO out CO n n n 
x 100

To give the efficiency of the overall process, including steam reforming and water gas shift (WGS), a theoretical hydrogen yield is also calculated (supposing a total conversion of C2H5OH into H2 + CO2) according to :

th H Y 2 = out 2 H out CO out 2 H n n n  x 2 H Y

Results

Performance of the catalyst for the ethanol steam reforming

The ethanol steam reforming was first performed using a pure ethanol and water mixture, i.e.

without impurity, in presence or absence (blank experiment) of the Rh (1%)/MgAl2O4/Al2O3 catalyst. Results are summarized in Table 1 where they are compared to the thermodynamic values at the equilibrium.

In absence of catalyst (blank reaction), an initial 66% conversion is observed. As the same result was obtained using a quartz reactor, it can be inferred that this conversion is due to the homogeneous gas phase thermal decomposition. The main products are H2, CO and CH4 (reaction 6). Ethylene, produced by dehydration, acetaldehyde, resulting from dehydrogenation, and ethane are also observed. The amount of water is higher than the amount introduced indicating that water not only does not react with ethanol in the absence of catalyst but is produced during the reaction by the dehydration of ethanol. It is important to note that, in absence of catalysts, the reaction is rapidly stopped, the tubes being blocked by the coke produced.

. In presence of catalyst, the initial conversion is of 93%.. The hydrogen yield is lower than the value predicted by the thermodynamic. This result is due in part to the incomplete conversion of ethanol and to (i) a relatively high methane yield, which is indicative of ethanol cracking (reaction 6) and (i) a non negligible amount of C2 compounds. 1) and those after 8h of time on stream (Table 3) shows that during the reaction, the yields of H2, CO and CH4 decrease as a consequence of the decrease of the ethanol conversion. On the contrary, the yield of other products, especially intermediate products such as ethylene and acetaldehyde, can be considered as unchanged. It is known that acetaldehyde can easily undergo decarbonylation to form CH4 and CO [30] or be converted by steam reforming. On the contrary, ethylene is an undesired product since it is a coke precursor at the origin, in part, of the deactivation observed. It should be noted that the deactivation may also result from the sintering of rhodium, involving a decrease in accessible metal surface, even if Frusteri et al. [31] have shown that Rh, when supported on MgO, is not sensitive to sintering during the ethanol steam reforming reaction compared to Ni, Co or Pd.

Influence of impurities on the ethanol steam reforming

The composition of crude bioethanol is presented in Table 2. The main impurities are alcohols accounting for 87% of the impurities contained in crude ethanol, the most important being propan-1-ol (27%) and methyl-3 butanol-1 (27%). One can also note the presence of esters, aldehydes, acetic acid and nitrogen-containing bases. In order to study the impact of each type of impurity on the ESR, "model" crude ethanol feeds were prepared by addition of 1 mol% of one impurity (0.01 mole per mole of ethanol) It is important to note that the quantities of impurities taken for the tests are much higher than the values mentioned in table 2. This choice was made to better see the role of each impurity on ESR during a short time on stream. The catalytic performances obtained with the "model" crude ethanol mixtures will be compared to those obtained with the pure ethanol mixture, considered as a reference.

Two series of impurities have been studied, the first one corresponding to molecules with four carbon atoms and different functions (butanal, diethylether, butanol, ethylacetate) and the second one to molecules with acidic and basic properties (acetic acid and diethylamine).

a. Effect of acidic and basic impurities

The impact of the acid-base properties of the impurities on the ESR was studied using model impurities as present in crude ethanol, namely, acetic acid and diethylamine. Acetic acid is a relatively weak acid whereas diethylamine is a strong base. The catalytic activity of the support for ethanol dehydration is associated with its acid-base properties. Then, it can be expected that both acid and basic substances will have an effect on ethanol dehydration [32]. For example, it was shown that basic molecules like pyridine may compete with alcohol molecules for the acidic sites, whereas acetic acid may either promote olefin formation or inhibit it while favoring the esterification of the alcohol by acetic acid, depending on the structure of the alcohol used [33]. On the other hand, the adsorption of acidic molecules on the surface causes the acidity of the surface to increase [33] and then may favor the catalyst deactivation.

