Tailoring PtâFe2O3 Interfaces for Selective Reductive Coupling

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Tailoring Pt-Fe2O3 interfaces for selective reductive coupling reaction to synthesize imine Bin Zhang, Xiao-Wei Guo, Haojie Liang, Huibin Ge, Xiaomin Gu, Shuai Chen, Huimin Yang, and Yong Qin ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01756 • Publication Date (Web): 24 Aug 2016 Downloaded from http://pubs.acs.org on August 24, 2016

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Tailoring Pt-Fe2O3 interfaces for selective reductive coupling reaction to synthesize imine ‡

Bin Zhang,† Xiao-Wei Guo, Haojie Liang, † Huibin Ge, † Xiaomin Gu, † Shuai Chen, † Huimin Yang, † Yong Qin*,† †

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of

Science, Taiyuan 030001, China ‡

Key Laboratory of Bio-Based Material Science and Technology, Ministry of Education,

Northeast Forestry University, Harbin 150040, China KEYWORDS: Imine, Pt-Fe3+-OH interface, Core-shell structure, Atomic layer deposition, Reductive coupling reactions ABSTRACT: The tailoring of metal-metal oxide interface has emerged as an effective method to improve the catalytic performance of heterogeneous catalysts. Herein, we report on the synthesis of a catalyst with Pt-Fe3+-OH interface based on a Fe2O3-Pt core-shell structure by atomic layer deposition. This tandem catalyst shows remarkably high catalytic efficiency and selectivity (> 89%) of imine for the reductive coupling of nitrobenzene and furfural. The Pt-Fe3+OH interface sites weaken the Pt-H bond and suppress the further hydrogenation of imine to amine. In contrast, monometallic Pt catalysts show a strong Pt-H interaction and dominant formation of amine. Further comparison revealed that the Fe2O3-Pt core-shell structure exhibited a higher TOF in the generation of imine and a better utilization of Pt than Pt-Fe2O3 core-shell structure.

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Introduction Heterogeneous catalysts, usually consisting of metal nanoparticles (NPs) and metal oxides, have been widely employed in a variety of important fields involving chemical industry, energy technology and environmental engineering.1 Generally, the metal oxides not only serve as supports to disperse metal NPs, but also function as electronic modulators and/or catalytic adsorption sites to improve the catalytic performances by interaction with metal NPs.2 The tailoring of metal-metal oxide interface has emerged as an effective way to improve the activity of heterogeneous catalysts.3 For example, Pt/FeOx interface or Pt-Fe3+-OH interface exhibit excellent performance in CO oxidation.3-4 The CO oxidation over catalysts using CeO2 as support is greatly enhanced at the ceria-metal interface sites for a range of metal (Ni, Pd, Pt) catalysts.5 Au(111) is inactive for the water-gas shift reaction, but the formation of CeO2/Au(111) and TiO2/Au(111) interfaces improve the activity of the water-gas shift reaction.6 The traditional way to introduce metal-metal oxide interface is dispersing metal NPs on the oxide support.1 However, most conventional support and metal NPs are structurally non-uniform, and the structures of supported metal catalysts are not well understood.7 The controlled assembly of metal NPs with metal oxide NPs can also realize the rational design of metal-metal oxide interfaces in nanostructured catalysts, 2c but introduces rather limited metal-metal oxide interface sites. Recently, the coating of metal NPs with ultra-thin films or porous shells of metal oxide have been extensively studied to maximize the ratio of metal-metal oxide interface.1,

7-8

Compared with the above-mentioned methods, the coating of metallic shells on metal-oxide NPs is more attractive due to its high atom utilization by improving the dispersion of the metal.9 However, it is difficult to control the composition and the thickness of metal shell in atomic level by traditional methods.


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Atomic layer deposition (ALD) produces films by self-limiting chemical reactions between gaseous precursors and a solid surface in an atomic layer-by-layer fashion.10 The self-limiting character of the ALD reactions makes it possible to synthesize uniform metal or metal oxides NPs and films to engineer metal-metal oxide interface in atomic scale.8b, 11 Coating metal NPs with metal oxide films and depositing metal NPs on the metal oxides supports are two general ways to fabricate metal-metal oxide interface by ALD.11c However, in the case of coating metal NPs with metal oxide films, the ratio of metal-metal oxide interface site to metal site increases with the increase of ALD cycle number of metal oxide. For example, a thin layer of ALD Al2O3 coating on surface of metal NPs can effectively reduce coke formation and sintering in high temperature reactions.8b, 11a However, the increase of stability and selectivity is at the cost of loss of catalytic activity. On the contrary, if small metal oxide NPs are first deposited and then selectively covered with ultra-thin metal, maximized metal-metal oxide interface sites can be produced.

