Vertical Graphene Foam for

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Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Hierarchical Petal-on-Petal MnO2/Vertical Graphene Foam for Postplasma Catalytic Decomposition of Toluene with High Efficiency and Ultralow Pressure Drop Shiling Yang, Zheng Bo,* Huachao Yang, Xiaorui Shuai, Hualei Qi, Xiaodong Li, Jianhua Yan, and Kefa Cen

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State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, College of Energy Engineering, Zhejiang University, Hangzhou, Zhejiang Province 310027, China S Supporting Information *

ABSTRACT: A hierarchical petal-on-petal MnO2/vertical graphene (VG) foam catalyst for high-performance toluene decomposition with low pressure drop in postplasma catalysis is demonstrated in this work. The as-fabricated hierarchical structure exhibits high toluene decomposition of ∼93% and ozone conversion of nearly 100% at a high flow rate of 500 mL min−1, significantly higher than those of MnO2 deposited on conventional activated carbon and reduced graphene oxide powders with pressure drop of 2−3 orders of magnitude higher than that of VG foam. The three-dimensional macroporous structure of VG foam is beneficial for gas permeability and thus significantly decreases the pressure drop. The nonagglomerated morphology, large surface area, and sharp exposed edges of VG foam could provide abundant sites for anchoring catalysts without obvious agglomeration. Besides, the unique petal-like structure and open channels of MnO2 prominently enlarge the accessible surface area exposed to gas, leading to enhanced decomposition efficiency. oxide catalysts in PPC, Dinh et al.20 achieved a high trichloroethylene decomposition efficiency of 100% and COx selectivity of 56%. However, VOCs are usually diluted in the huge streams of exhaust gases from industries, which yields high gas flow rates in the practical catalytic reactors (typically ranging from 100 to 10 000 mL min−1).2,24−26 Such high flow rate may decrease the residence time of VOCs in catalytic reactors, thus resulting in an obvious degradation of catalytic performance (i.e., CO2 selectivity and VOC decomposition efficiency).27−29 For example, with increasing the gas flow rate from 100 to 330 mL min−1, the CO2 selectivity decreased significantly from 81% to 23%.29 The catalytic performance can be improved with employing highly compact packings of catalysts, while the pressure drop will increase dramatically,9,11,15,22,30,31 leading to high operating costs.10 Therefore, PPC decomposition of VOCs with simultaneously high catalytic performance and ultralow pressure drop still remains a great challenge, especially at high gas flow rates. To address such issue, a rational design on the structure of catalyst supports is necessary. Vertical graphene (VG)

1. INTRODUCTION Emission control of airborne hazardous volatile organic compounds (VOCs) from mobile and stationary sources has become an important issue.1,2 Nonthermal plasma (NTP) produced by atmospheric pressure discharges has been widely used for VOC decomposition due to its fast ignition, moderate working condition (ambient temperature) and broad applicability.3,4 Moreover, coupling NTP with heterogeneous catalysts (i.e., plasma catalysis) exhibits a great potential for further improving the energy efficiency and suppressing the reaction byproducts.5,6 Postplasma catalysis (PPC) is a configuration of plasma catalysis that places heterogeneous catalysts downstream of NTP region. It is considered as a promising approach for VOC decomposition with high performance.4−6 In PPC, heterogeneous catalysts can effectively convert the long-lived species ozone into more active oxygen species such as O(1D), O(3P), O−2, and O2−2, which obviously accelerates the toluene reaction rate and thus promotes the oxidation of residual VOC and byproducts.7,8 Various heterogeneous catalysts (e.g., Mn,8−17 Co,18,19 Ce,19−21 Ag,22 Ti,23 etc.) and their combinations have been proposed for high VOC decomposition efficiency and superior product selectivity. For example, using TiO2 as catalyst, Huang et al.23 reported that the toluene decomposition efficiency could be remarkably improved from 28.5% to 96%. With employing Ce−Mn mixed © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

July 23, 2018 October 19, 2018 October 22, 2018 October 22, 2018 DOI: 10.1021/acs.iecr.8b03387 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Schematic diagram of the PPC experimental setup.

