Sites and Mechanism in Selective NOx Reduction

Transcription

1 Sites and Mechanism in Selective NOx Reduction William F. Schneider Dept. of Chemical and Biomolecular Engineering Dept. of Chemistry and Biochemistry CLEERS Workshop Ann Arbor, Michigan October 4, 2017

2 The NSF/DoE NO x SCR Team ND Chris Paolucci Atun Anggara Hui Li Sichi Li Purdue Fabio Ribeiro Raj Gounder Jeff Miller Nic Delgass WSU J-S McEwen PNNL Chuck Peden Janos Szanyi Feng Gao Cummins Alex Yezerets Neil Currier

3 Objectives of NSF/DoE NO x SCR Team This project seeks to build a microscopically detailed model of catalyst performance under all operating conditions and throughout the life cycle aiming to optimize engine efficiency within emission constraints and to circumvent catalyst deactivation. Precise Synthesis Operando Spectroscopies Quantitative Kinetics Atomistic to Microkinetic Modeling

4 NO x Selective Catalytic Reduction Standard SCR 4 NH NO + O 2 4 N H 2 O NH 3 + O 2 products Cu-SSZ-13 Catalyst Kwak, J. H., et al. J. Catal. 2012; 287,

5 Rates vs. Cu Si:Al 5: Standard SCR activity scales with number of isolated Cu 2+ Dry NO oxidation activity scales with Cu oxo species at higher Cu loadings Bates et al. J. Catal. 2014, 312,

6 Cu(II)-SSZ-13 Composition Space Cu/Al O O O Si O O Al O O O Si/Al

7 Cu Exchange in Zeolite Frameworks Cu 2+ NH 4 -SSZ-13 (Si:Al ) + Cu(NO 3 ) 2 (aq) (Cu 2+ ) Z 2 Cu Cu(II) exhibits a strong preference for binding in 6-fold rings near 2 Al Agreement with DFT, UV-vis, EXAFS, and XRD, acid titrations

8 Cu Exchange in Zeolite Frameworks NH 4 -SSZ-13 (Si:Al ) + Cu(NO 3 ) 2 (aq) (Cu 2+ ) Isolated Al bind Cu(II) as ZCu(II)OH ZCuOH Agreement from DFT, UV-vis, EXAFS, and XRD, acid titrations

9 Relative Site Energies Does exchange favor ZCuOH or Z 2 Cu? Construct connecting reaction ZH 0 ev Evaluate in reference models Z 2 CuH 2 O Paolucci et al. J. Am. Chem. Soc. 2016, 138, 6028

10 Relative Site Energies Does exchange favor ZCuOH or Z 2 Cu? Construct connecting reaction ZCuOH +0.6 ev Evaluate in reference models Z 2 H 2 Paolucci et al. J. Am. Chem. Soc. 2016, 138, 6028

11 In Situ Thermodynamic Screening Enumerate and rank all possible species by relative free energy

12 Relative Site Free Energies Hydrated Paolucci et al. J. Am. Chem. Soc. 2016, 138, 6028 Dry

13 How Many of Which Cu(II) Site? Large SSZ-13 supercell Pair site histogram Set Si:Al ratio Swap Al locations Obey Löwenstein s rule Verma et al. J. Catal., 2014, 312,

14 Cu-SSZ-13 Composition Phase Diagram Cu : Al ZCuOH Exchanges one proton 3-fold coordination Autoreduction CuOH : Cu total Z 2 Cu Exchanges two protons 4-fold coodination No autoreduction Synthesized Si : Al Paolucci et al. J. Am. Chem. Soc. 2016, 138, 6028

15 Ex situ Characterization of Cu Exchange Residual protons after exchange CuO H vibrational stretch Monte Carlo Cu II Exchange Co II Exchange Si:Al 15 Cu:Al 0.00 Cu:Al 0.12 Cu:Al 0.21 Cu:Al 0.37 Cu:Al 0.44 H:H(parent) Intensity [CuOH] + H Si:Al M:Al ν cm -1 Paolucci et al. J. Am. Chem. Soc. 2016, 138, 6028

16 Ex Situ Phase Diagram ZCuOH Z 2 Cu 5% H 2 O [ZH]/ [ZCu I ] [ZCu I ] [Z 2 Cu II ] 3 2 [ZCu II OH] [ZCu II O 2 ] 3 2 [Z 2 Cu II ]O 2 [ZCu I H 2 O] Z[Cu II (OH)(H 2 O) 3 ] 1 [ZCu II (OH)(H 2 O)] 1 [Z 2 Cu II H 2 O] Z 2 [Cu II (H 2 O) 4 ] 1 Atmosphere 2 O 2 3 He 1 Z[Cu II (OH)(H 2 O) 3 ] 2 [ZCu II OH] 3 [ZCu I ] 1 Z 2 [Cu II (H 2 O) 4 ] 2 3 [Z 2 Cu II ] Paolucci et al. J. Am. Chem. Soc. 2016, 138, 6028

