Insights into Activated Carbon-Supported Platinum Catalysts for

Dimitratos , N.; Messi , C.; Porta , F.; Prati , L.; Villa , A. Investigation on the Behaviour of Pt(0)/Carbon and Pt(0), Au(0)/Carbon Catalysts Emplo...
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Insights into Activated Carbon-Supported Platinum Catalysts for Base-Free Oxidation of Glycerol Jiaqi Lei,† Hua Dong,† Xuezhi Duan,† Wenyao Chen,† Gang Qian,*,† De Chen,‡ and Xinggui Zhou† †

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: Two kinds of activated carbon (AC)-supported Pt catalysts with similar particle sizes were respectively synthesized using the polyol method and incipient wetness impregnation method, and then tested for glycerol oxidation in a base-free condition. It is revealed that the initial reaction rate on the Pt catalyst prepared by the polyol method (Pt/ACPO) is nearly triple that on the Pt catalyst prepared by incipient wetness impregnation method (Pt/ACIWI), and the Pt/ACIWI catalyst favors further oxidation of glyceraldehyde to glyceric acid and more C−C bond cleavage, which might mainly arise from the difference in relative coverage of oxygen to glycerol caused by the different electronic properties of Pt as well as the difference in the catalyst surface properties. Moreover, the deactivation mechanisms of both catalysts were also investigated. The results show that strongly adsorbed intermediates contribute significantly to the deactivation of the Pt/ACPO catalyst, while both the oxygen poison and the adsorption of intermediates on Pt surfaces cause the deactivation of the Pt/ACIWI catalyst.

1. INTRODUCTION Glycerol is one of the most promising chemical building blocks for the synthesis of value-added chemicals because of its multiple functionalities and ready availability from biosustainable resources.1,2 Its conversion to valuable chemicals can be realized through several routes such as oxidation, hydrogenolysis, and dehydration.3−5 Among them, chemoselective catalytic oxidation of glycerol with air or oxygen to various valuable oxygenated derivatives has attracted much attention for its potential to replace the conventional nonenvironmentally friendly stoichiometric oxidation processes or low productivity fermentation processes.4,5 This route usually occurs in a basic or base-free condition. Compared with the basic condition, a base-free condition benefits the direct formation of free acids, which is favorable to the potential industrial application.6−8 Many monometallic and bimetallic supported catalysts have been tested for the reaction in a base-free condition.6−16 The Pt catalyst was found to be the most active among the monometallic catalysts.10,11,17 Carbon materials, especially activated carbons with large specific surface area and relatively low price, are usually employed as the supports because of their high resistance to acidic environments.18,19 Meanwhile, the extraordinary electrical conductivity of carbon materials may have a significant impact on the electronic properties of Pt nanoparticles.18,20 However, the activated carbon (AC)supported Pt catalyst has shown poor activity in glycerol oxidation under base-free conditions.7,21,22 In view of a practical application, it is highly desirable to prepare an AC-supported Pt catalyst with superior activity for glycerol oxidation. The stability of Pt catalysts is another important issue for its application in glycerol oxidation. As mentioned in the literature, Pt catalysts are susceptible to deactivation.23,24 Several causes for deactivation of Pt catalysts in glycerol oxidation have been © 2015 American Chemical Society

put forward. For hydrotalcite supported Pt catalyst, some products adsorbed on its surface were supposed to be the reason for its deactivation,25 while for N-doped carbon nanotubes (CNTs) supported Pt, oxygen poisoning rather than the adsorption of acid products was thought to be the fatal factor for the deactivation.26 Further probing the origin of the issue will be helpful to improve the stability of Pt catalysts. Catalyst preparation methods have been reported to affect properties of the catalysts, such as the particle size27 and the valence state of the metals,28 and thus ultimately controlling their catalytic activity and durability. Herein, the polyol method27,28 and an incipient wetness impregnation method were separately utilized to synthesize AC-supported Pt catalysts. The obtained similar sized catalysts were evaluated in the liquid-phase glycerol oxidation under a base-free condition. The relationship between catalyst structure and performance was correlated by using multiple techniques, such as N2 physisorption, transmission electron microscopy (TEM), CO chemisorption, and X-ray photoelectron spectroscopy (XPS). Moreover, the abilities of the two catalysts to resist oxygen poisoning were compared, and the possible reason for the deactivation of Pt catalysts in glycerol oxidation was also investigated.

