Article pubs.acs.org/est
Catalytic Control of Typical Particulate Matters and Volatile Organic Compounds Emissions from Simulated Biomass Burning Yaxin Chen,† Guangkai Tian,‡ Meijuan Zhou,† Zhiwei Huang,† Chenxi Lu,‡ Pingping Hu,† Jiayi Gao,† Zhaoliang Zhang,*,‡ and Xingfu Tang*,†,§ †
Institute of Atmosphere Sciences, Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China ‡ School of Chemistry & Chemical Engineering, University of Jinan, Jinan, Shandong 250022, China § Collaborative Innovation Center of Atmospheric Environment and Equipment Technology (CICAEET), Nanjing, Jiangsu 210044, China S Supporting Information *
ABSTRACT: Emissions of particulate matters (PMs) and volatile organic compounds (VOCs) from open burning of biomass often cause severe air pollution; a viable approach is to allow biomass to burn in a furnace to collectively control these emissions, but practical control technologies for this purpose are lacking. Here, we report a hollandite manganese oxide (HMO) catalyst that can efficiently control both typical PMs and VOCs emissions from biomass burning. The results reveal that typical alkali-rich PMs such as KCl particles are disintegrated and the K+ ions are trapped in the HMO “singlewalled” tunnels with a great trapping capacity. The K+-trapping HMO increases the electron density of the lattice oxygen and the redox ability, thus promoting the combustion of soot PMs and the oxidation of typical VOCs such as aldehydes and acetylates. This could pave a way to control emissions from biomass burning concomitant with its utilization for energy or heat generation.
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INTRODUCTION Severe haze pollution with high-level particulate matters (PMs), in particular the PMs with an aerodynamic diameter less than 2.5 μm, has influenced the environment, economy, and human health, especially in some cities of developing countries.1−3 A recent study reported that biomass burning contributes a high fraction to the PMs pollution in China.4 During severe haze events, a conservative estimate is that biomass open burning makes substantial contributions: ∼8% of primary PMs emissions such as alkali-rich PMs containing KCl and K2CO3 and soot PMs and ∼15% of secondary organic aerosol that mainly originates from photochemical oxidation of emitted volatile organic compounds (VOCs) such as formaldehyde (HCHO), ethanol (CH3CH2OH), ethyl acetate (CH2COOCH2CH3), and dicarboxylic acids.5−8 Hence, control of these emissions from biomass burning is in particular required to minimize impacts on air quality, climate, and health. One appealing alternative to open fires of agricultural and other waste9 or biomass combustion with individual traditional cookstoves10 is collective firing of biomass at power or heating plants.11 This approach takes some advantages such as reducing the PMs emissions especially with relatively big sizes in the stack gas by electrostatic equipment, lowering SO2 emissions, and gaining an extra energy. However, the small PMs and the © XXXX American Chemical Society
VOCs emissions from biomass burning can not yet be efficiently controlled. The alternative is not viable until a new practical technology for controlling both the PMs and the VOCs emissions is developed. Although alkali-rich PMs have important contributions to both primary PMs emissions and formation of secondary inorganic aerosol, alkali metals often serve as promoters in heterogeneous catalysis, which can give rise to the positive effects on many important catalytic reactions.12,13 Occasionally, alkali ions can give a similar promotion effect in oxidation reactions over oxide catalysts.14,15 Actually, depending on the electronic states and the dispersion states, alkali ions can also function as active components in catalytic reactions for controlling soot PMs and VOCs emissions.14−16 For instance, the K+ ions have been identified as catalytically active sites in the soot combustion, which can activate gaseous oxygen and enhance the oxidation activity of the catalysts.16 Upon arriving at the high or even atomic dispersion state on surfaces, alkalis become so active that they facilitate dissociation of molecular Received: December 14, 2015 Revised: April 28, 2016 Accepted: April 29, 2016
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DOI: 10.1021/acs.est.5b06109 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology oxygen (O2) by transferring electrons to the π* bonding orbitals of O2 molecule,14,17,18 and dissociated atomic oxygen is often extremely active in oxidation reactions of the soot PMs and the VOCs. This motivates us to develop an “ideal” catalyst that can initially disintegrate alkali-rich PMs and trap the alkali ions, which then produce the promotion effects in the combustion of the soot PMs and the oxidation of the VOCs. Hollandite manganese oxides (HMO) as one class of environmental catalysts have attracted great attention in both VOC oxidation and selective catalytic reduction NO by NH3 due to their strong redox ability and size-suitable tunnel structure.19,20 In this work, we use the HMO catalysts to control several typical emissions such as alkali-rich PMs, soot PMs, and VOCs from biomass burning under a simulated environment (Scheme 1), because the HMO has a one-
