| In the past two decades, a great deal of nitrogen oxides (NOx) has been emitted by both fixed and mobile sources, and has been found to contribute to acid rain and photochemical smog, hence converting NOx into innocuous gases has become one of the main challenges for environmental catalysis.The main ways for NOx removal can be classified into three types:selected catalytic reduction (SCR), NOx storage and reduction (NSR) and direct NOx decomposition. Even though SCR is practically very efficient with NH3as reductant, it has to deal with the risky job of transportation and storage of ammonia; and moreover, the dosage of NH3has to be well controlled so as to minimize its remaining in the exhaust. NSR also has attracted much attention in recent years for its efficiency; however, carbon dioxide and sulphur dioxide existing in exhaust gases decrease the catalytic NOx storage capacity by the formation of sulphates and carbonates. Thus, direct NOx decomposition is the most attractive method for NOx removal without any reductant added. This method, simple and cheap, would avoid a secondary pollution. Heteropoly acids(HPAs) or heteropoly compounds, as a new type of catalyst for NOx direct decomposition, have attracted the attention of worldwide researchers during the past years. Heteropoly compounds are environmental-friendly catalysts and have been applied successfully in organic synthesis, but to date there has been no report on the industrial application of the NOx catalytic decomposition by heteropoly compounds. Heteropoly acids have low surface area and poor thermal stability, which limits its use. Thus, solids with high surface area are employed as supports to enhance the surface of HPAs.Carbon nanotube (CNT) has been used in many fields due to its enormous versatility, however, very few reports have been given on its promotive effect as support on the adsorption of NOx with solid tungstophosphoric acid (H3PW12O40), and no results on the use of surface modified CNT in this process can be found in the literatures. In this study, we have conducted systematic works on this issue and found some relations between pre-treatment method of CNT and NOx adsorption efficiency of H3PW12O40/CNT, and the relations between differernt impregnating solvent used in loading process and NOx adsorption efficiency of H3PW12O4/CNT, which may be very helpful to the development of new effective adsorbent/catalyst for the adsorption-conversion of NOx. In our present work, intact CNT, surface modified CNT and pre-treated ACF (activated carbon fiber) were employed as supports for the adsorption of NOx with H3PW12O40. The main conclusions derived are as follows: absolute ethyl alcohol was the superior solvent to water for HPW loading on CNT; the resultant-OH containing CNT shows better promotive effect on the adsorption of NOx than that containing-COOH when using absolute ethyl alcohol as solvent; in both cases, with the increase of H3PW12O40loading, the NOx adsorption efficiency tends to reach a peak value close to that in the case of pure H3PW12O40, before dropping down; compared to pre-treated activated carbon fiber, modified CNT, especially CNT-OH, is favorable to be used as support of H3PW12O40for effective adsorption and even further conversion of NOxLayered double hydroxides (LDHs) has found applications in many fields due to its enormous versatility, however, very few report has been given on its promotive effect as host material on the adsorption of nitrous oxide (NOx) with solid HPAs, and no results on the use of LDHs contaning different transition metals in this process can be found in literatures. In this study, supermolecular compounds M-Al-LDH-HPAs (M=Co, Ni, Cu, Zn) were synthesized from M-Al-LDHs (M=Co, Ni, Cu, Zn) intercalated with HPAs (H3PW12O40, H3PM012O40, and H4SiW12O40) to improve the NOx adsorption efficiency of single HPAs. Our study indicates that these supermolecular compounds prepared can achieve higher deNOx capacity than HPAs, because of their special features caused by different types of cations M (M=Co, Ni, Cu, Zn) and anions (PW12O403-,PMo12O403-、and SiW12O404-) in the pillared clays, which have a major influence on their NOx adsorption performance. The nature of the cations M was found to be a main factor affecting deNOx efficiency which followed the order: Co-Al-LDH-HPAs> Zn-Al-LDH-HPAs> Ni-Al-LDH-HPAs> Cu-Al-LDH-HPAs. And the heteropoly anions were another main factor affecting deNOx efficiency in the order of M-Al-LDH-HPW>M-Al-LDH-HSiW> M-Al-LDH-HPMo, which was in accordance with the comparisons of the deNOx efficiency of pure HPAs:HPW> HSiW> HPMo. In this paper, Co-Al mixed oxides were employed as supports of H3PW12O40for the adsorption and decomposition of NOx. And the NOx adsorption efficiency was found to depend on varied preparing conditions. Co-Al mixed oxides calcinated at500℃was the suitable supports to load with HPW, and showed remarkable promotive effect on the adsorption of NOx. The mechanical grinding method is much more suitable for Co-Al mixed oxides to load with HPW than isovolume impregnation method. In the Co/Al ratio of2～4, the NOx adsorption efficiency was significantly influenced by the Co/Al ratio and when the ratio was4, the highest NOx adsorption efficiency occoured. Subsequent to the adsorption of NOx,29.7%of the adsorbed NOx was found to be converted to N2upon heating at a rate of50℃C·min-1from150℃to450℃. Both the NOx adsorption capacity and N2yield coefficient of70%PW/Co4A10M500are higher than pure HPW.Cerium oxide has broad range of applications in catalysis because of its unique elevated oxygen transport ability and its capacity to shift easily between Ce3+-Ce4+Nevertheless, the application of pure CeO2was discouraged because CeO2sintered easily at high temperature and then lost its oxygen storage and release characteristics. Hence, other metal ions (Si4+, Ti4+, Zr4+, etc) were introduced into the ceria cubic structure to improve its thermostability. Meanwhile, when the particle size of Ce-Zr mixed oxide was reduced, especially when the size decreased to around100nm, the material became nanophasic, and would provide more active sites for gas-solid catalysis. Thus, it should be advantageous for nano Ce-Zr mixed oxide to be used as suitable support. In our present work, nano Ce-Zr mixed oxides (CeZrO) were prepared to support HPW for NOx adsorption-decomposition. Relationship between preparing conditions and NOx adsorption efficiency of H3PW12O40/CeZrO was obtained; decomposition of NOx to N2over H3PW12O40and70%H3PW12O40/CeZrO was studied. Compared to single HPW, HPW/CeZrO was a favourable catalyst for NOx adsorption and decomposition. The nano Ce-Zr mixed oxide supports had positive contribution to the obtained catalysts. Furthermore, the adsorbed NOx is decomposed into N2, O2and N2O, where a yield of27.3%is achieved for N2over the supported catalyst at a temperature ramp of50℃·min-1 Ce-Ti mixed oxides (CeO2-TiO2) were prepared as the support for solid tungstophosphoric acid (H3PW12O40). FTIR, XRD, BET surface area measurement, SEM and TPD-MS were employed for characterization and mechanism analysis. CeO2-TiO2is an excellent support for H3PW12O40By loading on CeO2-TiO2, the NOx adsorption efficiency of H3PW12O40increases, with a peak efficiency of90%, which is much higher than that of single H3PWi2O40(60%). With the increase of H3PW12O40loading, the NOx adsorption efficiency tends to reach a peak value before dropping down. The mechanical grinding method is superior to the incipient impregnation method for preparing H3PW12O40/CeO2-TiO2. In NOx adsorption process, NOx reacts with H3PW12O40to produce NOH. The crystal water in the secondary structure of H3PW12O40plays an important role in NOx adsorption. And the lost crystal water and oxygen vacancy can be effectively compensated by adding water vapor to regenerate the catalyst. Furthermore, the adsorbed NOx is decomposed into N2, O2and N2O, and rapid heating contributes significantly to the decomposition of NOx over the catalyst, where a yield of30.5%is achieved for N2over the supported catalyst at a temperature ramp of50℃·min-1. |
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