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Physical and Computational Sciences Directorate Using in situ electron paramagnetic resonance (EPR) spectroscopy to probe reactivity and relocation of isolated Cu(II) active sites in Cu/SSZ-13 selective catalytic reduction (SCR) catalysts CLEERS Workshop, 09/16/2020 Yani Zhang, Yue Peng, Junhua Li, Kyle Groden, Jean-Sabin McEwen, Eric D. Walter, Ying Chen, Nancy M. Washton, Janos Szanyi, Yong Wang, Feng Gao Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA 99352, USA Introduction 0: Using EPR to study Cu/chabazite ► EPR can be applied to: ► Quantify the amount of isolated Cu(II) species, i.e., the SCR active sites, in Cu/zeolites. ► To gain information on coordination and local environments of isolated Cu(II) ions (EPR parameter analysis). ► EPR characteristics for Cu analysis. ► Only isolated Cu(II) ions can be detected. CuxOy clusters and CuO particles are EPR silent; Cu(I) ions are EPR silent. ► Isolated Cu(II) ions are not always EPR active. Signal loss when they are highly mobile and smear each other. Signal loss when excited states relax too rapidly to monitor. O Si O Si Al Si O Al O O Si Si 2+ Si Cu O O 2+ O O O Cu 2+ O Z Cu Si Si 2 Si Al Al Cu OH ZCuOH O O Al O O O O O Si Si Si Si Si O Si O 2 Introduction 1: EPR signals we couldn’t explain before Hydrated Cu/SSZ-13, Si/Al = 12, Cu loading 2.1% Dehydrated Cu/SSZ-13, Si/Al = 12, Cu loading 2.1% Fresh Fresh HTA-550 HTA-550 HTA-600 HTA-600 HTA-650 HTA-650 HTA-700 HTA-700 HTA-750 HTA-750 HTA-800 HTA-800 HTA-900 HTA-900 2400 2600 2800 3000 3200 3400 3600 2400 2600 2800 3000 3200 3400 3600 Field (G) Field (G) Weak hyperfine region and high field EPR signals only appear in hydrothermally aged and dehydrated Cu/SSZ-13 catalysts. J. Song, Y.L. Wang, E.D. Walter, N.M. Washton, D. Mei, L. Kovarik, M.H. Engelhard, S. Prodinger, Y. Wang, C.H.F. Peden, F. Gao, ACS Catalysis, 2017, 7: 8214-8227. 3 Introduction 2: in situ (operando) EPR during SCR https://en.wikipedia.org/wiki/Operando_spectroscopy Operando spectroscopy is an analytical methodology wherein the spectroscopic characterization of materials undergoing reaction is coupled simultaneously with measurement of catalytic activity and selectivity.[1] The primary concern of this methodology is to establish structure-reactivity/selectivity relationships of catalysts and thereby yield information about mechanisms. Other uses include those in engineering improvements to existing catalytic materials and processes and in developing new ones.[2] 1. Bañares, M. A. (2002). "Raman spectroscopy during catalytic operations with on-line activity measurement (operando spectroscopy): a method for understanding the active centres of cations supported on porous materials". Journal of Materials Chemistry. 12 (11): 3337-3342. doi:10.1039/b204494c. 2. "Operando Group Welcomes You". www.lehigh.edu. Retrieved 2019-09-26. ► Pulsed 2-dimensional electron paramagnetic resonance (EPR) techniques, for example hyperfine sublevel correlation spectroscopy (HYSCORE) studies at liquid helium temperature, are now routinely available to study SCR catalysts. Such techniques provide unprecedented details on local environments of the Cu active sites. ► Newly established in situ EPR capability to better understand the nature of Cu(II) species under chemical titration and SCR reaction conditions. 4 Model Cu/SSZ-13, fresh and mildly hydrothermally aged Cu/SSZ-13 via solution ion exchange: Si/Al = 6, Cu/Al = 0.1 EPR TPR ► Mild hydrothermal aging (600 °C, 20 h) converts some ZCuOH to Z2Cu, but generates no CuxOy clusters, and causes no loss of active sites, or degradation of the support. ► The unexplained weak EPR signals reproduced after catalyst hydrothermal aging and dehydration. 5 Nature of the weak, unusual EPR signals ► These split EPR signals are formed as a result of interactions between two isolated Cu(II) ions. ► The fact that they are not present in dehydrated fresh samples or any hydrated samples suggests that Cu-Cu D distance plays a decisive role for their appearance; only paired isolated Cu(II) sites spatially positioned within a relatively short distance give rise to these unusual split EPR signals. ► Distance between two unpaired electrons from EPR powder spectra can be estimated via = = . × . ► r = 3.90 Å based on EPR measurements. Eaton, S. S.; More, K. M.; Sawant, B. M.; Eaton, G. R., Use of the EPR half-field transition to determine the interspin distance and the orientation of the interspin vector in systems with two unpaired electrons. J. Am. Chem. Soc. 1983, 105 (22), 6560-6567. 