into the quartz tube as the precursor gases for VG growth. The chamber pressure was maintained at 100 Pa during the growth process. After growth, samples were cooled down to room temperature under the Ar atmosphere. Template Etching. The VG/Ni foam was immersed in poly(methyl methacrylate) (PMMA) solution (4 wt % in ethyl lactate) and then baked at 80 °C for 2 h. The PMMA-coated VG/Ni foam was rolled up into a cylinder (20 mm in length and a diameter of 10 mm) and placed in a cylindrical glass tube. The sample was then immersed in a 3 M HCl solution at 80 °C overnight to completely dissolve the Ni foam. After dissolving the PMMA with hot acetone at 50 °C, the freestanding and monolithic VG foam was obtained. MnO2 Deposition. 500 ppm of ozone was generated by the dielectric barrier discharge reactor using dry air as the carrier gas with a gas flow rate of 1000 mL min−1. The generated ozone was further humidified by a water bubble column and then used to functionalize VG petals at room temperature for 5 min. The ozone treated samples were immersed into KMnO4 (Sinopharm Chemical Reagent CO., Ltd.) solution (800 mg KMnO4 dissolved in 120 mL deionized water) and kept at 80 °C in an oven for 24 h. The samples were then washed with deionized water to remove the remaining KMnO4, followed by dryness at 60 °C overnight. After annealing at 400 °C for 4 h in N2, MnO2/VG foam was obtained. 2.2. Synthesis of rGO Powder. Graphene oxide (GO) was synthesized from graphite powder (XF010, XF NANO) based on a modified Hummer’s method. 100 mL prepared GO dispersion (containing 200 mg GO) was sonicated for 2 h and then transferred into a stainless-steel autoclave (a capacity of 200 mL) for 12 h hydrothermal treatment (160 °C). The autoclave was cooled down to ambient temperature to separate the precipitate, followed by a 12 h freeze-drying process. The obtained product was ground into powder (40−60 mesh). 2.3. Synthesis of MnO2/AC and MnO2/rGO. The AC and rGO powders were immersed into 120 mL KMnO4 solution (containing 800 mg KMnO4) and kept at 80 °C in an oven for 24h. The precipitates were filtered and rinsed with deionized water several times, followed by dryness at 60 °C overnight. MnO2/AC and MnO2/rGO powders were obtained after anneal in N2 at 400 °C (4 h). 2.4. Material Characterizations. The surface morphologies of VG foam and MnO2/VG foam were investigated by a SU-70 scanning electron microscope (SEM, Hitachi) at an acceleration voltage of 8 kV. Transmission electron microscopy (TEM, JEM-2100; JEOL, Tokyo, Japan) at 200 kV was

perpendicularly growing on 3D foam template via a plasmaenhanced chemical vapor deposition (PECVD) process enables three-dimensional (3D) architectures.32,33 The asobtained 3D VG foam combines the unique features of VG and foam-like structure. The 3D macroporous structure of VG foam facilitates gas permeability, which could obviously reduce the pressure drop at high gas flow rates.25,34−36 The nonagglomerated internetworked morphology, large surface area and sharp exposed edges of VG foam could provide abundant sites for anchoring catalysts without obvious agglomeration.37−39 Moreover, the free-standing and monolithic structure of VG foam makes it convenient for manipulation in practical applications. Thus, VG foam exhibits great potentials for employing as a catalyst support to achieve high catalytic performance with low pressure drop in PPC. Herein, we demonstrated a hierarchical petal-on-petal MnO2/VG foam catalyst for high performance of toluene decomposition in PPC. VG foam was prepared via a PECVD system by using Ni foam template. MnO2 nanopetals, one of the most effective catalysts for VOC decomposition, were fabricated on the surface of larger VG petals through a facile redox deposition. The catalytic performance (including toluene decomposition efficiency, CO2 selectivity and ozone conversion efficiency) of MnO2/VG foam catalyst, MnO2/ activated carbon (AC) and MnO2/reduced graphene oxide (rGO) powders at a high gas flow rate of 500 mL min−1 was investigated in detail. Moreover, a comparison of pressure drops between VG foam and conventional AC and rGO powders was conducted. We highlighted that the as-fabricated hierarchical MnO2/VG foam catalyst achieved excellent catalytic performances (toluene decomposition of ∼93% and ozone conversion of nearly 100%) and ultralow pressure drop (2−3 orders of magnitude lower than that of conventional powders) simultaneously.