Materials Characterizations. The synchrotron X-ray diffraction (SXRD) patterns were performed at the BL14B of the Shanghai Synchrotron Radiation Facility (SSRF) at a wavelength of 1.2398 Å. Rietveld refinement analyses of the diffraction profiles were conducted using a FULLPROF software package on the basis of the space group of I4/m.28 The X-ray absorption spectra (XAS) covering X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra of the samples were measured at BL4B7A of the Beijing Synchrotron Radiation Facility (BSRF) with an electron beam energy of 2.2 GeV and a ring current of 300−450 mA. The raw data were analyzed using the IFEFFIT 1.2.11 software package. X-ray photoelectron spectroscopy (XPS) analysis was undertaken using the Kratos Axis Ultra-DLD system with a charge neutralizer and a 150 W Al (Monochromatized) X-ray gun (1486.6 eV) with an energy resolution of ∼0.15 eV. The XPS spectra were referenced to the C 1s peak at the binding energy of 284.6 eV. The temperatureprogrammed reduction of hydrogen (H2-TPR) measurements were carried out on a 2920 adsorption instrument (Micromeritics, USA) equipped with a thermal conductivity detector (TCD). 20−30 mg samples were loaded and reduced in the stream of 10.0 vol % H2/Ar (50 mL min−1) at a ramp of 10 °C min−1. The TEM image in Figure S4 was carried out with a JEOL JEM-2100F filed-emission gun transmission electron microscope operating at an accelerating voltage of 200 kV. Removal of Alkali-Rich PMs. KCl was selected as one typical representative of the alkali-rich PMs. When the HMO was exposed on the stack gas, KCl was initially disposed on the HMO surfaces to form a KCl/HMO sample. We simulated this process by impregnating the HMO (1.00 g) with an aqueous solution of KCl (0.11 g) and drying at 110 °C for 12 h to give the KCl/HMO sample. The disintegration of KCl by the HMO was conducted by heating KCl/HMO up to 400 °C, and it was kept at this temperature for 12 h; then, the SXRD and the XAS analyses showed that KCl had been disintegrated and K+ had been trapped by the HMO to form KinHMO. The surfaces of the KinHMO were coated by an excess of K+ ions through impregnating the KinHMO (1.00 g) with an aqueous solution containing potassium carbonate (K2CO3, 0.03 g) and drying at 110 °C for 12 h, followed by being annealed at 400 °C for 12 h to give a K/KinHMO sample with a whole surface coating by K+ ions (detailed calculation in the Supporting Information). Combustion of Soot PMs. The combustion of the soot PMs was conducted by a temperature-programmed oxidation (TPO) procedure in a fixed bed microreactor.16 Soot PMs (Printex U, Degussa)29 were mixed with the catalyst in a weight ratio of 1:9 in an agate mortar for 30 min, which resulted in a tight contact between soot and catalyst. 50 mg of the mixture was pretreated in a flow of He (100 mL min−1) at 200 °C for 30 min to remove surface-adsorbed species. After cooling down to room temperature, a gas flow with 5 vol % O2 in He (100 mL min−1) was introduced and the TPO was started at a ramp of 5 °C min−1 up to 500 °C. COx (x = 1 or 2) products were converted into CH4 by a COx-CH4 convertor and then analyzed by an online gas chromatograph (GC) (SP-6890, China) with a flame ionization detector (FID). A referential sample of K/TiO2 and a blank test without any catalyst were conducted at a ramp of 5 °C min−1 up to 730 °C under the same conditions for comparison. The catalytic activity of the soot combustion was evaluated in terms of the reaction temperature (Tmax), at which the maximum rate in the soot
Scheme 1. Models Illustrating the Emissions Control of Alkali-Rich PMs, Soot PMs, and VOCs with the HMO Catalyst
dimensional tunnel structure with a suitable size of ∼0.46 nm, which allows the HMO to efficiently trap K+, NH4+, Na+, and H+ ions.21,22 Moreover, the HMO has a great trapping capacity due to the single-walled tunnel structure, which can be calculated to be 1450 μmol g−1 according to an equation □Mn8O16 + K+ → KMn8O16, where □Mn8O16 is the formula of HMO and □ represents the vacant sites in the HMO tunnels.23 First, we use the HMO to disintegrate KCl, one of the typical alkali-rich PMs emissions from biomass burning,24 and then to trap K+ ions at an atomically dispersed state. Next, we study the catalytic combustion of the soot PMs and the mineralization of several typical VOCs, such as HCHO, CH3CH2OH, and CH2COOCH2CH3, which are the typical representatives of VOCs emissions from biomass burning,25−27 over the K+-trapping HMO catalysts. Finally, we combine experimental results with theoretical calculations to investigate the nature of the promotion effects of the trapped K+ ions in the catalytic oxidation of the soot PMs and the VOCs.