The results obtained in the presence of the acidic and basic impurities, and in the presence of pure ethanol as a reference, are summarized in Table 3 and illustrated in Fig. 2. Figure 2 shows that, except at the starting point of the reaction, the dry gas flowrate in the presence of the amine is definitely higher than that of the reference test. This may be directly linked to the higher ethanol conversion observed in the presence of the amine (Table 3). This highest ethanol conversion has a direct consequence not only on the hydrogen yield but also on the CO, CO2 and CH4 yields (Table 2), which are also slightly higher in the presence of the amine than with pure ethanol. On the contrary the yields of intermediate products such as ethylene and acetaldehyde can be considered as identical in the presence of diethylamine compared to the pure ethanol/water mixture. If we look at the amount of water, it is also the lowest in the presence of diethylamine, which indicates that a more important amount of water has reacted in the presence of the base. These changes in the products yields, especially hydrogen, due to the addition of diethylamine in the ethanol/water mixture should be explained by the steam reforming of diethylamine even if only 1% of diethylamine was added to pure ethanol. Then, it is interesting to analyse the product distribution obtained with or without diethylamine at isoconversion of ethanol, i.e. at the same value of ethanol conversion. This can be performed by comparing the data obtained at 88% of ethanol conversion, corresponding to 8h of time on stream in the presence of diethylamine (Table 3), to those obtained at 2h of time on stream with pure ethanol (results not shown). In that case, the yields of the various products can be considered as identical. Consequently, it can be deduced that diethylamine does not modify significantly the reaction pathway during the ethanol steam reforming but it promotes the ethanol steam reforming by increasing its conversion. This promoting effect of diethylamine may be explained by the fact that this basic molecule may compete with the alcohol molecules for the acidic sites, in accordance with [33], thus inhibiting the dehydration of ethanol on these sites. However, in the presence of diethylamine in the stream, (i) the yield of ethylene and (ii) the rate of deactivation, illustrated by the evolution of the gas flowrate versus time on stream (Fig. 2) are similar to those obtained with the pure ethanol and water stream Then it can be inferred that the diethylamine transformation also yields ethylene and then does not limit coke production leading to catalyst deactivation. The promoting effect of diethylamine on ethanol conversion may also be explained by a modification of the metal electronic properties resulting from an electron transfer of the free nitrogen doublet toward the metal phase [34,35].

In the presence of acetic acid, the dry gas flowrate (fig. 2) is lower than that obtained with the reference mixture (water +ethanol), which is due to the lower ethanol conversion observed in the presence of the acid. As a result, the hydrogen yield and the yield of other products strongly decreased. At 88% of conversion (obtained at the beginning of the reaction, after 1h of time on stream) the hydrogen yield (2.37 mol/molethanol) is lower than that obtained with pure ethanol or with ethanol + diethylamine. The yield of CO, CO2, CH4 and acetaldehyde are also lower whereas the yield of ethylene and ethane are higher. Neither ester that may result from the reaction between ethanol and acetic acid nor acetone, as observed by Tanabe et al. [36,37] during acetic acid steam reforming over Pt/ZrO2 catalysts, were observed. This result confirms that the presence of an acid such as acetic acid may favour the production of ethylene by dehydration reaction followed by the deactivation of the catalyst by coke deposition. It is also known that acetic acid forms acetate species at low temperature on clean noble metal surfaces and that acetate species decompose to yield H2, CO2 and adsorbed carbon on the surface [38,39].

b. Effect of the other types of impurities Different molecules representative of raw bioethanol impurities were studied separately, namely an aldehyde, a heavier alcohol and an ester. An ether was also added in the series, although no ether was evidenced in raw bioethanol. In order to favor the comparisons, molecules with the same amount of carbon atoms were chosen, i.e. butan-1-al, diethylether, butan-1-ol and ethyl acetate.

Results are presented in Table 3 and Fig 3 . In presence of butanal, results are similar to those obtained with diethylamine: the gas flowrate and the ethanol conversion are enhanced compared the reference test.. The yields of the different products (Table 3) can also be considered as similar to those obtained with diethylamine. Conversely, the presence of the other impurities, i.e. the ester, ether, C4 alcohol, strongly affects the catalytic performance, especially the ethanol conversion.

Furthermore, the conversion, measured after 8 h of time-on-steam, approaches the value obtained without catalyst by thermal decomposition (Table 1). Nevertheless, the yields of hydrogen are higher than that obtained by thermal decomposition thus proving that, even if the catalyst is strongly poisoned by the presence of these impurities, it is not totally deactivated. The poisoning effect of these impurities can be classified in this increasing order: butanol< diethylether < ethyl acetate.

Whereas the yields of final products, i.e. hydrogen, CO, CO2 and methane are strongly decreased compared to the reference test, the yields of intermediate products, namely ethylene and acetaldehyde are higher than those obtained with the pure ethanol/water mixture. This proves that the presence of these impurities affects the overall kinetics of the reaction by probably decreasing the number of active sites. Then, it can be inferred that intermediate products are rapidly produced by ethanol dehydration or dehydrogenation on the catalyst but they are more slowly transformed.

