Shape-controlled synthesis of Pt particles and their catalytic performances in the n-hexadecane hydroconversion

Catalysis Today 259 (2016) 331–339

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Shape-controlled synthesis of Pt particles and their catalytic performances in the n-hexadecane hydroconversion Yudan Wang a,b,c , Zhichao Tao c , Baoshan Wu a,c,∗ , Huimin Chen a,b,c , Jian Xu a,c , Yong Yang a,c , Yongwang Li a,c,∗ a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China c National Energy Research Center for Coal to Clean Fuels, Synfuels China Co., Ltd., Beijing 101407, PR China b

a r t i c l e

i n f o

Article history: Received 15 January 2015 Received in revised form 10 June 2015 Accepted 26 June 2015 Available online 5 August 2015 Keywords: Shape-controlled Octahedron Cube Pt {111} Pt {100}

a b s t r a c t The platinum nanocrystals with truncated octahedral, spherical and cubic morphologies were synthesized and well dispersed onto the ZSM-22 support, in order to investigate the shape effect of Pt crystals on n-hexadecane hydroconversion. It is found that the crystal facets of Pt nanoparticles play more profound roles in determining the catalytic properties. The reaction test shows that both the conversion of n-hexadecane and selectivity of iso-hydrocarbons are higher for the catalysts with octahedron nanoparticles of Pt that are predominantly enclosed by Pt {111} crystal facets than those with spherical and cubic morphologies of Pt, whose surfaces consist of more Pt {100} facets. Combined with the results of CO-IR and TPHD, it is suggested that the activity and selectivity of the reaction are well correlated to the fractions of exposed Pt {111} crystal facets, which possess more activated surface defect sites and higher amounts of activated hydrogens either on the Pt surface or in the Pt phase. Meanwhile, the catalyst activity and selectivity are found to be highly sensitive to the Pt particle size. Smaller Pt particles have higher activity and lower isomerization selectivity. © 2015 Published by Elsevier B.V.

1. Introduction The catalytic hydroconversion of n-paraffin is an important reaction to improve the quality of diesel and gasoline with high octane number [1–4]. Hydroisomerization of light hydrocarbons can yield high octane fractions for gasoline blending, while hydroisomerization of long-chain alkanes can be employed for improving the low-temperature fluidity of middle distillates (e.g., jet fuels and diesels) or lubrication oils [5–7]. Platinum-supported catalysts, which are commonly employed for hydrotreating of oil fractions, are extensively used [8–11]. The activities and selectivities of a catalyst strongly depend on the shape (morphology) and size (dimension) of Pt crystals, and therefore synthesis of highly active Pt nanostructures of well-controlled shapes and sizes is one of the most attractive goals due to their potential application in heterogeneous catalysts[12–15]. Recently, a variety of methods have been reported for the synthesis of

∗ Corresponding authors at: State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China. Tel.: +86 010 69667788; fax: +86 010 69667635. E-mail addresses: [email protected] (B. Wu), [email protected] (Y. Li). http://dx.doi.org/10.1016/j.cattod.2015.06.017 0920-5861/© 2015 Published by Elsevier B.V.

nano-structured Pt with different sizes and shapes, such as photochemical, pulse-radiolytical, thermal methods, electrodeposition processes, supercritical CO2 deposition, toxic reducing agents, wet chemical, sonochemical, hard and soft template assisted approaches, etc [16–18]. In particular, it was found that the use of linear polymers like polyacrylate solution, poly(N-vinyl2-pyrrolidone), sodium polyacrylate, P123 and polyvinyl alcohol can control the size and shape of the nanoparticles without affecting their inherent catalytic activity [12,19,20]. Reportedly, polymers can stabilize metal nanoparticles through the steric bulk of their framework and also by binding weakly to the nanoparticle surface through heteroatoms that act as ligands [18,21]. Alkane hydrogenolysis rates showed sensitivity to metal nanocrystals size [22–24]. Previous research unambiguously suggested that the structure dependency of reactivity is more important for nanocrystals smaller than 10 nm, in particular, particle size of less than about 2 nm to be crucial for activity [25–27]. However, there is no consensus on the reason for this effect. Some argue that low-coordinatively unsaturated metal atoms presented in small particles are more active, another explanation is electronic effects based on the particle size or combinations of the two [28]. Recently, the formation of Pt with particular morphologies, including cubic,

