Design of Highly Efficient Pt-SnO2 Hydrogenation Nanocatalysts using

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Design of Highly Efficient Pt-SnO2 Hydrogenation Nanocatalysts Using Pt@Sn Core-shell Nanoparticles Miaomiao Liu, Weiqiang Tang, Zhaohui Xie, Hongbo Yu, Hongfeng Yin, Yisheng Xu, Shuangliang Zhao, and Shenghu Zhou ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03109 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 21, 2017

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Design of Highly Efficient Pt-SnO2 Hydrogenation Nanocatalysts Using Pt@Sn Core-Shell Nanoparticles Miaomiao Liu,†, ‡ Weiqiang Tang,† Zhaohui Xie,‡ Hongbo Yu,†, ‡ Hongfeng Yin,‡ Yisheng Xu,†, * Shuangliang Zhao,†, * and Shenghu Zhou†, * †

State Key Laboratory of Chemical Engineering, and Shanghai Key Laboratory of

Multiphase Materials Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P. R. China ‡

Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, 1219 Zhongguan West Road, Ningbo, Zhejiang 315201, P. R. China

*Corresponding author Fax: (+86) 21-64253159 E-mail: [email protected]; [email protected]; [email protected]

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Abstract In this work, Pt-SnO2 heteroaggregate nanocatalysts were synthesized by in-situ transformation of Pt@Sn core-shell nanoparticles and their catalytic performance for hydrogenation of various substituted nitroaromatics were investigated. The Pt@Sn nanoparticles were prepared by one-step method, and the alumina supported Pt@Sn nanoparticles were further in-situ transformed into Pt-SnO2 heteroaggregate nanostructures by calcination. The structures of Pt@Sn and Pt-SnO2 nanomaterials were characterized, and FT-IR with CO probes, HRTEM, XRD and XPS characterizations revealed that the as-synthesized Pt@Sn nanoparticles were core@shell like structures with Sn-rich shells and Pt-rich cores, and the obtained Pt-SnO2 heteroaggregate nanostructures consisted of close contact pure Pt and SnO2 phases. The Pt-SnO2/Al2O3 nanostructures demonstrated a better catalytic performance for hydrogenation of various substituted nitroaromatics relative to individual Pt/Al2O3 nanocatalysts. Theoretical calculations suggested that Pt-SnO2 nanocatalysts can slightly facilitate the adsorption of H2 and o-chloronitrobenzene, and strongly weaken the binding of Pt/o-chloroaniline, resulting in more available reactants and easier release of products from the catalyst surfaces. The theoretical calculations indicated that the enhanced catalytic performance may originate from the cooperative interaction between Pt and SnO2.

Keywords: Nanocatalysts; Pt-SnO2; hydrogenation; core@shell; nitroaromatics


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1.

Introduction Morphology-controlled bicomponent nanostructures1-8 have been applied in

many research fields, such as catalysis,9-11 gas storage and separation,12,13 electrochemistry,14,15 and biological sciences.16,17 For example, the well-controlled Ru@Pt core-shell nanostructures illustrated an enhanced catalytic performance for preferential CO oxidation relative to monometallic analogues,18 and Pt3Sn alloy nanoparticles (NPs) showed an enhanced activity for electro-oxidation of CO and methanol.19 The origin of performance enhancement of the aforementioned nanosystems is believed to be the metal-metal interactions,20-22 which adjusts the electronic effect of active metals and further facilitates the adsorption of reactants and the formation of the desired products.18,23,24 Among bicomponent nanostructures, nanostructures containing metals and metal oxides have close contact interfaces between metals and metal oxides, where many catalytic reactions can take place. The strong interactions between active metals and metal oxide supports, which have been found in traditional catalyst systems, illustrated enhanced catalytic performance in some reactions.25 Their nanostructure analogues such as Au-Fe3O4 dumbbell NPs and Au-NiO heteroaggregate nanostructures also demonstrated enhanced CO oxidation activity relative to those systems without cooperative metal-metal oxide interaction.26,27 The occurrence of strong metal-metal oxide interaction in traditional catalysts usually requires reducible metal oxide supports including CeO2,28 ZrO229 and Fe2O3,30 and sometimes also requires special treatments such as high temperature H2 treatment,31 and this limits the use of readily available silica or alumina supports in this type of catalysts. To obtain silica or alumina supported catalysts with cooperative


