Promoting role of bismuth and antimony on Pt catalysts for the selective oxidation of glycerol to dihydroxyacetone

Journal of Catalysis 335 (2016) 95–104

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Promoting role of bismuth and antimony on Pt catalysts for the selective oxidation of glycerol to dihydroxyacetone Xiaomei Ning a, Yuhang Li a, Hao Yu a,⇑, Feng Peng a, Hongjuan Wang a, Yanhui Yang b,⇑ a b

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore

a r t i c l e

i n f o

Article history: Received 26 October 2015 Revised 22 December 2015 Accepted 24 December 2015

Keywords: Platinum Bismuth Glycerol oxidation 1,3-Dihydroxyacetone Leaching Geometrical effect

a b s t r a c t The group 15 metals, bismuth and antimony, play important roles in promoting noble-metal-catalyzed oxidation reactions toward high value-added chemicals. Herein, we report that the selective oxidation of glycerol to 1,3-dihydroxyacetone (DHA) catalyzed by Pt supported on N-doped carbon nanotubes (Pt/NCNT) can be significantly promoted in the presence of Bi or Sb in reaction solution. This catalyst system showed not only comparable even better performance to the Pt/NCNT with pre-loaded Bi, but also the greatly simplified catalyst preparation. It was found that the Bi-promoted Pt/NCNT underwent dynamic surface reconstruction through leaching and adsorption of Bi adatoms, due to the formation of glyceric acid. By characterizing the adsorption of Bi on Pt catalyst with high-resolution transmission electron microscopy, CO-stripping, horizontal attenuated total reflection infrared spectroscopy and Xray photoelectron spectroscopy, it has been ascertained that Bi preferentially deposits on the step sites of Pt, and then blocks the terrace sites to promote the Pt catalyst mainly through a geometrical effect, which facilitates the activation and transformation of the secondary hydroxyl group of glycerol through the chelation between substrate and Pt–Bi sites. Ó 2016 Elsevier Inc. All rights reserved.

1. Introduction The selective oxidation of alcohols to carbonyl compounds catalyzed by noble metals, employing oxygen as ultimate oxidant, has received growing attentions because of its environmental benignness for the production of high value-added chemicals [1,2]. Promoters, such as Bi [3–5], Pb [6–8], Sb [9], Ag [10], and Te [11] and Sn [12], have significant impact on the activity and selectivity of these catalysts. The selective oxidation of glycerol to 1,3-dihydroxyacetone (DHA), via either a catalytic thermal oxidation [13–16] or an electro-oxidation [17,18], has been extensively studied using Pt or Pd catalysts promoted by bismuth. Recently, the continuous gas-phase oxidation of glycerol to DHA over iron-containing zeolites obtains good performance [19]. As one of the most valueadded derivatives of glycerol, DHA can be utilized in cosmetics and fine chemical industry [20,21]. As early as 1993, Kimura et al. [13] had found that the DHA yield increased from 4% to 20% (DHA selectivity from 10% to 80%) by incorporating bismuth in platinum. Since then, continuous research efforts have been ⇑ Corresponding authors. Fax: +86 20 8711 4916 (H. Yu). Fax: +65 67947553 (Y. Yang). E-mail addresses: [email protected] (H. Yu), [email protected] (Y. Yang). http://dx.doi.org/10.1016/j.jcat.2015.12.020 0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

devoted to optimize the Bi-promoted Pt- or Pd-based catalysts and corresponding reaction conditions [14,22–24]. Additionally, the Pt–Bi or Pd–Bi catalysts also have been widely applied in the oxidation of a wide spectrum of chain alcohols or aromatic alcohols to their corresponding carbonyl products, such as sorbitol [25], 1octanol [26], n-butanol [27], benzyl alcohol [28], cinnamyl alcohol [29], and 1-phenylethanol [30]. However, the essential promoting role of bismuth on Pt or Pd catalysts for the oxidation of alcohols is still under debate. Various interpretations for the role of promoters have been proposed: (i) the formation of a complex among the noble metal, promoter and reactant [17,31,32]; (ii) geometric blocking of active sites [13,28]; (iii) the formation of alloy [9,11,33]; (iv) preventing the noble metals from over-oxidation [28]; (v) inhibiting the corrosion of metal catalysts [11]; and (vi) forming new active centers, such as Bi–OHads [34,35]. In the case of Bi-promoted Pt for the selective oxidation of glycerol toward DHA, Kimura et al. have proposed a geometric effect in which bismuth adatoms functioned as site pffiffiffi pffiffiffi blockers on Pt(1 1 1) with ( 3  3) R30° structure at Bi coverage of 0.33 [13]. The similar steric effect was adopted to explain the promotion of Bi on Pt/Al2O3 for the oxidation of 1-methoxy-2propanol [34] and cinnamyl alcohol [29]. In these early efforts, electronic effect of Bi promoter was considered trivial. However,