Imines are important class of chemicals, which are extensively used for organic synthesis, agricultural chemicals, pharmaceuticals and other fine chemicals.12 The traditional method for imine synthesis has been proposed based on aldimine condensation between amines and carbonyl compounds.13 This process requires Lewis acid catalysts, dehydrating agents, activated aldehydes, and prolonged reaction time, restraining efficiency and economics. Recently, the direct synthesis of imines via tandem reductive coupling of nitroarenes and carbonyl compounds has been realized by using Au/TiO214, Ni/SiO215 or Au-Pd/Al2O316 as catalysts and becomes an attractive process. But this process shows low catalytic efficiency and requires high temperature (>100 oC) or hydrogen pressure. Pt-based catalysts are widely used in hydrogenation reactions. However, amine is the main product (selectivity > 48%) in the reductive coupling of nitro


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compounds with aldehydes over Pt catalysts at high temperature (over 100 oC),17 and nitrone is the main product at low temperature (35 oC)18. In this work, we applied ALD to engineer Pt-Fe3+-OH interface on multiwalled carbon nanotubes (CNTs) by selectively coating the supported Fe2O3 NPs with ultra-thin Pt shell, or depositing thin Fe2O3 films on the surface of supported Pt NPs. The catalysts with maximized ratio of Pt-Fe3+-OH interface showed high selectivity compared with the supported Pt/CNTs catalysts and the mixture of supported Pt and Fe2O3 catalysts in the one-pot synthesis of imine via tandem reductive coupling of nitrobenzene and furfural at normal temperature. The Pt-Fe3+OH interface increased the imine selectivity by weakening the Pt-H bond and suppressing the further hydrogenation of imine. Furthermore, the Fe2O3-Pt core-shell structure exhibited a higher TOF in the generation of imine than Pt-Fe2O3 core-shell structure. Experimental procedures Atomic Layer Deposition. ALD was performed in a home-made closed chamber-type ALD reactor at 230 oC, using ultrahigh purity N2 (99.999%) carrier gas at a flow rate of 50 sccm and a pressure of 60 Pa. Prior to ALD, 15 mg CNTs (Shenzhen Nanotech Port Co. Ltd., 40-60 nm in diameter, 5-15 µm in length) dispersed in ethanol were spread out on a quartz wafer and dried in air. The Pt or Fe2O3 deposition were performed by sequential exposure of the CNTs to (methylcyclopentadienyl)trimethylplatinum (MeCpPtMe3) or ferrocene (FeCp2) and O3, respectively.11e, 19 MeCpPtMe3 was heated to 60 oC, and FeCp2 was kept at 90 oC. The pulse, exposure, and purge times were 0.5 s, 8 s, and 20 s for MeCpPtMe3, 0.8 s, 8 s, and 20 s for FeCp2, and 1 s, 8 s, and 20 s for O3, respectively. The CNTs supported Fe2O3-Pt core-shell NPs (mPt-50Fe2O3/CNTs) are prepared by sequential deposition of 50 cycles of Fe2O3 and m cycles of Pt on CNTs by ALD, while CNTs supported Pt-Fe2O3 core-shell NPs (nFe2O3-20Pt/CNTs) are


