Size effects of Pt-Re bimetallic catalysts for glycerol hydrogenolysis

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Catalysis Today xxx (2014) xxx–xxx

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Catalysis Today

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ize effects of Pt-Re bimetallic catalysts for glycerol hydrogenolysis

henghao Denga, Xuezhi Duana, Jinghong Zhoua,∗, De Chenb, Xinggui Zhoua,eikang Yuana

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, ChinaDepartment of Chemical Engineering, Norwegian University of Science and Technology, Trondheim 7491, Norway

r t i c l e i n f o

rticle history:eceived 7 November 2013eceived in revised form 14 January 2014ccepted 17 February 2014vailable online xxx

eywords:lycerol hydrogenolysist-Re

a b s t r a c t

A series of Pt-Re/CNTs catalysts with different particle sizes have been synthesized and applied in glyc-erol hydrogenolysis to elucidate the size effects. The trend of turnover frequency (TOF) for these differentsized Pt-Re/CNTs catalysts follows a volcanic curve, in which the TOF for Pt-Re/CNTs catalyst with parti-cle size of 1.9 nm is ca. 7.5 times higher than that of 4.9 nm. X-ray photoelectron spectroscopy analysisrevealed that the surface Pt/Re ratio decreased with the decrease of particle size. The enrichment of sur-face rhenium species on smaller particles probably led to the increase in the surface acidity, which couldbe one reason for the enhanced activity over smaller sized Pt-Re/CNTs catalysts. However, too small sizedPt-Re/CNTs catalyst (e.g., 1.5 nm) suffered from severe coking, resulting in the lower activity. Moreover,

ize effectNTsoking

the hydrogenolysis of different substrates (glycerol, 1,2-propanediol and 1,3-propanediol) over differ-ent sized Pt-Re/CNTs catalysts was also investigated. It was revealed that the reactivity declined in thefollowing order: glycerol > 1,3-propanediol > 1,2-propanediol, and the cleavage of secondary C–O bondwas favored over larger sized Pt-Re/CNTs catalyst. Based on these results, a possible reaction pathwaydepending on Pt-Re particle size was proposed.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

The utilization of renewable biomass and biomass-derivativess alternatives for fossil fuels is essential for the sustainableevelopment of our society [1]. In order to better meet the sus-ainability goals, biodiesel production has increased dramatically inhe past few years. However, huge amount of surplus glycerol is co-roduced with biodiesel production (ca. 10 wt%), which is requiredo be addressed to make this process more profitable [2,3]. For thisurpose, various catalytic conversion processes, e.g., oxidation [4],ydrogenolysis [5–8], reforming [9,10], have been proposed for theransformation of the surplus glycerol to value-added chemicals.n this context, hydrogenolysis of glycerol to 1,3-propanediol (1,3-D), 1,2-propanediol (1,2-PD) and bio-propanols (1-propanol and-propanol) is one of the most promising processes due to theirersatile applications [5–8]. Propanediols can be used as monomersor the synthesis of various polymers, e.g., polypropylene tereph-

Please cite this article in press as: C. Deng, et al., Size effects of Pt-Re bimhttp://dx.doi.org/10.1016/j.cattod.2014.02.023

halate, polyester resins. Propanols are also useful chemicals usedainly as solvent, printing ink and so on.

∗ Corresponding author. Tel.: +86 21 64252169; fax: +86 21 64253528.E-mail address: [email protected] (J. Zhou).

ttp://dx.doi.org/10.1016/j.cattod.2014.02.023920-5861/© 2014 Elsevier B.V. All rights reserved.

Recently, the combination of noble metals, e.g., Pt, Ir, Rh, Ru, withRe or W species [5–8,11–16,25] has received much attention fortheir improved performance for the cleavage of C–O bond of glyc-erol. In the case of Pt-based catalysts, the cleavage of C–C bond ofglycerol can be greatly inhibited, promoting the generation of C–Ohydrogenolysis products. For example, the Pt-H4SiW12O40/ZrO2catalysts [15] exhibited 48% selectivity to 1,3-propanediol and17% selectivity to 1,2-propanediol at 24% conversion of glyc-erol in continuous operation. However, the specific formationrates with respect to propanediols were relatively lower, i.e.,0.11 mol1,3-PG molPt