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an electronic effect was recently observed in PtBi/C for the electrooxidation of alcohols in alkaline medium, featured by the electronic back donation from Bi to Pt and the shift of oxidation onset potential [4,36]. Nie et al. have reported that the Pt particles were wrapped by a layer of Bi2O2CO3, which suppressed the alloying and interaction of Bi and Pt [9]. Hu et al. [22] have optimized the performance of Pt–Bi catalysts for DHA production, where PtBi alloy was also not formed. However, PtBi alloy had been found to promote the activity for formic acid oxidation [33] and methanol tolerant oxygen-reduction [37]. To enhance the interaction between Pt and promoter, Bi is usually immobilized on Pt or Pd through impregnation [3,9], chemical reduction [22,35], polyol reduction [37,38] and electro-deposition [4,39]. However, Kwon et al. [17] have found that a Bi-saturated solution could enhance the DHA production from glycerol through electro-oxidation on Pt/C electrodes, with a comparable selectivity to the pre-loaded Bi modified Pt/C catalyst. The similar improvement by soluble Bi has been observed over Pd/C for glucose oxidation reaction [31]. These controversial results raise some questions about the nature of Bipromoted Pt catalyst: is it necessary to pre-load Bi on Pt catalysts? To what extent is a strong interaction between noble metal and promoter needed, e.g. alloying? Does geometric or electronic influence dominate the promotion effect? In this work, the catalytic performance and structural properties of Pt–Bi system were systematically studied to understand the promoting role of Bi for Pt, based on which a simple and feasible production of DHA from glycerol may be reached. A comparative study has been carried out between the Pt catalysts with immobilized Bi/Sb and with soluble Bi/Sb in reaction solutions. The catalysts were characterized by a combination of X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), electrochemical method and horizontal attenuated total reflection infrared spectroscopy (HATR-IR) before and after the oxidation reaction of glycerol. The promoting effect of Bi and Sb was elucidated by emphasizing the transfer of promoter via reaction solution, in-situ surface reconstruction and the effect of promoter as adatoms. 2. Experimental 2.1. Materials H2PtCl66H2O (>99%), glycerol (>99%), Bi(NO3)35H2O (>99%), BiCl3 (>99%) and SbCl3 (>99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). DL-glyceraldehyde (GLYAD, 90%), glyceric acid (GA, 20% in water), 1,3-dihydroxyacetone (DHA, 97%), glycolic acid (GLYCA, 99%), oxalic acid dihydrate (OA, 99.5%), tartronic acid (TA, 95%) and Bi2O3 (>99.9%) were purchased from Aladdin, Sigma Aldrich or TCI. All the reagents were used as received. 2.2. Catalyst preparation Nitrogen-doped carbon nanotubes (NCNT) were used as support of Pt, because of the excellent dispersion of Pt over NCNT and attractive interaction between Pt nanoparticles (NPs) and NCNT [40–42]. The NCNT were synthesized by a chemical vapor deposition (CVD) of xylene in NH3 atmosphere over a FeMo/Al2O3 catalyst in a horizontal tubular quartz furnace in 4 cm inner diameter. It has been proved that this NCNT improves the intrinsic activity of Pt compared with conventional CNTs. The details can be found elsewhere [42]. Pt/NCNT catalyst was synthesized by the ethylene glycol (EG) reduction method [43]. First, 100 mg NCNT was added into 60 ml EG and sonicated to obtain a homogeneous suspension. Then, H2-

PtCl6 aqueous solution (0.02 M) was added dropwise to the suspension with stirring. After additional 15 min sonication and continuously stirring for 30 min at room temperature, KOH aqueous solution (0.04 M) was added to adjust pH to 8.5, and then the suspension was refluxed at 140 °C for 2 h. Solid sample was obtained after filtration, rinsing and drying in vacuum at 75 °C for 24 h. Pt nominal loading was controlled at 5 wt.%. For loading Bi, 100 mg NCNT or Pt/NCNT were added to 60 ml EG and sonicated to obtain a homogeneous suspension. Owing to the insolubility of BiCl3 in nonacidic solution, the suspension was adjusted to pH of 1.5 by adding HCl solution. A certain amount of BiCl3 solution (0.02 M Bi3+ in 0.6 M HCl) was then added. After stirring for 2 h at room temperature, Bi was immobilized by the EG reduction as described above. The solids were rinsed and dried in vacuum at 75 °C for 24 h. The resultant catalysts are denoted as PtBiX/NCNT (X = 0.5, 1, 2, 5, 10, 20), where X is the nominal weight percentage of Bi. The Bi/NCNT has the nominal loading for 5 wt.%. In addition to the sequential procedure, a co-reduction procedure was also applied, where Pt and Bi were simultaneously added into the NCNT suspension, followed by the EG reduction. The resulted catalyst is denoted as PtBi5/NCNT(co-reduction). In addition, 100 mg Pt/NCNT stirred at 60 °C in 50 ml 10 wt.% glycerol aqueous solution containing a certain amount of Bi(NO3)35H2O for 6 h. Solid sample was obtained after filtration, rinsing and drying in vacuum at 75 °C for 24 h. The resultant catalysts are denoted as BiX–Pt/NCNT (X = 0.2, 0.5, 1, 5, 10), where X is the nominal weight percentage of Bi. It should be noted that the concentration of Bi (NO3)3 must be well controlled because of the low solubility of Bi (NO3)35H2O in water. We have evaluated the solubility of Bi (NO3)35H2O in water at R.T., which is about 412 mg/L determined by the appearance of milky white turbid liquid. The Bi concentrations used to prepare catalysts were kept lower than its solubility, ca. 202 mg/L for Bi10–Pt/NCNT. Pt–Sb catalysts were prepared by a sequential procedure modified from Nie et al. [9]. 100 mg Pt/NCNT catalyst was added to 60 ml EG and sonicated to obtain a homogeneous suspension. A certain amount of SbCl3 solution (0.02 M Sb3+ in 0.6 M HCl) was then added after adjusting the suspension pH to 1.5. After stirring for 12 h at room temperature, the solid was rinsed and dried in vacuum, followed by reduction with 40% H2 at 400 °C for 3 h. The catalysts are denoted as PtSbX/NCNT (X = 0.5, 1), where X is the nominal weight percentage of Sb.

2.3. Catalyst characterizations Electron probe microanalysis (EPMA, Shimadzu EPMA-1600) was used to analyze the content of Pt. Atomic absorption spectroscopy (AAS, Hitachi Z-5000) was used to detect Bi and Pt content in solution. X-ray photoelectron spectroscopy (XPS) was performed in a Kratos Axis ultra (DLD) spectrometer equipped with an AlKa X-ray source. The binding energies were referenced to the C1s peak at 284.6 eV. The morphology and microstructures of the catalysts were observed in a JEOL JEM2010 microscope operated at 200 kV. Specimens for TEM were prepared by ultrasonically suspending the sample in ethanol and depositing a drop of the suspen
was calculated as sion onto a grid. The average particle size (d) P P nidi3/ nidi2, where ni is the number of particles having a characteristic diameter of di. CO stripping experiments were conducted in a computercontrolled Autolab PGSTAT30 electrochemical analyzer (Eco Chemie B.V., Utrecht, Netherlands), with a three-electrode cell at room temperature to evaluate the coverage of Bi on Pt NPs. The cyclic voltammogram sweeps were performed in 0.1 M KOH solution and recorded from 1.0 V to 0.25 V with the sweep rate of 0.1 V s1. The catalyst on a modified glassy carbon electrode, a Ag/AgCl electrode saturated with KCl and a Pt electrode were used

