Synthesis of nitrogen-doped carbon coated TiO2

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product, hydrocinnamyl alcohol (HCOL), may also form by two-step ... bond presents a higher binding energy than the C_C bond (715 kJ/mol ... In a typical procedure, 0.2 g ... cobalt species were not presented for PtCo/CNx/TiO2, due to the low .... a Reaction conditions: 8.2 mmol CAL, 19.0 mL C2H5OH, 70 °C, 2.0 MPa, and ...
Catalysis Communications 61 (2015) 97–101

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Synthesis of nitrogen-doped carbon coated TiO2 microspheres and its application as metal support in cinnamaldehyde hydrogenation Zhengbin Tian, Qingyang Li, Yan Li ⁎, Shiyun Ai ⁎ College of Chemistry and Material Science, Shandong Agricultural University, 271018, PR China

a r t i c l e

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Article history: Received 4 November 2014 Received in revised form 19 December 2014 Accepted 21 December 2014 Available online 24 December 2014 Keywords: N-doped carbon TiO2 microspheres Cinnamaldehyde Selective hydrogenation

a b s t r a c t N-doped carbon coated TiO2 microspheres (CNx/TiO2) were synthesized by the carbonization of the polypyrrole (PPy) coating on the surface of TiO2 microspheres and used as support to disperse Pt and PtCo nanoparticles for investing the selective hydrogenation of cinnamaldehyde. The support and catalysts have been characterized in terms of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscope (TEM), scanning electron microscope (SEM), and X-ray photoelectron spectroscopy (XPS). The hydrogenation results showed the conversion increased with an increase of CNx amount until the CNx coated TiO2 microspheres completely. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Selective hydrogenation of α, β-unsaturated aldehydes at the carbonyl (C_O) and olefinic (C = C) groups is an important reaction, because most of their products are important intermediates for the syntheses of many chemicals [1]. Cinnamaldehyde (CAL) is a particularly important representative of α, β-unsaturated aldehyde, since its partial hydrogenation products, cinnamyl alcohol (COL), are important intermediates in the manufacture of chemicals (particularly perfumes, flavors, and pharmaceuticals) [2,3]. Also, it is considered as a good model for investing the catalytic behaviors of the microstructures of heterogeneous catalysts. Reaction pathways for the hydrogenation of CAL was described in Fig. 1; hydrocinnamaldehyde (HCAL) and COL will form in one-step hydrogenation and fully hydrogenated product, hydrocinnamyl alcohol (HCOL), may also form by two-step hydrogenation. Accordingly, the development of catalysts for the selective hydrogenation of CAL to COL is still challenging, owing to the fact that the C_O bond presents a higher binding energy than the C_C bond (715 kJ/mol and 615 kJ/mol, respectively) [4]; the reduction of the C_C bond is thermodynamically more favorable than that of the C_O bond. In the catalytic process, the undesirable product, HCAL, would be formed largely, decreasing the selectivity to the unsaturated alcohol. The selectivity to the unsaturated alcohol can be correlated with the metal d-band width. Generally, the larger the d-band, the stronger the repulsive four-electron interaction of the metal with the C_C double bond and the stronger the attractive interaction of the metal surface with the ⁎ Corresponding authors at: 271018, Daizong Street 61, Taian, Shandong, PR China. E-mail addresses: [email protected] (Y. Li), [email protected] (S. Ai).

http://dx.doi.org/10.1016/j.catcom.2014.12.019 1566-7367/© 2015 Elsevier B.V. All rights reserved.

C_O π-system, which results in good selectivity to the unsaturated alcohol (Ir, Os N Pt N Ru N Rh N Pd) [5]. Also, the selectivity can be promoted with lower redox potential exhibiting electropositive d-electron metals (Fe) or p-electron metals (Ge, Sn) [6,7]. Over the past few years, common noble metal (Pd, Ru and Au)-based catalysts have been used for the hydrogenation of CAL [8–10]. In fact, Pt is most commonly used for the purpose of C_O hydrogenations in the experiment. Maintaining high selectivity of Pt-based catalysts while increasing their activity remains an urgent task for further development in this field. It is well known that there is an interaction between the support and active metal sites that can modify the catalytic properties of the metal catalyst. It is a feasible way to search for appropriate materials used as catalyst supports for preparing highly active and selective catalysts for the hydrogenation of CAL. The selectivity to the unsaturated alcohol is generally considered to be improved by using a partially reducible support, for example TiO2. That is because there is a strong interaction between the carbonyl group and positively charged center (TiOδx +) [7]. However, we found that plenty of HCOL was produced when TiO2 supported Pt particles was used. Therefore, the surface modification of TiO2 is presented. Carbon materials such as active carbon, carbon nanotubes [11], and graphene [12] had been widely employed as support in heterogeneous catalysis. Recently, the introduction of electron-donating nitrogen heteroatoms to carbon materials has attracted global attention, due to its abundance, accessibility, low health risk, suitable surface areas, extreme chemical and thermal stabilities [13,14]. These materials with excellent properties have been extensively used for sorption, sensing, battery, storage, electrocatalyst and energy-sustainability applications, owing to their structural, morphological, and chemical properties. In many catalytic and supercapacitor applications, N-doped carbon materials are

