Interface Engineering of Graphene-Supported Cu Nanoparticles

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Functional Nanostructured Materials (including low-D carbon)

Interface Engineering of Graphene-Supported Cu Nanoparticles Encapsulated by Mesoporous Silica for SizeDependent Catalytic Oxidative Coupling of Aromatic Amines Chitra Sarkar, Saikiran Pendem, Abhijit Shrotri, Duy Quang Dao, Phuong Pham Thi Mai, Tue Nguyen Ngoc, Dhanunjaya Rao Chandaka, Tumula Venkateshwar Rao, Quang Thang Trinh, Matthew Sherburne, and John Mondal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18675 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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ACS Applied Materials & Interfaces

Interface Engineering of Graphene-Supported Cu Nanoparticles Encapsulated by Mesoporous Silica for Size-Dependent Catalytic Oxidative Coupling of Aromatic Amines Chitra Sarkar†, Saikiran Pendem†, Abhijit Shrotri‡, Duy Quang Dao¶, Phuong Pham Thi Mai#, Tue Nguyen Ngoc, Dhanunjaya Rao Chandaka†, Tumula Venkateshwar Rao†, Quang Thang Trinh*,¶,§, Matthew P. Sherburne*,∇,⊥ and John Mondal*,† † Catalysis & Fine Chemicals Division, CSIR-Indian Institute of Chemical Technology, Uppal Road, Hyderabad-500007, India. ‡ Institute for Catalysis, Hokkaido University, Kita 21 Nishi 10, Kita-Ku, Sapporo, Japan, 001-0021. ¶ Institute of Research and Development, Duy Tan University, 03 Quang Trung, Danang 550000, Viet Nam. # Advanced Institute of Science and Technology (AIST), Hanoi University of Science and Technology, 1 Dai Co Viet, Hanoi 10000, Vietnam.  School of Chemical Engineering, Hanoi University of Science and Technology, 1 Dai Co Viet, Hanoi 10000, Vietnam. § Cambridge Centre for Advanced Research and Education in Singapore (CARES), Campus for Research Excellence and Technological Enterprise (CREATE), 1 Create Way, 138602 Singapore. ∇ A Singapore Berkeley Research Initiative for Sustainable Energy, Berkeley Educational Alliance for Research in Singapore, 1 CREATE Way, 138602 Singapore. ⊥ Materials Science and Engineering Department, University of California, Berkeley, California 94720, United States.

*Email: [email protected] (Q.T.T); [email protected] (M.P.S); [email protected] (J.M)

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Abstract: In this study, graphene-nanosheet-supported ultrafine Cu nanoparticles (NPs) encapsulated with thin mesoporous silica (Cu-GO@m-SiO2) materials have been fabricated with particle sizes ranging from 60 nm to 7.8 nm; and are systematically investigated for the oxidative coupling of amines to produce biologically and pharmaceutically important imine derivatives. Catalytic activity has been remarkably boosted from 76.5% conversion of benzyl amine for 60 nm NPs to 99.3% conversion and exclusive selectivity of N-Benzylidene-1phenylmethanamine over 7.8 nm NPs. The superior catalytic performance along with outstanding catalyst stability of newly designed catalysts are attributed to the easy diffusion of organic molecules through porous channel of mesoporous SiO2 layers, which not only restricts the restacking of the graphene nanosheets, but also prevents sintering and leaching of metal NPs in extreme extent through nano-confinement effect. Density functional theory calculations were performed to shed light on the reaction mechanism and give insight into the trend of catalytic activity observed. The computed activation barriers of all elementary steps are very high on terrace Cu(111) sites which dominate the large-size Cu-NPs, but are significantly lower on step-sites which are presented in higher density on smaller-size CuNPs and could explain the higher activity of smaller Cu-GO@m-SiO2 samples. In particular, the activation barrier for the elementary coupling reaction is reduced from 139 kJ/mol on flat terrace Cu(111) sites to the feasible value of 94 kJ/mol at steps sites, demonstrating the crucial role of step site in facilitating the formation of secondary imine products. Keywords: Oxygen activation, Graphene nanosheet, Mesoporous silica, Size-dependent activity, Density functional theory calculations (DFT)

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1. Introduction. Metal nanoparticles, owing to their large surface-to-volume ratio along with quantum effects, are promising material for catalysis when compared to their bulk materials for several research techniques and industrial procedures such as oxidation of aldehyde, chlorination, conversion of bio-renewable feedstock to fine chemicals etc.1-5 The large surface area of ultrafine metal particles as well as the presence of a major percentage of unsaturated atoms at the surface, corners and edges of the nanoparticles results in this unique catalytic property. However due to their small sizes, they aggregate readily to form bulk materials greatly reducing the surface area and hence losing much of their catalytic activity.6 Utilisation of polymers, dendrimers, surfactants as stabilizers and various types of ligands can eliminate the aggregation of metal nanoparticles (NPs), but on another hand lowering their catalytic activity.5, 7-9 One of the best ways to alleviate this difficulty is to use a solid supports such as carbonaceous material, polymer, metal oxide, silica material etc. which provide large surface area and strong interactions to the nanoparticles to be adsorbed on their surface, strengthening the stability of metal nanoparticles as well as increasing their catalytic activity.10-11 In addition to that, these support materials also play an advantageous role in the catalysis by providing basicity/acidity or tuning the electronic property of the metal nanoparticles.12-13 Among the solid supports are being used to stabilize small size metal nanoparticles, Graphene oxide (GO) has drawn a staggering degree of attention in various applications such as nano-electronics, drug delivery, sensing, intercalation materials, photovoltaics and catalysis etc. due to their large specific surface area, electrical conductivity, mechanical strength and most importantly their robust nature.14-17 Although graphene supported metal nanocatalysts have been well explored in literature but the strong π-π interaction between graphene layers usually lead to the restacking of layers, consequently results in the reduction of surface area as well as diminishment in catalytic activity and limit the practical applications of graphene supported metal nanocatalysts.18 Another additional drawback of this type of catalytic system is associated with the sintering of metal NPs under thermally robust conditions owing to their high surface energy. Therefore, novel synthesis strategies need to be established for the high dispersibility and stability of metal NPs anchored onto graphene supports leading to the development of robust promising catalysts with improved activity. An interesting methodology is to use a chemically and thermally stable mesoporous silica layer (m-SiO2) on metal nanoparticle-graphene oxide.19 Mesoporous SiO2 offers confinement to the NPs attached to graphene oxide surface for accommodation as well as 3



