【作者】 魏翠；

【导师】 石川；

【作者基本信息】 大连理工大学 ， 物理化学， 2010， 硕士

【摘要】 随着人们对环境质量要求的不断提高,吸烟场所、厨房、汽车拥挤的公路上产生的低浓度氮氧化物造成的环境污染日益受到重视。低浓度氮氧化物的脱除对催化剂的性能提出了新的挑战。光催化技术是近几年发展起来的一项空气净化技术,该技术由于利用太阳光,可以在室温下进行,以及高效处理低浓度污染物(无需富集),具有反应条件温和、操作易于控制、能耗低、二次污染少等优点,在处理低浓度大气污染物方面具有很高的研究价值和潜在的应用背景。本论文以高吸附性的H-ZSM-5分子筛为载体,银离子改性制得的催化剂,在紫外光下对NO这一典型氮氧化物进行光催化脱除,并借助XRD、UV-vis漫反射、TPD、原位AgNO3程序升温分解等技术对催化剂表征,对反应中起重要作用的银物种及光催化分解NO机理进行了探讨和研究。为进一步提高光催化分解NO活性,同时还对催化剂做了进一步改性。论文取得了以下有意义结果：1.光催化分解NO反应是在一套自制的套管式固定床光催化反应器中进行。H-ZSM-5分子筛中活性组分银的引入提高了催化剂光催化分解NO反应活性,银担载量、反应气NO初始浓度、灯距对光催化分解NO反应有较大影响。2.通过在不同预处理条件下处理得到的Ag/H-ZSM-5催化剂,对NO进行光催化分解实验,实验表明,不同预处理条件能明显影响Ag/H-ZSM-5催化剂的活性。将XRD、UV-vis漫反射表征与活性数据相关联,发现随着预处理温度升高,单个的银原子为降低表面能会聚集,并随着温度升高颗粒逐渐长大,催化剂光催化分解NO活性逐渐下降；氧化性气氛处理的催化剂上银物种主要为银离子和银簇,光催化分解NO活性最高,而催化剂经还原性气氛处理使交换到阳离子位的部分Ag+被还原为Ag0,具有最差的光催化分解NO活性。结合包信和等人建立的定量区分分子筛内外表面银物种含量的方法,确立高度分散于分子筛阳离子位上的银离子及银簇是光催化分解NO的活性中心。3.为获得更多活性银物种提高催化剂的光催化分解NO活性,对银催化剂进行如下改性：首先,将银担载在不同的分子筛载体上进行光催化分解NO反应,实验结果表明,Ag/H-ZSM-5催化剂具有最好的光催化分解NO活性。结合XRD, UV-vis、原位AgNO3程序升温分解等技术发现,虽然Ag/HY催化剂上具有最多的活性银物种,但光催化分解NO活性并不是最高,这主要是由于孔径的限制,反应物NO分子不能与分子筛中的活性银物种有效接触。其次,通过采用银氨溶液作为银源,获得更多的活性银物种,催化剂具有较高的光催化分解NO活性。4.通过对新鲜催化剂的NO吸附饱和后程序升温脱附和反应4个小时后的催化剂程序升温脱附过程的研究,结果表明,光催化分解NO生成的氧吸附在催化剂活性位上难以脱附是催化剂失活的主要原因。惰性气氛500℃处理可以使催化剂部分再生。更多还原

【Abstract】 As people’s consciousness on environmental quality continues to increase, the indoor air pollution attracts people’s attention day by day. The low concentrations of nitrogen oxides mainly come from smoking areas and kitchen, car congested highway. Effective removal of low concentration of nitrogen oxides, i.e, the direct decomposition of NO into N2 and O2, has been a great challenge for many researchers. Photocatalysis is an attractive, clean, safe, low-temperature reaction and a nonenergy-intensitive approach, especilly, for chemical waste removal.In this thesis, the photocatalytic decomposition of NO under UV light was investigated over silver modified H-ZSM-5 catalysts. The characterizations of XRD, UV-vis diffuse spectra, TPD, temperature-programmed decomposition of AgNO3 were employed to discriminate different kinds of Ag species. By correlating silver species with their photo-catalytic behaviors in NO decomposition, the active species for NO decomposition is clarified. Some interesting results were got as follows:1. The photocatalytic decomposition of NO was carried out in a self-made fixed bed reactor. Silver introduced into H-ZSM-5 greatly enhanced the catalytic activity in photocacalytic decomposition of NO. Silver loadings, the initial NO concentration, lamp distance has great affects on the photocatalytic activities.2. Silver is shown to be a structure-sensitive catalyst. Pretreatment has different influence on the catalytic performance. Upon thermal treatment in nitrogen, thermal induced changes in silver morphology leaded to the formation of reduced silver clusters and nano-silver particles to achieve a minimization of surface free energy. With the increase of pretreatment temperatures, catalytic activity decreased. Upon oxygen treatment of silver catalysts at elevated temperatures, the oxygen oxidized silver species into silver ions and depresses the growing of silver patticles. While under thermal treatment in hydrogen, the reduction atmosphere enchanced silver ions self-reduced and aggregated into silver particles. Consequently, the the catalyst lost part of its activity. Using the method of quantitative discrimination of silver species in the zeolite channels and those deposited on the zeolite surface established by Bao Xinhe et.al, it was concluded that silver ions and silver clusters played an important role on the photocatalytic decomposition of NO.3. To improve the photocatalytic reactivity, silver was supported on different kinds of zeolites. It indicated that Ag/H-ZSM-5 catalyst has the best photocatalytic activity. Combined with XRD, UV-vis and in situ temperature-programmed decomposition of AgNO3 techniques, it is found that Ag/HY catalyst had the most active silver species, but the activity of which was worst. This is mainly due to aperture restriction. Size 6-ring window in Y zeolite is smaller than the kinetic diameter of NO. Therefore, NO is difficult to be activated over this catalyst. While aqueous solution of silver ammonia was used as a precursor for Ag to impregnate in H-ZSM-5 zeolite. The Ag(NH)/H-ZSM-5 catalysts with more active Ag species had better activity.4. Study on the TPD results over the fresh and used samples indicates that oxygen produced from NO dissociation strongly adsorbed on silver catalyst, which blocks the active sites. That might be the reason for catalyst deactivation.更多还原