2. EXPERIMENTAL SECTION 2.1. Synthesis of MnO2/VG foam. VG Growth. VG/Ni foam was synthesized via PECVD method employing inductively coupled plasma source with a radio frequency power of 250 W. The commercial 110 pores per inch Ni foam (100 mm × 150 mm × 3 mm) was rolled up into a cylinder and then placed in the cylindrical quartz tube (internal diameter: 43 mm). Before the growth, the quartz tube was pumped to a low pressure of 3 Pa. The growth substrate was heated to 700 °C under the vacuum condition. Subsequently, a mixture of CH4 (5 mL min−1) and H2 (5 mL min−1) was fed B

DOI: 10.1021/acs.iecr.8b03387 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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(FID). A reformer furnace was used in the GC instrument to determine the CO2 and CO concentration. The concentration of ozone was analyzed by ozone monitor (models 106-MH, 2B Technology). The discharge power (P) is calculated by using the voltage and charge (V−Q) Lissajous method. The area of V−Q parallelogram gives the discharge energy per voltage cycle:

employed to characterize crystal structures. The crystalline structures of catalysts were measured by X-ray diffraction (XRD) patterns using an XRD-6000 Diffractometer with Cu Kα source (λ = 0.15425 nm, Shimadzu). The surface chemical compositions were determined by an X-ray photoelectron spectroscopy (XPS, VG Escalab Mark II) with a monochromatic Mg Kα X-ray source (hv = 1253.6 eV, West Sussex). The Raman spectra were obtained with a DXR 532 Raman spectrometer (Thermo Fischer Scientific) with an excitation wavelength of 532 nm. The ion bombardment time of Raman measurements was 10 s. Fourier transform infrared (FT-IR) spectra were measured on Nicolet 5700 FT-IR spectrometer from 4000 to 400 cm−1. The weight percentage of K for MnO2/VG foam was obtained from inductively coupled plasma mass spectrometry (ICP-MS, XSENIES, Thermo Electron Corporation) tests. N2 adsorption−desorption measurements were carried out using an Autosorb-1-C instrument (Quantachrome Instrument Co.) at −196 °C (liquid N2). Brunauer−Emmett−Teller (BET) method was used to estimate the BET surface area. 2.5. Measurement of Pressure Drop. The pressure drops of VG foam, AC powder and rGO powder (weight of 20 mg) were determined by a manometer. During the measurement, samples were placed in a quartz tube (inner diameter of 10 mm). The pressure drops of samples at gas flow rates ranging from 250 to 3000 mL min−1 were tested. For VG foam, a gas velocity of 0.11 m s−1 corresponds to a space velocity of 19 100 h−1 at a gas flow rate of 500 mL min−1. 2.6. Experimental Setup for Toluene Decomposition. As shown in Figure 1, the experimental setup of PPC system for toluene decomposition consisted of a gas supply system, an alternating-current (AC) high voltage power supply (CTP2000K, Suman Plasma Technology Co., Ltd., Nanjing), a coaxial dielectric barrier discharge (DBD) reactor, a catalytic reactor and a gas-analysis system. The total gas flow rate was controlled by a mass flow controller (MFC, Sevenstars D08− 3F, China). Zero grade air (99.999%, Jingong, Hangzhou) was used as a carrier gas and then fed into a heating pipe with a gas flow rate of 500 mL min−1. Liquid toluene was injected into the heating pipe by a high-resolution syringe pump (LSP01− 1BH, Longer Precision Pump Co., Ltd.) to generate a steadystate gaseous toluene before flowing into the DBD reactor. The initial concentration of toluene was kept at 145 ppm. The DBD reactor was driven by the AC power supply with a frequency of 7.8 kHz and a maximum peak voltage of 30 kV. A 10 mm-long aluminum foil was wrapped around a quartz tube (with an outer diameter of 10 mm and a wall thickness of 1 mm) as a ground electrode. A stainless-steel rod with an outer diameter of 4 mm was placed on the axis of the quartz tube to act as the high voltage electrode, providing a discharge gap of 2 mm. A piece of nickel foam was packed into the gap. MnO2/ VG foam, MnO2/AC and MnO2/rGO (weight of 180 mg) were packed into the catalytic reactor located downstream of the DBD reactor. The experiments were carried out at room temperature and atmospheric pressure. The DBD reactor was energized with AC when the concentration of toluene at the exit of the catalytic reactor reached a steady state. Measurements were carried out after running the DBD reactor for approximately 30 min, when a steady-state was achieved. The discharge voltage and current were monitored by a four-channel digital oscilloscope (MDO 3034, Tektronix). Toluene was analyzed by gas chromatography (GC9790Plus, Fuli, Zhejiang) equipped with a flame ionization detector

P(W) = f × C × A

(1)

where f is the frequency, C is the capacitance of the external capacitor and A is the area of the Lissajous curve. The specific input energy (SIE) is defined as energy dissipated per unit volume of the gas flow as SIE(JL−1) =