2. EXPERIMENTAL SECTION 2.1. Pt Catalysts Preparation. Polyol method: Pristine AC (Sinopharm Chemical Reagent, containing about 1.2 wt % impurities) was added into an ethylene glycol (Sinopharm Chemical Reagent) solution of H2PtCl6·6H2O (Sinopharm Received: Revised: Accepted: Published: 420

August 20, 2015 December 20, 2015 December 24, 2015 December 24, 2015 DOI: 10.1021/acs.iecr.5b03076 Ind. Eng. Chem. Res. 2016, 55, 420−427

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Industrial & Engineering Chemistry Research

4f5/2 components was set to be equal, and (3) the peak area of the 4f7/2 and 4f5/2 components has been fixed at a 4:3 ratio. Pt 4f7/2 and 4f5/2 binding energies of different Pt oxidation states were obtained from the corresponding positions in the deconvoluted spectra. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) analyses were conducted on an IRIS 1000 instrument (Thermo Elemental, USA). The samples were dissolved by hydrochloric acid and nitric acid with a volume ratio of 3, and diluted with deionized water to a constant volume before detection. Fourier transform infrared (FTIR) spectra were measured with Nicolet instrument (Thermo Scientific, USA) using KBr plates. The spectra were recorded from 4000 to 400 cm−1. The surface acidities of Pt catalysts were measured by NH3temperature programed desorption (NH3-TPD) on an apparatus TP-5080 (Tianjin Xianquan Technology Development Co., Ltd., China). Typically, about 60 mg of sample was first pretreated in He gas flow at 120 °C for 1 h, and then ammonia adsorption was carried out. Physically adsorbed ammonia was removed by purging with He for 2 h, then NH3TPD was performed in a He flow by heating the sample to 800 °C at a rate of 10 °C·min−1. 2.3. Catalyst Evaluation. Glycerol oxidation was carried out in a 100 cm3 three-neck flask. The aqueous solution of glycerol (30 cm3, 0.1 g·cm−3) and an appropriate amount of catalysts were added into the reactor and agitated at 1200 rpm, in which the molar ratio of glycerol/Pt was 890. When the reaction temperature reached 60 °C, O2 (150 cm3·min−1) was bubbled into the suspension. Under these conditions, the effect of mass transport limitations was eliminated (Supporting Information, Figure S1). The quantitative analysis of the reaction mixtures was performed by a high-performance liquid chromatograph (HPLC, Agilent 1100) using the external standard method. The HPLC was equipped with a UV (210 nm) and a refractive index (RI) detector using a BP-OA column (300 mm × 7.8 mm) operating at 80 °C. The eluent was an aqueous solution of H3PO4 (0.001 g·cm−3) with a flow rate of 0.5 cm3·min−1. Glycerol oxidation over the two catalysts after pretreatment with oxygen was also performed. First, oxygen with a flow rate of 150 cm3·min−1 was introduced into the aqueous solution only containing the catalyst for 30 min. Then, a certain amount of the reactant, glycerol, was injected into the solution to initiate the reaction under the standard reaction conditions. Moreover, for the Pt/ACIWI catalyst, after reaction for 240 min, oxygen was replaced by an inert gas (argon) bubbled for another 120 min, and then oxygen was again introduced into the reaction liquid for the reaction to continue.