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METHODS Materials Preparation. The large-scale preparation of the HMO was done by a refluxing route23 with an aqueous solution (30 L) of manganese sulfate monohydrate (MnSO4·H2O, 1.0 mol L−1), ammonium persulfate ((NH4)2S2O8, 1.0 mol L−1), and ammonium sulfate ((NH4)2SO4, 2.5 mol L−1) at 105 °C for 12 h. After filtering and washing with deionized H2O, the obtained black solid was dried at 105 °C for 12 h, followed by being annealed at 400 °C for 4 h in air to achieve the HMO. A referential sample, K/TiO2 (8 wt % K), was prepared by impregnating TiO2 with an aqueous solution of K2CO3, then dried at 120 °C overnight, and calcined at 850 °C in air for 2 h. B
DOI: 10.1021/acs.est.5b06109 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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crystalline KCl is present at the external surface of HMO.30 As the KCl/HMO sample was treated at 400 °C to get a KinHMO, the diffraction of KCl vanished (the red curve in Figure 1), concomitant with the changes of the reflections of the HMO, possibly indicating the KCl particles have been disintegrated. To evidence the K+ positions after the removal of the KCl PMs by the HMO, we made the subsequent Rietveld refinement analyses of the SXRD patterns and the EXAFS spectroscopy measurements. Figure 2a shows the SXRD
combustion arrives. The carbon balance was calculated with an error of ±5%. Mineralization of Typical VOCs. The mineralization of the VOCs was performed in a fixed-bed quartz reactor (i.d. = 8 mm) under atmospheric pressure. The catalyst (50 mg, 40−60 mesh) was loaded for each run. Gaseous HCHO was generated by passing a N2 gas flow over paraformaldehyde (96%, Acros) in an incubator kept at 45 °C and then mixed with an O2 flow, leading to a feed gas containing 150 ppm of HCHO and 10.0 vol % O2 balanced by N2. The total flow rate was 100 mL min−1. Gaseous CH3CH2OH and CH2COOCH2CH3 were generated by passing a N2 gas flow over CH3CH2OH (99.7%) and CH2COOCH2CH3 (99.5%) solution, respectively, in an incubator kept at 0 °C, and then mixed with an air flow, leading to a feed gas containing 400 ppm of CH3CH2OH and 800 ppm of CH2COOCH2CH3, respectively. The total flow rate was 200 mL min−1. The effluents from the reactor were analyzed with an online GC (Agilent 7890A, USA) equipped with TCD and FID detectors. The data were recorded up to the steady state for each run. Theoretical Calculations. The general gradient approximation (GGA) functional and the projector augmented wave (PAW) pseudopotentials were used in the periodic density functional theory (DFT) calculations by using Vienna ab initio Simulation Package (VASP). The cutoff energy for the plane waves was equal to 450 eV. In the calculation of the HMO or the KinHMO, the lattice constants for a conventional tetragonal cell were 9.871 × 9.871 × 2.870 Å3, and a Monkhorst-Pack 2 × 2 × 6 mesh grid was used in the calculation. A supercell containing two formula units of Mn8O16 of the HMO was employed in the calculations for alkali cations in the terminal of the tunnels.