P × 60 Q

(2)

where Q is the total gas flow rate. The toluene decomposition efficiency (ηC7H8), the ozone conversion efficiency (ηO3), the selectivities of CO2(SCO2), CO(SCO), and COx(SCOX) are defined as ηC H (%) = 7

8

ηO (%) = 3

[O3]0 − [O3]out × 100 [O3]0

SCO2 (%) = SCO(%) =

[C7H8]0 − [C7H8]out × 100 [C7H8]0

[CO2 ]out × 100 7 × ([C7H8]0 − [C7H8]out ) [CO]out × 100 7 × ([C7H8]0 − [C7H8]out )

SCOx (%) = SCOx + SCO

(3)

(4)

(5)

(6) (7)

where [C7H8]0 and [C7H8]out are the initial and outlet concentrations of toluene, respectively; [O3]0 and [O3]out are the plasma generated and outlet concentrations of ozone, respectively; [CO2]out and [CO]out are the outlet concentrations of CO2 and CO, respectively.

3. RESULTS AND DISCUSSION 3.1. Morphological Characterization. The synthesis procedure of 3D hierarchical MnO2/VG foam is schematically displayed in Figure 2. The fabrication of MnO2/VG foam mainly consists of three steps: (I) growth of VG petals on the Ni foam substrate, (II) chemical etching of the Ni foam with HCl and (III) redox deposition of MnO2 nanopetals on the VG foam. The MnO2/VG foam composite was synthesized through a facile direct redox reaction (eq 8) in the neutral

Figure 2. Schematic illustration for the synthesis of MnO2/VG foam. C

DOI: 10.1021/acs.iecr.8b03387 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research solution (pH 7.1).40,41 MnO−4 ions are converted to MnO2 using VG petals as the sacrificial reductant. The C−C bonds in graphene layers are oxidized to form CO2−3 ions and MnO2 is deposited on VG petal surface by the spontaneous reduction of MnO−4 ions. Especially, the functional groups decorated on graphene layers are able to improve the hydrophilicity of VG petals, which facilitates the interactions between MnO−4 and graphene layers for developing self-assembled MnO2 nanopetals.42

Figure 3b shows that dense and smaller VG petals were uniformly grown on the surface of larger graphene sheets within the scaffold of VG foam. The high-magnification SEM images in Figure 3c and d reveal that VG petals exhibit exposed sharp edges, vertical orientation and nonagglomerated 3D morphology. The typical span width of VG petal is approximately 100 nm, and the intersheet distance between two adjacent VG petals is hundreds of nanometers. As presented in Figure 3e, the structural features of VG foam were well maintained, while their surface became rough after MnO2 deposition. Importantly, self-assembled MnO2 nanopetals with lateral size of several tens of nanometers were uniformly deposited on the larger VG petals (Figure 3f), developing a hierarchical petal-on-petal structure. High-resolution TEM was further employed to investigate the microstructure of VG foam and MnO2/VG foam. VG petals with crumpled structure were displayed in Figure 4a, in which no obvious defects could be found in the basal planes. After redox reactions, MnO2 were uniformly deposited on the surface of VG petals (Figure 4b). As shown in Figure 4c, the lattice spacing of MnO2 was 0.275 nm, corresponding to the (211) lattice plane of α-MnO2.43,44 According to previous literatures, the as-prepared α-MnO2 presented higher ozone conversion efficiency than those of β-MnO2 and γ-MnO2.11 3.2. Structural Characterization. XRD patterns of VG foam and MnO2/VG foam were displayed in Figure 5a. Characteristic peaks at around 26°, 43° and 54° were observed in all patterns, corresponding to the (002), (100), and (004) diffraction of graphitic carbon in VG petals, respectively.45,46 Besides, a characteristic peak at around 37°, referring to the diffraction peak of (211) lattice plane in standard αMnO2(JCPDS 44−0141), was observed in the MnO2/VG sample,47 which was in good accordance with TEM results. The broad and weak peak indicates the high dispersion of MnO2 on VG petals.48 The weight percentage of K for MnO2/ VG foam determined by ICP-MS was 4.13%. Besides, MnO2/ VG foam with higher MnO2 loading (redox deposition of 48 h) was also prepared and characterized by XRD measurement to further confirm the phase of α-MnO2. As shown in Supporting Information (SI) Figure S1, three characteristic diffraction peaks at around 12°, 18°, and 37° were observed, corresponding to (110), (200), and (211) lattice plane of αMnO2, respectively.47,49 In addition, the BET surface areas of MnO2/VG foam, MnO2/AC and MnO2/rGO catalysts were measured to be 33.9 m2 g−1, 23.4 m2 g−1 and 16.2 m2 g−1, respectively. Raman spectra were employed to characterize the structural and electronic characteristics of VG foam and MnO2/VG

