Size Effects of Pt Nanoparticles Supported on Carbon Nanotubes for

Sep 30, 2014 - ACS eBooks; C&EN Global Enterprise .... Size Effects of Pt Nanoparticles Supported on Carbon Nanotubes for Selective ... In particular,...
0 downloads 0 Views 5MB Size
Download PDF
Article pubs.acs.org/IECR

Size Effects of Pt Nanoparticles Supported on Carbon Nanotubes for Selective Oxidation of Glycerol in a Base-Free Condition Jiaqi Lei,† Xuezhi Duan,† Gang Qian,*,† Xinggui Zhou,† and De Chen‡ †

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China ‡ Department of Chemical Engineering, Norwegian University of Science and Technology, Trondheim 7491, Norway S Supporting Information *

ABSTRACT: Different sized Pt nanoparticles supported on carbon nanotubes were prepared and then applied in selective oxidation of glycerol in a base-free condition to understand the size effects. It is shown that the turnover frequency of glycerol increases with Pt particle size to a maximum at the mean size of 2.5 nm followed by a decline with a further increase in size, which may be due to strongly adsorbed intermediates blocking Pt active sites for smaller sized Pt catalyst. Moreover, compared with dihydroxyacetone, glyceraldehyde and subsequent glyceric acid (GLYA) are dominating products. In particular, smaller sized Pt catalyst favors the formation of GLYA, which could be related to the stronger oxidation. Unexpectedly, the selectivity of C3 products is insensitive to both Pt particle size and reaction time within 9 h.

1. INTRODUCTION Glycerol, an inevitable byproduct during the production of biodiesel and a highly functionalized molecule, can be used as a renewable feedstock for the production of valuable oxygenated derivatives. Particularly, its heterogeneous and chemo-selective catalytic oxidation with air or oxygen is a promising route for the production of various high-value fine chemicals such as glyceric acid (GLYA), glyceraldehyde (GLYD), dihydroxyacetone (DHA), and hydroxypyruvic acid (HA), which are currently produced either by costly and polluting stoichiometric oxidation processes or by enzymatic routes of low productivity.1−5 This route occurs in a base or base-free condition. The addition of base is found to facilitate the initial dehydrogenation, and thus leading to a significantly increased activity.6,7 However, in alkaline solution, the main products are in the form of sodium salts, which require additional acidification to get the free acids. In terms of the simplicity and cost of the route, catalytic oxidization of glycerol in a base-free condition is more attractive, which directly produces the final chemicals.8−12 Among the tested catalysts for glycerol oxidation in a basefree condition, Pt catalysts are found to be the most active.13,14 Nowadays, most Pt catalysts use carbonaceous materials as supports, which are well recognized in industrial chemistry for being stable in acidic or basic conditions and have easy recovery of Pt by burning off the support.15−17 It has been found that the type of carbonaceous materials, especially the textural and chemical properties, has a great impact on the performance of Pt catalysts.18,19 In view of the practical application of this system, the cost of the noble metal catalyst is crucial. It is a common method to achieve high Pt utilization by decreasing the particle size. For example, the smaller sized Pt nanoparticles supported on activated carbon (AC) (10 nm),20 and S-pretreated carbon nanotubes (CNTs) supported 2.0 nm Pt catalyst showed higher glycerol conversion, e.g., 90.4% at 6 h.21 In these © 2014 American Chemical Society

studies, the microporosity of AC inevitably leads to the hindrance of the accessibility of small Pt particles depositing in the micropore of AC, and the change in Pt particle size always accompanies the change in the supports. In other words, both size effects and support effects simultaneously exist. Keeping the supports unchanged and using micropore-free support to probe the real size effects is highly desirable, which will guide the rational design of Pt catalysts with higher activity and selectivity. In this work, closed CNTs, mainly consisting of external surfaces, were employed to support different sized Pt particles by varying Pt loading. The as-obtained catalysts were used to catalyze glycerol oxidation in a base-free condition. Dependence of the turnover frequency (TOF) of glycerol on Pt particle size was correlated. Moreover, effects of Pt particle size on the selectivities to GLYD, GLYA, and C3 products were also investigated. Finally, a possible reaction pathway of glycerol oxidation in a base-free condition was proposed.