restricts the restacking of the graphene layers due to high Van der Waal’s force of attraction.20 This mesostructured silica layer will also facilitate easy diffusion of reactants through the large surface area while also preventing sintering and leaching of metal NPs to a large extent through nanoconfinement effect.21 In this study, we apply the above-mention approach to fabricate the graphene-nanosheetsupported ultrafine Cu nanoparticles (NPs) encapsulated with thin mesoporous silica (CuGO@m-SiO2) materials with well-defined particle size as catalysts for the oxidative coupling of amines to produce the secondary imines products. These imines and their derivatives can serve as important intermediates and potentially useful chemicals in pharmaceutical and industrial applications. Conventional methods have mostly reported the coupling of aldehydes and amine to deliver corresponding imine, but offer low selectivity, low reaction rate, and often require harsh conditions such as at high temperature (150-170oC). Therefore the oxidative coupling process for the conversion of amine to imine by using conventional oxygen (O2) or air has emerged as an area of increasing research interest and offers a promising alternate method. Though significant progress has been made in oxidation based techniques they suffer from poor selectivity because they produce nitrile as a by-product through dehydrogenation and therefore designing catalyst with higher selectivity is always desirable.22-24 A comparison study for oxidative coupling of benzyl amine to imine with the literature reported catalysts are tabulated in Table S1, Supporting Information. Besides, particle size of the catalyst is also an important factor that significantly influence to its activity.25-27 However, the synthesis of supported catalysts considering well defined particle size and shape employing liquid-phase chemistry is a very challenging task where appropriate distribution of the colloidal particles throughout the pores of high-surface-area supports followed by removal of the organic templates in solution to expose the clean metal NPs surface is essential, avoiding the occurrence of particle agglomeration (sintering).28 A relationship between catalyst performance and the particles size and/or shape could be established once we successfully developed the catalyst structure including homogeneous particles dispersion and activation. Herein, we report on the design and synthesis of graphene-nanosheet-supported ultrafine Cu-NPs encapsulated with mesoporous silica (CuGO@m-SiO2) using three different synthesis procedures to furnish three catalysts CuGO@m-SiO2 samples with different copper particle sizes ranging from ~7.8-60 nm, thus allowing a controlled study of size effects on catalytic activity. The resulting catalysts were comprehensively characterized, activated, and executed for the promotion of liquid phase 4


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solvent-free oxidative coupling of aromatic amines to produce biologically and pharmaceutically important imine derivatives with high selectivity. We have also investigated the crucial role of m-SiO2 of the newly designed catalyst for the catalytic reactions of interest. This approach has been applied to Pt based catalyst designed with sandwiched structures by Zhang et al. and successfully implemented for oxidation and reduction reactions.29 However, this is the first systematic investigation of catalyst synthesis applying the various preparation methods and performing a comprehensive study on the evolution of shape/size-performance relationship in step-forwarding oxidative-coupling of amines of these supported heterogeneous catalysts with sandwiched structures. 2. Results and Discussion: 2.1 Synthesis and Characterisation: 2.1.1. Synthesis of Cu-GO@m-SiO2 materials. The procedures adopted for the deposition and activation of the copper NPs onto interface of graphene oxide (GO) and high-surface-area mesoporous silica (m-SiO2) solid supports are perhaps the most crucial aspect of our catalysts synthesis because catalyst nanoparticles loading into the sandwiched structures makes it convenient for recycling, avoiding aggregation and leaching. Three basic approaches have been explored and differ by the way of the colloidal copper particles reduction processed, as illustrated in the Figure 1. Firstly, a precursor of copper NPs (CuCl2.2H2O salt in the presence of urea) was anchored onto the hydrophilic surface of GO nanosheets by electrostatic interaction of surface -COOH group and Cu2+ ions follow the deposition-precipitation method.30 Afterwards, the structuredirecting agent or template cetyltrimethylammonium bromide (CTAB) has been introduced into the premixed solution followed by addition of an appropriate amount of tetraethyl orthosilicate (TEOS). The hydrolysis of TEOS has been catalyzed in basic medium by in situ production of NH4OH with decomposition of urea to initiate the gelling of silica solid which wrapped the surface of M-GO composite. Lastly, the in situ liquid phase reduction method is applied to encapsulate the colloidal copper particles within the pores of solid (GO-SiO2) support during the growth process by using different reagents including N2H4.H2O and NaBH4 to furnish the Cu-GO@m-SiO2-A and Cu-GO@m-SiO2-C materials, respectively. Besides, in another approach, we have annealed the dried grey powder in vacuum under an atmosphere of H2 at high temperature to produce the copper NPs encapsulated nanostructure, denoted as Cu-GO@m-SiO2-B. A similar approach has been successfully used by Zhang et 5



al. to encapsulate the expensive and rare platinum NPs for CO oxidation and nitrophenol reduction by solid-phase H2 reduction method.29 Details of catalyst preparation procedures are provided in the Supporting Information.

Figure 1: Schematic illustration for the synthesis of graphene-supported Cu nanoparticles encapsulated by mesoporous silica Cu-GO@m-SiO2. The TEM image of GO is inserted. 2.1.2. Structural Characterization of Cu-GO@m-SiO2 materials. After the reduction steps, the resulting dry catalysts displayed blackish grey color, indicating successful metal deposition and reduction. Subsequently, the catalyst were comprehensive characterized by employing powder XRD, N2 physisorption, high-resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray (EDX) analysis, scanning electron microscopy (SEM), H2-Temperature programme reduction (TPR) and Xray photoelectron spectroscopy (XPS) studies (details of characterization procedures and the data analysis are presented in the Supporting Information). The Cu contents in the assynthesized Cu-GO@m-SiO2-A, -B and -C materials are 0.653, 0.594 and 0.628 mmol.g-1, respectively, as measured by inductively coupled plasma optical emission spectrometer (ICPOES) analysis. Owing to the presence of chemically and thermally stable m-SiO2 layer which could inhibit the п-п stacking interactions among graphene nanosheet, the serious aggregation and restacking of graphene nanosheets which often responsible for the blocking of catalytic active sites could be restricted. Additionally, metal NPs could be nicely confined into the mesopores of m-SiO2 which could prevent the detachment of NPs during the reaction. Investigations on thermogravimetric analysis (TGA), H2-TPR and FE-SEM analysis have been carried out to explore the thermal stability and morphology of the catalysts and are 6


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shown in Figure S1-3, Supporting Information. These findings unambiguously reveal that, it is possible to obtain Cu-GO@SiO2 nanohybrids with diverse morphologies depending on the synthesis procedures. The molecular degradation followed by self-assembly of the nanoflakes towards one-dimension nanorod could be happened to take place during high temperature solid-phase reduction process as was reported ealier.31 In this study, we have attempted two approaches including (i) in situ reduction of the colloidal mixture in the presence of the support and (ii) direct solid phase reduction of the metal impregnated nanocomposite to furnish

Cu-GO@m-SiO2-A,

Cu-GO@m-SiO2-C

and

Cu-GO@m-SiO2-B

materials,

respectively. In the Fig. S4 (Supporting Information), the EDX spectra of Cu-GO@m-SiO2 clearly show that Cu and Si are well distributed with the simultaneous existence of elemental carbons.

