Graphene–MnO2 Hybrid Nanostructure as a New Catalyst for

Article pubs.acs.org/JPCC

Graphene−MnO2 Hybrid Nanostructure as a New Catalyst for Formaldehyde Oxidation Li Lu,†,‡,§ Hua Tian,†,§ Junhui He,*,† and Qiaowen Yang‡ †

Functional Nanomaterials Laboratory, Center for Micro/Nanomaterials and Technology, and Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ School of Chemical and Environmental Engineering, China University of Mining and Technology, Beijing 100083, China S Supporting Information *

ABSTRACT: Graphene-based hybrids for catalysis are currently attracting tremendous attention due to their unique and advantageous properties. However, the application in gas-phase thermal catalysis including the catalytic oxidation of volatile organic compounds (VOCs) remains a theoretical research stage. Here we developed a new use of graphene-based hybrid as a catalyst for formaldehyde (HCHO) oxidation. The hybrid design of MnO2 catalyst incorporated on graphene nanosheets not only exposes more active surface for catalysis, introduces expressways for charge travel during redox reaction, but also brings a large amount of surface OH− species, which simplifies the decomposition pathway of HCHO without the generation and oxidation of intermediate CO. Therefore, this hybrid design enables great performance enhancements in HCHO oxidation as compared to pure MnO2 and even other noble metal catalysts, displaying a much low 100% removal temperature of 65 °C. Highly stable performance and excellent recycling ability are also observed over graphene−MnO2 hybrids. Kinetic tests reveal that the introduction of graphene reduces activation energy of MnO2 catalyst from 65.5 to 39.5 kJ mol−1.

1. INTRODUCTION Volatile organic compounds (VOCs), such as formaldehyde (HCHO), ethane, benzene, toluene, and xylene, are always breathed in both outdoor and indoor environments.1 Even a low concentration of VOCs can cause human chronic immune diseases, harm nervous system, and even increase the risk of cancers.2 Current abatement technologies of VOCs in air include adsorption, photocatalytic decomposition and catalytic combustion.3−5 Among these methods, catalytic oxidation using noble metals or transition metal oxides as catalysts is the most convenient and effective technology. But this method always suffers from high cost, susceptibility to poisoning, or high reaction temperature,6−9 limiting its widespread applications. Thus, it is significant and urgent to exploit novel promising catalysts that overcome these limitations for VOC, especially for indoor VOC decontamination. Recent studies have demonstrated that the micromorphology of catalytically active centers and lattice oxygen species play important roles in VOC oxidation.10−13 In order to obtain more efficient catalysts, doping other metal ions (such as Na+ and K+) in noble metal catalysts has recently attracted attention. These metal ions introduce new or more active species (such as O2− and OH−),14−17 resulting in a great increase in reaction rate and a remarkable enhancement in catalytic activity. Tuning electronic interactions of metal supports were also selected to improve catalyst activity. Recently, Tang’s group found that the Ag atoms confined on © XXXX American Chemical Society

the surface of a hollandite manganese oxide can perturb the electronic states of active centers, leading to easier redox ability and higher catalytic activity in HCHO oxidation.18 Manganese oxides have been widely studied as catalysts, and show high efficiency in catalytic oxidation of VOCs because of their multiple valences. Previous studies have suggested that manganese oxides are the most active catalysts among transition metal oxides for HCHO oxidation.10,19−21 Essentially, the catalytic activity of manganese oxides is regarded as originating from a process of oxygen activation and oxygen transfer through redox cycles between manganese species (Mn4+/Mn3+), with an electron transfer on the catalyst surface.22 Thus, it is foreseeable that by enhancing the conductivity of manganese oxide, its catalytic performance could be greatly improved due to acceleration of charge transport during redox processes in HCHO oxidation. Graphene, a two-dimensional (2D) layered material, has exceptional electron conductivity, chemical stability, and large surface area. These unique characteristics open up its widespread applications in electronic and optoelectronic devices, chemical sensors, catalysis, and so on.23−26 For example, graphene hybrids have been developed for detection of NOx, NH3, and other VOCs.27−29 Recently, graphene-based Received: August 17, 2016 Revised: September 18, 2016

