Nanoscale Cobalt–Manganese Oxide Catalyst Supported on Shape

Article pubs.acs.org/Langmuir

Nanoscale Cobalt−Manganese Oxide Catalyst Supported on ShapeControlled Cerium Oxide: Effect of Nanointerface Configuration on Structural, Redox, and Catalytic Properties Brendan Hillary, Putla Sudarsanam,* Mohamad Hassan Amin, and Suresh K. Bhargava* Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Science, RMIT University, Melbourne, Victoria 3001, Australia S Supporting Information *

ABSTRACT: Understanding the role of nanointerface structures in supported bimetallic nanoparticles is vital for the rational design of novel high-performance catalysts. This study reports the synthesis, characterization, and the catalytic application of Co−Mn oxide nanoparticles supported on CeO2 nanocubes with the specific aim of investigating the effect of nanointerfaces in tuning structure−activity properties. Highresolution transmission electron microscopy analysis reveals the formation of different types of Co−Mn nanoalloys with a range of 6 ± 0.5 to 14 ± 0.5 nm on the surface of CeO2 nanocubes, which are in the range of 15 ± 1.5 to 25 ± 1.5 nm. High concentration of Ce3+ species are found in Co−Mn/CeO2 (23.34%) compared with that in Mn/CeO2 (21.41%), Co/CeO2 (15.63%), and CeO2 (11.06%), as evidenced by X-ray photoelectron spectroscopy (XPS) analysis. Nanoscale electron energy loss spectroscopy analysis in combination with XPS studies shows the transformation of Co2+ to Co3+ and simultaneously Mn4+/3+ to Mn2+. The Co−Mn/CeO2 catalyst exhibits the best performance in solvent-free oxidation of benzylamine (89.7% benzylamine conversion) compared with the Co/CeO2 (29.2% benzylamine conversion) and Mn/CeO2 (82.6% benzylamine conversion) catalysts for 3 h at 120 °C using air as the oxidant. Irrespective of the catalysts employed, a high selectivity toward the dibenzylimine product (97−98%) was found compared with the benzonitrile product (2−3%). The interplay of redox chemistry of Mn and Co at the nanointerface sites between Co−Mn nanoparticles and CeO2 nanocubes as well as the abundant structural defects in cerium oxide plays a key role in the efficiency of the Co−Mn/CeO2 catalyst for the aerobic oxidation of benzylamine.

1. INTRODUCTION

Understanding the properties of supported bimetallic nanoparticles at nanointerface sites is quite difficult owing to the structural complexity associated with each metal nanoparticle and the shape-controlled nanosupport. This is, however, extremely important for optimal tuning of these properties to enable the development of novel, promising heterogeneous catalysts. To elucidate this, we synthesized a bimetallic Co−Mn oxide catalyst supported on CeO2 nanocubes. Co- and Mn-containing oxides are widely used catalysts in many industrially important oxidation reactions, such as CO oxidation,9−11 methane oxidation,11 oxidation of volatile organic compounds,11,12 diesel soot oxidation,13 water oxidation,14 oxidative coupling of amines,15 oxidation of elemental mercury,16,17 and many others. This significance is due to the existence of multiple valences of Co (2+, 3+, and 4+) and Mn (2+, 3+, 4+, 6+, and 7+), resulting in abundant redox and catalytic properties. CeO2 has been widely employed as an active component of heterogeneous catalysts owing to its active Ce3+/4+ redox couple and its superior oxygen storage

Supported bimetallic nanoparticles are considered to be a unique class of heterogeneous catalytic systems owing to their attractive physicochemical properties and versatile catalytic applications.1−3 The properties of supported bimetallic nanoparticles originate from synergistic interactions at nanointerface sites and differ significantly from their monometallic analogues. Nanointerface synergy effects in supported bimetallic nanoparticles are possible between the metal nanoparticles or between the metal nanoparticles and the support or both.4 Such synergistic effects may be further enhanced when the bimetallic nanoparticles are dispersed on a shape-controlled support, such as CeO2 nanocubes, because of the shape-tuned metal−support interactions. CeO2 nanocubes preferentially expose a large fraction of reactive (100) crystal facets, which favor the generation of abundant structural point defects (oxygen vacancies and high concentrations of reduced Ce(III) species).5−8 Thus, the integration of bimetallic nanoparticles with CeO2 nanocubes can provide new enriched structural, redox, and catalytic properties as a result of the synergistic interactions between the reactive crystal planes of CeO2 nanocubes and bimetallic nanoparticles. © 2017 American Chemical Society