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metal-metal oxide interaction, a generic method is to use bimetallic NPs as precursors and carry out in situ transformation of the bimetallic NPs into metal-metal oxide heteroaggregate nanostructures on alumina or silica supports.32-34 By precisely controlling the structure and composition of bimetallic NP precursors, well-regulated metal-metal oxide heteroaggregate nanostructures can be obtained. We herein reported the alumina supported Pt-SnO2 heteroaggregate nanocatlysts prepared by using in-situ transformation of Pt@Sn core-shell NPs for hydrogenation of various substituted nitroaromatics. The 2~3 nm Pt@Sn NPs were obtained by reduction of Pt and Sn precursors. Due to the easier reduction of Pt precursors, the as-synthesized Pt@Sn NPs has a core@shell structure with Pt-rich cores and Sn-rich shells, which further facilitates the in-situ transformation of Pt@Sn NPs into 3~4 nm Pt-SnO2 heteroaggregate NPs. Scheme 1 presents the synthetic procedure of Pt-SnO2/Al2O3 nanocatalysts. The selection of SnO2 is based on not only the formation of catalytically active metal-metal oxide interfaces but also the fact that the Lewis acid SnO2 can enhance the catalytic hydrogenation or dehydrogenation activity.35-38 Various substituted nitroaromatics, such as chloronitrobenzene (CNB), nitrotoluene (NT), nitrophenol (NP), nitroacetophenone (NAP), and nitrobenzaldehyde (NBA) were chosen to investigate the catalytic hydrogenation performance of Pt-SnO2/Al2O3 at 45 °C and atmospheric H2 pressure. The results showed that the Pt-SnO2/Al2O3 catalysts illustrated an enhanced catalytic performance for the aforementioned substituted nitroaromatics relative to individual Pt/Al2O3 catalysts. Theoretical calculations upon density function theory (DFT) suggested that the Pt-SnO2 interaction slightly increased the adsorption of H2 and o-chloronitrobenzene, and strongly weakened the binding of Pt/o-chloroaniline, indicating more available


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reactants and easier release of products from catalyst surfaces. It is concluded that the enhanced catalyst performance of Pt-SnO2/Al2O3 originates from the well-controlled close-contact Pt-SnO2 heteroaggragate nanostructures.

Scheme 1. Synthesis of Pt-SnO2/Al2O3 nanocatalysts and the demonstration of catalytic hydrogenation of substituted nitroaromatics using o-CNB hydrogenation reaction as an example. 2. Experimental Section 2.1. Chemicals Sn (II) 2-ethylhexanote, chloroplatinic acid (IV) (H2PtCl6), sodium borohydride (NaBH4),

p-chloronitrobenzene

(p-CNB),

m-chloronitrobenzene

(m-CNB),

o-chloronitrobenzene (o-CNB), p-nitrotoluene (p-NT), m-nitrotoluene (m-NT), and o-nitrotoluene

(o-NT),

p-nitrophenol

(p-NP),

p-nitroacetophenone

(p-NAP),

p-nitrobenzaldehyde (p-NBA), o-nitrobenzaldehyde (o-NBA) were purchased from Aladdin. Polyvinylpyrrolidone (PVP-K30, GR), ethylene glycol (EG, AR), absolute ethyl alcohol (AR), and acetone (AR) were purchased from Sinopharm Chemical Reagent Co., Ltd.. Aluminum oxide powders calcined at 500 oC for 3 hours were purchased from Qingdao Haiyang Chemical Co., Ltd.. All of the reagents were used as


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received without further purification. 2.2. Catalyst Preparation

Synthesis of Pt@Sn Core-shell Nanoparticles. Pt@Sn NPs were synthesized by reduction of Pt and Sn precursors in one-step. 0.075 mmol of Sn (II) 2-ethylhexanote, 0.075 mmol of chloroplatinic acid (IV) (H2PtCl6), 300.0 mg of PVP-K30 and 20.0 mL of EG were transferred into a 100 mL three-necked round-bottomed flask. The mixture was further heated to 50 °C with vigorous magnetic stirring, and then 1.5 mmol NaBH4 in 2.0 ml absolute ethyl alcohol was injected to the flask. The yellow brown solution was instantly changed to dark colloid, indicating the occurrence of reduction reaction. The resultant mixture was maintained at 50 °C for 15 minutes under an N2 atmosphere. The mixture was cooled down to room temperature and followed by centrifugation with acetone and ethyl alcohol two times to obtain black solid for further characterization.

Synthesis of Pt Nanoparticles. The synthetic procedure of Pt NPs was similar to that of Pt@Sn NPs. 0.075 mmol of chloroplatinic acid (IV) (H2PtCl6), 150.0 mg of PVP-K30 and 20.0 mL of EG were first transferred into a 100 mL three-necked round-bottomed flask, and then the mixture was heated to 50 °C with vigorous magnetic stirring. After that, 0.75 mmol NaBH4 in 2.0 ml absolute ethyl alcohol was injected to the flask, and the yellow solution was instantly changed to dark colloid, suggesting the reduction of Pt precursors. The resultant mixture was maintained at 50 °C for 15 minutes under an N2 atmosphere. The mixture was further cooled down to room temperature followed by centrifugation with acetone and ethyl alcohol two times to obtain the black solid for further characterization.

