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New insights into the mechanism of NH3-SCR over Cu- and Fe-zeolite catalyst: Apparent negative activation energy at high temperature and catalyst unit design consequences
Applied Catalysis B: Environment and Energy ( IF 22.1 ) Pub Date : 2018-01-03 , DOI: 10.1016/j.apcatb.2017.12.076
Saurabh Y. Joshi , Ashok Kumar , Jinyong Luo , Krishna Kamasamudram , Neal W. Currier , Aleksey Yezerets

We demonstrate an unusual behavior of a practically-important reaction of selective catalytic reduction (SCR) of NOx with NH3 over a state-of-the-art Cu-SSZ-13 catalyst. In response to increasing temperature, the rate of SCR reaction increases initially, attains a maximum around 300 °C, and then declines, resulting in a negative apparent activation energy at higher temperatures. This behavior has never been previously reported because the side reaction of NH3 oxidation obfuscated the kinetic analysis of SCR reaction at high temperatures. We were able to discover this phenomenon by performing severe hydrothermal aging of the catalyst that suppressed the NH3 oxidation activity without a significant change in the SCR activity. Further, we show that the phenomenon is more general and also found in other Cu- and Fe-exchanged zeolite and V2O5-(WO3)/TiO2 catalysts. The behavior is explained based on the fact that the activation energy for NH3 desorption is higher than that for standard SCR reaction. Therefore, at higher temperatures, the increase in the SCR reaction rate constant with temperature gets outpaced by the decline in the NH3 coverage, resulting in the overall decline of the reaction rate.

One of the implications of the finding is that the studied Cu-SSZ-13 SCR catalyst never operates in a purely external mass transfer limited regime, under the range of conditions relevant to the practical SCR applications. In other words, there is always a certain contribution of slower reaction rate to the performance of the SCR catalyst, even after increasing the reaction rates by increasing catalyst amount, using the “fast” SCR reaction, or operating at elevated temperatures. This has significant consequences for the design of practical SCR catalysts, in particular related to the choices of the active material loading and channel hydraulic diameter, as empirically and analytically demonstrated in this paper.



中文翻译:

NH 3 -SCR在铜和铁沸石催化剂上的机理的新见解:高温下明显的负活化能和催化剂单元设计的后果

我们证明选择性催化还原(SCR)的NO的实用上重要的反应的异常行为X与NH 3在一个国家的最先进的Cu-SSZ-13的催化剂。随着温度的升高,SCR反应速率开始增加,在300°C左右达到最大值,然后下降,从而在较高温度下产生负的表观活化能。这种行为以前从未被报道过,因为NH 3氧化的副反应使高温下SCR反应的动力学分析变得模糊。通过对抑制NH 3的催化剂进行严格的水热老化,我们能够发现这种现象。氧化活性,SCR活性无明显变化。此外,我们表明该现象更普遍,并且在其他Cu和Fe交换的沸石和V 2 O 5-(WO 3)/ TiO 2催化剂中也发现。基于以下事实来解释该行为:NH 3解吸的活化能高于标准SCR反应的活化能。因此,在更高的温度下,随着温度的升高,SCR反应速率常数的增加被NH 3覆盖率的下降所抵消,从而导致反应速率的总体下降。

该发现的意义之一是,在与实际SCR应用相关的条件范围内,所研究的Cu-SSZ-13 SCR催化剂从未在纯外部传质受限的条件下运行。换句话说,即使在通过增加催化剂量,使用“快速” SCR反应或在升高的温度下操作来提高反应速率之后,总会有较慢的反应速率对SCR催化剂的性能做出一定的贡献。这对实际的SCR催化剂的设计具有重大影响,特别是与活性材料负载和通道水力直径的选择有关,如本文的经验和分析所示。

更新日期:2018-01-03
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