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prepared by sequential deposition of 20 cycles of Pt and n cycles of Fe2O3 on CNTs. For comparison, monometallic Pt catalysts (xPt/CNTs) are prepared by deposition of x cycles of Pt on CNTs. xFe2O3/CNTs were prepared by deposition of x cycles of Fe2O3 on untreated CNTs by ALD. Other supported catalysts, such as 1wt% Pt/SBA-15, 4wt% Pt/Al2O3, 4wt% Pd/SiO2 and 4wt% Ru/Al2O3 are prepared by incipient wetness impregnation method and further calcination at 500 oC for 2 h. Characterization and Equipment. Transmission electron microscopy (TEM) and highresolution TEM (HRTEM), were conducted using a JEOL JEM 2100F instrument. High-angle annular dark field scanning transmission electron microcopy (HAADF-STEM) were performed on an atomic resolution analytical microscope (JEM-ARM 200F) operating at 200 kV. Energy dispersive spectroscopy (EDS) measurements were also performed on JEM-ARM 200F. X-ray powder diffraction (XRD) analysis was carried out on a D/max-RA X-ray diffractometer (Rigaku, Japan) with Cu Kα radiation (λ = 0.154 nm) operated at 40 kV. The content of the metal in the catalyst was determined by inductively coupled plasma optical emission spectrometry (ICP-OES; Thermo iCAP 6300, USA). X-ray photoelectron spectroscopy (XPS) was recorded with a VG MiltiLab 2000 system at a base pressure of 1 × 10–9 mbar. Samples were excited with monochromatized MgKα radiation (hν = 1253.6 eV). The analyzer was operated in a constantpass energy mode (20 eV). Catalysts were pre-treated in H2/N2 at 80 oC for 2 h before XPS analysis. The number of surface metal sites was determined by hydrogen pulse adsorption in a Xianquan TP-5080 multi-functional atomic adsorption instrument with a thermal conductivity detector (TCD). Samples (100 mg) were reduced in situ at 200 °C for 2 h in a 10% H2/N2 flow, and then swept at 400 °C for 30 min and cooled to 40 °C in a N2 flow. Hydrogen pulse adsorption was performed at 40 °C in a N2 flow (40 mL/min). Hydrogen temperature-


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programmed desorption (H2-TPD) experiments were also performed in the Xianquan TP-5080 instrument. Briefly, 100 mg sample was pre-treated at 200 °C for 1 h in a 10% H2/N2 flow, purged with N2 at 400 °C to remove adsorbed species from catalyst surface, and then cooled down to 25 °C under a N2 flow. This was followed by adsorption of H2 at 25 °C for 30 min, purging with N2 for 15 min, and linear increase of temperature (10 °C/min) to 800 °C under a N2 flow (40 mL/min). A TCD was used for on-line monitoring of the TPD patterns. Catalytic performance. Catalytic reactions were performed in a 25 mL stainless steel autoclave with at a stirring speed of 600 rpm. In a typical run, 1 mmol furfural and 1 mmol nitrobenzene, 10 mg catalyst and 10 mL isopropanol were introduced into the autoclave. Hydrogen pressure and reaction temperature were 0.5 MPa and 40 oC, respectively. Conversion and selectivity were determined using a gas chromatograph (GC-9720, Zhejiang Fuli chromatogram analysis Co., Ltd, China) equipped with a FID detector and a 30 m HP-5 capillary column. Results and discussion


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Figure 1. TEM, HRTEM, representative high-angle annular dark-field (HAADF) STEM images, and particle size histograms of different samples: 20Pt/CNTs (A and D), 10Fe2O3-20Pt/CNTs (B and E), and 20Pt-50Fe2O3/CNTs (C and F). Figure 1 shows TEM images of 20Pt-50Fe2O3/CNTs, 10Fe2O3-20Pt/CNTs and 20Pt/CNTs. Particle size histograms show that the 20Pt-50Fe2O3/CNTs (2.8 ± 0.4 nm), 10Fe2O3-20Pt/CNTs (2.5 ± 0.5 nm) and 20Pt/CNTs (2.9 ± 0.6 nm) catalysts have a similar size distribution of NPs on the surface of CNTs. Both the lattice fringes with interplanar distances of 0.22 nm and 0.19 nm, corresponding to Pt(111) and Pt(100) planes, are observed on the HRTEM image of 20Pt/CNTs catalyst, respectively (Figure 1D). Aberration-corrected STEM was used to further reveal the microstructure of the typical NPs. HADDF-STEM images (Figure 1E and Figure S1A) of 10Fe2O3-20Pt/CNTs revealed the deposition of less electron-dense layers on the Pt NPs, and EDS analysis showed that this layer contained iron (Figure S1 B and C). Therefore, the surface of Pt NPs provides active sites for the deposition of Fe2O3 layers. Although the surface of Pt NPs