−1 h−1 and 0.04 mol1,2-PG molPt−1 h−1, respec-

tively. For Pt/WO3/ZrO2 catalysts [14,17,18], the specific formationrate of 1,3-propanediol was ca. 4.0 mol1,3-PG molPt

−1 h−1 withca. 28% selectivity to 1,3-propanediol in batch-wise operationand 46% in continuous operation. The specific formation rateof propanediols for glycerol hydrogenolysis over Pt-Re/C cata-lyst [5] was even better, 5.7 mol1,3-PG molPt

−1 h−1 (34% sel.) and5.3 mol1,2-PG molPt

−1 h−1 (33% sel.) at 20% conversion of glycerolin batch-wise operation. The Pt-Re based bimetallic catalysts isherein a promising candidate for hydrogenolysis of glycerol in the

etallic catalysts for glycerol hydrogenolysis, Catal. Today (2014),

consideration of both activity and selectivity to propanediols.Much attention has been paid to optimize the performance of

these catalysts due to the usage of noble metals [5–8,11–14,16],among which the modification of the acidic-basic properties of

dx.doi.org/10.1016/j.cattod.2014.02.023


http://www.sciencedirect.com/science/journal/09205861

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he catalysts was mainly focused [7,10,11,16]. However, the sizeffect, which is a key factor to influence the catalytic performancen many cases [11,19–21], has drawn little attention. Qin et al.11] reported that the performance of glycerol hydrogenolysis overt/WO3/ZrO2 catalyst was dependent on the Pt particle size, wherehe production of 1,3-propanediol and 1-propanol was preferredver larger sized Pt/WO3/ZrO2 catalyst. Nevertheless, in the case oft-Re bimetallic catalyst, the inherent correlation between the Pt-e bimetallic particle size and the catalytic performance for glycerolydrogenolysis is still unclear. Hence, there is a need to systemati-ally study the size effects to obtain fundamental insights into theeaction networks and to achieve highly efficient catalysts.

In this work, a series of carbon nanotubes (CNTs) sup-orted Pt-Re bimetallic catalysts were employed for the glycerolydrogenolysis. The particle sizes were adjusted by changing the

oading of Pt and Re, by which the surface chemistry of carbonanotubes was maintained as much as possible. The relation-hip between the Pt-Re particle size and the activity of glycerolydrogenolysis was correlated and interpreted based on theharacterizations of both fresh and used Pt-Re/CNTs catalysts.oreover, a comparative study on the hydrogenolysis of glycerol,

,3-propanediol and 1,2-propanediol over different sized Pt-Re cat-lysts was carried out to investigate the possible dependence ofeaction pathway on particle size.

. Experimental

.1. Catalyst preparation

CNTs supported Pt-Re bimetallic catalysts (Pt/Re molar ratio = 1)ere prepared by incipient wetness co-impregnation with aqueous

olutions of both chloroplatinic acid (Sinopharm Chemical Reagento., 99.9%-Pt) and perrhenic acid (Strem Chemicals, 99.99%-Re) [5].he CNTs was purchased from Tsinghua University without fur-her treatments. The impregnated samples were dried at 393 K for2 h, followed by direct activation at 723 K in hydrogen flow for 3 h.fter reduction, the catalysts were passivated with 1% (v/v) O2/Art room temperature for 1 h. The as-synthesized catalysts wereenoted as xPt-Re/CNTs, in which x denotes the nominal platinum

oading (wt%).

.2. Glycerol hydrogenolysis

Glycerol hydrogenolysis was carried out in a 100-ml stain-ess steel autoclave (Parr Instruments, USA) with an electronicemperature controller and a mechanical stirrer. In a typical run,0 g glycerol aqueous solution (1 wt%) and appropriate amountf pre-reduced Pt-Re/CNTs catalyst, equivalent to 14 �mol Pt forach case unless stated otherwise, were placed into the autoclave.fter being sealed, the reactor was purged thrice by flushing with

MPa hydrogen (99.99%). The autoclave was then heated to 443 Knd compressed to 4 MPa H2 at stirring rate of 500 rpm. After

h reaction, the reactor was cooled down to room temperaturend the used catalyst was separated and collected by centrifuga-ion. The reaction mixture was analyzed using a UPLC equippedith an RI detector (Waters 2414) and a C18 AQ column (Shi-

eido, 4.6 mm × 250 mm × 5 �m). The distilled water was used asobile phase at a flow rate of 0.6 ml/min. The temperature of

olumn and RI detector was set at 308 K. The products were quanti-ed with external standard method. Carbon balance was typicallyreater than 95%. As has been determined from plot of ln(C) ∼ t,


he reaction order with respect to glycerol in the hydrogenoly-is of glycerol over Pt-Re/CNTs was close to 1 (Fig. S1) under theeaction condition. The reaction rate constant is thus calculatedy k = 1/t × ln(1/(1 − X)), where X represents conversion of glycerol.