6 Nature of the weak, unusual EPR signals ► DFT calculations conducted by Kyle Groden and Prof. Jean-Sabin McEwen, WSU. ► The unusual EPR signals come from interactions between isolated Z2Cu ions within the same double 6-membered ring prism. ► It is readily understood why they are not present in fresh catalysts, or hydrated samples. ► Such structures cannot form during ion exchange; Cu ions tend to stay away from one another as much as possible to lower repulsive electrostatic interactions. ► In hydrated samples, such structures are not present due to solvation effects. 7 Why hydrothermal aging is needed ► But why their presence also requires the catalyst being hydrothermally aged? ► The ZCuOH + ZH → Z2Cu + H2O reaction must occur to form such structures. ► From DFT, the reaction heat is low for this reaction. ► We hypothesize that activation barriers for this reaction must be very high under “dry” conditions; hydrothermal aging facilitates this reaction, e.g., by facilitating proton transport. 8 The formation of the unusual structure is not all what’s happened during hydrothermal aging Standard SCR: 4NO + 4NH3 + O2 = 4N2 + 6H2O ) -1 100 s -1 Fresh HTA -5 80 10 Fresh E = 46.1±1.2 kJ/mol HTA a 60 40 -6 E = 40.2±0.7 kJ/mol 10 a 20 NOx Conversion (%) Conversion NOx 0 NOx Conversion Rate (mole g (mole Rate Conversion NOx 100 200 300 400 500 600 2.2 2.3 2.4 2.5 2.6 2.7 Reaction Temperature (°C) 1000/T (K-1) ► The unusual structures (2 Cu(II) ions in one double 6-membered ring prism) comprise only a few percent of all Cu(II) in the catalyst. ► However, low-temperature SCR rates dropped to a half after hydrothermal aging (without loss of active sites and support integrity). 9 NH3 titration of Cu(II) sites as probed with EPR superhyperfine structures of 14N in Cu-NH3 complexes. Much stronger high-field superhyperfine structures for the HTA sample demonstrates lower mobility of the Cu-NH3 complexes. Stay closer to the framework, motion restricted. 10 In situ EPR during standard SCR Fresh, steady-state HTA, steady state g = 2.26, A = 176G g// = 2.25, A = 172G // g// = 2.35, A = 135G g// = 2.35, A = 142G o o o o 350 C 300 C 350 C 300 C o o 280 o C 260 o C 280 C 260 C ~3245 o o o o ~3260 240 C 220 C ~3245 Intensity Signal 240 C 220 C Signal Intensity Signal 200 o C 180 o C 200 o C 180 o C o o o o 160 C 140 C 160 C 140 C o o o o 120 C 100 C ~3300 120 C 100 C ~3320 2400 2600 2800 3000 3200 3400 3600 2400 2600 2800 3000 3200 3400 3600 Field (G) Field (G) ► In situ EPR spectra under steady-state standard NH3-SCR. ► At all reaction temperatures, Cu(II) ions in the HTA catalyst are less mobile. 11 In situ EPR during standard SCR Cu(II) Reduction half-cycle NO + NH3 NO + O2 oxidation Cu(I) half-cycle ► Higher concentrations of Cu(II) detected over the HTA catalyst under identical reaction conditions. ► Hydrothermal aging slows down the “reduction half-cycle”: Cu(II) less reactive after aging. 12 Summary EPR, kinetic measurements, and DFT combine to give fundamental insights into the deactivation mechanism of a Cu/SSZ-13 SCR catalyst under mild hydrothermal ageing conditions. Hydrothermal treatment results in stronger Cu–zeolite interactions, including relocation of Cu ions. Such structural changes slow down the reduction half cycle of the SCR reaction. Zhang, Y., Y. Peng, J.H. Li, K. Groden, J.-S. McEwen, E. D. Walter, Y. Chen, Y. Wang, F. Gao, Probing Active-Site Relocation in Cu/SSZ-13 SCR Catalysts during Hydrothermal Aging by in situ EPR Spectroscopy, Kinetic Studies, and DFT Calculations. ACS Catalysis, 2020, 10, 9410-9419. Doi: 10.1021/acscatal.0c01590 13 In situ EPR for quantifying isolated Cu ions EPR active EPR active EPR active EPR silent EPR silent J. Song, Y.L. Wang, E.D. Walter, N.M. Washton, D. Mei, L. Kovarik, M.H. Engelhard, S. Prodinger, Y. Wang, C.H.F. Peden, F. Gao, ACS Catalysis, 2017, 7: 8214-8227. 14 In situ EPR for quantifying isolated Cu ions EPR active EPR active EPR silent EPR spectroscopy, coupled with hydration/dehydration, NH3 titrations, and NO+O2 titrations can be used to precisely quantify all isolated Cu EPR silent EPR active EPR silent species in Cu/SSZ-13 catalysts. EPR active Yani Zhang, Yiqing Wu, Yue Peng, Junhua Li, Eric D. Walter, Ying Chen, Nancy M. Washton, Janos Szanyi, Yong Wang, Feng Gao, Quantitative Cu counting methodologies for Cu/SSZ-13 selective catalytic reduction catalysts by EPR spectroscopy, submitted to JPCC. 15 Acknowledgements • Authors from PNNL acknowledge DOE Office of Energy Efficiency and Renewable Energy/Vehicle Technologies Office for the financial support of this work (managers Ken Howden and Gurpreet Singh). • Research conducted partially using DOE user facility, EMSL, located at PNNL.
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