Synthesis of Pt-SnO2/Al2O3 Nanocatalysts. Pt-SnO2/Al2O3 nanocatalysts were


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synthesized through calcining the Al2O3 supported Pt@Sn NPs. The calculated amount of as-synthesized Pt@Sn colloid containing 0.075 mmol Pt and 0.075 mmol Sn were centrifuged with acetone to collect Pt@Sn black powdery solid. The obtained solid was further re-dispersed in absolute ethanol in a 250 mL three-necked round-bottomed flask, and a calculated amount (Pt loading--0.5 wt %) of alumina was added into the Pt@Sn ethanol colloid.

The resultant mixture was further heated to 50 °C and

purged with N2 to remove ethanol to obtain the grey Pt@Sn/Al2O3 materials. The Pt@Sn/Al2O3 was then calcined at 500 °C in air for 3 hours to obtain Pt-SnO2/Al2O3 heteroaggregate nanocatalysts.

Synthesis of Pt/Al2O3 Nanocatalysts. The calculated amount of as-synthesized Pt colloid containing 0.075 mmol Pt were centrifuged with acetone to collect Pt black powdery solid. The obtained solid was further re-dispersed in absolute ethanol in a 250 mL three-necked round-bottomed flask, and a calculated amount (Pt loading--0.5 wt %) of alumina was added into the Pt ethanol colloid. The resultant mixture was further heated to 50 °C and purged with N2 to remove ethanol to obtain the grey Pt/Al2O3 materials, which was then calcined at 500 °C in air for 3 hours to obtain Pt/Al2O3 nanocatalysts. 2.3. Catalyst Characterizations X-ray diffraction (XRD) patterns of Pt@Sn and Pt-SnO2 nanomaterials were obtained using Bruker D8 Advance X-ray diffractometer with Cu Kα radiation in the 2θ range from 20° to 90°. Transmission electron microscopy (TEM) images with energy-dispersive spectroscopy (EDS) were obtained by using a JEOL 2100 transmission electron microscope operated at 200 kV. The high angle annular dark field scanning transition electron microscopy (HAADF-STEM) images were obtained using


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a Titan G2 80-200 ChemiSTEM, which is operated at 200 kV with 0.08 nm resolution for STEM images. X-ray photoelectron spectra (XPS) were taken on AXIS ULTRA DLD multifunctional X-ray photoelectron spectroscope with an Al source, and the data was calibrated by C 1s (284.8 eV) and processed using Casa XPS software. Temperature programmed oxidation (TPO) of Pt@Sn/Al2O3 was carried out on AutoChemII 2920. The product analyses of catalytic hydrogenation of various substituted nitroaromatics over Pt-SnO2/Al2O3 nanocatalysts were performed by gas chromatograph GC 2060 equipped with a flame ionization detector. The diffuse reflectance flourier transform infrared spectra (DRIFT-IR) with CO probes were carried out on a Nicolet-6700 Fourier transform infrared spectrometer. The Pt-SnO2/Al2O3, Pt@Sn/Al2O3 and Pt/Al2O3 nanomaterials were pretreated by Ar purging at room temperature for 30 minutes. The nanomaterials were then treated with pure CO at a gas flow rate of 20 ml/min at room temperature for 30 minutes, and then purged with Ar to remove the free CO before the DRIFT-IR spectra were recorded. 2.4. Catalysts Activity Measurements

Hydrogenation of Various Substituted Nitroaromatics with H2 at 45 °C and Ambient H2 Pressure. Hydrogenation of p-CNB, m-CNB, o-CNB, p-NBA, o-NBA, p-NAP, p-NP, p-NT, m-NT, and o-NT with H2 was carried out at 45 °C and atmospheric H2 pressure. Vigorous magnetic stirring was used to get rid of the influence of stirring rate on the reaction rate. A three-necked round-bottomed flask was charged with 0.100 g of supported nanocatalysts, 25.0 mL of absolute ethanol, and 1.0 g of various substituted nitroaromatics. The resultant mixture was heated up to and then maintained at 45 oC under H2 atmosphere for the defined reaction time. The liquid products were collected by centrifugation to remove the solid catalysts, and analyzed by a gas


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chromatograph equipped with a flame ionization detector. For cycle-to-cycle catalytic reactions, the solid catalysts were collected after each cycle by centrifugation and washed by ethyl alcohol, which was followed by drying in an oven at 60 °C overnight. Due to the loss of catalysts during the recovery process, the weights of reactants (substituted nitroaromatics) and solvents (absolute ethanol) were proportional to the weight of recovered catalysts in the subsequent cycle to keep the ratio of reactants/catalysts and reactants/solvents constant. 3.