17 SCR Gas Binding Energies Cu(II) Sites ZCuOH Z 2 Cu Cu(I) Sites ZCu ZCu/ZNH 4 HSE06+vdW

18 Z 2 Cu/NH 3 /H 2 O/O 2 phase diagram NH 3 covers all sites below 400 C Oxidized and reduced Cu states close in free energy at 200 C Paolucci et al. J. Am. Chem. Soc. 2016, 138, 6028

19 Operando X-ray Spectroscopy vs Theory Paolucci et al. J. Am. Chem. Soc. 2016, 138, 6028

20 SCR: Cu Redox on Solvated Cu Ions SCR? Cu II OH(NH 3 ) 3 Cu I (NH 3 ) 2

21 Cu II + NH3 + NO à Cu I + N 2 + H 2 O Cu 2+ Cu + /H + Paolucci et al. Angew. Chemie 2014

22 SCR: Cu Redox on Solvated Cu Ions NO + NH 3 Reduction halfcycle N 2 + x H 2 O? ZCu II OH(NH 3 ) 3 ZCu I (NH 3 ) 2 Ishant Khurana poster O 2 + NO + NH 3

23 Operando SCR Rate and Cu Oxidation State rate~ k (Cu) 2 Paolucci et al., Science 2017 TOF and Cu oxidation state sensitive to Cu density

24 Transient Cu(I) Cu(II) Cu I + O 2! Cu II rate k [Cu I ] [Cu I ] 1 2 Approximately second order reoxidation k increases systematically with initial Cu density Recalcitrant Cu I fraction decreases systematically with initial Cu density All at densities << 1 Cu/cage Paolucci et al., Science 2017 Not single site Not homogeneous Notmean-field kinetics

25 Intercage Cu Transport Paolucci et al., Science 2017

26 Intercage Cu Reactions J. Phys. Chem. B 1999, 103, Cluster Model Studies of Oxygen-Bridged Cu Pairs in Cu-ZSM-5 Catalysts B. R. Goodman Department of Chemistry, UniVersity of Illinois, Urbana, Illinois K. C. Hass* and W. F. Schneider Ford Research Laboratory, MD 3028/SRL, Dearborn, Michigan J. B. Adams Department of Chemical, Bio and Materials Engineering, Arizona State UniVersity, Tempe, Arizona ReceiVed: June 29, 1999; In Final Form: September 7, 1999 Paolucci et al., Science

27 Intercage Cu Reactions (B) Energy (kj mol -1 ) (A) S 3 to S 1 Cu Oxidation State (C) (D) Reaction Coordinate (E) O 2 Cu I 2 Cu II 2O 2 Paolucci et al., Science 2017

28 How far can a Cu I travel? CPMD metadynamics, 473 K Metadynamics free energy Coulombic potential 2 F, E (kj/mol) Cu-Al distance (Å) 1 ~ 9 Å diffusion radius from home Al Paolucci et al., Science 2017

29 How Many Unique Pairs? # pairs Cu density Fraction of pairable Cu 18 Å collision diameter Cu density

30 Recalcitrant Cu I fraction Distribute Cu onto framework at appropriate density Count pairable Cu Within diffusion radius No double counting Paolucci et al., Science 2017 Electrostatically tethered, dynamic site pairs

31 Dual Single Site SCR Mechanism Single site reduction Insensitive to framework topology Si/Al, Cu/Al Cu(II) N 2 + H 2 O NO N H 2 O NO + 2 NH 3 NO + 2 NH NH 4 Cu(II) N H 2 O NO N 2 + H 2 O + + NH 4 Single site reduction Insensitive to framework topology Si/Al, Cu/Al Cu(I) Cu(I) O 2 Dual site oxidation Sensitive to framework topology Si/Al, Cu/Al

32 Mechanistic Implications At least at low temperature, not all atomically dispersed Cu participate in SCR cycle Apparent SCR rates connected to dynamic Cu ion mobility Microkinetics have a transport component that lies outside the domain of traditional mean field models

33 Transient Oxidation Rates F, E (kj/mol) Metadynamics free energy Coulombic potential % O Cu-Al distance (Å) n O2 ~ 0.2 rate(d) = kt h e ( G(d)+Ea)/kT (O 2 ) + Kinetic Monte Carlo n O2 ~ 0.3 n O2 ~ 1

34 Mechanistic Implications At least at low temperature, not all atomically dispersed Cu participate in SCR cycle Apparent SCR rates connected to dynamic Cu ion mobility Microkinetics have a transport component that lies outside the domain of traditional mean field models Needs/Opportunites How does SCR cycle close? What is an appropriate, general rate model? How do mechanism and rates change away from standard conditions? How does reaction get short-circuited? Eg, to make N 2 O? How do catalytic sites change with sulfur and/or aging?