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tetrahedral, octahedral, hexagonal, polyhedral, spherical, irregular prismatic, icosahedral and cuboctahedral have also been reported. In addition, many investigations presented some close relationship between shapes and the catalytic properties of metal nanocrystals, which were performed on the exposed facets of metal polyhedral nanocrystals. For instance, Somorjai et al. [29,30] found that only cyclohexane were produced on Pt (100) or Pt nanocubes enclosed by {100} planes, whereas both cyclohexane and cyclohexene could be obtained over Pt (111) or cuboctahedrons enclosed by both {111} and {100} planes in Benzene hydrogenation reaction. They also observed that Pt nanocubes enhanced ring-opening ability and thus showed a higher selectivity to n-butylamine as compared to nanopolyhedra for pyrrole hydrogenation [22]. El-Sayed and co-workers [31–35] discovered that the average rate constant in the Suzuki cross-coupling reaction increased exponentially as the percentage of surface atoms at the corners and edges increased. Besides, Lee’s reported [36,37] that the isomerization of trans 2butene to their cis counterparts was promoted by (111) facets of platinum and that such selectivity was reversed on more open surfaces. They also pointed out that the dehydrogenation of cyclohexene was found to be faster on Pt (111) than on Pt (100) single-crystal surfaces and surface-science investigations on the isomerization of unsaturated olefins strongly suggest that selectivity toward the formation of the cis-isomer may be favored by Pt (111) facets [36]. Some works have been carried out to study the catalytic behaviors of different shapes, but there are few reports on what are the factors contributed to the effect. In this study, the Pt nanocrystals with octahedral, spherical and cubic morphologies, which are typically enclosed by equivalent {111} and {100} facets were synthesized and supported on ZSM-22 as the model catalysts. The n-hexadecane hydroconversion performance of catalysts was tested in a fixed-bed reactor. The catalysts were characterized by H2 chemisorption, Transmission Electron Microscope (TEM), FT-IR spectroscopy of adsorbed CO, Temperature-programmed hydride decomposition (TPHD) techniques to investigate the reason for the different isomerization performance caused by shapes. 2. Experimental 2.1. Materials ZSM-22 (Si/Al = 100) used as the support of the catalysts was provided by Shanghai Novel Chemical Technology Co., Ltd., China. K2 PtCl4 (AR) were supplied by Xi’An catalyst chemical Co., Ltd. H2 PtCl6 ·6H2 O (AR) was purchased from Tianjin Guangfu Fine Chemical Research Insititute. Poly (vinyl pyrrolidone) (PVP, Mw = 55,000) were purchased from Sigma–Aldrich. Cetyltrimethyl Ammonium Bromide (CTAB) and NaBH4 were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. All Other chemicals used in this study were of analytical grade (purity > 99%). All the chemical reagents were used without further purification. 2.2. Synthesis of Pt nanocrystals with different shapes Pt truncated octahedral nanoparticles were synthesized according to literature methods [25,38,39]. In detail, 1.5 ml of 0.077 M H2 PtCl6 and PVP were dissolved in 15 mL mixed solution of deionized water and Ethylene glycol with a volume ratio of 1:4 hosted in a three-neck flask equipped with a reflux condenser and a Tefloncoated magnetic stirring bar. The solution was heated to 90 ◦ C and refluxed for 4 h. The products were collected by centrifugation and washed with acetone once and a mixed solution of deionized water and ethanol with a volume ratio of 1:4 several times to remove excess PVP and physically adsorbed chloridion ions, and then dried in air at 60 ◦ C for 10 h. For the Synthesis of Pt nanospheres, an

