| More and more research institutions pay close attention to nitrogen oxides (NOx) abatement, due to the increasing NOx emission and the introduction of more rigorous environmental laws. NOx catalytic decomposition has been one of the most attractive methods, for no reducing agent is consumed and pollution-free N2and O2are the only products. Heteropoly compounds (HPCs) have been widely used in the catalytic reaction because of its special crystal structure and catalytic properties. The application of HPCs on NOx elimination have been researched for many years, however, they are mainly used as NOx absorbent or used in NOx catalytic reduction. Keggin type phosphotungstic acid (HPW) is the most studied HPCs, while there are many HPCs that have not been explored in NOx abatement. In this thesis, novel Keggin and Dawson type heteropoly acids (HPAs), the lanthanide complexes and cesium salt of HPW and the supported catalyst were prepared and used in the NOx adsorption and decomposition. The content of this thesis contains the following five parts.I) NOx Adsorption-Decomposition on Keggin Type Heteropoly Acids Containing GermaniumFour kinds of keggin stucture HPAs employing germanium as central atom, including tungstogermanic heteropoly acid (abbr. HGeW), tungstovanadogermanic heteropoly acid H5GeW11V040(abbr. HGeWV), molybdovanadogermanic heteropoly acid H5GeMo11V040(abbr. HGeMoV) and molybdotungstovanadogermanic heteropoly acid H5GeW9Mo2V04o (abbr. HGeWMoV) were first applied as catalysts to remove NOx in lean exhaust gas, and they were synthesized and characterized by IR measurements. The adsorption tests showed that at the optimum adsorption temperature of230℃, the synthesized catalysts showed excellent NOx adsorption ability in the following order:HGeW> HGeWV> HGeWMoV> HGeMoV, and among which HGeW had the highest NOx adsorption efficiency and capacity of80% and16.2mg NOx/g. Two new IR absorption bands at2210and1851cm-1appeared after NOx being adsorbed on HGeW, the latter band was observed for the first time on HPAs adsorbed NOx, and was assigned to nitrosyl radical (NO-). Experiments on NOx desorption and the reusability of absorbent were carried out on HGeW by increasing oven temperature and cooling in wet air. The results showed that NOx could be desorbed through both two methods and the reusability of HGeW could be realized by the latter method. Temperature-programmed desorption-mass spectroscopy (TPD-MS) was carried out to investigate the decomposition of NOx on the synthesized catalysts and and purchased phosphotungstic acid (HPW) and for the first time, the formation of O2was observed on all of the catalysts mentioned above, and the decomposition products were comprised of N2, O2and N2O. The effect of heteroatom and polyatom on NOx decomposition was investigated, and the result showed that heteroatom Ge was superior to P, polyatom W was superior to Mo for NOx transformation, while it was inferior to Mo as for N2selectivity, and after substitute by V the catalytic activity would decrease. The effects of heating rate and gas velocity on NOx decomposition were investigated and the results showed that increasing the heating rate and decreasing the gas velocity promoted the NOx transformation, however, too high heating rate and too low gas velocity decreased the N2selectivity.Ⅱ) NOx Adsorption-Aecomposition on Dawson Type Phosphotungstic AcidThe Dawson type phosphotungstic heteropolyacid (HP2W) was synthesized and characterized by IR, XRD and TGA measurements. NOx adsorption, desorption and decomposition were explored on HP2W. The influences of temperature, gas velocity, crystal water and pretreatment of HP2W on NOx adsorption were examined, the results showed that the NOx adsorption optimum temperature was200℃, the NOx adsorption efficiency had an inverse relationship with gas velocity, and the maximum NOx adsorption efficiency of78%was observed at the gas velocity of5000-1h. Two types of crystal water were observed on HP2W, among which the physical adsorbed water hindered the NOx adsorption, while the chemical adsorbed water improved it. After pretreated in the atmosphere of oxygen, hydrogen and nitrogen individually, the HP2W showed no significant difference on NOx adsorption. NOx desorption by improving temperature and cooling in wet air were comparatively studied, and the result was the same as that of HgeW explored in I), that is to say, desorption by cooling in wet air had a better performance for the reusability of HP2W could be realized through this method. TPD-MS was taken to investigate the NOx decomposition on Dawson type HP2W. Compared with Keggin type HPW, Dawson type phosphotungstic acid had lower catalytic activity, and the reason was attributed to the fact that less NOx was adsorbed on the second crystal structure of HP2W in the form of (HNO)+, which would improve the NOx decomposition by weakening the N-O bond.