Experimental and kinetic modeling study of NH3-SCR of NOx on Fe-ZSM-5, Cu-chabazite and combined Fe- and Cu-zeolite monolithic catalysts
Graphical abstract
Highlights
► First experimental and kinetic modeling study of Cu-chabazite SCR catalyst. ► 1+1 dimensional monolith model combines multi-reaction global kinetics and mass transport. ► Model predicts conversion and selectivity trends for standard, fast, and NO2 SCR. ► Model confirms expanded high NOx conversion temperature window for combined Fe/Cu catalysts.
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.
Section snippets
Catalyst samples
The commercial Cu-zeolite catalyst was supplied by BASF (Iselin, NJ). It is a small-pore Cu-chabazite (CHA) type catalyst, established in patents and communicated in recent papers to possess excellent hydrothermal stability (Bull et al., 2009, Fickel et al., 2011, Kwak et al., 2010). The Cu loading was about 2.5%. The commercial washcoated Fe-zeolite (ZSM-5 type) catalyst was supplied by an unnamed catalyst manufacturer. The sample had a Fe loading of about 3 wt% in the monolith washcoat. Both
Reactor model
We used a 1+1 dimensional reactor model to simulate various reactions involved in the NH3-SCR system. The model accounts for convection (in axial direction), gas to solid external transport, and diffusion in the washcoat and reaction. The model is based on the assumptions of laminar flow and isothermal operation. The concentrations of the reactants in the experiments were quite small (about 500 ppm NOx, and 500 ppm NH3). So, while most of the reactions are exothermic, the heat generation rate and
Kinetic model
For the purpose of the current study, we develop a global kinetic model for the NOx reduction reactions occurring during NH3-SCR. The use of a global model with assumed first-order dependencies on reactants, with a few exceptions such as NH3 in the standard SCR reaction, has the advantage of fewer kinetic parameters but admittedly lacks mechanistic sophistication. This approach builds on those of previous works (e.g. Olsson et al., 2008, Wang et al., 2011), but has the added important feature
Ammonia adsorption–desorption
Ammonia adsorption is an important step in the NH3-based SCR reaction system. We carried out NH3-TPD studies on both catalysts using 150 °C as the initial adsorption temperature while spanning a wide temperature range for desorption, with the intent to quantify the temperature and coverage dependence. A feed containing 500 ppm NH3 in Ar (and 2% H2O) was introduced during the first 60 min. NH3 uptake occurred during the induction period preceding ammonia breakthrough. After achieving saturation,
Conclusions
We have presented a comprehensive experimental study combined with the global kinetic modeling of various reactions occurring in the selective catalytic reduction of NOx with NH3 on both commercial Fe-ZSM-5 and Cu-chabazite catalysts. The kinetic model accounts for NH3 adsorption-desorption, NH3 oxidation, NO oxidation, standard SCR, fast SCR, NO2 SCR, N2O formation and decomposition, NH4NO3 formation and decomposition, etc. To our knowledge, this is the first study focused on the detailed
Nomenclature
- Aib
reverse pre-exponential factor for step i
- Aif
forward pre-exponential factor for step i
- Cfm
cup-mixing concentration in fluid phase
- Cs
fluid–washcoat interphase concentration
- Cwc
volume-averaged concentration in the washcoat
- Cst
total concentration of adsorption sites
- CTm
total molar concentration (mol/m3)
- Df
diffusivity of a species in fluid phase (m2/s)
- De
effective diffusivity of a species in washcoat (m2/s)
- j
various species (NH3, NO, NO2, N2O, N2, NH4NO3) participating in a reaction
- kbi
reverse rate
Acknowledgment
This study was funded by grants from the U.S. DOE National Energy Technology Laboratory as part of the Vehicles Technologies Program (DE-FC26-05NT42630 and DE-EE0000205).
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