4MnO−4 (aq) + 3C(s) + H 2O(l) ↔ 4MnO2 (s) + CO32 −(aq) + 2HCO−3 (aq)

(8)

As shown in the optical images in Figure 2, the as-obtained sample is free-standing and monolithic. With adjusting the template size, the cylindrical VG foam can be fabricated into various lengths and diameters, making it favorable for manipulation and collection in practical applications. The surface morphology of VG foam and MnO2/VG foam was characterized by SEM images. VG foam presented 3D interconnected macroporous structure with pore size of few hundreds of micrometers (Figure 3a). A close-up view in

Figure 3. SEM images of (a−d) VG foam and (e−f) MnO2/VG foam.

Figure 4. TEM images of (a) VG foam and (b) MnO2/VG foam. (c) High-resolution TEM image of MnO2. D

DOI: 10.1021/acs.iecr.8b03387 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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to the stretching of OH groups.54 The adsorption bands at ∼1630 cm−1 are corresponded to the vibration of tunnel water in MnO2.12 The peaks of ∼1575 cm−1 are ascribed to the bending vibration of adsorbed water and OH groups.11,55 Compared with the spectrum of VG foam, two new adsorption bands at ∼490 cm−1 and ∼610 cm−1 are observed, indexed to the Mn−O and Mn−O−Mn vibrations, respectively.49,56 XPS spectra of VG foam and MnO2/VG foam were presented in Figure 6. The binding energies of elements

Figure 5. (a) XRD patterns, (b) Raman spectra, and (c) FT-IR spectra of VG foam and MnO2/VG foam.

foam. The D band at ∼1338 cm−1 and the G band at ∼1565 cm−1 are related to the disorder induced phonon-mode and ordered sp2-hybrid orbits of graphitized structure, respectively.50,51 The intensity ratio of D band and G band (ID/IG) has been widely used to reveal the defect level in graphitic structure.52 The ID/IG was calculated as ∼0.26 for MnO2/VG foam, which was obviously lower than that of VG foam (∼0.70). According to previous studies of depositing MnO2 on graphene nanosheets,48,50 such small ID/IG intensity ratio verified the chemical interactions between MnO2 and VG petals. The covalent bonding between MnO2 and graphene formed a conducting channel at the graphene-MnO2 interface, which could decrease the interfacial resistance to accelerate the electron transfer between MnO2 and VG petals for promoting the catalytic reaction in PPC.41,48 Besides, a new peak corresponding to the Mn−O lattice vibration was observed at ∼636 cm−1 in MnO2/VG foam.48,53 FT-IR spectra of VG foam and MnO2/VG foam with the wavelength ranging from 400 to 4000 cm−1 were presented in Figure 5c. The broad adsorption band at ∼3390 cm−1 is related

Figure 6. (a) XPS survey spectra of VG foam and MnO2/VG foam. Gaussian line fitted (b) Mn 2p spectra and (c) O 1s spectra of MnO2/ VG foam.

were calibrated with the C 1s peak centered at 284.6 eV.57 In the full-range spectra of MnO2/VG foam (Figure 6a), the binding energy peaks of O 1s, Mn 2p and Mn 3p were observed. As presented in Figure 6b, the spin-energy separation between Mn 2p1/2 (653.5 eV) and Mn 2p3/2 (641.8 eV) is ∼11.7 eV, in good accordance with the Mn (2p3/2, 2p1/3) in MnO2.58 As displayed in Figure 6b, the Mn 2p spectrum of MnO2/ VG foam can be fitted with two peaks, that is, Mn3+ peak (641.4 eV) and Mn4+ peak (642.6 eV), implying the presence of mixed-valency manganese ions in MnO2/VG foam.12,50 The area ratio of Mn3+/Mn4+ was calculated as 1.33, suggesting the high content of Mn3+ in MnO2/VG foam, which mainly E