Moreover, the presence of high amounts of ethylene may enhance the deactivation of the catalysts by formation of carbonaceous products. It is difficult to compare the product distribution obtained in the presence of these various impurities to that obtained on the pure ethanol feed at isoconversion, the maximum of conversion reached with these impurities being much lower than that obtained in the reference test. Nevertheless, the calculation of the selectivity, calculated from the data given in Table 3 by dividing the amount of product (in mol) by the amount of ethanol converted (in mol), demonstrates that the selectivity for hydrogen is significantly decreased by the presence of these impurities: it is equal to 2.03, 2.2 and 1.7 in the presence of the ether, butanol and ester, respectively, whereas it is equal to 3.01 with the pure ethanol feed. Then, it can be inferred that these impurities strongly poisoned the catalyst surface, but in a different way. In the case of diethylether, the enhanced production of ethylene may be explained by the dehydration of this impurity on the acidic sites of the support, as described by Knözinger and Köhne in [40]. The deactivation observed in the presence of butanol may be linked to the production of butene, as intermediate product, by dehydration reaction. Although this olefin has not been detected in the reaction products, the formation of butene may contribute to the coke formation. Indeed, it can be inferred that butanol may be either dehydrated to olefin or dehydrogenated to aldehyde according to a similar mechanism to that proposed for ethanol [8,9]. However, the dehydrogenation of butanol, yielding butanal, is probably not favored on the Rh/MgAl2O4/Al2O3 catalyst, since butanal may promote the ethanol conversion. As far as the deactivation by the ester is concerned, it may be explained in part by the hydrolysis of ethylacetate on the acidic sites of the support, yielding ethanol and acetic acid [41], and then to the presence of acetic acid. However, the deactivation observed is much more important than that observed in the presence of acetic acid (see previous paragraph).

Another possibility will be a competitive adsorption, where ethylacetate will be more strongly adsorbed on active sites of the catalyst than ethanol and/or water. It has been reported in the literature that ethylacetate is more strongly adsorbed on alumina than ethanol [41] and that this stronger adsorption leads to an inhibition of ethanol oxidation. As shown by results presented in Table 3, the yield of water obtained during the steam reforming of ethanol in the presence of ethyl acetate is equal to 4.06 mol per mol of ethanol introduced, that is slightly higher than the water to ethanol molar ratio introduced (R = 4). Consequently, it can be deduced that the activation of water on the support is likely to be hindered when ethyl acetate is present; furthermore, the amount of water produced during the reaction is higher than the amount consumed, proving that the dehydration reaction yielding ethylene is predominant.

Conclusion

The present paper aimed to evidence the effect of various impurities present in raw bioethanol on the catalytic performance of a Rh/MgAl2O4/Al2O3 catalyst for ethanol steam reforming. The impurities chosen were globally representative of those present in raw bioethanol, namely acetic acid, diethylamine and various type of molecules with the same number of carbon atoms, i.e. a higher alcohol, butanol, an aldehyde, butanal, an ester, ethyl acetate and an ether, diethylether. The impurities can be classified in two categories, corresponding to impurities with either a promoting effect (diethylamine and butanal) or a poisoning effect (butanol, diethylether and ethyl acetate). In the last case the deactivation of the catalyst is mainly due to coke deposition.

(T = 675°C, P = 2 bar, R = 4) yield (mol mol

g m -3 g m -3 g m -3 g m -3 g m -3 g m -3 g/ m 3 g m -3 g m -3 g m -3 g m -3 g m -3 g m -3 g m -3 mg dm -3 mg kg --1

  )  CO(g) + CH4(g) + H2(g)

Figure 1

 1 Figure 1 presents the evolution of the dry gas flowrate and ethanol conversion as a function of time

Figure 1 :

 1 Figure 1: Dry gas flowrate and ethanol conversion as a function of time on stream obtained

Figure 2 :

 2 Figure 2: Dry gas flowrate as a function of time on stream obtained during pure ethanol steam

Figure 3 :

 3 Figure 3: Dry gas flowrate as a function of time on stream obtained during pure ethanol steam

  Fig.1

Fig. 2

 2 Fig.2

  Fig.3

Table 2 :

 2 Composition of rectified and crude alcohol. Analysis performed by the alcohol distillers union (nd : non detected).

	%vol. (@ 20°C)	96.3	92,9
		0.8	04
		20	39
		<1	123
		<1	108
		42.5	94
		<0.5	<10
		<0.5	581
		<1	304
		n.d.	<10
		<0.5	<10
		n.d.	273
		n.d.	582
		<0.5	855
		<1	1746
		<0.2	<0.2
		<0.2	1.2

Table 3 :

 3 Performances in steam reforming of ethanol with or without 1% of impurity using Rh(1%)/MgAl2O4/Al203 catalyst after 8 h of time

	Characteristic of the feed	Unities	Rectified	Raw
			Alcohol	Alcohol
	Alcohol percentage
	Total acidity (Acetic acid)
	Dry extract
	Esters
	Aldehydes
	Methanol
	Butan-2-ol
	Propan-1-ol
	Methyl-2 Propanol-1
	Propen-2 ol-1
	Butan-1-ol
	Methyl-2 butanol-1
	Methyl-3 butanol-1
	Methyls-butanol-1
	Total higher alcohols
	Total sulphur
	Volatile nitrogenated bases

c

Taking into account the amount of CO, considering that all the CO produced reacts with H2O to give CO2 and H2 Captions to figures


