Journal of Catalysis 292 (2012) 73–80

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Journal of Catalysis

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Bismuth as a modifier of Au–Pd catalyst: Enhancing selectivity in alcoholoxidation by suppressing parallel reaction

Alberto Villa a, Di Wang b, Gabriel M. Veith c, Laura Prati a,⇑a Università di Milano, Dipartimento CIMA ‘‘L.Malatesta’’, via Venezian 21, I-20133 Milano, Italyb Institut für Nanotechnologie, Karlsruher Institut für Technologie, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germanyc Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States

a r t i c l e i n f o

Article history:Received 15 February 2012Revised 24 April 2012Accepted 28 April 2012Available online 9 June 2012

Keywords:Bi-modified catalystAuPd alloySelective alcohol oxidationTartronic acid

0021-9517/$ - see front matter � 2012 Elsevier Inc. Ahttp://dx.doi.org/10.1016/j.jcat.2012.04.021

⇑ Corresponding author.E-mail address: Laura.Prati@unimi.it (L. Prati).

a b s t r a c t

Bi has been widely employed as a modifier for Pd and Pt based catalyst mainly in order to improve selec-tivity. We found that when Bi was added to the bimetallic system AuPd, the effect on activity in alcoholoxidation mainly depends on the amount of Bi regardless its position, being negligible when Bi was0.1 wt% and detectably negative when the amount was increased to 3 wt%. However, the selectivity ofthe reactions notably varied only when Bi was deposited on the surface of metal nanoparticles suppress-ing parallel reaction in both benzyl alcohol and glycerol oxidation. After a careful characterization of allthe catalysts and additional catalytic tests, we concluded that the Bi influence on the activity of the cat-alysts could be ascribed to electronic effect whereas the one on selectivity mainly to a geometric modi-fication. Moreover, the Bi-modified AuPd/AC catalyst showed possible application in the production oftartronic acid, a useful intermediate, from glycerol.

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1. Introduction

The liquid phase oxidation of alcohols to the corresponding car-bonyl compounds over supported metal catalysts has been exten-sively studied in the last decade [1,2] especially involving the useof molecular oxygen as the oxidant because of its attractive‘‘green’’ technology, with water as main co-product. Research hashistorically focused on the use of Pt and Pd catalysts [3–5]; how-ever, a drawback in using these catalysts is the deactivation dueto metal overoxidation and irreversible adsorption of (by)productsor intermediates [6]. On the contrary, gold has been demonstratedto be an active and long-life catalyst for oxidizing alcohol in thepresence of O2 [7–9]. However, gold as the catalyst presentsthe limitation of the often compulsory presence of a base [7–9].The base enhances the rate of the reaction but also drastically af-fects the selectivity, mainly producing carboxylate instead of thedesired aldehyde. This problem was partly over passed by usingbimetallic systems based on Au and Pd or Pt metals [10–18].

Pd or Pt based catalysts have also been modified by metal pro-moters, such as Bi and Pb [19,20]. When used alone Bi and Pb areinactive in the alcohol oxidation, but when associated with noblemetal, they increase the catalytic performance [21] also modifyingthe selectivity [21,23]. Despite the large number of studies carried

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out on this subject, the nature of the promoter role is still underdebate. Proposed models include the formation of a complexamong the noble metal, the promoter and the reactant [24–26];a bifunctional catalyst [27,28]; an alloy formation [29–31], oreven a homogeneous catalysis by leached promoter [32]. Themost accepted model involves the blocking of a fraction of activesites, thus providing different substrate coordination route that inturn modified the activity and the selectivity of the original cata-lyst [33].

Different preparation routes were developed aiming the bis-muth atoms being on the metal and not on the support[28,34,35]. Indeed, it is generally accepted that bismuth directlydeposited on the noble metal is mostly effective, whereas bis-muth atoms on the support are only ‘‘spectator species’’ [35].We already reported the preparation of completely alloyed Au–Pd on carbon catalyst and ascribed its high catalytic performancesin the liquid phase oxidation of alcohol to the isolation of Pd sin-gle sites [13,20]. In the present paper, we investigated the effectof Bi addition to the fully characterized, well alloyed Au–Pd bime-tallic catalyst in the oxidation of benzyl alcohol and glycerol inorder to study the activity and the regio- as well as chemo-selec-tivity of these modified catalysts. We added different amount ofBi (0.1–3%) using two different procedures, the first for assuringthe deposition of Bi on Au–Pd nanoparticles, the second for thedirect deposition on the support. This latter methodology allowedus also to study the effect of Bi on the formation of the alloyedphase.