DOI: 10.1021/acs.iecr.8b03387 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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to a high space velocity of 166 667 mL h−1 g−1, which was comparable or obviously higher than those reported in the state-of-art literatures (18 900 to 150 000 mL h−1 g−1).9,18,22 The residence time of gas in the plasma reactor was 0.045 s (the length of the discharge zone is 10 mm). All the catalytic reactions were carried out at room temperature (25 °C). As shown in Figure 8a, the toluene decomposition efficiency increased significantly with the increment of SIE. Specifically, for plasma process, the toluene decomposition efficiency was prominently improved from ∼30% to ∼59% with SIE increasing from 103 to 304 J L−1. This was mainly due to the promoted generation of active species (i.e., electrons, •OH and •O radicals) by discharge at high SIE.65 Similar trends were observed for PPC processes with MnO2/VG foam, MnO2/AC and MnO2/rGO catalysts. The toluene decomposition efficiencies of PPC processes were obviously higher than that of plasma counterpart, which could be interpreted by the enhanced generation of active oxygen species from ozone conversion.8,10 Moreover, toluene decomposition efficiency of MnO2/VG foam catalyst (e.g., ∼93% at a SIE of 292 J L−1) was significantly higher than those of MnO2/AC and MnO2/ rGO catalysts (e.g., ∼68% at a SIE of 301 J L−1 and ∼74% at a SIE of 317 J L−1, respectively). As indicated in Figure 8b, the ozone concentration in plasma process grew dramatically with the increment of SIE, reaching up to 461.8 ppm at a SIE of 304 J L−1.MnO2/AC and MnO2/rGO catalysts presented similar trends (e.g., 416.5 ppm at a SIE of 301 J L−1 and 350.5 ppm at a SIE of 317 J L−1, respectively), while a nearly 100% ozone conversion efficiency was recognized on the MnO2/VG foam counterpart. Considering the strong oxidation capacities of active oxygen species converted from ozone, the total conversion of ozone over MnO2/VG foam catalyst played a crucial role in the effectively decomposition of toluene,8 indicating that VG foam is a promising catalyst support for efficient toluene decomposition in PPC. Catalysts with good stability are critical to the practical applications. As shown in Figure 8c, MnO2/VG foam catalyst exhibited no obvious activity loss (less than ∼3%) after 6 h test at a SIE of 292 J L−1, demonstrating the excellent stability to withstand deactivation. The observed deactivation of catalysts was predominantly ascribed to the accumulation of organic intermediates on the catalyst surface.66,67 In the current work, ozone was totally converted to active oxygen species (nearly 100%) because of the abundant oxygen vacancies and low Mn average oxidation state on the surface of MnO2/VG foam catalyst. The generated abundant active oxygen species can effectively oxidize the intermediate products accumulated on the catalyst surface (e.g., hydrocarbons, acids and aldehydes et al.), thus exhibiting outstanding catalytic stability to resist catalyst deactivation.68 To interpret the as-obtained results, the underlying mechanisms were further explored. According to previous studies, VOC oxidation by ozone is based on eqs 9−13, where VOCs and ozone are adsorbed on Mn sites to form adsorbed species and the reactions proceed between the adsorbed species based on Langmuir−Hinshelwood mechanism.60,69 According to reaction (eq 14), Mn atoms with lower oxidation state are beneficial for transferring electrons to ozone, allowing faster ozone conversion rate to generate active oxygen species and thus promoting toluene decomposition.60

stemmed from the high dispersion of MnO2 on large VG petals. The high content of Mn3+ was beneficial for generating more oxygen vacancies and enhancing the electron transfer, thereby promoting the ozone conversion in PPC.5,59,60 As shown in Figure 6c, two peaks, corresponding to the lattice oxygen (529.8 eV, Olatt) of Mn−O−Mn and surface adsorbed oxygen (531.7 or 532.1 eV, Oads) of Mn−O−H, were observed in the Gaussian line fitted O 1s spectrum of MnO2/VG foam.12,61 Besides, the ratio of Oads/Olatt of MnO2/VG foam (0.90) was obviously higher than those reported in previous work (0.28−0.61),12,62,63 which was correlated with the coupling of MnO2 to VG petals. The Mn 2p and O 1s spectra of different catalysts were given in SI Figure S2. The ratios of Mn3+/Mn4+ and Oads/Olatt in MnO2/VG foam, MnO2/AC and MnO2/rGO were presented in SI Table S2. The ratio of Mn3+/ Mn4+ in MnO2/VG foam (1.33) was higher than that of MnO2/AC (1.11) and MnO2/rGO (1.16). Besides, MnO2/VG foam exhibited higher ratio of Oads/Olatt (0.90) compared with MnO2/AC (0.83) and MnO2/rGO (0.85). The average oxidation state (AOS) of different catalysts were calculated from Mn 3s spectra (SI Figure S3).57,59,64 The Mn AOS of MnO2/VG foam (3.37) was lower than that of MnO2/AC (3.87) and MnO2/rGO (3.81), which was ascribed to the good dispersion of Mn on VG foam, facilitating the ozone conversion.5 3.3. Pressure Drop. The pressure drops of catalyst supports including VG foam, rGO and AC powders were further measured. As shown in Figure 7, the pressure drop