Anthony Le Valant

Fabien Can

Nicolas Bion

Daniel Duprez

Florence Epron
email: florence.epron.cognet@univ-poitiers.fr

HYDROGEN PRODUCTION FROM RAW BIOETHANOL STEAM REFORMING: OPTIMIZATION OF CATALYST COMPOSITION WITH IMPROVED STABILITY AGAINST VARIOUS IMPURITIES

Keywords: ethanol steam reforming, crude bioethanol, rhodium catalysts

published or not. The documents may come

Hydrogen Production from raw bioethanol steam reforming: optimization of catalyst composition with improved stability against various impurities

INTRODUCTION

With the aim of reducing the global emissions of greenhouse gases, hydrogen should be produced from renewable resources such as bioethanol, which can be obtained by fermentation of several biomass sources. The catalytic steam reforming of bioethanol constitutes thus a promising route to hydrogen production [START_REF] Garcia | Hydrogen production by the steam reforming of ethanol: Thermodynamic analysis[END_REF][START_REF] Cavallaro | Ethanol steam reforming in a molten carbonate fuel cell. A preliminary kinetic investigation[END_REF][START_REF] Aupretre | Bio-ethanol catalytic steam reforming over supported metal catalysts[END_REF][START_REF] Liguras | Production of hydrogen for fuel cells by steam reforming of ethanol over supported noble metal catalysts[END_REF][START_REF] Frusteri | H2 production for MC fuel cell by steam reforming of ethanol over MgO supported Pd, Rh, Ni and Co catalysts[END_REF][START_REF] Fatsikostas | Reaction network of steam reforming of ethanol over Nibased catalysts[END_REF][START_REF] Sun | from steam reforming of ethanol at low temperature over Ni/Y2O3, Ni/La2O3 and Ni/Al2O3 catalysts for fuel-cell application[END_REF][START_REF] Aupretre | Ethanol steam reforming over MgxNi1-xAl 2O3 spinel oxide-supported Rh catalysts[END_REF][START_REF] Palmeri | hydrogen production from ethanol on Rh/MgO based catalysts. The influence of rhodium precursor on catalytic performance[END_REF][START_REF] Sanchez-Sanchez | Ethanol steam reforming over Ni/MxOy-Al2O3 (M=Ce, La, Zr and Mg) catalysts: Influence of support on the hydrogen production[END_REF][START_REF] Profeti | Production of hydrogen via steam reforming of biofuels on Ni/CeO2-Al2O3 catalysts promoted by noble metals[END_REF][START_REF] Andréia | Bimetallic catalysts performance during ethanol steam reforming: Influence of support materials[END_REF] since the CO2 produced during this process is consumed by the plant during its growth. The main products of ethanol steam reforming are H2, CO2, CO (Eq. 1 and 2) whereas the hydrogenation of ethanol is responsible for the formation of methane (Eq. 3). C2H5OH(g) + 3H2O(g)  2CO2(g) + 6H2(g) H 0 298K = 173.3 kJ mol 1 (eq. 1)

C2H5OH(g) + H2O(g)  2CO(g) + 4H2(g) H 0 298K = 255.0 kJ mol 1 (eq. 2)

C2H5OH(g) + 2H2(g)  2CH4(g) + H2O(g) H 0 298K = 156.7 kJ mol 1 (eq. 3)

Methane and carbon monoxide produced during the reaction can be easily removed from the gas flow.

The major intermediate products are acetaldehyde and ethylene, resulting, respectively from ethanol dehydrogenation (Eq. 4), and ethanol dehydration, (Eq. 5).

C2H5OH(g)  CH3CHO(g) + H2(g) H 0 298K = 68.0 kJ mol 1 (eq. 4)

C2H5OH(g)  C2H4(g) + H2O(g) H 0 298K = 45 kJ mol 1 (eq. 5)

The thermal decomposition of ethanol may also yield methane, CO and H2 (Eq. 6).

C2H5OH(g)  CO(g) + CH4(g) + H2(g) H 0 298K = 50 kJ mol 1 (eq. 6)

The ethanol dehydration reaction occurs mainly on the acid sites of the support. The formation of ethylene favors the catalyst deactivation, ethylene being a coke precursor [START_REF] Duprez | Selective steam reforming of aromatic hydrocarbons. IV. Steam conversion and hydroconversion of selected monoalkyl-and dialkylbenzenes on Rh catalysts[END_REF]. The ethanol dehydrogenation reaction depends not only on the basicity of the supports but also on the activity of supported metals in dehydrogenation.

Among noble metals, rhodium is the most active for ethanol steam reforming [START_REF] Cavallaro | Ethanol steam reforming in a molten carbonate fuel cell. A preliminary kinetic investigation[END_REF][START_REF] Aupretre | Bio-ethanol catalytic steam reforming over supported metal catalysts[END_REF][START_REF] Liguras | Production of hydrogen for fuel cells by steam reforming of ethanol over supported noble metal catalysts[END_REF][START_REF] Frusteri | H2 production for MC fuel cell by steam reforming of ethanol over MgO supported Pd, Rh, Ni and Co catalysts[END_REF]. As far as the support is concerned, it may favor water splitting into OH groups and promote the migration of these reactive species toward the metal particles, it contributes to the formation of intermediate acetaldehyde and/or ethylene and it may affect the stability of the metal particles at high temperature under steam.