Received: September 19, 2016 Revised: January 31, 2017 Published: February 2, 2017 1743

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2. EXPERIMENTAL SECTION

infrared 785 nm laser as the source. The instrument was equipped with a liquid-N2-cooled charge-coupled device detector and a confocal microscope. The reported wavenumber values were accurate to within ∼3 cm−1. A Thermo K-α XPS instrument at a pressure less than 10−7 Torr and an electron take-off angle of 90° was used for the XPS analysis of the Co−Mn/CeO2 catalyst. Al Kα radiation (1486.6 eV) at a pass energy of 50 eV was used for recording the Co 2p, Mn 2p, Ce 3d, and O 1s core-level spectra of the Co−Mn/CeO2 catalyst. A survey scan was performed at a pass energy of 200 eV to determine possible contaminants in the Co−Mn/CeO2 catalyst. The obtained binding energies were charge-corrected using the carbon (C 1s) peak at 284.6 eV, which were within a precision of ±0.2 eV. 2.3. Catalytic Activity Measurements. The catalytic efficiency of the Co−Mn/CeO2 catalyst along with that of Mn/CeO2, Co/CeO2, and CeO2 samples was tested for solvent-free oxidation of benzylamine under an air flow. In a typical experiment, 100 mg of the catalyst and 3 mmol benzylamine were placed into a 25 mL round-bottom flask. Reactions were carried out at 120 °C, 50 mL/min air flow, and 500 rpm stirring speed. After the reaction, the liquid products and the catalyst were separated using centrifugation. The products were confirmed using a Varian CP3800 GC/Saturn 2200MS instrument equipped with a DB-5 capillary column. Samples were taken periodically and analyzed using a Shimadzu GC 2010 plus GC equipped with an Rxi-5ms capillary column and a flame ionization detector. In addition to GC−MS, the key products in this reaction, namely, dibenzylimine and benzonitrile, were also confirmed by comparing the retention times of the purchased chemicals with that of the reaction mixture using a Shimadzu GC 2010 plus GC instrument.

2.1. Catalyst Preparation. A template-free hydrothermal method was used for the synthesis of CeO2 nanocubes using Ce(NO3)3·6H2O and NaOH. In brief, a required amount of cerium nitrate was dissolved in 10 mL of Milli-Q water and stirred for 10 min at room temperature to obtain a homogeneous solution. To this solution, 60 mL of aqueous NaOH solution (6 M) was added dropwise for approximately 10 min at room temperature, and the solution was then aged while stirring for a further 30 min. The resultant solution was transferred into a 100 mL Teflon bottle sealed in a stainless-steel autoclave, and the hydrothermal treatment was performed at 180 °C for 24 h. Afterward, the sample was collected, washed to neutrality with Milli-Q water, and oven-dried at 100 °C for 10 h. The powder sample was then calcined at 500 °C for 5 h in static air with a heating ramp of 5 °C/min. The Co−Mn/CeO2 catalyst was prepared using a wet impregnation method with 5 wt % Co and 5 wt % Mn with respect to Ce. In a typical procedure, estimated amounts of Co(NO3)2·4H2O (Aldrich, AR grade) and Mn(NO3)2·4H2O (Aldrich, AR grade) were dissolved in Milli-Q water under stirring conditions at room temperature. The required quantity of CeO2 nanocubes was then added to the above solution, and the excess water was evaporated using a hot plate at approximately 105 °C while stirring. The obtained sample was ovendried at 100 °C for 10 h and then calcined at 500 °C for 5 h under static air conditions with a heating ramp of 5 °C/min. A similar procedure was used for the synthesis of Co/CeO2 nanocubes (10 wt % Co with respect to Ce) and Mn/CeO2 nanocubes (10 wt % Mn with respect to Ce), using the respective metal-nitrate precursors. 2.2. Catalyst Characterization. The HRTEM studies of the prepared catalysts were carried out on a JEOL JEM-2100F microscope equipped with a Gatan Orius SC1000 charge-coupled device camera. The accelerating voltage of the electron beam was 200 kV. An EELS spectrometer (Gatan GIF Tridium) was used for the elemental mapping of the materials in the STEM mode. For the analysis, the sample was sonicated in ethanol for approximately 3−5 min followed by the deposition of one or two drops on a copper grid supporting a perforated carbon film. The powder XRD pattern of the Co−Mn/CeO2 catalyst was recorded using a Rigaku diffractometer with Cu Kα radiation (0.1540 nm) as the source. The XRD data were obtained in the two theta range of 10°−90°, with a step size of 0.02° and a step time of 2.4 s. Raman spectroscopy analysis was carried out using a Perkin ElmerRaman Station 400F spectrometer equipped with a 350 mW near-