Figure 2: TEM images (a, b & c), (e, f & g), (i, j & k) along with the HR-TEM images (d, h & l) of Cu-GO@m-SiO2-A, Cu-GO@m-SiO2-B & Cu-GO@m-SiO2-C, respectively. To understand the nanostructure, morphology, mean particle size and location of the copper species anchored on the graphene surface of the respective three catalysts, we characterized with the transmission electron microscopy (TEM) analysis which has provided insight into the structural details. HR-TEM images (Figure 2a-c) of Cu-GO@m-SiO2-A catalyst showed uniform rod like morphology with the average length of 1.82-2.49 µm where 7



some agglomerated spherical copper particles are tightly anchored within the graphene layer and mesoporous silica. We found very large diameter (~60 nm) of copper metallic NPs dispersed over graphene oxide from the closer inspection to HR-TEM images (Fig. 2b-c) with some open ends, which enable them to have potential application in the catalysis. Careful observation of the TEM image (Fig. 2b) of the Cu-GO@m-SiO2-A catalyst revealed some agglomerates of copper crystallites with irregular shape are randomly connected to each other and also strongly wrapped with the GO nanosheet layer shell (Fig. 2c). HR-TEM image (Fig. 2d) displayed the existence of distinguished inter-planar lattice spacing of about 0.201 nm, consequent to the (111) lattice spacing of the face-centered cubic (fcc) arrangement of the Cu0 NPs.32 A homogeneous distribution of relatively smaller sized copper NPs having dimension ~13.2 - 16.6 nm on graphene oxide nanosheet was observed from TEM images (Fig. 2e-f) of catalyst Cu-GO@m-SiO2-B compared with Cu-GO@m-SiO2-A catalyst. The HR-TEM images (Fig. 2g-h) of Cu-GO@m-SiO2-B sample clearly showed the spherical copper particles (~12.3 nm) are located on the external surface of GO-mSiO2 with the more exposed crystalline phase. TEM images of Cu-GO@m-SiO2-C catalyst (Fig. 2i-j) demonstrate fibrous morphology where short ribbons (0.24-0.37 µm) are fused together with fine homogeneous dispersion of copper nanograins (~7.8-9.6 nm) at the interface. TEM image (Fig. 2k) indicates that the homogeneity in dispersion of copper NPs appeared by some black spots for Cu-GO@m-SiO2-C catalyst is so nice with the proper encapsulation between the GO and m-SiO2 layers without any aggregation, indicative to distinct an individual particle is hardly visible with closer inspection. The appearance of well-defined crystalline lattice fringes with d-spacing of about 0.248 nm in the high-resolution HR-TEM image (Fig. 2l) demonstrates the (111) planes of cubic Cu2O nano-clusters, and the lattice fringe with interplanar distances of 0.207 nm could be attributed to the Cu (111). This result shows that highly crystalline Cu-Cu2O nano-clusters have been formed during the synthesis of CuGO@m-SiO2-C sample by NaBH4 treatment method, in agreement with powder XRD pattern shown in Fig. S5, Supporting Information. Our observation is consistent with the previous findings on the Cu/Cu2O/carbon spheres composite for enzymaticless glucose sensors by Yin et al.33 Additional inspection using transmission electron microscopy (TEM) specify (Fig. S6, Supporting Information) that hierarchical mesopores (visible bright spots) as originated from silica layer with mean diameter of ca. 4.54 nm are uniformly distributed throughout the regions, whereas the black color spherical Cu NPs are well strongly anchored between the GO and m-SiO2 layers. The 8


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smallest Cu particle size with more step sites as obtained by NaBH4 reduction method compared with the other reduction methods could be attributed to the dual role of NaBH4 acting as both reducing as well as stabilizing agent during nanoparticles formation. NaBH4 reduces metal ions to zero valent metal nanoparticles and subsequently stabilizes the metal nanoparticles by being adsorbed on the surface of NPs. Adsorption of ions to the surface creates an electrical double layer which results in a Columbic repulsion force between individual particles, thereby preventing uncontrollable growth of particles and aggregation.25

Figure 3: X-ray photoelectron spectra in C-1s region of Cu-GO@m-SiO2-A (a),-B (b), -C (c) and Cu-2p (d) core-regions. Powder X-ray diffraction (PXRD) patters of the series of Cu-GO@m-SiO2-A, B and C nanocomposites are shown in the Fig. S5, Supporting Information. Cu-GO@m-SiO2-A and B exhibited broad diffraction peak in the range of 2θ =15-35º which could be assigned to the graphene nanosheet supported metal nanoparticles tightly covered by mesoporous silica (mSiO2) layers.34-35 One strong peak along with the two very weak diffraction peaks appeared at 2θ = 43.3°, 50.4° and 74.1°, respectively, in Cu-GO@m-SiO2-A, B catalysts which could be 9



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nicely indexed to the (111), (200), and (220) crystalline reflections planes, respectively, corresponding to cubic Cu(0)-NPs (JCPDS card No. 04-0836).13,

36-37

In contrast, Cu-

GO@m-SiO2-C material exhibited three distinct peaks, in addition other diffraction peaks are noticed at 2θ =16.2, 17.5º, 31º, 32.4º, 39.8º, 50.0º, 53.6º and 57.3º, which could be readily assigned to (111), (003), (021), (113), (024), (033), (220) and (223) planes, respectively, of (Cu2Cl(OH)3) (JCPDS card no. 87-0679).38 XPS analysis has been conducted to investigate the elemental composition of the surface and modification of chemical or electronic state of each component on the surface and is shown in Figure 3. Four distinctive peaks located in the C 1s XP spectra of Cu@GO sample (Fig. S7, Supporting Information) at the binding energy 284.6, 286.6, 287.5 and 288.7 eV, respectively, corresponding to the functional groups C=C sp2, C-O, C=O and C-C=O, respectively, which matches nicely with previous literature reports.38-39 For all the three catalysts (Figure 3a-c), two peaks attributing to C-O and C=O functional groups slightly down shifted due to the reduction of GO and decrease in oxygen containing groups, and a new C 1s peak was assigned to C-Si bond (~281.5 eV), thus confirming the combination of SiO2 with GO with strong interaction.40 For Cu-GO@m-SiO2-B catalyst a better fit was obtained by including a small peak for the sp3 hybridized carbon at 282.3 eV. NaBH4 exhibited more efficient reducing agent for reduction of GO than N2H4.H2O. The main disadvantage of using N2H4.H2O is that this method introduced some heteroatom impurity (N covalently bonded with the GO) while eliminating the oxygen containing functional groups. Due to the presence of C-N functional groups in Cu-GO@m-SiO2-A which commonly act as n-type dopant created the high sheet resistance compare to Cu-GO@m-SiO2-C catalyst. High resolution deconvoluted Cu-2p3/2 XP spectra (Fig. 3d) illustrated the appearance of characteristic binding energy peaks at around ~933.0-933.6 eV and 935.0-935.8 eV, which could be attributed to the presence of Cu0 and Cu+2 oxidation states respectively, which is in good agreement with previous reports.41-43 Furthermore, we found another additional shakeup peak with the binding energy at ~944 eV in the Cu-2p3/2 XP spectra, corresponding to the Cu-O bond in CuO species, which is consistent with previous observations of copper catalyst on the nanoscale dimension, confirming the chemical composition of the catalyst.44 Here, we have also noticed a positive binding energy displacement (~0.7 eV) for Cu-GO@m-SiO2-C catalyst in Cu-2p3/2 XPS compared with the other two catalysts, which is due to the smaller Cu-particle size. The same scenario was observed by Balamurugan and co-workers where 10


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Cu3N nanoparticles clearly reveal a shift in their peak positions towards the higher binding energy side with decreasing particle size.45 The interaction between SiO2 and Cu-NPs could be nicely explained by performing XPS analysis of Cu@GO and Cu-GO@SiO2 materials at Cu-2p regions. From the Cu-2p XPS analysis (Fig. S8, Supporting Information) we have found out a negative displacement in Cu0 binding energy (~0.9 eV) in Cu-GO@SiO2 in comparison with the Cu@GO. This binding energy shift could be emphasized by the interaction of surface -OH groups from SiO2 causing changes to the electron density over Cu NPs after wrapping with mesoporous silica layer.