Figure 7. Pressure drops of VG foam, rGO, and AC powders under different gas velocities.

increased monotonously with the increment of gas velocity. For example, with the gas velocity increasing from 0.05 to 0.27 m s−1, the pressure drops of rGO and AC powders were significantly enlarged from 3.11 to 19.76 kPa cm−1 and from 0.20 to 1.11 kPa cm−1, respectively. With the same bed weight and gas flow rate, the pressure drop of VG foam decreased obviously (e.g., 0.03 kPa cm−1 at 0.27 m s−1), 2−3 orders of magnitude lower than those of rGO and AC powders, which is mainly attributed to the 3D macroporous foam structure. This result indicates that VG foam can obviously reduce the resistance at high gas flow rates, making it a promising catalyst support to handle with high VOC throughput in PPC. 3.4. Toluene Decomposition and Ozone Conversion. The catalytic performances of MnO2/VG foam, MnO2/AC and MnO2/rGO catalysts toward toluene decomposition and ozone conversion were measured. The experiments were carried out at a gas flow rate of 500 mL min−1, corresponding

O3 + Mn → O2 + O − Mn F

(9) DOI: 10.1021/acs.iecr.8b03387 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 8. (a) Toluene decomposition efficiency, (b) ozone conversion efficiency, (c) stability test of MnO2/VG foam catalyst, and (d) schematic illustration of the toluene decomposition by hierarchical MnO2/VG foam catalyst.

O3 + O − Mn → O2 +O2 −Mn

(10)

O2 − Mn → O2 + Mn

(11)

VOC + Mn → VOC‐Mn

(12)

VOC − Mn + O − Mn → products

(13)

O3 + Mn n + → O2 + O2 − + Mn(n + 2) +

(14)

MnO2/VG foam (from 1.33 to 1.21, by 9.02%) was much lower than that of MnO2/AC (from 1.11 to 0.79, by 28.83%) and MnO2/rGO (from 1.16 to 0.84, by 27.59%). Besides, the descent of Oads/Olatt ratio in MnO2/VG foam (from 0.90 to 0.82, by 8.89%) was also lower than that in MnO2/AC (from 0.83 to 0.64, by 22.89%) and MnO2/rGO (from 0.85 to 0.75, by 11.76%), demonstrating the excellent catalytic performance of MnO2/VG foam. 3.5. CO 2 and CO x Selectivities. CO 2 selectivity demonstrates the degree of complete oxidation of toluene and COx selectivity (the carbon balance) represents the percentage of reduced toluene transformed into gaseous byproducts.23,70 Higher CO2 and COx selectivities indicate that more toluene is decomposed into COx and less byproducts are formed in PPC.17,19 As shown in Figure 9a and b, the CO2 and COx selectivities as a function of SIE in the plasma and PPC processes were presented. It was observed that CO2 and COx selectivities were improved with the increase of SIE. For plasma process, with increasing SIE from 103 to 304 J L−1, CO2 and COx selectivities increased from ∼20% to ∼35% and ∼40% to ∼60%, respectively. Such poor selectivity in plasma process was mainly ascribed to the incomplete toluene oxidation and formation of intermediates at low energy density.23,65 After combining plasma with catalyst, CO2 and COx selectivities were further promoted, which was mainly attributed to the efficient oxidation of byproducts and residual toluene by ozone converted active oxygen species.5,12 Especially, the CO2 and COx selectivities of MnO2/VG foam catalyst (∼60% and ∼78% at a SIE of 292 J L−1) were obviously higher than those of MnO2/AC (∼44% and ∼65% at a SIE of 301 J L−1, respectively) and MnO2/rGO (∼51% and ∼71% at a SIE of 317 J L−1, respectively). Moreover, CO2 yield can also be used to evaluate the percentage of toluene concentration converted to CO2. As

