Structural Identification and Enhanced Catalytic Performance of

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Structural Identification and Enhanced Catalytic Performance of Alumina Supported Well-defined RhSnO2 Close-contact Heteroaggragate Nanostructures Caiyun Xu, Qi Li, Qiuyang Zhang, Kaijie Li, Hongfeng Yin, and Shenghu Zhou ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b01000 • Publication Date (Web): 23 Jul 2019 Downloaded from pubs.acs.org on July 29, 2019

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ACS Applied Nano Materials

Structural Identification and Enhanced Catalytic Performance of Alumina Supported Well-defined Rh-SnO2 Close-contact Heteroaggragate Nanostructures Caiyun Xu,† Qi Li,† Qiuyang Zhang,† Kaijie Li,† Hongfeng Yin‡ and Shenghu Zhou†,* †Shanghai

Key Laboratory of Multiphase Materials Chemical Engineering, School of

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]

ACS Paragon Plus Environment

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Abstract This work illustrates direct evidences of the formation of close-contact Rh-SnO2 heteroaggregate interfaces, and their enhanced catalytic hydrogenation performance was demonstrated. Alumina supported Rh-SnO2 heteroaggregate nanocatalysts were prepared by in-situ transformation of Rh@Sn core-shell nanostructures. Various characterization techniques, especially the studies of HAADF-STEM with phase mappings, confirm that Rh-SnO2 close-contact heteroaggragate nanostructures were obtained by in-situ calcination and following reduction of Rh@Sn core-shell nanoparticle precursors. The catalytic studies for hydrogenations of various substituted

nitroaromatics

show

that

the

Rh-SnO2/Al2O3

heteroaggregate

nanocatalysts not only improve the catalytic conversions but also enhance the catalytic selectivity. The combination of characterization and catalytic studies suggests that a synergistic effect originated from close-contact Rh-SnO2 heteroaggregate interfaces exists in the Rh-SnO2 system, which enhances their catalytic hydrogenation performance.

Key words: core-shell; heteroaggregates; Rh-SnO2; synergistic effect; hydrogenation


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1. Introduction Noble metals are considered as good catalysts for various hydrogenation,1-5 oxidation,6-8 olefination9-10 and hydrogenolysis11-12 reactions. Especially for hydrogenation reactions, noble metals demonstrate excellent performance due to their good coordination with H2 and unsaturated functional groups. For example, mesoporous silica loaded palladium nanocatalysts can selectively catalyze the hydrogenation of alkynes,13 and Pt is a good catalyst for 1,4-butynediol hydrogenation.14 Although noble metal catalysts are widely used in hydrogenation reactions, the catalytic selectivity is relatively low for hydrogenations of substrates containing several functional groups. One way to improve the hydrogenation selectivity of noble metal catalysts is to alloy them with other metals.15-18 Through alloying, the electronic states of noble metals are adjusted so that they can selectively coordinate the desired functional group to form the expected products.19 However, in some cases, the selectivity enhancement is accompanied by the activity loss due to the dilution of noble metal surface by additional metal atoms. Another way to improve selectivity of noble metal catalysts is to employ metal-support interaction (SMSI).20 By H2 reduction of Pt/TiO2 at high temperatures, the partially reduced Ti4O7 migrates onto Pt particles to form Pt-Ti4O7 interfaces that are highly active for methanation reactions. Generally, the traditional SMSI catalysts require the reducible supports, such as TiO2,21 Fe3O4,22 CeO2,23 and ZrO2,24 limiting



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the use of non-reducible alumina and silica in SMSI catalysts. To extend SMSI to alumina supported catalysts, we previously developed a methodology to synthesize Pd-M

Ⅱ xOy/Al2O3

(M

Ⅱ

=Fe, Co, Ni)25 and M

Ⅰ

-SnO2/Al2O3 (M

Ⅰ

=Pt, Pd)26-27

heteroaggregate nanocatalysts by in-situ transformation of bimetallic MⅠMⅡ alloy or M Ⅰ @M Ⅱ core-shell nanoparticles (NPs). Usually, core-shell structured NPs are desired materials with interesting properties.28-29 In some cases, M Ⅰ @M Ⅱ NPs are also ideal precursors to prepare such M Ⅰ -M Ⅱ xOy structures with thin M Ⅱ xOy in the outside surfaces. The underlining principle of the developed methodology is to in situ convert supported bimetallic NPs into close-contact M Ⅰ -M Ⅱ xOy heteroaggregate nanostructures which interfaces are highly active for hydrogenation reactions. By using pre-synthesized bimetallic NP precursors, the precise control of interface is possible, and the random distribution of promoters and active NPs is avoided by anchoring promoters onto active NPs. Although the modification of metal oxide on noble metal catalysts has been well accepted, directly obtaining the evidence of close-contact interface is extremely difficult for those small noble metal NPs modified by metal oxide promoters with thin layers. Some progress has been achieved with SMSI catalysts with reducible metal oxide supports as promoters,30-31 but research about heteroaggregate interfaces obtained from transformation of small bimetallic precursors is rare.32 Lacking direct evidences of the formation of MⅠ-MⅡxOy heteroaggregate nanostructures comes from the difficulty of characterization of supported small heteroaggreagte nanostructures,