Chemical Reagent) and stirred for 2 h under nitrogen at room temperature. The resulting suspension was heated under reflux at 130 °C for 3 h after the pH was adjusted to 13 by the addition of 1 M NaOH. The suspension was then cooled to room temperature in nitrogen. After that, the pH of the suspension was adjusted to ca. 3 by the addition of 1 M HCl. Then the suspension was stirred for another 1 h, filtered, and extensively washed with water. The as-obtained Pt catalyst was dried in vacuum at 80 °C overnight, and denoted as Pt/ACPO. Incipient wetness impregnation method: A certain volume of chloroplatinic acid aqueous solution corresponding to the water absorbance of AC (1.1 cm3·gAC−1) was impregnated onto pristine AC. The impregnated sample was first dried in stagnant air at ambient temperature for 12 h, further dried at 80 °C for 12 h in an oven, and subsequently reduced at 250 °C for 3 h in a hydrogen flow (30 cm3·min−1). After reduction, 0.92% (v/v) O2/Ar was used for the passivation of the catalysts at room temperature for 0.5 h. The as-synthesized catalyst was denoted as Pt/ACIWI. 2.2. Catalyst Characterization. Nitrogen adsorption− desorption isotherms were obtained on an ASAP 2020 system (Micromeritics, USA). Before the measurements, the samples were degassed at 120 °C for 6 h. The Brunauer−Emmett− Teller (BET) method was used to calculate the specific surface areas. The total pore volume was estimated from the adsorption branch at a relative pressure (P/P0) of 0.998. The micropore volume was determined according to the t-plot method. Pt dispersion was determined by CO chemisorption using an Autochem 2920 (Micromeritics, USA). A certain amount of sample was in situ reduced in H2/Ar (30 cm3·min−1) at 250 °C for 2 h, and then purged with Ar (30 cm3·min−1) at 280 °C for 30 min. After that, the sample was cooled to 25 °C and CO pulses from a calibrated online sampling valve were injected. The calculation was based on the assumption of a Pt/CO stoichiometric ratio of 1:1. The microstructure and Pt particle size of the two catalysts were characterized by high resolution transmission electron microscopy (HRTEM, JEOL JEM-2100F, Japan) operated at 200 kV. The specimens were prepared by ultrasonically suspending the sample in ethanol and depositing a drop of the suspension onto a grid. The average particle size was calculated by d = ∑nidi/∑ni, where ni is the number of particles with a diameter of di. More than 150 random particles were selected to estimate the average particle size. Thermal gravimetric analysis (TGA) was carried out on a SDT-Q600 thermo balance (TA Instruments, USA). To determine the Pt loadings, TGA experiments were performed in air flow (50 cm3·min−1) at a heating rate of 5 °C·min−1 to 800 °C, while to measure the surface concentration of oxygen groups on Pt catalysts, it was performed under N2 gas flow (50 cm3·min−1) at a heating rate of 10 °C·min−1 to 800 °C. XPS was performed on an XSAM-800 instrument (Kratos, UK) using Al Kα X-ray with 1486.6 eV as the excitation source. The binding energies were calibrated based on the graphite C 1s peak at 284.6 eV. The raw spectra were curve-fitted by the Gaussian−Lorentzian method, after background subtraction using the Shirley method, and deconvoluted into Pt (0), Pt (II), and Pt (IV) species. The deconvolution was performed with several constraints: (1) the energy separation between the 4f7/2 and 4f5/2 components of the Pt (0), Pt (II), and Pt (IV) peaks was restricted, and the energy separation between Pt (0) and Pt (II) and the separation between Pt (II) and Pt (IV) were restricted, (2) the full width at half-maximum of the 4f7/2 and

3. RESULTS AND DISCUSSION 3.1. Influence of Preparation Methods on the Performance of AC-Supported Pt Catalysts. The polyol method and the incipient wetness impregnation method were respectively employed to synthesize AC-supported Pt catalysts in order to probe the effects of preparation methods on the catalytic performance in glycerol oxidation. First, HRTEM and CO chemisorption measurements of the two catalysts (i.e., Pt/ ACPO and Pt/ACIWI) were carried out, and the results are shown in Figure 1 and Table 1. The two catalysts with the same Pt loading are found to have similar average size of Pt nanoparticles observed by HRTEM, and the Pt/ACIWI catalyst has a slightly higher Pt dispersion than the Pt/ACPO catalyst. 421

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Figure 1. Typical HRTEM images of the Pt/ACIWI and Pt/ACPO catalysts.