Figure 4: (a) Raman Spectra and (b) N2-adsorption/desorption analysis. Raman Spectroscopy is utilised as a structural fingerprint to define the framework of chemical compounds (Figure 4a). Generally Raman spectrum of pristine graphite appeared at 1581 cm-1 due to sp2 defect which is referred as G band, symbolizing the graphitic nature of parent powder. Here we observed two sharp peaks located at 1352 cm-1 and 1595 cm-1 corresponding to D and G bands, respectively. D band appeared due to sp3 defects in sp2 lattice. ID and IG ratio of three catalysts Cu-GO@m-SiO2-A,-B and -C are 0.90, 0.88 and 1, respectively. Inverse of the in plane crystalline dimension has a linear relationship with this intensity ratio, indicates here the deposition of Cu nanoparticles on GO are slightly less crystalline in nature.46 In the N2 adsorption/desorption analysis (Fig. 4b), we observe a typical type IV isotherm with a moderate H2 type of characteristic hysteresis loop for Cu-GO@mSiO2-B catalyst. The synthetic procedure of preparing Cu-GO@m-SiO2-B, reduction has been carried out under H2 gas at 250ºC, as a result of using such high temperature some organic compound come out through the pore and surface of catalyst, deformation of pores occurred, which introduced a hysteresis loop. A small hysteresis loop in the P/P0 region 0.60.8 with type IV isotherm also occurred for Cu-GO@m-SiO2-C and Cu-GO@m-SiO2-A 11



catalysts, indicating the mesoporous nature of the catalysts, in good agreement with previous literature. The Brunauer-Emmett-Teller (BET) surface areas of these three catalysts CuGO@m-SiO2-A, Cu-GO@m-SiO2-B, Cu-GO@m-SiO2-C were observed to be 35.7, 225 and 60.6 m2g-1, respectively. It is clear that the high temperature solid state hydrogenation assisted reaction leads to a crucial increase of a high specific surface area (SBET= 225 m2/g) of Cu-GO@m-SiO2-B indicating that three dimensional silica network size is enlarged with modified nanostructure with the complete successful removal of organic surfactant after appropriate thermal treatment. For other catalysts which have been prepared under liquid phase conditions, the pores have been blocked in somewhat extent providing comparatively less surface areas and the mesopores can be developed on the graphene oxide carbon surface or in the SiO2 matrix itself. Cu particles also tend to self-assemble together during thermal treatment onto SiO2-GO patterned surfaces in a vertically oriented fashion leading to development of interparticle void spaces, resulting in an enormous increase in the specific surface area.47 2.2. Catalytic activity of material Cu-GO@m-SiO2-A, -B and -C towards the oxidative coupling of aromatic amine. We have explored the catalytic performance of these newly designed Cu-GO@m-SiO2 catalysts in liquid phase solvent-free oxidative coupling of aromatic amines to the corresponding imine under aerobic conditions. In this investigation, our initial catalytic experiment began by conducting aerobic oxidation of benzyl amine (1 mL, 9 mmol) over CuGO@m-SiO2 catalyst (50 mg) in a round-bottomed flask at 110°C oil-bath temperature. The contents of products and reactants were determined using GC-FID based on authentic samples. The corresponding reaction parameters such as reaction temperature, atmosphere, amount of amine used and different aromatic amines were also tested under identical reaction conditions for comparison study. Evolution in the reactant and product distributions with the progress of the catalytic oxidation (Time-on-stream profile diagram) demonstrated that among all the examined catalysts Cu-GO@m-SiO2-C exhibited an enhanced catalytic performance providing 99.3% conversion of benzyl amine with exclusive selectivity of NBenzylidene-1-phenylmethanamine (imine) as the major product (Figure 5a). In strong contrast, we have also successfully achieved 96.5% and 76.5% conversions of benzyl amine with the corresponding selectivity of the desired imine product of 86.5% and 92.1% with the Cu-GO@m-SiO2-B and Cu-GO@m-SiO2-A catalysts, respectively. The reaction was 12


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accompanied by a progressive increase in imine selectivity ( ∼ 98.2%) and conversion of benzyl amine ( ∼ 44.5%) the initial 1 h, which revealed that dehydrogenation of amine to form imine is the first step followed by subsequent coupling between amine and imine with the elimination of NH3 to yield as N-Benzylidene-1-phenylmethanamine the primary product.48

Figure 5: (a) Evolution of reactant and product distributions as a function of time, (b) influence of reaction temperature and (c) effect of atmosphere; Reaction conditions: Benzyl amine (1 mL, 9 mmol), catalyst (50 mg), reaction temperature (110ºC). The influence of temperature on the catalytic activities of Cu-GO@m-SiO2-A, CuGO@m-SiO2-B and Cu-GO@m-SiO2-C were evaluated by performing the oxidative 13



coupling reaction of benzyl amine at three different temperatures (90ºC, 100ºC and 110ºC) in air under solvent free condition which produced N-Benzylidene-1-phenylmethanamine (imine) with an undesired product benzonitrile (Fig. 5b). The catalytic activity was calculated in terms of percentage conversion of benzyl amine to N-benzylidene-1-phenylmethanamine and its percentage selectivity at these temperatures. Enhancement in percentage benzyl amine conversion was found with increasing temperature, for each the catalyst Cu-GO@m-SiO2-A, Cu-GO@m-SiO2-B & Cu-GO@m-SiO2-C, which reflected the improvement of catalytic activity with temperature. An increasing trend in benzyl amine conversion was found for catalyst Cu-GO@m-SiO2-A as it showed 55.3% conversion at 90ºC, 65.1% at 100ºC and 77.6% at 110ºC, whereas the selectivity for imine product was observed to decrease from 100% to 90% with the enhancement of temperature from 90ºC to 110ºC. Cu-GO@m-SiO2-B catalyst demonstrated enhancement in benzyl amine conversion from 76.5% to 92.3% to 97.4% at 90ºC, 100ºC and 110ºC respectively, but the selectivity for imine remained almost unchanged with temperature. Excellent catalytic efficiency was noticed with elevating temperature for Cu-GO@m-SiO2-C to produce 90.6% benzyl amine conversion at 90ºC and complete conversion at 100ºC. Selectivity for the desired imine product remained unaltered at 98.6% for Cu-GO@m-SiO2-C at these temperatures. An enhancement in the catalytic activity has been observed with the gradual decrease in the Cu-NPs size from 60 nm to 10 nm. The catalytic activity of Cu-GO@m-SiO2-A catalyst is poor due to its larger aggregated particles with diminished available surface area, whereas Cu-GO@m-SiO2-B catalyst offered better catalytic activity than Cu-GO@m-SiO2-A due to its homogeneous dispersion of particles as well as their smaller size with greater surface area availability. In strong contrast, catalyst CuGO@m-SiO2-C exhibited superior activity and selectivity for the oxidative coupling of amine due to its widespread homogeneous dispersion of fine copper nanograins on graphene oxide with mesoporous silica layer as found from HR-TEM analysis (Fig. 2). Catalytic performance of these three catalysts was compared by conducting the oxidative coupling reaction of benzyl amine in different atmospheres including air, N2 and Ar, respectively (Fig. 5c). It was observed that in presence of air, N-benzylidene-1phenylmethanamine formed with small amount of benzonitrile, whereas in inert atmosphere (Ar & N2) imine formed as major and nitrile as minor product; which exhibited diminution selectivity for each of the catalyst in absence of air or oxygen. The amplification in catalytic activity in air may be attributed due to the formation of copper-oxide transient species, from Cu(0) present in the catalysts, by capturing oxygen present in air which was utilised for the 14