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and the metal-metal oxide heteroaggregate nanostructures are just speculated from those indirect evidences, such as DRIFT-IR spectra with CO probes, H2-TPR studies and particle size analyses. Here, we select Rh-SnO2/Al2O3 system to first illustrate the direct evidence of in situ transformation of M Ⅰ @M Ⅱ core-shell NPs into close-contact M Ⅰ -M Ⅱ xOy heteroaggregate nanostructures by the studies of HAADF-STEM with elemental phase mappings, and the selective hydrogenations of substituted nitrobenzene over Rh-SnO2/Al2O3 were investigated. Rh catalysts have wide applications in small molecule electroreduction,33-36 syngas conversion,37 hydrogen generation38 and reaction.39 However, they are less active than Pd and Pt catalysts in hydrogenations of nitrobenzenes. In this study, by transformation of Rh@Sn structures into Rh-SnO2 close contact heteroaggregate nanostructures, the Rh-based catalysts show a significantly enhanced catalytic performance for substituted nitrobenzenes. Scheme 1 illustrates the synthetic route for Rh-SnO2/Al2O3 nanocatalysts. In this work, a sequential reduction method was used to obtain Rh@Sn core-shell NPs, which is further supported onto alumina to give Rh@Sn/Al2O3. After calcination at 550 oC and following reduction at 200 oC, the Rh@Sn/Al2O3 was converted into Rh-SnO2/Al2O3 nanocatalysts. For a series of substituted nitrobenzene hydrogenations, the Rh-SnO2/Al2O3 shows better catalytic performances relative to Rh/Al2O3 catalysts, due to the Rh-SnO2 interaction. In this study, the real structures of supported metal-oxide

heteroaggregate

nanostructures

were


first

characterized

by


HAADF-STEM with EDS phase mappings, and the close contact nanostructures were clearly confirmed. By designing Rh-SnO2 heteroaggregate nanostructures, the catalytic performance of Rh-based catalysts is first comparable to those active Pd or Pt-based catalysts, where the Rh-based catalysts are previously considered as poor catalysts for hydrogenation of nitrobenzenes.

Scheme 1. Schematic demonstration of the synthesis of Rh-SnO2/Al2O3 heteroaggregate nanocatalysts by Rh@Sn core-shell NP precursors. 2. Experimental 2.1 Chemicals Tin(II) 2-ethylhexanoate (C16H30O4Sn), Rhodium(II) trifluoroacetate dimer [Rh2(COOCF3)4, 98 %], sodium borohydride (NaBH4), chloronitrobenzene (CNB), nitrotoluene (NT) and nitroacetophenone (NAP) 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 (Al2O3) powders calcined at 550 °C for 2 h were purchased from Qingdao Haiyang Chemical Co., Ltd.. All reagents were purchase and


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used without further purification. 2.2 Catalyst Preparation. 2.2.1 Synthesis of Rh icosahedral NPs. The Rh icosahedral NPs were prepared by reduction of rhodium(II) trifluoroacetate dimer with EG in the presence of sodium trifluoroacetate under an N2 atmosphere.40 0.050 mmol Rh2(COOCF3)4, 2.0 mmol of PVP-K30 and 10.0 mL EG were charged into a 50 mL three-necked round-bottomed flask with magnetic stirring. The system was heated to 65 oC, and 0.40 mmol sodium trifluoroacetate in 2.0 mL ethanol were then added into the flask. After that, the mixture was further heated to and maintained at 110 oC for 15 min. The mixture’s color gradually changed from cyan to dark brown, indicating the reduction of Rh precursors. The mixture was subsequently heated to and maintained at 145 oC for 75 min, resulting in a black colloid. After cooled down to room temperature, the Rh NPs were collected by centrifugation with acetone followed by acetone washing several times, and then dispersed in 27.0 mL EG for further coating of Sn to obtain Rh@Sn core-shell NPs. 2.2.2 Synthesis of Rh@Sn NPs. The aforementioned 27.0 mL re-dispersed Rh EG colloids containing 0.10 mmol Rh, 0.10 mmol tin(II) 2-ethylhexanoate and 400.0 mg PVP were charged into a 50 mL three-necked round-bottomed flask with magnetic stirring under an N2 atmosphere. The mixture was heated to 50 oC, and 2.0 mmol NaBH4 in 3.0 mL ethanol were then added into the mixture. The system was kept at 50 oC for 15 min to complete the reduction of Sn precursors. After cooled down to



