Pd/TiO_2催化剂上氢气选择性催化还原NO_x的研究（英文）
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摘要
氮氧化物(NO_x)的控制是当前环境催化领域研究的热点.氢气选择性催化还原NO_x (H_2-SCR)近年来引起研究者的高度关注.在H_2-SCR反应中, NO_x可在较低的温度(100–300°C)下被还原,并且过量氢气会与氧气反应生成水,不会导致二次污染.研究表明, Pd基催化剂在H_2-SCR反应中具有良好的活性,并且其催化活性与Pd的化学价态密切相关, Pd~0较Pd2+具有更高的催化活性.鉴于Pd的化学价态与催化剂的制备方法密切相关,本文通过浸渍法、沉积–沉淀法和聚乙二醇还原法分别制备了Pd/TiO_2 (IM)、Pd/TiO_2 (DP)和Pd/TiO_2 (PR)催化剂,研究了不同方法制得的Pd/TiO_2催化剂对H_2-SCR的催化性能.实验结果表明,与Pd/TiO_2 (IM)和Pd/TiO_2 (DP)相比, Pd/TiO_2 (PR)催化剂的催化活性明显提高,并且温度窗口显著拓宽.为了揭示Pd/TiO_2 (PR)催化剂具有高活性的原因,对Pd/TiO_2 (IM)、Pd/TiO_2 (DP)和Pd/TiO_2 (PR)催化剂进行了一系列分析表征.XRD分析表明,在三种不同方法制备的Pd/TiO_2催化剂上,活性组分Pd高度分散在TiO_2表面.TEM结果也证实了Pd高度分散在TiO_2表面,并且在Pd/TiO_2 (PR)催化剂上, Pd颗粒粒径(1.02 nm)最小,这可能是Pd/TiO_2 (PR)催化剂具有高活性的原因之一.XPS分析表明,在Pd/TiO_2 (IM)和Pd/TiO_2 (DP)催化剂中, Pd主要以Pd2+的形式存在;而在Pd/TiO_2 (PR)中, Pd则以Pd~0的形式存在,高分散的Pd~0有利于H_2-SCR反应的进行.同时, Pd/TiO_2 (PR)中表面吸附氧(Oα)含量明显高于Pd/TiO_2 (IM)和Pd/TiO_2 (DP),较高含量的Oα能有效促进NO_x的吸附与活化,从而促进NO_x还原反应的进行.通过原位漫反射红外光谱(In-situDRIFTS)分析发现,与Pd/TiO_2 (IM)催化剂上NO+O_2稳态吸附的光谱相比, Pd/TiO_2(PR)催化剂上螯合亚硝酸根和单齿亚硝酸根的特征峰明显增强.同时观测到Pd~0上吸附的单齿亚硝酰基特征峰,由此进一步证实在Pd/TiO_2 (PR)催化剂中Pd以Pd~0的形式存在.NO+O_2→H_2 (或H_2+O_2)→NO+O_2瞬态吸附研究表明, Pd/TiO_2 (PR)催化剂上吸附的NO_x具有高的反应活性,并且吸附态NO_x和H_2(或H_2+O_2)反应可生成中间产物NH3, NH3可进一步与NO_x反应.Pd/TiO_2 (PR)催化剂上高分散的Pd~0以及在反应条件下表面生成的更多螯合亚硝酸根和单齿亚硝酸根反应中间体是其具有高H_2-SCR催化性能的主要原因.
Pd/TiO_2 catalysts prepared by three different methods(impregnation, deposition-precipitation, and polyethylene glycol reduction) were investigated in the selective catalytic reduction of NO_x by H_2(H_2-SCR). It was found that the preparation method exerted a significant effect on the activity of the Pd/TiO_2 catalyst, and that the catalyst prepared by the polyethylene glycol reduction method exhibited the highest activity in the reduction of NO_x. Characterization of the catalyst showed that, in the Pd/TiO_2 catalyst prepared by the polyethylene glycol reduction method, the existing Pd species was Pd0, which is the desirable species for the H_2-SCR of NO_x. In situ DRIFTS studies demonstrated that over this catalyst, more chelating nitrite and monodentate nitrite species formed, both of which are reactive intermediates in the H_2-SCR of NO_x. All of these factors account for the high activity of Pd/TiO_2 prepared by the polyethylene glycol reduction method.
Pd/TiO_2 catalysts prepared by three different methods(impregnation, deposition-precipitation, and polyethylene glycol reduction) were investigated in the selective catalytic reduction of NO_x by H_2(H_2-SCR). It was found that the preparation method exerted a significant effect on the activity of the Pd/TiO_2 catalyst, and that the catalyst prepared by the polyethylene glycol reduction method exhibited the highest activity in the reduction of NO_x. Characterization of the catalyst showed that, in the Pd/TiO_2 catalyst prepared by the polyethylene glycol reduction method, the existing Pd species was Pd0, which is the desirable species for the H_2-SCR of NO_x. In situ DRIFTS studies demonstrated that over this catalyst, more chelating nitrite and monodentate nitrite species formed, both of which are reactive intermediates in the H_2-SCR of NO_x. All of these factors account for the high activity of Pd/TiO_2 prepared by the polyethylene glycol reduction method.
引文
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