Article pubs.acs.org/EF
Improvement in the Water Tolerance of SiO2‑Modified Semicoke Catalysts for the Low-Temperature NO + CO Reaction Luyuan Wang,† Xingxing Cheng,*,† Zhiqiang Wang,*,† Chunyuan Ma,† and Yukun Qin†,‡ †
National Engineering Lab for Coal-fired Pollutants Emission Reduction, Shandong Provincial Key Lab of Energy Carbon Reduction and Resource Utilization, Shandong University, Jinan, China, 250061 ‡ School of Power Engineering, Harbin Institute of Technology, Harbin, China, 150001 S Supporting Information *
ABSTRACT: Activated semicoke (ASC) was modified with 3 wt % SiO2 and loaded with Fe-Co mixed oxides. This prepared catalyst exhibited excellent denitrification (deNO) activity, even when the flue gas contained 10 vol % water vapor, from 150 to 300 °C. To understand the water resistance mechanism, the catalysts were characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, and Raman, X-ray, Fourier-transform infrared, and UV−vis spectroscopies, and H2-temperature-programmed reduction, as well as NO and/or water adsorption testing. In addition, quantum mechanical calculations were performed. The results indicate that the SiO2 doped onto the semicoke surface repels both water and NO molecules. However, the repulsive force toward NO is smaller, resulting in the adsorption of NO on the surface, while the adsorption of water was reduced. The reduction in the quantity of adsorbed water improved the water tolerance of the catalysts significantly. The adsorption results demonstrate that, when the water adsorption was less than 0.345− 0.442%, water vapor had no influence on the deNO activity of ASC supported catalysts.
1. INTRODUCTION Nitric oxides (NOx) are major air pollutants, threatening the environment with urban smog, acid rain, ozone depletion, and photochemical smog.1 As environmental laws and regulations become more rigorous and stringent, greater attention has been paid to NOx removal technologies. The catalytic elimination of NO by CO (CO-deNO) is an efficient process for the abatement of power plant NO emissions.2−4. The most widely investigated CO-deNO reaction is the NO + CO reaction over transition metal oxides. In fact, Cu-,2,4−8 Co-,9−11 Fe-,12 and Mn-based2,13 oxides, as either loaded or nonloaded catalysts, exhibit excellent CO-deNO efficiency at temperatures between 150 and 300 °C. Furthermore, CeO2 or ZrO2 mixed with transition metals and carbon supported metal catalysts also promote the efficient reaction of CO with NO.6,14,15 As for the reaction mechanism, at relatively low temperatures, NO coordinates to the metal cation, generating nitrites.12,16,17 Subsequently, the formed nitrites can react with CO, affording N2O and CO2.16,17 However, as the temperature increases, the coordinated NO is transformed to nitrates that react with CO, generating N2 and CO2.12,16,17 Nevertheless, there are two intractable problems: The oxygen in the flue gas competitively consumes CO, preventing reaction of CO with NO, and, in addition, water in the flue gas inhibits the reaction severely,16,18 although this effect is negligible at high temperature (≥350 °C).19 Concerning the adverse effects of oxygen, the NOx adsorption−reduction (proposed by Prof. Bi and used in power plants) process prevents the mixing of CO and O2 (schematic diagram in Figure S1).20,21 However, in this process, an appreciable quantity of water can desorb from the adsorbents. Furthermore, in practice, the water content cannot be easily reduced. As shown in our previous studies,16,22 the © XXXX American Chemical Society
addition of 10% water reduces the NO conversion by 35% at 250 °C. Therefore, understanding the water tolerance of this reaction has become vital. In the literature,2−4,12,14,23 there are many reports of the low-temperature selective catalytic reduction of ammonia (NH3-SCR) reaction, whereas the water tolerance of low-temperature CO-deNO reactions has not been widely reported. The effect of water on NH3-SCR catalysts can be classified into three categories: (1) The competitive adsorption of H 2O and NO with an inhibition of the reaction intermediates;23 (2) the interaction of H2O and surface species24 with some damage to the surface structure;24,25 and (3) a decrease in the redox performance of catalysts in the presence of H2O.26 Of these categories, the second is the most problematic at high temperatures, while the third can severely