10th International Symposium “Scientific Bases for the Preparation of Heterogeneous Catalysts” E.M. Gaigneaux, M. Devillers, S. Hermans, P. Jacobs, J. Martens and P. Ruiz (Editors) © 2010 Elsevier B.V. All rights reserved.

Catalytic wet air oxidation of succinic acid over monometallic and bimetallic gold based catalysts: Influence of the preparation method Radka Nedyalkova, Michèle Besson and Claude Descorme* Institut de recherches sur la catalyse et l’environnement de Lyon (IRCELYON), UMR 5256 CNRS – Université de Lyon, 2 avenue A. Einstein, 69626 Villeurbanne, France *E-mail : claude.descorme@ircelyon.univ-lyon1.fr

Abstract Different methods for the preparation of gold catalysts (mono and bimetallic) were used – the modified deposition-precipitation (MDP), the deposition-precipitation by ammonia (DPA) and the colloidal method (CM). The catalytic performances of all samples were evaluated in the catalytic wet air oxidation (CWAO) of succinic acid under mild conditions (190°C, 50 bar total pressure). The results showed that the preparation procedure and the addition of a second metal (Pt or Ru) clearly influence the catalytic activity and selectivity, depending on the size of the gold particles and the nature of the second metal. Keywords: Au-Pt(Ru) bimetallic catalysts, catalytic wet air oxidation (CWAO), organic compounds

1. Introduction Gold has long been disregarded for catalytic applications, due to its inert nature in the bulk state. Since Haruta’s [1,2] discovery of the remarkable activity of supported gold nanoparticles in oxidation reactions, different methods to prepare highly active gold catalysts have been developed. The same group has developed/adapted four different techniques that allow the deposition of gold nanoparticles on certain metal oxides: the co-precipitation (CP), the co-sputtering, the deposition-precipitation (DP) and the gas-phase grafting [3]. Between all techniques developed so far, it appears that the deposition-precipitation is the most successful for preparing highly dispersed Au catalysts. The DP method has numerous variations, depending on the pH, the temperature of deposition, the precipitation agent and the state of the support (oxide or hydroxide). However, the DP method still cannot completely avoid the adsorption of gold hydroxyl chlorides species onto the support or the wrapping of chlorides in the precipitate, which may cause the deactivation of the Au catalysts. Grunwaldt et al. [4] first developed a two-stage method, based on Au colloids, for the preparation of Au/TiO2 and Au/ZrO2 catalysts employing tetrakis(hydroxymethyl)-phosphonium chloride (THPC) as the reducing and capping agent. Later, Porta et al. [5] employed poly(vinylalcohol) (PVA) as the protective agent and prepared Au/C and Au/TiO2 catalysts. The simplicity of the colloidal method and the possibility to prepare chlorine free catalysts are the main advantages, motivating his large application not only for the preparation of mono but also bimetallic catalysts. The interest towards bimetallic heterogeneous catalysts is increasing since the presence of a second metal can influence the catalytic properties, improving their activity, stability and/or selectivity. The presence of one less reducible component, which strongly interacts with the support,

178 R. Nedyalkova et al.

may stabilise the second, more noble metal, in the highly dispersed state. Recently, Pd and Pt were used for the preparation of colloidal bimetallic gold catalysts and apply in different oxidation reactions [6]. Furthermore, the catalytic properties of metal-oxide-supported gold catalysts strongly depend on the nature, the texture and the structure of the support. The support must present a defective surface available for strong interactions with the gold precursor. It is well known that reducible metal oxide supports (TiO2, Fe2O3, Co3O4), supplying reactive oxygen to the active gold sites, are more active in oxidation reactions than non reducible supports. Ceria has been regarded as one of the most important component in many catalytic systems due to its remarkable redox properties [7]. Recently, it has been shown that gold catalysts supported on ceria exhibit higher activity in the succinic acid wet air oxidation than Au/TiO2 [8]. Evidence was provided that the activity depended on the gold particle size.