X. Ning et al. / Journal of Catalysis 335 (2016) 95–104

as the working electrode, reference and counter electrode, respectively. The detailed process can be found in Ref. [42]. The coverage of Bi was calculated by

Coverage ð%Þ ¼ 100 

Q 0CO  mPt=NCNT  wPt=NCNT ; Q COPt=NCNT  m0  w0

where QCO–Pt/NCNT and Q0 CO are the quantity of electric charge for CO oxidation over Pt/NCNT and BiX–Pt/NCNT or PtBiX/NCNT, respectively, mPt/NCNT and m0 are the catalyst weight of Pt/NCNT and BiX–Pt/NCNT or PtBiX/NCNT on the working electrode, and wPt/NCNT and w0 are the mass fraction of Pt. HATR-IR spectroscopy was employed to study the CO adsorption on catalysts in a Nicolet 6700 spectrometer with a trapezoidal ZnSe prism [44]. 4 mg catalyst was firstly suspended in 12 ml cyclohexanol under sonication to form an ink, and then 300 ll of the ink was coated onto the ZnSe prism and dried overnight to form a thin catalyst layer [45]. Before measurements, the catalyst layer was reduced in 10% H2/N2 at 200 °C for 1.5 h. After purged with N2 for 30 min and cooled down to room temperature, a background spectrum was collected. A flow of 10% CO/N2 was then introduced for the CO adsorption and IR spectra were collected at certain temperatures. 2.4. Glycerol oxidation The selective oxidation of glycerol was carried out in a 150 mL four-neck flask. The aqueous solution of glycerol (50 g, 0.1 g/g) and 100 mg PtBiX/NCNT catalyst were added into the reactor with stirring at 600 rpm to minimize the effect of mass transport (see Fig. S1). In the case of Pt/NCNT, a certain amount of Bi additive was added into the reaction solution. Accordingly, the catalysts are denoted as BiX–Pt/NCNT(in-situ) (X = 0.1, 0.2, 0.5, 1, 5, 10), where X is the weight percentage of added Bi based on Pt/NCNT. Once the reactor was stabilized at 60 °C in oil bath, O2 was bubbled into the suspension at 150 Ncm3/min. After a certain time interval, the reaction was stopped quickly by turning off oxygen supplying and taking away from the heater. The reactor containing all catalysts, reactants and products was weighed before and after reaction to calculate the weight gain due to the oxidation reaction. The liquid mixed with catalysts was transferred to a vial and separated by filtration. Products were analyzed using highperformance liquid chromatography (HPLC, Agilent 1260) equipped with an ultraviolet (210 nm) and a refractive index detector in series. An Aminex HPX-87H column (Bio-Rad) was employed for product separation with diluted H2SO4 (0.025 M) as eluent. For the analysis, products were added with 10.0 wt.% H2SO4 to adjust pH. A measuring time of 20 min, a column temperature of 333 K and a flow of 0.6 ml min1 were applied. The identification of the possible products was performed by comparison with authentic samples. For the quantification of the product, the external standard method was employed. The potential effect of Bi3+ ions on HPLC analysis has been investigated. According to a typical reaction result, we made two solutions containing 2 mg/ mL GA with or without 4.81 mg/L Bi3+, and analyzed them by HPLC. There was not any difference for the peak of GA with or without Bi3+, in terms of appearance time, peak shape and area. In addition, the guard column before analytical column in our analysis system could also minimize the interferences caused by ions. 3. Results and discussion 3.1. Catalytic performance of PtBi/NCNT In this work, NCNT were selected as support for Pt NPs based on our previous report demonstrating the improved metal dispersion

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compared with conventional carbon materials as support [42]. The Pt/NCNT catalyst is featured as homogeneous NPs with average diameter of 3.2 nm [42]. As shown in Table 1, the Pt/NCNT catalyst selectively catalyzed the oxidation of glycerol to GLYAD with 60.5% selectivity at 31.5% conversion, while the selectivity to DHA was quite low (11.1%). The pre-loaded Bi significantly elevated the DHA selectivity of PtBi5/NCNT to 55.5%. The initial rate of glycerol over PtBi5/NCNT was improved, suggesting the promoting effect of Bi on activity. In a control experiment, the Bi/NCNT was totally inactive, indicating that Pt is the active site and Bi is a promoter. It was exceptional that, when 100 mg Pt/NCNT and 50 mg Bi/ NCNT were subjected to the oxidation reaction together, not only the initial rate of Pt/NCNT was enhanced, but also the DHA selectivity was significantly improved from 11.1% to 53.3% at 34% conversion, which is almost the same as PtBi5/NCNT. It is indicative that Bi leaching from Bi/NCNT and Bi adsorption on Pt/NCNT plays an essential role. On one hand, there must be considerable Bi transferring from Bi/NCNT to the liquid phase; on the other hand, the similar active sites to that of PtBi/NCNT could be generated via re-adsorption of Bi onto Pt/NCNT. It should be noted that, if the similar performance can be achieved by using such a simple mechanical mixture, the preparation of catalyst might be simplified. This guess was verified by directly adding bismuth salts or oxide into the reaction solution. As shown in Table 1, the additive Bi(NO3)35H2O, BiCl3 and Bi2O3, in an equivalent of 5 wt.% Bi of Pt/ NCNT, resulted in tremendous improvement of the DHA selectivity from 11.1% to 64.1%, 57.5% and 59.2%, respectively. At the same time, the activity remained almost the same in the case of Bi (NO3)35H2O, while loss of activity was observed in the cases of BiCl3 and Bi2O3, probably due to the low solubility. Above results indicate that, regardless of the way how Bi is introduced to the reaction system, the similar enhancement of DHA selectivity can be reached. Among the additive Bi sources, bismuth nitrate offered the highest DHA selectivity. The effect of amount of bismuth nitrate was further investigated by varying Bi (NO3)35H2O concentration from 0.1% to 5% of Pt/NCNT. As shown in Fig. 1, the initial rate of glycerol conversion decreased slightly as 0.1–0.2% Bi were added because of the occupation of Pt surface by Bi; then the rate increased with Bi amount till X = 1, indicating the promoting effect of Bi on activity [9,22]. A significant decrease in activity was observed at high Bi dosing up to 5%, probably due to the serious surface blocking. At the meantime, the DHA selectivity continuously increased from 14% to 78% with increasing Bi(NO3)35H2O amount from 0% to 1%. This result demonstrates that the additive soluble Bi3+ can be used as an effective means for tuning the DHA selectivity in the glycerol oxidation. Fig. 2 compares the time courses of glycerol oxidation catalyzed by BiX–Pt/NCNT (X = 0, 0.5, 1, 5) (in-situ) and PtBiX/NCNT (X = 0.5, 1, 2, 5, see Table S1 for the compositions and surface areas). In the both systems, the activity increased with the amount of Bi from 0.5% to 1%, and then decreased with further increasing Bi content. The activity improvement may be due to the structure modification of Pt by Bi, which changed the accessible contact of Pt active sites. However, too high Bi content would block the active sites resulting in the loss of active sites. Compared to PtBi1/NCNT, the Bi1–Pt/NCNT(in-situ) system displayed the higher activity by about 10 percentages conversion at 6 h. In all the cases, the DHA selectivity declined with reaction time. Although the highest DHA selectivity, 78%, was reached at 0.5 h for Bi1–Pt/NCNT(in-situ), the catalyst showed significant drop of selectivity along with the reaction time. The DHA selectivity of Bi5–Pt/NCNT(in-situ) reached 64% at 29% conversion and 6 h. However, the activity was much lower than Bi1–Pt/NCNT(in-situ). Taking into account the yield of DHA, the Bi1–Pt/NCNT(in-situ) had the better catalytic performance. In the PtBiX/NCNT system, the best performance was achieved at X = 1, where 56% DHA selectivity can be reached at 36% conversion. In

