Schottky Barrier-Induced Coupled Interface of Electron-Rich N-Doped

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Schottky Barrier-Induced Coupled Interface of ElectronRich N-Doped Carbon and Electron-Deficient Cu: In-Built Lewis Acid-Base Pairs for Highly Efficient CO Fixation 2

Yong-Xing Liu, Hong-Hui Wang, Tian-Jian Zhao, Bing Zhang, Hui Su, Zhong-Hua Xue, Xin-Hao Li, and Jie-Sheng Chen J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b08267 • Publication Date (Web): 10 Dec 2018 Downloaded from http://pubs.acs.org on December 10, 2018

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Journal of the American Chemical Society

Schottky Barrier-Induced Coupled Interface of Electron-Rich NDoped Carbon and Electron-Deficient Cu: In-Built Lewis Acid-Base Pairs for Highly Efficient CO2 Fixation Yong-Xing Liu, Hong-Hui Wang, Tian-Jian Zhao, Bing Zhang, Hui Su, Zhong-Hua Xue, Xin-Hao Li,* and Jie-Sheng Chen* School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China Supporting Information ABSTRACT: Highly efficient fixation of CO2 for the

synthesis of useful organic carbonates has drawn much attention. The design of sustainable Lewis acid-base pairs, which has mainly relied on expensive organic ligands, is the key challenge in the activation of the substrate and CO2 molecule. Here, we report the application of Mott-Schottky type nanohybrids composed of electron-deficient Cu and electron-rich N-doped carbon for CO2 fixation. A ligand-free and additive-free method was used to boost the basicity of the carbon supports and the acidity of Cu by increasing the Schottky barrier at their boundary, mimicking the beneficial function of organic ligands acting as the Lewis acid and base in metal-organic frameworks (MOFs) or polymers and simultaneously avoiding the possible deactivation associated with the necessary stability of a heterogeneous catalyst. The optimal Cu/NC-0.5 catalyst exhibited a remarkably high turnover frequency (TOF) value of 615 h-1 at 80 oC, which is 10 times higher than that of the state-of-the-art metal-based heterogeneous catalysts in the literature.

Excessive emission of carbon dioxide has triggered numerous environmental problems.1-3 Efficient reuse of CO2, which is an inexpensive, abundant and nontoxic gas, is consistently considered a promising green chemistry process.1-5 However, the high chemical stability of CO2 makes its conversion difficult.6 Current strategies for chemical fixation of CO2 mainly focus on the conversion of CO2 into useful organic carbonates, carbamates, ureas and carboxylic acids.7 Cycloaddition of CO2 to epoxides is an important method for the synthesis of cyclic carbonates,8,9 which are of great interest in industry as lithium battery electrolytes, nonprotic polar solvents and monomers used for generating polycarbonates.10 Recently, metal-organic frameworks (MOFs)11,12 and porous ionic polymers13 have been developed as reusable heterogeneous catalysts for the coupling reaction. This pioneering work has clearly demonstrated the importance of Lewis acid and/or base groups in facilitating the fixation of CO2 and the activation of epoxides,14-16 even though the

expensive and complex method for the synthesis of MOF materials will limit their practical applications. In principle, electron-rich and electron-deficient areas at the boundary of a Mott-Schottky heterojunction could also function as Lewis base and acid pairs, e.g., in metal-semiconductor nanocomposites.17,18 Modification of the electron distribution within the metal-semiconductor nanocatalyst is an alternative and straightforward way to trigger a possible cycloaddition reaction without involving expensive ligands.19 Herein, we report a proof-of-concept method for constructing bifunctional Lewis acid-base nanocatalysts by coupling copper nanoparticles to nitrogen-doped carbon (Cu/NC) with a rectifying contact for highly efficient cycloaddition of CO2 to epoxides under mild conditions. Such a contact between copper and supports has been applied to increase the amounts of Lewis acid-base sites simultaneously by optimizing the Schottky barriers at the interface, significantly promoting the cycloaddition reaction. As depicted in Figure 1a, the Cu/NC material was prepared from thermal condensation of a mixture of glucose (carbon source), dicyandiamide (nitrogen source) and CuCl2 at temperatures higher than 750 oC (Table S1) to obtain a fluffy black powder. The Cu/NC-x dyads, where x (0.1, 0.5 or 1) refers to the weight percentage of Cu in the carbon-copper precursor, were used for further characterization and catalytic reactions. The scanning electron microscopy (SEM) images (Figure S1) reveal the layered morphology of carbon supports as the main components. The samples show similar structures after the introduction of various amounts of Cu, and the different expected amounts of embedded Cu nanoparticles were found by the transmission electron microscopy (TEM) observations (Figures 1b-d and Figures S23). It is observed from the bright and dark field TEM observations that the Cu nanoparticles are uniformly distributed in carbon supports with a mean size of approximately 6 nm (Figures 1b-c and Figures S2-4). An examination of the HRTEM image (Figure 1d) of the Cu/NC0.5 sample shows that the lattice fringes display an interplanar spacing of 0.21 nm, matching well with the (111) plane of the metallic copper nanocrystal. The powder X-ray diffraction (XRD) patterns (Figure S5) indicate that metallic