Auprêtre et al [START_REF] Aupretre | Ethanol steam reforming over MgxNi1-xAl 2O3 spinel oxide-supported Rh catalysts[END_REF] have optimized the catalyst formulation in order to obtain active and stable catalyst for H2 production by ethanol steam reforming at 700°C and 10 bar and the best results were obtained with the Rh/MgAl2O4 catalyst supported on alumina beads.

Except some papers [START_REF] Vargas | Study of Ce-Zr-Co fluorite-type oxide as catalysts for hydrogen production by steam reforming of bioethanol[END_REF][START_REF] Akande | Synthesis, characterization and performance evaluation of Ni/Al 2O3 catalysts for reforming of crude ethanol for hydrogen production[END_REF], studies reported in the literature on bioethanol steam reforming generally deal with the use of a mixture of water and pure ethanol. However, the use of a raw bioethanol feed, limiting the purification steps, is of major importance for a cost effective industrial application. But the presence of impurities in the feed could induce the deactivation of the catalyst.

The aim of this paper is (i) to show the effect of impurities present in raw bioethanol, such as esters, aldehydes, amine, acetic acid, methanol, linear or branched alcohols, on the stability of a Rh/MgAl2O4 catalyst during bioethanol steam reforming for hydrogen production and (ii) to improve the stability of the catalyst against deactivating impurities by modifying the support and metallic phase composition.

EXPERIMENTAL

For the reference catalyst Rh/MgAl2O4, the support was prepared by impregnation of magnesium acetate onto γ-Al2O3 beads (diameter 1-2 mm, AXENS). The amount of salt was adjusted to obtain 5 wt.% Mg in the support. The spinel structure was obtained by calcination under air at 1000°C for 15h [START_REF] Frusteri | H2 production for MC fuel cell by steam reforming of ethanol over MgO supported Pd, Rh, Ni and Co catalysts[END_REF]. The 1wt..%Rh/MgAl2O4 catalyst was prepared by impregnation of a rhodium chloride precursor salt on the support, followed by calcination under air at 700°C for 4h.

For the modified support, the alumina beads were modified by wet impregnation using an appropriate amount of nitrate precursors of rare earth elements (Sc(NO3)3, Y(NO3)3, La(NO3)3, Er(NO3)3 and Gd(NO3)3), in order to obtain 10 wt.% of rare earth element on the support. After impregnation and drying, supports were calcined at 900°C for 15 h at 5°C min -1 . The nonimpregnated alumina support was submitted to the same calcination treatment. The supports are referred as Al2O3, Sc-Al2O3, Y-Al2O3, La-Al2O3, Er-Al2O3 and Gd-Al2O3. Bimetallic Rh-Ni, Rh-Pt and Rh-Pd catalysts were obtained by coimpregnation of the precursor salt (1wt.% of Rh, Pt, Pd and 10 wt.% of Ni) and calcination under air at 700°C for 4h.

The steam reforming of ethanol was carried out in a fixed bed reactor (L = 550 mm; int = 12.5 mm). The catalyst (0.25 g) mixed with carborundum (2.75 g) was pretreated under flowing H2 for 15 h at 675°C. Then the reactor was purged under flowing N2 at the same temperature. After suppressing the gas flowing, ethanol and water were introduced in the reactor (molar ratio water/ethanol = 4, 0.23 mL min -1 of liquid flowrate) with a HPLC pump. No carrier gas was added.

The reaction was performed at 675°C and a pressure of 2 Bar. The experimental conditions, especially the weight hourly space velocity (WHSV = 19.5 h -1 ), determined by dividing the ethanol flowrate (in g per h) by the mass of catalyst (in g), and the temperature were chosen in order to obtain less than 100% of ethanol conversion at the beginning of the reaction and to evidence the deactivation of the catalyst as a function of time, coke formation being favored at this temperature.

In these experimental conditions, the yields in gaseous products are the 3.78 mol/molethanol for H2, 0.54 mol/molethanol for CO, 0.71 mol/molethanol for CO2 and 0.1 mol/molethanol for CH4. The ethanol chosen for the reaction is either pure ethanol or modified by one selected impurity (ethyl acetate, isobutanol, acetic acid …) or raw bioethanol. The catalytic performances were examined taking into account the ethanol conversion and the hydrogen yield after 8h of time-on-stream.

RESULTS AND DISCUSSION

Effect of impurities present in raw bioethanol on catalyst performances

The main impurities present in raw bioethanol obtained from sugar beet are higher alcohols accounting for 87% of the impurities, the most important being propan-1-ol (27%) and methyl-3 butan-1-ol (27%), and also esters, aldehydes, acetic acid and nitrogen-containing bases [START_REF] Valant | Hydrogen production from raw bioethanol over Rh/MgAl2O4 catalyst. Impact of impurities: Heavy alcohol, aldehyde, ester, acid and amine[END_REF].