3. RESULTS AND DISCUSSION 3.1. Morphological Properties. The detailed physicochemical, redox, and morphological properties of CeO2 nanocubes, Co/CeO2 nanocubes, and Mn/CeO2 nanocubes can be found elsewhere.24,25 In this study, HRTEM studies have been carried out to obtain an overview of the morphology, lattice d-spacings, and particle size of the species present in the Co−Mn/CeO2 catalyst. A representative selection of HRTEM images of the Co−Mn/CeO2 catalyst is shown in Figure 1. The lattice fringes of CeO2 nanocubes are clearly visible in all images of the Co−Mn/CeO2 catalyst. The d-spacings for the lattice fringes of CeO2 were found to be ∼0.273 nm. This dspacing corresponds to the distance between the adjacent (100) crystal planes of fluorite-structured CeO2.26 The particle size of CeO2 nanocubes was estimated to vary from 15 ± 1.5 to 25 ± 1.5 nm. Various Co oxide and Mn oxide phases with clearly visible lattice fringes can be noticed in the Co−Mn/CeO2 catalyst. As shown in Figure 1B, the estimated d-spacing of ∼0.467 nm can be assigned to the (111) crystal plane of spinel Co3O4, whereas the 0.166 nm d-spacing corresponds to a Mn2O3 (110) phase.24,27 In Figure 1D, different crystal planes of Mn oxide and Co oxide phases are found, such as the MnO2 (121) plane (d-spacing = 0.236 nm) and the Co3O4 (110) plane (d-spacing = 0.286 nm). Figure 1E,F shows a semioctahedral particle on the edge of a CeO2 nanocube with different d-spacings of Mn oxide and Co oxide phases. As shown in Figure 1F, a decrease to 0.228 nm (i.e., lattice contraction) in the d-spacing of the CeO2 (100) plane is found at the interface of Co−Mn nanoparticles and CeO2 nanocubes. This is the proof for the existence of strong interface interactions in the Co−Mn/CeO2 catalyst. The Co−Mn/ CeO2 catalyst shows different types of Co−Mn nanoalloys, exhibiting single phase and phase-segregated solid solution states.28,29 In Figure 1B,D, Co and Mn oxides share an interface, forming segregated Co−Mn nanoalloys. By contrast, mixed Co−Mn nanoalloys and intermetallic Co−Mn nano-