From the environmental point of view, the removal of toxic organic compounds from aqueous wastewaters is drawing a lot of attention and the wet air oxidation (WAO) is a suitable technology for that. The main disadvantage is that WAO requires high temperature and pressure (200-350°C, 70-230 bar), conditions that severely affect the economics of this technology. Using a catalyst, the operating conditions can be made significantly milder (120-220°C, 5-50bar). Since heterogeneous catalysts might easily be removed, their development and optimization has been the subject of several works in the recent decades. For the first time, Besson et al. [9] have reported that the Au/TiO2 catalyst is a promising candidate in the CWAO of succinic acid. The main disadvantage is that gold catalysts are not stable upon long term reactions and recycling. The important challenge is then to get stable gold catalysts in the CWAO. Deactivation may occur by sintering, poisoning of the active sites or “fouling” of the catalyst surface by adsorption of intermediate reaction products. Also, in hot acidic environments, the active components might be dissolved into the liquid phase (leaching).

In order to reduce leaching and prevent the gold particles from sintering, the active phase might be incorporated into a catalyst support. On the other hand, the presence of a second metal may induce significant changes in both activity and stability. In our study, Pt and Ru were chosen as the second metal. Both concepts are the basis of the present study aiming to achieve an active and stable gold catalyst in the CWAO of succinic acid.

2. Experimental

2.1. Catalyst preparation A total of six gold catalysts were prepared by different methods. The total metal loading was fixed at 3wt.%. Two monometallic gold on ceria catalysts were prepared by the modified deposition precipitation method (MDP) and the deposition-precipitation by ammonia (DPA). First of all, the Ce(NO3)3.6H2O aqueous solution was precipitated with K2CO3 at 60°C and pH=9. In the case of the MDP, just after aging of the Ce(OH)4 precipitate for 1h at 45°C, the deposition of HAuCl4 was performed at pH=7. The resulting precipitates were aged for 1 h at 45°C, filtered and washed until no Cl - and NO3

- could be detected. For the DPA method, the as prepared Ce(OH)4 precipitate was carefully washed to eliminate the K+ and NO3

- ions and dried at 80°C. Noteworthy, to achieve a 3wt.% metal loading, a test was performed to determine the weight lost upon the transformation from Ce(OH)4 to CeO2 at 400°C for 2h. After that, the deposition of gold was performed as follows: the support was suspended in deionizer water, the gold precursor (HAuCl4.3H2O 6 10-4 M) was added and pH was adjusted to 11 with ammonia and maintained for 1 h. After washing and drying at 80°C under vacuum, the solid was

Catalytic wet oxidation of succinic acid 179

calcined under flowing air (6 L h-1) at 400°C for 2h. Three bimetallic gold catalysts were synthesized using the MDP method: 2wt.%Au-1wt. %Pt/CeO2 MDP I, 2wt.%Au-1wt.%Pt/CeO2 MDP II and 2wt.%Au-1wt.%Ru/CeO2 MDP II. I indicates that the two salts (HAuCl4 and Pt(NH3)4(NO3)2) were introduced simultaneously, while II indicates that Pt or Ru, respectively, was introduced first. As a precursor ruthenium nitrosil nitrate was used. After aging for 1h at 45°C, the received precipitate was washed carefully, until no Cl- and NO3

- could be detected, and dried at 80°C under vacuum. The solids were calcined at 400°C for 2h under flowing air (6 L h-1). One bimetallic catalyst noted 2wt.%Au-1wt.%Ru/CeO2 CM was prepared via the colloidal method as follows: the gold precursor was dissolve in 400 mL deionised water in the presence of polyvinyl alcohol (PVA 2wt.% solution) under vigorously stirring. After that, the ruthenium precursor was added and the slurry was kept under stirring for 3 min. Then, the 0,1M NaBH4 solution, freshly prepared, was added to the solution to obtain a colloidal sol. Once the sol was obtained, the immobilisation on the Ce(OH)4 support was carried out for 2h. The resulting solid was centrifuged, carefully washed, dried at 80°C under vacuum and finally calcined at 400°C for 2h under flowing air. For the bimetallic catalysts, the thermal treatment is an important step, not only as far as the transformation of Ce(OH)4 into CeO2 is concerned but considering the interaction between the metal particles and the support. Before reaction all samples were reduced at 300°C for 2h under flowing H2 (12L h-1).