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is covered by Fe2O3 layers, lattice fringes with interplanar distances of 0.22 nm and 0.19 nm, corresponding to Pt(111) and Pt(100) planes, are still observed for the 10Fe2O3-20Pt/CNTs (Figure 1E). 20Pt-50Fe2O3/CNTs were prepared by sequential deposition of 50 cycles of Fe2O3 and 20 cycles of Pt on untreated CNTs by ALD. Without surface functionalization (covalent or nocovalent) to form enough anchoring sites, an island growth mode is favored for the untreated CNTs to produce Fe2O3 NPs by ALD.20 After 50 cycles of Fe2O3 ALD, most of the anchoring sites on CNTs, even generated from O3 oxidation, were occupied by Fe2O3 NPs. Fe2O3 NPs provide sites for the active adsorption of O3. Because Pt has higher binding energies with iron oxide,21 it tends to selectively grow onto the surface of Fe2O3 NPs. HADDF-STEM images (Figure 1F and Figure S2A) of 20Pt-50Fe2O3/CNTs revealed the deposition of Pt atoms on the less electron-dense Fe2O3 NPs, and EDS analysis showed that these NPs contained iron (Figure S2B). In particular, Pt atoms were incompactly distributed on the surface of Fe2O3 NPs. The EDS line-profile analysis of 20Pt-50Fe2O3/CNTs also revealed the Fe2O3-core and Pt-shell nanostructure (Figure S3). No clear diffraction peaks of Fe2O3 were found in XRD (Figure S4), which is probably due to the poor crystallinity and low content of Fe2O3 in the composites. Besides, the HRTEM images of 50Fe2O3/CNTs and 100Fe2O3/CNTs show no obvious lattice fringes (Figure S5), which also indicated the poor crystallinity of Fe2O3 NPs.


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Table 1. Physicochemical characteristics of 20Pt-50Fe2O3/CNTs, 10Fe2O3-20Pt/CNTs, and 20Pt/CNTs catalysts and summary of XPS results for O 1s and Pt 4f. (wt%)a

O 1s D

Catalyst Pt

20Pt-50Fe2O3/CNTs

10Fe2O3-20Pt/CNTs

20Pt/CNTs

1.6

2.3

2.4

Fe

3.8

1.9

—

(nm)b

2.8

2.5

2.9

H2 uptake (µmol/g)c

12.4

21.8

23.1

Pt 4f

Surface Pt/Fed

2.5

2.2

—

BE

Rate

BE

Rate

(eV)

(%)

(eV)

(%)

529.8

9

-Fe-O-

71.4

50

Pt0

531.3

66

-OH

72.4

35

Pt2+

533.0

25

-C=O

74.5

15

Pt4+

529.8

9

Fe-O-

71.4

51

Pt0

531.3

67

-OH

72.4

37

Pt2+

533.0

24

-C=O

74.6

12

Pt4+

529.8

0

-Fe-O-

71.4

33

Pt0

531.3

51

-OH

72.4

33

Pt2+

533.0

49

-C=O

74.6

34

Pt4+

Assign ment

Assign ment

a

Measured by ICP-OES; b Average particles size (D) calculated by counting a minimum of 100 particles in TEM images; c Measured by hydrogen pulse adsorption; d Calculated from XPS of the element.

Figure 2. XPS analysis of the catalysts. (1) 20Pt/CNTs; (2) 10Fe2O3-20Pt/CNTs; (3) 20Pt50Fe2O3/CNTs.