capacity, both of which have resulted in significant attention from both industry and academia.18−21 Therefore, the combination of bimetallic Co−Mn nanoparticles with CeO2 nanocubes seems to be a suitable model for examining the physicochemical and redox properties at the nanointerface sites of supported bimetallic nanoparticles. A wide range of analytical techniques such as high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy−electron energy loss spectroscopy (STEM−EELS), powder X-ray diffraction (XRD), Raman spectroscopy, and Xray photoelectron spectroscopy (XPS) have been used in this study. The catalytic efficiency of a Co−Mn/CeO2 catalyst along with that of monometallic Mn/CeO2 and Co/CeO2 catalysts was tested for solvent-free oxidation of benzylamine under an air flow. Irrespective of the catalysts employed, a high selectivity toward the dibenzylimine product was observed in this reaction. Imines are important building blocks in the synthesis of diverse organic molecules for a variety of fields, ranging from fine chemicals, agrochemicals, pharmaceuticals, to biology.22,23 Until now, there are no reports on the design of Co−Mn nanoparticles dispersed on shape-controlled CeO2 nanocubes. Therefore, significant efforts have been made to understand the effect of the nanointerface structure on physicochemical, redox, and catalytic properties of the Co−Mn/CeO2 catalyst.

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agglomerated, and the Co and Mn species can be observed on the surface of CeO2 nanocubes. The nanoscale EELS spectrum was recorded at the nanointerface sites between Co− Mn and CeO2 nanocubes to estimate the redox properties of Mn, Co, and Ce. For comparison, the EELS spectrum of the Co/CeO224 and Mn/CeO225 catalysts, recorded at the nanointerface sites (Figure S2), is also shown in Figure 3.

Figure 3. Nanoscale EELS spectra recorded at the nanointerfaces of the (A) Mn/CeO2, (B) Co/CeO2, and (C) Co−Mn/CeO2 catalysts. The corresponding EELS images of the Mn/CeO2 and Co/CeO2 catalysts and the location of the EELS spectrum recorded are shown in Figure S2.

The noticed ionization edges at approximately 626.96, 780.35, and 880.63 eV indicate the Mn-L, Co-L, and Ce-M excitations, respectively.32−34 There are noticeable differences between the EELS spectrum of the Co−Mn/CeO2 catalyst and those of the Co/CeO2 and Mn/CeO2 catalysts. For example, the Mn L3 and L2 edges of the Co−Mn/CeO2 catalyst are shifted to lower energy levels compared with those of the Mn/CeO2 catalyst. This indicates that the valence of Mn in the Co−Mn/CeO2 catalyst is lower than that in the Mn/CeO2 catalyst.32 By contrast, the Co L3 and L2 edges of the Co−Mn/CeO2 catalyst are shifted to slightly higher energy levels compared with those of the Co/CeO2 catalyst, indicating an increase in the oxidation state of Co after the addition of Mn. The Ce M5 and Ce M4 edges of the Co−Mn/CeO2 catalyst are shifted to lower energy levels compared with those of the Co/CeO2 and Mn/CeO2 catalysts. Furthermore, the intensity of the Ce M5 peak is higher than that of the Ce M4 peak in the Co−Mn/CeO2 catalyst compared with that of the Mn/CeO2 and Co/CeO2 catalysts. These observations indicate the improved reducible properties of Ce in the bimetallic Co−Mn/CeO2 catalyst.34

Figure 1. HRTEM images of the Co−Mn/CeO2 catalyst at different magnifications.

alloys are formed, as shown in Figures 1F and S1, respectively. The mixed Co−Mn nanoalloy shape is semioctahedral, which could contain low coordination numbers at the edge and corner sites (Figure 1F). The particle size of Co−Mn nanoalloys was estimated to vary from 6 ± 0.5 to 14 ± 0.5 nm. The CeO2 lattice contraction and semioctahedral particles in the Co−Mn/ CeO2 catalyst are highly energetic catalytically active phases, which could show a beneficial role in heterogeneous catalysis.28,29 3.2. Metal Dispersion and Redox Properties at the Nanointerfaces of the Co−Mn/CeO2 Catalyst. STEM− EELS analysis is a powerful tool for estimating the metal distribution in heterostructured materials and for determining the redox properties at nanointerface sites.30,31 The STEM− EELS elemental mapping images of Co−Mn/CeO2 are shown in Figure 2. It is noticed that the CeO2 particles are

Figure 2. STEM−EELS elemental mapping images of the Co−Mn/CeO2 catalyst: gray = Ce; green = Mn; and yellow = Co. The red circle indicates the location of the EELS spectrum recorded, which is shown in Figure 3. 1745

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Figure 4. (A) Powder XRD pattern and (B) Raman spectrum of the Co−Mn/CeO2 catalyst.