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step, 0.18 g of GO was dissolved in 90 mL distilled water, and sonicated for 30 min to form a homogeneous dispersion. The GO dispersion was then moved into a Teflon-lined stainlesssteel autoclave (100 mL) and maintained at 120 °C for 12 h to generate reduced graphene oxide (denoted as G). G-Mn hybrid was formed just by immersing G nanosheets into KMnO4 solution at room temperature for 12 h. The purification and freeze-drying procedures were similar to the aforementioned method for synthesizing MnO2. G-Mn hybrids with various mass contents of graphene were achieved by varying KMnO4 concentration associated with G mass. In detail, the products marked as G-Mn-1, G-Mn-2, and G-Mn-3 were prepared with the KMnO4 concentration of 1, 2, and 3 mg mL−1, respectively. 2.3. Characterization. XRD patterns of the prepared products were measured on a Bruker D8 Focus X-ray diffractometer with a Cu Kα X-ray source (40 mA, 40 kV). Scanning electron microscopy (SEM) images were obtained on a Hitachi S-4300 field emission scanning electron microscope operating at 10 kV. Transmission electron microscopy (TEM) images were recorded on a JEOL JEM-2100F electron microscope. Nitrogen adsorption−desorption measurements were carried out at 77 K using a Quadrasorb SI automate surface area and pore size analyzer. Specific surface areas and pore volumes were calculated by the Brunauer-Emmett-Teller (BET) method. Raman spectra were obtained on a Raman spectrometer (Via-Reflex, Renishaw, U.K.) with 532 nm wavelength incident laser light. Fourier-transform infrared (FT-IR) spectra were recorded in the range of 300−4000 cm−1 at a resolution of 2 cm−1 on a Varian Excalibur 3100 spectrometer. The thermal stability was assessed by thermogravimetric analysis (TGA) on a SDT-Q600 thermal gravimetric analyzer in dry air with a heating rate of 10 °C min−1. X-ray photoelectron spectroscopy (XPS) profiles were recorded on an ESCALAB 250Xi (Thermo Fisher Scientific) spectrometer, equipped with monochromatized Al Kα radiation (hν = 1486.6 eV) with an anode operated at 225 W and 15 kV. All spectra were calibrated with the binding energy of the C 1s peak at 284.8 eV. Electrochemical experiments were performed with a CHI 660D Electrochemical Workstation in a common threeelectrode electrochemical cell. All experiments were done using a three-electrode cell configuration with a catalyst modified conductive glass (prepared by dip-coating and drying in air at 70 °C) as the working electrode, a platinum wire as the auxiliary electrode and a saturated silver electrode as the reference electrode. 2.4. HCHO Oxidation over G-Mn Catalysts. The catalytic activity of as-prepared catalysts for HCHO oxidation were evaluated in a fixed-bed reactor under atmospheric pressure. A total of 100 mg of catalyst (40−60 mesh) was loaded in a quartz tube reactor (length = 500 mm, diameter = 4 mm). Gaseous HCHO was generated by passing a purified air flow over HCHO solution in an incubator kept at 0 °C, leading to a feed gas with 100 ppm of HCHO. The total flow rate was 50 mL min−1 in a gas hourly space velocity (GHSV) of 30000 mL· (gcat·h)−1. The effluents from the reactor were analyzed with an online 8600 gas chromatograph equipped with FID and Ni catalyst converter, which was used for converting carbon oxides quantitatively into methane in the presence of hydrogen before the detector. For the kinetic measurement, the HCHO conversion was controlled below 15%. Kinetics data were recorded under the conditions of HCHO 80 ppm and GHSV 1170000 mL·(gcat·h)−1 for G-Mn-2, and HCHO 70 ppm and GHSV 1190000 mL·(gcat·h)−1 for MnO2. No other carbon