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oxidation of aromatic amine to imine.49 The higher reactivity of Cu-GO@m-SiO2-C catalysts than Cu-GO@m-SiO2-A and Cu-GO@m-SiO2-B in air may be due to the presence of mixture of Cu(0) and Cu2O species, as reflected from XRD analysis of Cu-GO@m-SiO2-C (Fig. S5, Supporting Information). The lattice oxygen present in Cu-GO@m-SiO2-C took part in the reaction along with oxygen taken from air, which could explain the higher reactivity of CuGO@m-SiO2-C than catalysts A and B. Drop in catalytic activity as well as selectivity in inert atmospheres may be due to dehydrogenation of imine produced in-situ, to form nitrile in absence of oxygen, rather than coupling with another amine molecule to form N-benzylidene1-phenylmethanamine, as explained by Singuru et al.48 Finally, the influence of benzyl amine concentration on the catalytic performance of CuGO@m-SiO2-A, -B and -C was examined by reacting different % mol of benzyl amine with fixed amount of catalysts under solvent free conditions at a particular temperature in presence of air. Three concentrations of benzyl amine as 1.8 mmol, 2.7 mmol and 9.0 mmol were taken and reacted with each catalyst separately under optimized conditions and percentage formation of imine and nitrile product were presented separately in Fig. S9, Supporting Information. When catalyst Cu-GO@m-SiO2-A reacted with 1.8 mmol of benzyl amine, 72% of imine product with very small amount of nitrile was found to be formed, whereas with 2.7 mmol of benzyl amine, a decrease in selectivity (56% imine & 6% nitrile) was observed. With excess benzyl amine (9.0 mmol), N-benzylidene-1-phenylmethanamine was formed as the sole product with small amounts of benzonitrile, which was attributed to the availability of excess benzyl amine to couple with the benzylimine formed in-situ. Poor selectivity was noticed in case of catalyst Cu-GO@m-SiO2-B when reacted with these three different concentrations of benzyl amine. In case of Cu-GO@m-SiO2-C catalyst, initially decrease in catalytic activity as well as imine selectivity was detected when benzyl amine concentration was varied from 1.8 mmol to 2.7 mmol. But with the use of excess benzyl amine (9.0 mmol), 98% selectivity in imine formation was found. When the catalytic activity of Cu-GO@mSiO2-A, -B and -C was compared with respect to different benzyl amine concentrations, the higher selectivity of imine in each condition again lead us to conclude that Cu-GO@m-SiO2C catalyst exhibited highest reactivity. 2.3. Mechanistic study by Density Functional Theory (DFT) calculations: Density Functional Theory (DFT) calculations were performed to explain the catalytic activity trend of different Cu-GO@m-SiO2 samples and to shed light into the reaction 15



mechanism. All the calculations for Cu and Cu2O systems have been done by the ab-initio total-energy and molecular-dynamics program VASP (Vienna ab-initio simulation program) developed at the Fakultät für Physik of the Universität Wien50-51 (details of computational method and the justification on using Cu metallic state to model Cu NPs in the simulation are presented in Supporting Information). For the simulations on Cu systems, the optimized Becke88 functional (optB88) including the non-local vdW-DF correlation (optB88-vdW exchange-correlation function) developed by Klimeš et al.52-53 was used to capture the concerted effect of covalence and van der Waals bonding between aromatic compounds and transition metal surfaces. It was reported that explicit coupling of optimized exchange functionals within the non-local vdW-inclusive DFT schemes is mandatory to describe correctly the interaction between aromatic compounds and transition metal surfaces.54-56 Optimized lattice constant for Cu using the optB88-vdW functional is 3.635 Å, which agrees very well with experimental value of 3.603 Å.56-57 For the simulations on Cu2O, the Generalized Gradient Approximation (GGA) and the Hubbard correction U = 4.5 eV within the GGA+U scheme was used to correct the electron delocalization that occurs in strongly correlated systems such as transition metal oxides.42, 58-60 The optimized lattice parameters for Cu2O with GGA+U method is 4.278 Å, which also matches very well with experimental data of 4.269 Å.61

Figure 6: Models used for DFT evaluation of the conversion of Bezylamine. (a) p(44) Cu(111) slab; (b) p(48) Cu(111) with 3 missing rows on the top layer. Different step sites (B5 and F4) are highlighted; (c) p(42) Cu2O(111) slab. Unit-cell sizes are also indicated. Detailed mechanism of benzyl amine activation and conversion was evaluated on Cu terrace sites, which was modelled as a periodic four-layers p(44) Cu(111) slab as is shown in Figure 6a. Step sites were described by a model of periodic three layers p(48) Cu(111) 16


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with 3 missing rows on the top layer as is depicted in Fig. 6b. The use of p(48) Cu(111) with missing rows on the top layer is very helpful in investigating the activity of step sites since there are the presence of both two most popular types of step sites on it (illustrated as inserts in Fig. 6b), denoted as B5 step site which are formed by the corrugation between a (111) and a (100) facets, and F4 step site which are formed by the corrugation between two (111) facets.55,

57

This model has also been successfully used to study the formation of

nanoisland of Co catalyst during the Fischer-Tropsch synthesis62-64 and account the contribution of step sites in overall activity of the catalyst in the activations of methane,57, 65 toluene55 and CO66 in literature. To evaluate the activity of Cu2O phase presented in the CuGO@m-SiO2-C catalyst, Cu2O(111) surface is modelled with a periodic p(42) slab which is illustrated in Fig. 6c. A vacuum thickness of 15 Å above the topmost layer was used for all of those models to avoid the interactions between different repeated slabs. The two top layers and the adsorbates were fully relaxed and optimized, while the remaining bottom layers were constrained at the bulk positions in those models to reduce computational cost while maintaining the high accuracy.67-69 The mechanism for Benzylamine activation and coupling on Cu-based catalyst has been studied in literature48, 70-73 and is proposed in Scheme S1, Supporting Information. The whole conversion involve three stages, the first stage generates Benzyl imine as the key intermediate.48, 72-73 This stage 1 was reported as the key step that controls both the activity and selectivity of the whole process. The subsequent C-N coupling and H-transfer between Benzylimine and Benzylamine occur simultaneously in stage 2, before the deammoniation in stage 3 to form the end-product of secondary imine. The reaction in stage 3 are fast on all Cubased catalysts,48, 70-71 hence we do not evaluate this stage in detail.

