Experimental and kinetic modeling study of NH₃-SCR of NOx on Fe-ZSM-5, Cu-chabazite and combined Fe- and Cu-zeolite monolithic catalysts

Abstract

A comprehensive experimental and modeling study of selective catalytic reduction of NOx with NH₃ was carried out on Fe-ZSM-5 and Cu-chabazite (CHA) catalysts. The experiments reveal that Cu-CHA catalyst has a higher NH₃ storage capacity and activity for NH₃ oxidation and standard SCR compared to Fe-ZSM-5. The NOx reduction activity on the Fe-ZSM-5 catalyst was found to be strongly dependent on the NO₂ feed fraction in contrast to Cu-CHA catalyst for which NOx conversion was much less sensitive to NO₂. In the presence of excess NO₂, both N₂O and ammonium nitrate were produced on both catalysts although Fe-ZSM-5 catalyst had a higher selectivity towards these byproducts compared to Cu-CHA. For different feed conditions (NO₂/NOx=0–1), Cu-CHA was a more active NOx reduction catalyst at lower temperatures (<350 °C) while Fe-ZSM-5 was more active at higher temperatures (>400 °C). Global kinetic models were developed to predict the main features of several SCR system reactions investigated experimentally. The models account for NH₃ adsorption, NH₃ oxidation, NO oxidation, standard SCR, fast SCR, NO₂ SCR, ammonium nitrate formation and its decomposition to N₂O, N₂O decomposition and N₂O reduction by NH₃. The 1+1 dimensional reactor model accounts for potential washcoat diffusion limitations. The model accurately predicts the steady state NOx and NH₃ conversions and the selectivity of the different products formed during these reactions. The model was used to predict the performance of standard and fast SCR reactions on combined systems of Fe- and Cu-zeolite monolithic catalysts which were found to have higher NOx conversion activity over a wider temperature range than with individual Fe- and Cu-zeolite catalysts as reported in our earlier study (Metkar et al., 2012b). Among various configurations of the combined catalysts, either a single brick made up of a dual-layer catalyst with a thin Fe-zeolite layer on top of a thick Cu-zeolite layer or a sequential arrangement of short Fe-ZSM-5 brick followed by longer Cu-CHA brick resulted in high NOx removal efficiency over a wide temperature range of practical interest.

Introduction

Selective catalytic reduction (SCR) of NOx with NH₃ is considered as the most promising technology to meet the stringent EPA standards of NOx emissions for heavy duty vehicles. The SCR of NOx with NH₃ has been studied extensively on various catalysts including vanadia–titania catalysts (Ciardelli et al., 2004, Ciardelli et al., 2007, Koebel et al., 2001), Cu-zeolites (Colombo et al., 2010, Komatsu et al., 1994, Sjovall et al., 2006) and Fe-zeolites (Coq et al., 2000, Grossale et al., 2008a, Grossale et al., 2008b, Metkar et al., 2011b).

The standard SCR reaction between NO and NH₃ occurs in the presence of oxygen:

4NH₃+4NO+O₂→4N₂+6H₂O.

Many studies have shown that the NOx reduction activity increases if NO and NO₂ are fed in equimolecular amount (Colombo et al., 2010, Devadas et al., 2006). The reaction between equimolar amounts of NO and NO₂ with NH₃ is known as the fast SCR reaction:

2NH₃+NO+NO₂→2N₂+3H₂O.

The third important reaction in this chemistry is NO₂ SCR reaction:

4NH₃+3NO₂→3.5N₂+6H₂O.

This reaction is important when the feed NOx comprises mostly NO₂. While the presence of NO₂ accelerates the NOx reduction rate on various catalysts (Colombo et al., 2010), its presence complicates the overall NH₃-SCR reaction chemistry (Schwidder et al., 2008). NO₂ has been proposed as the key species for the SCR of NOx with NH₃ on various zeolite catalysts, and the oxidation of NO to NO₂ is the likely rate-limiting step (Kiovsky et al., 1980). Stevenson et al. (2000) and Wallin et al. (2003) argued that oxidation of NO is the rate determining step in the reduction of NO by NH₃ on H-ZSM-5. The oxidation of NO to NO₂ has been proposed to be the rate limiting step for the standard SCR reaction on Fe-zeolite catalysts (Devadas et al., 2006, Huang et al., 2002, Metkar et al., 2011b). Many other studies have focused on the mechanistic issues for the SCR chemistry. Komatsu et al. (1994) suggested a mechanism for the standard SCR reaction on Cu-zeolite catalysts that involves the formation of a bridging NO₃ molecule which further reacts with NO to form NO₂. The NO₂ then reacts with NH₃ to produce N₂ and H₂O. Sun et al. (2001) presented a mechanistic model on Fe-MFI catalysts in which the preferred path for NOx reduction with ammonia occurs via ammonium nitrite which decomposes to N₂ and H₂O. Other studies have suggested that the reduction of NO and NO₂ by NH₃ involves the formation of HNO₂ and HNO₃ (Choi et al., 1996, Eng and Bartholomew, 1997, Richter et al., 1998, Yeom et al., 2005).