aqueous H2 PtCl6 solution (0.077 M, 2.5 mL) were mixed with 10 mL of aqueous D-glucose solution (0.02 M) and stirred for 10 min. Then, an aqueous NaBH4 solution was added dropwise under stirring for another 1 h. The product was washed with water and centrifuged to collect the catalyst [16]. In a typical synthesis of Pt nanocubes, CTAB were used because bromide species selectively adsorbed onto Pt (100) crystal faces and induced the formation of Pt nanocubes. Aqueous solutions of CTAB (100 mM) and K2 PtCl4 (1 mM) were dissolved in deionized water at room temperature. The solution was heated at 60 ◦ C for about 5 min until the solution became clear. Freshly prepared NaBH4 (30 mM) used as reducing agent were added dropwise. Finally, the reaction continued for 5 h at 60 ◦ C. The products were collected by centrifugation and washed with ethanol several times [40]. 2.3. Catalysts preparation The catalysts studied in this work were prepared by impregnation of the zeolitic supports with the platinum nanoparticle colloid following a previously reported methodology [41]. The impregnation was carried out by mixing a known amount of support with an adequate amount of the purified nanoparticle colloid suspension in order to obtain a final metallic loading of 0.5 wt.%. The mixtures were vigorously stirred for 12 h using a magnetic stirrer to guarantee similar metal loading and distribution in all the catalysts and then the samples were heat treated at 60 ◦ C to remove the solvent. Finally, the catalysts were washed several times with a cold mixture of H2 O/EtOH (50:50, v/v), dried at 60 ◦ C for 12 h and calcinated in air at 200 ◦ C to remove the stabilising agents [36]. The resulting catalysts were denoted as oct-111, sph-111+100 and cub-100. 2.4. Catalyst characterization TEM measurement was performed with a Tecnai G2 F30 electron microscope operating at 300 kV voltages. The reduced catalysts (in H2 flow at 300 ◦ C for 1 h) were suspended in ethanol with an ultrasonic dispersion for 20 min and deposited on copper grids coated with amorphous carbon films. H2 chemisorption was performed on AutoChem II 2920 equipment (Micromeritics, USA) with a TCD detector at 50 ◦ C. Before the test, the catalysts were in situ reduced by flowing pure H2 (30 ml/min) at 300 ◦ C for 1 h and then purged with Ar for 1 h. After cooled down to 50 ◦ C, several pluses of H2 were injected at regular intervals until saturation with H2 for the sample. FTIR study of CO adsorption was performed on an infrared spectrometer (VERTEX70, Bruker, Germany), equipped with KBr optics working at the liquid nitrogen temperature. The infrared cell with ZnSe windows was connected to a gas-feed system with a set of stainless steel gas lines, allowing the in situ measurement for the adsorption of CO. At first, the catalysts were reduced in H2 flow (20 ml/min) at 300 ◦ C for 1 h. Then the system was cooled down to 25 ◦ C in He flow and pretreated in He flow (20 ml/min) at 25 ◦ C to clean the surface for 1 h. The CO-FTIR spectra were recorded following adsorption of CO at 25 ◦ C and subsequently desorption of CO in He flow at a higher temperature. Temperature-programmed hydride decomposition (TPHD) was used to examine the thermal decomposition of the Pt-H phase that was either adsorbed on or absorbed in the catalyst [42]. A 150 mg of catalyst was reduced in a quartz reactor in a flow of 10% H2 /Ar (20 ml/min) at 300 ◦ C for 2 h. After reduction, the sample was cooled down to room temperature in H2 /Ar flow (20 ml/min) to avoid the thermal decomposition of the Pt-H phase and it was purged with Ar gas (20 ml/min) for 30 min to remove the weakly adsorbed hydrogen. The catalyst temperature was increased from 30 ◦ C to 300 ◦ C at a rate of 5 ◦ C/min under H2 /Ar with a flow rate 20 ml/min. Since the samples had already been reduced, the aim of

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333

Fig. 1. TEM images of the nanoparticles (A) octahedral (B) spherical and (C) cubic.