In the current work, the nonagglomerated morphology, large surface area and sharp exposed edges of VG foam facilitate catalyst dispersion, which could provide abundant oxygen vacancies and low Mn average oxidation state on catalyst surface for VOC decomposition. The chemical bonding between MnO2 and graphene could accelerate electron transfer for reactions. Moreover, at high gas flow rates, ozone conversion reaction is mass-transfer-limited because of the short contact times.24,27 As shown in Figure 8d, the unique petal-like structure and open channels of MnO2 could enlarge the accessible surface area exposed to VOCs, leading to enhanced decomposition efficiency. Ozone consumption using different catalysts was provided in SI Figure S5. With enlarging the SIE, ozone consumption increased monotonically. Especially, MnO2/VG foam catalyst achieved higher ozone consumption (160−452 ppm) than that of MnO2/AC (72−41 ppm) and MnO2/rGO (91−112 ppm). Thus, more active oxygen species could be generated on the surface of MnO2/VG foam catalyst to enhance the toluene decomposition reaction. XPS measurements of MnO2/VG foam, MnO2/AC and MnO2/rGO catalysts after toluene decomposition reactions were performed to further demonstrate the superior toluene decomposition of MnO2/VG foam. As shown in SI Figure S4 and SI Table S3, the area ratios of Mn3+/Mn4+ and Oads/Olatt of three catalysts decreased after the toluene decomposition reactions. Especially, the decrease of Mn3+/Mn4+ ratio for G

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conditions) were also presented in SI Table S1. Specifically, the initial concentrations of toluene varied from 50 to 240 ppm; most of the gas flow rates were in the range of 200 to 1000 mL min−1 and the corresponding space velocities increased from 18 900 to 166 667 mL h−1 g−1; all the experiments were carried out using dry air as carrier gas and DBD as plasma source at room temperature and atmosphere pressure. It is found that the achieved high COx selectivity (∼78%) and high toluene decomposition efficiency (∼93%) of MnO2/VG foam catalyst is among the best performances of the state-of-art literatures.8,9,17,68,71

4. CONCLUSION In this work, we demonstrated the hierarchical MnO2/VG foam catalyst for high performance of toluene decomposition in PPC. MnO2 with petal-like morphology was deposited on the 3D VG foam, exhibiting a hierarchical petal-on-petal structure. The as-obtained hierarchical MnO2/VG foam catalyst presented outstanding catalytic performance toward postplasma catalytic decomposition of toluene. Efficient toluene decomposition of ∼93% and ozone conversion of nearly 100% were achieved, corresponding to high CO2 and COx selectivities of ∼60% and ∼78%, respectively, which was significantly higher than those of MnO2/AC and MnO2/rGO counterpart. The achieved excellent catalytic performance was ascribed to the hierarchical petal-on-petal structure of MnO2/ VG foam. The nonagglomerated morphology, large surface area and sharp exposed edges of VG foam could provide abundant sites for MnO2 dispersion. The unique petal-like structure and open channels of MnO2 could enlarge the accessible surface area exposed to VOCs, resulting in enhanced decomposition efficiency. Moreover, VG foam presented an ultralow pressure drop at high gas flow rates, which was 2−3 orders of magnitude lower than those of conventional AC and rGO powders, realizing high catalytic performance and low pressure drop simultaneously. This work could provide new insights into constructing 3D hierarchical graphene-based catalyst for efficient VOC decomposition with high throughput in PPC.

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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.iecr.8b03387.

Figure 9. (a) COx selectivity and (b) CO2 selectivity in plasma and PPC processes. (c) Comparison of COx selectivity and toluene decomposition efficiency between our work and previous studies (see details in SI Table S1).

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shown in SI Figure S6, MnO2/VG foam exhibited higher CO2 yield (∼56% at a SIE of 292 J L−1) than that of MnO2/AC (∼30% at a SIE of 301 J L−1) and MnO2/rGO (∼37% at a SIE of 317 J L−1). The corresponding CO concentrations using different catalysts (as presented in SI Figure S7) also validated the superior catalytic performance of MnO2/VG foam. The asobtained advancement was attributed to the unique hierarchical petal-on-petal structure of MnO2/VG foam catalyst for highly efficient ozone conversion (nearly 100%). Comparison of COx selectivity and toluene decomposition efficiency between our work and previous studies was also presented in Figure 9c and SI Table S1. The specific experimental conditions (including concentrations of toluene and ozone, gas flow rate, space velocity and operating

Comparation of toluene decomposition with various catalysts in PPC (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel: +86 571 87951369.Fax: +86 571 87951616. E-mail: [email protected]. ORCID

Zheng Bo: 0000-0001-9308-7624 Xiaodong Li: 0000-0002-5331-5968 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation of China (No. 51576175). H

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