Figure 2. Pore-size distributions of AC, Pt/ACIWI, and Pt/ACPO catalysts.

These results indicate that for the Pt/ACIWI catalyst, most of Pt particles locate outside the pores of AC. Furthermore, N2 physisorption has been employed to characterize the two catalysts. The results show that the pore-size distribution of the two catalysts is similar as that of AC (Figure 2), which also suggests that most of the Pt particles were deposited on the external surface of AC.29 The similar sized Pt/ACPO and Pt/ACIWI catalysts were tested for glycerol oxidation in a base-free condition. Activated carbon was also investigated at the same conditions and found to be entirely inactive for the reaction. Figure 3a shows the evolution of glycerol conversion over the two catalysts as a function of time. From the slopes of the curves in the initial reaction period, the initial reaction rates can be determined, which are also presented in Figure 3a. Clearly, the initial reaction rate on Pt/ACPO catalyst is nearly triple that on Pt/ ACIWI catalyst. As the Pt/ACPO and the Pt/ACIWI catalysts have similar particle sizes, the different activities of the two catalysts may result from the difference in the electronic properties of Pt nanoparticles. XPS was employed to probe the electronic properties of Pt/ ACPO and Pt/ACIWI catalysts. The Pt 4f XPS spectra of the two catalysts are presented in Figure 3b. It is observed that the Pt/ ACPO catalyst has a higher binding energy of metallic Pt 4f7/2 than the Pt/ACIWI catalyst, which may be due to the different surface chemistries of the catalysts.30,31 From the characterization results of TGA, FTIR, and XPS shown in Figures S2, S3, and S4, it can be revealed that the polyol method in comparison with the incipient wetness impregnation method leads to the catalyst (i.e., Pt/ACPO) with more surface oxygen groups and lower binding energy of O 1s. Correspondingly, the binding energy of Pt 4f7/2 for the Pt/ACPO catalyst has a positive shift, which is beneficial to the enhancement of the initial dehydrogenation rate of the primary OH groups of glycerol,30 the rate-determining step of glycerol oxidation in base-free conditions,11 thus leading to a higher reaction rate. Moreover, compared with the Pt/ACIWI catalyst, the Pt/ACPO catalyst has more surface oxygen groups and thus is more hydrophilic,31,32 which will favor the adsorption of reactants. This may also

contribute to the enhancement of the initial reaction rate of the Pt/ACPO catalyst. Furthermore, Table 2 lists glycerol conversion and product distribution of the two catalysts at a longer reaction time (i.e., 6 h). The Pt/ACPO catalyst is still more active (i.e., higher glycerol conversion) than the Pt/ACIWI catalyst, which is similar to that of the reported Pt/CNTs catalyst.33 On both catalysts, the product distribution is similar. Glyceraldehyde (GLYD), glyceric acid (GLYA), and dihydroxyacetone (DHA) are observed to be the main products. Other products, such as, hydroxypyruvic acid (HA) and glycolic acid (GLYCA) are also detected. Moreover, trace amounts of tartronic acid (TA) and oxalic acid (OA) are detected in the liquid phase, and CO2 is detected in the exit gas. It should be noted that the mass balance, determined by the moles of C2 and C3 products detected by HPLC, is less than 100%, which may be due to the sequential oxidation of C2 and C3 products to C1 products.11 Meanwhile, even at a higher conversion, the selectivity to GLYD over the Pt/ACPO catalyst is higher than that over the Pt/ACIWI catalyst. To compare the selectivity over the two catalysts at the same conversion, the evolution of selectivities to the main products as a function of glycerol conversion is depicted in Figure 4. Clearly, for both catalysts, with the increase of glycerol conversion, the selectivity to GLYD decreases, while that to GLYA increases as a result of more GLYD oxidized to GLYA. However, the relative content of GLYD and GLYA is different on the two catalysts. A higher selectivity to GLYD is apparent on the Pt/ACPO catalyst, whereas a higher selectivity to GLYA rather than GLYD is observed on the Pt/ACIWI catalyst. This is most likely due to the different coverage of oxygen on the surface of the catalysts and the different acid strength of the supports. On the one hand, Pt/ACIWI has a relatively lower binding energy of Pt 4f7/2, which may result in a higher coverage of oxygen,34 and favor the consecutive oxidation of GLYD to GLYA. On the other hand, the acidic sites on the Pt/ ACPO catalyst are stronger than those on the Pt/ACIWI catalyst (Figure S5), leading to a higher selectivity to GLYD over the