such experiments was to monitor hydrogen evolution in the process of hydride decomposition. The hydrogen that evolved during this process was monitored using a thermal conductivity detector (TCD). 2.5. Catalyst test n-Hexadecane isomerization was carried out in a stainless steel fixed bed reactor (i.d. 12 mm, length 600 mm). A quantity of 2–3 g catalyst (20–40 mesh) was loaded into the isothermal region of the reactor tube for all reaction tests. Prior to each test, the catalyst was in situ reduced in flowing H2 (100 ml/min) at 300 ◦ C for 2 h at atmospheric pressure. After reduction, n-hexadecane at 0.1 ml/min was pumped continuously into the reactor together with a flow of 100 ml/min co-feed H2 . The standard reaction conditions were 4.0 MPa, H2 /n-hexadecane = 1000:1 (molar ratio), WHSV = 1.0 h−1 and in the temperature range of 260–320 ◦ C. The tail gas was analyzed by a gas chromatography using capillary column (OV-101, 60 m × 0.25 mm) and an FID detector. The liquid products were determined by a gas chromatography with a capillary column (DB-WAX, 30 m × 0.32 mm) and an FID detector. 3. Results and discussion Fig. 1 shows some representative TEM images of the Pt nanocrystals with different shapes. Pt nanoparticles with a preferential rhombus and some small amount of triangles can be easily discerned in Fig. 1A, which correspond to octahedral and tetrahedral shapes [12,14]. In any case, both shapes suggest the presence of a preferential {111} surface structure. Fig. 1B demonstrates clearly some representative images of the Pt spherical shapes. As can be appreciated, the shape of the Pt nanoparticles is predominantly roundness suggesting the coexistence of {100} and {111} Pt surface domains. As can be observed from Fig. 1C, a preferential cubic shape is obtained which suggests the existence of a {100} preferential surface structure [43]. The Pt cubes could be formed when Br ions was introduced [44]. TEM and HRTEM images of Pt/ZSM-22 were given in Fig. 2. The images of Fig. 2 show that the Pt nanoparticles are in good

dispersion on the surface of ZSM-22 support and the shapes of Pt nanoparticles loaded on the support have no obvious change. The lattice spacing of 2.26 A˚ is observed on the HRTEM image (Fig. 2A), corresponding to the (111) planes of Pt, which proved that there were Pt {111} existing on the surface of the catalyst oct-111. The ˚ lattice spacing of Pt nanoparticles in the catalyst cub-100 is 1.96 A, corresponding to the (100) planes of cubic Pt (Fig. 2C). The average sizes of the octahedral, spherical and cubic Pt nanoparticles on the catalysts were statistically 12.25 ± 2.5 nm, 12.77 ± 0.8 nm and 13.54 ± 0.8 nm, respectively in Table 1 and the detailed size distributions are presented in Fig. 2. H2 chemisorption on the catalyst samples was also carried out to determine the particle size (dPt ) (Table 1). It should be noticed that the particle sizes deduced from H2 chemisorption are significantly different from those by the statistic averages of TEM measurements. A plausible reason could be strong interactions between particle and support [45,46]. Besides, particle Pt dispersion (DPt ), specific surface area (SPt ) of the catalyst samples were also measured by H2 chemisorption. It can be seen that the shapes impose prominent effects on the Pt particle structure. The H2 chemisorption result indicates that oct-111 exhibits the highest Pt dispersion (22.32%) and the lowest particle size (4.23 nm), while the catalyst with cubic Pt nanoparticles (cub-100) shows the lowest dispersion (18.43%) and the largest particle size (5.12 nm). Average particle size calculated by TEM of the catalyst samples also confirms the results. It has been shown by simulation that truncated octahedron supported on the ZSM-22 are favored at small sizes, spherical at intermediate sizes, and cubes at large sizes [47]. 3.1. The catalytic performance of Pt nanocrystals with different shapes The conversions of the catalysts in the n-hexadecane hydroisomerization versus temperature are shown in Fig. 3. At all reaction temperatures, the activities of the catalysts show the following order oct-111 > sph-111+100 > cub-100, consistent with the sequence of platinum dispersion on the surface of the catalysts. There is a correlation between platinum dispersion on the surface and the fraction of surface atoms. Generally speaking, higher

Table 1 Dispersion and Pt particle size of the sample catalysts. Cat.

Surface-bound H2 (cm3 /g STP)

Subsurface H2 (mmol/gPt )

DPt (%)a

SPt a (m2 /gPt )

dpt (nm)a

dpt (nm) b

oct-111 sph-111+100 cub-100

0.14 0.12 0.10

5.52 1.83 1.18

22.32 20.40 18.43

55.14 50.40 45.53

4.23 4.63 5.12

12.25 ± 2.5 12.77 ± 0.8 13.54 ± 0.8

a b

Determined by H2 chemisorption. Average Pt particle size calculated by TEM.