2. EXPERIMENTAL SECTION 2.1. Pt Catalysts Preparation. Carbon nanotube supported Pt (Pt/CNTs) catalysts with different Pt loadings were prepared by incipient wetness impregnation method. An aqueous solution of chloroplatinic acid (Sinopharm Chemical Reagent) was impregnated onto CNTs surfaces, which were purchased from CNano Technology without further treatment. The impregnated samples were first dried in stagnant air at ambient temperature for 12 h, further dried at 353 K for 12 h in an oven, and then activated at 473 K for 3 h in a hydrogen flow (30 mL·min−1). After reduction, 0.92% (v/v) O2/Ar was used for the passivation of the catalysts at room temperature for 0.5 Received: Revised: Accepted: Published: 16309

July 7, 2014 September 25, 2014 September 30, 2014 September 30, 2014 dx.doi.org/10.1021/ie502706r | Ind. Eng. Chem. Res. 2014, 53, 16309−16315

Industrial & Engineering Chemistry Research

Article

Figure 1. Typical HRTEM images of Pt/CNTs catalysts with different nominal Pt loadings: (a) 2.0, (b) 3.0, (c) 4.0, (d) 4.5, (e) 5.0, and (f) 8.0 wt %.

h. The as-synthesized catalysts were denoted as x wt % Pt/ CNTs, in which x wt % denotes the nominal platinum loading. 2.2. Catalyst Characterization. The microstructure of the Pt/CNTs catalysts and the Pt particle size were characterized by high resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F, Japan). The mean particle size (d = ∑nidi/ ∑ni, where ni is the number of particles with a diameter of di) and distribution of Pt were estimated by analyzing the particle sizes of 120 randomly selected particles. X-ray diffraction (XRD) was performed on a Rigaku D/ Max2550VB/PC X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation (λ = 1.54056 Å).

The dispersion of Pt was examined by CO adsorption using an Autochem 2920 (Micromeritics, USA). About 100 mg sample was first in situ reduced in H2/Ar (30 mL·min−1) at 473 K for 2 h and purged with Ar (30 mL·min−1) at 503 K for 30 min. Then, CO pulses from a calibrated online sampling valve were injected after the sample was cooled to 298 K. An assumption of the CO/Pt adsorption stoichiometry ratio of 1 was used for calculation. Pt loadings of Pt/CNTs catalysts were determined by thermal gravimetric analysis (TGA) according to the previous report.22 TGA experiments in air flow (50 mL·min−1) at a heating rate of 5 °C·min−1 to 800 °C were carried out on a 16310

dx.doi.org/10.1021/ie502706r | Ind. Eng. Chem. Res. 2014, 53, 16309−16315

Industrial & Engineering Chemistry Research

Article

be seen that the Pt loadings are close to the nominal values for all the catalysts. With increasing Pt loading, the dispersion of Pt decreased, resulting in a larger Pt particle size. Figure 2 shows XRD patterns of different sized Pt/CNTs catalysts. For the nominal Pt loading of 2.0 and 3.0 wt %, no