This obviously reveals the existence of synergistic interactions between Co−Mn nanoparticles and the CeO2 nanocubes. 3.3. Structural Properties. The powder XRD pattern of the Co−Mn/CeO2 catalyst is shown in Figure 4A. Various XRD peaks were observed at the 2θ values of 28.64° (111), 33.15° (200), 47.45° (220), 56.42° (311), 59.12° (222), 69.49° (400), 76.82° (331), and 79.14° (420). These peaks correspond to fluorite-structured CeO2.35,36 By contrast, no XRD peaks corresponding to Mn oxide or Co oxide phases were found in the Co−Mn/CeO2 catalyst. This is either due to the high dispersion of Co oxide and Mn oxide species on the surface of CeO2 nanocubes and/or due to the small amount of Co and Mn in the Co−Mn/CeO2 catalyst. Figure 4B shows the Raman spectrum of the Co−Mn/CeO2 catalyst. A strong Raman band is noticed at about 464.3 cm−1 in Figure 4B. This band indicates fluorite-structured CeO2, in line with the XRD results (Figure 4A).37,38 This Raman peak arises from the symmetrical stretching (F2g) mode of the CeO8 unit. Various Raman bands at approximately 193, 510, and 681 cm−1 were observed. These bands indicate the formation of a spinel Co3O4 phase in the Co−Mn/CeO2 catalyst.39,40 A broad Raman band is observed at ∼631 cm−1, indicating the M−O stretching vibrations of a Mn2O3 phase.25 In addition, a Raman band can be noticed at around 589 cm−1, corresponding to the presence of structural point defects, that is, oxygen vacancies in the cerium oxide lattice of the Co−Mn/CeO2 catalyst.41 Oxygen vacancies are formed to balance the lattice charge when the Ce species are reduced, that is, Ce4+ → Ce3+. A weak band is noticed at ∼253.3 cm−1, which is attributed to the displacement of oxygen atoms from their ideal fluorite lattice positions in CeO2.42 3.4. Surface Redox Properties. To further understand the redox nature of the Co−Mn/CeO2 catalyst, surface XPS analysis was conducted. Figure 5A shows the Mn 2p XPS spectrum of the Co−Mn/CeO2 catalyst. The Co−Mn/CeO2 sample shows two XPS peaks at ∼641.74 and 653.31 eV, corresponding to Mn 2p3/2 and Mn 2p1/2, respectively. As shown in Figure 5A, the Mn 2p3/2 peak can be deconvoluted into three characteristic peaks centered at ∼640.8, 641.9, and 644.1 eV. The first peak corresponds to Mn2+, the second peak indicates Mn3+, and the third peak is assigned to Mn4+.43 Only the Mn3+ and Mn4+ species are found in the monometallic Mn/ CeO2 catalyst.25 These observations indicate the transformation of Mn4+/3+ to Mn2+ in the Co−Mn/CeO2 catalyst, in line with the nanoscale EELS results (Figure 3). The Co 2p XPS curve of Co−Mn/CeO2 shows two major peaks at approximately 780.03 and 795.21 eV, corresponding to Co 2p3/2 and Co 2p1/2 spin− orbital peaks, respectively (Figure S3). The spin−orbit split energy of the Co 2p spectrum is usually associated with the Co

Figure 5. (A) Mn 2p XPS spectrum of the Co−Mn/CeO2 catalyst, (B) Ce 3d XPS spectrum of the Co−Mn/CeO2 catalyst, and (C) relative Ce3+ concentration of the CeO2, Co/CeO2, Mn/CeO2, and Co−Mn/CeO2 samples: error bar is ±0.5%.