PRESSy xxx (2014) xxx–xxx

Accordingly, the initial reaction rate is calculated by k* molar ofglycerol charged/(mol of Pt charged), while the TOF is calculatedby initial reaction rate/Dispersion.

2.3. Catalyst characterization

H2 chemisorption was carried out at 318 K using an Autochem2920 (Micromeritics). Prior to the measurement, the pre-reducedand passivated samples were in-situ reduced at 473 K for 1 h inH2/Ar with flow rate of 40 ml/min, then purged with Ar at 503 Kfor 30 min (40 ml/min), and finally cooled to 318 K. The number ofcatalytic sites was taken to be equal to the irreversible H uptake.Temperature-programmed reduction experiments (TPR) were car-ried out in a U-shaped quartz reactor equipped with a thermalconductivity detector using 5% H2/Ar (30 ml/min) (Micromeritics,AutoChem 2920). The amount of sample was 30–50 mg, and thetemperature was increased from room temperature to 1073 K at aheating rate of 10 K/min.

X-ray diffraction (XRD) was performed on a Rigaku D/Max2550VB/PC diffractometer using Cu K� radiation. The TEM char-acterization was carried out for reduced catalysts with a JEOL JEM2100F with accelerating voltage of 200 kV and a point resolution of0.18 nm. Metal particle sizes and distributions were determined bymeasuring more than 150 randomly selected particles. The metalloadings of all samples were determined by ICP-AES.

X-ray photoelectron spectroscopy (XPS) was conducted on aMultilab 2000 spectrometer (Thermo VH Scientific) using Al K�

radiation (1486.6 eV). The aluminum anode was operated at anaccelerating voltage of 15 kV, 15 mA, 20 V. The pressure in the anal-ysis chamber was maintained in the range of 5 × 10−9 mbar. Thebinding energy of samples was calibrated using the binding energyof the C 1s peak (284.6 eV) as a reference.

An SDT Q600 (TA Instrument) instrument was employed forthermogravimetric analysis of the used catalysts. A small quantityof the sample was placed in an aluminum sample crucible and thetemperature was raised from room temperature to 1073 K at a rateof 10 K/min in the flowing air (100 ml/min). The coke content wasmeasured for used samples in the temperature range of 423–573 K,where the derivative peak centered at 523 K.

3. Results and discussion

3.1. Particle size and morphology

Different sized Pt-Re/CNTs catalysts with a fixed Pt/Re ratio (i.e.,1/1) were obtained by varying the metal loading with platinumloading in the range of 1–30 wt%. In this way, the surface chem-istry of the CNTs, particularly the oxygenated groups on the CNTssurface, were maintained as much as possible.

TEM studies were conducted for all reduced and passivated sam-ples. As can be seen in Fig. 1, the bimetallic particles were highlydispersed on the CNTs, even when the platinum loading was as highas 30 wt% (i.e., the total metal loading of ca. 60 wt%). Histograms ofover 150 metal particle diameters measured directly from theseimages are also provided in Fig. 1. Monomodal particle size dis-tributions were observed for all samples. The average particle sizegradually increased from 1.5 to 4.9 nm with the platinum loadingfrom 1 to 30 wt%.

Typically, high resolution TEM images of 20Pt-Re/CNTs wereobtained to identify the morphology of the bimetallic particles, asshown in Fig. 2. Spherical shaped particles were mostly observed forthe bimetallic catalysts, along with several elliptical shaped parti-


cles resulting from aggregation of metals. Only Pt crystalline latticecan be clearly distinguished, consistent with the XRD patterns. Thed-spacing values of 0.195 and 0.225 nm were ascribed to the (2 0 0)and (1 1 1) planes of platinum (PDF#70-2431), respectively.



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Fig. 1. Representative TEM images and the corresponding particle size distributionhistograms for the xPt-Re/CNTs catalysts with different metal loading: (a) 1 wt%; (b)3 wt%; (c) 5 wt%; (d) 10 wt%; (e) 20 wt%; (g) 30 wt%.