2.2. Characterization of the catalysts XRD patterns were obtained on a Siemens D5005 diffractometer (Cu Kα, 0.15406 nm). The metal concentration in the liquid phase after 8 h reaction was repeatedly measured by ICP-OES. The metal concentration in the solution was systematically lower than 0.5 ppm (detection limit), indicating that no leaching occurred.

2.3. Catalytic activity Experiments were carried out in a 300 mL autoclave made of Hastelloy C22 (model 4836, Parr Instrument Inc.). In a typical run, the autoclave was loaded with 150 mL succinic acid aqueous solution (5 g L-1, i.e. initial total organic carbon (TOC) = 2032 mg L-1) and 0.5 g catalyst. After the reactor was out gassed under argon, the mixture was heated to the reaction temperature (190°C) under stirring. Then, the stirrer was stopped and air was admitted into the reactor until the predefined pressure was reached (50 bar total). The reaction finally started when the stirrer was switched on again. This point was taken as ‘‘zero time’’ and one sample was withdrawn to measure the exact initial concentration of succinic acid. The liquid samples were periodically withdrawn from the reactor, centrifuged to remove any catalyst particle in the liquid sample and further analyzed. The substrate and the reaction intermediates (acetic acid and acrylic acid) were analyzed by HPLC (Shimadzu) using an ICSep Coregel 107H column. The mobile phase was a 0.005N H2SO4 aqueous solution (0.5 mL min-1). The HPLC system was equipped with a UV–vis detector set at 210 nm.

The TOC in the liquid samples was measured with a Shimadzu 5050 TOC analyzer after subtraction of the inorganic carbon (IC) contribution from the total carbon (TC). Furthermore, the carbon mass balance in the liquid phase could be checked by comparing the TOC values with the total carbon concentrations in the liquid phase derived from the HPLC analysis.

3. Results and discussion Table 1 summarizes the metal loadings as measured by ICP. These results show a good agreement between the theoretical and experimental values for Au and Ru, especially

180 R. Nedyalkova et al.

for the catalysts prepared by DPA and CM, keeping in mind that the support was in the form of Ce(OH)4 and transformed into the oxide form during the calcination in air at 400°C. In the case of Pt, the experimental values are almost twice lower than the expected values. This unusually low platinum loading could indeed be connected with the nature of the precursor, whom precipitation at pH 7 and 45°C was not complete. Andreeva et al. [10] have applied the same method for the preparation of 3wt.%Au/CeO2 catalysts, but at higher temperature (60°C). They found that Ce3+ ion act as a reducing agent, converting Au3+ to Au0 during the preparation. In turn, Ce3+ ions are oxidized to Ce4+. Using XRD [10], the average gold particle size was estimated to be about 15 nm. In our study we decreased the temperature to 45°C in order to slow down the reduction process and decrease the gold particle size. At such low temperature and pH 7, the gold loading decreased slightly and part of the Pt was again lost upon washing since the Pt complex was not fully hydrolyzed. However, lowering the precipitation temperature, the average gold particle size decreased to 8 nm (Table 1).

Table 1. Catalysts chemical composition measured by ICP-OES and average gold particle size derived from XRD measurements.

Samples Au , wt%

Pt, wt% Ru, wt% DAu a, nm

3wt.%Au/CeO2 MDP 2.7 - - 8.0

3wt.%Au/CeO2 DPA 2.9 - - 6.0

2wt.%Au-1wt.%Pt/CeO2 MDP I 1.74 0.45 - 25.0

2wt.%Au-1wt.%Pt/CeO2 MDP II 1.78 0.40 - 10.0

2wt.%Au-1wt.%Ru/CeO2 MDP II 1.82 - 0.85 10.0

2wt.%Au-1wt.%Ru/CeO2 CM 2.0 - 0.8 6.0 a derived from XRD diffractograms using Debye-Scherrer equation