SDT-Q600 thermobalance (TA Instruments, USA). From the weight of the remaining solids, Pt loadings were calculated. The presence of adsorbed species on Pt/CNTs catalysts was verified by TGA. The pristine CNTs support, the used catalysts with reaction time of 9 h were heated at 5 °C·min−1 in an inert atmosphere (N2, 50 mL·min−1) from room temperature to 800 °C. In order to discriminate the adsorption sites, CNTs were first treated by immersing into the filtered reaction solution after 9 h with stirring at 60 °C for 1 h and then characterized by TGA. X-ray photoelectron spectroscopy (XPS) was performed on a Kratos XSAM-800 instrument using Al Kα X-ray with 1486.6 eV as the excitation source. The binding energy of C 1s (284.6 eV) was taken as a reference to correct the binding energies of the samples. 2.3. Catalyst Evaluation. Glycerol oxidation was performed in a three-neck flask (100 mL) equipped with gas supply system, magnetic stirrer, condenser, and thermocouple. In a standard reaction, 30 mL of aqueous solution of glycerol (0.1 g·mL−1) and an appropriate amount of catalyst were added into the reactor and stirred at a speed of 500 rpm, in which the molar ratio of glycerol/Pt was 890. When the reaction temperature reached 60 °C, 150 mL·min−1 O2 was introduced into the reactor. Under these conditions, the effect of mass transport limitations was eliminated (Figures S1 and S2). The samples were filtered using 0.22 μm NY filters before analysis using a high performance liquid chromatograph (HPLC, Agilent 1100). The HPLC was equipped with a UV (210 nm) and a refractive index (RI) detector using a BP-OA column (50 × 4.6 mm) operating at 353 K. The eluent was an aqueous solution of H3PO4 (0.001 g·mL−1) operating at 0.5 mL·min−1. The retention time and calibration curves for observed products were determined by injecting the standard samples with known concentrations.

Figure 2. XRD patterns of Pt/CNTs catalysts with different nominal Pt loadings: (a) 2.0, (b) 3.0, (c) 4.0, (d) 4.5, (e) 5.0, and (f) 8.0 wt %.

legible diffraction peaks associated with Pt are present, which could be related to the high Pt dispersion and the relative low Pt loading. However, for higher Pt loadings, such as 4.5, 5.0, and 8.0 wt %, the typical diffraction peaks of 39.7°, 46.3°, and 67.5° are observed, which are assigned to face-centered cubic (fcc) Pt crystalline. The performances of different sized Pt/CNTs catalysts for glycerol oxidation in base-free conditions were studied as a function of time. For all the catalysts, glycerol conversion increases with time (as shown in Figure 3), but at a same

3. RESULTS AND DISCUSSION 3.1. Size-Dependent Activity of Glycerol Oxidation over Pt/CNTs Catalysts. Particle sizes of Pt/CNTs catalysts with different nominal Pt loadings (i.e., 2.0, 3.0, 4.0, 4.5, 5.0, and 8.0 wt %) were characterized by HRTEM. Figure 1 shows typical HRTEM images of different sized Pt/CNTs catalysts and the corresponding particle size distributions and average sizes. All the catalysts are observed to be well dispersed on the external surfaces of CNTs with narrow particle size distributions, and the average Pt particle size ranges from 1.5 to 2.9 nm when the Pt nominal loading increases from 2.0 to 8.0 wt %. Table 1 lists the average particle size of Pt particles obtained by HRTEM, and Pt loading and dispersion determined by TGA and CO adsorption, respectively. It can Figure 3. Glycerol conversion as a function of time over Pt/CNTs with different Pt loadings. Reaction conditions: 30 mL glycerol aqueous solution (0.1 g·mL−1), glycerol/Pt molar ratio 890, T = 60 °C, FO2 = 150 mL·min−1, 500 rpm.

Table 1. Characterization of the Pt Catalysts with Different Loadings catalyst 2.0 3.0 4.0 4.5 5.0 8.0

wt wt wt wt wt wt

% % % % % %

Pt/CNTs Pt/CNTs Pt/CNTs Pt/CNTs Pt/CNTs Pt/CNTs

Pt loading (%)a

Pt dispersionb

d (nm)c

1.9 3.0 4.0 4.5 4.9 7.8

0.54 0.42 0.35 0.31 0.25 0.23

1.5 1.8 2.0 2.2 2.5 2.9

reaction time, the conversion varied with different catalysts. When these catalysts were prepared, only Pt loadings were changed, and other preparation and evaluation conditions were kept the same. Therefore, the different catalyst activities are due to the different Pt loadings, and furthermore the size of Pt particles. Figure 4 shows the turnover frequency (TOF) of glycerol as a function of Pt particle size. The TOF of glycerol increases

