Au–Carbon Electronic Interaction Mediated Selective Oxidation of

Letter pubs.acs.org/acscatalysis

Au−Carbon Electronic Interaction Mediated Selective Oxidation of Styrene Ben Liu,† Pu Wang,§ Aaron Lopes,† Lei Jin,† Wei Zhong,‡ Yong Pei,*,§ Steven L. Suib,*,†,‡ and Jie He*,†,‡ †

Department of Chemistry and ‡Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269, United States Department of Chemistry, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Hunan 411105, China

§

S Supporting Information *

ABSTRACT: The rational design of the Au−support electronic interaction is crucial for Au nanocatalysis. We herein report our observation of electronic perturbation at the Au−carbon interface and its application in controlling the reaction selectivity in styrene oxidation. Ultrasmall Au nanocatalysts were grown in situ on a nitrided carbon support where the nitrogen-doped carbon supports enriched the surface charge density and generated electron-rich Au surface sites. The Au− carbon interaction altered the binding behavior of CC bonds to catalytic centers, leading to a solvent-polarity-dependent selectivity in CC oxidation reactions. A high selectivity of 90% to benzaldehyde was achieved in an apolar solvent, and a selectivity of 95% to styrene epoxide was attained in a polar solvent. The Au−carbon electronic perturbation, originating from surface functional groups on the carbon support, may provide an alternative avenue to tune the selectivity and activity of more complex reactions in heterogeneous catalysis. KEYWORDS: Au catalysts, nanocatalysis, metal−support interaction, selective oxidation reaction, electronic perturbation, styrene oxidation

U

acetonitrile and H2O. The cause of solvent polarity-dependent selectivity in SOR is due to the Au−carbon electronic interaction where charge transfer from N-dopants in the carbon support to the surface Au atoms of AuNCs occurs. Such electron-rich AuNCs significantly changed the binding of CC to metal atoms and the reaction pathway of CC oxidation. Density functional theory (DFT) simulations indicated that the electron-rich surface of AuNCs decreased the activation energy of the intermediates to styrene epoxide in polar solvents. Compared to traditional Au−oxide electronic interactions, the Au−carbon interaction can be readily tuned by simply varying the surface charge density of carbon supports through nitrogen doping; this, therefore, illustrates an alternative way to control the electronic nature of Au catalytic sites that is vital for tuning the catalytic activity and selectivity of Au. AuNCs supported on nitrided carbon (Activated Charcoal from Sigma-Aldrich, layered carbon with a surface area of 286.3 m2 g−1) were prepared via a soft “nitriding” method as reported elsewhere.8a Ultrasmall AuNCs are highly uniform with a diameter of 1.8 ± 0.3 nm and a loading amount of 2.3 wt %