room temperature, the Rh@Sn NPs were collected by centrifugation with acetone and followed by acetone washing several times for further use. The as-prepared Rh@Sn NPs are calcined at 550 oC in air for 2.0 h and subsequently reduced by H2 at 200 oC for 3.0 h to give unsupported Rh-SnO2 NPs for further characterization. 2.2.3 Synthesis of Rh-SnO2/Al2O3 and Rh/Al2O3 Nanocatalysts. The Rh@Sn absolute ethyl alcohol colloids containing the desired amount of Rh were charged into a 100 mL three-necked round-bottomed flask. Alumina oxide (Al2O3) powders were added to the flask under magnetic stirring to achieve a theoretic Rh loading of 1.0 wt %. The resultant mixture was heated to and maintained at 60 oC, and purged with N2 continually to remove the alcohol, resulting in grey Rh@Sn/Al2O3 solids. The obtained Rh@Sn/Al2O3 powders were calcined at 550 oC in air for 2.0 h to obtain Rh2O3-SnO2/Al2O3, which is subsequently reduced by H2 at 200 oC for 3.0 h to give Rh-SnO2/Al2O3 nanocatalysts. The synthetic procedure of Rh/Al2O3 nanocatalysts is the same with that of Rh-SnO2/Al2O3 except that the Rh absolute ethyl alcohol colloids were used. The theoretic Rh loading of Rh/Al2O3 nanocatalysts is fixed at 1.0 wt %. The real metal loadings of Rh-SnO2/Al2O3 and Rh/Al2O3 nanocatalysts are determined by ICP-OES. 2.3 Catalyst Characterization The images of high resolution transmission electron microscopy (HRTEM) were obtained by using a JEM-2100 transmission electron microscope with an accelerating voltage of 200 kV. The images of high angle annular dark field scanning transition


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electron microscopy (HAADF-STEM) with an energy dispersive spectrometer were taken on a Talos F200X instrument, which is operated at 200 kV with 0.25 nm resolution for STEM images. X-ray diffractions (XRD) of various NPs were measured on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation in the 2θ range from 20 to 90°. The studies of X-ray photoelectron spectroscopy (XPS) of Rh@Sn and Rh-SnO2 NPs were carried out with an ESCALAB 250Xi multifunctional X-ray photoelectron spectroscope with an Al Kα radiation using C1s 284.8 eV as calibration. The XPS data were processed by XPS Peak 4.1 software. The H2 temperature programmed reduction (H2-TPR) of Rh2O3-SnO2/Al2O3 was investigated with an AutoChemII 2920 instrument in the temperature range from room temperature to 800 oC.

The diffuse reflectance Fourier transform infrared spectra (DRIFT-IR) with CO

probes of various materials were taken on a Nicolet-6700 Fourier transform infrared spectrometer. The alumina-supported catalysts were first purged by Ar at room temperature for 30 min and then purged with pure CO at a gas flow rate of 20 mL/min. The DRIFT-IR spectra were recorded after removing free CO by Ar purging. The real metal loadings of various catalysts were measured by inductively coupled plasma optical emission spectrometer (ICP-OES). 2.4 Catalytic Hydrogenations of Substituted Nitrobenzenes over Rh/Al2O3 and Rh-SnO2/Al2O3 Nanocatalysts A portion of 1.0 g of various substituted nitrobenzenes, 0.1 g of nanocatalysts and 25 mL absolute ethyl alcohol were charged into a 50 mL three-necked



round-bottomed flask. The catalytic reactions were carried out at 60 oC for a desired reaction time under 0.1 MPa H2 atmosphere, and magnetic stirring at 1100 rpm was applied to eliminate the mass transport effect. The product solution was analyzed by a gas chromatograph equipped with a flame ionization detector. For reusability test of Rh-SnO2/Al2O3 nanocatalysts, after first cycle, the catalysts were collected by centrifugation, followed by absolute ethyl alcohol washing several times, and dried in an oven at 60 oC overnight. Due to the catalyst loss in the recovery process, the amounts of substituted nitrobenzenes and absolute ethyl alcohol in the subsequent cycle were proportional to the amount of recovered Rh-SnO2/Al2O3 nanocatalysts. The detailed product analysis by gas chromatograph is shown in Figure S1-S9 in Supporting Information. 3. Results and Discussion 3.1 Synthesis and Characterization of Rh-SnO2/Al2O3 Nanocatalysts A sequential reduction method was used to synthesize Rh@Sn core-shell NPs. In the presence of NaCOOCF3, icosahedral Rh NPs were formed by reduction of Rh precursors with ethylene glycol. The obtained Rh NPs were used as seeds to deposit Sn through reduction of Sn precursors with NaBH4 to form Rh@Sn NPs. To clearly identify the real structures of Rh@Sn NPs and Rh-SnO2/Al2O3 nanocatalysts, characterization techniques of XRD diffractions, particle size analysis, EDS phase mappings and XPS studies are used. Figure 1 shows the XRD patterns of Rh, Rh@Sn and Rh-SnO2 NPs. As shown in the XRD pattern of Rh NPs in Figure 1a, the Rh