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Fig. 1. Reaction pathways in the hydrogenation of CAL.

also used as supports to facilitate dispersion of noble and non-noble catalysts. Notably, it has been proven that the presence of N-species in the carbon matrix could lead to a high dispersion of metal nanoparticles [15], undoubtedly, which would tremendously increase the catalyst activity. N-doped carbon materials are often fabricated using expensive resources and energy-consuming carbonization. In this article, we reported that the selective hydrogenation of CAL over N-doped carbon materials coated TiO2 microspheres (CNx/TiO2) composites supported PtCo catalyst for the first time. The CNx/TiO2 composites were fabricated by forming PPy polymer on the surface of TiO2 microspheres and calcining at N2 atmosphere. N-doped carbon coating provides an effective method to modify the surface of TiO2, leading to the change of catalytic property. The catalytic activity of the catalyst was investigated under mild reaction conditions. 2. Experimental Mesoporous TiO2 microspheres were prepared via a nonaqueous solvothermal process with anhydrous acetone as solvent. CNx/TiO2 was synthesized by the carbonization of the PPy coating on the surface of TiO2 microspheres (PPy/TiO2), which was formed by the oxidation of pyrrole (Py) in a solution containing FeCl3. Pt based catalysts supported on CNx/TiO2 were prepared by the conventional impregnation method with NaHB4 as reductant. The detailed procedure was described in the Supplementary information. The hydrogenation reactions were carried out in a stainless autoclave reactor with a 50 mL Teflon sleeve. In a typical procedure, 0.2 g catalyst was dispersed in 19 mL ethanol and then 8.2 mmol CAL was added into solution. The reactor was sealed, purging with H2 for 3 times respectively and then pressurized to 2.0 MPa. The reaction was conducted at 70 °C with a stirring speed of 750 rpm for 2.0 h. The reaction mixture was analyzed by Shimadzu GC-2010 gas chromatograph with a flame ionization detector (FID) system. 3. Results and discussion 3.1. Catalyst characterization The X-ray diffraction pattern of an ensemble of the samples is compared in Fig. 2. As shown in Fig. 2a and b, there are two phases in the TiO2 and PPy/TiO2 samples, in which TiO2 microspheres were calcined at 400 °C. The sharp Bragg diffraction peaks (2θ = 25.3°, 37.9°, 48.1°, 54.0° and 55.1°) in good agreement with the standard anatase TiO2 pattern (JCPDS card No. 21-1272), could be clearly observed, which are indexed as the (101), (004), (200), (105) and (211) planes, respectively. The weaker peaks (2θ = 27.4°, 36.1°, 41.2°, 54.3°) belong to the rutile phase, which could be well assigned to the (110), (101), (111) and (211) planes. The intensity of PPy/TiO2 composites decreases slightly

Fig. 2. XRD pattern of (a) TiO2, (b) 0.3-PPy/TiO2, (c) 0.3-CNx/TiO2 and (d) PtCo/0.3-CNx/ TiO2.

