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首页> 外文期刊>Catalysis science & technology >Mechanistic studies of NH3-assisted reduction of mononuclear Cu(ii) cation sites in Cu-CHA zeolites
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Mechanistic studies of NH3-assisted reduction of mononuclear Cu(ii) cation sites in Cu-CHA zeolites

机译：NH3-assisted减少机械的研究在Cu-CHA单核铜(ii)阳离子网站沸石

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摘要

Cu-Exchanged zeolites catalyze various redox reactions including the selective catalytic reduction (SCR) of NOx with NH3 and the partial oxidation of hydrocarbons. The reduction of Cu(ii) cations to Cu(i) by NH3 alone has been observed experimentally, yet fundamental details regarding the Cu active site requirements, reaction stoichiometry, and reaction mechanism remain incompletely understood. Here, we synthesized model Cu-exchanged chabazite (Cu-CHA) zeolites with varying Cu ion densities and distributions of mononuclear Cu(ii) ion site types (Cu2+, (CuOH)(+)) and studied NH3-assisted Cu(ii) reduction reactions using a combination of spectroscopic, titrimetric, and gas-phase product analysis methods. In situ UV-visible and X-ray absorption spectroscopies were used to monitor and quantify the transient reduction of Cu(ii) to Cu(i) during exposure to NH3 (473 K), in concert with titration methods that use NO and NH3 co-reductants to fully reduce to the Cu(i) state any residual Cu(ii) ions that remained after treatments in NH3 alone for a given time period. The techniques provide quantitative evidence that both mononuclear Cu(ii) site types are able to reduce in NH3 alone, and do so to similar extents as a function of time. NH3 temperature programmed reduction (TPR) revealed that the reaction stoichiometry of NH3-assisted reduction forms approximately one equivalent of N-2 per 6 Cu sites, regardless of Cu speciation or density, consistent with a six-electron reduction process whereby two NH3 molecules react with six Cu(ii) species to produce one N-2 molecule and six Cu(i) species. These findings provide new insights into the reaction pathways and mechanisms by which NH3 alone reduces mononuclear Cu(ii) sites in zeolites, which are undesired side-reactions that occur during steady-state NOx SCR and can unintentionally influence SCR-relevant spectroscopic or titrimetric characterization experiments.
机译：Cu-Exchanged沸石催化各种氧化还原包括选择性催化的反应(SCR)减少氮氧化物NH3和部分碳氢化合物的氧化。铜(ii)铜(i)的阳离子NH3孤独观察实验,但基本的细节关于铜活性部位的需求,化学计量学的反应和反应机理仍不完全理解。合成模型Cu-exchanged菱沸石(Cu-CHA)沸石不同铜离子密度和单核铜(ii)离子的分布类型(Cu2 + (CuOH)(+))和研究NH3-assisted铜(ii)减少使用的结合反应光谱滴定,气相产物分析方法。吸收光谱被用来监控和量化的瞬态减少铜(ii)铜(i)在接触NH3 (473 K),在音乐会使用没有和NH3的滴定方法co-reductants充分减少到铜(i)状态任何残留的铜(ii)离子了治疗仅在NH3对于一个给定的时期。技术提供定量的证据两个单核铜(ii)网站类型是可以的减少仅在NH3,和类似的区段作为时间的函数。减少(TPR)显示,反应化学计量学NH3-assisted减少形式大约相当于每6 - 2铜网站,不管铜物种形成或密度,符合six-electron还原过程即两个氨分子与六个铜(ii)反应物种产生一个n - 2分子和六个铜(i)物种。氨的反应途径和机制仅减少单核铜(ii)网站沸石是不受欢迎的副反应发生在稳态氮氧化合物可控硅和可以无意中影响SCR-relevant光谱或滴定的表征实验。

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