Results and Discussion

3.1. Synthesis and Characterization of Pt-SnO2/Al2O3 catalysts Pt@Sn bimetallic NPs were synthesized by reduction of Sn (II) 2-ethylhexanote and H2PtCl6 using NaBH4 as reduction agents. Because Pt precursors were more easily reduced than Sn precursors, the Pt precursors were first reduced to form the Pt cores and followed by reduction of Sn precursors to form the Pt@Sn core-shell structures. Fig. 1a, 1b and 1c illustrated the XRD patterns of individual Pt, as-synthesized Pt@Sn and Pt-SnO2 NPs obtained by calcination of Pt@Sn NPs, respectively. As shown in Fig. 1b, the nearly pure Pt diffractions were observed and Sn diffractions were invisible for Pt@Sn NPs, suggesting a structure of Pt cores covered by amorphous Sn. The clear Pt and SnO2 diffractions for Pt-SnO2 NPs in Fig. 1c are consistent with the presence of Sn in the Pt@Sn NPs although the Sn diffractions were not observed in Fig. 1b. The Pt@Sn core-shell structures were further confirmed by XPS, DRIFT-IR and TEM studies, and will be discussed later.


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Pt(111) Pt(200)

Pt(220)

Pt(311)

a)

Pt(111)

intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Pt(111)

b) SnO2(101) SnO2(110)

Pt(200) SnO2(211)

Pt(220)

Pt(311)

c)

Pt(PDF#04-0802)

SnO (PDF#41-1445) 2

20

40

60

80

2θ/° Fig. 1. XRD patterns showing a), Pt NPs; b), Pt@Sn NPs; c), Pt-SnO2 NPs obtained by calcination of Pt@Sn NPs at 500 oC for 3 hours. The TEM studies of Pt NPs, Pt@Sn NPs, Pt /Al2O3 and Pt-SnO2/Al2O3 were shown in Fig. 2a, 2b, 2c and 2d, respectively, and their size analyses were presented in Fig. 3a, 3b, 3c and 3d, respectively. As shown in Fig. 2a and 3a, the Pt NPs demonstrated a spherical shape with an average particle size of 2.1 nm. The Pt@Sn NPs in Fig. 2b were spherical with an average particle size of 2.3 nm (Fig. 3b). Moreover, the insert in Fig. 2b only showed a lattice of 0.233 nm, which is consistent with the Pt (111) lattice of 0.227 nm. Furthermore, the lattices of PtSn alloy and individual Sn were not observed in Fig. 2b, indicating that the Pt of Pt@Sn is nearly phase-pure and the Sn in the Pt@Sn is amorphous. The average particle size of Pt in Pt/Al2O3 in Fig. 2c was 3.9 nm (Fig. 3c), suggesting a slight increase of particle size after removing the stabilizing agents by calcination. The insert in Fig. 2d showed the Pt (111) lattice of 0.232 nm for Pt-SnO2/Al2O3, suggesting the presence of phase-pure


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Pt in heteroaggregate nanostructures. The similar but smaller particle size increase was observed for Pt-SnO2/Al2O3 in Fig. 2d and 3d, indicating the Pt-SnO2 interaction can decrease the agglomeration of particles.

Fig.2. TEM images showing a), Pt NPs; b), Pt@Sn NPs; c), Pt/Al2O3; d), Pt-SnO2/Al2O3; e), HAADF-STEM image of one Pt@Sn nanoparticles showing the Pt (200) lattice spacing (0.195 nm); and f), HAADF-STEM image of one Pt@Sn nanoparticle showing the Pt (111) lattice spacing (0.227 nm). The inserts of a), b), c) and d) show enlarged TEM images of individual nanoparticle. Scale bars in a−d are 10 nm; scale bars in the insets of a), b), c) and d) are 2 nm; scale bars in e) and d) are 1 nm and 2 nm, respectively. To further confirm the core-shell structure of Pt@Sn NPs, HAADF-STEM images were obtained. As shown in Figure 2e and 2f, the Pt@Sn NPs illustrated a core-shell structure, which consists of crystallized bright cores and amorphous dark shells. According to the dark field mode, the crystallized bright cores were assigned to


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Pt cores (heavy atoms), and the amorphous dark shells were assigned to Sn shells (relatively light atoms), where the crystallized Pt and amorphous Sn are consistent with the XRD pattern in Figure 1b. Moreover, due to the extremely high resolution (0.08 nm for STEM mode) with Titan G2 80-200 ChemiSTEM, the individual Pt atoms in Pt cores can be clearly differentiated in Figure 2e and 2f. The lattices of 0.195 nm and 0.227 nm were observed in the cores in Figure 2e and 2f, which is consistent with the Pt (200) and (111) lattice spacing, respectively, further confirming the core-shell structures of Pt cores and Sn shells. More HAADF-STEM images of Pt@Sn NPs were shown in Figure S1 in SI.

a)

b)

2.1+0.3nm

2.3+0.3nm

0.4

0.4

Relative Frequency

Relative Frequency

0.5

0.3 0.2 0.1

0.3

0.2

0.1

0.0

0.0 1.0

1.5

2.0

2.5

1.5

3.0

2.0

Particle Size (nm)

c)