compared with the bare TiO2, probably due to the formation of amorphous polymer onto the surface of TiO2 nanoparticles [16]. However, the PPy/TiO2 composites have not changed in peak positions and shapes in comparison with the pure TiO2, indicating that the lattice structure of TiO2 is well maintained after the PPy coating process. Fig. 2c and d shows an XRD pattern of the CNx/TiO2 and PtCo/PPy/TiO2 revealing only the anatase phase, which is mainly attributed to the transition of the rutile phase to anatase phase in high temperature. Additionally, the diffraction peaks of the samples are broader, indicating that the crystalline sizes of these TiO2 are smaller than that calcined at low temperature. The XRD characteristic peaks corresponding to platinum and cobalt species were not presented for PtCo/CNx/TiO2, due to the low amount of metal loading and, probably, the very low intensity of the metal peaks compared to the crystallite size of the TiO2 [17]. The SEM image shown in Fig. 3a revealed that the TiO2 microspheres are nearly perfectly spherical architectures with diameters of 300–500 nm. From a closer observation of the magnified SEM image, the TiO2 microspheres were uniformly arranged with diameters of ca. 20 nm. As shown in Fig. 3b, the microspheres were completely coated by the CNx material, when r was 0.3. Fig. 4 shows the representative TEM images of the reduced the PtCo/ 0.3-CNx/TiO2 catalysts. In Fig. 4a, the microspheres were found to be coated by CNx completely, which was consistent with that observed in the SEM image. The CNx layer could be clearly observed with the thickness of ca. 40 nm. Fig. 4b was the amplification of the interface of crystalline TiO2 and amorphous CNx, in which (101) plane (d = 0.352 nm) was presented. The selectivity of COL may depend upon the nature of the exposed crystal faces. The Pt (111) surface is more compact, which prefers the adsorption of the C_O bond. The adsorption of the C_C bond is substituted and thus hindered. Nevertheless, the (100) surface is more open and cannot be affected to such a great extent [18–20]. Also, large metal particles (N3 nm) was found to favor the high selectivity for COL [21,22]. As shown in the Fig. 4c, the agglomeration of the PtCo particles occurred and the (111) surface was exposed more on the surface of the catalyst. The average size of the PtCo particles was approximately 4–5 nm. XPS analysis is a practical surface analysis method, through which chemical and structural information on the material surface can be obtained. Also, XPS is very sensitive to the state of sample, and can be used to analyze all elements except hydrogen and helium. Therefore, to obtain information about the chemical environment and atomic concentrations presented in catalyst PtCo/0.3-CNx/TiO2, XPS analysis was performed. The typical XPS survey spectrum with binding energies ranging from 0 to 1000.0 eV is indicated in Fig. 5a. The results obviously revealed the presence of platinum, nitrogen, carbon, oxygen and cobalt elements, which was demonstrated by the peaks of Pt4f (at 75 eV), C1s (at 284 eV), O1s (at 533 eV), N1s (at 400 eV) and Co2p (at 780 eV). It's

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Fig. 3. SEM images of (a) TiO2 and (b) 0.3-CNx/TiO2.

worth mentioning that the N/C ratio was 0.098, which was much lower than the theory value of 0.292 in Py. This is because the loss of nitrogen is quicker than that of carbon in the high temperature. The high resolution N1s spectra can be mainly deconvoluted into two components with binding energies of 398.3 eV and 400.5 eV that correspond to pyridinic N and pyrrolic N (Fig. 5b) [23,24], meaning that there exist two chemical bonding states for N atoms in the catalyst. The emergence of these two peaks is relevant to the structure of PPy. A slightly asymmetric tail was shown in the deconvoluted C1s spectra at higher binding energy (Fig. 5c), which is a common characteristic for nitrogen-doped carbon materials [25]. The high resolution C1s spectra were deconvoluted into four different peaks. The peak located at 284.7 eV was assigned to sp2-hybridized graphite-like carbon (C_C bond), while the one at

285.8 eV was attributed to sp3-hybridized diamond-like carbon (C\C bond) [26]. In addition, the peaks at 286.7 eV and 287.6 eV represented the contributions from C\N and C_N, respectively [27]. Moreover, the XPS spectrum for Pt nanoparticles showed two peaks corresponding to Pt4f7/2 and Pt4f5/2, respectively, positioned at 72.1 eV and 75.5 eV, respectively, which was attributed to zerovalent Pt0 in the metallic state [28,29]. 3.2. Hydrogenation reaction CNx/TiO2 supported PtCo bimetallic catalysts were tested for the selective hydrogenation of CAL in the mild condition. The hydrogenation of CAL typically produced a mixture of the desired COL, the undesired

Fig. 4. The representative TEM images of the PtCo/0.3-CNx/TiO2 catalysts.

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Fig. 5. XPS spectra of the PtCo/0.3-CNx/TiO2 catalysts.