In spite of a host of experimental studies focused on mechanistic issues and reaction pathways, a much smaller number of studies have focused on the kinetic modeling of NH₃-SCR reactions. Several studies presented steady-state global kinetic models for the standard SCR for vanadia-based catalysts (Dumesic et al., 1996, Roduit et al., 1998, Willi et al., 1996), Cu-ZSM-5 (Baik et al., 2006, Olsson et al., 2008) and Cu-faujasite (Delahay et al., 2004). Nova et al. developed a detailed transient kinetic model for the SCR reaction system on vanadia-based catalyst (Nova et al., 2006, Nova et al., 2009) while Chatterjee et al., 2006, Chatterjee et al., 2007, Chatterjee et al., 2010) developed a global kinetic model for both vanadia- and Fe-zeolite catalysts. Their model describes well the transient and steady-state NOx conversion and the effect of feed NO/NO₂ ratio on the NH₃-SCR chemistry. Olsson and co-workers developed both global and detailed kinetic models for NH₃-SCR reactions on Fe-ZSM-5 and Cu-ZSM-5 catalysts (Olsson et al., 2008, Olsson et al., 2009, Sjovall et al., 2009a, Sjovall et al., 2009b, Sjovall et al., 2010). Their kinetic models account for transient effects for various feed concentrations of NO₂. Finally, some other studies present kinetic models predicting transient behaviors of NH₃-SCR reactions on Fe-zeolite catalysts (Colombo et al., 2012, Malmberg et al., 2007, Sjovall et al., 2010).

The main objective of the current study is to develop a global kinetic model that predicts the main features of various reactions occurring during NH₃-SCR on both Fe-ZSM-5 and Cu-chabazite (CHA) catalysts. In spite of the above mentioned progress in the kinetic modeling of NH₃-SCR, including the availability of kinetic models for various Fe- and Cu-exchanged zeolites, to our knowledge there is no kinetic model available for the recently commercialized small pore Cu-CHA catalyst. [Remark: Cu-CHA has emerged as the NH₃-SCR catalyst of choice because of its combination of high activity, and hydrothermal stability.] Moreover, the aforementioned models do not account for potential external transport or washcoat diffusion limitations. Our recent study determined the extent of washcoat diffusion limitations in Fe- and Cu-zeolite catalysts for NH₃-SCR reactions (Metkar et al., 2011a). Such limitations were found to be important for each of the SCR reactions and should be included in any SCR reactor model. For example, for the standard SCR on Fe-ZSM-5, diffusion limitations are important for temperatures above ca. 300 °C; for fast SCR the threshold temperature is closer to 200 °C (Metkar et al., 2011a). To this end, not including washcoat diffusion prevents consideration of more complex multi-layer catalysts for which diffusion and reaction coupling is integral to the overall performance; examples include the multi-metal exchanged zeolite catalyst in the current study, dual-layer ammonia slip catalysts, and dual-layer LNT/SCR catalysts, among others.

In the current study, we present a 1+1 dimensional reactor model, which includes external transport and washcoat diffusion processes, for both the Fe-ZSM-5 and Cu-CHA catalysts. The kinetic model considers ammonia adsorption and desorption, NH₃ oxidation, NO oxidation, standard SCR, fast SCR, NO₂ SCR, ammonium nitrate formation, N₂O formation, N₂O decomposition and N₂O SCR reactions. In addition to predicting the performance of NH₃-SCR on Fe-ZSM-5 and Cu-CHA catalysts, the model is used to simulate combined Fe- and Cu-zeolite catalysts, including sequential brick and dual-layer configurations. In a recent study (Metkar et al., 2012b), we showed how the catalyst performance in terms of the high temperature performance window can be improved through the judicious combinations of Fe- and Cu-exchanged zeolites. We use the model to predict these improvements and go further to estimate compositions and architectures that provide improved performance.