Fig. 2. High resolution TEM images and the corresponding XRD pattern of 20Pt-Re/CNTs catalyst.

3.2. Catalyst compositions and crystallinity

The metal loadings determined by ICP analysis are summarizedin Table 1. As can be seen, the metal loading values of Pt and Re wereclose to the nominal values in all cases, revealing the equal molarratio of Pt to Re dispersed on the CNT supports. Meanwhile, thesurface compositions of Pt and Re were determined by XPS analysis.The surface Pt/Re ratio increased with the decrease of particle sizewithin the size range of 2.3–4.9 nm (Table 1).

Fig. 3 depicts XRD patterns of the CNTs support and the Pt-Re/CNTs catalysts with different metal loadings after reduction andpassivation. The XRD patterns of the CNTs displayed two distinctpeaks at 2-theta = 25.6◦ and 42.8◦. No obvious peaks ascribing toplatinum and rhenium were observed when 1 wt% Pt and Re wereintroduced onto the CNTs support. This indicated that the bimetallicPt-Re particles were rather small and highly dispersed on the CNTs


surface, which was consistent with the TEM observation. Furtherincreasing the metal loadings resulted in the appearance of peaks at2-theta = 39.8◦, 46.2◦ and 67.5◦, which could be ascribed to (1 1 1),(2 0 0) and (2 2 0) planes of metallic platinum, respectively. Another

Fig. 3. Wide angle XRD patterns of the CNTs and xPt-Re/CNTs catalysts: (a) CNTs; (b)1Pt-Re/CNTs; (c) 3Pt-Re/CNTs; (d) 5Pt-Re/CNTs; (e) 10Pt-Re/CNTs; (f) 20Pt-Re/CNTs;(g) 30Pt-Re/CNTs.



4 C. Deng et al. / Catalysis Today xxx (2014) xxx–xxx

Table 1Summary of mean particle sizes and bulk/surface compositions.

Catalyst Pta (wt%) Rea (wt%) H/Ptb dav.c Ptd (atom %) Red (atom %)

5Pt/CNTs 5.1 – 0.66 – – –1Pt-Re/CNTs 1.06 0.96 0.23 1.5 –e –e

3Pt-Re/CNTs 2.95 2.80 0.27 1.9 –e –e

5Pt-Re/CNTs 5.11 4.84 0.30 2.3 0.25 0.2510Pt-Re/CNTs 10.23 9.71 0.26 2.6 0.47 0.4120Pt-Re/CNTs 21.13 19.27 0.28 3.6 1.95 0.9130Pt-Re/CNTs 29.76 28.66 0.17 4.9 2.36 0.95

a Results from ICP analysis.b Results from H2 chemisorption, assuming that only Pt is responsible for H2 chemisorption (H/Re = 0.03);

ions c

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c Estimated from TEM observations.d Surface contents from XPS.e Not characterized because the disturbance of contributions from bulk composit

nteresting phenomenon was that the peak due to metallic rheniumhase was not observed even when the rhenium loading was asigh as ca. 30 wt% (Fig. 3g), possibly due to the high dispersion ofhenium species [13,22,23].

.3. Chemisorption properties and reducibilities

The chemisorption results for Pt/CNTs, Re/CNTs and xPt-e/CNTs are summarized in Table 1. The H2 uptake value forRe/CNTs was very low (8 �mol/g, i.e., H/Re = 0.03), consistent withhe previous reports [27,29]. It should be noted that the additionf Re to Pt/CNTs decreased H2 uptake ability of Pt/CNTs [27–29],hich was interpreted by the coverage of Re on the particle sur-

ace and/or altered electronic properties of Pt [28]. Specifically, theispersion value based on Pt (assuming that only Pt is responsi-le for H2 chemisorption) did not monotonously increased withecreasing the particle size, which could be explained by the dif-erent coverage of Re on the different sized particle surface, asetermined by XPS analysis (Table 1).