a

Obtained by TGA. bDetermined by CO adsorption. cEstimated by HRTEM. 16311

dx.doi.org/10.1021/ie502706r | Ind. Eng. Chem. Res. 2014, 53, 16309−16315

Industrial & Engineering Chemistry Research

Article

used Pt/CNTs catalysts after 9 h reaction present much more mass loss, indicating the adsorption of oxygenate species on the surface of the Pt. Furthermore, the mass loss on used 3.0 wt % Pt/CNTs catalyst is larger than that on used 5.0 wt % Pt/ CNTs, illustrating more species adsorbed on the surface of 3.0 wt % Pt/CNTs catalyst, i.e., the smaller sized Pt/CNTs adsorbing more oxygenate species. Moreover, the XPS results (as shown in Figure S3 and Table S1) show that Pt 4f binding energies of the used Pt/CNTs catalysts (after 9 h reaction) shift toward lower values with respect to the fresh catalysts, and the used 3.0 wt % Pt/CNTs catalyst has a larger shift, which may be due to the more adsorbed oxygenate species on the surface of used 3.0 wt % Pt/CNTs catalyst.28 3.2. Size-Dependent Selectivity of Glycerol Oxidation over Pt/CNTs Catalysts. The oxidation of glycerol can produce a large number of products such as DHA, GLYD, GLYA, and HA. Therefore, control of the selectivity to the desired product is highly required. However, the relationship between the product selectivity and Pt particle size in a basefree condition has been scarcely studied. In this part, our work focuses on the effect of Pt particle size on the selectivities of main products. Table 2 lists the selectivities to different products of glycerol oxidation over different sized Pt/CNTs catalysts at a reaction time of 6 h. On all the catalysts, the product distribution is similar, and GLYD, GLYA, and DHA are observed to be the main products. Compared with DHA, GLYD and its oxidation products GLYA are produced dominantly as a result of the preferential oxidation of the primary hydroxyl of glycerol over Pt/CNTs catalysts. The minor products are HA and some products from oxidative cleavage, such as glycolic acid (GLYCA) and CO2. Moreover, trace amounts of tartronic acid (TA) and oxalic acid (OA) are also detected. It should be noticed that at the reaction conditions a large amount of GLYD is produced. It is generally accepted that glycerol oxidation over supported Pt catalysts proceeds by dehydrogenation to an aldehyde or ketone intermediate, followed by oxidation to the acid product. During the oxidation of aldehyde to acid, there exists an alkoxy intermediate of the germinal diol, which is formed by the reaction of the aldehyde with hydroxide.29 The subsequent C−H activation of the germinal diol to form acid occurs quite readily over Pt catalysts.14 In a base-free condition, there are not so many hydroxides to be used to form germinal diol as that in the presence of base. Therefore, the oxidation rate from GLYD to GLYA is slow, that is why lots of GLYD can be detected in our experiments. This is in agreement with the previous result19 (Table S2). However, in a base-free condition, Liang et al. obtained a very low GLYD selectivity (Table S2), which may be due to the smaller molar ratio of glycerol to Pt. As shown in Table S2, our results indicated that the molar ratio of glycerol to Pt also had some effects on GLYD selectivity. Higher molar ratio preferred to a larger GLYD selectivity. This trend had also been reported in Carrettin’s work.7 Table 2 also shows that, at the same reaction time (6 h), the conversion and selectivity varied with the Pt loading. Meanwhile, the mass balance, based on the selectivities to C2 and C3 products detected by HPLC, also changed, which means that the extent of oxidative cleavage to C1 products also varied with the Pt loading. However, the product selectivity always varies with glycerol conversion (as shown in Figure S4). Therefore, to investigate the effect of Pt particle size on the selectivity of glycerol oxidation accurately, the selectivities to the two main products, GLYD and GLYA, over different sized

Figure 4. TOF of glycerol as a function of Pt particle size for Pt/CNTs catalysts. Reaction conditions: 30 mL of glycerol aqueous solution (0.1 g·mL−1), glycerol/Pt molar ratio 890, 30 min, T = 60 °C, FO2 = 150 mL·min−1, 500 rpm.