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Cu and graphitic carbons were mainly formed. The N2 adsorption-desorption isotherms (Figure S6) reveal that the as-obtained Cu/NC-x samples have excellent surface areas of between 450 and 600 m2/g that should be highly beneficial for the exposure of the active sites. It should be noted that approximately 79% (Figure S7) of the Cu loaded in the Cu/NC-0.5 sample could be removed via a mild etching process, indicating that the exposed Cu species comprised most of the total Cu amount.

effect between Cu and supports for facilitating the cycloaddition. The introduction of metallic Cu nanoparticles, that act as a Lewis acid as shown in previous work,20 also increased the Lewis basicity of the carbon supports by donating electrons. We note that the nitrogen contents of Cu/NC-x samples are nearly the same (Figure S9), indicating the presence of similar amounts of the Lewis base sites contributed by the carbon supports themselves. The typical X-ray photoemission spectroscopy (XPS) Cu peaks (Figure 2c) shifted to a higher binding energy by 0.4 eV after the introduction of more Cu. Such a shift in the binding energy of Cu directly demonstrates the role of metallic Cu as electron donors (Figures S10-11). Presumably, the interface Mott-Schottky effect (Figure 2d) gives rise to an electron-rich surface and, thus, an enhanced Lewis basicity of the carbon supports. This electronic increase in basicity is also well demonstrated by the CO2 temperature-programmed desorption (TPD) analysis (Figure 2e). The benchmarked Cu/NC-0.5 catalyst has the highest CO2 desorption peak at the highest temperature of approximately 150 oC and the highest Lewis base strength and density (Figure 2f), verifying the interfacial synergies as the main source of the catalytic activity. No further shift in the binding energy (Figure 2c) of the Cu/NC-1 compared to Cu/NC-0.5 and the presence of even fewer basic sites (Figure 2f) indicate the key importance of the optimal Cu-NC ratios and their synergistic effect for stimulating the basicity of the Cu/NC-x dyads.

Figure 1. Formation and structure of Cu/NC. (a) Synthetic process for Cu/NC from dicyandiamide, glucose and CuCl2·2H2O. Bright (b) and dark (c) field TEM and HRTEM (d) images of Cu/NC-0.5. Initially, the cycloaddition of CO2 and styrene oxide over Cu/NC-x catalysts under mild conditions was tested to investigate the Mott-Schottky effect on the formation of Lewis acid-base pairs and, thus, on the final catalytic activity. The content of Cu in the Cu/NC-x samples was gradually increased from 0.16% to 0.66% to 0.95%, as estimated by inductively coupled plasma optical emission spectrometry (ICP-OES) analysis (Figure S8). The Cu/NC-x samples were then used to confirm the function of Cu as the electron donor to enhance the Lewis basicity of the carbon supports. Only a moderate amount of styrene oxide could be converted without a catalyst (Figure 2a; Entries 1-2, Table S2) or over bare NC (Entries 3-4, Table S2) and Cu/C without nitrogen doping (Entry 5, Table S2). Under fixed conditions, the Cu/NC catalysts exhibited significantly increased conversions to 80% with high selectivities (>99%) (Figure 2a; Entries 6-13, Table S2). The Cu/NC-0.5 sample is the optimal catalyst in this work and was used for the following reactions. A further increase in the reaction time achieved a complete conversion of the substrate with constant selectivity (Figure 2b; Entries 8-11, Table S2). Increasing the Cu content for the Cu/NC-1 sample led to an even lower yield (Figure 2a; Entries 12-13, Table S2). Meanwhile, a mechanical mixture of Cu/C and NC sample showed lower conversion (44%) (Entry 14, Table S2) than Cu/NC-0.5. All of these results directly confirm the key importance of the synergistic

Figure 2. (a) Conversions of styrene oxide over Cu/NC-x and Cu/C+NC catalysts or in a blank reaction. Reaction conditions: 20 mmol styrene oxide, 50 mg catalyst, TBAB (5 mol%), 1 atm CO2, 40 oC, 48 h. (b) Conversions of styrene oxide over Cu/NC-0.5 at different reaction times. (c) Cu 2p XPS spectra of Cu/NC-x catalysts. (d) Proposed basic sites induced by the Mott-Schottky effect at the interface. CO2-

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Journal of the American Chemical Society TPD results (e) and the amounts of basic sites (f) of typical Cu/NC-x catalysts. The possible effect of the copper oxide layer that naturally forms on the surface of metallic Cu nanoparticles was excluded. The similar contents of metallic Cu calculated from the Cu 2p XPS spectra (Figures S12-13) of 55-66% for all of typical Cu/NC-x samples suggest that the Cu2+ species made only a minor contribution to the significant improvement in the catalytic activity. The catalytic activity of the CuO/NC0.5 with a conversion of 52% (Entry 15, Table S2) directly excluded the possible promotion effect of the Cu2+ species on the performance of the Cu/NC-x catalyst.