materials have received great attention for various heterogeneous catalytic reactions, such as electrocatalysis and photocatalysis (Table S1).30−32 As we know, heterogeneous reactions generally occur on the interface of two different phases and involve three consecutive steps: adsorption, chemical reaction, and desorption. The 2D architecture and surface hydroxyl groups of graphene are welcome for the adsorption of HCHO molecules. In some cases, by loading a catalyst on a 2D planar support, the exposed active sites of the resulting system can far surpass what would be expected from simple catalyst nanoparticles, achieving the maximum efficiency in the use of active sites.33 Furthermore, graphene exhibits outstanding and tunable electrical properties, facilitating charge transport at room temperature. Accordingly, graphene-based materials display excellent performance in oxygen reduction reaction, oxygen evolution reaction, photocatalytic hydrogen production, and catalytic dehydrogenation.33,34 Studies of various manganese oxides supported on graphene nanosheets have shown that the graphene plays a key role in increasing the electrical conductivity, which greatly improves the activity of oxygen reduction.35,36 For instance, Xu et al. reported that graphene could appear to be electron transfer channels, and enhance the electrical conductivity of manganese oxides, which is beneficial for catalytic oxidation of Hg0.37 However, it should be noted that all of these heterogeneous reactions occur in liquid phase or electrolyte solution. Few graphene−manganese oxide hybrids have been used as catalysts in gas-phase reactions. Therefore, the rational design of graphene−manganese oxide hybrid catalysts for gasphase thermal catalysis is highly desirable yet a great challenge to date. Herein, MnO2 catalyst was self-assembled onto the surface of graphene nanosheets forming graphene−MnO2 hybrid nanostructures. The graphene−MnO2 hybrids were subsequently employed for catalytic oxidation of gaseous HCHO. As compared to MnO2, the planar structure of graphene could help expose more surfaces of MnO2 for adsorption and desorption of reagents and more active sites for catalysis during HCHO oxidation. In addition, the extra interface at the hybridized areas of graphene−MnO2 nanostructures facilitates charge transport during Mn4+/Mn3+ redox processes, offering high reaction rate and good catalytic property. On the basis of experimental results, the mechanism of HCHO oxidation over graphene−MnO2 catalyst was elucidated.

2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals in this study were of analytical grade and used without further purification. For the catalyst preparation, ultrapure water obtained through a three-stage Millipore Mill-Q Plus 185 purification system was used. 2.2. Synthesis of Catalysts. Synthesis of MnO2. MnO2 were synthesized by a hydrothermal method. The precursor solution was prepared by mixing KMnO4 and MnSO4 at a 6:1 molar ratio in 80 mL of deionized water. The solution was stirred magnetically for 1 h and then transferred into a 100 mL stainless-steel autoclave with Teflon liner. Hydrothermal reaction was carried out at 120 °C for 12 h. After cooling to room temperature and washing with water and ethanol, the product was obtained via freeze-drying at −50 °C. Synthesis of G-Mn Hybrids. Graphene oxide (GO) was obtained via oxidation and exfoliation of graphite by a modified Hummers’ method.38 G-Mn hybrid nanostructures were prepared by a simple and economical method. In the first B

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hydrocarbonate. This process can be formally expressed by the following transformation path:

containing compounds except CO2 were detected in the effluents for all tested catalysts. Thus, the HCHO conversion was calculated from the CO2 yield as follows:

4MnO4 − + 3C + H 2O ⇋ 4MnO2 + CO32 − + 2HCO3−

[CO2 ]out HCHO conversion(%) = CO2 yield(%) = × 100% [HCHO]in

SEM, TEM, and high-resolution TEM (HRTEM) analyses were employed to visualize the morphology and structure of asprepared hybrids (Figure 2). It is clear that G is a 2D planar structure with a well-defined and interconnected porous network (Figure 2A). This 2D planar architecture facilitates the adsorption of reagents and exposes more active sites for catalysis. In addition, many wrinkles and folds are obtained, revealing the flexibility of G nanosheets. After reduction of KMnO4, small MnO2 nanoparticles are densely and homogeneously dispersed on the G nanosheet surface, and no free nanoparticles are formed outside the G nanosheets. Figure 2E,F shows the details of the morphologies for MnO2 nanoparticles, which adopt a nanoneedle shape. These nanoneedles have width of approximately 3 nm and length of about 36 nm. Interestingly, the planar architecture of G nanosheets leads to high dispersion of MnO2 nanoneedles, while the loaded MnO2 nanoneedles act as nanoscale spacers to prevent the G nanosheets from aggregation and stack to some extent, thus further improving the porous structure of G-Mn hybrids. Representative HRTEM image (Figure 2F) shows clear lattice fringes with a d-spacing of 0.70 nm assignable to Mn (003),40 confirming the crystalline feature of MnO2 nanoneedles. SEM and TEM were also used to analyze the control sample of single MnO2. The single MnO2 is composed of flower-like nanospheres with diameter of about 260 nm (Figure 2C,G), significantly different from the morphology of graphenesupported MnO2. Each nanosphere is constructed by a large amount of radially standing nanosheets emanated from the spherical center. Typical XRD patterns of the prepared samples are shown in Figure 3A. In the XRD pattern of MnO2, diffraction peaks at 2θ = 12.5°, 24.9°, 37.2°, and 66.1° are consistent with the standard values of birnessite phase (JPCDS card no. 86−0666). After incorporating MnO2 with graphene, all diffraction peaks of MnO2 are detected but lower a lot. A broad peak at 2θ = 26° with a lower intensity compared to G is detected and indexed (002) of graphene nanosheets. This confirms again the well incorporation of MnO2 and graphene nanosheets.

where [CO2]out is the CO2 concentration in the effluent, and [HCHO]in is the HCHO concentration of the feed gas.

3. RESULTS AND DISCUSSION 3.1. Structural Features of Materials. The schematic representation for the synthesis of G-Mn hybrid nanostructures is displayed in Figure 1. The overall synthesis consists of two

Figure 1. Schematic illustration for the preparation of G-Mn hybrid nanostructures.

steps: the hydrothermal reduction of GO to G, and the nucleation and growth of MnO2 nanoparticles on the surface of G nanosheets. G is obtained by reduction of GO through hydrothermal dehydration.39 When aqueous MnO4− ions are dripped into the G dispersion, the strong interactions between MnO4− and carbon atoms in the G nanosheet lead to the adsorption of the MnO4− ions to the surface of G nanosheets. The adsorbed MnO4− ions are reduced to MnO2 by electron transfer from the carbon atoms to the adsorbed MnO4− ions, bringing the nucleation and growth of MnO2 nanoparticles on the surface of G nanosheets. Meanwhile, G nanosheets are partly consumed with the oxidation of carbon to carbonate and

Figure 2. SEM and TEM images of G (A, D), G-Mn-2 (B, E), and MnO2 samples (C, G); (F) HRTEM image of G-Mn-2. C

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Figure 3. Characterizations of G, MnO2 and G-Mn hybrids. (A) XRD patterns. (B) Thermogravimetry analysis; the R1, R2, and R3 regions indicated by different colors show the weight loss of three regions. (C) Raman spectra. (D) FT-IR spectra.

Figure 4. (A) Catalytic conversions of HCHO over G, MnO2, and G-Mn hybrids. (B) HCHO conversion over G, MnO2, and G-Mn hybrids at 65 °C. (C) Cycle tests of G-Mn-2 at 65 °C. (D) Stability test of G-Mn-2 at 65 °C. Reaction conditions: 100 ppm of HCHO, GHSV = 30000 mL·(gcat· h)−1.