Ⅲ) NOx Adsorption-Decomposition on Salts of Phosphotungstic AcidLnL (Ln=La, Ce, Pr, Nd, Sm, Eu, Gd, Yb, Y, Lu, Ho, Er, Tb, Dy, Tm; L=PW11O40), LnL2type K8H3[Eu(PW,,039)2] and Cs3-xHxPW1204o(x=0,0.5,1,1.5,2,2.5,3) were prepared and used for NOx adsorption and decomposition. The adsorption results showed that NOx adsorption efficiency and capacity on LnL were lower, and for the NOx adsorption efficiency, LnL exhibited the following order: GdPW> PrPW> YPW> SmPW=CePW> LaPW> YbPW> NdPW> EuPW, while for the adsorption capacity the order was:LaPW> GdPW> YPW> PrPW> CePW> NdPW> YbPW> SmPW> EuPW. After doped with HPW, the NOx adsorption ability of LnL/HPW was improved significantly. The NOx decomposition on LnL/HPW was explored by rapid heating, and the results showed that the catalytic decomposition of NOx on LnL/HPW were superior than that of HPW, and for the N2yeild the catalytic activity followed the order:YPW/HPW> TmPW/HPW YbPW/HPW> PrPW/HPW> TbPW/HPW> SmPW/HPW> LuPW/HPW> EuPW/HPW> LaPW/HPW> CePW/HPW> ErPW/HPW> GdPW/HPW> DyPW/HPW> HoPW/HPW> NdPW/HPW. The effects of doping ratio between EuPW and HPW on adsorption and decomposition of NOx were studied. The results showed that HPW played the role of NOx adsorbent while EuPW as catalyst for decomposition of NOx, and more HPW in EuPW/HPW contributed to higher NOx adsorption efficiency and capacity, while more EuPW contributed to high NOx transformation and N2selectivity. Microwave was introduced to NOx decomposition on the catalyst of EuPW/HPW/CNTs, and the result showed that the decomposition of NOx was related to ratio of CNTs, and more CNTs improved the NOx decomposition. Compared with LnL type EuPW/HPW, the LnL2type K8H3[Eu(PW11O39)2]/HPW had higher NOx adsorption efficiency for the K ion increased the special surface thus improved the HPW dispers on K8H3[Eu(PW11O39)2], while the NOx decomposition on K8H3[Eu(PW11O39)2]/HPW was inferior to that of EuPW/HPW for the defect site and K ion on K8H3[Eu(PW11O39)2] captured O and inhibited the O removal from catalyst. NOx adsorption and decomposition on Cs3_xHxPW12O040were studied, the adsorption results showed that due to the high special surface of CS3PW, and the Cs3_xHxPW12O40exhibited fine NOx adsorption ability with the following order: CS1H2PW> Cs1.5H1.5PW> Cs2H1PW> Cs2.5H0.5PW> Cs0.5H2.5PW. TPD-MS was taken on Cs,H2PW and Cs2H1PW to investigate the effect of ratio between Cs3PW and HPW on NOx decomposition, the result showed that because the alkalis metal Cs had the O adsorption ability and O inhibited the NOx catalytic decomposition, so the more Cs, the less decomposition activity on CS3-XHxPW12O40.Ⅳ) NOx Adsorption-Decomposition on H3PW12O40/CNTSA series of HPW/CNTs catalysts were prepared by impregnation and mechanical grinding methods. For this purpose, CNTs was pre-treated separately by nitric acid vapor, mixture of nitric acid and sulphuric acid, and subsequently calcined. The performances of catalysts on NOx adsorption-decomposition were studied.0.5g catalyst was used to adsorb1000ppm NOx under the space velocity of1000h-1at200℃, and the results showed that the catalyst prepared by mechanical grinding methods supporting HPW on CNTs pretreated by nitric acid vapor and calcined at300℃had the highest NOx adsorption efficiency and capacity of54%and16.6mg NOx/g·h, respectively. TPD-MS was carried out on the catalyst adsorbed NOx, and the result showed that N2, O2and N2O were formed during rapid heating, among which O2was observed for the first time. Two modes of decomposition were employed, and one was by electric oven at a ramping rate of150℃/min, and the other was by microwave oven. Compared to the mode of the former, the latter has higher N2yield with a N2yield up to33.3%corresponding to the power of microwave oven being700W. The catalyst used can be reusable by water vapor reactivation, and the recycling results showed that there were no significant performance degradation for NOx adsorption and catalytic decomposition.V) NOx Adsorption-Decomposition on "Ship in Bottle" Type HPW-USYTo synthesis pure HPW encaged in USY zeolite (HPW-USY), NH4PW encaged in the USY (NH4PW-USY) was firstly prepared by the methods of microwave radiation and hydrothermal synthesis. FT-IR, XRD, BET, pore size and pore volume measurements indicated that microwave radiation method was superior to hydrothermal synthesis in the preparation of NH4PW-USY. Then NH4PW-USY was used to reduce NOx, and accompanied with the consumption of NH4+and formation of H+, NH4PW-USY was turned into HPW-USY simultaneously, which was confirmed by FT-IR and in site pyridine adsorption IR. NOx adsorption-desorption behavior on HPW-USY was studied. The NOx adsorption capacity was2.7mg NOx/g, and the adsorbed NOx was desorbed when the temperature was decreased to ca.100℃in wet air. TPD-MS was carried out on HPW-USY adsorbed NOx to explore the performance of catalytic decomposition NOx. For the first time, O2production was observed a little latter than the formation of N2O and N2, the NOx conversion was64.9%and N2selectivity was74.3%on HPW-USY, which was higher than pure HPW. |
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