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Table 1 Composition, surface area and catalytic performance of Pt–Bi catalysts in the selective oxidation of glycerol.a Ptb (wt.%)

Catalyst

Pt/NCNT PtBi5/NCNT (co-reduction) PtBi5/NCNT Bi/NCNT Pt/NCNT + Bi/NCNT Pt/NCNT + Bi(NO3)35H2O Pt/NCNT + BiCl3 Pt/NCNT + Bi2O3

3.87 5.08 4.39 0 3.87 3.87 3.87 3.87

Bic (wt.%)

SBET (m2/g)

0 2.19 2.76 2.62 2.62 5.0d 5.0d 5.0d

107.5 91.2 85.7 96.9 – – – –

r0e (mmol/m2 h)

66.2 62 107.4 0 81.4 64.6 56.4 58.8

Conv. (%)

31.5 25.1 30.4 0 34 29.3 29.1 24.8

Selectivity (%) GA

GLYAD

DHA

Othersf

26.7 20.3 18.1 – 20.1 9.6 14.1 11.7

60.5 31.4 24.4 – 24.7 24.8 26.9 26.8

11.3 46.9 55.5 – 53.3 64.4 57.5 59.2

1.5 1.4 2 – 1.9 1.2 1.5 2.3

a

Reaction conditions: 50 g of 10 wt.% glycerol aqueous solution, 0.1 g catalyst, 60 °C, O2 flow at 150 Ncm3/min, 600 r/min, 6 h. Pt contents were determined by EPMA. c Bi contents were determined by EPMA. d Bi of 5.0 wt.% of Pt/NCNT was added. glycerol converted ½mmol e Initial reaction rates, defined as catalyst particle , are calculated at 0.5 h. All the conversions are lower than 13%. The catalyst surface areas are calculated surface area ½m2   time ½h from the average diameter of catalyst NPs by TEM. The catalyst particles are considered as semispheres. f Other products include glycolic acid, oxalic acid and tartronic acid. b

120 80 70

DHA GA GLYAD

80

60 50 40

60 30

Selectivity (%)

Initial rate (mmol m-2 h-1)

100

The conversion of glycerol was higher at high pH because the basic environment favored the deprotonation of the hydroxyl group [47] or benefited the desorption of acid products from the metal [48]. However, the high pH led to the selective production of GA with very low DHA selectivity. The acidic condition was beneficial for the selectivity of DHA, on the cost of lower glycerol conversion. For a good compromise between activity and selectivity, the neutral condition is favored, which is also desired from the viewpoint of environmental benignness.

3.2. Dynamic leaching and adsorption of Bi

20

40

10 20

0 0.0

0.5

1.0

4.8

5.0

X of BiX-Pt/NCNT(in-situ) (wt%) Fig. 1. Dependences of initial reaction rate and selectivity of Pt/NCNT on the amount of added Bi(NO3)35H2O in reaction solution. Reaction conditions: 50 g of 10 wt.% glycerol aqueous solution, 0.1 g catalyst, 60 °C, O2 flow at 150 Ncm3/min, 600 r/min, 0.5 h. The conversion was in the range from 9.1% to 12.5%.

general, the BiX–Pt/NCNT(in-situ) system shows the better performance for the selective oxidation of glycerol toward DHA. The improved DHA selectivity at low Bi ratios was resulted from the complex interplay among Bi promoter and Pt NPs with the reactant, which made the Pt NPs more accessible to oxidize the sechydroxyl of glycerol. With the increase of Bi amount, Bi might selectively block those sites being more active for the further oxidation and transformation of DHA. With a certain amount of Bi, the DHA selectivity would be stable due to the dynamic equilibrium between adsorption and leaching. Hence, the DHA selectivity shows small change with reaction time at high Bi ratios, despite the expense of catalytic activity to some degree. It was the first time to report that Pt catalysts with additive Bi in liquid selectively catalyze the glycerol to DHA. The activity and DHA selectivity were comparable to other reported performance of loaded PtBi catalyst [9,22,46] (see Table S2). Fig. 3 shows the effect of initial pH on the catalytic property. We have investigated the influence of pH on catalyst leaching to exclude the effect of catalyst stability under acidic condition. Our results indicated negligible leaching of Pt and Bi caused by the acidity of solution (see Fig. S2 for details). At acidic and neutral pH, Bi significantly promoted the selective production of DHA.