ltrasmall Au nanocatalysts (AuNCs, 99% in ethanol was found as given in entry 7 in Table 1 (Figure S8). The solvent polarity-dependent selectivity of the SOR has been confirmed at a similar styrene conversion of 8−12% (Figure S12). The low conversions of styrene (50% in apolar solvents (hexane, toluene, and acetonitrile), compared to that of 33.7% in a mixed polar solvent of acetonitrile/H2O10% (Figure 1d,e and entries 14,15 in Table 1). The reaction kinetics and the product selectivity of Au-4/C are similar to those reported previously.1c,10d,f However, when physically adsorbing Au NPs on the nitrided carbon support (denoted as Au-4/N-C), an increase in the selectivity to styrene epoxide from 50.2% to 69.3% was observed when adding 10 vol % of H2O to acetonitrile (entries 16,17 in Table 1 and Figure S11). Second, the Au−carbon interaction is governed by the size of Au analogous to that of Au−oxide interactions.11 To evaluate the size effect of Au, ligand-free Au NPs with an average diameter of 6.2 ± 1.6 nm and 10.4 ± 1.9 nm were grown on the nitrided carbon support with a similar loading amount (denoted as Au-6@N-C and Au-10@N-C) (see Figure S5).8a The solvent polarity became less effective for the SOR using both catalysts with a larger size (entries 10−13 in Table 1, and Figure S10), compared to that of ultrasmall AuNCs. The increase in the selectivity of the SOR was from 37.8% to 71.2% for Au-6@N-C when adding H2O (10 vol %) to acetonitrile. This trend became less obvious for Au-10@N-C (see Figure S10). These findings indicated that N sites played an essential role in controlling the SOR selectivity. Since the carbon support was usually considered to be chemically and electronically inert to Au, the observed metal−carbon interaction likely originates from the electronic interaction between surface Au atoms of AuNCs and N-dopants in the carbon support. When recycling AuNC@N-C, a similar tendency of the sizeeffect was observed. The initially recycled AuNC@N-C with an average diameter of 2.2 nm showed reasonably high selectivity to styrene epoxidation in acetonitrile/H2O10%, ca. 88.1% (entry 4 in Table 1 and Figure S9). After the second reuse, the size of AuNCs further increased to 5.3 nm and the selectivity to styrene epoxidation dropped to 73.8% (entry 5 in Table 1). The decrease in selectivity to epoxidation was attributed to the increase in size of Au nanocatalysts on the carbon support; however, the Au−carbon interaction was weakened with less available surface area of Au nanocatalysts. Additionally, the oxidative reaction condition may partially disrupt the surface nitrogen doping on carbon, which lowered the Au−carbon interaction. To understand the nature of the Au−carbon interaction and how this controls the catalytic selectivity of the SOR, the structural models of ligand-free AuNCs on the nitrided carbon as well as their bare counterparts were built to simulate the surface charges using DFT (Figures 3a and S21−22). We used Au nanoclusters supported on the nitrided carbon to explore the interfacial electronic interaction. To conduct the simulation in a reasonable computational time frame, model catalysts with small Au clusters (e.g., Au3, Au16, Au28, and Au55) were used for DFT calculations. In the case of a face-centered cubic Au16 cluster, the Hirshfeld charge analysis showed that surface Au atoms on the nitrided carbon support are negatively charged by −0.33|e| with an average charge of −0.021|e| per surface Au atom (Figure 3a). The loading of Au16 cluster on the nitrided

styrene epoxide using Au-4/C (activated carbon support without nitriding; see Figure 1d,e). Figure 2a shows the representative high-annular dark-field scanning transmission electron microscope (STEM) images of

Figure 2. Characterizations of AuNC@N-C catalysts. (a) Dark-field STEM image and (b) STEM-EDX mapping of AuNC@N-C prior to use. (c) TEM image of AuNC@N-C after use. The insets in (a) are the diameter distribution of AuNCs (left) measured from dark-field STEM images by averaging >300 particles and a zoom-in view of AuNCs (right).

AuNC@N-C prior to use. AuNCs are homogeneously dispersed on the surface of the layered carbon support. No aggregation and overgrowth of AuNCs were observed in the low-magnification image (see more TEM images in Figure S1). The Brunauer−Emmett−Teller surface area of AuNC@N-C was measured to be 286 m2 g−1 using N2 sorption isotherms (Figure S2). The STEM energy-dispersive X-ray (EDX) mapping reveals the presence of N and the well-distributed Au throughout the carbon support (Figure 2b). N-doping enhanced the interaction between AuNCs and carbon supports resulting in the formation of ultrasmall and ligand-free AuNCs as mentioned in our previous report.8a After the SOR, a slightly larger size of 2.2 ± 0.4 nm for AuNCs was found (see Figure 2c and Figure S3). Selective epoxidation of other alkene monomers was examined to confirm our experimental observations. Excellent selectivity to epoxidation (>97%) for styrene derivatives containing electron-donating groups on the phenyl ring (e.g., α-methylstyrene, 4-methylstyrene, and 4-methoxystyrene) was observed in the solvent mixture acetonitrile/H2O10%; however, only ca. 40−50% of selectivity to epoxidation was observed in pure acetonitrile (entries 1−3 in Table S2; Figure S14−16). In addition, cyclic and linear alkenes (e.g., cyclohexane, cyclooctene, and 1-hexene) also showed an obvious increase in selectivity to epoxidation using AuNC@N-C in acetonitrile/ H2O10% (entries 4−6 in Table S2; Figures S17−19), compared to that in acetonitrile. For example, the oxidation of 1-hexene showed a selectivity of 94.2% to 1,2-epoxyhexane in acetonitrile/H2O10%, compared to that of 60.4% in pure acetonitrile. Therefore, the selective epoxidation of CC dominated by the solvent polarity is universal but, again, very 3485