In order to study the impact of each type of impurity on the ethanol steam reforming, "model" raw ethanol feeds were prepared by addition of 1 mol% of one impurity in ethanol. Three series of impurities have been studied, namely (i) molecules with four carbon atoms and different functions (butanal, diethylether, butanol, ethylacetate) (ii) molecules with acidic and basic properties (acetic acid and diethylamine) and (iii) linear or branched alcohols (methanol, propan-1-ol, butan-1-ol, pentan-1-ol, isopropanol, 2-methylpropan-1-ol, 3-methylbutan-1-ol). The catalyst performances (hydrogen yield and ethanol conversion) were determined after 8h of time-on-stream. In the absence or in the presence of impurities, the initial ethanol conversion was always high (98-99%), but depending on the experimental conditions, the ethanol conversion decreased as a function of time, due to the deactivation of the catalyst, thus leading to a decrease in the hydrogen yield.

The results obtained in the presence of the acidic and basic impurities, and in the presence of pure ethanol as a reference, are summarized in Fig. 1.

Fig. 1 shows that the presence of diethylamine favors the ethanol conversion, and slightly increases the hydrogen yield. This promoting effect of diethylamine may be explained by a competition of this basic molecule with the alcohol molecules for the acidic sites, in accordance with [START_REF] Jain | Catalytic dehydration of alcohols over alumina. Mechanism of ether formation[END_REF], thus inhibiting the dehydration of ethanol on these sites. The promoting effect of diethylamine on ethanol conversion may also be explained by a modification of the metal electronic properties resulting from an electron transfer of the free nitrogen doublet toward the metal phase [START_REF] Boitiaux | Additive effects in the selective hydrogenation of unsaturated hydrocarbons on platinum and rhodium catalysts. I: Influence of nitrogen-containing compounds[END_REF][START_REF] Nohair | Palladium supported catalysts for the selective hydrogenation of sunflower oil[END_REF].

A lower ethanol conversion is observed in the presence of acetic acid compared to the conversion obtained without impurity. As a result, the hydrogen yield is decreased. It has been shown in the literature that acetic acid may either promote olefin formation by increasing the acidity of the support surface or inhibit it while favoring the esterification of the alcohol by acetic acid, depending on the structure of the alcohol used [START_REF] Jain | Catalytic dehydration of alcohols over alumina. Mechanism of ether formation[END_REF]. In the present study, neither ester nor acetone was observed. This result confirms that the presence of an acid such as acetic acid may favor the production of ethylene by dehydration reaction thus favoring the catalyst deactivation by coke deposition. It is also known that acetic acid forms acetate species at low temperature on clean noble metal surfaces and that acetate species decompose to yield H2, CO2 and adsorbed carbon on the surface [START_REF] Li | Acetate formation, stabilisation and surface explosion on Rh(111)[END_REF][START_REF] Wang | Catalytic steam reforming of biomass-derived oxygenates: Acetic acid and hydroxyacetaldehyde[END_REF]. Then, the deactivation of the catalyst observed in the presence of acetic acid may be explained by the formation of coke, favored in the presence of this acidic molecule.

The results obtained in the presence of various impurities (aldehyde, ether, higher alcohol and ester)

with the same amount of carbon atoms, i.e. butan-1-al, diethylether, butan-1-ol and ethyl acetate are summarized in Fig. 2.

In presence of butanal, similarly to what was observed with diethylamine, the ethanol conversion is increased compared to the reference test (without impurity). The hydrogen yield is also as similar to that obtained with diethylamine. Conversely, the presence of the other impurities, i.e. the ester, ether, C4 alcohol, strongly affects the ethanol conversion thus decreasing the hydrogen yield. The poisoning effect of these impurities can be classified in this increasing order: butanol< diethylether < ethyl acetate. This poisoning effect may be explained by an enhanced production of coke in the presence of these impurities. In the case of diethylether, ethylene, which is coke precursor, may be produced by the dehydration of this impurity on the acidic sites of the support, as described by Knözinger et al. [START_REF] Knözinger | The dehydration of alcohols over alumina. I. The reaction scheme[END_REF]. The deactivation observed in the presence of butanol may be linked to the production of butene by butanol dehydration. The deactivation by the ester may be due to the hydrolysis of ethylacetate on the acidic sites of the support, yielding ethanol and acetic acid [START_REF] Sawyer | Reaction pathways during the oxidation of ethyl acetate on a platinum/alumina catalyst[END_REF], and then to the presence of acetic acid. However, as the deactivation observed is much more important than that observed in the presence of acetic acid, the deactivation may be explained also by a competitive adsorption, since it has been reported in the literature that ethylacetate is more strongly adsorbed on alumina than ethanol [START_REF] Wang | Catalytic steam reforming of biomass-derived oxygenates: Acetic acid and hydroxyacetaldehyde[END_REF] and that this stronger adsorption leads to an inhibition of ethanol oxidation.