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RESULTS AND DISCUSSION The emissions of PMs from biomass burning often include alkali-rich inorganic ash particles. We demonstrate the capture and disintegration of the KCl PMs and the K+-trapping process by the HMO, because the KCl PMs are typically inorganic ash particles emitted from biomass (e.g., straw or wood) burning.24 We deposited the KCl particles on the surface of the HMO to form a KCl/HMO structure similar to that resulting from the initial accumulation of the inorganic ash particles on the HMO surfaces, as evidenced in the SXRD patterns of the KCl/HMO and the HMO of Figure 1. The reflections of both the KCl particles and the HMO can be observed in the SXRD pattern of the KCl/HMO, and the diffraction due to HMO is essentially the same as that of the pristine HMO, indicating that the
Figure 2. (a) SXRD pattern of the KinHMO sample. The short green vertical lines below the SXRD patterns mark the peak positions of all of the possible Bragg reflections. The blue curve is the differential SXRD pattern. (b) χ(R) k2-weighted FT EXAFS spectra of the KinHMO, KCl/HMO, KCl, and K@HMO samples at the K K-edge.
pattern of KinHMO. Subsequent Rietveld refinement analysis gives a coordination configuration of MnO6 polyhedron (see Table S1). The tetragonal structure of KinHMO with an I4/m space group is preserved after the K+ trapping (see Table S2) in comparison to that of HMO (see Figure S1 and Tables S2 and S3). The accurate position of K ions can be assigned to the Wychoff 2b site,31 where each K+ is coordinated by eight lattice oxygen atoms on the tunnel walls to arrive at a high dispersion and steady state.32,33 For more detailed results from the SXRD analyses, please see the Supporting Information. The immediate environment of K+ of the KinHMO was investigated by using EXAFS spectroscopy. Figure 2b displays the Fourier transform (FT) amplitudes of the χ(R) k2-weighted EXAFS spectra of the KinHMO and two references, KCl (K+ precursor) and K@HMO (@ represents K+ at the Wychoff 2b site of the HMO tunnels), at the K K-edge.33 The local environments of the K+ in the KinHMO and the K@HMO are similar to each other judging from the similarity of their FT EXAFS spectra, but significantly different from that of the K+ precursor, KCl/HMO, or KCl (Figure 2b). The structural parameters obtained by fitting the spectra with theoretical
Figure 1. SXRD patterns of the KinHMO, the KCl/HMO, and the HMO samples. C
DOI: 10.1021/acs.est.5b06109 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology models are listed in Table S4,34 and the curve-fitting of R-space and inverse FT spectra are given in Figure S2. The interatomic distance in the first shell for the KinHMO can be attributed to the K−O bonds with the bond length of ∼2.9 Å and a coordination number of 8, similar to the corresponding parameters of
[email protected] The EXAFS results fairly agree with the SXRD data above, indicating that the KCl was disintegrated by the HMO and the K+ ions were trapped in the HMO tunnels. The HMO has a large capacity to trapping alkali ions due to its “single-walled” MnO6 structure,30 and its maximal trapping capacity is calculated to be ∼1450 μmol alkali ions per gram of the HMO.23 At a normal biomass power plant, an average accumulation of alkali ions on the surfaces of catalysts is ∼360 μmol gcat−1 y−1,35 and thus, the lifetime of the HMO is calculated to be ∼4 years. The deactivated HMO can be regenerated with ease by acidic washing to recover its K+trapping capacity.36 Therefore, the HMO can efficiently control alkali-rich PMs emissions from biomass burning. Soot, one of the main PMs emissions from biomass burning, also has an important contribution to air pollution, and thus, simultaneous control of the alkali-rich PMs and soot PMs is extremely desired. We initially investigated the impact of the trapped K+ ions in the HMO tunnels on catalytic performance in the combustion of the soot PMs. Only CO2 was detected in the outlet gas during the soot combustion over all the tested catalysts except the K/TiO2 catalyst and for the pure soot combustion, CO can also be detected (Figure S3). Figure 3
on some transitional metal oxides like TiO2 in this work (Figure 3).37 Hence, we further coated the excess of alkali-rich PMs on the surfaces of the KinHMO with full K+-trapped tunnels to simulate the practical reaction conditions and investigated impact of surface PMs on catalytic performance of the HMO. Clearly, the K+ coating on the HMO surfaces did not have an important influence on the catalytic activity of the HMO in the soot combustion, taking the similar activity in Figure 3 into account. More accurately, the HMO has a rodshaped morphology (see Figure S4),23 and the K+ coating should predominantly occur on the side-facets of the HMO with semitunnel structures consisting of surface lattice oxygen ions (O2−) due to charge appealing forces between K+ and O2− (see Figure S5). Due to the K+-K+ Coulomb repulsions and the K+ trapped top-facets of the KinHMO rods, it appears impossible to further coat the K+ ions on the KinHMO topfacets. Therefore, this evidence indicates that the soot combustion mainly took place on the HMO top-facets and the excess of alkali-rich PMs accumulating on the HMO sidefacets did not have a great influence on the catalytic performance. The VOCs emitted from biomass burning are important sources for the formation of secondary organic aerosol in the atmosphere. After the removal of these alkali-rich and soot PMs, the catalytic performance of the HMO was tested in the complete oxidation of typical VOCs emitted from biomass burning.25−27 Figure 4 shows the catalytic activity of the
Figure 4. Conversion (X) of HCHO (red), CH3CH2OH (blue), and CH3COOCH2CH3 (black) over the KinHMO (solid circle) and the pristine HMO (hollow circle).