17



Figure 7: (a) Adsorption of Benzylamin on Cu(111) surface via the Parallel configuration; (b) Transition state of Initial C-H activation; (c) Transition state of subsequent N-H activation forming Benzylimine; (d) Transition state of concerted C-N coupling and H transferring occur during the coupling between Benzylamine and Benzylimine. Adsorption energy, activation barriers and bond distance are inserted. Color code: peach, white, grey, and red balls represent Copper (Cu), Hydrogen (H), Carbon (C), and Oxygen (O) atoms respectively. Firstly, we studied the adsorption and activation of Benzylamine on terrace site Cu (111) surface. Benzylamine can adopt two different adsorption configurations; with the most stable one is the Parallel configuration (Figure 7a). Another structure is the Up perpendicular configuration which is 0.45 eV less stable (Supporting Information, Figure S11). The Parallel configuration is the more stable structure due to the strong van der Waals interaction between the benzene ring with the flat Cu(111) surface54, 56 and the computed adsorption energy of Benzylamine in this configuration is -1.41 eV. The initial activation of Benzylamine is the CH activation with the activation barrier of 1.76 eV and the transition states structure of this 18


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step is shown in Fig. 7b, while the N-H activation is less favourable with the higher barrier of 1.98 eV (Figure S12, Supporting Information). In this case, the transition state of C-H activation on Cu(111) surface occurred via the radical H-abstraction mechanism. The more feasible CH activation than NH activation could be explained by the stabilized conjugation effect of the produced intermediate. The product of CH activation has the high planar structure and the HOMO-LUMO (highest occupied molecular orbitals - lowest unoccupied molecular orbitals) analysis confirms its conjugation electronic structure, while this affect is not observed in the product of NH activation (Supporting Information, Figure S10). It is widely accepted that the delocalization of charge through resonance and hybridization energy makes the conjugated molecular system more stable than the non-conjugated one.55, 74-75 This is all due to the positioning of the pi orbitals and ability for overlap occurring to strengthen the single bond between the two double bond. For this reason, the more conjugated singly occupied molecular orbitals (SOMO) of the CH radical make it more stable than the NH radical. The conjugation-driven type radical activation on heterogeneous catalyst at low temperature (< 200ºC) has also been reported for CO bond activation in furfural on Cubased catalyst by Minorenko et al.75 Detailed electronics properties and energetics evaluation of Benzylamine activation in gas-phase is presented in the Supporting Information. After the initial C-H activation, the subsequent N-H activation with the barrier of 1.05 eV forms the key intermediate Benzyl imine (transition state structure is shown in Fig. 7c). After being formed, the coupling reaction between Benzylamine and Benzylimine (Stage 2 in Scheme S1, Supporting Information) occurs on Cu(111) surface with the activation barrier of 1.39 eV. The transition state of this reaction step involves the concerted reaction wherein the C-N bond formation and the H-transfer are processed simultaneously (Fig. 7d). It should be noted that the computed barrier for this step without the catalyst is 2.01 eV (Supporting Information, Fig. S12c), demonstrating the catalytic role of Cu catalyst. However, the barriers of the activation and conversion of benzaldehyde on terrace site of Cu(111) surface in Fig. 7 above are all higher than 1.0 eV, making those reactions difficult to occur at reaction temperature of 110oC76 and explain why the conversion of Benzylamine on Cu-catalyst under inert atmosphere (Ar or N2) is low as was observed in Fig. 5.

19



Figure 8: Energy profile of benzyalmine activation and conversion on terrace site of Cu(111) surface and at B5 step site. Activation barriers (in eV) for the transition states are indicated. To explain the catalytic trend of Cu-based catalyst with different sizes of particle as observed in Fig. 5, we compare the activation and conversion of Benzylamine at terrace site of Cu(111) surface and at step sites of the p(48) Cu(111) slab with 3 missing rows on the top layer. As it was mentioned earlier, there are two most popular step sites presented in the model of p(48) Cu(111) slab with 3 missing rows on the top layer, called B5 step site and F4 step site.55, 57, 62, 64, 66 We have evaluated the activity of both B5 step site and F4 step site for the Benzylamine activation and conversion, and it was observed that their activities are quite similar (Supporting Information, Figure S13 and S14). Therefore in the next sections, we only discuss the activity of B5 step site and use the calculated energies for reactions at B5 site to represent for both those step sites. Since the density of step sites is higher (presenting with the larger partial ratio) at smaller-size particle (e.g. Cu-GO@m-SiO2-C catalyst with the average size of 7.8nm), the contribution of step sites in the activity of that smaller-size catalyst become more important than it was on bigger-size particles (e.g. Cu-GO@m-SiO2-A catalyst with the average size of 60nm).26,

77-80

The energy profiles for Benzylamine

activation and coupling on terrace site Cu(111) surface and at B5 step site are shown in Figure 8 and the adsorption configuration and transition state structure of Benzylamine activation at step site are presented in Figure S13-S15, Supporting Information.

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At step site, Benzylamine adsorbs 0.12 eV stronger than at terrace site (Fig. S13a). The stronger interaction between step site and the reactant (Benzylamine) reduces the activation barrier for initial C-H dissociation of Benzylamine to 1.44 eV (transition state in Fig. S14a), which is 0.32 eV lower than the corresponding barrier of initial C-H dissociation on terrace site of Cu(111) surface (Fig. 7b). The subsequent N-H activation forming Benzylimine at step site also has lower barrier (0.89 eV, Fig. S15a) than the barrier of same reaction on terrace site (1.05 eV). More importantly, the coupling reaction between Benzylamine and Benzylimine at step site has the activation barrier of only 0.97 eV (Fig. S15b), which is 0.42 eV lower than the barrier of the coupling reaction on terrace site, making it feasible at reaction temperature of ~100ºC. The lower activation barriers for all those steps demonstrate the higher activity of step site than the terrace site (Fig. 8). The density of step sites at smaller-size particle is much higher than it is for larger particles,26, 66, 77-80 this could explain the activity trend of Cu-GO@m-SiO2-C (particle size of 8 nm) > Cu-GO@m-SiO2-B (particle size of 13 nm) > Cu-GO@m-SiO2-A (particle size of 60 nm). Highest activity was observed for sample C, which has the smallest particle size and hence has the highest density of step sites, while sample A has the largest particles with the density of terrace sites dominating the surface and hence correlates to its lowest activity.