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ACS Catalysis

surface Au atom in all cases). The decrease in the charge per surface Au atom agreed with the observed catalytic performances where the larger size of Au lowered the selectivity of the SOR in different solvents. As a comparison, bare Au nanoclusters have an almost neutral surface (Figure S21). This result reveals that the nature of the Au−carbon interaction is the electronic perturbation of Au in contact with the electron-rich, nitrided carbon support. However, it is currently unclear whether the charge transfer was mediated by carbon or directly through Au−N. X-ray photoelectron spectroscopy (XPS) was used to examine the surface charge state of AuNCs.12 The highresolution XPS spectrum of the Au 4f region of AuNCs@N-C is given in Figure 3b. Two asymmetric peaks appeared at 87.2 eV (Au 4f5/2) and 83.5 eV (Au 4f7/2) for AuNC@N-C, corresponding to a downshift of 0.5 eV compared to that of Au foil. The decrease in the binding energy of Au 4f is due to the increase of surface charge density of AuNCs from the nitrided carbon support, also known as the initial state effect. The downshift of Au 4f peaks was also observed in Au-6@N-C having slightly larger Au (Figure S20). The XPS results are consistent with the Hirshfeld charge analysis. We further used

Figure 3. Surface electronic effect of AuNCs induced by N-dopants in nitrided carbon support. (a) A model of Au16@N-C with its corresponding Hirshfeld charge analysis. The colors in AuNCs suggest the Bader charge of each Au atom. (b) High-resolution XPS Au 4f spectrum of AuNC@N-C (top) and Au thin film (bottom).

carbon resulted in a strong absorption energy of −2.96 eV. The average surface charge per Au atom is strongly correlated to the size of the Au cluster, as indicated by the decrease in average charge from −0.040 |e| for Au3, to −0.021 |e| for Au16 and −0.012 |e| for Au28, eventually to −0.011 |e| for Au55 (per

Figure 4. (a) Proposed mechanisms of solvent polarity-dependent selectivity of SOR using AuNC@N-C. Paths 1 and 2 result in the selectivity to styrene epoxide and benzaldehyde, respectively. (b) Comparisons of the activation energy barriers of the rate-determining step (the formation of TS3) between the reaction Path 1 and Path 2 calculated from DFT. The ΔE is calculated using, ΔE = E(TS3-Path 1) − E(TS3-Path 2). (c) The typical reaction pathway of the SOR using AuNC@N-C catalysts in a polar solvent with a small amount of H2O. The red line represents the generation of the styrene epoxide, whereas the blue line indicates the formation of benzaldehyde. 3486

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electron-rich Au catalytic sites. The Au−carbon electronic interaction led to the solvent-polarity-dependent selectivity in the SORs by altering the binding of CC bonds to catalytic centers. Our study may open a new avenue in fundamentally understanding metal−carbon interaction, elucidating their catalytic mechanisms, and controlling the product selectivity of Au catalysis. The use of inexpensive and abundant activated carbon as supports in tuning the complex reactions will be of broad interest for various industrial reactions using supported noble-metal catalysts.