+
Figure 3. Effect of the tunnel-trapped and surface-coated K ions on the catalytic activity of the HMO in the soot combustion. The soot combustion as a function of reaction temperature over the K/TiO2 catalyst together with the blank test is also shown for comparison. CCO2 represents the concentration of the produced CO2.
pristine HMO in the oxidation of the VOCs and the impact of the trapped K+ ions after the removal of the alkali-rich PMs. As expected, the HMO shows high activity and can completely mineralize all these VOCs at temperature lower than 300 °C. Similar results were also reported in the previous work,15,38,39 because the openings of the HMO tunnels on the top facets served as highly active catalytic sites. After the removal of alkalirich PMs, the structure of these catalytic sites had changed; that is, the empty openings were occupied by the K+ ions. The results shown in Figure 4 demonstrate that the trapped K+ ions on the openings of the HMO tunnels have distinct promotion effects, and the temperatures required for the complete mineralization of the VOCs reduced. Therefore, the data above indicate that under operation temperatures of 300−450 °C the HMO can realize simultaneous control of typical PMs and VOCs emissions from biomass burning.
shows the CO2 concentrations as a function of reaction temperature in the soot combustion. The soot combustion over the pristine HMO starts at a low temperature of 300 °C and gives a maximal CO2 yield at Tmax = ∼ 400 °C, 230 °C lower than that (Tmax = 630 °C) of the pure soot combustion. Note that, after the K trapping, the promotion effect of the K+ ions can be observed and the Tmax decreases by ∼20 °C, demonstrating that our designed idea about simultaneous control of alkali-rich PMs and soot PMs emissions is feasible. Under a practical condition, alkali-rich PMs often accumulate on surfaces of catalysts and even coat the whole surfaces. In fact, alkali-rich PMs are not active in the combustion of soot PMs and also show low activity even though they are supported D
DOI: 10.1021/acs.est.5b06109 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Figure 5. (a) XANES spectra of the KinHMO and the KCl samples at the K K-edge together with the corresponding pre-edge XANES spectra (inset). (b) O 1s XPS of the KinHMO and the HMO and (c) differential XPS with respect to that of the HMO. I represents intensity.
To understand the nature of the promotion effects of the K+ ions in the simultaneous control of these emissions, we investigated the change of the electronic properties of the HMO after the removal of alkali-rich PMs by using XANES spectroscopy and XPS coupled with DFT calculations. Figure 5a depicts the XANES spectra of the KinHMO and KCl samples at the K K-edge, and the spectrum of the KinHMO is distinguished from that of the KCl by the presence of a lowintensity pre-edge absorption, which can be assigned to the 1s → 3d transition, indicating the presence of the hybridized d-sp orbitals after interacting with the lattice oxygen ions of the HMO.14 Accordingly, the electronic property of the lattice oxygen ions was determined by the O 1s XPS, as shown in Figure 5b. A differential XPS of the KinHMO by subtracting the XPS of the HMO appears with a negative peak at the high binding energy regime and a positive peak in the low binding energy regime (Figure 5c), strongly evidencing that the electron density of the surface oxygen ions of the HMO increases through the hybridization of the trapped K+. The oxidation reactions over heterogeneous catalysts, which follow Mar-van Krevelen models, essentially proceed with electron exchanges between reactants/products and catalysts by redox cycle.15,38,39 For given reactants under a certain condition, there are two factors closely associated with the reaction rates: (i) the electronic states of catalytically active sites; (ii) the ability of electron transport of catalysts. All the typical VOCs above have an electrophilic carbonyl group (>Cδ+O), and thus, the higher electron density of lattice oxygen has the stronger ability to attack the electrophilic group, thereby enhancing the reaction rates. The electron densities of the lattice oxygen of the active sites of the HMO and the KinHMO were calculated, and their differential charge densities are shown in Figure 6a. The surface cavities are active sites consisting of four surface lattice oxygen ions with sp3 hybridization, and the electron densities of these Osp3 ions increase after the K+ trapping (Figure 6b), especially around the active center; the electron structure also has significant polarization characteristics, increasing the ability to attack the electrophilic carbonyl groups. Hence, the K+ trapping is favorable for the enhancement of the oxidation performance of the VOCs. Another important factor that influences the reaction rate is the electron transport ability of the catalysts. For oxidation reactions, dissociation of O2 is often one of the rate-limiting
Figure 6. (a) The differential charge density of the HMO after the K+ trapping. Purple, red, and light green balls represent K, O, and Mn atoms, respectively. (b) The differential charge density contour of the catalytically active sites on the relaxed (001) surface of the HMO after the K+ trapping. (c) The total DOS of the HMO and the KinHMO around the Fermi level.