The results obtained in the presence of various alcohols, linear or branched, are reported in Fig. 3a and Fig. 3b, respectively.

In the presence of methanol (Fig. 3a), the ethanol conversion is not modified but the hydrogen yield is slightly increased, compared to the reference test. Then, it can be inferred that this alcohol has no effect on ethanol steam reforming, probably because methanol is easily converted by steam reforming, thus producing also hydrogen. As far as the higher alcohols are concerned, one can see that the ethanol conversion and the hydrogen yield decrease when the amount of carbon atoms in the molecule is increased. This effect is more pronounced in the presence of branched alcohols compared to the linear ones. Once again, the deactivation of the catalyst in the presence of this type of impurity may be explained in terms of coke deposition: alcohols are dehydrated to the corresponding olefin, which is then polymerized to yield coke, the coke extent increasing with the amount of carbon atoms in the olefin. The more important deactivating effect of the branched alcohols may be explained by the formation of more stable carbocations thus facilitating the olefin production.

In conclusion, except methanol, diethylamine and butanal, all the impurities identified in raw bioethanol lead to the deactivation of the Rh/MgAl2O4 catalyst, mainly by coke deposition. The coke deposition has been confirmed by performing temperature programmed experiments on the deactivated catalyst. These experiments allowed us to quantify the amount of coke deposited onto the catalyst surface. Compared to the experiment performed in the presence of pure ethanol, the presence of impurities increased the amount of coke by a factor 2 to 3. Consequently, the catalyst formulation must be improved, by modifying the support and then metallic phase, in order to find a stable catalyst able to convert raw bioethanol by steam reforming.

Modification of the support

Rare earth element added to a support such as alumina are known to improve the catalytic performance in steam reforming and the stability of the catalysts, by decreasing its acidity, thus disfavoring the olefin formation, and by increasing the basicity, necessary to activate water. In order to improve the catalyst stability in the presence of raw bioethanol, various rare earth (RE) elements, namely RE = Sc, Y, La, Er and Gd, were added to the alumina support instead of Mg and the catalytic properties of the Rh/RE-Al2O3 catalysts were compared to those of a Rh/Al2O3 catalyst. The characterization of this catalyst series has been described in detail by Can et al. [START_REF] Can | New active and selective Rh-REOx-Al2O3 catalysts for ethanol steam reforming[END_REF].

The surface acidity was determined by infra-red (IR) spectroscopy of adsorbed pyridine. It was demonstrated that the concentration of strong and medium Lewis acid sites decreases as a function of the support as the following :

Al2O3 > La-Al2O3 > Er-Al2O3 > Gd-Al2O3 > Sc-Al2O3 > Y-Al2O3.
It is known that surface acidic sites favors the ethanol dehydration reaction leading to ethylene, which is coke precursor. Consequently, the lower the acidity, the higher stability of the catalysts. On the other hand, basic sites of the support favor the dehydrogenation route yielding acetaldehyde. The basicity of the supports was determined by carbon dioxide thermodesorption followed by IR spectroscopy. These results allowed us to establish a ranking of the support basicity as follows:

La-Al2O3 > Y-Al2O3 > Sc- Al2O3 > Er-Al2O3 > Gd-Al2O3> Al2O3.
The catalytic performances of the Rh/(RE)-Al2O3 catalysts for ethanol steam reforming were determined in the presence of pure ethanol, i.e. without impurities. Results, reported in Fig. 4, show that the addition of a rare-earth element onto alumina allows one to increase the hydrogen yield.

Moreover, in all cases, the conversion is near 100% and the hydrogen yield is much higher than that obtained with the reference Rh/MgAl2O4 catalyst.

As far as the hydrogen production is concerned, the following ranking can be suggested: Y-Al2O3> La-Al2O3 > Sc-Al2O3> Gd-Al2O3 > Er-Al2O3 = Al2O3. The highest hydrogen yield is obtained with the from these results that the presence of Pd or Ni decreases slightly the activity of the catalysts for the water-gas shift reaction but increases in a major extent the activity in methane steam reforming. As this reaction yields 3 molecules of hydrogen per molecule of methane converted, the highest hydrogen yield obtained with the Rh-Ni catalyst may be explained by its higher activity in methane steam reforming compared to the Rh catalysts.