with Pt particle size to a maximum at an average size of 2.5 nm followed by a decline with a further increase in size. Previous studies showed that, with the decrease of Pt particle size, the population of Pt atoms at vertices and edges increases rapidly,23 and these atoms with low coordination number have strong abilities to adsorb oxygenate species.24−26 For glycerol oxidation, the strong adsorbed oxygenated intermediate can act as a poison to the reaction, and thus decreasing the reaction rate, while the adsorbed hydroxide intermediates are beneficial to the dehydrogenation of glycerol to an aldehyde or ketone intermediate, and the following oxidation to the acid product.14 Therefore, the combination of the two different particle size effects could result in a maximum glycerol oxidation rate on Pt/ CNTs catalysts. To probe the presence of oxygenate species adsorbed on the surface of Pt/CNTs, the used catalysts were characterized by using TGA and XPS. The TGA results are shown in Figure 5.

Figure 5. TGA profiles of pristine CNTs, CNTs treated by reaction solution, and used Pt/CNTs catalysts with different loadings after 9 h reaction.

The pristine CNTs support has a mass loss of about 3 wt %, which is due to the decomposition of the oxygen groups on the surface of CNTs.27 However, after being treated with filtered reaction solution with a reaction time of 9 h at 60 °C for 1 h, the CNTs support has a larger mass loss, suggesting that some oxygenated species are adsorbed on the support surface. Compared with the pristine CNTs and the treated CNTs, the 16312

dx.doi.org/10.1021/ie502706r | Ind. Eng. Chem. Res. 2014, 53, 16309−16315

Industrial & Engineering Chemistry Research

Article

Table 2. Oxidation of Glycerol over Pt/CNTs Catalysts in a Base-Free Conditiona product selectivityb (%) Pt catalyst 2.0 3.0 4.0 4.5 5.0 8.0

wt wt wt wt wt wt

% % % % % %

Pt/CNTs Pt/CNTs Pt/CNTs Pt/CNTs Pt/CNTs Pt/CNTs

conversion (%)

GLYA

GLYD

DHA

GLYCA

HA

mass balancec (%)

46.2 49.5 52.4 58.3 63.0 53.0

54.1 47.0 45.5 44.3 37.6 32.4

18.6 23.7 30.4 32.5 36.2 43.5

12.9 10.5 9.3 9.9 8.8 8.6

2.4 2.5 2.5 2.9 2.5 2.2

0.8 1.8 1.0 1.9 0.7 1.0

88.8 85.5 88.7 91.5 85.8 87.7

Reaction conditions: 30 mL of glycerol aqueous solution (0.1 g·mL−1), glycerol/Pt molar ratio 890, T = 60 °C, FO2 = 150 mL·min−1, 6 h, 500 rpm. Calculated as (mol of product in reaction mixture)*(the number of carbon atom in the product)/(mol of glycerol converted × 3) × 100%. Trace amounts of tartronic acid (TA) and oxalic acid (OA) detected in HPLC were not displayed in the table. Abbreviations: GLYA = glyceric acid; GLYD = glyceraldehyde; DHA = dihydroxyacetone; GLYCA = glycolic acid; HA = hydroxypyruvic acid. cDetermined on the basis of detected C2 and C3 products.

a b

Figure 6. Selectivities to different products of glycerol oxidation over different sized Pt/CNTs catalysts in a base-free condition. (a) GLYD, (b) GLYA, and (c) C3 products. Reaction conditions: 30 mL of glycerol aqueous solution (0.1 g·mL−1), glycerol/Pt molar ratio 890, T = 60 °C, FO2 = 150 mL·min−1, 500 rpm.

found that the oxidation rate of GLYD varies with the particle size of Pt/CNTs catalysts. As shown in Figure 7, at the same reaction time, the conversion of GLYD on smaller sized Pt/ CNTs is higher than that on larger one, which further demonstrates that smaller sized Pt/CNTs catalyst favors the oxidation of GLYD, and thus a lower selectivity to GLYD during glycerol oxidation. Compared with GLYD and GLYA,