2.5

3.0

Particle Size (nm)

3.9+0.6nm

0.5

Relative Frequency

0.4

Relative Frequency

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.3

0.2

0.1

d)

3.2+0.4nm

0.4 0.3 0.2 0.1 0.0

0.0 2.5

3.0

3.5

4.0

4.5

5.0

5.5

2.0

2.5

Particle Size (nm)

3.0

3.5

4.0

4.5

Particle Size (nm)

Fig. 3. Particle size analyses of a), Pt NPs; b), Pt@Sn NPs; c), Pt/Al2O3 and d), Pt-SnO2/Al2O3. The XPS spectra of Pt@Sn and Pt-SnO2 obtained by calcination of Pt@Sn NPs at 500 oC in air for 3 hours were shown in Fig. 4a and 4b, respectively. To obtain the


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better XPS signals, the above mentioned samples were not supported on alumina. The method of Doniach and Sunjic39 was employed for curve fitting, and the binding energy assignment is according to the literature.40-42 Accordingly, the binding energies at 70.8/74.1, 72.0/75.2 in Fig. 4a can be assigned to 4f7/2/4f5/2 of Pt0 and Pt2+ species, respectively. The binding energies at 71.1/74.5, 72.5/75.8, and 74.7/78.0 in Fig. 4b could be assigned to 4f7/2/4f5/2 of Pt0, Pt2+ and Pt4+ species, respectively. The binding energies at 486.6/494.9 in Fig. 4b could be assigned to 3d5/2/3d3/2 of Sn4+, which is consistent with the XPS database of SnO2 and their XRD patterns in Fig. 1c. For as-synthesized Pt@Sn NPs in Fig. 4a, two Sn species were observed. One species with the binding energies of 484.8/493.3 could be assigned to 3d5/2/3d3/2 of Sn0.43,44 Another Sn species with the binding energies of 485.1/493.6 is oxidized Sn species. Since XPS could not well distinguish between Sn2+ and Sn4+ species,45,46 and the binding energies 485.1/493.6 is smaller than 486.6/494.9 of Sn4+ in Fig. 4b, the ＋

binding energies of 485.1/493.6 is assigned to Snδ . b)

a)

4+

0

Sn 3d5/2

0

Intensity (a.u.)

Sn 3d5/2 Intensity (a.u.)

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0

Sn 3d3/2

Pt 4f7/2 0

Pt 4f5/2

0

Pt 4f5/2 2+ Pt 4f5/2

4+

Sn 3d3/2

4+

Pt 4f5/2

4+

Pt 4f7/2 2+

Pt 4f7/2 0 Pt 4f7/2

δ+

Sn 3d3/2 Snδ+ 3d 5/2 495

490

2+

Pt 4f 5/2 Pt2+ 4f 7/2

485 480 78 75 Binding Energy (eV)

72

69

495

490

485 80 75 Binding Energy (eV)

70

Fig. 4. XPS spectra showing a) as-synthesized Pt@Sn, and b) Pt-SnO2. As shown in Fig. 4a, small percentages of Sn

δ＋

and Pt2

＋

were observed for

Pt@Sn NPs presumably due to the NPs exposure to air. Table 1 illustrated a total Sn/Pt ratio of 1.81/1.00 for Pt@Sn by XPS, which is significantly higher than


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0.86/1.00 obtained by EDS analysis. Since XPS is a surface sensitive technology, information collected by XPS mainly comes from the surface. The higher Sn/Pt ratio for Pt@Sn by XPS is consistent with Pt@Sn core@shell structures. Fig. 4b showed XPS spectra of Pt-SnO2 obtained by calcinations of Pt@Sn NPs, and only Sn4+ species for Sn species was observed. Table 1 illustrated a Pt/ oxidized Pt ratio of 27.68/72.32 and a total Sn/Pt ratio of 2.79/1.00 by XPS for Pt-SnO2. The Pt/ oxidized Pt ratio of 27.68/72.32 for Pt-SnO2 indicated that more surface Pt atoms were oxidized by calcination, and the total Sn/Pt ratio of 2.79/1.00 by XPS suggested the heteroaggregate nanostructures with a distribution of SnO2 on Pt surfaces. Table 1. Atomic Ratios of Sn and Pt with Different Oxidation States from the XPS Analysis for Pt@Sn, Pt-SnO2 NPs and EDS Analysis for Pt@Sn NPs. Sample

Sn0/oxidized Sn

Pt0/oxidized Pt

Sn/Pt (XPS)

Sn/Pt (EDS)