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Fig. 2. TEM images and corresponding particle size distribution with an average size (dpt ) of the catalysts (A) oct-111, (B) sph-111+100 and (C) cub-100.

dispersion of platinum particles may stand for more surface atoms even larger amount of defect sites. While the larger the percentage of edge and corner atoms that a nanoparticle has, the more catalytically active it is. So, it needs to be verified that if there is a difference in the fraction of defect sites presented on the surfaces of the catalysts with different shapes. The isomerization selectivity over different catalysts decreases with the fraction of {100} facets increase. As a result, the sample of Pt octahedron supported on ZSM-22 (oct-111) shows apparently

higher isomerization selectivity than the Pt (cube)/ZSM-22 catalyst (cub-100). The selectivity is heavily influenced by the amount of the adsorbed hydrogen atoms present on the surface (surface-bound hydrogen) [48,49]. What is the effect of shapes on the amount of the surface-bound hydrogen and if there are other factors that are also contributors to the reaction performance also need to be further studied. The yields of cracked product fractions distributed according to the carbon numbers at conversion of ca. 30% are depicted in Fig. 4.

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Table 2 The reaction products yield at cat.30% conversion over various catalysts.a

Pt shapes Conversion (%) iC16 Yield (wt.%) iC16 selectivity (%) 7MP-C15 6MP-C15 5MP-C15 4MP-C15 3MP-C15 2MP-C15 Mono-C15 MultiCraC1-15

Yield (wt.%)

C3 /C1 (molar) C3 –C13 (mol%) Mono/Multi (mol/mol) I/C

oct-111

sph-111+100

cub-100

Octahedral 29.82 21.29 71.40

Spherical 34.39 19.41 56.45

Cubes 31.68 16.81 53.05

4.72 2.20 2.28 2.01 3.14 6.32 20.67 0.62 8.53

3.81 2.18 2.40 1.81 3.04 5.59 18.83 0.58 14.98

3.56 2.14 2.10 1.60 2.56 4.32 16.28 0.53 14.87

7.37 95.96 33.34

6.98 98.11 32.47

6.96 97.98 30.72

2.50

1.30

1.13

Reaction conditions: 280–290 ◦ C; H2 /n-C16 = 1000; WHSV = 1.0 h−1 ; P = 4 MPa. M: mono-branched P: n-pentadecane I/C: isomers/cracked products a

For all the samples, the maximum of cracked products are C3 –C13 hydrocarbons and the similar carbon number distribution is slightly asymmetrical centered at C5 over the three catalysts, indicative of secondary cracking and the (s,s) ␤-scission as the main cracking mechanism [50,51]. Besides, a trace of hydrogenolysis reaction may

80 oct-111

70

Selectivity(%)

60

sph-111+100 cub-100

50 40 30 20 10

260 270

280

290

Tem per a

300

310

ture o (C )

0

80

100

) (% ion s r e nv Co

20 320

60

40

Fig. 3. Variation of n-hexadecane conversion and isomerization selectivity with temperature.

Yield (mol/100 mol cracked)

25 oct-111 sph-111+100 cub-100

20

15

10

5

0 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10C11C12C13C14C15

Carbon number Fig. 4. Yield of cracked product fractions per carbon number at the conversion of ca.30% over catalysts.

