TiO2 Sandwich Catalyst for Highly Selective

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Porous TiO2/Pt/TiO2 sandwich catalyst for highly selective semi-hydrogenation of alkyne to olefin Haojie Liang, Bin Zhang, Huibin Ge, Xiaomin Gu, Shufang Zhang, and Yong Qin ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02032 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 21, 2017

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ACS Catalysis

Porous TiO2/Pt/TiO2 sandwich catalyst for highly selective semi-hydrogenation of alkyne to olefin Haojie Liang1,2, Bin Zhang1*, Huibin Ge1,2, Xiaomin Gu1,2, Shufang Zhang1,2, Yong Qin1* 1 State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China. 2 University of Chinese Academy of Sciences, Beijing, 100049 China.

ABSTRACT: The tailoring of metal-oxide interface is a powerful approach to enhance catalytic efficiency of heterogeneous catalyst. However, the function of the metal-oxide interface is still not clearly understood in most catalytic processes. The construction of heterogeneous catalyst with single interface sites will be a straightforward way to reveal the interface effect. In this work, we introduced a simple strategy to synthesize porous TiO2/Pt/TiO2 sandwich catalyst by atomic layer deposition. All Pt nanoparticles were covered by two porous TiO2 layers in this sandwich structure, creating dominant Pt-TiO2 interface sites. The TiO2/Pt/TiO2 sandwich catalyst shows good catalytic performance in the tandem ammonia borane decomposition and semi-hydrogenation of various alkynes with high selectivity and stability. In contrast, the Pt nanoparticles without complete coverage of porous TiO2 layers have a low selectivity in semihydrogenation of alkynes. The catalyst also exhibits high selectivity in hydrogenation of -C=O bond of α,β-unsaturated aldehyde. The high selectivity of the TiO2/Pt/TiO2 sandwich catalyst can

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be ascribed to the electron-rich property of the Pt-TiO2 interface sites, which favor the adsorption of alkyne with electrophilicity but inhibit the over-hydrogenation of C=C bond.

KEYWORDS: tandem catalyst, atomic layer deposition, semi-hydrogenation, sandwich structure, metal-oxide interface Introduction Heterogeneous catalysts, especially metal oxide supported metal nanoparticles, are widely used in petrochemicals, fine chemicals, oil refinery, polluted gas purification, and hydrogen production.1-3 In order to obtain excellent catalytic efficiency, many approaches have been developed, including controlling the sizes and shapes of metal nanoparticles, adding promoters, and enhancing metal-oxide interaction.4-7 Other than function as supports to disperse metal nanoparticles, metal oxides also promote the catalytic performance by interplaying with metal nanoparticles to form the metal-oxide interface.5, 8-11 Generally, the active sites induced by the metal-oxide interface on the supported catalyst account for only a small portion of all surface sites. For reactions that metal-oxide interface sites show dramatically higher catalytic activity than other sites, such as CO oxidation12, the outstanding property of metal-oxide interface sites is clearly observed. However, for reactions that metal sites show higher catalytic activity than metal-oxide interface sites, the function of the interface sites cannot be easily revealed. Therefore, in order to reveal the advantage of metal-oxide interface, it is necessary to maximize the ratio of metal-oxide interface by forming only metal-oxide interface sites or blocking the metal sites. The semi-hydrogenation of alkyne to olefin is an important industrial process in both the production of intermediates for fine chemicals and purification of alkene for the production of polyolefin.4 It is a structure-sensitive reaction, and over-hydrogenation is thermodynamically


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favored.13-14 Both homogeneous catalysts15-17 and heterogeneous catalysts18 are used in this certain reaction. Pd-based catalyst with additives is most widely used for this type of reaction, but the selectivity of Pd catalyst often decreases sharply at a high conversion rate4, 19-20. Ni-, Cu-, and Au-based catalysts suffer from low activity and selectivity.21-22 Pt-based catalyst is a widely used highly active catalyst in hydrogenation reaction, but there are only few reports on its use in alkyne hydrogenation in recent years due to low selectivity.15, 23-24 The precise interface tailoring of Pt-based catalyst is an alternative strategy to achieve semi-hydrogenation with both high activity and selectivity. In general, traditional methods have limitations in tailoring metal-oxide interface because of low precision and complicated operation, thus it is difficult to correlate the catalytic performance and the interface structures especially in structure-sensitive reactions.14,