Table 1Statistical median and standard deviation of particle size analysis for Au–Pd–Bicatalysts.

Catalyst Statistical median (n) Standard deviations

1% Au–Pd 3.4 1.41% Au–Pd@ 0.1% Bi 3.5 1.41% Au–Pd@ 3% Bi 3.6 1.40.1%Bi@ 1% Au–Pd 3.4 1.43%Bi@ 1% Au–Pd 3.4 1.4

74 A. Villa et al. / Journal of Catalysis 292 (2012) 73–80

2. Experimental

2.1. Materials

NaAuCl4�2H2O, Na2PdCl4, and BiO(NO3) were from Aldrich(99.99% purity) and activated carbon from Camel (X40S;SA = 900–1100 m2/g; PV = 1.5 ml/g; pH 9–10). Before use, the car-bon was suspended in HCl 6 M and left under stirring for 12 h andthen washed several times with distilled water by decantation untilthe pH of the solution reached values of 6–6.5. At the end, the carbonwas filtered off and dried for 5–6 h at 150 �C in air. The final watercontent was evaluated to be <3%. NaBH4 of purity >96% from Fluka,polyvinylalcohol (PVA) (Mw = 13,000–23,000 87–89% hydrolysed)from Aldrich was used. Gaseous oxygen from SIAD was 99.99% pure.

2.2. Catalyst preparation

2.2.1. Au–Pd bimetallic catalystsNaAuCl4�2H2O (0.072 mmol) was dissolved in 140 ml of H2O, and

PVA (2% w/w) was added (0.706 ml). The yellow solution was stirredfor 3 min, and 0.1 M NaBH4 (2.15 ml) was added under vigorousmagnetic stirring. The ruby red Au(0) sol was immediately formed.An UV–visible spectrum of the gold sol was recorded to check thecomplete AuCl�4 reduction and the formation of plasmon peak.Within a few minutes of sol generation, the gold sol (acidified untilpH 2 by sulfuric acid) was immobilized by adding activated carbonunder vigorous stirring. The amount of support was calculated ashaving a final metal loading of 0.73 wt% when Au/Pd was prepared.After 2 h, the slurry was filtered and the catalyst washed thoroughlywith distilled water (neutral mother liquors). ICP analyses were per-formed on the filtrate using a Jobin Yvon JV24 to verify the total me-tal loading on carbon. The Au/C was then dispersed in 140 ml ofwater; Na2PdCl4 (10 wt% in Pd solution) (0.0386 ml) and PVA (2%w/w) (0.225 ml) were added. H2 was bubbled (50 ml/min) underatmospheric pressure and room temperature for 2 h. After addi-tional 18 h, the slurry was filtered and the catalyst washed thor-oughly with distilled water. ICP analyses were performed on thefiltrate using a Jobin Yvon JV24 to verify the metal loading on car-bon. 1 wt% has been the total metal loading.

2.2.2. Au–Pd–Bi trimetallic catalysts2.2.2.1. Procedure A. Catalysts were prepared following the proce-dure reported for Au–Pd bimetallic catalyst, except that activatedcarbon (Camel X40S; SA = 1100–1200 m2 g�1; pH 8–9) that wasloaded by different amount of Bi (0.1–3 wt%) priori. To acidicH2O (pH 1 H2SO4), a 10�2 M stock solution of BiO(NO3) and PEG–BDE (polyethyleneglycol reacted with bisphenol diglycidyl ether)(2% w/w stock solution) were added, under nitrogen and vigorousmagnetic stirring. After 3 min, an alkaline NaBH4 solution (14 M)was added obtaining a brown sol. An UV–visible spectrum of theBi sol was recorded, checking for complete reduction. After1 min, 1 g of carbon was added as having the proper Bi loading(0.1; and 3 wt%) and it was maintained under vigorous stirringfor 0.5 h. The slurry was then filtered, and the catalyst washedthoroughly with distilled water (neutral mother liquors). The twocatalysts have been labeled as Au–Pd@0.1%Bi and Au–Pd@3%Bi.

