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Experimental and Computational Interrogation of Fast SCR Mechanism and Active Sites on H-Form SSZ-13 Sichi Li, Yang Zheng, Feng Gao, János Szanyi, and William F. Schneider ACS Catal., Just Accepted Manuscript • Publication Date (Web): 21 Jun 2017 Downloaded from http://pubs.acs.org on June 21, 2017
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Experimental and Computational Interrogation of Fast SCR Mechanism and Active Sites on H-Form SSZ-13 Sichi Li,† Yang Zheng,‡ Feng Gao,‡ Janos Szanyi,‡ and William F. Schneider∗,† †Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556, United States ‡Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, United States E-mail:
[email protected] 1
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Abstract Experiments and density functional theory (DFT) models are combined to develop a unified, quantitative model of the mechanism and kinetics of fast selective catalytic reduction (SCR) of NO/NO2 mixtures over H-SSZ-13 zeolite. Rates, rate orders, and apparent activation energies collected under differential conditions reveal two distinct kinetic regimes. First-principles thermodynamics simulations are used to determine the relative coverages of free Brønsted sites, chemisorbed NH4+ and physisorbed NH3 as a function of reaction conditions. First-principles metadynamics calculations show that all three sites can contribute to the rate-limiting N−N bond forming step in fast SCR. The results are used to parameterize a kinetic model that encompasses the full range of reaction conditions and recovers observed rate orders and apparent activation energies. Observed kinetic regimes are related to changes in most-abundant surface intermediates.
Keywords: SCR catalysis, H-SSZ-13, density functional theory, kinetic modeling, ab inito molecular dynamics
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Introduction Nitrogen oxides NOx produced by industrial processes, during the combustion of fossil fuels, and by operation of light and heavy duty vehicles contribute adversely to air quality. The selective catalytic reduction (SCR) of NOx with ammonia is a widely used approach for abating these emissions. 1–4 Cation-exchanged zeolites have long been known to be active for NOx SCR and are the preferred catalysts for applications that demand high thermal stability and wide temperature windows of operation. 1,5,6 Exchanged redox-active metal ions, such as copper or iron, are usually required for a material to be active for “standard” SCR: 7,8
4 NO + O2 + 4NH3 −−→ 4N2 + 6H2 O
(1)
Catalytic performance for “fast” SCR, in which equimolar amounts of NO and NO2 are consumed: 2NO + 2 NO2 + 4 NH3 −−→ 4N2 + 6H2 O
(2)
is less sensitive to the nature of exchanged ions, and even Brønsted acid, or H-form, zeolites have sufficient SCR activity for some stationary source applications. 3,9–11 Even metalexchanged zeolites typically contain significant densities of residual Brønsted sites due to incomplete metal exchange. 12 Thus, it is important to isolate the contributions of these Brønsted acid sites to fast SCR even in studies related to the performance of metal exchange sites. The precise role of Brønsted sites in fast SCR remains unclear. NH3 binds to Brønsted sites as NH4+ , and a variety of experiments and theoretical studies on metal oxides 13–15 and H-form zeolites 16–18 implicate NH4+ as a key fast SCR reaction intermediate. Br¨ uggemann et al. 19 used density functional theory (DFT) calculations and cluster models to explore the reaction of N2 O3 with a zeolitic NH4+ : ZNH4 + N2 O3 −→ ZH + H2 NNO + HONO 3
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Here we adopt “Z” as a symbol for an Al-substituted tetrahedral site in a zeolite. In this reaction NH4+ is oxidized by N2 O3 postulated to form in the well-known equilibrium: 20,21 NO + NO2 − ↽− −⇀ − N2 O3
(4)
Following reaction 3, decomposition of H2 NNO and reaction between HONO and NH3 catalyzed by Brønsted sites take place, producing final products N2 and H2 O. Br¨ uggemann et al. subsequently proposed NO adsorbed at Brønsted sites as an alternative fast SCR intermediate that is quickly consumed by NH3 to recover active Brønsted sites: 22 ZH + N2 O3 −−→ ZNO + HONO
(5)
ZNO + NH3 −−→ ZH + H2 NNO
(6)
Computed activation energies of reaction 3, reactions 5 and 6 are ≈ 100, 22 and 12 kJ mol−1 , respectively. Microkinetic modeling based on these reaction pathways can reproduce NOx conversion at high temperature reasonably close to experimental results but significantly underestimate low temperature ( 500 ppm at 633 K, while at 663 K total rate is constant up to 800 ppm NH3 .