Figure 9: (a) Initial N-H activation and (b) Subsequent C-H activation of Benzylamine by surface Oxygen on Cu(111) surface; (c) Coupling reaction between Benzylamine and Benzylimine on Cu2O(111) surface. Finally, since the activity of the Cu-GO@m-SiO2 catalysts are much better when the reaction was processed in an air atmosphere, we also performed DFT simulations to evaluate the role of surface Oxygen on Cu(111) surface and the results are presented in Figure 9. Its been reported that the dissociation of O2 on Cu(111) surface is reasonable (reported activation barrier of only 0.29 eV by Amaniampong et al.36 and 0.27 eV by Xu et al.81). The 21



presence of chemisorbed oxygen atom on Cu(111) surface (called O/Cu(111) surface) could alter the activations of reactant and even be able to change the most favourable reaction pathway as reported earlier.27, 36, 82 Indeed, the computed barriers for both initial C-H and NH activations of Benzylamine are significantly reduced on O/Cu(111) surface. While the initial C-H activation barrier is reduced from 1.76 eV to 0.89 eV (Supporting Information, Figure S16), the initial N-H activation is more strongly promoted by chemisorbed O on Cu(111) and the barrier is lowered to 0.74 eV from 1.98 eV, making the initial N-H activation more kinetically favourable (Fig. 9a). After the initial N-H activation, the subsequent C-H activation is also facilitated by chemisorbed O with the barrier of only 0.79 eV (Fig. 9b). Those results therefore could explain the much higher conversion of Benzylamine in air atmosphere than under an inert atmosphere of N2 or Ar. Despite the strong promotional effect to the C-H and N-H dissociation, the chemisorbed O does not promote the coupling of Benzylamine and Benzylimine and therefore the selectivity towards the end-product coupled secondary imine of sample A and sample B are not much enhanced. However, due to the presence of small amount of Cu2O phase in sample C, the Cu2O(111) surface also promotes the coupling reaction (besides the promotional effect of steps sites for this reaction) with the activation barrier of only 0.74 eV (Fig. 9c) and contributes to highest selectivity towards secondary product imine of Sample C. It is also important to mention that although owing the promotional affect in facilitating the C-N coupling reaction, Cu2O(111) surface does not enhance the C-H or N-H activations as chemisorbed O on Cu(111) does (Supporting Information, Figure S17). 2.4. Catalytic stability testing. Control experiments were carried out to understand the role of mesoporous silica (m-SiO2) on catalytic behaviour by performing oxidative coupling of Benzylamine with catalyst CuGO@m-SiO2-C, Cu-GO (etched) and Cu-GO (without mesoporous silica) under identical reaction conditions (Figure 10). Catalytic activity of Cu-GO@m-SiO2-C was studied by removing the mesoporous silica layer with sodium hydroxide, which we abbreviated as CuGO (etched). TEM images confirmed the absence of silica layer (Fig. 10a & b) for Cu-GO (etched). Oxidative coupling reaction using Cu-GO (etched) show a decrease in conversion percentage of benzyl amine (98% to 85%) as well as imine selectivity (97% to 90%) in comparison to catalyst Cu-GO@m-SiO2-C under optimized reaction conditions (Fig. 10d). Experiments in which, Cu-GO was synthesised by conjugated only Cu nanoparticles to 22


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graphene oxide layer without any silica achieved only 40% benzyl amine conversion though having high imine selectivity. TEM image of the Cu-GO catalyst is provided in the Fig. 10c. So, mesoporous silica layer plays a pivotal role in catalytic oxidative coupling of amine to deliver imine. The mesoporus silica layer not only acts as a space barrier to impede the sintering of the nanoparticles from graphene oxide surface but also due to the porous nature of mesoporus silica layer, the reactant molecules are allowed to diffuse throughout the porous channel of silica to interact with catalytic active centre to furnish the product.

Figure 10: TEM images (a & b) of Cu-GO (etched) and (c) of Cu-GO catalyst. (d) Comparison in catalytic performance for aerobic solvent-free oxidative coupling of benzyl amine for the respective catalysts. We have placed a particular emphasis on the substrate scope for Cu-GO@m-SiO2-C catalyst with different aromatic amine molecules having various functionalities under optimized reaction conditions (Fig. S18, Supporting Information). Time dependent study was 23



drawn against percentage conversion/selectivity of amine/imine for each molecule revealed that our catalyst not only works well with aromatic amine having electron withdrawing or electron releasing group at ortho, meta or para substitution but also compatible with heterocycle amines with good conversion (> 90%) and high selectivity (> 80%). Aromatic amine having electron withdrawing group at -meta position of benzyl amine produced imine with 92% conversion and 82% selectivity after 9 h of reaction (Fig. S18a, SI). A striking increase in the conversion of imine (98%) as well as selectivity (95%) was observed for heterocycle amines after 12 h of reaction with catalyst Cu-GO@m-SiO2-C in identical reaction conditions (Fig. S18b, SI). Substitution at -para position didn’t have significant effect on the percentage conversion of imine and desired product selectivity (Fig. S18c, SI). Ortho substituted benzyl amine could be transformed into corresponding imine with 96% conversion and 96% selectivity after 12 h of reaction with Cu-GO@mSiO2-C catalyst under solvent free condition (Fig. S18d, SI). To assess the efficiency of the catalysts Cu-GO@m-SiO2-A, -B and -C, recyclability test was carried out for the oxidative coupling of amine under solvent free conditions considering benzyl amine as the model compound. Time dependent study plotted against imine selectivity was chosen as the parameter to evaluate their efficacy for each catalytic run (Fig. S19, Supporting Information). The solid catalyst was recovered after the reaction by centrifugation followed by washing with methanol 2-3 times, drying at 80ºC in an oven overnight, and was then used for the next reaction. Details about the recyclability test are provided in the Experimental Section. The catalyst was used for 4 continued cycles. Increase in percentage imine selectivity with time was observed for each of the catalyst, which remained pristine up to the 3rd cycle and then dropped gradually (Fig. S19a-c, SI). A drop in the amine conversion and desired product imine selectivity for the Cu-GO@m-SiO2 catalysts in the recycling test at the 4th cycle could be attributed to the accumulation of carbonaceous deposits on the catalyst surface or slight trapping of residual reactants or products inside the pores, thus blocking accessibility to the active sites.83 Finally, the Cu-GO@m-SiO2 catalyst integrity after the exposure to the catalytic reaction conditions was examined with HR-TEM analysis (Fig. S20, Supporting Information). Surface reconstruction with the reshaping of original metal NPs, thereby reducing homogeneous dispersibility was observed to take place towards exposure of supported metal particles to reactive environments. However, analyzing TEM images after the catalytic reaction revealed negligible modification in the size of the copper particles and no obvious aggregation (Fig. 24