Pt@Sn

80.48:19.52

86.85:13.15

1.81:1.00

0.86:1.00

Pt-SnO2

0:100

27.68:72.32

2.79:1.00

Fig. 5a showed the TPO study of Pt@Sn/Al2O3, which further confirms the complete oxidation of Sn of Pt@Sn below 300 oC. The TPO results are consistent with the XPS study of Pt-SnO2, where only Sn4+ species for Sn species was observed for Pt-SnO2. DRIFT-IR spectra with CO probes of Pt-SnO2/Al2O3, Pt/Al2O3 and Pt@Sn/Al2O3 were shown in Fig. 5b. Only CO bands on Pt were observed since CO is not adsorbed on tin.23,47 As shown in Fig. 5b, Pt/Al2O3 illustrated a characteristic band at 2060 cm-1, and this can be ascribed to linear CO on Pt step and kink sites.48 The small CO band at 2058 cm-1 for Pt@Sn/Al2O3 revealed that the surfaces mainly consisted of Sn and the majority of Pt was in the cores. After calcinations of


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Pt@Sn/Al2O3, two characteristic bands at 2054 cm-1 and 2083 cm-1 were observed for Pt-SnO2/Al2O3, where the former is assigned to linear CO on Pt step and kink sites and the latter is ascribed to linear CO on Pt terrace sites.47 b)

a)

2083

Absorbance (a.u.)

O2 Consumption (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2054

2060 Pt-SnO2/Al2O3 Pt/Al2O3

Pt@Sn/Al2O3

100

200

300

400

500

2058

2150

2100

Temperature (°C)

2050

Pt@Sn/Al2O3 2000

1950 -1

Wavenumber (cm )

Fig. 5. a), TPO profile showing Pt@Sn/Al2O3, b) DRIFT-IR spectra with CO probes showing Pt-SnO2/Al2O3, Pt/Al2O3 and Pt@Sn/Al2O3. 3.2. Catalytic Performance of Pt-SnO2/Al2O3 and Pt/Al2O3 Nanocatalysts. Catalytic hydrogenation of various substituted nitroaromatics with H2 was used to

investigate

the

catalytic

performance

of

Pt-SnO2/Al2O3

nanocatalysts.

Pt-SnO2/Al2O3 catalysts with Pt/Sn ratios of 3/1, 1/1 and 1/3 were prepared using Pt@Sn NP precursors with Pt/Sn ratios of 3/1, 1/1 and 1/3, respectively, and the Pt loadings of the above mentioned catalysts were maintained at 0.5 wt %. The synthesis of the Pt-SnO2/Al2O3 with the Pt/Sn ratios of 3/1 and 1/3 were shown in SI, and the catalytic hydrogenation of o-chloronitrobenzene over Pt-SnO2/Al2O3 catalysts with different Pt/Sn ratios was summarized in Table S1 in SI. The Pt-SnO2/Al2O3 with a Pt/Sn ratio of 1/1 illustrated the highest catalytic activity and excellent catalytic selectivity, and was chosen for catalytic hydrogenation of a series of substituted nitroaromatics. Table 2 summarized the catalytic results of substituted nitroaromatics hydrogenation over Pt-SnO2 nanocatalysts with a Pt/Sn ratio of 1/1 and individual Pt


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nanocatalysts. The substituted nitroaromatics are mainly divided into 3 groups: Group 1, chloronitrobenzene, the by-product is dechlorination product (aniline);49 Group 2, the nitrobenzene containing methyl or hydroxyl substituents;50,51 Group 3, nitrobenzene containing carbonyl substituents, the by-reaction in catalytic hydrogenation is hydrogenation of carbonyl substituents.52,53 All the reactions were carried out at atmospheric hydrogen pressure and 45 °C using ethanol as solvents, and vigorous magnetic stirring was employed to eliminate the mass transport effect. As presented in Table 2, for Group 1 reactants including p-CNB, m-CNB and o-CNB, Pt-SnO2/Al2O3 nanocatalysts illustrated higher activity and selectivity than Pt/Al2O3 nanocatalysts except that the selectivity of catalytic m-CNB hydrogenation over Pt-SnO2/Al2O3 is lower than that over Pt/Al2O3. For Group 2 reactants including p-NP, p-NT, m-NT and o-NT, the catalytic activities for nitrophenol and nitrotoluene hydrogenation over Pt-SnO2/Al2O3 are significantly higher than those over Pt/Al2O3 while the selectivity over Pt-SnO2/Al2O3 and Pt/Al2O3 are same (100 %). For group 3 reactants including p-NAP, p-NBA and o-NBA, the catalytic activity and selectivity for hydrogenation of nitrobenzaldehyde and nitroacetophenone over Pt-SnO2/Al2O3 are higher than those over Pt/Al2O3 except that the catalytic activities for p-NAP hydrogenation over Pt-SnO2/Al2O3 and Pt/Al2O3 are same (100 %).