CO stripping voltammetry to confirm the change in surface charge density of Au. No obvious CO stripping peak was detected for AuNCs@N-C, due to the weak binding between Au and CO. However, the CO stripping voltammetry can be performed using Pd and Pt nanoclusters both of which were grown on the nitrided carbon support using the same method.8a Pd and Pt nanoclusters exhibited a 50 and 100 mV higher CO stripping potential on the nitrided carbon support, respectively, compared to those of commercialized Pd/C and Pt/C catalysts (Figure S23). The shift of CO stripping potential is indicative of the electron-rich surface of metal nanomaterials.13 On the basis of these results, we conclude that the Au−carbon interaction leads to an electron-rich Au surface that is different from metal−oxide interactions. On the basis of the above results, it is reasonable to assume that the negatively charged, electron-rich surface of Au contributes to the solvent polarity-dependent selectivity of SOR by altering the adsorption behavior of alkene monomers. DFT calculations were used to investigate the reaction mechanism of the SOR to gain further insight into the influence of Au−carbon interactions on the reaction selectivity. To simplify the simulation, we used Au16 clusters as model systems to examine the impact of surface charge density of clusters on the reaction mechanism. The dissociation of peroxides on nanoclusters was neglected and oxygen atoms were preadsorbed to interact with styrene which forms a fivemember cycle oxametallacycle (OM-type) intermediate (Figure 4a).14 On a negatively charged Au16 nanocluster, the electrophilic α-C of HCCH2 bonds (C1) favors the direct binding to the surface of Au nanoclusters. The formation of TS-3 (Figure S24), also known to be the rate-determining step, is the key to tuning the reaction selectivity, via the binding of C1 to different oxygen atoms (i.e., Path 1 and Path 2). The difference between TS3-Path 1 and TS3-Path 2, defined as ΔE calculated using E(TS3-Path 1) − E(TS3-Path 2) (Figures 4b), is strongly dependent on the surface charge state of AuNCs. On a negatively charged AuNC, the formation of styrene epoxide is thermodynamically favorable in a polar solvent, particularly in the presence of a highly protonatable solvent (e.g., H2O, see Figures 4c and S24−26). In the presence of water, the activation energy to form styrene epoxide is 1.1 eV, which is 0.08 eV lower than that to form benzaldehyde (Figure 4c). On the contrary, using positively charged AuNCs as catalysts, benzaldehyde is the dominant product regardless of the solvent polarity (Figure S27). These findings are consistent with our key experimental observations. Similar results were seen for other alkene monomers (Figure S28). However, we cannot rule out the contribution of the highly protonatable solvent to promote the formation of key transition states because the selectivity of epoxidation showed a large jump in aqueous or alcoholic solvents. The solvents as a proton source possibly interact with bound oxygen atoms and dissociate/adsorb on the surface of Au (e.g., to form OM2OH* intermediates; see Figures 4c and S24). The possible “cage” effect of protonatable solvents also reduced the reactivity of oxygen atoms to promote the formation of styrene epoxide as partial oxidation products, and it slowed the reaction rate as observed. A similar water-assisted epoxidation mechanism has been proposed previously in Fe-catalyzed oxidation of olefins.15 To summarize, we demonstrated the first observation of electronic perturbation at the Au−carbon interface. N-dopants on the carbon support changed the charge density of the surface Au atoms of AuNCs and resulted in the formation of

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b01048. Synthetic details of catalysts, additional electron microscopic images, SOR results, and DFT simulation data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for Y.P.: [email protected]. *E-mail for S.L.S.: [email protected]. *E-mail for J.H.: [email protected]. ORCID

Ben Liu: 0000-0003-1305-5900 Yong Pei: 0000-0003-0585-2045 Steven L. Suib: 0000-0003-3073-311X Jie He: 0000-0003-0252-3094 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.H. recognizes financial support from the University of Connecticut. S.L.S. acknowledges the support of the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical, Biological and Geological Sciences under Grant DE-FG02-86ER13622.A000. Y.P. recognizes the support of the National Natural Science Foundation of China (21373176, 21422305). TEM studies were performed using facilities of UConn/FEI Center for Advanced Microscopy and Materials Analysis (CAMMA). This work was partially supported by the Green Emulsions Micelles and Surfactants (GEMS) Center, and the FEI Company under an FEI-UConn partnership agreement, a UConn Research Excellence Program Award.

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