2.2.2.2. Procedure B. In this case, the Bi nanoparticles, prepared asin A, have been added after the immobilization of AuPd alloy.The two catalysts have been labeled as 0.1%Bi@Au–Pd and3%Bi@Au–Pd.

2.3. Catalytic tests

Reactions were carried out in a 30 ml glass reactor equippedwith a thermostat and an electronically controlled magnetic stirrer

connected to a 5000 ml reservoir charged with oxygen (300 kPa).The oxygen uptake was followed by a mass-flow controller con-nected to a PC through an A/D board, plotting a flow time diagram.

Glycerol oxidation: glycerol 0.3 M and the catalyst (substrate/total metal = 1000 mol/mol) were mixed in distilled water (totalvolume 10 ml) and 4 eq of NaOH. The reactor was pressurized at300 kPa of oxygen and set to 50 �C. Once this temperature wasreached, the gas supply was switched to oxygen and the monitor-ing of the reaction started. The reaction was initiated by stirring.Samples were removed periodically and analyzed by high-performance liquid chromatography (HPLC) using a column(Alltech OA- 10,308, 300 mm_7.8 mm) with UV and refractive in-dex (RI) detection to analyze the mixture of the samples. AqueousH3PO4 solution (0.1 wt%) was used as the eluent. Products wereidentified by comparison with the original samples.

Benzyl alcohol oxidation: benzyl alcohol and the catalyst (alco-hol/total metal = 5000 mol/mol) were mixed in cyclohexane (ben-zyl alcohol/cyclohexane 50/50 vol/vol; total volume, 10 ml). Thereactor was pressurized at 200 kPa of oxygen and set to 80 �C.The reaction was initiated by stirring. Periodic removal of samplesfrom the reactor was performed. Identification and analysis of theproducts were done by comparison with the authentic samples byGC using a HP 7820A gas chromatograph equipped with a capillarycolumn (HP-5 30 m � 0.32 mm, 0.25 lm Film, made by AgilentTechnologies) and TCD detector. Quantification of the reactionproducts was done by the external calibration method.

2.4. Characterization

2.4.1. Catalyst characterization

(a) The metal content was checked by ICP analysis of the filtrateor alternatively directly on catalyst after burning off the car-bon, on a Jobin Yvon JY24.

(b) Morphology and microstructures of the catalysts are charac-terized by transmission electron microscopy (TEM). Thepowder samples of the catalysts were ultrasonically dis-persed in ethanol and mounted onto copper grids coveredwith holey carbon film. A Philips CM200 FEG electron micro-scope, operating at 200 kV and equipped with EDX DX4 ana-lyzer system and a FEI Titan 80–300 electron microscope,operating at 300 kV and equipped with EDX SUTW detectorwere used for TEM observation.

(c) XPS – X-ray photoelectron spectroscopy (XPS) measurementswere performed with a PHI 3056 spectrometer equipped withan Al anode source operated at 15 kV and an applied power of350 W and a pass energy of 5.85 eV. Samples were mountedon In foil since the C1s binding energy, from the carbon inthe samples, was used to calibrate the binding energy shiftsof the sample (C1s = 284.8 eV).

3. Results and discussion

The two step procedure for preparing Au–Pd on carbon catalystwas demonstrated to produce AuPd alloyed NPs of single composi-

Fig. 1. Overview TEM image of 1% Pd–Au@0.1% Bi on AC.

A. Villa et al. / Journal of Catalysis 292 (2012) 73–80 75

tion with multiply twinned structure highly dispersed on activatedcarbon [13]. In the liquid phase oxidation of alcohols, this catalysthighlighted an extraordinary synergistic effect between gold and

Fig. 2. The overview TEM image of 4Pd@6Au/3% Bi on AC from (a) the area with small pcorresponding to (a and c).

palladium [14]. It was assumed that the synergistic effect can beconsidered as a sum of two different effects: the geometric andthe electronic one, the geometric deriving from the isolation ofPd active site by Au [13,22,23].