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XPS analysis was used to measure surface composition of 20Pt/CNTs, 10Fe2O3-20Pt/CNTs, and 20Pt-50Fe2O3/CNTs. The C 1s spectra (Figure 2A) of both 20Pt-50Fe2O3/CNTs and 10Fe2O3-20Pt/CNTs are nearly the same as that of 20Pt/CNTs, indicating that no new carbon species are generated after Fe2O3 ALD. The binding energy of Fe 2p orbital (Fe 2p3/2, Figure 2B) on both 20Pt-50Fe2O3/CNTs and 10Fe2O3-20Pt/CNTs is located at 711.1 eV, corresponding to the Fe3+ in Fe2O322. Therefore, both 20Pt-50Fe2O3/CNTs and 10Fe2O3-20Pt/CNTs have the same Fe3+ species on the surface of the catalysts. Deconvolution of the O 1s XPS peak indicates the presence of some carbolylic (C=O, 531.2 eV) and hydroxyl function (–OH, 533.0 eV) on the surface of 20Pt/CNTs23. In addition to the two peaks observed in 20Pt/CNTs, a new O 1s XPS peak for 20Pt-50Fe2O3/CNTs or 10Fe2O3-20Pt/CNTs appears at 529.8 eV, corresponding to bridging oxo groups of Fe-O bond24. Besides, the intensity of the peak at 533.0 eV (–OH) for 20Pt-50Fe2O3/CNTs and 10Fe2O3-20Pt/CNTs, corresponding to Fe-OH or C-OH species, is obviously higher than that of 20Pt/CNTs (Figure 2C, Table 1). The above results suggest the presence of Fe-OH and bridging oxo groups on the surface of 20Pt-50Fe2O3/CNTs and 10Fe2O320Pt/CNTs since no new carbon species are generated as revealed by XPS.3 Deconvolution of the XPS of Pt 4f peak shows that the ratio of metallic Pt (Pt0, 50% or 51%) on the surface of 20Pt-50Fe2O3/CNTs and 10Fe2O3-20Pt/CNTs is higher than that of 20Pt/CNTs (32%) (Figure 2D, Table 1). Moreover, the surface Pt/Fe ratio is similar on both 10Fe2O3-20Pt/CNTs (2.2) and 20Pt-50Fe2O3/CNTs (2.5). Therefore, both strategies, deposition Fe2O3 on Pt NPs or coating Fe2O3 NPs with thin Pt film, are favorable for the formation of Pt-Fe3+-OH interface sites, which are the major species on the surface of 10Fe2O3-20Pt/CNTs and 20Pt-50Fe2O3/CNTs.


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Table 2. Catalytic performance of various catalysts for the reductive coupling of nitrobenzene and furfural to imine.a O NO2 + 1

+

3

O O

H2,(0.5MPa) 313K, Cat.

O N

O

O OH 6

Selectivity [%]

Rate of TOF

Conv.

[h-1]b

[%]

imine formation [mmol·gCat-1·h-1]

a

5

2 4

Catalyst

O

H N

N

Imine

Nitrone

Amine

Alcohol

3

4

5

6

5Fe2O3-20Pt/CNTs

4

—

46

23

39

12

1

10Fe2O3-20Pt/CNTs

10

224

31

91

4

0

4

15Fe2O3-20Pt/CNTs

0.1

—

1

24

2

0

0

10Pt-50Fe2O3/CNTs

22

—

71

89

7

0

3

20Pt-50Fe2O3/CNTs

27

1071

87

89

8

0

4

30Pt-50Fe2O3/CNTs

23

—

94

71

28

0

2

50Fe2O3/CNTs

0

—

0

—

—

—

—

20Pt/CNTs +50Fe2O3/CNTs

4

92

72

17

20

46

13

20Pt/CNTs

2

52

77

9

6

54

21

10Pt/CNTs

13

253

85

45

15

37

0

5Pt/CNTs

4

54

72

16

45

35

0

1Pt/CNTs

1

538

7

44

29

23

0

1wt% Pt/SBA-15

1

—

12

30

9

18

33

4wt% Pt/Al2O3

0.5

—

27

5

22

19

36

4wt% Pd/SiO2

2

—

70

9

6

71

2

4wt% Ru/Al2O3

0.3

—

6

15

17

3

12

Reaction condition: 40 °C, 0.5 MPa, 3 h, Catalyst (0.01 g), Furfural (1 mmol), Nitrobenzene

(1 mmol), Isopropanol (10 mL); b Turnover frequency (TOF) is based on the yield of imine 3.