35 Fast SCR on Brønsted Sites Fast SCR 2 NH NO + 2 NO 2 2 N H 2 O Li et al, ACS Catal NH NO 2 2 N 2 O + 3 H 2 O N2O pathway?

36 Sulfur on Cu-SSZ-13 Z 2 Cu ZCuOH Temperature Cu(II) Cu(II) [NH3]=300 ppm Cu(I) Cu(II) Cu(II) Cu(I) Nature and reducibility of dominant sulfur species differs between Z2Cu to ZCuOH Arthur Shih poster

37 Pathways to Dealumination Löwenstein s rule violation, 1NN 2NN, 4MR 2NN, meta-z 2 Cu 3NN, para-z 2 Cu 3NN in 8MR 4NN in 8MR 4NN PBE relative Energy (ev) Al-Al distance (Å) Nomenclature: 6 membered-ring (6MR), 2 nearest-neighbor (2NN) GGA/PBE energies Normalized to E=0 for infinite separation E Z = E - E 2 H 2 Z-H Z Energy minimized for Al-O-Al configuration Energy approach 0 at larger Al-Al distances Brønsted sites (H + ) exchange

38 Pathways to Dealumination PBE relative Energy (ev) Cation site (Cu 2+ ) exchange GGA/PBE energies Normalized to E=0 for para-z 2 Cu structure Energy minimized for para-z 2 Cu, 3NN in 6MR meta-z 2 Cu 0.15 ev higher All other structures at least 0.8 ev higher in energy Cu prefers to located in 6MR at larger Al-Al distances Löwenstein s rule violation, 1NN 2NN, 4MR 2NN, meta-z 2 Cu 3NN, para-z 2 Cu 3NN in 8MR 4NN in 8MR 4NN Conclusions Löwenstein s rule (no Al-O-Al) for synthesized zeolites is not a consequence of the energy landscape Low energy for Al-O-Al structures may be the cause of Al aggregation during hydrothermal aging Exchanged Cu 2+ can help stabilizing 6MR and the zeolite structure