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The Journal of Physical Chemistry C Thermogravimetry analysis was chosen to confirm the graphene contents in G-Mn hybrid nanostructures. As shown in Figure 3B, the themogravimetry curves of all samples can be mainly divided into three consecutive weight loss regions (R1, R2, and R3 regions). The first weight loss (R1), about 10−20% at ≤200 °C, is attributed to desorption of physisorbed and chemisorbed water. A large weight loss is observed in the R2 region (200−420 °C), corresponding to the release of water from manganese oxide crystallites.41 The last weight loss occurring between 420 and 800 °C (R3) is generally due to the lattice oxygen release from the transformation of Mn4+ to Mn3+42 and the graphene decomposition. Calculated by the data at 800 °C, the graphene contents of 3.7, 18.8, and 35.8 wt % could be obtained for G-Mn-1, G-Mn-2, and G-Mn-3, respectively. Raman spectroscopy is a very useful optical technique to acquire the structural information and electronic properties of carbonaceous materials. We employed this technique to investigate the structure of samples, and the results are shown in Figure 3C. In the spectra of G and G-Mn, characteristic D and G bands of graphene occurring at about 1360 and 1590 cm−1, respectively, are observed. The ID/IG intensity ratio is an indicator of the disorder degree of sp2 domains and the defect degree of graphene.13,18 The ID/IG value of G-Mn was calculated to be 0.88, obviously lower than the 1.00 of G. This offers a powerful evidence of the oxidation of graphene nanosheets induced by the reduction of KMnO4 to MnO2 and also verifies the chemical interaction between MnO2 and graphene nanosheets. This strong interaction will benefit the electron transformation and enhance the electrochemical performance of G-Mn nanostructure. It is also noticed from the Raman spectrum of G-Mn that a new peak centered at 575 cm−1 is present, corresponding to the characteristic Mn−O band.43 The chemical structures of samples are further investigated by FT-IR spectra. Figure 3D shows the spectra of G, G-Mn, and MnO2 samples. G shows many absorption peaks like C−H, CO, O−CO, and C−O−C stretching that are attributed to various oxygen functional groups. After incorporating with MnO2, these peaks are lower or not apparent in the resultant hybrid, owing to the existence of MnO2 dispersed on the surface of G nanosheets. In the cases of G-Mn hybrid and MnO2, two peaks at 528 and 445 cm−1 correspond to Mn−O vibrations,44 while that at 1626 cm−1 is attributed to the OHbending of interlayer water in birnessite nanostructure. The FT-IR results further confirm the successful integration of MnO2 on graphene. 3.2. Catalytic Activity. The catalytic activity of samples was evaluated via gaseous HCHO oxidation tests under different reaction conditions. Figure 4A portrays the resultant HCHO conversions to CO2 at different reaction temperatures associated with the as-prepared samples. The G-Mn hybrids exhibit much higher activity than MnO2. The catalytic activities of the as-prepared samples follow the order: G-Mn-2 > G-Mn-1 > G-Mn-3 > MnO2 > G, indicating the improvement of catalytic performance for G-Mn hybrid nanostructures. Compared to G-Mn-1 and G-Mn-3, G-Mn-2 exhibits the best catalytic activity with the lowest complete-conversion temperature of 65 °C (Figure 4B). This temperature is among the most active even for noble metal catalysts (see Table 1). The G-Mn-1, G-Mn-3, and MnO 2 catalyst obtained 100% conversion of HCHO at 70, 90, and 140 °C, respectively. In contrast, G appeared to be inert for catalytic decomposition of

Table 1. Temperatures for 100% HCHO Conversion and Activation Energies for HCHO Oxidation over Various Catalysts catalyst G-Mn-2 MnO2 Ag/TiO2 5 wt % CuOx/Mn0.5Ce0.5O2 1.7%K−Ag/Co3O4 3D Co3O4 Mn0.75Co2.25O4

T100% (°C)

activation energy (kJ mol−1)