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Atomic layer

deposition (ALD) is a high-level film growth technology, which can deposit films of metals, oxides, polymers and other materials with uniform thickness, excellent conformity, and high reproducibility due to its self-limiting feature.25 Relying on its outstanding advantages, ALD can be used to design nano-catalysts and tailor metal-oxide interface with atomic-scale precision.26-28 Recently, we reported the tailoring of Ni-Al2O329, Cu-ZnO30, Pt-Al2O331, and Pt-Fe(3+)OH interfaces32 via a series of ALD strategies. We also synthesized a tandem catalyst with multiple metal-oxide interfaces by ALD, which showed remarkably high catalytic efficiency in the tandem reaction of hydrazine hydrate decomposition on Ni/Al2O3 interface and nitrobenzene hydrogenation on Pt/TiO2 interface.33 In this work, we report the synthesis of TiO2/Pt/TiO2 catalysts with a tubular sandwich structure by ALD. Pt nanoparticles are confined in-between two porous TiO2 layers. The TiO2/Pt/TiO2 catalysts show excellent olefin selectivity (95%) and high stability for tandem


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ammonia borane decomposition and semi-hydrogenation of alkyne. The superior catalytic performance can be ascribed to the fact that no Pt metal surface sites, but only Pt-TiO2 interface sites, including Pt species at interface and Pt species not at interface but influenced by the interface, are exposed in the designed structure. Results and discussion

Scheme 1. Schematic illustration of the synthesis process of porous xTiO2/Pt/TiO2 sandwich nanostructures. The sandwich catalysts (xTiO2/Pt/TiO2) were synthesized by sequential ALD of TiO2 (300 cycles), Pt (20 cycles), and TiO2 (x cycles) on carbon nanofibers (CNFs) template, followed by calcination in air at 500 °C (Scheme 1). Figure 1A shows the TEM image of the Pt/TiO2 catalyst before coating the outer TiO2 layer. The average particle size of Pt nanoparticles is 3.0 nm (Figure S1). Figures 1C and 1D show that one layer of Pt nanoparticles are encapsulated by two TiO2 layers in the 30TiO2/Pt/TiO2 and 50TiO2/Pt/TiO2. The inner TiO2 layer of sandwich structure keeps a constant thickness as that of Pt/TiO2 catalyst (42.0 nm), while the thickness of


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outer TiO2 layer linearly increases from 0 to 15.5 nm with the increase of ALD cycle number from 0 to 100 (Figures S1, S2). After 30 ALD cycles for the outer TiO2 layer, the two TiO2 layers in the sandwich structure result in a full encapsulation of Pt nanoparticles. Furthermore, the porous structure in the TiO2 layer was observed (Figure S1), which was resulted from the crystallization of TiO2 film by calcination34. N2-physical absorption further reveals the pore structure of the sandwich catalysts (Table S1). The BJH pore diameter of the sandwich catalysts is 2.6 ± 0.3 nm.

Figure 1. TEM images of A) Pt/TiO2, B) and C) 30TiO2/Pt/TiO2, D) 50TiO2/Pt/TiO2. XPS was applied to measure the surface electronic properties of the catalysts (Figure 2A). The signal intensity of Pt 4f peak distinctly decreased with the increase of ALD cycle numbers of the


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outer TiO2 layer from 0 to 30, indicating an increased coverage of TiO2 on Pt. The outer TiO2 layer also resulted in the shift of Pt 4f spectrum to low binding energy from 71.1 eV (Pt/TiO2) to 70.7 eV (30TiO2/Pt/TiO2) due to the increase of surface Pt species interacted with TiO2. This indicates that the Pt species interacted with TiO2 have a low binding energy with an electron-rich property. The electron donation from interface TiO2 (Figure S4) is responsible for this electronrich property of surface Pt species in the sandwich structure.