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Table 2 shows the performance of different catalysts in the reductive coupling of nitrobenzene with furfural at 40 °C. The 10Fe2O3-20Pt/CNTs, 10Pt-50Fe2O3/CNTs, and 20Pt50Fe2O3/CNTs showed high selectivity (≥ 89%) for imine 3, while monometallic catalysts give imine 3 with low selectivity. For example, the 20Pt/CNTs, Pt/SBA-15, Pt/Al2O3, Pd/SiO2, and Ru/Al2O3 produced imine 3 with low selectivity (< 30%), and the reduction of furfural to alcohol 6 and imine 3 to amine 5 were two predominate reactions. XPS results show that Pt-Fe3+-OH interface sites are dominated on the surface of 10Fe2O3-20Pt/CNTs and 20Pt-50Fe2O3/CNTs. Thus, the Pt-Fe3+-OH interface sites promoted the yield of imine 3 by inhibiting the hydrogenation of –N=C– bond and –C=O bond. Furthermore, the imine 3 formation rate and TOF over the 20Pt-50Fe2O3/CNTs catalyst (27 mmol·gCat-1·h-1, TOF = 1071 h-1) were significantly higher than those over the 10Fe2O3-20Pt/CNTs catalyst (10 mmol·gCat-1·h-1, TOF = 224 h-1), and even the reported Ni/SiO2 catalyst (7.6 mmol·gCat-1·h-1, 105 °C, 1.4 MPa)15 or AuPd/Al2O3 catalyst (2.0 mmol·gCat-1·h-1, 180 °C, 2 MPa)16, indicating the advantages of the Fe2O3Pt core-shell structure. The Pt-Fe3+-OH interface sites can be easily tailored by changing the ratio of Pt/Fe2O3 on the surface of mPt-50Fe2O3/CNTs catalysts and nFe2O3-20Pt/CNTs catalysts via ALD. The catalytic activity of mPt-50Fe2O3/CNTs catalysts increases with the Pt ALD cycle number (m), and a similar high selectivity of imine 3 (~88%) is obtained over both 10Pt-50Fe2O3/CNTs and 20Pt50Fe2O3/CNTs catalysts (Table 2). But the selectivity of imine 3 decreases to 71% over the 30Pt50Fe2O3/CNTs catalyst. These results indicate that the content of the Pt-Fe3+-OH interface sites reach a maximum over the 20Pt-50Fe2O3/CNTs catalyst. For the nFe2O3-20Pt/CNTs catalysts, the content of Pt-Fe3+-OH interface sites can also increase with the increase of ALD cycle number of Fe2O3 (n). Table 2 shows that the selectivity of imine 3 over nFe2O3-20Pt/CNTs


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catalysts increases with the increase of ALD cycle number (n) of Fe2O3. However, the selectivity increase is at the cost of the catalytic activity. After 15 cycles of Fe2O3, the conversion drops to 1% for the 15Fe2O3-20Pt/CNTs catalyst, indicating that the surface of Pt NPs is fully covered by Fe2O3 layers. Therefore, the deposition of Pt onto the surface of Fe2O3 NPs generates more PtFe3+-OH interface sites with high selectivity and yield for imine 3. We have also investigated the catalytic performance of the 50Fe2O3/CNTs and physical mixture of the 20Pt/CNTs catalyst and 50Fe2O3/CNTs catalyst (20Pt/CNTs + 50Fe2O3/CNTs, Table 2). The 50Fe2O3/CNTs catalyst shows no catalytic activity, suggesting that Fe2O3 cannot separately provide active sites in the tandem reaction. The physical mixture of 20Pt/CNTs + 50Fe2O3/CNTs catalysts have a similar activity as 20Pt/CNTs, and the selectivity of imine 3 increases to 17%. Therefore, the direct interaction between Fe2O3 and Pt is essential for a high selectivity of imine 3 in this tandem reaction.

Figure 3. Time course of reductive coupling of nitrobenzene and furfural over the following catalysts: A) 20Pt-50Fe2O3/CNTs, B) 10Fe2O3-20Pt/CNTs, and C) 20Pt/CNTs (0.01 g Catalyst, 40 oC, 0.5 MPa, 1 mmol Furfural and 1 mmol Nitrobenzene); D) The stability of 20Pt50Fe2O3/CNTs in four runs at 40 °C.