65 140 95 230

39.5 65.5 67 37.8

this study this study

70 110

28.5 59.7 55

15

reference

45 61

15 59

HCHO in the measurement temperature range, which confirms that the high activity of the G-Mn hybrids comes from the supported MnO2 nanoneedles. Good stability is another critical aspect in the development of catalysts. Figure 4C displays that the G-Mn hybrid exhibits excellent cycle life. No noticeable decrease in the catalytic performance was recorded after five times of on/off cycles. Furthermore, to probe the durability of the catalyst, continuous catalytic HCHO decomposition reaction was carried out at 65 °C. As shown in Figure 4D, no apparent activity loss can be observed even after a long-time test up to 72 h. This confirms the catalyst is highly stable to withstand deactivation. Furthermore, XPS analysis was performed to investigate the structural stability of G-Mn hybrid during catalytic reaction. As shown in Figure S1, negligible changes in the Mn 2p and O 1s before and after catalytic tests are observed, confirming the excellent stability of G-Mn-2. Overall, these excellent catalytic activity and good durability of G-Mn hybrids open a promising route to develop noble metal-free catalysts for low-temperature catalytic oxidation of HCHO. 3.3. Reaction Mechanism of HCHO Oxidation over GMn Catalyst. In graphene-based hybrid catalysts, graphene usually serves as a support, where heterogeneous catalysis is carried out at the exposed active centers of supported catalysts. In the current case, the planar graphene nanosheets are a key factor to enhance the catalytic performance of G-Mn hybrids. As mentioned above, the 2D planar nanostructure of graphene introduces high dispersion of MnO2 nanoparticles and abundant active centers are exposed to reactants. These can lead to the maximum efficiency in the use of active centers for catalysis and consequently enhance the catalytic performance of supported catalyst. The enhanced catalytic activity by graphene nanosheets may be further related to other two events. First, the enhancement is generated from the change of surface physicochemical property by incorporating graphene nanosheets. Second, the promoted activity of hybrid catalyst should be related to the electrochemical property of the hybrid nanostructure. As a proof of the above concept, we first implemented kinetics tests with both MnO2 and G-Mn-2 catalysts to study the HCHO oxidation mechanism. The catalytic reaction rates at different temperatures were first obtained by assuming the first-order reaction for HCHO oxidation with high GHSV condition,45 and then the Arrhenius plots were obtained by plotting the logarithm of rate versus 1/T, as shown in Figure S2. The apparent activation energy was acquired from the Arrhenius plots. Table 1 portrays the activation energy of the G-Mn hybrid catalyst as compared to those of the pure MnO2 and some recently reported catalysts used in HCHO oxidation. E