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ACS Catalysis

Table 2. Catalytic Hydrogenation of Various Substituted Nitroaromatics over Pt-SnO2/Al2O3 and Pt/Al2O3 Nanocatalysts. Reactants

Main

Raction

Products

Timea

Cl

Cl

NO 2

NH2

Cl

Cl

NO2

NH2

Cl

Cl

NH2

NO2

CH3

CH3

NO 2

NH2

CH3

CH3

NH2

NO2

CH3

CH3 NO2

OH

OH

NO 2

NH 2

CH 3

O

O

CHO

CHO

NO2

NH2

CHO

CHO NO2

Conversion

Selectivity

（%））

(%)

1h

Pt-SnO2/Al2O3

100.0

86.9

1h

Pt/Al2O3

65.1

85.5

1h

Pt-SnO2/Al2O3

100.0

80.8

1h

Pt/Al2O3

64.5

89.0

2h

Pt-SnO2/Al2O3

98.2

94.1

2h

Pt/Al2O3

82.6

77.7

0.5h

Pt-SnO2/Al2O3

66.2

100.0

0.5h

Pt/Al2O3

50.8

100.0

2h

Pt-SnO2/Al2O3

90.5

100.0

2h

Pt/Al2O3

60.8

100.0

2h

Pt-SnO2/Al2O3

70.1

100.0

2h

Pt/Al2O3

45.4

100.0

2h

Pt-SnO2/Al2O3

100.0

100.0

2h

Pt/Al2O3

53.8

100.0

2h

Pt-SnO2/Al2O3

100.0

70.6

2h

Pt/Al2O3

100.0

54.9

2h

Pt-SnO2/Al2O3

90.7

59.9

2h

Pt/Al2O3

39.5

54.9

2h

Pt-SnO2/Al2O3

55.5

26.6

2h

Pt/Al2O3

26.3

16.7

CH 3

NH 2

NO 2

a

NH2

Catalysts

NH2

Reaction conditions: Reactants-1.0 g; Supported catalysts-0.1 g; EtOH-25.0 mL;

H2-0.10 MPa; Reaction temperature-45 °C; Speed of agitation-500 rpm. Pt/Al2O3 (Pt loading-0.50 wt %); Pt-SnO2/Al2O3 (Pt loading-0.50 wt %).


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3.3. Catalytic Stability of Pt-SnO2/Al2O3 Nanocatalysts The catalytic hydrogenation of o-CNB was selected for the investigation of catalytic stability of Pt-SnO2/Al2O3. Due to the loss of catalysts during the recovery process in each cycle, the weights of ethanol and o-CNB in the subsequent cycle are proportional to that of the recovered catalysts to keep the ratios of o-CNB/catalysts and ethanol/catalysts constant. Table 3 summarized the results of catalytic stability for o-CNB hydrogenation over Pt-SnO2/Al2O3. Within the first five cycles, the catalytic activity of Pt-SnO2/Al2O3 was stable, and the catalytic selectivity was slightly increased, suggesting a good structural stability of Pt-SnO2 heteroaggregate nanocatalysts. We speculate that the reduction of a tiny part of SnO2 or the slight change of catalytic activity center may cause the selectivity increase in the recycle experiments. Table 3. Cycle to Cycle o-CNB Hydrogenation over Pt-SnO2/Al2O3 Nanocatalysts Cycle

Conversion

o-CAN

(%)

selectivity (%)

1

98.5

91.9

0.0942

0.942

98.3

89.3

3

0.0517

0.517

98.8

91.3

4

0.0391

0.391

98.5

92.2

5

0.0327

0.327

98.7

94.9

Catalyst (g)

o-CNB (g)

1

0.1

2

Index

a

a

Reaction conditions: EtOH-25.0 mL in cycle 1; the volume of EtOH in the

following cycles decreased according to the weight of o-CNB at a fixed EtOH/o-CNB

ratio;

H2-0.10

MPa;

Reaction

time-2.0

temperature-45 °C; agitation speed-500 rpm.


hours;

reaction

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ACS Catalysis

3.4. The Origin of Enhanced Catalytic Performance of Pt-SnO2 Nanocatalysts In order to understand the enhanced catalytic performance of Pt-SnO2 nanostructures, theoretical calculation upon density function theory (DFT) was performed.54,55 Table 4 summarized the adsorption energies of hydrogen atom, o-CNB and o-CAN on Pt (111) and Pt (111)-SnO2 surfaces. The calculation details were supplied in the Supporting Information (SI). According to the calculations, the binding energy between H and Pt (111)-SnO2 (-2.75 eV) is slightly stronger than that between H and pure Pt (111) surface (-2.73 eV),56 suggesting that the partial coverage of Pt (111) surfaces with SnO2 slightly promote the H adsorption on Pt (111) surfaces. As for the adsorption of the reactant o-chloronitrobenzene (o-CNB), the binding energies for the o-CNB/Pt (111)-SnO2 and o-CNB/Pt (111) are -0.84 eV and -0.82 eV, respectively, indicating that the Pt-SnO2 nanostructures can slightly facilitate the adsorption of o-CNB. Moreover, the binding energies for the o-CAN/Pt (111)-SnO2 and o-CAN/Pt (111) are -0.98 eV and -1.16 eV, respectively, and this shows that the cooperative Pt-SnO2 interaction can significantly weaken the binding of o-CAN on Pt (111) surface, which indicates easier desorption of products, resulting in more catalytic surface available and a fast reaction rate. Basing on the calculations, it is concluded that the enhanced catalytic performance of Pt-SnO2/Al2O3 nanocatalysts originates