steps.40 One O2 molecule is dissociated into lattice oxygen ions (O2 + 4e− → 2O2−), and the active site has to provide four electrons. Often, one active site consisting of 1−2 atoms can not provide four electrons to O2 once by itself, and thus electrons, which are transported from other atoms of the catalyst to the active site, will be favorable for the O2 dissociation. Hence, the ability of the electron transport has a crucial influence on the O2 dissociation rate. For this, the electron density of states (DOS) of the HMO and the KinHMO were calculated, and Figure 6c shows the DOS around the Fermi level. The HMO has the charge localization at the Fermi level, while the charge of the KinHMO is delocalized (like a conductor), which should increase the electron transport in the redox cycle including the O2 dissociation above. Furthermore, the reaction rates of the combustion of the PMs and the oxidation of the VOCs could be accelerated because the KinHMO has the strong electron transport ability, which allows one oxidation reaction and another reduction reaction to occur simultaneously between two different active sites, and thus, electrons can easily transport between the two active sites to finish an “electrode-couple” electron transport redox cycle. As a E
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(6) Wang, G.; Kawamura, K.; Watanabe, T.; Lee, S.; Ho, K.; Cao, J. High loadings and source strengths of organic aerosols in China. Geophys. Res. Lett. 2006, 33, L22801. (7) Atkinson, R.; Arey, J. Atmospheric degradation of volatile organic compounds. Chem. Rev. 2003, 103 (12), 4605−4638. (8) Kumar, S.; Aggarwal, S. G.; Gupta, P. K.; Kawamura, K. Investigation of the tracers for plastic-enriched waste burning aerosols. Atmos. Environ. 2015, 108, 49−58. (9) Zhang, H.; Hu, D.; Chen, J.; Ye, X.; Wang, S.; Hao, J.; Wang, L.; Zhang, R.; An, Z. Particle size distribution and polycyclic aromatic hydrocarbons emissions from agricultural crop residue burning. Environ. Sci. Technol. 2011, 45 (13), 5477−5482. (10) Mobarak, A. M.; Dwivedi, P.; Bailis, R.; Hildemann, L.; Miller, G. Low demand for nontraditional cookstove technologies. Proc. Natl. Acad. Sci. U. S. A. 2012, 109 (27), 10815−10820. (11) Wieck-Hansen, K.; Overgaard, P.; Larsen, O. H. Cofiring coal and straw in a 150MWe power boiler experiences. Biomass Bioenergy 2000, 19 (6), 395−409. (12) Nielsen, A. Ammonia synthesis: Exploratory and applied research. Catal. Rev.: Sci. Eng. 1981, 23 (1−2), 17−51. (13) Herzog, K.; Gaube, J. Kinetic studies for elucidation of the promoter effect of alkali in Fischer−Tropsch iron catalysts. J. Catal. 1989, 115 (2), 337−346. (14) Xu, F.; Huang, Z.; Hu, P.; Chen, Y.; Zheng, L.; Gao, J.; Tang, X. The promotion effect of isolated potassium atoms with hybridized orbitals in catalytic oxidation. Chem. Commun. 2015, 51 (48), 9888− 9891. (15) Santos, V. P.; Soares, O. S. G. P.; Bakker, J. J. W.; Pereira, M. F. R.; Ó rfão, J. J. M.; Gascon, J.; Kapteijn, F.; Figueiredo, J. L. Structural and chemical disorder of cryptomelane promoted by alkali doping: Influence on catalytic properties. J. Catal. 2012, 293, 165−174. (16) Li, Q.; Wang, X.; Xin, Y.; Zhang, Z.; Zhang, Y.; Hao, C.; Meng, M.; Zheng, L.; Zheng, L. A unified intermediate and mechanism for soot combustion on potassium-supported oxides. Sci. Rep. 2014, 4, 4725. (17) Janiak, C.; Hoffmann, R.; Sjövall, P.; Kasemo, B. The potassium promoter function in the oxidation of graphite: An experimental and theoretical study. Langmuir 1993, 9 (12), 3427−3440. (18) Lamoen, D.; Persson, B. N. J. Adsorption of potassium and oxygen on graphite: A theoretical study. J. Chem. Phys. 1998, 108 (8), 3332−3341. (19) Wang, R. H.; Li, J. H. Effects of precursor and sulfation on OMS-2 catalyst for oxidation of ethanol and acetaldehyde at low temperatures. Environ. Sci. Technol. 