HCAL and HCOL. Aldol condensation of acetal (ACE) and other unidentified products may be produce in small amounts particularly over nonselective catalysts [30]. Transition metals have been employed widely as co-catalysts in a large variety of hydrogenation reactions. The selectivity and the activity of the catalysts could be considerably modulated by combining a transition metal with some noble metals [31]. The representative of transition metals, cobalt, has been used in the selective hydrogenation of CAL and exhibited a high selectivity toward the C_O bond, resulting a high yield toward the COL [11]. Table 1 lists the catalytic properties of the CAL hydrogenation over the TiO2 supported catalysts and the CNx/TiO2 supported catalysts with different CNx amounts. It was clear that Pt/TiO2 showed a great hydrogenation activity but poor selectivity for COL. Table 1 The hydrogenation of CAL over CNx/TiO2 supported Pt and PtCo metallic catalysts. Catalyst

Pt/TiO2 Pt/0.1-CNx/TiO2 Pt/0.2-CNx/TiO2 Pt/0.3-CNx/TiO2 Pt/0.4-CNx/TiO2 Pt/0.5-CNx/TiO2 PtCo/TiO2 PtCo/0.1-CNx/TiO2 PtCo/0.2-CNx/TiO2 PtCo/0.3-CNx/TiO2 PtCo/0.4-CNx/TiO2 PtCo/0.5-CNx/TiO2 a b

Conv.a (%)

99.4 38.7 65.9 95.3 93.1 96.4 78.6 80.9 87.7 95.3 94.6 97.1

Sel. (%) COL

HCAL

HCOL

ACEb

3.4 59.0 54.3 60.8 70.0 68.7 81.9 78.1 73.3 70.5 75.3 79.3

30.1 27.6 33.3 27.0 20.5 23.4 6.7 2.5 5.3 2.8 4.6 1.1

62.3 9.6 8.3 11.0 7.5 5.9 10.0 17.3 19.9 22.6 11.5 18.9

4.2 3.9 4.1 1.2 2.0 2.2 1.5 2.2 1.4 2.0 2.5 0.7

Reaction conditions: 8.2 mmol CAL, 19.0 mL C2H5OH, 70 °C, 2.0 MPa, and 2.0 h. ACE: acetal.

The addition of Co to Pt/TiO2 evidently improved the selectivity. When r was 0.1, Pt/CNx/TiO2 showed a low activity, which was presumably caused by the incomplete CNx coating on the TiO2. It was found that the conversion of CAL over the Pt/CNx/TiO2 catalysts increased with the rise of the CNx amount until the CNx coated TiO2 microspheres completely. The conversion with PtCo/CNx/TiO2 was much higher than that with Pt/CNx/TiO2 under the same conditions. The conversion and the selectivity of COL catalyzed by the Pt catalyst were dramatically improved; nevertheless, the conversion increased slightly over the PtCo catalyst. Additionally, the selectivity of COL catalyzed by the PtCo catalyst had a slight change, which could be attributable to the excellent activity of Co for the transformation of CAL to COL. The stability of the PtCo/0.5-CNx/TiO2 catalyst was evaluated by recycling experiment. It was noted that the catalytic activity decreased as the recycle times increased, while the COL selectivity still remained at ca. 65.5% after the fourth recycle. Many factors contributed to the low catalytic activity on the hydrogenation of CAL. The detachment of metal particles on the catalysts decreased the number of active sites for hydrogen. The formation of carbon deposits on the surface of the catalyst partly covered active center and resulted in the decreasing of the catalytic activity.

4. Conclusion In the article, N-doped carbon coated TiO2 microspheres were successfully prepared and firstly used as catalyst support for the hydrogenation reaction of CAL. The catalysts were prepared by conventional impregnation and characterized in terms of XRD, FTIR, TEM, SEM, and XPS. The hydrogenation results indicated that the PtCo catalysts presented a higher activity than the Pt catalysts. It was found that the

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conversion increased with an increase of CNx amount until the CNx coated TiO2 microspheres completely. Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21203113) and Project of Development of Science and Technology of Shandong Province, China (No. 2013GZX20109). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2014.12.019. References [1] H.G. Manyar, B. Yang, H. Daly, H. Moor, S. McMonagle, Y. Tao, G.D. Yadav, A. Goguet, P. Hu, C. Hardacre, ChemCatChem 5 (2013) 506–512. [2] S. Bhogeswararao, D. Srinivas, J. Catal. 285 (2012) 31–40. [3] B.-H. Zhao, J.-G. Chen, X. Liu, Z.-W. Liu, Z. Hao, J. Xiao, Z.-T. Liu, Ind. Eng. Chem. Res. 51 (2012) 11112–11121. [4] K. Taniya, H. Jinno, M. Kishida, Y. Ichihashi, S. Nishiyama, J. Catal. 288 (2012) 84–91. [5] P. Claus, Y. Önal, Regioselective hydrogenations, in: G. Ertl, H. Knözinger, F. Schüth, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, Wiley-VCH, Weinheim, 2008, p. 3308. [6] V. Ponec, Appl. Catal. A Gen. 149 (1997) 27–48.

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