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Page 11 of 34

(111) diffraction was clearly observed while the high 2θ degree diffractions are weak, indicating the formation of relatively small Rh NPs. The XRD pattern of as-synthesized Rh@Sn NPs in Figure 1b only exhibit the (111) diffraction of Rh without the XRD diffractions of Sn, suggesting the formation of amorphous Sn shells. The phenomenon of formation of amorphous Sn shells in core-shell nanostructures has been reported in literature.26 Accordingly, the XRD pattern of Rh-SnO2 materials in Figure 1c, obtained through calcination at 550 oC and subsequent H2 reduction at 200 oC of Rh@Sn NPs, only show Rh diffractions, again indicating the formation of amorphous SnO2. The XRD pattern of Rh-SnO2/Al2O3 is shown in Figure S10 in SI. Due to the low Rh loading (theoretic loading-1.0 wt % ), only alumina diffractions are observed. The similar phenomenon is also observed in the XRD pattern of Rh2O3-SnO2/Al2O3. Rh(111)

a) 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


Rh(200)

b)

Rh(220)

Rh(311)

c)

Rh(PDF#05-0685)

SnO2(PDF#41-1445) Sn(PDF#04-0673) 20

40

60

2 Theta (degree)

80

Figure 1. XRD patterns showing: a), Rh NPs; b), Rh@Sn NPs; c), Rh-SnO2 NPs. In the 2θ range from 25 to 35 degree, as prepared Rh (Figure 1a) and Rh@Sn (Figure 1b) NPs show similar weak diffractions while the calcined and reduced



Rh-SnO2 NPs (Figure 1c) do not show diffractions in the mentioned 2θ range, suggesting that the weak diffractions in the 2θ range are possibly due to the organic capping agents. Figure S11 illustrates the XRD pattern of unsupported Rh2O3-SnO2 NPs, which is obtained from calcination of as-prepared Rh@Sn NPs at 550 oC. As shown in Figure S11, the Rh2O3-SnO2 only shows XRD diffractions of Rh2O3, further suggesting the formation of amorphous SnO2 by calcination of Rh@Sn NPs. As shown in Figure 2a and 3a, Rh NPs illustrate faceted shapes with an average particle size of 4.6 nm. In contrast, Rh@Sn NPs in Figure 2b basically show spherical shapes, with an average particle size of 5.4 nm (Figure 3b). The change of particle shapes from Rh to Rh@Sn as well as the particle size increase is consistent with the deposition of Sn onto Rh NP surfaces by a sequential reduction method. The lattice spacing of 0.196 nm and 0.222 nm in the inserts of Figure 2a and 2b respectively correspond to the lattice spacing of (200) and (111) planes of Rh fcc structures, and the absence of Sn lattice spacing in the insert of Figure 2b further suggests the formation of amorphous Sn shells. TEM images of Rh/Al2O3 catalysts are shown in Figure 2c, and their particle size analysis is shown in Figure 3c. After calcination and subsequent reduction, the shapes of Rh NPs change from faceted to spherical due to thermal treatment, with a particle size increasing from 4.6 nm of individual Rh NPs to 6.2 nm of supported Rh NPs. The TEM images of Rh2O3-SnO2/Al2O3 obtained from calcination of Rh@Sn/Al2O3 are shown in Figure S12 in SI, where basically spherical oxide nanoparticles (~ 6.8 nm) are clearly visible. Figure 2d and 3d illustrate TEM


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images and particle size analysis of Rh-SnO2/Al2O3 catalysts, respectively. A smaller particle size increase from 5.4 nm of Rh@Sn (Figure 3b) to 5.8 nm of supported Rh-SnO2 (Figure 3d) is observed for Rh-SnO2/Al2O3 catalysts, indicating the stabilizing effect of SnO2 on Rh particle size in Rh-SnO2 nanostructures. Moreover, the lattice spacing in the inserts of Figure 2c and 2d respectively correspond to the lattice spacing of the lattice spacing of (111) planes of fcc structures, and the absence of SnO2 lattice spacing in the insert of Figure 2d suggests the formation of amorphous SnO2 in Rh-SnO2 nanostructures. The as-prepared Rh@Sn NPs are calcined and subsequently reduced to give unsupported Rh-SnO2 NPs, and their TEM images with corresponding particle size analysis are shown in Figure S13. Due to heat treatment, these unsupported Rh-SnO2 NPs aggregate and are no longer dispersed.

Figure 2. TEM images showing: a), Rh NPs; b), Rh@Sn NPs; c), Rh/Al2O3 catalysts; d), Rh-SnO2/Al2O3 catalysts. The inserts of a)-d) showing HRTEM images with lattice spacing. The scale bars in a)-d) are 20 nm, and those in the inserts are 2 nm.