Pt/CNTs catalysts are compared at the same glycerol conversion. As shown in Figure 6a,b, larger sized Pt/CNTs catalyst enhances the formation of GLYD, whereas smaller sized Pt/CNTs catalyst favors the further oxidation of GLYD to GLYA. As mentioned above, smaller Pt particles have more atoms at vertices and edges, which have strong abilities to adsorb hydroxide, and favor the formation of germinal diol. As a result, on the surface of smaller sized Pt/CNTs catalysts, GLYD is easier to be further oxidized into GLYA, leading to a lower selectivity to GLYD and a higher selectivity to GLYA. The C3 products of glycerol oxidation are potentially valuable chelating agents and useful intermediates in organic synthesis. Figure 6c shows the histograms of the selectivities to total C3 products over different sized Pt/CNTs catalysts as a function of time. The selectivity to total C3 products is found to be insensitive to the Pt particle size. That is, all the catalysts have similar initial selectivities of about 90%, and the selectivities keep constant for about 9 h. After that, they begin to decrease and have values of around 75% at 24 h, which may be due to the oxidative cleavage of the intermediate products.21,30 To understand the possible reaction pathway of glycerol oxidation over Pt/CNTs catalysts in a base-free condition, the oxidation of some intermediate products was also investigated at the reaction conditions. It is found that GLYD and GLYA are not stable under the reaction conditions. The major products of GLYD oxidation are GLYA and HA, while those of GLYA oxidation are HA, GLYCA, and some C1 products. It is also

Figure 7. Oxidation of glyceraldehyde over Pt/CNTs catalysts with different Pt loadings. Reaction conditions: 30 mL of glyceraldehyde aqueous solution (0.05 wt %), glyceraldehyde/Pt molar ratio 15, T = 60 °C, FO2 = 150 mL·min−1, 500 rpm. 16313

dx.doi.org/10.1021/ie502706r | Ind. Eng. Chem. Res. 2014, 53, 16309−16315

Industrial & Engineering Chemistry Research

Article

Scheme 1. Possible Reaction Route of Selective Oxidation of Glycerol in a Base-Free Conditiona

a

Red dash line describes the reaction route that favors on smaller sized Pt/CNTs catalyst, blue dash line describes the reaction route that favors on larger sized Pt/CNTs catalyst, and black solid line describes the reaction route that happens on all Pt/CNTs catalyst.

■

DHA is a little more stable, although it can also be oxidized to HA, GLYCA, and C1 products. Based on these results, a possible reaction route is proposed as shown in Scheme 1.