Temperature-programmed reduction (TPR) was performed tovaluate the catalyst reducibility. Fig. 4 shows the TPR profiles forPt/CNTs, 5Re/CNTs and xPt-Re/CNTs. The addition of Pt to Re/C


aused the reduction peak shifted to lower temperature comparedith that of Re/C, demonstrating that the presence of Pt promoted

he reduction of the Re precursor [29]. On the other hand, the addi-ion of equal molar of Re to Pt/C resulted in the shift of reduction

ig. 4. TPR profiles of 5Pt/CNTs (a), 5Re/CNTs (b) and xPt-Re/CNTs (c–h) with dif-erent Pt loading (wt%): 1.06 (c); 2.95 (d); 5.11 (e); 10.03 (f); 20.13 (g); 29.76 (h).onditions: sample (30–50 mg), H2/Ar (5%, v/v, 40 ml/min) at the heating rate of0 ◦C/min.

ould not be ignored since XPS detection depth is ca. 2 nm.

peak to lower temperature [29]. In addition, the shift of maxi-mum of reduction peak for Pt-Re/CNTs to higher temperature wasobserved when the metal loading decreased. This can be ascribedto the increase of the support-metal interaction resulted from bet-ter dispersion of Pt and Re on the support [30]. Notably, all of theTPR profiles contained a broad hydrogen consumption peak after317 ◦C, which was accompanied by methane emission, resultingfrom hydrogenation of surface functional groups on the carbona-ceous support [29].

3.4. Catalytic performance and reaction pathway

3.4.1. Catalytic performanceThe time course of the glycerol hydrogenolysis catalyzed by

5Pt-Re/CNTs catalyst is shown in Fig. 5. The initial selectivity to1,2-propanediol and 1,3-propanediol was 64% and 23% after 0.67 h,respectively. As the conversion of glycerol increased, an obviousdecay of the selectivity to propanediols and a concomitant increaseof the selectivity to 1-propanol were observed. In the case of eth-ylene glycol and 2-propanol, the selectivity with time course wasnearly unchanged.

We note that the product distribution over Pt-Re/CNTs in ourwork differ from the results over Pt-Re/C reported by R.J. Daviset al. [5]. In our work, for 5Pt-Re/CNTs reduced at 723 K, theselectivity to 1,3-propanediol, 1,2-propanediol and 1-propanol


was ∼21%, 57% and 12% at 20% conversion, where the 1,2-propanediol was the dominant product. For Pt-Re/C (5.7 wt% Pt,4.6 wt% Re) reduced at the same temperature, the selectivity to1,3-propanediol, 1,2-propanediol and 1-propanol was 26%, 41%

Fig. 5. The time course of glycerol hydrogenolysis over 5Pt-Re/CNTs catalysts:Conversion (�); 1,2-PD = 1,2-propanediol (�); 1,3-PD = 1,3-propanediol (©); 1-PO = 1-propanol (�); 2-PO = 2-propanol (�); EG = ethylene glycol (�).



C. Deng et al. / Catalysis Today xxx (2014) xxx–xxx 5

Fig. 6. TOF and surface Pt/Re ratio as a function of particle size for Pt-Re/CNTs cat-as

aPst[taskvf[

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decreased in the following order: glycerol > 1,3-propanediol > 1,2-propanediol. The ratio of 1-PO/2-PO selectivity was defined inorder to compare the competitive cleavage of primary and sec-ondary C–O bond over different sized Pt-Re/CNTs catalysts. The

Table 2Summary of TG analysis results for xPt-Re/CNTs catalysts after 8 h catalytic use.

Catalyst Cokecontenta (%)

TOF (h−1) Coking rateb

(gcoke molPt−1 h−1)

5Pt-Re/CNTs 3.59 51.0 18.410Pt-Re/CNTs 2.22 35.0 5.7

lysts (443 K, 4 MPa, 1 wt% glycerol aqueous solution) (�) and the correspondingurface Pt/Re ratio (�).

nd 22%, respectively, at 20% conversion of glycerol [5]. When thet-Re/C catalyst was reduced at higher temperature (973 K), theelectivity to 1,3-propanediol increased to 34%, while the selec-ivity to 1,2-propanediol was 33%, at 20% conversion of glycerol5]. The difference in product distribution could be resulting fromhe differences in the applied reaction conditions (substrate/metaldsorption site: 520 vs. 350 [5]) and the employed carbonaceousupports (carbon nanotubes vs. activated carbon (Norit)). It isnown that the surface chemistry and textural properties of acti-ated carbon and carbon nanotubes was quite different, which mayurther affect the catalytic performance of the supported catalysts28,31].