We proceeded to modify this catalyst by the addition of con-trolled amounts of Bi (0.1–3 wt%). On the basis of the literature[36,37], it was expected that by depositing Bi on the preformedAu–Pd/AC, a selective deposition of Bi on the metallic NPs can be ob-tained. On the contrary, by previously impregnating the support, arandom dispersed Bi NPs could be expected. A perturbation in theAuPd alloy formation could be also envisaged in this latter case. Acomplete characterization of the catalysts by TEM and ICP analyseshas been then performed. The preparation and the detailed charac-terization of the Au–Pd/AC homogenously alloyed single phasecatalyst have been already reported somewhere else [13]. The syn-thesized single alloyed Au6Pd4 particles have a median particle sizeof 3.4 nm (Table 1). By replacing pristine activated carbon with 0.1%Bi on activate carbon (0.1 wt%Bi/AC), the resulted tri-metallic cata-lyst shows a very similar structure as the Au6Pd4/AC. An overviewTEM micrograph is shown in Fig. 1. The metal particles are homoge-neously distributed on the activated carbon, and the size histogramcan be fitted by LogNormal function with median value of 3.53 nm(Table 1). The 0.1 wt% Bi could not be detected by EDX, but the ICPanalysis carried out by burning off the carbon revealed its presence.Despite the Bi concentration on each particle was too low to be de-tected by EDX, we could also assume that the dispersion is goodsince no segregation of Bi or Bi-rich alloy particles have been ob-served. By increasing the Bi loading on AC to 3 wt%, the characteris-

articles and (c) the area with big and irregular particles. (b and d) The EDX spectra

76 A. Villa et al. / Journal of Catalysis 292 (2012) 73–80

tics of the catalyst changed while maintaining a similar particle size,(median value of the size 3.58 nm) (Table 1). In this case, inhomoge-neous composition is observed in different regions. Fig. 2a and c arethe TEM micrographs from two randomly selected regions of the 3%Bi catalyst. From the corresponding EDX spectra and the quantita-tive data shown in Fig. 2b and d, it can be seen that the ratios amongAu, Pd, and Bi vary largely even without obvious change of the par-ticle morphology. Therefore, we concluded that Bi/AC (especiallywhen 3 wt% loaded) produces an inhomogeneous catalyst, differing

Fig. 3. The overview STEM image of (a) 4Pd@6Au/0.1% Bi on AC and (b) 4Pd@6Au/3% Bi o3% Bi on AC with the integrated EDX spectrum from the mapping area.

from AC alone, which allows the uniform Au–Pd alloy formation. Onthe contrary when Bi was added after the immobilization of AuPdNPs, the alloyed structure of AuPd is maintained at any loading ofBi (0.1% or 3%). Fig. 3a shows the overview STEM image of0.1%Bi@Au–Pd/AC, where EDX spectrum (Fig. 3b) was acquired fromthe area within the red box and Bi signal was only at noise level butthe ICP analysis carried out by burning off the carbon revealed itspresence. In the case of 3% Bismuth, different Bi forms coexisted,the Bi alloyed with Au–Pd and the segregated Bi. A typical overview

n AC; (c) the EDX spectra corresponding to (a and d) element mapping on 4Pd@6Au/

Fig. 4. Bi segregation in 3%Bi/AuPd/AC.

Fig. 5. Benzyl alcohol oxidation with 1% Au–Pd and 1% Au–Pd + 0.1%Bi.

A. Villa et al. / Journal of Catalysis 292 (2012) 73–80 77

STEM image of trimetallic particles supported on carbon is shown inFig. 3c. Fig. 3d is the STEM–EDX spectrum imaging of some particles,where each particle consists of all the three metals. The integratedspectrum from the whole mapping area and the quantification re-sult are also shown in Fig. 3d. The Bi concentration is much lower

Table 2Comparison of bi and tri-metallic catalyst activities in benzyl alcohol oxidation.