(1) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering. Chem. Rev. 2006, 106, 4044. (2) Czernik, S.; French, R.; Feik, C.; Feik, C.; Chornet, E. Hydrogen by Catalytic Steam Reforming of Liquid Byproducts from Biomass Thermoconversion Processes. Ind. Eng. Chem. Res. 2002, 41, 4209. (3) Zhou, C. H.; Beltramini, N. J.; Fan, Y. X.; Lu, G. Q. Chemoselective Catalytic Conversion of Glycerol as a Biorenewable Source to Valuable Commodity Chemicals. Chem. Soc. Rev. 2008, 37, 527. (4) Katryniok, B.; Kimura, H.; Skrzyńska, E.; Giradon, J. S.; Fongarland, P.; Capron, M.; Ducoulombier, R.; Mimura, N.; Paul, S.; Dumeignil, F. Selective Catalytic Oxidation of Glycerol: Perspectives for High Value Chemicals. Green Chem. 2011, 13, 1960. (5) Behr, A.; Eilting, J.; Irawadi, K.; Leschinski, J.; Lindner, F. Improved Utilisation of Renewable Resources: New Important Derivatives of Glycerol. Green Chem. 2008, 10, 13. (6) Ketchie, W. C.; Murayama, M.; Davis, R. J. Promotional Effect of Hydroxyl on the Aqueous Phase Oxidation of Carbon Monoxide and Glycerol over Supported Au Catalysts. Top. Catal. 2007, 44, 307. (7) Carrettin, S.; McMorn, P.; McMorn, P.; Griffin, K.; Kiely, C. J.; Johnston, P.; Hutchings, G. J. Oxidation of Glycerol Using Supported Pt, Pd and Au Catalysts. Phys. Chem. Chem. Phys. 2003, 5, 1329. (8) Villa, A.; Veith, G. M.; Prati, L. Selective Oxidation of Glycerol under Acidic Conditions Using Gold Catalysts. Angew. Chem., Int. Ed. 2010, 122, 4601. (9) Tsuji, A.; Rao, K. T. V.; Nishimura, S.; Takagaki, A.; Ebitani, K. Selective Oxidation of Glycerol by Using a Hydrotalcite-Supported Platinum Catalyst under Atmospheric Oxygen Pressure in Water. ChemSusChem 2011, 4, 542. (10) Liu, S. S.; Sun, K. Q.; Xu, B. Q. Specific Selectivity of AuCatalyzed Oxidation of Glycerol and Other C3-polyols in Water without the Presence of a Base. ACS Catal. 2014, 4, 2226. (11) Takagaki, A.; Tsuji, A.; Nishimura, S.; Ebitani, K. Genesis of Catalytically Active Gold Nanoparticles Supported on Hydrotalcite for Base-free Selective Oxidation of Glycerol in Water with Molecular Oxygen. Chem. Lett. 2011, 40, 150. (12) Nie, R. F.; Liang, D.; Shen, L.; Gao, J.; Chen, P.; Hou, Z. Y. Selective Oxidation of Glycerol with Oxygen in Base-Free Solution over MWCNTs Supported PtSb Alloy Nanoparticles. Appl. Catal. B: Environ. 2012, 127, 212. (13) Garcia, R.; Besson, M.; Gallezot, P. Chemoselective Catalytic Oxidation of Glycerol with Air on Platinum Metals. Appl. Catal. A: Gen. 1995, 127, 165. (14) Zope, B. N.; Hibbitts, D. D.; Neurock, M.; Davis, R. J. Reactivity of the Gold/Water Interface during Selective Oxidation Catalysis. Science 2010, 330, 74.

4. CONCLUSIONS Different sized Pt particles were supported on the external surface of closed CNTs, and the size effects of Pt/CNTs catalysts on glycerol oxidation in a base-free condition were investigated. The TOF of glycerol increases with Pt particle size to a maximum at the mean size of 2.5 nm followed by a decrease with a further increase in size. The lower catalytic activity for too small sized Pt/CNTs may be due to stronger adsorption of the intermediates, leading to a decreased reaction rate. At the reaction conditions, the primary hydroxyl of glycerol prefers to be oxidized over Pt/CNTs catalysts, and thus, compared with DHA, GLYD and its oxidation product GLYA are dominantly produced. Moreover, on the surface of smaller sized Pt/CNTs catalysts, GLYD is easier to be further oxidized to GLYA, leading to a lower selectivity to GLYD and a higher selectivity to GLYA. The selectivity of C3 products is insensitive to Pt particle size and reaction time within 9 h, but it decreases because of the oxidative cleavage.

■

ASSOCIATED CONTENT

S Supporting Information *

Figures of the influence of agitation speed and oxygen flow rate on the catalytic activity of 5.0 wt % Pt/CNTs, XPS spectra, and catalytic performance of 5.0 wt % Pt/CNTs. Table of Pt catalysts binding energies and table of the comparison between our results and those reported in literature. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-21-64250937. Fax: +86-21-64253528. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is financially supported by the Shanghai Natural Science Foundation (12ZR1407300) and the 111 Project of Ministry of Education of China (B08021). 16314