Figure 1 compares the XRD patterns of mono and bimetallic catalysts. For all samples, the diffraction lines for CeO2 are typical of the cubic structure of fluorite type oxides. In the case of the monometallic catalysts, the main line characteristic for Au (2θ=38.2°) is more intense for the catalyst prepared by MDP than DPA, indicating that gold was better dispersed on the catalyst synthesized by DPA. In the case of bimetallic catalysts, although the diffraction lines characteristic for Au, Pt and Ru are very close in position, we could clearly see the difference between the MDP I and MDP II preparation routes. Bimetallics prepared by mixing the two salts led to catalysts with a lower dispersion (DAu=25 nm). On the opposite, when the salts were precipitated one after the other, the gold particles dispersion was improved (DAu=10 nm). Finally, gold particles of approximately 6 nm were obtained by the colloidal method, probably because of the presence of the protecting agent (PVA) that could stabilized the gold colloids at a higher dispersion state. It is noteworthy that, as far as the amount of Ru and Pt was 1wt.% and 0.5wt.%, respectively, that is below the detection limit for XRD, the dispersion of the second metal could not accurately be estimated from the XRD patterns.

Catalytic wet oxidation of succinic acid 181

20 30 40 50 60 70 80

A

Au

CeO

2

CeO

2 CeO

2

CeO

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nsity

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u.]

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Pt Au

Au

CeO

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CeO

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CeO

2

CeO

2

4

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Inte

nsity

, [a.

u.]

2θ, deg Figure 1. XRD patterns for: A – (1) - pure support, (2) - 3Au/CeO2 MDP, (3) - 3Au/CeO2 DPA; B - (1) - 2Au-1Pt/CeO2 MDP I, (2) - 2Au-1Pt/CeO2 MDP II, (3) – 2Au-1Ru/CeO2 MDP II, (4) – 2Au-1Ru/CeO2 CM.

The catalytic performances of the monometallic gold catalysts in the catalytic wet air oxidation of succinic acid are presented on Fig. 2 and on Fig. 3 for the bimetallic Au-Pt and Au-Ru catalysts. As a preliminary test, a blank was performed to confirm that succinic acid is stable under the applied reaction conditions in the absence of catalyst. Furthermore, it is known that succinic acid might be intermediately degraded to acetic and acrylic acids or directly mineralized into CO2 and H2O [11]. In the presence of the ceria support 100% succinic acid conversion was achieved after 6h. Acrylic acid concentration was systematically very low and was not reported in the figures.

0 1 2 3 4 5 6 7 80

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without catalyst CeO2 3Au/CeO2 MDP 3Au/CeO2 DPA

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aci

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20

30

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cetic

aci

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mol

L-1

Time, h

Figure 2. Evolution of the succinic and acetic acid concentrations as a function of time upon the CWAO of succinic acid over pure ceria and monometallic catalysts.

For the monometallic Au catalysts, 100% conversion was reached after only 3h for the catalyst prepared by DPA and 4h for the corresponding one prepared by MDP.

182 R. Nedyalkova et al.

0 1 2 3 4 5 6 7 80

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without catalyst 2Au-1Pt/CeO2 MDP I 2Au-1Pt/CeO2 MDP II 2Au-1Ru/CeO2 MDP 2Au-1Ru/CeO2 CM

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Ace

tic a

cid,

mm

ol L

-1

Time, h Figure 3. Evolution of the succinic and acetic acid concentrations as a function of time upon the CWAO of succinic acid over bimetallic catalysts.

The acetic acid concentration reached 26.8 and 29.0 mmol L-1 in the case of the DPA and MDP catalysts, respectively. In the case of the bimetallic catalysts, the highest activity was observed for the 2wt.%Au-1wt.%Ru/CeO2 MDP II catalyst.