occur on the platinum metal surface because there are a very small amount of C1 + C2 and C14 + C15 presented in the cracked products. A further observation is the highest C3 /C1 ratio and the lowest amount of linear hydrocarbons (C3 –C13 ) fragments in the cracked products on the oct-111 catalyst (see Table 2). The formation of C3 can also be taken as a measure of the acid function. Another product of the reaction is C1 , a typical product of the hydrogenolysis on the metallic function [52,53]. For metal-acid bifunctional catalysts, the most accepted mechanism is that the reaction begins with the paraffin dehydrogenation on the metallic sites, the olefin so produced is isomerized on the acid sites and the iso-olefin is hydrogenated on the metal sites. The balance between the metal and acid functions is important [54]. Especially, the catalyst oct111, which presents strongest hydrogenolysis activity, the largest acid function and the highest acid/metal ratio, is the most active and selective catalyst. 3.1.1. Effect of defect sites presented on the metal atoms The reactivity of metal nanocatalysts depend strongly on the crystallographic planes exposed on the surface of the particles especially on the defect sites (corners and steps) and can therefore be tuned by controlling the morphology of these particles [55]. From the results of the TEM, it can be seen that the octahedron particles on the catalyst oct-111 are small and have relative sharp edges and corners. It is expected that the fraction of atoms in the small octahedral particles at these sites is probably sizable, making these particles the most catalytically active. The particle sizes of the cubic nanoparticles are the largest and most of its surface atoms are located on their {100} facets, which are reported to be the least active due to their high value of activation energy [33]. The spherical shapes nanoparticles have both the {111} and {100} facets, explaining its intermediate catalytic activity. To quantify the above conclusions, more supporting evidence is given to calculate the fraction of the surface atoms for the different shape particles located on the corners and edges by formulas in Table 3 [56]. The diameter of Pt atoms (dpt atom ) is 0.276 nm. The particle size (dpt ) and the total number of atoms (NT ) involved in one shaped particle follows the relationship: dpt = 1.105 × NT 1/3 × dpt atom . The octahedron, cube, and cubo-octahedron models of the fcc crystal structure are used for the octahedral, cubic, and spherical nanoparticles, respectively. The formulas for the calculation of the total atom number NT , the surface atom number NS , the corner atom number Ncorner , the edge atoms number Nedge , the

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Table 3 Formulas and results to calculate the atom numbers at different positions for each Pt particle as well as the number of Pt particles. oct-111

sph-111+100

cub-100

Models dpt (nm) m

– – –

f.c.c. octahedron 12.25 45.98

f.c.c. cubo-octahedron 12.77 17.30

f.c.c. cube 13.54 28.47

NT

Formula Value

1/3m(2m2 + 1) 64802.86

16m3 − 33m2 + 24m − 6 73410.57

4m3 − 6m2 + 3m 87506.82

NS

Formula NS /NT (%)

4m2 − 8m + 6 12.49

30m2 − 60m + 32 10.86

12m2 − 24m + 14 10.35

Ncorner

Formula Ncorner /NT (%)

6 9.26 × 10−3

24 3.27 × 10−2

8 9.14 × 10−3

Nedge

Formula Nedge /NT (%)

12(m − 2) 0.82

36(m − 2) 0.75

12(m − 2) 0.36

N100

Formula N100 /NT (%)

– –

6(m − 2)2 1.91

6(m − 1)2 + 6(m − 2)2 9.98

N111

Formula N111 /NT (%)

4(m − 3)(m − 2) 11.67

8(3m2 − 9m + 7) 8.17

– –

m: the number of atoms lying on an equivalent edge (corner atoms included).

atom number on the {100} planes N100 and the atom number on the {111} planes N111 can be obtained based on the size of the nanoparticles. From the TEM images, mean diameters were estimated to be 12.25, 12.77, and 13.54 nm for catalysts with different shapes. Octahedral nanoparticles are relative small (12.25 nm) with sharp edges and corners, composed entirely of {111} facets, which comprise 12.49% of the surface atoms, 0.82% of the defect site atoms and 11.67% of the {111} site atoms. The spherical nanoparticles are assumed to follow a cubo-octahedron structured model, formed with {100} and {111} facets with numerous edges and corners. The mean particle sizes of spherical shape are 12.77 nm, the number of atoms on the corners and edges comprise 10.86% of the surface atoms, 0.75% of the defect site atoms, 8.17% of the {111} site atoms and 1.91% of the {100} site atoms. Cubic nanoparticles are larger (13.54 nm) and composed entirely of {100} facets with a smaller fraction of atoms on their edges and corners, which comprise 10.35% of surface atoms, 0.36% of the defect site atoms and 9.98% of the {100} site atoms. From this analysis, if the atoms on the corners and edges (or defects resulting from them) are the dominantly active sites in the catalysis, one would predict that octahedral nanoparticles are the most active and the cubic nanoparticles are the least active, while the spherical nanoparticles are in between. This is consistent with our reaction results. Besides, it can be deduced that {111} site atoms are favor for n-hexadecane hydroconversion than {100} site atoms. The result is in accordance with the result of Somorjai et al. [57]. They also found the flat hexagonal (111) crystal face to be more active than the square and flat (100) surfaces in the isomerization of n-hexane reaction. The fraction of surface sites on the corners and edges of the three types of nanoparticles are also included in Table 3. The surface structure of the catalysts was also examined by analyzing the FT-IR spectra of CO that was adsorbed on the catalyst surface, as shown in Fig. 5. The spectra show three adsorption modes of CO. The peaks observed between 2040 and 2130 cm−1 are assigned to the CO species that are linearly bound to the surface sites include corner, facet and edge sites. As shown in Fig. 5, IR spectra of oct-111 display a strong vibration band at around 2075 cm−1 with a very weak shoulder at 2089 cm−1 , ascribing to linear CO adsorbed on {111} facet and defect Pt sites, respectively. There are three CO vibration bands at around 2071, 2083, and 2100 cm−1 in the IR spectrum of sph-111+100, assigning to linear CO adsorbed on {111} facet atoms, {100} facet atoms and defect Pt sites. However, IR spectra of cub-100 show only one strong shoulder at 2083 cm−1 , belonging to CO adsorbed on {100} facet Pt sites