2010, 44, 4282−4287. (20) Wang, Z. M.; Tezuka, S.; Kanoh, H. Characterization of the structural and surface properties of a synthesized hydrous hollandite by gaseous molecular adsorption. Chem. Mater. 2001, 13, 530−537. (21) Liu, J.; Makwana, V.; Cai, J.; Suib, S. L.; Aindow, M. Effect of alkali metal and ammonium cation templates on nanofibrous cryptomelane-type manganese oxide octahedral molecular sieves (OMS-2). J. Phys. Chem. B 2003, 107 (35), 9185−9194. (22) Kumar, R.; Sithambaram, S.; Suib, S. Cyclohexane oxidation catalyzed by manganese oxide octahedral molecular sievesEffect of acidity of the catalyst. J. Catal. 2009, 262, 304−313. (23) Hu, P.; Schuster, M. E.; Huang, Z.; Xu, F.; Jin, S.; Chen, Y.; Hua, W.; Su, D. S.; Tang, X. The active sites of a rod-shaped hollandite DeNOx catalyst. Chem. - Eur. J. 2015, 21, 9619−9623. (24) Hosseini, S.; Urbanski, S. P.; Dixit, P.; Burling, I. R.; Yokelson, R. J.; Johnson, T. J.; Shrivastava, M.; Jung, H. S.; Weise, D. R.; Miller, J. W.; Cocker, D. R., III Laboratory characterization of PM emissions from combustion of wildland biomass fuels. J. Geophys. Res. 2013, 118 (17), 9914−9929. (25) Lee, M.; Heikes, B. G.; Jacob, D. J.; Sachse, G.; Anderson, B. Hydrogen peroxide, organic hydroperoxide, and formaldehyde as primary pollutants from biomass burning. J. Geophys. Res. 1997, 102 (D1), 1301−1309. (26) Lee, M.; Heikes, B. G.; Jacob, D. J. Enhancements of hydroperoxides and formaldehyde in biomass burning impacted air
consequence, the redox ability of the HMO increases after the K+ trapping, as shown in the H2-TPR profiles of Figure S6. In conclusion, the HMO was applied to simultaneously control both the PMs and the VOCs emissions from simulated biomass burning. The alkali-rich PMs were initially disintegrated, and then, the alkali ions were trapped by the HMO. The promotion effect of the trapped K+ ions was observed in the combustion of the soot PMs and the mineralization of the VOCs by increasing the electron density of the surface lattice oxygen and the electron transport ability of the HMO. This work not only gives a general strategy for simultaneously controlling the typical PMs and the VOCs emissions from biomass burning but also provides an alternative to enable the environmentally friendly utilization of biomass in energy or heat generation.
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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.est.5b06109. Related calculation details, tables, and figures. (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*Phone: +86-21-65642997; fax: +86-21-65643597; e-mail:
[email protected]. *Phone: +86-531-82765338; fax: +86-531-89736032; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the NSFC (21277032, 21277060, 21477023, and 21477046), the STCSM (14JC1400400). The SXRD and XAS measurements were conducted at the SSRF and BSRF, respectively. We acknowledge Fei Xu for conducting the catalytic oxidation of the VOCs and Professor Xiao Gu for the useful discussion on the theoretical calculations.
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REFERENCES
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