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NOx removal over modified CMS catalysts using a combined adsorption-discharge plasma catalytic process Dong Li, Xiaolong Tang, Hong hong Yi, Ding Ma, and Fengyu Gao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01672 • Publication Date (Web): 31 Aug 2015 Downloaded from http://pubs.acs.org on August 31, 2015
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NOx removal over modified CMS catalysts using a combined adsorption-discharge plasma catalytic process *
Dong Li, Xiaolong Tang , Honghong Yi, Ding Ma, Fengyu Gao College of Civil and Environmental Engineering, University of Science and Technology Beijing, Beijing, 100083, China
Abstract: Carbon molecular sieves (CMS), 13X zeolite, and γ-Al2O3, were selected as catalyst support to investigate the NOx adsorption capacity. And a series of Cu modified CMS-based catalysts were used to investigate the NOx adsorption and discharge plasma catalytic removal capacity. Results showed CMS has larger NOx adsorption amount and lower desorption temperature. The addition of Cu benefit the NOx adsorption and NTP removal capacity, and the NOx removal capacity and the ratio of NTP/(NTP+TPD) achieved 96.2% and 68.39% over 15%Cu-CMS in 30 minutes. Water vapor has an obvious effect on NOx adsorption and discharge plasma catalytic process. In cyclic operation, 15%Cu-CMS has a better NOx adsorption-discharge property. BET showed the average pore width, surface area and pore volume of the sample after cyclic operation has no significant change. XPS showed a new lattice oxygen peak appeared in O1s spectra, and the Cu2O peak disappeared in Cu2p spectra after cyclic operation. Keywords: NOx, CMS, adsorption, discharge plasma
1 Introduction With rapid economic development, energy consumption has increased significantly1.Some energy consumption, such as coal consumption, has generated large amounts of nitrogen oxides (NOx). Several NOx control technologies, such as selective catalytic reduction (SCR) 2, selective non-catalytic reduction (SNCR) 3, and catalytic decomposition4, have been developed. Among the existing De-NOx technologies, the catalytic decomposition of NOx to N2 and O2 (2NO→N2+O2) would probably represent the most attractive way since it does not require the use of a reducing agent. However, NOx direct catalytic decomposition is a phenomenon which happens at high temperature, and the coexistence of O2 inhibits the decomposition of NOx5. Atmospheric-pressure non-thermal plasmas (NTP), a new air pollution control technology, 1
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have been paid more attention because of its lower gas temperature, and higher efficiency6. For this reason, NOx removal by NTP has been proposed and explored recently7-9. Cycle operation of NOx adsorption and discharge plasma catalytic process was been thought a promising method. The catalyst was first used as a sorbent for NOx trapping with plasma off, and then the NOx were removed by discharge plasma. Proper ratio of adsorption time and NTP discharge time can decrease the energy consumption effectively10. In addition, the low discharge voltage applied in the NTP discharge process also can decrease the thermal conversion, and enhance discharge energy utilization. In this investigation, 13X zeolite, γ-Al2O3, and carbon molecular sieves (CMS) were selected as catalyst support to investigate the NOx adsorption capacity. Then, a series of Cu modified CMS-based catalysts were used to investigated the NOx adsorption and discharge plasma catalytic removal capacity. In addition, the impact of water vapor was discussed and cycle operation was also conducted to evaluate the catalytic stability.
2 Experimental and preparation 2.1 Catalyst preparation The preparation of catalyst was divided into three steps: (1)The CMS, Al2O3, 13X were ground into small particles, sieved to 20-40 mesh, washed with deionized water to remove dirt and fines and then dried at 110℃ for 8h in a dying oven; (2)The CMS was treated with Cu(NO3)2·3H2O solution of an appropriate concentration by incipient wetness impregnation and then dried in 110℃ for 12h; (3)The samples were calcined at 350℃ for 3h in a pipe furnace in N2 atmosphere.
2.2 Activity test The combined adsorption-discharge plasma catalytic process consisted of three processes (Fig. 1). First, the mixed gas containing NO (0.05%), O2 (3%) and N2 as balance gas was introduced into the reactor starting adsorption process at ambient temperature (plasma off). The total gas flow was 300 mL/min, the space velocity was 6600h–1 and catalyst packed 1g. The concentrations of NOx (NO and NO2) were analyzed by flue gas analyzer (Kane, KM9106), and N2O was analyzed by On-line MS (Extrel CMS, LLC, MAX300-LG). Then, cut off the input and output gas, started discharge process at ambient temperature (plasma on). The input voltage of the 2
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plasma was 7.6 kV, and frequency was 8.9 KHz, it lasted 20minutes at room temperature. Third, the N2 was switched into the reactor a total flow rate 300ml/min starting TPD process, the catalyst was heated from 25℃ to 600℃ at a heating rate of 10℃/min. When the NOx concentration reached zero, switch the mixed gas into the reactor and started the second adsorption process (plasma off). The water was introduced using a thermostatic water bath and the water vapor content was 8.5% NTP was generated in a cylinder-type dielectric barrier discharge (DBD) reactor, and the schematic diagram of DBD reactor was detailed in S1 (Supporting Information). The reactor consists of a coaxial cylindrical tube of quartz with dielectrical properties (i.d 10mm). The high voltage electrode is a copper bar, with 3 mm in diameter. The ground electrode is aluminum foil surrounding the external dielectric cylinder. The plasma electrical source was produced by Nanjing Suman electron LTD (model: CTP-2000P). The output voltage and frequency were measured by a digital oscilloscope (Rigol, DS1072U).
Fig. 1 The total NOx removal efficiency (η), and the NOx adsorption amount (S) was defined as follows: η=
c0 − ci × 100 % c0 tm
∫( c
S ( mmol / g ) =
0
− c i ) vdt
0
G
where Ci was the concentration of NOx in the outlet (ppm); C0 was the concentration of NOx in the inlet (ppm); v was the flow rate (mL/min); tm was adsorption time (min); G was the mass of catalysis (g).
2.3 Catalyst Characterization The N2 adsorption–desorption isotherms were obtained on a Micromeritics ASAP 2020 at liquid nitrogen temperature. Specific areas were computed from these isotherms by applying the Brunauer–Emmett–Teller (BET) method and the pore size distribution was calculated by the BJH method. Fourier transform infrared spectroscopy (FTIR) spectra were recorded on a Magna-750 Fourier transform infrared spectrometer using KBr pellets. X-ray photoelectron spectroscope (XPS) (PHI 5500) analysis used Al Kߙ radiation with energy of Al rake and power 200W. 3
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3 Results and discussion 3.1 NOx adsorption of different supports The adsorption and catalytic characteristic of different supports were different, because of its pore structure and surface chemical properties. So we compared the NOx adsorption ability of CMS, 13X, Al2O3 for 30min, respectively.
Fig. 2 As shown in Fig.2, the η and S of CMS, 13X, and γ-Al2O3 were presented in the following order: Al2O3