0.0 0.2 0.4 0.6 0.8 1.00.0

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CeO2

acetic acid acrylic acid mineralization

Overall conversion

Yie

ld to

Yie

ld to

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acetic acid acrylic acid mineralization

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0.4

0.6

0.8

1.0

3Au/CeO2DPA

acetic acid acrylic acid mineralization

Figure 4. Distribution of the reaction products as a function of the overall conversion.

Comparing the curves for the reaction product distribution as a function of the overall conversion (Fig. 4) for the ceria support and the monometallic gold catalysts, the direct mineralization pathway becomes predominant in the case of the catalysts. The faster direct mineralization for the DPA catalyst is certainly to be connected with the higher dispersion of the gold particles. These results are in good agreement with the results reported by Bond et al. [12] who showed that the catalytic activity rapidly increases as the gold particle size decreases. Another phenomenon concerns the reaction mechanism: acrylic acid was not formed as an intermediate product in the presence of highly dispersed gold particles. For the bimetallic catalysts, the reaction pathway

Catalytic wet oxidation of succinic acid 183

depends of a nature of the second metal (Fig. 5). In the presence of Pt, no acrylic acid was detected, while for Ru the production of acrylic acid was much more significant.

0.0 0.2 0.4 0.6 0.8 1.00.0

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Overall conversionOverall conversion

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acetic acid acrylic acid mineralization

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2Au-1Pt/CeO2 MDP I

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acetic acid acrylic acid mineralization

The most active 2wt.%Au-1wt.%Ru/CeO2 MDP II catalyst was then submitted to a stability test (Fig. 6). Noteworthy, as the experiments were carried out in a batch reactor, the stability was studied by recycling the catalyst. For that reason, the first run was repeated three times in order to recover enough catalyst to perform a second run using the same amount of catalyst (0.5g). After every run, the catalyst was washed with cold water and dried overnight in air at 80°C. The results obtained upon three independent runs performed on the fresh catalyst showed a perfect reproducibility. Furthermore, deactivation was observed upon recycling. The total organic carbon (TOC) at the end of the 8 h run increased from 125 to 450 ppm. A similar deactivation was observed by Besson et al. [9] on a 2.2wt.%Au/TiO2 catalyst prepared by DP using NaOH and tested under the same reaction conditions. To get a better idea about the possible reasons for this deactivation, the catalyst was reduced again at 300°C for 2h under flowing H2 (12 L h-1) in between the two runs. The results showed that deactivation is partially reversible and might be connected with the metal particle surface re-oxidation. In that case, the TOC in the liquid phase after 8 h reaction reached 280 ppm. No Au or Ru leaching could be detected (< 0.1 ppm).

Figure 5. Distribution of the reaction products as a function of the overall conversion.

184 R. Nedyalkova et al.

0 1 2 3 4 5 6 7 80

500

1000

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2500 only dried after direct reduced after direct direct direct direct

CO

T, m

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-1

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1

2

CeO

2CeO

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CeO

2

CeO

2

Au

Inte

nsity

, [a.

u.]

2θ, deg

Figure 6. Stability test of the 2Au-1Ru/CeO2 MDP II catalyst in the CWAO of succinic acid. Full symbols : first run (three independent tests); dried only; re-reduced

Figure 7. Comparison of the XRD patterns of the fresh (1) and re-reduced (2) 2wt.%Au-1wt.%Ru/CeO2 MDP II catalyst

In Fig. 7 the XRD patterns of the fresh and used catalysts are compared. A slight increase in the gold particle size might be evidenced. As a conclusion, gold sintering is mainly responsible for the observed deactivation. This phenomenon is certainly related to the intrinsic instability of gold at elevated temperature which might somehow be related, in combination with particle size effects, to the lower melting point of bulk gold compared to other noble metals.

4. Conclusions The CWAO of succinic acid under mild reaction conditions over monometallic and bimetallic gold catalysts strongly depends on the applied preparation method. High dispersion of gold resulted in higher performances. The presence of a second metal has a beneficial effect on the catalytic activity and stability. The reaction product distribution was also affected by the nature of the second metal.

Acknowledgments We gratefully acknowledge the financial support from the Agence Nationale de la Recherche (Project ANR Blanc CatOxOr).

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