35

a)

b)

4.6±0.5 nm Distribution (%)

Distribution (%)

5.4±0.8 nm

20

30 25

15

20

10

15 10

5

5 0 3.5 60

0 4.0

4.5

5.0

Particle Size (nm)

c)

5.5

6.0

6.2±0.7 nm

4.0

40

4.5

5.0

5.5

6.0

6.5

7.0

Particle Size (nm)

d)

7.5

5.8±0.5 nm

Distribution (%)

50

Distribution (%)

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

Page 14 of 34

40 30 20 10

30

20

10

0

0 5

6

7

Particle Size (nm)

8

4.5

5.0

5.5

6.0

Particle Size (nm)

6.5

7.0

Figure 3. Particles size analyses showing: a), Rh NPs; b), Rh@Sn NPs; c), Rh/Al2O3; d), Rh-SnO2/Al2O3. To investigate the Rh and Sn distribution in the Rh@Sn NPs, HAADF-STEM studies were carried out. Figure 4a shows the HAADF-STEM images of Rh@Sn NPs, which is selected for EDS phase mapping. As showed in Figure 4b and 4c, the Sn phase mapping is larger than the Rh phase mapping. The combination of Rh and Sn phase mapping in figure 4d further shows that Rh atoms are mainly distributed in the cores while Sn atoms are mainly present in the shells, further confirming the formation Rh@Sn core-shell like structures. Figure 5a-5d show HAADF-STEM images of Rh-SnO2/Al2O3 catalysts. As shown in Figure 5b and 5c, the Rh distribution is more concentrated in some places while the Sn distribution is sparser, indicating that Rh is in the centers and Sn is located in the outside surfaces. Moreover, the combination of Rh and Sn phase mapping in Figure 5d shows that Rh and SnO2


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are in close-contact, further confirming the formation Rh-SnO2 close-contact heteroaggragate nanostructurs by in-situ transformation of Rh@Sn core-shell NPs.

Figure 4. a)-d) showing HAADF-STEM images of Rh@Sn NPs: a), the selected areas for phase mappings; b), Rh phase mappings; c), Sn phase mappings; d), the combined Rh and Sn phase mappings. The scale bars are 10 nm.

Figure 5. a)-d) showing HAADF-STEM images of Rh-SnO2/Al2O3: a), the selected areas for phase mappings; b), Rh phase mappings; c), Sn phase mappings; d), the combined Rh and Sn phase mappings. The scale bars are 50 nm. Figure 6a and 6b respectively demonstrate the XPS spectra of Rh@Sn and Rh-SnO2, where Rh-SnO2 is obtained by calcination of Rh@Sn NPs at 550 °C and



subsequent reduction by H2 at 200 oC. To increase the signal/noise ratio, unsupported Rh@Sn and Rh-SnO2 are used in XPS studies. The method of Doniach and Sunjic is employed for curve fitting, and the binding energy assignments are in accordance with those in the literature.41-42 The binding energies at 306.6/311.4 and 307.2/312.0 eV in Figure 6a can be assigned to 3d5/2/3d3/2 of Rh0 and Rhδ+ species, respectively, while the binding energies at 486.1/494.0 and 486.7/495.2 eV are assigned to 3d5/2/3d3/2 of Sn0 and Sn4+ species, respectively. As for Rh-SnO2 in Figure 6b, the binding energies at 485.5/493.9, 486.4/494.9, and 486.8/495.2 eV are assigned to 3d5/2/3d3/2 of Sn0, Sn2+, and Sn4+ species, respectively, while the binding energies at 307.0/311.7 and 308.8/313.5 eV are respectively assigned to 3d5/2/3d3/2 of Rh0 and Rh3+ species.

b) Rh03d3/2

Rhδ+3d3/2 310

Intensity (a.u.)

a) 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

Page 16 of 34

Sn03d3/2 Sn03d5/2

Rh03d5/2

Sn4+3d3/2

Rhδ+3d5/2 305

500

495

490

485

0 Rh 3d3/2

3+ Rh 3d3/2

Sn4+3d5/2 480

Sn4+3d5/2

0 Rh 3d5/2

315

Sn4+3d3/2

Sn2+3d3/2

3+ Rh 3d5/2 310

Binding Energy (ev)

Sn03d3/2 Sn03d5/2

305

500

495

Sn2+3d5/2 490

485

Binding Energy (ev)

Figure 6. XPS spectra showing; a), as-synthesized Rh@Sn NPs; b), Rh-SnO2. Table 1 summarizes the Sn/Rh atomic ratios of Rh@Sn and Rh-SnO2 NPs by XPS and EDS. The Sn/Rh ratio of 0.8/1 is observed by EDS analysis, which is close to that of the starting materials. However, a Sn/Rh ratio of 2.3/1.0 is obtained by XPS analysis, which is significantly higher than that obtained by EDS analysis. Since XPS is a surface-sensitive technique, the high Sn/Rh ratio suggests that Sn atoms are


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mainly present in the shells of Rh@Sn NPs. The similar high Sn/Rh ratio of 3.4/1.0 is also observed for Rh-SnO2, further confirming the transformation of surface Sn into surface SnO2 by calcination. As shown in Table 1, 17.5 % of oxidized Rh species and 24.6 % of oxidized Sn species are observed for Rh@Sn NPs, due to their exposure to air. For calcined and subsequently reduced Rh-SnO2, 35.4 % of oxidized Rh species and 13.3 % of Sn0 species are observed, where the former is ascribed to the oxidation of some surface Rh atoms and the latter is due to the H2 reduction of some Sn4+ species. Table 1. The Sn/Rh atomic ratios obtained by XPS and EDS analysis for Rh@Sn and Rh-SnO2 NPs. Sample

oxidized Rh/Rh0 oxidized Sn/Sn0 Sn/Rh (XPS) Sn/Rh (EDS)