[58]. Obviously, the relative intensity of the linear CO adsorbed on defect Pt sites is the highest for oct-111, followed by sph-111+100 and cub-100, indicating a larger fraction of the defect sites on the {111} planes, especially on oct-111 planes. It correlates well with the above results and that is also one of the most important reasons for oct-111 exhibiting the best isomerization activity.

3.1.2. Effect of surface-bound hydrogen of Pt and Pt hydride The amount of surface-bound hydrogen can be calculated from the results of H2 chemisorption (Table 1), which demonstrate that Pt oct-111(0.14 cm3 H2 /g) exhibits much higher numbers than Pt sph-111+100 (0.12 cm3 H2 /g) and Pt cub-100 (0.10 cm3 H2 /g), which are in line with the order of isomerization selectivity. However, Pt nanoparticles are known to accommodate two types of activated hydrogens. There are surface-bound hydrogen on the metal surface and subsurface hydrogen, which is absorbed in the metal phase as Pt hydride [43,59,60]. As known, the subsurface hydrogen that emerged toward the metal surface is more energetic than surfacebound hydrogen and participates more actively in hydrogenation and Pt surfaces with low-coordination sites (e.g., steps or corners) are more efficient in activating the subsurface hydrogen than those with high-coordination sites (e.g., terraces) [61]. The Pt oct-111 catalyst with higher amounts of surface hydrogen and more surface defects of low-coordination sites exhibits higher activity and selectivity, as showed in Fig. 3.

oct-111 1950

2000

2050

2100

2150

2200

2250

2300

2050

2100

2150

2200

2250

2300

2050

2100

2150

2200

2250

2300

sph-111+100 1950

2000

cub-100 1950

2000

-1

Wavenumber (cm ) Fig. 5. FT-IR spectra of the CO-absorbed on the surface of the Pt catalysts.

Y. Wang et al. / Catalysis Today 259 (2016) 331–339

337

H2 evolution( A.U.)

cub-100

sph-111+100

oct-111 60

80

100

120

140

160

180

O

Temperature( C) Fig. 6. Temperature programmed hydride decomposition (TPHD) of the catalysts.

Fig. 7. TEM images of the catalysts with small particle size and large particle size with exhibit {111} facet (A) 111-1.2 (B) oct-111.

111-1.2nm

100

Conversion(%)

TPHD experiments were performed to determine whether the structure of the Pt surface with different shapes affected the activation of subsurface hydrogen [60,62]. Fig. 6 shows that the decomposition temperature of Pt hydride is lowest for Pt oct-111, (65.0 ◦ C), while that for Pt sph-111+100 and Pt cub-100 are 66.4 and 67.6 ◦ C, indicating that the catalyst with octahedron nanoparticles that contained {111} surface sites are more effective in hydride decomposition than the catalyst with cubic ones that contained {100} surfaces. The catalysts with the lower Pt hydride decomposition temperature exhibit higher conversion in the reaction. Meanwhile, the peak area of TPHD also correlated well with the activity and selectivity. The amount of subsurface hydrogen of the catalysts was also provided in Table 1, which demonstrate that Pt oct-111 (5.52 mmol H2 /gPt ) exhibits much higher numbers than Pt sph-111+100 (1.83 mmol H2 /gPt ) and Pt cub-100 (1.18 mmol H2 /gPt ). Pt oct-111 exhibits the highest peak area in the TPHD and demonstrates the highest conversion and selectivity. In a word, the amount of subsurface hydrogen and how easily the Pt hydride is decomposed have a greater effect on the hydrogenation reaction. These results suggest that the Pt-oct-111 catalyst with more surface defect sites, higher amounts of surface-bound hydrogen on Pt surface and Pt hydride absorbed in the metal phase exhibits higher activity and selectivity than the Pt-cub-100 catalyst.