Figure 11: Computed natural logarithm of total reaction rate versus logarithm of PNH3 at (a) 633 K and (b) 663 K. P ◦ = 1 atm. This behavior reflects the competing contributions of the three underlying routes to the total rate. The NH4+ route is least sensitive to PNH3 . It decreases slightly with PNH3 (< 0.5 in ln(rate) between 100 and 800 ppm NH3 ). The rate of the ZH route (reaction 5) also decreases 23
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with PNH3 but with a much larger relative amplitude (≈ 2.5 in ln(rate) in same PNH3 range). The physisorbed NH3 route, conversely, increases with PNH3 (≈ 2.0 in ln(rate) in same PNH3 range). These rate-PNH3 trends of different routes can be rationalized by different dependence of site coverages on PNH3 as each individual rate is proportional to the corresponding site coverage. Within the simulated PNH3 range, coverage of ZNH4 (θZNH4 ) stays high and decreases slightly with PNH3 . In contrast, coverages of both ZNH4 · NH3 and ZH (θZNH4 · NH3 and θZH ) are relatively low and very sensitive to PNH3 that increase significantly with PNH3 . Under low PNH3 , NH4+ and ZH routes contribute more than physisorbed NH3 route, so the total rate follows the decreasing trend of NH4+ and ZH routes. As PNH3 increases further, rate of physisorbed NH3 route becomes comparable to the other two. When increase of rate of physisorbed NH3 route balances the decrease of the other two, total rate reaches the flat region. When rate of physisorbed NH3 route surpasses the other two, the total rate begins to increase with PNH3 . These simulated results are qualitatively consistent with experimental measurements, rationalizing the experimentally observed negative-zero(-positive) transition of NH3 rate order (Fig. 2c).
Conclusions The role of zeolitic Brønsted acid sites in fast SCR has been debated for over a decade. What remains unclear is the mechanistic origin of high fast SCR activity observed at low temperature. We show here through theory that governed by NH3 adsorption thermodynamics, the state of Brønsted sites in H-SSZ-13 changes dynamically as a function of temperature, from NH3 -solvated NH4+ to free NH4+ under reaction conditions. Abundance of physisorbed NH3 hydrogen bonded to NH4+ ions and low energy barrier of reaction between N2 O3 and such physisorbed NH3 species rationalize low temperature catalytic performance. Further, we show through experiment that fast SCR exhibits two distinct kinetics regimes in the temperature window between 473 and 673 K that is rationalized by change of active sites for
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N2 O3 consumption revealed by first-principle kinetic modeling. Since this mechanism has no dependence on zeolitic topology, we expect the same kinetics can be observed in H-form zeolites with different framework topologies, such as H-ZSM-5. These findings demonstrate the importance of isolating reaction conditions in comprehensive investigation of kinetics of a complex reaction network. The DFT approach based on non-equilibrium AIMD and first-principles thermodynamics is particularly powerful to study mechanisms of catalytic systems, especially for those containing multiple competing reactive intermediates, adsorbates and/or sites, such as SCR on metal exchanged zeolites.
Supporting Information Available XRD pattern of the catalyst, NOx conversion plots under different space velocities of gas feed, rate orders of NOx , computational details are included in the associated PDF file.
Acknowledgement Financial support was provided by the National Science Foundation GAOLI program under award number 1258690-CBET. We thank the Center for Research Computing at Notre Dame, and EMSL, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory, for support of computational resources. Authors from PNNL (YZ, FG and JZ) gratefully acknowledge supports from the United States Department of Energy (DOE), Energy Efficiency and Renewable Energy, Vehicle Technologies Office.
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