from

the

cooperative interaction between Pt and SnO2

inside

the

heteroaggregate nanostructures. The interaction between metal and metal oxide supports has been reported by Tauster several decades before,25 and recently this type of catalysts were found to have more applications in various fields.11,33,57-59 Usually, the strong metal-support interaction catalysts require CeO2,28 ZrO229 and Fe2O330 as supports. The method developed by this work can provide another choice to use


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bimetallic NPs as precursors and in-situ transform the precursors to metal-metal oxide heteroaggregate nanostructures on readily available Al2O3 and SiO2 supports, which could enhance catalytic activity and selectivity in hydrogenation reactions. Table 4. The adsorption energies Eads of hydrogen atom, o-CNB molecule and o-CAN molecule on Pt (111) and Pt (111)-SnO2 surfaces calculated in the present work. Structures

H/ Pt(111)

H/ Pt(111)-SnO2

o-CNB/ Pt(111)

o-CNB/ Pt(111)-SnO2

o-CAN/ Pt(111)

o-CAN/ Pt(111)-SnO2

Eads (eV)

-2.73

-2.75

-0.82

-0.84

-1.16

-0.98

Literature reported that the decoration of inert metal oxides onto active metal could cause blockage effect, which may enhance catalytic selectivity.60-62 Noble metal catalysts are excellent catalysts for hydrogenolysis of C-O linkage while the by-reactions of hydrogenation of aromatic rings is also accelerated, resulting in a poor selectivity. Due to the blockage effect of metal oxides decorated on noble metals, the coordination of aromatic rings was inhibited. For example, the Rh-NiOx catalysts containing NiOx illustrated the enhanced hydrogenolysis selectivity.60 In the current Pt-SnO2/Al2O3 system, the hydrogenation of aromatic rings could not occur at the selected reaction conditions, and the selectivity enhancement may also come from the blockage effect of SnO2, which inhibits the coordination of the substituent (for example –Cl) and prevent the by-reactions (for example hydrodechlorination). 4.

Conclusion In the present work, Pt@Sn core-shell NPs were synthesized by one-step method,

and the prepared Pt@Sn NPs were used as precursors to be in-situ transformed into Pt-SnO2

heteroaggregate

nanostructures

on

Al2O3

supports.

The

catalytic

hydrogenations of various substituted nitroaromatics were selected to investigate the


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ACS Catalysis

catalytic performance of Pt-SnO2 nanocatalysts. The DRIFT-IR with CO probes, HRTEM and XPS studies confirmed Pt-SnO2 heteroaggregate nanostructures, where SnO2 partially covered the Pt nanoparticle surface. Compared to the control Pt nanocatalysts, the Pt-SnO2 nanocatalysts exhibited higher catalytic activity and selectivity for hydrogenation of p-CNB, o-CNB, p-NBA and o-NBA, and higher activity for hydrogenation of p-NT, m-NT, o-NT and p-NP with the same 100% selectivity as well as higher selectivity for hydrogenation of p-NAP with the same 100% conversion. DFT calculations showed that the adsorptions of hydrogen and the substituted nitroaromatics on Pt-SnO2 do not decrease although the Pt surfaces are partially covered by SnO2 species, and the product molecules are more easily released from the catalytic surfaces, resulting in a fast reaction rate. It is concluded that the enhanced catalytic performance of Pt-SnO2/Al2O3 originates from the cooperative Pt-SnO2 interaction inside the heteroaggregate nanostructures. This method of synthesizing catalysts with cooperative metal-metal oxide interaction using bimetallic NPs as precursors may extend to other systems, and this type of catalysts can be applied in other catalytic reactions. Supporting Information More catalytic results, HAADF-STEM images, and details of DFT calculations. Acknowledgment S. Zhou and H. Yin thank programs of Ningbo Municipal Science and Technology Innovative Research Team (Grant No. 2014B81004 and 2016B10005). This work is also supported by the National Natural Science Foundation of China (Grant No. 21571183, 91434110 and 21676089). References (1) Zhu, C.; Zeng, J.; Tao, J.; Johnson, M. C.; Schmidt-Krey, I.; Blubaugh, L.; Zhu, Y.; Gu, Z.; Xia,


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Design of Highly Efficient Pt-SnO2 Hydrogenation Nanocatalysts using

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