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Figure 3 shows the time profile for the reductive coupling of nitrobenzene and furfural over 20Pt-50Fe2O3/CNTs, 10Fe2O3-20Pt/CNTs, and 20Pt/CNTs catalysts. Interestingly, 20Pt50Fe2O3/CNTs gave imine 3 in an 88% yield with 89% selectivity in 5 h (Figure 3 A). It is notably observed that 20Pt-50Fe2O3/CNTs could strictly prevent the over-hydrogenation of imine 3, and the selectivity of imine 3 was maintained even when prolonging the reaction time after full consumption of furfural. The over-hydrogenation of imine 3 is also prevented over 10Fe2O3-20Pt/CNTs, but the catalytic activity is low (Figure 3 B). The selectivity of imine 3 is decreased to 75% in 9 h due to the formation of more nitrone 4 over 10Fe2O3-20Pt/CNTs. The catalytic activity of 20Pt/CNTs (Figure 3 C) is similar to that of 20Pt-50Fe2O3/CNTs. However, the selectivity of imine 3 is very low (88%) of corresponding imine at high conversion were achieved from the reductive coupling of 4nitrophenol with furfural or nitrobenzene with 5-methylfurfural to imines. However, the selectivity of imine was decreased to 64% due to the hydrogenation of C=C bond on vinyl group in the 4-nitrostyrene. For the reductive coupling of nitrobenzene with 5-HydroxyMethyl-2-


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furaldehyde, the yield of imine was decreased to 66% due to the instability of 5-HydroxyMethyl2-furaldehyde. Our previous research results have shown that the size of Pt particles supported on CNTs increases with the ALD cycle numbers and more electronegativity of Pt species are generated.11e But the catalytic performance of xPt/CNTs suggests that there is no clear correlation between the selectivity imine 3 and Pt particle size (Table 2). Meanwhile, the formation of more electronegative Pt species is not the key reason for the selectivity increase of imine 3 over both 20Pt-50Fe2O3/CNTs and 10Fe2O3-20Pt/CNTs. The similar distribution of NPs for 20Pt50Fe2O3/CNTs, 10Fe2O3-20Pt/CNTs, and 20Pt/CNTs (Figure 1) also eliminates the size effect of Pt NPs in the formation of imine 3. Cismeros et al. have reported that the selectivity of imine can be increased by raising contact area between Pt and TiO2 on the surface of Pt NPs.18 Our XPS results indicate that Pt-Fe3+-OH interface sites are dominating on the surface of the 20Pt50Fe2O3/CNTs and 10Fe2O3-20Pt/CNTs. Therefore, the selectivity of imine 3 depends on the PtFe3+-OH interface sites. The low selectivity of imine 3 over the physical mixture of 50Fe2O3/CNTs and 20Pt/CNTs catalysts further illustrates the critical role of Pt-Fe3+-OH interface in the yield of imine 3. ICP results show that the content of Pt on 20Pt-50Fe2O3/CNTs (1.6 wt%) is lower than that on 10Fe2O3-20Pt/CNTs (2.3 wt%) (Table 1), although the catalytic activity over 20Pt-50Fe2O3/CNTs is obviously higher than that over 10Fe2O3-20Pt-/CNTs. This indicates more available Pt-Fe3+-OH interface sites on 20Pt-50Fe2O3/CNTs for the Fe2O3 core and thin Pt shell structure.


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Figure 4. H2-TPD profiles of (1) 20Pt/CNTs, (2) 10Fe2O3-20Pt/CNTs and (3) 20Pt50Fe2O3/CNTs. The formation of Pt-Fe3+-OH interface influences the chemadsorption of reactants. H2-TPD was conducted for 20Pt-50Fe2O3/CNTs 10Fe2O3-20Pt/CNTs, and 20Pt/CNTs catalysts (Figure 4). The desorption peak at low temperature (around 80 °C) and high temperature (maxima around 550 °C) are generally assigned to hydrogen at metal surface and spillover hydrogen, respectively.25 The desorption peak at low temperature (around 80 °C) observed on 20Pt/CNTs disappears on 10Fe2O3-20Pt/CNTs and 20Pt-50Fe2O3/CNTs, indicating the adsorption of active hydrogen on metal surface (Pt-H bond) is weaker on these two catalysts. The weak adsorbed hydrogen on these two catalysts is purged away by carrier gas prior to applying the temperature program to the sample. XPS results have indicated that the Pt-Fe3+-OH interface sites show higher electron-donating properties, which weakens Pt-H bond strength. Generally, the weaker Pt-H bond can results in a lower coverage of hydrogen on the catalytic surface during reaction, and therefore suppress the further hydrogenation of C=N bond of imine 3 in kinetics. Zheng and co-workers have revealed that the electron donation from ethylenediamine makes the surface of platinum nanowires highly electron-rich, and therefore this catalytic surface favors the adsorption of electron-deﬁcient reactants over electron-rich substrates, thus preventing full