oxide phase. In this study, the estimated spin−orbit splittings are 15.19 and 15.16 eV for Co/CeO2 and Co−Mn/CeO2 catalysts, respectively, which indicates the formation of the spinel Co3O4 phase, in line with the Raman studies (Figure 4B).39 A notable observation is that the Co 2p3/2 peak of the Co−Mn/CeO2 catalyst is shifted to a higher binding energy compared with that of the Co/CeO2 catalyst, in line with the nanoscale EELS results (Figure 3). This obviously indicates the transformation of Co2+ to Co3+ in the Co−Mn/CeO2 catalyst. The O 1s spectrum of the Co−Mn/CeO2 catalyst shows two components at around 529.1 and 531.2 eV (Figure S4). The first peak corresponds to surface lattice oxygen, whereas the second peak can be assigned to adsorbed oxygen (O−, O2− or O22−) species.44 The Ce 3d spectrum of the Co−Mn/CeO2 catalyst is shown in Figure 4B. The Ce 3d curve is fitted with eight peaks: u‴, u″, u′, u and v‴, v″, v, v′.45 The peaks labeled as u‴, u″, u and v‴, v″, v can be assigned to Ce4+ 3d3/2 and Ce4+ 3d5/2 spin−orbit components, respectively. The peaks labeled as u′ and v′ can be attributed to Ce3+ 3d3/2 and Ce3+ 3d5/2 spin−orbit components, respectively. Estimating the relative concentration of Ce3+ and Ce4+ provides insight into the synergistic interaction between CeO2 and the surrounding atoms in the CeO2-supported 1746

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Langmuir catalysts.45 The relative concentration of Ce3+ is thus estimated using the following equation

using the Co−Mn/CeO2 catalyst (Table 1). This could be due to the competition between minor side reactions that occur simultaneously during the oxidation of benzylamine.46 Irrespective of the catalyst used, a high selectivity toward the dibenzylimine product was observed (Table 1). A high selectivity toward dibenzylimine was found when using pure CeO2 nanocubes and the Co/CeO2 catalyst (entries 1 and 2, Table 1). This is probably an artifact of low benzylamine conversions, resulting in nondetectable peaks for the minor products. The reusability of the Co−Mn/CeO2 nanocubes was studied for the oxidation of benzylamine for three cycles. The reaction conditions were 3 mmol benzylamine, 120 °C, 3 h, 500 rpm, and 100 mg catalyst. After each run, the catalyst was washed with methanol and acetone to remove the reactant and products adsorbed on the catalyst surface and then oven-dried at 393 K for 12 h before catalytic runs. As shown in Figure 6,

Relative Ce3 + concentration (%) = [A(Ce3 +)]/[A(Ce3 +) + A(Ce 4 +)] × 100

(1)

3+

For comparison, the Ce concentration of pure CeO2, Co/ CeO2,24 and Mn/CeO225 catalysts is also shown in Figure 5C. It was found that the Co−Mn/CeO2 catalyst contains higher concentrations of Ce3+ ions than monometallic catalysts. This reveals that the reducible properties of the CeO2 support is improved in the Co−Mn/CeO2 catalyst, indicating the existence of strong metal−support interactions in supported bimetallic catalysts, again in line with the nanoscale EELS analysis (Figure 3). 3.5. Catalytical Properties. The catalytic performance of the Co−Mn/CeO2 material was compared with that of pure CeO2 nanocubes, Mn/CeO2, and Co/CeO2 for the oxidation of benzylamine. The catalytic experiments were performed without using any solvent and using air as a “green” oxidant. The results are provided in Table 1. The detailed reaction Table 1. Solvent-Free Oxidation of Benzylamine Using Coand Mn-Containing Catalysts under Air Flow Conditions selectivity (%) catalyst

benzylamine conversion (%)

dibenzylimine

benzonitrile

blank CeO2a Co/CeO2 Mn/CeO2 Co−Mn/CeO2 Co−Mn/CeO2b

3.2 3.7 29.2 82.6 89.7 91.5

100 100 99 97 97 97

2 3 3 3

Figure 6. Reusability of the Co−Mn/CeO2 catalyst for the oxidation of benzylamine. Reaction conditions: 3 mmol amine, 120 °C, 3 h, 500 rpm, and 100 mg catalyst.