Figure 2. A) XPS analysis of the catalysts (Pt/TiO2, 10TiO2/Pt/TiO2, and 30TiO2/Pt/TiO2). The normalized intensity of Pt L3-XANES spectra (B) and corresponding Fourier transform k3weighted EXAFS spectra (C) of Pt/TiO2 and 50TiO2/Pt/TiO2. X-ray absorption fine spectra (XAFS) emphasized on electronic structure and nearest-neighbor atoms structure of Pt nanoparticles. As shown in Figure 2B, the “white line” intensities of Pt L3edge normalized X-ray absorption near edge structure (XANES) spectra for both Pt/TiO2 and 50TiO2/Pt/TiO2 catalysts, which is based on the density of unoccupied state and oxidation state35, are similar to that of Pt foil, indicating the dominance of Pt0. The Fourier transforms (R space,


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Figure 2C) of extended X-ray absorption ﬁne structure (EXAFS) data of both Pt/TiO2 and 50TiO2/Pt/TiO2 samples are similar, and both have one prominent peak at 2.5 Å from Pt-Pt contribution and one weak peak at 1.6 Å from Pt-O contribution, indicating that the coordination and valence states of bulk phase of Pt nanoparticles did not change after the formation of sandwich structure.

Figure 3. Catalytic performance of the catalysts for semi-hydrogenation of phenylacetylene to styrene and reusability of 30TiO2/Pt/TiO2 catalyst for 5 run (The reaction time is 4 h for each run). Reaction conditions: 10 mg catalyst, 0.18 mmol phenylacetylene, 40 mg ammonia borane, 12 mL H2O, 8 mL ethanol, and at 30 °C.


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Figure 3 and Table 1 show the catalytic performance of xTiO2/Pt/TiO2 in tandem ammonia borane decomposition and phenylacetylene hydrogenation, which considerably depends on the thickness of the outer TiO2 layer. The Pt/TiO2 catalyst without the outer TiO2 layer shows a high catalytic activity but no selectivity of styrene (Figure 3A, Table 1). The catalytic performance of xTiO2/Pt/TiO2 (Figure 3B, Figure S5A) was similar to Pt/TiO2 when the ALD cycle number was less than 30 (thickness below 4.2 nm), and the selectivity of styrene decreased significantly with the conversion elevation due to the further hydrogenation. However, a high styrene selectivity (≤ 95%) was observed on sandwich xTiO2/Pt/TiO2 even at 100% conversion of phenylacetylene when the ALD cycle number was over 30 (Figure 3C, Figures S5B and C). The turnover frequency (TOF) decreased from 352.9 h-1 of Pt/TiO2 to 232.4 h-1 of 30TiO2/Pt/TiO2 due to the coverage of TiO2. When the thickness of porous TiO2 outer layer is greater than 4.2 nm, the catalytic performance is not sensitive to the outer layer thickness any more. For instance, 30TiO2/Pt/TiO2 and 100TiO2/Pt/TiO2 have a similar TOF (232.4 h-1 and 222.1 h-1, respectively). Thus, the property of catalytic active sites is similar for xTiO2/Pt/TiO2 (x ≥ 30). These results suggest that a 4.2 nm porous TiO2 outer layer is enough to encapsulate the platinum nanoparticles together with the inner TiO2 layer to form a sandwich nanostructure, which is critical to the high selectivity of styrene. Although the outer TiO2 layer plays so important role in the selectivity of sandwich structure, the TiO2 itself has no intrinsic reactivity to the hydrogenation of phenylacetylene. When TiO2 was used as catalyst, the catalytic performance was same as the reaction without catalyst (Table 1). Pt metal particles size is an important factor in catalytic performance of supported metal catalyst. Generally, the size of supported Pt particles increases with the ALD cycle numbers. The influence of Pt particles size was investigated for the tandem semi-hydrogenation of alkyne by


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changing the ALD cycle number (n= 10, 20 or 30) of Pt for 50TiO2/nPt/TiO2 (Table 1 and Figure S6). For sandwich 50TiO2/nPt/TiO2, the selectivity of olefin was higher than 90% and almost unchanged with the increase of ALD cycle number. The Pt particle size had less effect to the selectivity of olefin, although the activity was improved on the larger Pt particle due to the increase of Pt loading.