Above results definitely demonstrate that the Bi-promoted Pt catalyst could be in-situ formed during the reaction, suggesting that the catalyst undergoes leaching and adsorption of Bi in the course of reaction. To understand the formation of active sites, the Bi content in the reaction solution was tracked by AAS. As shown in Fig. 4(a), the evolution of Bi content of Bi1–Pt/NCNT(insitu) system reflects how Bi3+ ions are adsorbed onto Pt/NCNT. In the first two hours, almost all Bi3+ ions were adsorbed by Pt/NCNT, which rationalized the selective oxidation of glycerol toward DHA. After 2 h, the leaching process dominated, evidenced by the increased Bi content in solution. The dependence of Bi adsorption on reaction extent implies that some reaction products might affect the process. It is supported by the fact that no any leaching was detected in the case of inactive Bi/NCNT (Table 1). For PtBi1/ NCNT, although the pre-immobilization of Bi can relieve the leaching, considerable Bi can be detected in solution after 6 h. The effect of reaction products on the adsorption and leaching of Bi was investigated by measuring the content of Bi in 10 g aqueous solutions containing 0.1 g/g glycerol, 0.003 g/g GLYAD, 0.003 g/ g DHA and 0.003 g/g GA, respectively. As shown in Fig. 4(c), 20 mg Pt/NCNT is able to completely adsorb the additive Bi3+ to form the Bi0.5–Pt/NCNT catalyst in the presence of glycerol and GLYAD. In the presence of DHA, ca. 10% additive Bi remained in solution (Fig. 4c). The adsorption was significantly suppressed by GA. 10 mg Bi/NCNT was used to study the leaching behavior. Similarly, GA is the major reason for the leaching of Bi, as shown in Fig. 4(d). A control experiment was carried out to distinguish the influence of solution acidity accompanied with GA on adsorption and leaching of Bi. The same adsorption and leaching experiments as Fig. 4 (c) and (d) were conducted by using a solution without GA but at the same pH of 2.36 controlled by adding hydrochloric acid. The amount of Bi in solution was measured by Atomic Fluorescence spectroscopy. In the adsorption experiment, the Bi content in solution was negligible, indicating a complete adsorption at this pH.

99

(a)

(b)

40

Conversion (%)

X. Ning et al. / Journal of Catalysis 335 (2016) 95–104

30

30

20 Pt/NCNT

10

Bi0.5 -Pt/NCNT(in-situ)

80

Bi1-Pt/NCNT(in-situ)

20

10 PtBi0.5/NCNTs PtBi1/NCNTs

80

DHA selectivity (%)

DHA selectivity (%)

Conversion (%)

40

Bi5-Pt/NCNT(in-situ)

70 60 50

PtBi2/NCNTs PtBi5/NCNTs

70 60 50 40

12 8

30 0

1

2

3

4

5

0

6

1

2

3

4

5

6

Time (h)

Time (h)

Fig. 2. Time courses of the glycerol conversion and DHA selectivity over (a) BiX–Pt/NCNT(in-situ) and (b) PtBiX/NCNT systems. Reaction conditions: 50 g of 10 wt.% glycerol aqueous solution, 0.1 g catalyst, 60 °C, O2 flow at 150 Ncm3/min, 600 r/min.

(b)

(a)

Bi1-Pt/NCNT(in-situ)

Pt/NCNT

80

Conversion or Selectivity (%)

Glycerol DHA GA GLYAD

60

40

20

80

60

40

20

0

2

4

6

8

10

12

14

Glycerol DHA GA GLYAD

80

60

40

20

0

0

0

PtBi1/NCNT

100

Glycerol DHA GA GLYAD

Conversion or Selectivity (%)

100

Conversion or Selectivity (%)

(c)

100

0

2

4

pH

6

8

pH

10

12

14

0

2

4

6

8

10

12

14

pH

Fig. 3. Effect of initial pH value on the glycerol oxidation over (a) Pt/NCNT, (b) Bi1–Pt/NCNT(in-situ) and (c) PtBi1/NCNT. Reaction conditions: 50 g of 10 wt.% glycerol aqueous solution, 0.1 g catalyst, 60 °C, O2 flow at 150 Ncm3/min, 600 r/min, 6 h. The lines are used to guide eyes.

For the leaching experiment, only 0.0079 mg/L Bi was detected, much lower than the case of GA, indicating that the acidic condition has negligible effect on the leaching of Bi. These results suggested that the chelation effect of generated GA with Bi3+ ions, but not acidity, is responsible for the dynamic behavior of Bi adsorption and leaching during reaction. As can be seen in Fig. 4 (a) and (b), the leaching amount of Bi during reaction displays a strong correlation with the formation of GA in the case of PtBi1/ NCNT. The considerable leaching of Bi after 2 h in the case of Bi1–Pt/NCNT(in-situ) can also be explained by the increase of product GA. Similar situation has been observed for Bi0.5–Pt/NCNT(insitu) (Fig. S3). The leaching of Bi in Bi-promoted noble metal catalysts has been found to be dependent upon the composition of the catalyst [31,49] and the reactant solution [31,50]. In this work, it was shown that the leaching might occur during reaction associated

with the formation of GA, probably due to the chelation effect of Bi species with GA, which has carboxylic and a-hydroxy groups [32,49,51]. Then, the re-adsorption of Bi may reconstruct the surface of Pt catalysts, resulting in the in-situ formation of active sites for the selective production of DHA, as will be discussed in Section 3.4. 3.3. Selective oxidation of glycerol over Pt–Sb catalysts Sb has been reported as a promoter of noble metals for the selective oxidation of glycerol [9,12]. In this work, small amount of soluble Sb3+ ions was added to the reaction solution to explore the promoting effect of Sb. As shown in Table 2, the improved activity was observed for PtSbX/NCNT(X = 0.5, 1). Similar to the case of Bi, the initial reaction rate first decreased for Sb0.5–Pt/ NCNT(in-situ), and then maximized at Sb1–Pt/NCNT(in-situ), indi-