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The Journal of Physical Chemistry C The G-Mn hybrid nanostructures have the ability to lower activation energy for HCHO oxidation by approximately 40% compared to the pure MnO2 and by 28−37% compared to conventional oxide materials (i.e., 3D Co 3 O 4 and Mn0.75Co2.25O4) and some noble-metal catalysts (i.e., Ag/ TiO2 nanoparticles). These data suggest that the G-Mn-2 hybrid nanostructure is extremely efficient as a noble metal-free catalyst for HCHO oxidation,17,45 and preliminarily confirms that the introduction of graphene changes the reaction mechanism. The activity of oxide catalyst in HCHO oxidation is expected to be sensitive to the valence state of the metal center and surface oxygen species.10,46 The above-mentioned results indicate that graphene has no catalytic activity for HCHO oxidation, and the active sites should exist on the graphenesupported MnO2. To probe the states of the surface Mn and O elements, XPS spectra of MnO2 and G-Mn-2 were measured. In the full-range G-Mn-2 spectrum (Figure S3), the binding energy peaks of C 1s at 284.8 eV, O 1s at 529.81 eV, and Mn 2p at 642.32 eV clearly reveal the presence of graphene and MnO2. In the Mn 2p spectra (Figure 5A), two apparent peaks are observed and indicate the identical chemical state of Mn atoms in MnO2 and G-Mn-2. The splitting energy of the Mn 2p peak (11.8 eV) approaches that of Mn4+, which is in accord with the energy separation between Mn 2p3/2 and Mn 2p1/2 reported previously.47−49 Moreover, after the introduction of graphene, noticeable downshifts of the Mn 2p3/2 and Mn 2p1/2 peaks (642.6 vs 642.1 eV and 654.4 eV vs 653.9 eV, respectively) occur. The downshift results in the increase of π electron cloud density that originated from the presence of residual functional groups in graphene. Regarding oxygen (Figure 5B), a well-formed peak at 530.1 eV (O 1s A) and a shoulder at 532.2 eV (O 1s B) are observed. The binding energy at 530.1 eV is ascribed to Mn−O−Mn (lattice O), while the binding energy at 531.8 or 532.2 eV refers to adsorbed oxygen species within the surface hydroxyl groups (Mn−OH) and H−OH, respectively.50−53 This result demonstrates that the interaction between Mn atoms and O atoms of the residual functional groups on graphene forms via a covalent coordination bond or a hydrogen bond. In addition, the area ratio of O 1s B/O 1s A (the relative abundance of two kinds of oxygen species) significantly increases after the coupling of MnO2 nanoparticles to graphene nanosheets, suggesting an increase of surface Mn−OH amount of G-Mn-2 hybrid nanostructure. The HCHO oxidation over MnO2 is known to follow a metal-assisted Mars-van Krevelen mechanism, in which adsorption and dissociation of O2 and oxidation of adsorbed HCOO −, an intermediate of HCHO during oxidation, are regarded as critical steps to control the apparent reaction rate.10,54 It has been suggested that surface OH− and lattice oxygen concentration are the main factors for determining reaction rate.22 For example, Tang et al. reported that relative abundance of lattice oxygen was responsible for the higher catalytic activity in HCHO oxidation. Zhang et al. found that the active lattice oxygen species played an important role in HCHO oxidation.10 In the case of G-Mn, however, lattice oxygen species seem to be notably unnecessary for superior catalytic performance in HCHO oxidation, as evidenced by that G-Mn-2 with an increased area ratio of O 1s B/O 1s A compared to that of MnO2, shows a higher catalytic performance than MnO2. More surface hydroxyl groups (Mn−OH) are desired to promote the catalytic activity for HCHO oxidation.

Figure 5. (A, B) XPS spectra of Mn 2p and O 1s, respectively, for MnO2 and G-Mn-2. (C) Cyclic voltammetry plots of G, MnO2, and GMn hybrids at a scan rate of 10 mV·s−1.

Graphene has been believed to serve as a host to provide metal oxides additional catalytic centers due to its 2D plane architecture and high specific surface area, and also improve electrical conductivity, decrease the electron-transfer resistance of metal oxides.55 To confirm this point of view suitable for the G-Mn hybrid nanostructures, surface properties and electronic properties of G-Mn samples were investigated by BET gas adsorption and CV analysis, respectively. As expected, higher F