Reaction conditions: 3 mmol benzylamine, 120 °C, 3 h, 500 rpm, and 100 mg catalyst. b4 h reaction time.

a

the conversion of benzylamine was decreased slightly during the first two recycle runs with no significant variation in the selectivity of dibenzylimine. After the third reuse, the catalytic performance of the Co−Mn/CeO2 nanocubes was considerably decreased, and 75% benzylamine conversion was obtained. This decrease in activity after several recycling experiments is still being investigated. 3.6. Structure−Activity Relationships. The performance of supported metal nanocatalysts is determined by several parameters, such as the composition, particle size, structure of metal nanoparticles, and the interfaces between metal nanoparticles and supports.47,48 Highly energetic, catalytically active sites can be created at the interface of metal nanoparticles and supports that originate from the synergistic effects of the respective components.49 Therefore, much attention is currently being devoted to understanding the role that interfaces play, particularly in the nanoscale range, when tuning the properties of supported nanocatalysts. This is particularly important when bimetallic nanoparticles are dispersed on a nanoscale support because of the complexity involved between the nanostructure of each metal particle and the support. In this study, the Co−Mn/CeO2 catalyst exhibits a higher catalytic efficiency in benzylamine oxidation compared with the Co/CeO2 and Mn/CeO2 catalysts (Table 1). The Mn4+/2+ redox couples and the structural defects and the redox properties of CeO2 (i.e. oxygen vacancies and Ce3+-ions) play a favorable role in the oxidation of benzylamine.15 The Mn4+ species can provide active oxygen species for the oxidation of benzylamine, whereas the Mn2+ species could activate benzyl-

pathways for the oxidation of benzylamine to yield dibenzylimine (via the oxidative coupling route) and benzonitrile (via the total oxidation route) can be found elsewhere.46 The benzylamine oxidation reaction was initially conducted in the absence of a catalyst (Table 1), and only 3.2% benzylamine conversion was found. This result indicates the necessity of a catalyst in the benzylamine oxidation reaction. Furthermore, very low benzylamine conversion was observed (3.7%) using CeO2 nanocubes (Table 1). When using the Co/ CeO2 catalyst, the conversion of benzylamine increased to 29.2% (Table 1). Surprisingly, higher conversions of benzylamine were found when using Mn-containing catalysts: ∼82.6 and 89.7% conversion of benzylamine was observed for Mn/ CeO2 and Co−Mn/CeO2 catalysts, respectively (Table 1). These results indicate that the Mn phase is catalytically superior to the Co phase for the oxidation of benzylamine. Although there was only 5 wt % Mn in the Co−Mn/CeO2 catalyst (whereas 10 wt Mn was present in the Mn/CeO2 catalyst), a higher benzylamine conversion was found for the Co−Mn/ CeO2 catalyst (Table 1). The performance of the Co/CeO2 catalyst was significantly improved after the addition of Mn; the conversion of benzylamine increased from 29.2 to 89.7%. This high catalytic performance could be due to the existence of strong synergistic interactions in the Co−Mn/CeO2 catalyst. However, there was not much improvement in benzylamine conversion when the reaction time was increased from 3 to 4 h 1747