X. Ning et al. / Journal of Catalysis 335 (2016) 95–104

(c)

Bi1- Pt/NCNT(in-situ)

20

0.008 15 0.006 0.004

5

0.002

0

0.000

20

PtBi1/NCNT

GA Content (g/g)

(b)

Bi Content (mg/L)

10

0.008

15 0.006 10

0.004

5

0.002

0

0.000 0

1

2

3

4

5

Residual Bi (mg/L)

(a)

10

Initial Bi content

8 6 4 2 0

(d) Bi leaching (mg/L)

100

5 4 3 2 1 0 l D ro A O YA H 2 lyce DH GL G

6

Time (h)

GA

Fig. 4. Variations of concentrations of Bi and GA in reaction solution along with the reaction time catalyzed by (a) Bi1–Pt/NCNT(in-situ) and (b) PtBi1/NCNT. Reaction conditions: 50 g of 10 wt.% glycerol aqueous solution, 0.1 g catalyst, 60 °C, O2 flow at 150 Ncm3/min, 600 r/min. (c) and (d) The influences of major products from glycerol oxidation on Bi adsorption and leaching. For the adsorption experiment (c), 20 mg Pt/NCNT and 0.23 mg Bi(NO3)35H2O were added in 10 g aqueous solutions containing 0.1 g/g glycerol, 0.003 g/g GLYAD, 0.003 g/g DHA and 0.003 g/g GA, respectively. For the leaching experiment (d), 10 mg Bi/NCNT were added in 10 g aqueous solutions containing 0.1 g/g glycerol, 0.003 g/g GLYAD, 0.003 g/g DHA and 0.003 g/g GA, respectively. After stirring for 6 h, Bi contents in the solutions were determined by AAS.

Table 2 Composition, surface area and catalytic performance of Pt–Sb catalysts in the selective oxidation of glycerol.a Catalyst

Pt/NCNT PtSb0.5/NCNT PtSb1/NCNT Sb0.5–Pt/NCNT(in-situ) Sb1–Pt/NCNT(in-situ) Sb5–Pt/NCNT(in-situ)

Ptb (wt.%)

3.87 5.10 5.22 4.60 4.60 4.60

Sbc (wt.%)

0 0.53 1.00 0.5d 1.0d 5.0d

SBET (m2/g)

107.5 112.1 85.7 – – –

r0e (mmol/m2 h)

66.2 56.6 70.4 59.8 83.8 40.2

Conv. (%)

31.5 49.0 50.9 45.8 46.1 32.7

Selectivity (%) GA

GLYAD

DHA

Othersf

26.7 40.5 34.7 28.7 25.7 25.1

60.5 32.6 24.9 33.2 30.8 30.7

11.1 24.9 38.1 37.0 42.5 43.5

1.7 2.0 2.3 1.0 1.0 0.7

a

Reaction conditions: 50 g of 10 wt.% glycerol aqueous solution, 0.1 g catalyst, 60 °C, O2 flow at 150 Ncm3/min, 600 r/min, 6 h. Pt contents were determined by EPMA. c Sb contents were determined by EPMA. d Sb was added according to the certain nominal ratio to Pt/NCNT. glycerol converted ½mmol e Initial reaction rates, defined as catalyst particle , are calculated at 0.5 h. All the conversions are lower than 15%. The catalyst surface areas are calculated surface area ½m2   time ½h from the average diameter of catalyst NPs by TEM. The catalyst particles are considered as semispheres. f Other products include glycolic acid, oxalic acid and tartronic acid. b

cating the promotion effect of Sb on activity. The additive Sb3+ obviously enhances the DHA selectivity. Compared to PtSbX/ NCNT(X = 0.5, 1), the SbX–Pt/NCNT(in-situ) had the higher DHA selectivity and slightly lower conversion. The time course of Sb1– Pt/NCNT(in-situ) (Fig. S4) shows a high DHA selectivity up to 80.5% at 15% conversion, being close to the state-of-art result of selective oxidation of glycerol over noble metal catalysts [10]. Above results suggest that the promotion mechanism might be universal across the group 15 metals, and thus an in-depth study on the formation and structure of active sites is desired, as detailed in the following sub-sections. 3.4. Characterizations of BiX–Pt/NCNT and PtBiX/NCNT XPS, HRTEM, CO-stripping and HATR-IR techniques were used to explore the structure and surface chemistry of BiX–Pt/NCNT and Pt–BiX/NCNT catalysts for understanding the role of promoter Bi. Special attentions have been devoted to reveal the structure of BiX–Pt/NCNT system.

Knowledge on the spatial distribution of Bi and Pt is significant for understanding the adsorption process of Bi. Fig. 5 compares the morphology and elemental distribution of PtBiX/NCNT and BiX–Pt/ NCNT. No matter how the Bi was introduced, homogeneous catalyst particles can be formed on NCNT. For the PtBiX/NCNT, the average particle diameter increased with the content of Bi, from 3.3 nm at X = 1 (Fig. 5a) to 5.4 nm at X = 5 (Fig. 5b), because of the wrapping of Bi and the formation of larger Bi-containing particles. The HRTEM measurement (Fig. 5c) shows the formation of Bi2O3, which covers active Pt sites and results in the low activity at high Bi contents (see Fig. 2b). The formation of Bi2O3 particles may be originated from bismuth hydroxide in basic aqueous solution during the preparation of PtBiX/NCNT by the EG reduction method. For BiX–Pt/NCNT(in-situ), the average diameter and size distribution remain almost the same when X increases from 0 to 1 to 5 before the reaction (Fig. 5e, S5). An obvious increase of particle size was observed with increased reaction duration. After 6 h reaction, the average diameter of catalysts over Bi5–Pt/NCNT increased from 3.1 nm to 3.7 nm. Scanning transmission electron micrographs

X. Ning et al. / Journal of Catalysis 335 (2016) 95–104

101

Fig. 5. TEM images of (a) PtBi1/NCNT, (b) PtBi5/NCNT, (e) Bi5–Pt/NCNT and (f) Bi5–Pt/NCNT(in-situ)–6 h; HRTEM images of (c) PtBi5/NCNT, (g) Bi0.5–Pt/NCNT and (h) Bi5–Pt/ NCNT(in-situ)–6 h; element mapping of (d) PtBi1/NCNT and (i) Bi1–Pt/NCNT.