Table 1. Catalytic performance of catalysts in tandem ammonia borane decomposition and phenylacetylene semi-hydrogenation. Thickness of outer TOFc Reaction Styrene Conv. % b -1 TiO2 layer /nm /h Time/min Sel. /% Pt/TiO2 0 352.9 60 100 0 5TiO2/Pt/TiO2 60 100 2 10TiO2/Pt/TiO2 1.4 80 99 1 20TiO2/Pt/TiO2 80 97 4 30TiO2/Pt/TiO2 4.2 232.4 220 98 95 50TiO2/Pt/TiO2 7.8 220 98 96 100TiO2/Pt/TiO2 15.5 222.1 220 99 96 100Al2O3/Pt/TiO2 7.5d 120 99 16 50TiO2/50Al2O3/Pt/TiO2 120 100 16 TiO2 220 14 82 None 220 14 80 e 50TiO2/10Pt/TiO2 220 78 90 50TiO2/30Pt/TiO2f 120 92 89 a Reaction conditions: 10 mg catalyst (The ALD cycle number of Pt is 20), 0.18 mmol Catalysta

phenylacetylene, 40 mg ammonia borane, 12 mL H2O, 8 mL ethanol, 30 °C. bBased on TEM image. cTOF of phenylacetylene conversion is calculated based on hydrogen chemisorption. dThe thickness of porous Al2O3 outer layer. eThe ALD cycle number of Pt is 10. fThe ALD cycle number of Pt is 30.


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Table 2. Catalytic performance of 50TiO2/Pt/TiO2 and Pt/TiO2 in tandem reaction of ammonia borane decomposition and semi-hydrogenation of alkyne, alkene, and α,β-unsaturated aldehydea. Entry

Catalyst

1

50TiO2/Pt/TiO2

Reagent

1a 2

Product

3

5

1b

1c

1a

1b

50TiO2/Pt/TiO2

50TiO2/Pt/TiO2 2a

6

50TiO2/Pt/TiO2

8

50TiO2/Pt/TiO2

3a

3b

4a

4b

5a

9d

6a 10

96

7

27

-

0.5

100

-

23

86

83

5

96

82

5

99

88

5

97

85

3

100

92

12

48

87 C/T=5

5b 6b

50TiO2/Pt/TiO2

98

2b

50TiO2/Pt/TiO2

7

4

1c

Pt/TiO2

4c

Conv. /% Sel. /%

1b

50TiO2/Pt/TiO2 1b

Timeb /h

6c

50TiO2/Pt/TiO2

3 100 99 7b 7a a Reaction conditions: 10 mg catalyst (3wt.% Pt), 0.18 mmol reagent, 40 mg ammonia borane (or

atmospheric pressure H2), 12 mL H2O, 8 mL ethanol, 30°C, except for entry 3, 4 and 10. b

Reaction time depend on when the conversion up to 90%. cAtmospheric pressure H2 was used

as hydrogen source. d8 mL methanol replace ethanol to avoid transesterification. C/T represents the ratio of cis-product 7b to trans-product 7c.


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The unique catalytic performance of sandwich TiO2/Pt/TiO2 was also demonstrated in the hydrogenation of styrene (1b, Table 2, entry 2) as a starting reagent, which shows a very low catalytic activity. However, the Pt/TiO2 with a similar Pt nanoparticle size shows a high activity in styrene hydrogenation (Table 2, entry 3). This demonstrates a high catalytic potential of sandwich TiO2/Pt/TiO2 for semi-hydrogenation of alkynes. Moreover, the sandwich catalyst is very stable. After being reused five times, no obvious deactivation or selectivity decline was observed over 30TiO2/Pt/TiO2 (Figure 3F). When the outer layer of the sandwich catalyst was replaced with a 7.5 nm porous Al2O3 layer (100Al2O3/Pt/TiO2) produced by annealing an alucone (-(Al-O-CH2CH2-O)n-) hybrid film deposited by molecular layer deposition36, the selectivity of styrene decreased to 16% at a 99% conversion of phenylacetylene (Table 1, Figure 3D). Therefore, the formation of Pt-Al2O3 interface cannot control the selectivity of styrene. This further indicates the advantage of sandwich TiO2/Pt/TiO2 catalyst. We have coated the surface of 50Al2O3/Pt/TiO2 with 50 cycles of TiO2 to prepare 50TiO2/50Al2O3/Pt/TiO2. However, the selectivity of 50TiO2/50Al2O3/Pt/TiO2 also decreased at high conversion (Table 1, Figure 3E), similar to that of 100Al2O3/Pt/TiO2. Therefore, the selectivity of the styrene is not determined by the pore structure of the TiO2 layer. To further explore the catalytic mechanism, we separately studied the two processes of the tandem reaction. Figure S7 and S8 show that the decomposition of ammonia borane is near quasi-zero-order reaction, consistent with previous reports37. Pt/TiO2 shows a high activity in ammonia borane decomposition (2.41 mol H2/gCat/h), while the catalytic activity of 30TiO2/Pt/TiO2 sandwich catalyst for this reaction obviously decreased (0.19 mol H2/gCat/h). However, the rate of H2 generation is still higher than the rate of hydrogen consumption for the