80 60

oct-111-10nm

40 20

3.2. The catalytic performance of Pt nanocrystals with different particle sizes

0 250 260 270 280 290 300 310 320 330 340 o

In our previous work [58], we have prepared two catalysts with possessed {111} facets of 1.2 nm particle size (denoted as 1111.2) (see Fig. 7) and exhibited {100} facets of 1.9 nm particle size (denoted as 100-1.9), which were used for comparison with the catalysts of larger particle size. As shown in Fig. 8.and Fig. 9, the catalyst activity and selectivity are found to be highly sensitive to the Pt particle size. The smaller Pt particles have higher activity and lower isomerization selectivity. It is well-known that the change in the fraction of specific active sites (e.g., corner, edge, {111}, or {100} site) with Pt particle size can be determined only when the particle shape is known. It is reported that when the platinum atoms at the facet of the crystallites showed a greater activity than those at the low-coordination sites (e.g., steps or edges), the specific activity will increase with increasing particle size, the reverse holding true when the atoms at the facets were less active than the other ones [63]. It is no doubt that the defect sites are favor for the catalysts activity. Meanwhile, these results also suggested that Pt {111} plane sites of the Pt nanoparticles, not only the defect sites, are all contributors to the isomerization selectivity, confirming the

Selectivity(%)

Temperature ( C)

75

oct-111-10nm

70

111-1.2nm

65 60 55 50 45 40

0

20

40

60

80

100

Conversion(%) Fig. 8. n-Hexadecane (A) conversion and (B) isomerization selectivity over the catalysts of different particle size with possessed Pt {111} facets.

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References

100

Conversion(%)

100-1.9nm

80 60 cub-100-10nm

40 20 0 250 260 270 280 290 300 310 320 330 340 o

Selectivity(%)

Temperature ( C) cub-100-10nm

55 50 45 40 35 30 25 20 15 10 5

100-1.9nm

0

20

40

60

80

100

Conversion(%) Fig. 9. n-Hexadecane (A) conversion and (B) isomerization selectivity over the catalysts of different particle size with possessed Pt {100} facets.

importance of the Pt {111} structure in catalyst design for selective n-paraffin hydroconversion. 4. Conclusion The activity and selectivity of n-hexadecane hydroconversion was investigated using platinum nanocrystals supported catalysts onto the ZSM-22 support with octahedral, spherical and cubic morphologies. The catalyst with octahedral shapes that predominantly expose Pt {111} crystal faces exhibit significantly better catalytic hydrogenation performance than the platinum with spherical or cubic morphologies that possessed the Pt {100} crystal facets as the basal plane for n-hexadecane hydroisomerization. The activity and selectivity decrease with the fraction of {100} facets increase. The superior performance of oct-111 is attributed to the higher fraction of defect sites, which facilitates the activity of Pt-H in the metal phase and more amount of surface-bound hydrogen of Pt. The catalyst activity and selectivity are also found to be highly sensitive to the Pt particle size. The smaller Pt particles have higher activity and lower isomerization selectivity, suggesting Pt {111} plane sites of the Pt nanoparticles, not only the defect sites, may be all contributors to the isomerization selectivity. The insights reported here may pave the way for the rational design of highly active and isomerization selectivity Pt catalysts for hydroconversion. Acknowledgments This work was supported by National Natural Science Foundation of China (No. 21273261), National Basic Research Program of China (No. 2011AA05A205), Chinese Academy of Science and Synfuels China Co., Ltd. We also acknowledge the physical and chemical analysis testing centre in Beijing City.

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