(STEM) and energy dispersive spectroscopy (EDS) mapping were employed to investigate the spatial distribution of Bi. As shown in Fig. 5(i), S6(b), the distributions of Bi and Pt show a strong spatial relevance, which is indicative of the selective adsorption of Bi on or near Pt NPs, although the adsorption on NCNT cannot be excluded (see Fig. S6 for more details.). HRTEM measurements show amorphous matters on the surfaces of or nearby Pt NPs (see Fig. 5g and h, S6 c and d). Bi can be detected by EDS analysis in these areas, evidencing the adsorption of Bi species. It could be rationalized by the fact that the Bi species are simply adsorbed on Pt particles without any further treatment, such as heating or aging. Thus, crystal structures cannot be formed. For the similar reason, we did not observe any Pt–Bi alloy in TEM. In the case of pre-loaded PtBiX/NCNT, the obvious segregation of Bi and Pt can be observed, as highlighted by the arrows in Fig. 5(d). These results suggest that, compared with the pre-loaded PtBi/NCNT, the Bipromoted Pt catalysts could be more effectively generated by the adsorption of soluble Bi in reaction solution. The CO stripping technique was applied to quantitatively characterize the coverage of Bi on Pt NPs. As shown in Fig. 6, the CO electro-oxidation onset potential and peak potential shift to higher potential by 0.18 V and 0.02 V, respectively, with increasing the content of adsorbed Bi on Pt/NCNT from 0% to 0.5%, because the blocking effect of Bi prevents adsorbed CO from being oxidized [52,53]. Besides, there is a couple of redox peaks at ca. 0.06 V and 0.20 V, which is attributed to the oxidation and reduction of Bi species [38,54]. The coverage of Bi on Pt can be estimated through the area of CO oxidation peak. In our calculation, the coverage of Pt/NCNT is defined as zero. The full layer coverage is hard to be determined because of the possible nonlinear dependence of electrochemical current on adatom coverage [55]. As an estimation, we simply defined the full layer coverage as the point where no CO adsorbed, although there may still exist exposed active Pt sites at this point. Based on this assumption, the coverage of Bi was calculated as 14.1% and 40.6% for Bi0.2–Pt/NCNT and Bi0.5–Pt/ NCNT, respectively. The complete inhibition of CO adsorption was reached at 1% Bi addition. Taking into account the best initial catalytic performance of Bi1–Pt/NCNT(in-situ), it could be concluded that Pt NPs covered by a layer of Bi atoms, probably with

a certain amount of exposed active Pt sites, are the optimized structure of the Bi-promoted Pt catalyst. Besides, it should be mentioned that the surface structure undergoes a continuous renewal because of the dynamic adsorption and leaching during the glycerol oxidation reaction. Therefore, there would update the exposed active Pt sites over Bi1–Pt/NCNT(in-situ). As can be seen in Fig. 6, over the Bi0.5–Pt/NCNT(in-situ) after 0.5 h reaction, a weak H2 desorption peak appears at 0.79 V. It is an evidence for the surface reconstruction of catalyst. The similar Bi coverage was also observed over the pre-loaded PtBiX/NCNT catalysts (Fig. S7). However, the complete coverage was reached at PtBi2/NCNT, indicating the less homogeneity of Bi coverage as revealed by the TEM study. The CO adsorption behavior over BiX–Pt/NCNT was further investigated by HATR-IR spectroscopy to explore the details of the formation of Bi adatoms on Pt NPs. Fig. 7 displays the HATRIR spectra of the BiX–Pt/NCNT catalyst saturated with CO. The signal at ca. 2141 cm1 origins from the residual CO in gas phase [56]. The dispersive asymmetric spectral signal at ca. 2030 cm1 can be assigned as CO linearly bonding to Pt (COL) [57]. The COL band is composed of two components, corresponding to the shoulder on the lower frequency side from CO adsorbed on the step sites of Pt NPs, and the higher frequency side from CO on the terrace sites [57,58]. Because of the dipole–dipole coupling of the vibrational modes of atop CO on step and terrace sites, the CO spectral signal from the two species is not well separated. As increased the content of Bi, the contribution of COL on steps gradually reduced, suggesting that Bi adatoms preferentially occupied the high energy step sites, then the extended faces of Pt NPs. According to literatures [59,60], the selective blocking effect can be attributed to the work function difference between the adatoms and substrate. The Bi adatoms with lower work function than platinum will preferentially deposit on the step sites. Above results agree with an adatom model of Bi-promoted Pt catalysts, which have been proposed by Mallat et al. [29] for Bi– Pt/Al2O3. In this explanation, the promotion effect of Bi is attributed to a geometrical effect [13,29]. In this work, XPS was employed to examine whether there is any electronic effect involved. As shown in Fig. 8(a), three chemical states of Bi, i.e. Bi(0) [33], Bi2O3 [29,33] and BiO(OH) [33,61] are found. The Bi 4f7/2 appearing

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QCO

0.0002

4.0

Gas phase CO

0.0000

Pt/NCNT Pt/NCNT

-0.0002

COL

Coverage (%): 14.1%

Bi0.2-Pt/NCNT

0.0000 -0.0002 0.0002

Bi0.2-Pt/NCNT Coverage (%): 40.6%

Absorbance (

.)

0.0002

3.8

3.6

Bi0.5-Pt/NCNT

3.4

Bi1-Pt/NCNT

0.0000

I (A)

3.2 -0.0002

Bi0.5-Pt/NCNT

Bi5-Pt/NCNT

0.0002 3.0 0.0000 -0.0002

Bi1-Pt/NCNT

2200

2100

2000

1900

Wave number

0.0002

1800

1700

(cm-1 )

Fig. 7. HATR-IR spectra of Pt/NCNT and BiX–Pt/NCNT with pre-absorbed CO.