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Langmuir amine to yield the dibenzylimine product. Moreover, Ce3+ ions and oxygen vacancies present in cerium oxide could play a key role in the adsorption and activation of oxygen species and benzylamine molecules to yield dibenzylimine. The nanoscale EELS (Figure 3), Raman spectroscopy (Figure 4B), and surface XPS (Figure 5A and C) analyses reveal that the Co−Mn/CeO2 catalyst has Mn4+, Mn3+, and Mn2+ species, abundant oxygen vacancies, and high concentrations of Ce3+ ions. By contrast, the Mn/CeO2 catalyst has only the Mn3+ and Mn4+ species.25 In addition, the nanoscale EELS (Figure 3) and Co 2p XPS analyses (Figure S3) indicate the transformation of Co2+ to Co3+ in the Co−Mn/CeO2 catalyst. Adding a second metal can be an effective strategy to tune the surface composition, atomic arrangement, and electronic state of bimetallic catalysts.50 Thus, bimetallic catalysts can exhibit higher catalytic activity and superior selectivity than their monometallic counterparts. As shown in Figure 7, the electron resulting from the transformation of Co2+ to Co3+ could be responsible for the

as evidenced by the XPS analysis. Raman analysis shows the presence of cobalt and manganese oxide structures as well as structural point defects (oxygen vacancies) in the ceria lattice. The nanoscale EELS and surface XPS studies reveal the transformation of Co2+ to Co3+ and simultaneously Mn4+/3+ to Mn2+ in the Co−Mn/CeO2 catalyst as a result of the cooperative effects of Mn and Co at the nanointerface. Among the catalysts tested for solvent-free benzylamine oxidation, the Co− Mn/CeO2 catalyst shows a high benzylamine conversion (89.7%) followed by Mn/CeO2 (82.6%) and Co/CeO2 (29.2%) after 3 h at 120 °C. The high catalytic performance of the Co−Mn/CeO2 material is due to the synergistic interplay of redox chemistry of Mn and Co oxides as well as the abundant structural defects of cerium oxide.

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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/acs.langmuir.6b03445. TEM image, EELS mapping images, Co 2p XPS, and O 1s XPS of the catalysts (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +61-3-99252330 (P.S.). *E-mail: [email protected] (S.K.B.). ORCID Figure 7. Schematic representation of the oxidative coupling of benzylamine to dibenzylimine over Co−Mn nanocatalyst-supported CeO2 nanocubes.

Suresh K. Bhargava: 0000-0002-9298-5112 Notes

The authors declare no competing financial interest.

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transformation of Mn4+/Mn3+ to Mn2+ in the Co−Mn/CeO2 catalyst, a clear proof for the existence of metal−metal interactions at the nanointerface sites. Another novel observation from the HRTEM studies is that the CeO2 present in the Co−Mn/CeO2 catalyst shows a lattice contraction at the nanointerface sites between the Co−Mn nanoparticles and CeO2 nanocubes (Figure 1F). The contraction of cerium oxide lattice leads to the formation of oxygen vacancies and hence the transformation of Ce4+ to Ce3+.51 Therefore, the enhanced redox properties of Mn and the abundant structural defects of the CeO2 nanocubes are due to the interplay of metal−support interface effects in the Co−Mn/CeO2 catalyst. These properties are responsible for the improved catalytic efficiency of the Co− Mn/CeO2 catalyst compared with its monometallic counterparts in the oxidation of benzylamine.

ACKNOWLEDGMENTS The authors thank Dr. Matthew Field and Edwin Mayes, RMIT University, for their immense help with technical assistance for characterizations. The authors duly acknowledge the RMIT Microscopy and Microanalysis Facility (RMMF) for providing access to the instruments used in this study.

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4. CONCLUSIONS A novel bimetallic Co−Mn nanocatalyst supported on CeO2 nanocubes was developed using a combination of hydrothermal and wet impregnation methods, and its catalytic activity was tested for the oxidation of benzylamine under practical conditions. The HRTEM studies show the formation of shape-controlled CeO2 nanocubes in the range of 15 ± 1.5 to 25 ± 1.5 nm. Different types of Co−Mn nanoalloys with a range of 6 ± 0.5 to 14 ± 0.5 nm were found on the surface of CeO2 nanocubes. The Co−Mn/CeO2 catalyst exhibits a high concentration of Ce3+ species (23.34%) than the Mn/CeO2 (21.41%), Co/CeO2 (15.63%), and CeO2 (11.06%) materials, 1748

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