Catalysta TOF (h�1)b Selectivity (%)c

Toluene

1% Au–Pd 2261 81% Au–Pd@ 0.1% Bi 2278 81% Au–Pd@ 3% Bi 1245 90.1%Bi@ 1% Au–Pd 2546 23%Bi@ 1% Au–Pd 1643 1

a Reaction condition: alcohol/metal 5000/1 (mol/mol), 80 �C, pO2 2 atm, 1250 rpm.b TOF calculated after 15 min of reaction.c Selectivity at 90% conversion.

Table 3Comparison of bi and tri-metallic catalyst activities in glycerol oxidation.

Catalysta TOF (h�1)b Selectivity (%)c

Glycerate

1% Au–Pd 3503 761% Au–Pd@ 0.1% Bi 3538 741% Au–Pd@ 3% Bi 1856 730.1%Bi@ 1% Au–Pd 3897 753%Bi@ 1% Au–Pd 2856 74

a Reaction condition: alcohol/metal 1000/1 (mol/mol), 50 �C, pO2 3 atm, 1250 rpm.b TOF calculated after 15 min of reaction.c Selectivity at 90% conversion.

than the initial feed, implying Bi segregation. The segregated Biwas detected in some areas, as the example shown in Fig. 4 withthe STEM image and the corresponding EDX spectrum.

The catalytic performances of Au–Pd and Au–Pd–Bi catalystshave been evaluated in the oxidative dehydrogenation of benzylalcohol (Table 2) and glycerol (Table 3).

Benzyl alcohol oxidation has been performed in a batch reactorat 200 kPa of oxygen and thermostatted at 80 �C, using cyclohexaneas solvent (benzyl alcohol/cyclohexane 50/50 vol/vol, AuPd/sub-strate molar ratio 1/5000). When a small amount of Bi (0.1 wt%)was added directly to the support, a negligible effect on both activityand selectivity was detected compared to Au–Pd/AC (TOF 2278 h�1

and 2261 h�1, respectively) (Table 2). However, when the Bi amountwas increased from 0.1 wt% to 3 wt%, the catalytic performance ofthe modified catalyst decreased (TOF 2278 h�1 for Au–Pd/0.1%Bi,and 1245 h�1 for Au–Pd/3%Bi) even the selectivity remained similar(selectivity to benzaldehyde at 90% conversion = 88–89%). It shouldbe noticed that the catalyst obtained using the support pre-treated

Benzaldehyde Benzoic acid Benzyl benzoate

89 1 288 1 389 0 295 0 397 0 2

Glycolate Tartronate Oxalate

19 2 118 4 020 5 112 10 115 11 1

78 A. Villa et al. / Journal of Catalysis 292 (2012) 73–80

with 3 wt% Bi did not allow the formation of a homogeneous alloy.Therefore, we considered that the results could also be affected byother effects than the presence of Bi, such as the different composi-tion, not the size as it was similar, of metallic nanoparticles. We thusconsidered the catalytic behaviors of the systems obtained by thesubsequent modification of Bi on preformed Au–Pd/AC that assuredthe presence of a single alloy composition. Surprisingly, we ob-served the same trend of activity as the previous one (Table 2):the slight increase of the catalytic activity observed when the0.1 wt% of Bi was added (TOF 2278 h�1 for AuPd/AC and 2546 h�1

for 0.1%Bi@AuPd/AC) drastically drop down when the amount ofBi was increased to 3 wt% (TOF 1643 h�1). We could thus concludethat the decreasing of activity could be correlated with the increas-ing of Bi from 0.1 wt% to 3 wt%. However, as the characterization of3%Bi@AuPd/AC revealed a partial segregation of Bi, we also tested aphysical mixture of 3%Bi/AC and AuPd/AC in order to determine thepotential role of isolated Bi nanoparticles on AC. This experiment