dx.doi.org/10.1021/ie502706r | Ind. Eng. Chem. Res. 2014, 53, 16309−16315

Industrial & Engineering Chemistry Research

Article

(15) Zhu, J.; Holmen, A.; Chen, D. Carbon Nanomaterials in Catalysis: Proton Affinity, Chemical and Electronic Properties, and Their Catalytic Consequences. ChemCatChem 2013, 5, 378. (16) de Jong, K. P.; Geus, J. W. Carbon Nanofibers: Catalytic Synthesis and Applications. Catal. Rev. Sci. Eng. 2000, 42, 481. (17) Auer, E.; Freund, A.; Pietsch, J.; Tacke, T. Carbons as Supports for Industrial Precious Metal Catalysts. Appl. Catal. A: Gen. 1998, 173, 259. (18) Serp, P.; Corrias, M.; Kalck, P. Carbon Nanotubes and Nanofibers in Catalysis. Appl. Catal. A: Gen. 2003, 253, 337. (19) Rodrigues, E. G.; Pereira, M. F.; Chen, X. W.; Delgado, J. J.; Ó rfão, J. J. M. Selective Oxidation of Glycerol over Platinum-Based Catalysts Supported on Carbon Nanotubes. Ind. Eng. Chem. Res. 2013, 52, 17390. (20) Liang, D.; Gao, J.; Wang, J. H.; Chen, P.; Hou, Z. Y.; Zheng, X. M. Selective Oxidation of Glycerol in a Base-free Aqueous Solution over Different Sized Pt Catalysts. Catal. Commun. 2009, 10, 1586. (21) Liang, D.; Gao, J.; Wang, J. H.; Chen, P.; Hou, Z. Y.; Zheng, X. M. Selective Oxidation of Glycerol with Oxygen in a Base-free Aqueous Solution over MWNTs Supported Pt Catalysts. Appl. Catal. B: Environ. 2011, 106, 423. (22) Knupp, S. L.; Li, W. Z.; Paschos, O.; Murray, T. M.; Snyder, J.; Haldar, P. The Effect of Experimental Parameters on the Synthesis of Carbon Nanotube/Nanofiber Supported Platinum by Polyol Processing Techniques. Carbon 2008, 46, 1276. (23) Tritsaris, G. A.; Greeley, J.; Rossmeisl, J.; Nørskov, J. K. AtomicScale Modeling of Particle Size Effects for the Oxygen Reduction Reaction on Pt. Catal. Lett. 2011, 141, 909. (24) Shao, M. H.; Peles, A.; Shoemaker, K. Electrocatalysis on Platinum Nanoparticles: Particle Size Effect on Oxygen Reduction Reaction Activity. Nano Lett. 2011, 11, 3714. (25) Mayrhofer, K. J. J.; Blizanac, B. B.; Arenz, M.; Stamenkovic, V. R.; Ross, P. N.; Markovic, N. M. The Impact of Geometric and Surface Electronic Properties of Pt-Catalysts on the Particle Size Effect in Electrocatalysis. J. Phys. Chem. B 2005, 109, 14433. (26) Gamez, A.; Richard, D.; Gallezot, P. Oxygen Reduction on WellDefined Platium Nanoparticles Inside Recast Ionomer. Electroehim. Acta. 1996, 41, 307. (27) Muthuswamy, N.; Chen, D. Towards a Highly-Efficient FuelCell Catalyst: Optimization of Pt Particle Size, Supports and SurfaceOxygen Group Concentration. Phys. Chem. Chem. Phys. 2013, 15, 3803. (28) Chen, W. Y.; Ji, J.; Duan, X. Z.; Qian, G.; Li, P.; Zhou, X. G.; Chen, D.; Yuan, W. K. Unique Reactivity in Pt/CNT Catalyzed Hydrolytic Dehydrogenation of Ammonia Borane. Chem. Commun. 2014, 50, 2142. (29) Besson, M.; Gallezot, P. Selective Oxidation of Alcohols and Aldehydes on Metal Catalysts. Catal. Today 2000, 57, 127. (30) Porta, F.; Prati, L. Selective Oxidation of Glycerol to Sodium Glycerate with Gold-on-Carbon Catalyst: An Insight into Reaction Selectivity. J. Catal. 2004, 224, 397.

16315

dx.doi.org/10.1021/ie502706r | Ind. Eng. Chem. Res. 2014, 53, 16309−16315