Table 1. Characterization of the Support and the Pt Catalysts

a

sample

specific surface area (m2/g)

total pore volume (cm3/g)

micropore volume (cm3/g)

Pt loading (%)a

Pt dispersionb

d (nm)c

AC Pt/ACIWI Pt/ACPO

828 777 736

0.471 0.493 0.429

0.315 0.251 0.259

4.75 (4.68d) 4.74 (4.70d)

0.20 0.16

2.4 ± 0.8 2.6 ± 0.4

Obtained by TGA. bDetermined by CO chemisorption. cEstimated from HRTEM images. dRepresenting the Pt loadings of the used catalysts. 422

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Figure 3. (a) Glycerol conversion as a function of time over the Pt/ACIWI and Pt/ACPO catalysts. Reaction conditions: 30 cm3 glycerol aqueous solution (0.1 g·cm−3), glycerol/Pt molar ratio 890, T = 60 °C, FO2 = 150 cm3·min−1, 1200 rpm. (b) XPS spectra for the Pt 4f regions of the Pt/ACIWI and Pt/ACPO catalysts.

Table 2. Oxidation of Glycerol over Pt Catalysts in a Base-Free Conditiona product selectivityb (%) Pt catalyst

conversion (%)

GLYA

GLYD

DHA

GLYCA

HA

mass balancec (%)

Pt/ACIWI Pt/ACPO

18.8 55.0

30.8 35.3

20.3 28.6

15.7 10.9

1.1 1.6

0.4 2.6

68.3 79.0

Reaction conditions: 30 cm3 glycerol aqueous solution (0.1 g·cm−3), glycerol/Pt molar ratio 890, T = 60 °C, FO2 = 150 cm3·min−1, 6 h, 1200 rpm. Calculated as (mol of product in reaction mixture) × (the number of carbon atom in the product)/(mol of glycerol converted ×3) × 100. Trace tartronic acid (TA) and trace 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 4. Selectivities to (a) GLYD, (b) GLYA, (c) DHA, and (d) C3 products of glycerol oxidation over Pt catalysts in a base-free condition. Reaction conditions: 30 cm3 glycerol aqueous solution (0.1 g·cm−3), glycerol/Pt molar ratio 890, T = 60 °C, FO2 = 150 cm3·min−1, 1200 rpm.

423

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Industrial & Engineering Chemistry Research Pt/ACPO catalyst.35 As to DHA, both catalysts present a relatively low selectivity, which decreases with an increase in glycerol conversion. At the same conversion, DHA selectivity over the Pt/ACIWI catalyst is a little higher than that over the Pt/ACPO catalyst. In addition, the selectivities to C3 products, potentially valuable chelating agents and useful intermediates in organic synthesis, are also shown in Figure 4d. Compared with the Pt/ACPO catalyst, the Pt/ACIWI catalyst showed a lower selectivity to C3 products, indicating that the Pt/ACIWI catalyst facilitates the C−C bond cleavage because of the higher coverage of oxygen and thus the overoxidation of C3 products. 3.2. Deactivation of Pt Catalysts in Base-Free Oxidation of Glycerol. As has been reported, one of the major challenges of glycerol oxidation catalyzed by supported Pt catalysts is the progressive loss of catalyst efficiency with an increase in reaction time.36,37 This is also true for the two catalysts. As shown in Figure 3a, the reaction rates of glycerol oxidation over the two catalysts decrease gradually after a reaction time of 30 min, which may be due to catalyst deactivation or decrease of glycerol concentration. Therefore, the effect of initial glycerol concentration on the oxidation rate was studied, and the typical results are shown in Figure 5. It can