Rh@Sn

17.5/82.5

24.6/75.4

2.3/1.0

0.8/1.0

Rh-SnO2

35.4/64.6

86.7/13.3

3.4/1.0

0.8/1.0

Figure 7a shows H2-TPR studies of Rh2O3/Al2O3 and Rh2O3-SnO2/Al2O3 that were respectively prepared from calcination of as-synthesized Rh/Al2O3 and as-synthesized Rh@Sn/Al2O3. There is only one H2 consumption peak at 85 oC for Rh2O3/Al2O3 while four H2 consumption peaks at 85, 209, 407, 563 oC exist for Rh2O3-SnO2/Al2O3. The peak at 85 oC is ascribed to the reduction of Rh3+ species,43-44 while the peak at 209 oC is possibly due to the reduction of Rh3+ species in the Rh2O3-SnO2 interfaces. The interaction between Rh2O3 and SnO2 makes Rh2O3 be reduced at relatively high temperatures, and the similar phenomenon has been



reported in other systems.44 The H2 consumption peak at 563 oC is due to the reduction of SnO2,45-46 and that at 407 oC is possibly due to the reduction of oxidized Sn species with different chemical states. a)

b)

Rh2O3-SnO2/Al2O3

100

200

300

400

500 o

Temperature ( C)

600

Absorbance (a.u.)

Rh2O3/Al2O3

Hydrogen Signal (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

Page 18 of 34

700 2150

2063 2025

2095

Rh-SnO2/Al2O3

2071

2091

2029

Rh@Sn/Al2O3

2059 2025

1915 Rh/Al2O3

2100

2050

2000

1950

1900

1850

1800

Wavenumber (cm-1)

Figure 7. a), H2-TPR profiles of Rh2O3/Al2O3 and Rh2O3-SnO2/Al2O3; b), DRIFT-IR spectra with CO probes of Rh/Al2O3, Rh@Sn/Al2O3 and Rh-SnO2/Al2O3. DRIFT-IR spectra with CO probes of various materials are shown in Figure 7b. As shown in the black spectra of Rh/Al2O3, three peaks appeared at 2059, 2025 and 1915 cm-1 can be assigned to CO adsorption on (100), (111) and (100) Rh surfaces, respectively.47-48 As for the red spectra of Rh@Sn/Al2O3, the bands at 2071 and 2029 cm-1 and the shoulder band at 2091 cm-1 can be assigned to CO adsorption on (100), (111) and (220) of Rh Surfaces.49-51 No bands for CO adsorption on Sn surfaces are observed, and this phenomenon is normal since CO is not adsorbed on Sn.52-53 Because DRIFT-IR with CO probes is a surface-specific technique, the appearance of CO bands for CO adsorption on Rh surfaces indicates that some Rh atoms are present on the surfaces of Rh@Sn NPs. Similarly, three CO bands at 2095, 2063 and 2025 cm-1 appear for Rh-SnO2/Al2O3, indicating the heteroaggragate nanostructures of Rh-SnO2 with naked Rh surfaces. Through the combination of XPS, XRD,


Page 19 of 34 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


HAADF-STEM with phase mappings and DRIFT-IR with CO probes, it is concluded that Rh@Sn NPs are core-shell like nanostructures with some Rh atoms on the surfaces, and the transformation of Rh@Sn NPs by calcination and reduction will result in Rh-SnO2 close-contact heteroaggregate nanostructures. 3.2 Catalytic Performance of Rh/Al2O3 and Rh-SnO2/Al2O3 Nanocatalysts The catalytic hydrogenations of substituted nitrobenzenes over Rh/Al2O3 and Rh-SnO2/Al2O3 nanocatalysts are carried out at the conditions of 60 oC and a 0.1 MPa H2 atmosphere. As shown in Figure S14, a vigorous stirring is required to eliminate the mass transport effect. When the magnetic stirrings are above 700 rpm, the catalytic yields are basically same, indicating the elimination of the mass transport effect. Therefore, a vigorous stirring at 1100 rpm is applied to all catalytic hydrogenation experiments. The theoretic Rh metal loadings of these catalysts are 1.0 wt %. The real Rh loadings of Rh-SnO2/Al2O3 and Rh/Al2O3 nanocatalysts obtained by ICP-OES are 0.95 wt %, and the Sn loading of Rh-SnO2/Al2O3 is 0.89 wt %. The Sn/Rh ratio of 0.81/1.00 obtained by ICP-OES is consistent with that of 0.83/1.00 obtained by EDS analysis in Table 1. The N2 adsorption-desorption isotherms and pore width distributions by NLDFT method of Rh-SnO2/Al2O3 nanocatalysts are shown in Figure S15 in SI, and the BET surface area of Rh-SnO2/Al2O3 nanocatalysts is 197.4 m²/g. Hydrogenations of m-CNP over Rhx-(SnO2)y/Al2O3 with different Rh/Sn ratios are shown in Table S1. Rh-SnO2/Al2O3 shows the best catalytic performance with a 100 % m-CNB conversion and 97.7 % m-chloroaniline selectivity