Stability of the optimized catalyst in the presence of raw bioethanol

It has been shown that the reference catalyst, Rh/MgAl2O4, is deactivated in the presence of a lot of impurities as identified in raw bioethanol. Then, the catalyst formulation was improved and a Rh-Ni/Y-Al2O3 allowed us to increase the ethanol conversion and the hydrogen yield. These two catalysts were then tested in the raw bioethanol steam reforming. This figure clearly shows that the optimized catalyst, Rh-Ni/Y-Al2O3, is very stable, in spite of the severe reaction conditions chosen, since the conversion is only slightly decreased after 24 h of timeon-stream (97% of conversion compared to 100% at the beginning of the reaction). After 24 h of timeon-stream, the hydrogen yield is very high (3.49 mol/molethanol), slightly lower than the value at the thermodynamic equilibrium (3.78 mol/molethanol). On the contrary, the reference catalyst, Rh/MgAl2O4 is strongly deactivated, probably by coke deposition, in the same reaction conditions, especially during the first 2 h of reaction.

CONCLUSIONS

A reference catalyst, Rh/MgAl2O4 , was tested in the presence of various impurities as identified in raw bioethanol from sugar beet. Whereas the presence of diethylamine, butanal and methanol has no negative effect on the ethanol conversion after 8 h of reaction, the presence of the other impurities (esters, acids, higher alcohols, linear or branched) strongly deactivates the catalyst, probably by coke deposition. Consequently, the catalyst formulation was modified to improve the catalytic performances (activity and stability) in the presence of raw bioethanol. In order to disfavor the coke formation, the acidity of the support was decreased by addition of rare earth elements, the acidic sites being responsible for olefin formation at the origin of coke production. The metallic phase was also modified by adding a second metal favoring either the water gas shift or the methane steam reforming reaction.

It was shown that the addition of nickel leads to a higher hydrogen yield, probably due to the higher methane steam reforming activity of the resulting catalyst. Thus, the optimized Rh-Ni/Y-Al2O3 catalyst is very stable in the presence of raw bioethanol, allowing us to obtain a high ethanol conversion under the severe experimental conditions chosen, with a high hydrogen yield.

Fig. 7

 7 reports the ethanol conversion as a function of time-on-stream determined in the presence of the Rh/MgAl2O4 and Rh-Ni/Y-Al2O3 catalysts.

Fig. 1 .Fig. 2 .Fig. 3 :Fig. 4 :

 1234 Fig.1. Hydrogen yield and ethanol conversion with or without impurities (diethylamine or acetic acid) determined after 8h of time-on-stream in the presence of Rh/MgAl2O4.

Fig 5 :

 5 Fig 5 : Hydrogen yield and ethanol conversion in the presence of 2-methyl-propan-1ol as impurity, determined after 8h of time-on-stream in the presence of various metal phase (Rh, Rh-Ni, Rh-Pt, Rh-Pd) supported on the Y-Al2O3 support.

Fig. 6 :

 6 Fig. 6: Carbon monoxide and methane yields in the presence of 2-methyl-propan-1ol as impurity, determined after 8h of time-on-stream in the presence of various metal phase (Rh, Rh-Ni, Rh-Pt, Rh-Pd) supported on the Y-Al2O3 support.

Fig 7 :

 7 Fig 7: Ethanol conversion as a function of time-on-stream during the steam reforming of raw bioethanol in the presence of the Rh/MgAl2O4 and Rh-Ni/Y-Al2O3 catalysts.

Rh catalyst supported on Y-Al2O3, which is the less acidic support and one of the most basic.

Consequently, this support has been chosen for the following in order to improve the catalytic phase.

Modification of the metallic phase

In order to continue to improve the catalytic performance in terms of catalyst stability and hydrogen yield, the metallic phase was modified by adding a second metal, namely: Ni, Pt or Pd. As it was shown before that in the presence of the Rh/Y-Al2O3 catalyst, the conversion after 8h of time-onstream is slightly lower than 100% in the presence of pure ethanol, we have chosen to compare the catalytic performances of the Rh-Ni/Y-Al2O3, Rh-Pt/Y-Al2O3, Rh-Pd/Y-Al2O3 for ethanol steam reforming, in the presence of methyl-2-propanol, which is one of the most deactivating impurity identified in raw bioethanol. Results are reported in Fig. 5.

These results show that the presence of nickel or palladium allows one to increase the hydrogen yield, whereas platinum has no beneficial effect. Nevertheless, the promoting effect of Ni is more pronounced than that of palladium. The better performances of the Rh-Ni catalysts may be explained either by a better water-gas shift activity (eq.7) or by a better methane steam reforming activity (eq.8), both reactions leading to the formation of hydrogen according to: CO + H2O ↔ CO2 + H2 [START_REF] Sun | from steam reforming of ethanol at low temperature over Ni/Y2O3, Ni/La2O3 and Ni/Al2O3 catalysts for fuel-cell application[END_REF] CH4 + H2O  CO + 3H2

In Fig. 6 are reported the yield in carbon monoxide and in methane obtained after 8 h of time-onstream.

Fig. 6 shows that the yield in CO is slightly increased in the presence of Rh-Pd and Rh-Ni catalysts compared to that obtained in the presence of Rh (from 0.7 to 0.79 and 0.75 respectively), but the methane yield is much more decreased (from 0.55 to 0.49 and 0.46 respectively). It can be inferred