0.0000 -0.0002

Bi5-Pt/NCNT

0.0002 0.0000 -0.0002

Bi1-Pt/NCNT(in-situ)-0.5h -1.2

-0.9

-0.6

-0.3

0.0

0.3

E (V vs. Ag/AgCl) Fig. 6. Cyclic voltammetric curves of CO electro-oxidation over Pt/NCNT and BiX–Pt/ NCNT. Legend: ———— the tenth cycle of sweep before CO adsorption, — (red) the first and — (green) the second cycle of sweep for CO electro-oxidation.

at ca. 157.7 eV is assigned as the Bi in zero valent state, in the form of mono-layer adatoms [29]. The percentage of Bi(0) decreased with the increasing Bi content, from 36% for Bi0.2–Pt/NCNT to 1% for Bi10–Pt/NCNT (see Table S3). When the Bi content was higher than 1%, BiO(OH) dominated the Bi species. The binding energy (B.E.) of Pt4f has negligible change with X values, suggesting that the adsorbed Bi had no detectable influence on the electronic state of Pt NPs. Therefore, the electronic structure of the Pt–Bi catalysts was unlikely to be the essential reason for the improved DHA selectivity. The weak electronic interaction between Pt and Bi has also been demonstrated in the cases of pre-loaded Pt–Bi catalysts (see Fig. S8). 3.5. Discussion A plausible model is shown in Fig. 9 to elucidate the formation of Bi-promoted Pt catalysts and its promoting role in the selective oxidation of glycerol to DHA. The pristine Pt/NCNT catalyst is active and selective for the production of GLYAD from glycerol. In the presence of soluble Bi3+ in the reaction solution, Bi3+ will be adsorbed onto Pt NPs to firstly occupy the high-energy step sites, and then the terrace sites. On the one hand, the selectivity

may be tuned by blocking the high-energy Pt sites where the adsorption and activation of primary hydroxyl groups or overoxidation and transformation of DHA are preferred. On the other hand, the Bi–Pt sites will be formed on the terrace surfaces of Pt NPs through the geometrical blocking effect. On these sites, the DHA selectivity is improved, probably due to the chelation effect of Bi–Pt catalysts with the secondary hydroxyl group of glycerol [17]. The formation of adatom layer of Bi promoter over Pt NPs might be general in the presence of soluble Bi, being similar to the observations by Herrero et al. [59] and Jones et al. [60] in the cases of Pt and Pd nanocrystal surfaces. In principle, this adsorption behavior might stem from the stereochemical lone pair electrons of Bi3+ [60,62]. Based on this mechanism, it is natural that there is an optimum Pt–Bi ratio as usually reported in literatures [13,22], 1% soluble Bi in our work, because the overwhelming Bi deposition would finally block the terrace sites on Pt NPs. In this work, the promotion of Bi is dominated by the geometrical effect. However, the weak electronic interaction between Pt and Bi could also work to some extent through activating oxygen and suppressing the over-oxidation of Pt NPs [38]. Through the in-situ adsorption of Bi, the comparable even better catalytic performance can be obtained over BiX–Pt/NCNT(insitu), compared with the pre-loaded PtBix/NCNT. The analysis of the Pt/Bi ratio, an important factor influencing catalysis, showed that the Pt/NCNT afforded very close surface Pt/Bi ratios to PtBix/ NCNT, after adsorption in a Bi-containing glycerol aqueous solution (see Fig. S9). Obviously, the introduction of Bi promoter in solution is appealing, because it simplifies the production of Pt catalysts, and makes the promotion effect could be separately controlled by the concentration of promoter, as revealed by Fig. 1. It should be stressed that the soluble Bi can either be introduced before reaction, or be in-situ produced through leaching. In particular, the GA as a by-product of glycerol oxidation plays an important role, which is capable to dissolve or chelate with Bi. Therefore, the adatom model of Bi-promoted Pt catalysts may be valid even for the catalysts with pre-loaded Bi. Furthermore, the dynamic structural evolution of Pt–Bi catalysts during reaction might be

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Pt4f7/2

(a) Pt

0

Pt

(b)

Pt4f5/2

2+

Pt

Bi1-Pt/NCNT

Bi0.2-Pt/NCNT

Bi0.5-Pt/NCNT

Intensity (a.u.)

Bi0.5-Pt/NCNT

Bi4f5/2

BiO(OH)

Bi2O3

Bi0.2-Pt/NCNT

Intensity (a.u.)

Bi4f7/2 Bi(0)

Pt/NCNT

4+

Bi1-Pt/NCNT

Bi5-Pt/NCNT

Bi5-Pt/NCNT

Bi10-Pt/NCNT Bi10-Pt/NCNT

70

72

74

76

78

80

Binding Energy (eV)

158

160

162

164

166

Binding Energy (eV)

Fig. 8. (a) Pt4f and (b) Bi4f XPS spectra of BiX–Pt/NCNT catalysts with varied Bi contents.

Fig. 9. A proposed mechanism for the selective oxidation of glycerol (a) to GLYAD over Pt/NCNT and (b) to DHA in the BiX–Pt/NCNT(in-situ) system. The size of Pt NPs was ca. 3.2 nm.

indispensable for understanding the noble metal catalysts promoted by the group 15 metals, represented by Bi and Sb.

4. Conclusions For the first time, it was found that the soluble Bi species in reaction solution have great impact on the catalytic performance of Pt catalysts for the selective oxidation of glycerol. Through a dynamic adsorption and leaching, depending on the content of GA product, Bi-promoted Pt can be in-situ formed during the oxidation of glycerol, which significantly improves the selectivity of

DHA. This phenomenon has been demonstrated universal either for the group 15 metal salts, or for the catalyst with pre-loaded Bi, which led to a selective production of DHA from glycerol in a more simple and controllable manner. The formation of highly selective Pt–Bi active sites can be rationalized by an adatom model. It was clarified that (i) the promoting role of Bi could mainly be attributed to the geometrical effect caused by the blocking of Pt surface by Bi; (ii) the electronic effect is trivial for the Pt–Bi system; (iii) the formation of alloy is not a prerequisite. This work paved a new way to optimizing the selective production of DHA and to a rational design for the noble metal catalysts promoted by Bi and Sb.

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