decrease the low-temperature activity. To improve the water resistance of the catalysts, efficient catalysts have been developed. Recent investigation has mainly focused on the optimization of the metal formulas and physicochemical properties of the catalysts. For example, Qiu et al.27 synthesized an ordered mesoporous MnCo2O4 catalyst, and the activity testing showed that this sample exhibits excellent water tolerance because of the enlarged surface and optimized mesoporous structure. In contrast, other researchers such as Liu et al.26 have proposed mixed metal oxides (Mn-Ce-TiOx) that are rich in surface oxygen vacancies, which are beneficial in preventing the adsorption of water. Furthermore, similar results have been reported by Lee et al.;24 that is, when a second metal ion is introduced into a single-metal-exchanged zeolite, the Received: March 14, 2017 Revised: June 6, 2017 Published: June 8, 2017 A
DOI: 10.1021/acs.energyfuels.7b00750 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels water tolerance of the catalysts (deNOx by hydrocarbons (HC)) improved. In addition, the addition of alkali or alkalineearth metals also increases the water resistance of SCR catalysts.28 Meanwhile, in HC-SCR research,24,28−30 the use of zeolites, especially ZSM-531 and MFI-6,32 has shown that increasing the Si/Al ratio is an efficient way to improve the water tolerance of these catalysts. This improvement is attributed to the hydrophobic properties of Si species.29 In addition, silica exhibits excellent properties for the production of mesopores.33 Recently, Dong19 and co-workers proposed that the deposition of trace SiO2 could enlarge the pore size of Mn-Ti catalysts, and the larger pores contributed to a significant improvement in the water tolerance of the catalyst for the low-temperature NH3-SCR. In our previous studies,16,17,21 we have demonstrated that commercial semicoke could be activated by nitric acid. Moreover, transition metal doping of the semicoke surface resulted in a very high CO-deNO activity between 150 and 350 °C. The highest efficiency was obtained when the transition molar ratio of Fe:Co was 0.8:0.2.16 Nevertheless, as discussed above, the water vapor inhibited the CO-deNO activity. Therefore, we have analyzed the literature results and developed a novel method for the modification of activated semicoke, that is, the doping of a small amount of crystalline SiO2 on the semicoke. The activity testing demonstrated that this method improves the water tolerance of samples for COdeNO. To understand the mechanism, scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray photoelectron (XPS), and Fourier-transform infrared (FTIR), UV−vis, and Raman spectroscopies, and temperature-programmed reduction of hydrogen (H2-TPR), as well as NO and/or water adsorption testing, were performed.
Table 1. Composition and Loading Amounts of the Catalysts catalysts Fe0.8Co0.2/ASC Fe0.8Co0.2/ASC-SiO2
concentration of precursor liquid (mol/L)
SiO2 expected loading (%)
Fe(NO3)3: 1.653 Co(NO3)3: 0.413 Fe(NO3)3: 1.653 Co(NO3)3: 0.413
0 3
(DFT) from the N2 adsorption/desorption isotherms. XRD patterns were recorded on a Rigaku D/max 2400 diffractometer using Cu−Kα radiation (λ = 1.5056 Å) at a scanning rate of 8°/min with a step size of 0.02° over a 2θ range of 10−80°. The surface atomic states of the catalysts were analyzed by XPS (Axis UltraDLD) with Al−Kα radiation (hv = 1486.7 eV, 225 W, 15 mA, 15 kV). The binding energies were calibrated using the C 1s peak at 284.5 eV as a reference, and experimental data were fitted with a Gaussian−Lorentzian mixed function as implemented in the Casa XPS software. H2-TPR was performed using a Chemisorb instrument (Chembet Pulsar TPR/TPD 2139). These tests were conducted using a quartz U-type reactor, which was connected to a thermal conductivity detector. The module reductant gas was composed of 10 vol % H2 balanced by Ar at a flow rate of 40 mL/min. Before the reduction, the sample (100 mg) was pretreated in a N2 stream at 300 °C for 1 h, and then TPR was started from room temperature to 900 °C at a rate of 10 °C/min. FTIR spectra were recorded from 650 to 4000 cm−1 at a spectral resolution of 4 cm−1 (number of scans: 100) on a Nicolet 6700 FTIR spectrophotometer equipped with a high-sensitive MCT detector cooled by liquid N2. Before the performance, all the samples were ground into a fine powder (