Page 20 of 34

even at a shorter reaction time. As shown in Table S1, more SnO2 will decrease the conversion with a slightly increased selectivity while less SnO2 will decrease conversion as well as selectivity. Therefore, Rh-SnO2/Al2O3 is selected for hydrogenations of various substrates. In this study, alumina mainly functions as supports to disperse the Rh-SnO2 NPs. As shown in Table S1, the alumina itself shows a negligible catalytic activity. Table 2 illustrates the hydrogenations of various substituted nitrobenzene over Rh/Al2O3 and Rh-SnO2/Al2O3 nanocatalysts. As shown in Table 2, for hydrogenations of m-, o-, and p-CNB, Rh-SnO2/Al2O3 shows significantly enhanced conversions and selectivity relative to Rh/Al2O3. For example, a 100.0 % m-CNP conversion and 97.7 % m-chloroaniline selectivity are achieved with Rh-SnO2/Al2O3 while those with Rh/Al2O3 are 11.1 % and 80.8 %, respectively. For hydrogenations of m-, o-, and p-NT, Rh-SnO2/Al2O3 illustrates significantly enhanced NT conversions while maintaining the basically same aminotoluene selectivity as those with Rh/Al2O3. The similar trend is also observed for hydrogenations of m-, o- and p-NAP. Rh-SnO2/Al2O3

illustrates

significantly

enhanced

NAP

conversions

or

aminoacetophenone selectivity. In this study, nitrobenzenes with different groups (CH3, carbonyl and chloride) at different substitution positions (m-, o-, and p-) are investigated.

Generally,

for

hydrogenations

of

nitroacetophenone

and

chloronitrobenzene, the catalytic activity as well as selectivity is significantly enhanced while the activities of nitrotoluenes are significantly improved by using


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Rh-SnO2/Al2O3. In this work, the Rh-SnO2/Al2O3 nanocatalysts show enhanced catalytic performance for all these nine substrates, indicating the wide adaptability of such metal-oxide heteroaggregate nanostructures. Table 2. Catalytic hydrogenations of various substituted nitroaromatics over Rh/Al2O3 and Rh-SnO2/Al2O3 nanocatalysts. Reactants

Main products

Cl

Cl

NH2

NO2

Cl

Cl

NO2

NH2

Cl

Cl

NH2

NO2

CH3

CH3

NH2

NO2

CH3

CH3 NO2

NH2

CH3

CH3

NO2

NH2

NO2

O

CH3

NH2

O

CH3

NO2 O

CH3

Conversion (%)

Selectivity (%)

2h

Rh/Al2O3

11.1

80.8

2h

Rh-SnO2/Al2O3

100.0

97.7

3h

Rh/Al2O3

16.9

78.3

3h

Rh-SnO2/Al2O3

100.0

96.6

4h

Rh/Al2O3

25.8

74.8

4h

Rh-SnO2/Al2O3

99.3

94.1

3h

Rh/Al2O3

11.6

76.7

3h

Rh-SnO2/Al2O3

93.2

97.1

2h

Rh/Al2O3

15.5

100.0

2h

Rh-SnO2/Al2O3

95.9

96.7

3h

Rh/Al2O3

7.3

100.0

3h

Rh-SnO2/Al2O3

98.7

100.0

4h

Rh/Al2O3

23.5

100.0

4h

Rh-SnO2/Al2O3

100.0

100.0

4h

Rh/Al2O3

100.0

18.7

O

CH3

4h

Rh-SnO2/Al2O3

100.0

93.4

4h

Rh/Al2O3

58.1

54.5

4h

Rh-SnO2/Al2O3

100.0

100.0

NH2

CH3

aReaction

Catalysts

NH2

NO2

O

Timea

O

CH3

conditions: reactants-1.0 g; supported catalysts-0.1 g with Rh loadings of 0.95 wt %; EtOH-25.0

mL; H2-0.10 MPa; reaction temperature-60 oC; speed of agitation-1100 rpm;

The TEM images of Rh-SnO2/Al2O3 after m-CNB hydrogenation are shown in



Figure S16 in SI. After reaction, no significant changes about particle size and morphology are observed, indicating the structural stability at the mentioned reaction conditions. The catalytic enhancement of close contact Rh-SnO2 nanostructures is due to the interaction between Rh and SnO2 in the Rh-SnO2 interfaces. The enhancement due to metal-oxide interaction is well known. In this study, the Rh-SnO2 interfaces function as the catalytic centers, where the SnO2 could modify the electron distribution of Rh centers and may facilitate the reactant adsorption and product desorption. Moreover, the introduction of metal oxide onto metal surfaces sometimes result in a blockage effect, which also could enhance catalytic performance.54-55 3.3. Reusability of Rh-SnO2/Al2O3 nanocatalysts Cycle to cycle m-CNB hydrogenations were carried out to investigate the reusability of Rh-SnO2/Al2O3 catalysts. Roughly, 10~15 % of catalysts were lost during the recovery process after each cycle. To maintain the ratios of reactants/catalysts and reactants/solvents, the amounts of absolute ethyl alcohol and reactants are decreased in the subsequent cycle due to catalyst loss in the recovery process. Table 3 presents the results of 5 cycles of hydrogenations over Rh-SnO2/Al2O3 catalysts. As shown in Table 3, during the recycling experiments, the conversions of m-CNB are above 99.5 % while the selectivity of m-chloroaniline is above 97.7 %, indicating a good catalytic stability of Rh-SnO2 heteroaggregate nanostructures.