Scheme 1. Proposed mechani

Scheme 2. Reaction scheme for glycero

showed a perfectly overlapped reaction profile for the reaction car-ried out with the AuPd/AC or with a physical mixture of 3%Bi/AC andAuPd/AC. This finding confirmed the spectator role of Bi on AC(Fig. 5). Therefore, we were also able to state that the detrimental ef-fect of Bi on the activity of AuPd/AC is due to Bi near located to theactive sites and possibly a consequence of a selective inhibition ofsome AuPd active sites by an intimate contact with Bi. Probablywhen the amount of Bi is low (0.1 wt%), this inhibiting effect onthe catalytic activity is reduced. This proximity effect can be re-vealed in almost the same extent regardless the Bi position (un-der/on) with respect to AuPd nanoparticles. Therefore, we mainlyascribed the detrimental effect of the Bi presence to an electronic ef-fect. To investigate the electronic properties of the catalysts, X-rayphotoelectron spectroscopy (XPS) measurements were performedon three AuPd/C based samples (Au–Pd/C; 0.1%Bi@AuPd/C and3%Bi@AuPd/C). However, due to peak overlap between the gold4d5/2 and Pd 3d5/2 peaks at 335 eV, and the low concentrations

sm for toluene formation.

l oxidation under basic conditions.

Table 4Selectivity of AuPd/AC and 0.1%Bi@AuPd/AC at different temperature.

Catalysta Selectivityb

50 �C 60 �C 70 �C

GLYC TART GLY GLYC TART GLY GLYC TART GLY

Au–Pd 76 2 19 70 5 23 22 20 450.1%Bi Au–Pd 75 10 12 54 34 13 10 34 42

a Reaction condition: alcohol/metal 1000/1 (mol/mol), pO2 3 atm, 1250 rpm.b Selectivity at 90% conversion. GLYC = Glycerate, TART = Tartronate, GLY = Glycolate.

Fig. 6. Reaction profile of (a) 0.1%Bi@AuPd/AC and (b) AuPd/AC at 60 �C.

A. Villa et al. / Journal of Catalysis 292 (2012) 73–80 79

of metals, relative to support material, identification of peak shiftsdue to changes in electronic structure is impossible. Therefore,XPS technique cannot definitively confirm the origin for this changein activity to electronic effects. However, given the incorporation ofBi, it is highly probably that the electronic structures of individualclusters changes significantly affecting the bonding between thereactant and the catalyst. However, looking at the selectivity ofthe benzyl alcohol oxidation, we found an additional effect thatstrictly depended on the position of Bi. In fact, interestingly, whenBi is deposited on the metallic nanoparticles, a significant increasein the selectivity to benzaldehyde was observed mainly at the ex-pense of toluene formation that in both cases is reduced to 1–2%from 8% to 9% in the other cases when Bi modified the AC (Table2). Toluene is expected to be formed by reaction of the intermediatemetal-hydride with alcohol instead of O2 [38] (Scheme 1). Thus,most probably the blocking of a fraction of active sites occurredand, according to previous studies on Bi-modified catalysts [33], thiscan also provide a different substrate coordination route slowingdown its reaction with the metal-hydride. On the contrary whenBi was deposited on the AC, this effect on selectivity resulted negli-gible thus supporting that a geometric effect could be the mainresponsible of selectivity modification. However, it cannot be ex-cluded that in this latter case also an electronic effect could be activeas well.

For better understanding this latter aspect, we tested the cata-lyst in the glycerol oxidation, where different OH groups couldbe preferentially oxidized. This reaction was performed in waterunder basic conditions (0.3 M Glycerol solution, glycerol/NaOH = 4 mol/mol, glycerol/metal = 1000 mol/mol) (Table 3). Thetrend of the activity of all the modified catalysts with respect tothe AuPd/AC one is in line with the activities observed in the ben-zyl alcohol oxidation. The addition of 0.1 wt% Bi slightly increasedthe activity, in particular when Bi has been deposited on preformedAuPd/AC, whereas a consistent decreasing of activity occurredwhen 3 wt% Bi is present. However, the most interesting aspect lieson the selectivity of the reaction. The selectivity to the main prod-uct glycerate did not show significant variations. However, con-cerning with the distribution of the other reaction products,consistent differences have been evidenced. Indeed, consideringthe two reaction pathways (a and b) of Scheme 2, it should benoted that tartronate is formed through the consecutive oxidationof glycerate, whereas glycolate represented a probe for the oxida-tive degradation [8]. Therefore, from the data of Table 3, we couldconclude that when Bi is deposited on the metal nanoparticles, itpromotes the consecutive reaction of glycerol to tartronate (pathb, Scheme 2) instead of the degradation (path a, Scheme 2). Thisfinding confirmed what reported in the literature in the case ofPd or Pt catalysts modified by Bi [36,38,39]. The maximum yield(58%) of tartronate was obtained under basic conditions by Kimuraet al. doping a Ce–Pd–Pt/AC catalyst with Bi [39].