the Pt particle sizes of the two catalysts slightly increased (from 2.4 to 2.6 nm for Pt/ACIWI and 2.6 to 2.7 nm for Pt/ACPO). These results indicate that the leaching or sintering of Pt does not contribute significantly to the observed deactivation. To study the ability of the two catalysts to tolerate oxygen, glycerol oxidation was also performed over the Pt/ACPO and Pt/ACIWI catalysts after pretreatment with oxygen for 30 min. As shown in Figure 7, after the oxygen treatment, the initial oxidation activity of Pt/ACIWI decreases, while that of Pt/ACPO is almost unchanged. This may result from the difference in the relative coverage of oxygen to glycerol on Pt surface. For the Pt/ACIWI catalyst with a lower Pt binding energy of metallic Pt 4f7/2, after the oxygen treatment, more active sites of the catalyst are covered by oxygen, which suppresses the adsorption of glycerol, thus resulting in a decrease in initial reaction rate, while for the Pt/ACPO catalyst with a higher Pt binding energy, the strong adsorption of glycerol results in a relatively higher coverage of glycerol on a Pt surface even in the case of preadsorption of oxygen, and thus an almost unchanged initial reaction rate. This is further verified by the fact that the catalytic activity of Pt/ACIWI can be partially recovered by flowing Ar instead of oxygen. When inert gas is introduced, the surface oxygen over Pt/ACIWI catalyst can be gradually removed, and the active sites expose again. As a result, the activity of the Pt/ACIWI catalyst for glycerol oxidation regenerates. Moreover, the effect of Ar flowing on as-prepared Pt/ACIWI catalyst was also investigated and the result is shown in Figure 7a. By flowing Ar instead of oxygen, the catalytic activity of as-prepared Pt/ACIWI can also be partially recovered. This further confirms the role of surface oxygen concentration in affecting the initial reaction rate of Pt/ACIWI. Furthermore, XPS was employed to probe the electronic properties of the Pt/ACPO and Pt/ACIWI catalysts after glycerol oxidation, and the results are shown in Figure 6c,d and Table 3. As can be seen from Table 3, the binding energies of metallic Pt 4f7/2 for used catalysts shift to a lower level, which may be due to the electronic modification caused by the adsorbed intermediates of glycerol oxidation on the metal surface. The degree of shifting for the used Pt/ACIWI catalyst is less than that for the used Pt/ACPO, suggesting that less intermediates adsorbed on the surface of the Pt/ACIWI catalyst. Moreover, the XPS signal intensity ratio of Pt to carbon (IPt/IC) of the used catalyst was compared with that of the fresh catalyst [i.e., (IPt/ IC)used/(IPt/IC)fresh], which had been used to evaluate the amount of species adsorbed on the catalyst surface.41 As listed in Table 3, the (IPt/IC)used/(IPt/IC)fresh ratio for Pt/ACIWI is larger than that for Pt/ACPO, meaning a lower coverage of the intermediates on the Pt nanoparticles over the used Pt/ACIWI catalyst. All the above results show that Pt leaching and the sintering of Pt nanoparticles are not mainly responsible for the deactivation of the two catalysts. Both the adsorption of oxygen and of intermediates on Pt surfaces lead to the deactivation of the Pt/ACIWI catalyst, while the strong adsorption of intermediates mainly causes the deactivation of the Pt/ACPO catalyst.