Page 22 of 34

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Table 3. Cycle to cycle m-CNB Hydrogenation Rh-SnO2/Al2O3 Nanocatalysts. cycle indexa

m-CNB

conversion

(g)

(g)

(%)

1

0.1000

1.00

100.0

97.7

387.5

2

0.0876

0.876

100.0

100.0

391.9

3

0.0788

0.788

99.7

98.0

376.6

4

0.0602

0.602

99.7

98.0

357.0

5

0.0494

0.494

99.5

98.8

370.1

aReaction

m-CAN selectivity

TOF (h-1)b

Catalyst

(%)

conditions: EtOH, 25.0 mL in cycle 1; the volume of EtOH in the following cycles

decreased according to the weight of m-CNB at a fixed EtOH/m-CNB ratio; H2, 0.10 MPa; reaction time, 2.0 h; reaction temperature, 60 °C; agitation speed, 1100 rpm. bTOF

is obtained at the reaction time of 1.5 hour (~ 90 % of m-CNB conversion).

The m-CNB removal and m-CAN selectivity with the reaction evolution during once more five cycle experiments are shown in Figure 8a and 8b, respectively. Apparently, during these cycles, conversion and selectivity slightly decrease, suggesting the structural stability of Rh-SnO2 heteroaggregate nanostructures under these mild reaction conditions. Moreover, the m-CAN selectivity increases with the reaction time increasing. It is well known that a series of intermediates will form during the hydrogenation of nitrobenzene. The selectivity increase is due to the continuous transformation of the intermediates into final products. The turnover frequency per hour (TOF per hour) at reaction time of 1.5 hour is calculated by dividing the total number of converted m-CNB molecules by the total number of Rh atoms in catalysts. As shown in Table 3, the TOFs show a slight decrease, further confirming the catalytic stability of Rh-SnO2 nanostructures.



80

100

a) 1 2 3 4 5

60 40 20 0.5

1.0

Time (h)

1.5

2.0

80 Selectivity (%)

100

Conversion (%)

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

Page 24 of 34

b) 1 2 3 4 5

60 40 20 0.5

1.0

1.5 Time (h)

2.0

Figure 8. a), the effect of reaction time on the m-CNB conversion during five cycle experiments; b), the effect of reaction time on the m-CAN selectivity during five cycle experiments. Reaction conditions: EtOH, 25.0 mL in cycle 1; the volume of EtOH in the following cycles decreased according to the weight of m-CNB at a fixed EtOH/m-CNB ratio; H2, 0.10 MPa; reaction temperature, 60 °C; agitation speed, 1100 rpm.

4. Conclusion In this work, various characterization techniques are employed to obtain the direct evidences of the formation of Rh-SnO2 heteroaggregate nanostructures, and the correlation between the structures and their catalytic performance is elucidated. Rh-SnO2 heteroaggregate nanostructures were prepared by in-situ transformation of Rh@Sn core-shell like NPs, and Rh-SnO2/Al2O3 shows enhanced catalytic performance of various substituted nitrobenzenes relative to the control Rh/Al2O3. The use of Rh@Sn NPs facilitates the formation of well-defined Rh-SnO2 interfaces, which is highly active for hydrogenations of substituted nitrobezenes. The


Page 25 of 34 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


performance enhancement is ascribed to the interaction between Rh and SnO2, and the synergetic effect not only promotes the catalytic activity but also the catalytic selectivity. Through the interaction between Rh and SnO2, the thermal and catalytic stability of Rh-SnO2 nanostructures are enhanced. It is believed that this synthetic method can be extended to other systems, and this type of nanostructures may find more applications in heterogeneous catalysis. Supporting Information The synthesis of Rhx-(SnO2)y/Al2O3 (x/y=2/1, 1/2) and their catalytic performance, the

GC

analysis

details,

more

XRD

patterns

and

TEM

images,

N2

adsorption-desorption isotherm, and more comparisons of different catalysts. Conflicts of interest There are no conflicts to declare. Acknowledgment S. Zhou and H. Yin thank the National Natural Science Foundation of China for financial supports (Grant No. 21776090 and 21571183), and this work is also partially supported by Industrial R&D Foundation of Ningbo (Grand No. 2017B10040). Author Information Corresponding Author *E-mail: [email protected] ORCID S. Zhou: 0000-0002-8203-6546



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Structural Identification and Enhanced Catalytic Performance of

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