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hydrogenation of nitrobenzene.26 Analogously, the Pt-Fe3+-OH interface sites prevent the full hydrogenation of imine to amine due to higher electron-donating properties. The TOF of imine 3 over the 20Pt-50Fe2O3/CNTs catalyst (TOF = 1071 h-1) is significantly higher than that over the 10Fe2O3-20Pt/CNTs catalyst (TOF = 224 h-1), although XPS analyses suggest that they have a similar surface. The obvious difference for these two catalysts is the surface position of Pt and Fe2O3. In the reductive coupling of nitrobenzene and furfural, the first step is the hydrogenation of nitrobenzene to aniline, which further reacts with furfural to produce imine. It is more favorable for the first hydrogenation steps over 20Pt-50Fe2O3/CNTs, in which Pt is located on the outer layer of Fe2O3 NPs. For the 10Fe2O3-20Pt/CNTs catalyst, the Fe2O3 layer is located on the surface of Pt NPs, thus nitrobenzene will first interact with this Fe2O3 layer and then diffuse to Pt-Fe3+-OH interface for hydrogenation. This additional step results in a low reaction rate. Therefore, Pt-Fe3+-OH interface sites on 20Pt-50Fe2O3/CNTs catalyst with a Pt thin shell on the surface of Fe2O3 NPs is more favorable for the synthesis of imine via the tandem reductive coupling of nitrobenzene and furfural. Conclusion We have demonstrated a new approach to tailor the Pt-Fe3+-OH site by depositing Pt on the surface of Fe2O3 NPs via ALD. After optimizing the ratio of the Pt-Fe3+-OH interface sites, the yield of imine (89%) from the reductive coupling of nitrobenzene and furfural is maximized over the 20Pt-50Fe2O3/CNTs catalyst. The formation of Pt-Fe3+-OH interface sites weakens the Pt-H bond and suppresses the further hydrogenation of imine to amine. The 20Pt-50Fe2O3/CNTs catalyst with a Fe2O3 core and Pt thin shell structure shows a higher utilization of Pt for the generation of Pt-Fe3+-OH interface sites compared with the 10Fe2O3-Pt20/CNTs catalyst with a


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Pt core and Fe2O3 shell structure. The special Fe2O3 core and Pt thin shell structure of the 20Pt50Fe2O3/CNTs catalyst results in a higher TOF in the generation of imine. ASSOCIATED CONTENT Supporting Information. Representative HAADF-STEM image of 10Fe2O3-20Pt/CNTs, STEM-EDS elemental mapping and EDS line analysis of NPs on 10Fe2O3-20Pt/CNTs; Representative HAADF-STEM image of 20Pt-50Fe2O3/CNTs, STEM-EDS elemental mapping of NPs on 20Pt-50Fe2O3/CNTs; HAADF-STEM image and EDX line analysis of the 20Pt50Fe2O3/CNTs; XRD of 20Pt-50Fe2O3/CNTs and 10Fe2O3-20Pt/CNTs; TEM and HRTEM images of different samples: 50Fe2O3/CNTs, 100Fe2O3/CNTs. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing ﬁnancial interest. ACKNOWLEDGMENT We appreciate the financial support from the National Natural Science Foundation of China (21403271, 21173248 and 21403272), the Hundred Talents Program of the Chinese Academy of Sciences, the Hundred Talents Program of Shanxi Province, and the Natural Science Foundation of Shanxi Province (201411012-1 and 2015021046).


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ABBREVIATIONS NPs, nanoparticles; ALD, Atomic layer deposition; CNTs, multiwalled carbon nanotubes; MeCpPtMe3,(methylcyclopentadienyl)trimethylplatinum; FeCp2, ferrocene; TEM, Transmission electron microscopy; HAADF-STEM, High-angle annular dark field scanning transmission electron microcopy; EDS, Energy dispersive spectroscopy; ICP-OES, Inductively coupled plasma optical emission spectrometry; XRD, X-ray powder diffraction; XPS, X-ray photoelectron spectroscopy; TCD, Thermal conductivity detector; H2-TPD, Hydrogen temperature-programmed desorption. REFERENCES 1.

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Tailoring PtâFe2O3 Interfaces for Selective Reductive Coupling

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