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The Journal of Physical Chemistry C adsorption performances could be clearly observed for G and G-Mn (Figure S5), with much higher specific surface areas and pore volumes compared to the unsupported MnO2 (Table S2). High specific surface area and large pore volume of G-Mn not only favor the HCHO molecule uptake, but also facilitate the access of HCHO molecules to active sites and the transport of intermediates. Nevertheless, it can be observed that the surface properties of G-Mn catalysts are not the dominant impact of factors on catalytic performance. The surface area and pore volume do not follow the order of catalytic activities for HCHO oxidation among these three G-Mn samples. G-Mn-3 with the highest specific surface area and the largest pore volume displays the lowest catalytic activity. In many heterogeneous catalytic system, the electron transfer process is much more important for catalytic reactions.56 Subsequently, we attempted to estimate the contribution of graphene on the electron transfer properties of G-Mn by utilizing cyclic voltammetry (CV) tests at a scan rate of 10 mV·s−1 with the potential range of 0−0.8 V. As clearly shown in Figure 5C, the resulting areas surrounded by the CV loops of G-Mn-1 and G-Mn-2 hybrids are greatly larger than that for pure MnO2 nanospheres, indicating that the G-Mn hybrid nanostructures have brought significant enhancement of electrochemical performance. The CV peak currents (at 0.8 V) are summarized in Table S2. An obvious enhanced current value is observed when using the graphene-supported MnO2. Ignoring the pure graphene that shows no catalytic activity for HCHO oxidation, the electrochemical performances are ranked in the order of G-Mn-2 > GMn-1 > G-Mn-3 > MnO2, which is consistent with the catalytic ability sequence. The interfacial electron transfer of samples accelerates with increase of CV peak current.57 Thus, we can demonstrate that the incorporation of graphene favors the charge transport and enhances the interfacial electron transfer during the Mn4+/Mn3+ redox processes by building abundant interfaces at the hybridized areas of G-Mn catalyst. As such, the enhanced catalytic activity observed with the G-Mn hybrids could be largely ascribed to their improved electrical characteristics. The heterogeneous catalysis is usually an adsorption− chemical reaction−desorption process over catalysts.41 Based on the above results, we can depict the predominant pathways of HCHO oxidation over G-Mn hybrid. HCHO and O2 molecules are first adsorbed on the hybrid surface, forming adsorbed molecules. The 2D geometry and surface hydroxyl groups of graphene greatly facilitate the adsorption of HCHO and O2 molecules. After that, the adsorbed HCHO is oxidized to form a formate intermediate. During this process, the oxygen activation and oxygen transfer through the redox cycles between manganese species (Mn4+/Mn3+) occur, that is, the oxygen transfer from molecular oxygen to active sites of MnO2 achieves the activation of molecular oxygen,47,58 as shown in Figure 6. The hybridized G-Mn interlayer areas provide important interfaces, where the conducting graphene greatly accelerates charge transfer between Mn4+ and Mn3+ species, and then enhancing the catalytic performance of G-Mn catalyst. Under different conditions, the formate intermediate is decomposed via different pathways, which are distinguished by the abundance of surface hydroxyl groups.14,17 The first pathway is a relatively complex pathway in which CO is first formed from formate and then oxidized, generating CO2 molecule. If a catalyst possesses abundant surface hydroxyl groups, the formate intermediate will be directly oxidized to CO2 by the surface hydroxyl groups. For this process, the

Figure 6. Proposed pathways for catalytic oxidation of HCHO over GMn hybrid nanostructure.

surface hydroxyls group is one of the main factors for determining the reaction rate of HCHO oxidation.17,59,60 In the current case, the G-Mn catalyst has more surface hydroxyl groups in comparison to MnO2. Therefore, direct decomposition of the adsorbed formate on G-Mn dominates the reaction pathway, producing a CO2 molecule and a H2O molecule (HCOO-Mn + Mn−OH → H2O + CO2 + 2Mn). Meanwhile, we also found that consumed hydroxyl groups could be supplemented (Figure S6), probably by the H2O vapor on stream.

4. CONCLUSIONS Overall, we have described a new use of G-Mn hybrid as a catalyst for gaseous HCHO oxidation. This G-Mn hybrid nanostructure fabricated by a facile method exhibits impressive catalytic performance and excellent stability in HCHO oxidation. A total of 100% HCHO (100 ppm) conversion was achieved just at 65 °C, and such high catalytic property could be maintained up to 72 h in our tests. High activity and good stability of G-Mn hybrid make it a good candidate for the development of noble metal-free catalysts for HCHO oxidation. More importantly, the current results open a window to insight the applications of graphene-based composites in gas-phase thermal catalysis. The association of graphene with manganese oxide catalyst offers great potential to be explored for developing efficient noble metal-free catalysts to decontaminate HCHO and other VOCs at low or even room temperature.

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ASSOCIATED CONTENT

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b08312. Additional information as noted in the text (PDF).

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AUTHOR INFORMATION

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*E-mail: [email protected]. Tel./Fax: (+86) 10 82543535. Author Contributions §

These two authors contributed equally (L.L. and H.T.).

G

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21571182 and 21271177), the Science and Technology Commission of Beijing Municipality (Z151100003315018), and Technical Institute of Physics and Chemistry, Chinese Academy of Sciences.

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