electron-deficient Cu site as estimated by the UPS analysis. As a result, the Cu/N0.24C sample has the largest Schottky barrier (Figure 3b insets). The upshift of the Cu 2p XPS peaks (Figure 3c) of the Cu/NyC samples from Cu/N0.08C to Cu/N0.13C to Cu/N0.24C directly indicates the transfer of additional electrons from Cu to the supports in Cu/N0.24C. The acidity of Cu/N0.24C was indeed significantly enhanced, as revealed by the new and stronger NH3 desorption peaks (Figure 3d) and more acidic sites (Figure 3e) compared with the corresponding behavior of Cu/N0.08C. Considering the negative contribution of N dopants (usually as Lewis basic sites) to the Lewis acidity, the increased acidity of Cu/NyC was mainly attributed to the interfacial effect. Thus, the fact that the Cu/N0.24C catalyst gave the highest conversion under fixed conditions (Figure 3f; Entries 1-3, Table S4) is compatible with all of the above-described mechanistic assumptions. Moreover, the control sample Cu/NC-H+ (Figure S7) showed only highly depressed activity (Entry 4, Table S4), again demonstrating the key role of electrondeficient Cu in boosting the final catalytic activity.22 Unlike the acid-base adducts in common nanocomposites that usually compensate each other, the Mott-Schottky effect of the Cu/N0.24C can make the Lewis acid-base pairs even more pronounced at the interface. As depicted in the inset of Figure 4, the enhanced density and strength of the Lewis acid-base sites at the boundary strengthens the preadsorption of CO2 and the activation of styrene oxide for the final reaction with the assistance of Br anions.23,24 As the “best-in-class” catalyst of this paper, Cu/N0.24C-0.5 offers the highest turnover frequency (TOF) values of 76 h-1 and 615 h-1 at 40 and 80 oC (Entry 8, Table S2; Entry 1, Table S5 and Table S6), respectively. This by far surpasses the state-of-theart heterogeneous catalysts reported in the literature (Figure 4 and Table S6).

Figure 3. (a) Electron redistribution at the interface of Cu and supports due to the Mott-Schottky effect. (b) Work functions (Φ) of the Cu/NyC samples measured by the ultraviolet photoelectron spectroscopy (UPS). Cu 2p XPS spectra (c), NH3-TPD results (d) and the amounts of acidic sites (e) of typical Cu/NyC catalysts. (f) Conversions of styrene oxide over the Cu/NyC catalysts. Reaction conditions: 20 mmol styrene oxide, 50 mg catalyst, TBAB (5 mol%), 1 atm CO2, 40 oC, 24 h. Instead, the interfacial Schottky contact (Figure 3a) enabled the further enhancement in the acidity of the Cu species by introducing additional N heteroatoms to the carbon supports (Figure S14 and Table S3).21 We controlled the mean sizes (Figures S2-4) and loading (approximately 0.5 wt.%, Figure S15) of the Cu nanoparticles in supports to be nearly the same in order to eliminate the direct influence of the size effect. It is observed that the increased N contents clearly increased the band gap and the work functions (Figure 3b and Figure S10) of the supports of the Cu/NC samples with the increased Schottky barrier and, thus, the

Figure 4. TOF values of Cu/NC-0.5 and reported values of the state-of-the-art catalysts reported in the literature (see Table S6 for detailed information). Inset: proposed mechanism for cycloaddition of CO2 with styrene oxide over Cu/NC catalyst. Schottky barrier-promoted Lewis acid-base pairs at the boundary were found to also be generally effective for the cycloaddition with other epoxides, including epoxy propane and epoxy chloropropane, to give the corresponding carbonates in high yields (Table S7). The Mott-Schottky

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effect also enhanced the long-term stability of the Cu/N0.24C catalyst, which is essential for the industrial applications. The activity of reused Cu/N0.24C catalyst was maintained at a relatively stable level (Figure S16) with only a slight decrease in the conversions throughout the following two runs, which was attributed to the inevitable loss of the catalysts during the separation process. The identical XRD patterns (Figure S17) of the Cu/N0.24C before and after the uses further verify its chemical stability during the reaction. In conclusion, we describe a Mott-Schottky catalyst composed of copper nanoparticles and nitrogen-doped carbon (Cu/NC). The reactivity of the coupled hybrid can be understood as a heterogeneous version of Lewis acid-base pairs. This was exemplified by the catalysis of highly efficient coupling of CO2 with styrene oxide into styrene carbonate under mild conditions. The amounts and the strength of Lewis acid and base sites were simultaneously promoted by optimizing the Schottky barriers at the interface of Cu and NC: the induced charge transfer from the acid to the base strengthened both reactivities and made the pair “more frustrated”. Given the wide scope of chemical processes involving frustrated Lewis acid-base pairs, this study paves the way for the development of Mott-Schottky type nanocatalysts with tailorable metal components to obtain high activity for green organic synthesis with low cost and emission levels.

ASSOCIATED CONTENT Supporting Information Experimental details, more characterization results, and detailed discussion are included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected] (X. H. Li), [email protected] (J. S. Chen)

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21720102002, 21722103, and 21673140), Shanghai Basic Research Program (16JC1401600), SJTUMPI partner group and Shanghai Rising-Star Program (16QA1402100).

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