Fig. 6 depicts the TOF as a function of particle size for Pt-Re/CNTsatalysts, while the detailed data of conversion and selectivity ishown in Table S1. The TOF was found to reach the maximum atround 1.9 nm. The TOF of the Pt-Re/CNTs catalyst with particle sizef 1.9 nm was ca. 7.5-fold of that of 4.9 nm. Moreover, it was shownhat the decrease of particle size from 4.9 to 1.9 nm resulted in a

onotonously increase of TOF. The XPS characterization (Table 1,ig. S2) on surface composition showed that the surface Pt/Re ratioecreased with the decrease of particle size within the range of.3–4.9 nm for Pt-Re/CNTs catalysts, as shown in Table 1 and Fig. 6.eanwhile, the tendency for the enrichment of surface rheniumas suggested to increase with the decreasing metal dispersion for

t-Re/C catalyst [27], indicating that the Pt-Re/CNTs catalysts withather smaller size (i.e., 1.5 and 1.9 nm) may possess even moreurface rhenium. In addition, studies on the effect of Pt/Re ration glycerol hydrogenolysis showed that the reactivity increasedonotonously with the loading amount of Re in Pt-Re/CNTs (Table

2), and analysis of NH3-TPD-MS revealed an increased acidity withhe loading amount of Re (Fig. S4). In this respect, it has been sug-ested that Pt-Re bimetallic may be oxidized by dissociated waterith OH attached to Re and generate surface acidity [24], and that

he concentration of acid sites increases with Re loading [10]. Andhe increase of surface acidity may enhance the hydrogenolysisctivity since acid sites were proposed to involve in the dehydrationf glycerol [16,25,26]. Accordingly, it was suggested that the highermount of surface Re for smaller particle sized Pt-Re/CNTs catalystould be responsible for the increased activity with the decrease ofarticle size.

However, the activity was not increased monotonously with the


ecreasing particle size within the investigating particle size range.emarkable decrease of TOF was observed for Pt-Re/CNTs catalystith particle size of 1.5 nm. The lowered TOF may be due to the

Fig. 7. DTG profiles of xPt-Re/CNTs catalysts after 8 h reaction.

inhibition of catalytically active sites by much faster accumulationof coke species on smaller sized catalyst.

In order to quantitatively characterize the coking behavior of theused typical catalysts, the TG analysis was performed (Figs. 7 andS3). For 5 Pt-Re/CNTs and 10 Pt-Re/CNTs, one major peak of deriva-tive weight centered at 250 ◦C was observed (Fig. 7), which could bedue to the burning of coke formed during reaction. And the inten-sity of the derivative weight increased with decreasing the particlesize. The corresponding data are summarized in Table 2. For the rel-atively smaller sized catalyst (5Pt-Re/CNTs, 2.3 nm), the coking ratewas ca. 10-fold higher than that of the larger one (20Pt-Re/CNTs,3.6 nm). This tendency could be ascribed to the much higher activ-ity for smaller sized catalysts, ca. 6-fold higher than 20Pt-Re/CNTscatalyst, possibly leading to higher deposition rate of carbonaceousspecies on the metal surface.

Moreover, product distribution at ∼40% conversion vs. particlesize is plotted in Fig. 8, and the detailed data are shown in Table S3.The results indicated that the product distribution changes withthe particle size. The smaller sized Pt-Re/CNTs generated more1,2-propanediol and ethylene glycol, while the larger sized onegenerated more 1,3-propanediol. This means that the scission ofC–C bond and primary C–O bond of glycerol were favored oversmaller sized Pt-Re/CNTs catalysts, producing ethylene glycol and1,2-propandiol, while the scission of the secondary C–O bond ofglycerol was favored over larger sized Pt-Re/CNTs catalysts, leadingto the formation of 1,3-propanediol.

3.4.2. Dependence of reaction pathway on particle sizeTo further understand the chemistry of glycerol hydrogenolysis

over Pt-Re/CNTs catalysts, results of hydrogenolysis of glyc-erol, 1,3-propanediol and 1,2-propanediol are summarized inTable 3. Results showed that the reactivity of these three alcohols


20Pt-Re/CNTs 1.14 8.9 1.5

a The coke content data was collected in the temperature range of 423–573 K withthe peak centered at 523 K.

b Coking rate = mass of coke formation/(molar of platinum charged × time).



6 C. Deng et al. / Catalysis Today xxx (2014) xxx–xxx

Table 3Hydrogenolysis of alcohols over different sized Pt-Re/CNTs.