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S20, SI). It appears that the Cu NPs show mechanical and chemical stability, no associated structural degradation of the catalyst is observed. In order to investigate any leaching that took place during the reaction we have conducted hot-filtration test under optimized reaction conditions. Our examination results definitely reveal that the nanoparticles were strongly anchored between the GO and m-SiO2 layer with sandwiched structures with no confirmation of metal leaching in the solution, as confirmed by AAS (Atomic Absorption Spectroscopy) analysis. 3. Conclusions. In this study, we have fabricated graphene-nanosheet-supported ultrafine Cu-NPs with well-defined sizes that are encapsulated with thin mesoporous silica (Cu-GO@m-SiO2). Cunanoparticles (NPs) with different particle sizes of 7.8nm, 13.2nm and 60nm have been synthesized via three different routes and were shown to effectively promote liquid phase solvent-free oxidative coupling of aromatic amines to produce biologically and pharmaceutically important imine derivatives with high selectivity. A prominent sizedependent catalytic activity of various sized Cu-NPs in this oxidative coupling of aromatic amines reaction was reported, with the catalytic activity has been remarkably boosted with the decrease of Cu-NPs size from 60±0.5 to 7.8±0.8 nm, affording 99.3% conversion of benzyl amine with the exclusive selectivity of N-Benzylidene-1-phenylmethanamine (imine) over 7.8 nm NPs in comparison with 76.5% conversion for 60 nm NPs. Furthermore, this newly designed sandwich catalyst shows good tolerance to a diversity of amine substrates and exhibits excellent stability in cyclic tests. By adopting the density-functional-theory (DFT) calculations for all the elementary reaction steps on different active sites of Cu NPs, our results reveal that the step sites play a crucial role in promoting the reactant activation and facilitating the C-N coupling reaction which correlates nicely to the experimental observation. On terrace Cu(111) sites which dominate the large-size of Cu-NPs, the activation barriers of the elementary steps during the conversion are very high, correlating with its low conversion. However, all those barriers are lower by magnitude of 15-30% on step-sites which are presented in higher density on smaller-size Cu-NPs and it could explain the higher activity of smaller Cu-GO@m-SiO2 samples. Insights from DFT simulations could also be able to explain the higher performance of the catalyst under the aerobic condition, with the surface chemisorbed oxygen brings down the barrier to smaller than 0.8 eV, making the reaction feasible at experimental conditions. This synthetic strategy thus demonstrates 25



that sandwiched structures with engineering active sites can provide a multipurpose approach to refine catalysts, in efforts to improve the selectivity of industrially valuable catalytic reactions. The idea demonstrated here emphasizes the significance of catalyst stability with size-dependent activity to utilization in oxidative chemical transformation, and calls for future efforts on tailoring these sandwich style catalyst structures to atomic precision. Supporting Information: Experimental section, Catalytic reactions, Characterization technique details, TGA, H2-TPR, FE-SEM, EDX-analysis, wide angle powder XRD patterns of Cu-GO@m-SiO2, effect of benzyl amine concentration for catalytic oxidative coupling, TEM images of as-synthesized catalysts, C1s XPS analysis of Cu@GO samples, and Cu-2p XPS analysis of Cu@GO and Cu-GO@m-SiO2 catalysts, Computational details, gas phase CH and N-H bond activations in Benzylamine, Optimized structures, LUMOs, SOMOs and HOMOs distribution of C-H and N-H radicals, Adsorption configurations of Benzyl amine on Cu (111) surface, Activation and conversion of Benzylamine on step sites of Cu(111) surface, Activation and conversion of Benzylamine on O/Cu(111), time-activity profiles, recyclability test, TEM images of used catalyst. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Q.T.T); [email protected] (M.P.S); [email protected] (J.M) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS C.S. and S.P wish to thankfully acknowledge the Council of Scientific and Industrial Research (CSIR)-University grant commission (UGC), New Delhi, for their respective junior research fellowships. J.M. thanks the Department of Science and Technology, India, for DST-INSPIRE Faculty Research project grant (GAP-0522) at CSIR-IICT, Hyderabad. DQD is also grateful to the Gridchem (www.seagrid.org) for allocating computer times. This program used the Extreme Science and Engineering Discovery Environment (XSEDE) 26


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facilities that are supported by USA National Science Foundation grant number ACI-10535. PTTM thanks the support from the B2017-BKA-35 project funded by Ministry of Education and Training (MOET), Vietnam. QTT and MPS acknowledge the financial support by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme. The computational work for this research is performed with resources of the National Supercomputing Centre, Singapore (NSCC). References 1. Fihri, A.; Bouhrara, M.; Nekoueishahraki, B.; Basset, J.-M.; Polshettiwar, V., Nanocatalysts for Suzuki Cross-Coupling Reactions. Chem. Soc. Rev. 2011, 40, 5181-5203. 2. Bronstein, L. M.; Shifrina, Z. B., Dendrimers as Encapsulating, Stabilizing, or Directing Agents for Inorganic Nanoparticles. Chem. Rev. 2011, 111, 5301-5344. 3. Balanta, A.; Godard, C.; Claver, C., Pd Nanoparticles for C–C Coupling Reactions. Chem. Soc. Rev. 2011, 40, 4973-4985. 4. Gross, E.; Liu, J. H.-C.; Toste, F. D.; Somorjai, G. A., Control of Selectivity in Heterogeneous Catalysis by Tuning Nanoparticle Properties and Reactor Residence Time. Nat. Chem. 2012, 4, 947. 5. Nihei, M.; Ida, H.; Nibe, T.; Moeljadi, A. M. P.; Trinh, Q. T.; Hirao, H.; Ishizaki, M.; Kurihara, M.; Shiga, T.; Oshio, H., Ferrihydrite Particle Encapsulated within a Molecular Organic Cage. J. Am. Chem. Soc. 2018, 140, 17753-17759. 6. White, R. J.; Luque, R.; Budarin, V. L.; Clark, J. H.; Macquarrie, D. J., Supported Metal Nanoparticles on Porous Materials. Methods and Applications. Chem. Soc. Rev. 2009, 38, 481-494. 7. Prieto, G.; Zečević, J.; Friedrich, H.; de Jong, K. P.; de Jongh, P. E., Towards stable catalysts by controlling collective properties of supported metal nanoparticles. Nature Materials 2012, 12, 34. 8. Lopez-Sanchez, J. A.; Dimitratos, N.; Hammond, C.; Brett, G. L.; Kesavan, L.; White, S.; Miedziak, P.; Tiruvalam, R.; Jenkins, R. L.; Carley, A. F.; Knight, D.; Kiely, C. J.; Hutchings, G. J., Facile Removal of Stabilizer-ligands from Supported Gold Nanoparticles. Nat. Chem. 2011, 3, 551. 9. Astruc, D.; Lu, F.; Aranzaes, J. R., Nanoparticles as Recyclable Catalysts: The Frontier between Homogeneous and Heterogeneous Catalysis. Angew. Chem. Int. Ed. 2005, 44, 7852-7872. 10. Son, S. U.; Jang, Y.; Yoon, K. Y.; Kang, E.; Hyeon, T., Facile Synthesis of Various Phosphine-Stabilized Monodisperse Palladium Nanoparticles through the Understanding of Coordination Chemistry of the Nanoparticles. Nano Lett. 2004, 4, 1147-1151. 11. Wilson, O. M.; Scott, R. W. J.; Garcia-Martinez, J. C.; Crooks, R. M., Synthesis, Characterization, and Structure-Selective Extraction of 1−3-nm Diameter AuAg DendrimerEncapsulated Bimetallic Nanoparticles. J. Am. Chem. Soc. 2005, 127, 1015-1024. 27



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Interface Engineering of Graphene-Supported Cu Nanoparticles

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