Figure 5. Glycerol conversions as a function of time over the Pt/ACPO catalyst with different initial glycerol concentrations. Reaction conditions: 30 cm3 glycerol aqueous solution, glycerol/Pt molar ratio 890, T = 60 °C, FO2 = 150 cm3·min−1, 1200 rpm.

be seen that the glycerol conversion is almost independent of the initial glycerol concentration, suggesting that the observed decrease in the reaction rate should be a result of catalyst deactivation. Previous studies have proposed that the main causes of catalyst deactivation during alcohol oxidation are metal leaching, particle sintering, oxygen poisoning, and poisoning of active sites by strongly adsorbed intermediates.38−40 To determine which factor exhibits the major influence on the deactivation of the two catalysts in glycerol oxidation, the Pt loadings of the used catalysts were first detected by TGA, and the results are listed in Table 1. As can be seen, the loss of active metal is less than 2% for both catalysts. Meanwhile, the filtrates after reaction of 6 h were characterized by ICP-AES, and the results show that the amount of Pt is under the limit of detection. This suggests that the Pt nanoparticles did not leach into the solution under the reaction conditions. The sintering of Pt nanoparticles in the oxidation process was also explored in this work. As shown in Figure 6 panels a and b, after reaction,

4. CONCLUSIONS In summary, we have elucidated the effects of catalyst preparation methods on the performance of AC-supported Pt catalysts in base-free oxidation of glycerol. The polyol method in comparison with the incipient wetness impregnation method leads to the catalyst (i.e., Pt/ACPO) with more surface oxygen 424

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Figure 6. Typical HRTEM images of used Pt catalysts: (a) used Pt/ACIWI catalyst, (b) used Pt/ACPO catalyst. XPS spectra for the Pt 4f regions of used Pt catalysts: (c) used Pt/ACIWI catalyst, (d) used Pt/ACPO catalyst.

Figure 7. Reaction rate as a function of time over Pt catalysts without and after pretreatment with O2: (a) Pt/ACIWI, (b) Pt/ACPO.

nor dissolution of Pt nanoparticles was significant enough to account for the observed deactivation of the two catalysts, whereas the strongly adsorbed intermediates may cause the deactivation of the Pt/ACPO catalyst, and both oxygen poison and intermediates adsorption result in the deactivation of the Pt/ACIWI catalyst. These results demonstrate that the electronic and surface properties of catalysts play an important role in the optimization of highly active Pt catalysts for glycerol oxidation in a base-free condition.

Table 3. XPS Results of Fresh and Used Pt Catalysts 0

Pt 4f7/2 binding energy (eV) catalyst

fresh

useda

ΔEb

(IPt/IC)used/(IPt/IC)fresh

Pt/ACIWI Pt/ACPO

71.35 71.88

71.30 71.65

0.05 0.23

95% 76%

Pt catalyst after reaction time of 6 h. bΔE is the Pt0 4f7/2 binding energy of fresh catalyst minus that of used catalyst.

a

■

groups and higher binding energy of Pt 4f7/2, and thus more hydrophilic and lower oxygen coverage. Correspondingly, the Pt/ACPO catalyst exhibits higher activity and selectivity to glyceraldehyde and less C−C bond cleavage than the Pt/ACIWI catalyst. Their different electronic and surface properties are mainly responsible for the difference in their activities and selectivities. In addition, TEM, TG, and XPS analyses between fresh catalysts and used ones indicate that neither sintering of

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b03076. Figures of the influence of agitation speed on the catalytic activity of Pt/ACIWI catalyst, TGA profiles, and FTIR spectra of pristine AC, Pt/ACIWI, and Pt/ACPO 425

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catalysts, and O 1s XPS spectra and NH3-TPD curves of Pt/ACIWI and Pt/ACPO catalysts (PDF)

AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work is financially supported by the 111 Project of Ministry of Education of China (B08021), the Fundamental Research Funds for the Central Universities (WA1514013), and the Open Project of State Key Laboratory of Chemical Engineering (SKL-Che-15C03).

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