Reactant TOF (h−1) 1-PO/2-PO (sel.)

1.9 nm 2.6 nm 3.6 nm 1.9 nm 2.6 nm 3.6 nm

Glycerol 61.9 35.0 8.9 6.6 7.2 6.91,3-Propanediol 16.7 10.0 4.6 –a –a –a

1.

otfpolPTfwsy1

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1,2-Propanediol 5.9 4.2

a The hydrogenolysis of 1,3-propanediol generated only 1-propanol.

btained 1-PO/2-PO ratio in glycerol hydrogenolysis was higherhan that in 1,2-propanediol hydrogenolysis (Table 3), especiallyor smaller sized catalyst, indicating that the hydrogenolysis of 1,2-ropanediol and 1,3-propanediol contributed to the accumulationf 1-propanol. Specifically, hydrogenolysis of 1,2-propanediol overarger sized Pt-Re/CNTs catalysts showed higher ratio of 1-PO/2-O than that over smaller sized catalysts, i.e., 5.7 compared to 3.7.hese results indicated that the larger sized Pt-Re/CNTs catalystavored the scission of secondary C–O bond of 1,2-propanediol,hich was similar to the case for glycerol. In addition, production of

mall amount of ethylene glycol was observed for the hydrogenol-sis of 1,2-propanediol, demonstrating the cleavage of C–C bond of,2-propanediol.

According to the above discussions, a possible reaction path-ay dependent on particle size is proposed, as shown in Scheme 1.


he reactant glycerol molecule contains C–O bond on all threearbons of the backbone, among which the two on both endsre primary C–O bonds and the middle one is secondary C–O

ig. 8. Product distribution as a function of particle size at 40 ± 2% conversion forhe Pt-Re/CNTs catalysts (443 K, 4 MPa, 1 wt% glycerol aqueous solution).

cheme 1. Possible reaction pathway for glycerol hydrogenolysis over differentized Pt-Re/CNTs catalysts (solid line represents the favored route for smaller sizedt-Re/CNTs catalyst, dash line represents the favored route for larger sized Pt-e/CNTs catalyst).

8 3.7 4.9 5.7

bond. The cleavage of primary or secondary C–O bond of glyc-erol gives 1,2-propanediol or 1,3-propanediol, respectively. Furtherhydrogenolysis of 1,2-propanediols produces 1-propanol and 2-propanol, while only 1-propanol forms from 1,3-propanediol. Theformation of ethylene glycol through cleavage of C–C bond resultsfrom hydrogenolysis of both glycerol and 1,2-propanediol. Notably,the scission of secondary C–O bond of glycerol and 1,2-propanediol,giving 1,3-propanediol and 1-propanol, respectively, was relativelyfavored over larger sized Pt-Re/CNTs catalysts, marked as dash linein Scheme 1.

4. Conclusions

In summary, different sized Pt-Re/CNTs catalysts were pre-pared and tested to investigate the particle size effect in glycerolhydrogenolysis. The relationship between the TOF and the Pt-Reparticle size was found to follow a volcanic curve, in which thereaction rate of Pt-Re/CNTs catalyst with particle size of 1.9 nmwas ca. 7.5 times higher than that of 4.9 nm. As determined byXPS, Re was enriched on the smaller sized Pt-Re bimetallic particlesurface, resulting in the increase of the surface acidic properties,which could be one reason for the enhanced activity for smallersized Pt-Re/CNTs catalysts. However, too small sized Pt-Re/CNTscatalyst (e.g., 1.5 nm) suffered from severe coking, resulting indecreased activity. Moreover, a comparative study on the productdistribution of the hydrogenolysis of glycerol, 1,3-propanediol and1,2-propanediol over different sized Pt-Re catalysts showed thatthe smaller sized Pt-Re/CNTs catalysts favored the scission of theC–C bond and primary C–O bond, while the larger Pt-Re/CNTs cata-lysts favored the scission of the secondary C–O bond of both glyceroland 1,2-propanediol.

Acknowledgements

This work was financially supported by the NSFC (21106047),CSC (201206745001) and the 111 Project of Ministry of Educationof China (B08021).

Appendix A. Supplementary data

Supplementary material related to this article can be found,in the online version, at http://dx.doi.org/10.1016/j.cattod.2014.02.023.

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