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tandem reaction (0.02 mol H2/gCat/h) over the 30TiO2/Pt/TiO2 sandwich catalyst. Thus, the hydrogenation reaction is a rate-determining step in the tandem reaction. When atmospheric H2 gas was used as the hydrogen source, the TiO2/Pt/TiO2 catalyst showed a high selectivity of olefin but a rather low activity (Table 2, entry 4), revealing an extremely low active hydrogen concentration. The ammonia borane decomposition and hydrogenation shared the same active sites according to control experiments (Figure S7). Thus, in-situ generated active hydrogen with high concentration from ammonia borane decomposition accelerates the hydrogenation in the tandem reaction.33 The sandwich TiO2/Pt/TiO2 catalyst was also applied to the semi-hydrogenation of some other alkynes (Table 2, entries 5-9). Different alkynes (2a-6a) were hydrogenated to the corresponding olefins (2b-6b) and the selectivity reach >80% at high conversion (>90%) over 50TiO2/Pt/TiO2. Furthermore, the successful semi-hydrogenation of substrate 3a, which contains both alkynyl and alkenyl groups, demonstrated that the C=C bond was protected from overhydrogenation during the semi-hydrogenation of alkynyl group. For internal alkynes (6a), the C/T ratio of 6a is high, which indicated the cis-hydrogenation is preferred in the semihydrogenation due to the stability of surface cis-intermediates17. This sandwich TiO2/Pt/TiO2 can also be applied in the selective hydrogenation of compounds simultaneously having both C=C double bonds (to be retained) and other bonds. For example, in the hydrogenation of cinnamyl aldehyde (7a, Table 2, entry 10) over the sandwich TiO2/Pt/TiO2 catalyst, the selectivity of C=O bond hydrogenation to C-O bond is 99% at high conversion, although the hydrogenation of C=C bond is thermodynamically favored. This further indicates the advantages of sandwich TiO2/Pt/TiO2 catalyst in selective hydrogenation.


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TEM images show that the thickness of the outer TiO2 layer increase with the ALD cycle numbers, and the TiO2/Pt/TiO2 sandwich nanostructure is formed. Correspondingly, the content of Pt-TiO2 interface sites increase by covering free Pt species with porous TiO2 layer. Here, the Pt-TiO2 interface sites included the Pt atoms at the interface and the Pt atoms not at the interface but strongly influenced by the formation of interface. Meanwhile, XAFS reveals no clear change in coordination state of bulk phase Pt after the formation of the Pt/TiO2 interface. However, a red shift of XPS spectrum suggests the formation of more Pt species with an electron-rich property with the thickness increase of porous TiO2 outer layer. Therefore, the formation of sandwich nanostructure results in the electron state change of surface Pt atoms. On the surface of a supported Pt catalyst (such as Pt/TiO2), there are two types of Pt species: the dominant free Pt species with an electron-deficient property on the surface of the nanoparticles, and the interface Pt species due to the formation of Pt-TiO2 interface. The content and ratio of interface Pt species increase with the coverage of porous TiO2 shell, and only interface Pt species existed in the sandwich nanostructure. Experimental results show that the Pt/TiO2 has a high activity in tandem ammonia borane decomposition and alkyne hydrogenation with no selectivity for olefin, which can be attributed to the dominant free Pt species. But the selectivity of olefin is still low (

TiO2 Sandwich Catalyst for Highly Selective

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