39 Group Publications Author's personal copy CHAPTER ONE Catalysis Science of NO x Selective Catalytic Reduction With Ammonia Over Cu-SSZ-13 and Cu-SAPO-34 C. Paolucci*, J.R. Di Iorio, F.H. Ribeiro, R. Gounder, W.F. Schneider*,1 *University of Notre Dame, Notre Dame, IN, United States School of Chemical Engineering, Purdue University, West Lafayette, IN, United States RESEARCH ARTICLES 1 Corresponding author: address: wschneider@nd.edu Contents Cite as: C. Paolucci et al., Science /science.aan5630 (2017). Dynamic multinuclear sites formed by mobilized copper 1. Introduction Selective Catalytic Reduction ions in NOx selective 6 catalytic reduction 1.2 Metal-Exchanged Zeolite Catalysts 7 Christopher Paolucci, 1 Khurana, 2 Atish A. Parekh, 2 Sichi Li, 1 Arthur J. Shih, 2 Hui Li, 1 John R. Di Iorio, Scope and Structure of This Review Ishant 11 Albarracin-Caballero, Jonatan D. 2 Aleksey Yezerets, 3 Jeffrey T. Miller, 2 W. Nicholas Delgass, 2 2. Synthesis of Zeolites William 13F. Fabio H. Ribeiro, 2 Schneider, 1 * Rajamani Gounder 2 * 2.1 Synthetic Control of Al Distribution in Zeolites Biomolecular 13 1 Department of Chemical and Engineering, University of Notre Dame, Notre Dame, IN 46556, USA. 2 Charles D. Davidson School of Chemical Engineering, 2.2 Synthesis of SSZ-13 Zeolites Drive, 15 Purdue University, 480 Stadium Mall West Lafayette, IN 47907, USA. 3 Cummins Inc., 1900 McKinley Avenue, MC 50183, Columbus, IN 47201, USA. 2.3 Synthesis of SAPO-34 Molecular Sieves *Corresponding author. wschneider@nd.edu (W.F.S.); rgounder@purdue.edu (R.G.) Copper Exchange in Zeolite and SAPO Frameworks Copper ions exchanged into zeolites are active for the selective catalytic reduction (SCR) of NOx with NH3, 20 but the low-temperature rate dependence on Cu volumetric density is inconsistent with reaction at single 3. Ex Situ Characterization of Cu-SSZ-13 and Cu-SAPO-34 sites. We combine steady-state 23 and transient kinetic measurements, x-ray absorption spectroscopy, and 3.1 DFT-Based Analysis of Cu Speciation first-principles calculations 24 to demonstrate that under reaction conditions, mobilized Cu ions can travel 3.2 Ambient Conditions through zeolite windows and form transient ion pairs that participate in an O2-mediated Cu 29 Cu II redox step integral to SCR. Electrostatic tethering to framework Al centers limits the volume that each ion can 3.3 High-Temperature Oxidative Conditions explore and thus its capacity 36 to form an ion pair. The dynamic, reversible formation of multinuclear sites 3.4 Vacuum and Inert Pretreatments from mobilized single atoms 47 represents a distinct phenomenon that falls outside the conventional boundaries of a or homogeneous catalyst. 3.5 Hydrogen Temperature-Programmed Reduction heterogeneous Characterization Following NO Dosing 53 catalysts Single-site heterogeneous promise to combine the onic charge that is balanced by extra-lattice cations. After 4. In Situ and Operando Characterization attractive features of homogeneous 59 and heterogeneous catalysts: Cu ion exchange and high temperature oxidation treatment, 4.1 DFT Models of NH 3 Adsorption in Cu-SSZ-13 active sites of regular 59and tunable architecture that two isolated Cu site motifs are present: discrete Cu II ions integrated into a thermally-stable, porous solid 4.2 Selective NH 3 Titration of H + provide precise catalytic Sites in H-Form and Cu-Exchanged Zeolites function, 62 host that that balance two proximal Al centers and [Cu II OH] + ions facilitates access of substrates that balance single Al centers (10, 13, 14). Under low tem- to those sites and separation 66 of products from the perature (<523 K) standard SCR conditions, ammonia coor- 4.3 Vibrational Spectroscopy 4.4 Magnetic Spectroscopy With NH3 catalyst (1). In the conventional 70 definition, a single-site catalyst dinates to and liberates Cu ions from direct association with contains functionally isolated active sites, such that re- the zeolite support, and these solvated Cu ions act as the 4.5 X-Ray Spectroscopy With NH3 70 action rates per active site are independent of their spatial redox-active catalytic sites (15). At typical Cu ion volumetric 4.6 Operando X-Ray Spectroscopy proximity (2). Single metal 74atoms incorporated into solid densities, standard SCR rates increase linearly with Cu density, 5. Catalytic Activity and Mechanism oxide supports are reported 76to follow this conventional single-site as expected for a single-site catalyst. As shown below, 5.1 Differential Standard SCR Kinetics behavior in catalytic CO oxidation to CO2 (3 5), se- however, experimental observations in the low-cu-density 78 lective hydrogenation (6, 7), and water-gas shift (8, 9). Here, limit reveal a portion of the catalytic cycle in which activation by transiently formed Cu pairs becomes rate-limiting. O2 5.2 Standard SCR Mechanism Near 200 C we report that a nominally 83 single-site catalyst (10) operates by dynamic, reversible, and density-dependent (non-meanfield) These Cu pairs form from NH3-solvated Cu ions with mobili- interaction of multiple ionically tethered single sites, a ties restricted by electrostatic attraction to charge- behavior that lies outside the canonical definition of a single-site compensating framework Al centers, leading to catalytic 2016 Elsevier heterogeneous Inc. catalyst 1 (11). function that is neither single-site nor homogeneous. Advances in Catalysis, Volume 59 # ISSN AllWe rights discovered reserved. this phenomenon in the quest for a molecularly Recognizing the intermediacy of this distinct catalytic detailed model to unify the seemingly disparate state reconciles a number of controversies in Cu-zeolite SCR observations of the catalytic function of copper-exchanged catalysis, including the role of the zeolite support in the catalytic mechanism, the sensitivity of SCR rates to Cu density chabazite (Cu-CHA) zeolites, materials used in emissions control for the standard selective catalytic reduction (SCR) under different conditions of observation, the extent to of nitrogen oxides (NOx, x = 1, 2) with ammonia (12): which standard and the closely-related fast SCR cycles (16) 4NO+ O + 4NH 4N + 6H O (1) are connected via common intermediates, the chemical processes that limit low-temperature NOx SCR reactivity, and Chabazite is a small-pore zeolite composed of cages (8 the origins of the apparent change in mechanism at elevated 8 12 Å) interconnected by 6-membered-ring (6-MR) prisms temperatures. These observations provide insight into the and 8-membered ring (8-MR) windows (Fig. 1A). Substitution design of improved catalysts for SCR. More broadly, they of Si 4+ by Al 3+ within the framework introduces an ani- Downloaded from on August 18, 2017