As tartronate represents a useful chemical for pharmaceuticalapplication [40–42] and as an anticorrosive agent [43], we studiedmore in detail this reaction focusing our attention on the0.1%Bi@AuPd/AC catalyst, being the catalyst showing the highest

activity and highest selectivity to tartronate. By increasing the tem-perature from 50 �C to 60 �C (Table 4), the selectivity to tartronateincreased to 34% from 10% without any appreciable increasing ofselectivity to glycolate. When the temperature was raised to 70 �C,the selectivity to tartronate did not vary but a strong increase of gly-colate selectivity (42%), deriving from oxidative C–C carbon bondcleavage. Therefore, for optimizing the tartronate production, wemaintained the temperature in the range of 50–60 �C but we pro-longed the reaction time. Fig. 6 showed the reaction profiles of glyc-erol oxidation carried out at 60 �C in the presence of AuPd/AC(Fig. 6a) and of 0.1Bi%@AuPd/AC (Fig. 6b). As clearly shown byFig. 6, AuPd/AC is not able to perform the consecutive oxidation ofglycerate to tartronate, whereas 0.1Bi%@AuPd/AC efficiently cata-lyzed the reaction providing the complete conversion of glycerateto tartronate after 14 h with a isolated yield of tartronate of 78%.

80 A. Villa et al. / Journal of Catalysis 292 (2012) 73–80

4. Conclusions

Bi has been used as modifier of AuPd/AC catalyst and tested inthe catalytic oxidation of benzyl alcohol in cyclohexane and glyc-erol in water. The deposition of Bi on AC has been shown to haveno effect on particle size of the AuPd metallic nanoparticles butdid not allow a homogeneous composition of the alloyed phaseeven no segregation of the active metal (Au or Pd) has been de-tected. Conversely, the deposition of Bi on preformed AuPd/ACdid not affect the alloy composition even partial segregation of Bihas been observed by increasing the loading of Bi.

From a catalytic point of view, in both the catalytic tests, it hasbeen observed that from an activity point of view, Bi has a detri-mental effect more consistent for a higher loading (3 wt%). The ef-fect on the activity of the catalysts was observed either the Bi wasdeposited under or on the AuPd nanoparticles but experiments car-ried out on a physical mixture of Bi/AC and AuPd/AC allowed toconclude that the effect is determined by the proximity of Bi toAuPd nanoparticles and could be mainly electronic. However, asensible effect on the selectivity was observed depending of therelative position between Bi and AuPd NPs, being present onlywhen Bi is deposited on preformed AuPd alloyed particles. We as-cribed the effect mainly to the blocking of a fraction of active sitesthat possibly provided a different substrate coordination routeeven we could not exclude an additional electronic effect. In thecase of benzyl alcohol oxidation, this produced a suppression ofthe parallel reaction pathway that yield to toluene thus increasingthe benzaldehyde production; in the case of glycerol oxidation, thispromoted the consecutive reactions increasing the tartronate pro-duction. As the production of tartronate is of importance from anindustrial point of view, the reaction was optimized obtaining ayield of 78% at full conversion of glycerol.

Acknowledgments

Financial support by Fondazione Cariplo and Euminafab aregratefully acknowledged. A portion of this research was sponsoredby the Laboratory Directed Research and Development Program ofOak Ridge National Laboratory, managed by UT-Battelle, LLC, forthe U.S. Department of Energy (GMV).

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