Structure/Activity Relationships in Cu/CHA-Based NH 3 SCR Catalysts

Transcription

1 Structure/Activity Relationships in Cu/CHA-Based NH 3 SCR Catalysts Feng Gao, Yilin Wang, Márton Kollár, Eric Walter, János Szanyi, Chuck Peden Institute for Integrated Catalysis Pacific Northwest National Laboratory

2 Location for Cu in SSZ 13 catalysts at low Cu loadings Cu/SSZ-13 contains mononuclear Cu 2+ species, located in the face of the double-6-ring subunit of the zeolite after calcination where it remains under reaction conditions U Deka, A Juhin, EA Eilertsen, H Emerich, MA Green, ST Korhonen, BM Weckhuysen, AM Beale, J Phys Chem C 116 (12) 489. This location also identified in: DW Fickel, RF Lobo, J Phys Chem C 114 (1) J-S McEwen, T Anggara, WF Schneider, VF Kispersky, JT Miller, WN Delgass, FH Ribeiro, Catal Today 184 (12) 129. a = b = Å c = Å d = 3.8 Å Higher Cu loadings result in multiple Cu locations in the zeolite cages. 2

3 Effect of Cu Loading on the Reduction of Cu Species in Cu/SSZ-13 Zeolites Catalysts A 55K 23 o C 615K 3 o C 1% Cu 8% Cu 6% Cu % Cu % Cu Temperature( o C) JH Kwak, H Zhu, JH Lee, CHF Peden, J Szanyi, Chemical Communications 48 (12) At low loading, only a single H 2 TPR reduction peak at ~3 ºC. At higher loadings, a second TPR peak at ~23 ºC. Reduction of Cu + to Cu does not occur until T>8 ºC (data not shown). F Gao, ED Walter, EM Karp, J Luo, RG Tonkyn, JH Kwak, J Szanyi, CHF Peden, Journal of Catalysis 3 (13). 3

4 Absorbance Perturbation of Zeolite Framework Vibrations also Suggest Multiple Cu Sites Cu/% 6 1 A ~9 cm -1 ~9 cm ~9 cm Wavenumbers/cm -1 Absorbance B Wavenumbers/cm -1 H 2 reduction IR peak at ~9 cm -1 in T-O-T (Si-O-Si, Si- O-Al) region grows in with Cu loading. This ~9 cm -1 peak is removed first during H 2 reduction, followed by loss of ~899 cm -1 peak. Two Cu +2 locations as a function of loading are clearly evident in FTIR spectra after NO adsorption. EPR results also strongly suggest two Cu species as a function of loading. JH Kwak, H Zhu, JH Lee, CHF Peden, J Szanyi, Chemical Communications 48 (12)

5 A couple more significant open questions Cu-SSZ-13 % 1% H 2 O 21 o C % 6% 8% 1% Cu at low loadings is more difficult to reduce. In presence of 1% H 2 O, H 2 -TPR shows only low-temp peak for all Cu loadings. It appears Cu species may move under reaction conditions. Relevant to catalytic reactivity? Cu-SSZ o C 3 o C 1% Temperature( o C) JH Kwak, H Zhu, JH Lee, CHF Peden, J Szanyi, Chemical Communications 48 (12) % 6% % % Temperature( o C)

6 The Seagull Shape in Conversion-Temperature Curves Example: Standard SCR on Cu-SAPO-34 at a relatively high SV. 8 Cu-SAPO-34-TEA GHSV ~, h Reaction Temperature ( o C) Some industrial colleagues indicated that this phenomenon only occurs for Cu-SAPO-34. However, we found this same effect at lower Cu-SSZ-13 catalysts at lower Cu loadings. 6

7 Dependence of SCR Performance on Cu Loading for SSZ-13 Catalysts The Seagull shape in present for both Cu-SSZ-13 and Cu-SAPO-34 On high Cu loading samples (IE level > 6%), the seagull flies away Cu-SSZ-13 (Si/Al ~ 6) Cu Loading (wt%) Reaction Temperature ( o C) F Gao, ED Walter, M Kollár, Y Wang, J Szanyi, CHF Peden, Journal of Catalysis, submitted. What might be responsible for the drop in reactivity between 25 and 35 C? 7

8 Behind the Seagull Shape Key questions: 1. Is this caused by mass transfer limitations, or reactant or product inhibitions? NO 2. Is this caused by changes in reaction mechanisms as a function of reaction temperature? 3. Or is this intrinsically correlated with changes in the nature and/or location of the catalytically active centers? 8

9 SSZ-13 with very low Cu loadings Why very low Cu loadings? (1) Hope to minimize mass transfer limitations. Si Si Si Al Si Si (2) Simplify the catalysts: eliminate Cu-Cu interactions, eliminate multiple Cu positioning. (3) Verify that we still see the seagull shape. Cu Si Cu loadings determined with EPR using standard solutions of Cu(II)-imidazole. Cu-SSZ-13 catalysts Signal Intensity (a.u.) 1.31 wt% 5.15 wt% Field (G) 9

10 Seagull shape especially evident at low Cu loaded SSZ-13 catalysts BG Cu Loading (wt%) Temperature ( o C) Remove BG 8 6 Cu (wt%) Temperature ( o C) Considering just temperatures below 3 ºC: Why conversions drops from 25 to 3 ºC? Why large increase in conversions above 35 ºC? F Gao, ED Walter, M Kollár, Y Wang, J Szanyi, CHF Peden, Journal of Catalysis, submitted. 1

11 Behind the Seagull Shape Key questions: 1. Is this caused by mass transfer limitations, or reactant or product inhibitions? NO 2. Is this caused by changes in reaction mechanisms as a function of reaction temperature? 3. Or is this intrinsically correlated with changes in the nature and/or location of the catalytically active centers? 11

12 Low-Temperature Reaction Kinetics Does the reaction mechanism change from 25 to 3 ºC? If yes, we might expect some change in the power-law pressure dependence Temp. ( o C) Temp. ( o C) NO input (ppm) In this temperature range: 1 What about changes in the nature of Cu? Recall that Cu is more readily reduced in the presence of water NH 3 Input (ppm) F Gao, ED Walter, M Kollár, Y Wang, J Szanyi, CHF Peden, Journal of Catalysis, submitted. 12

13 Behind the Seagull Shape Key questions: 1. Is this caused by mass transfer limitations, or reactant or product inhibitions? NO 2. Is this caused by changes in reaction mechanisms as a function of reaction temperature? NO 3. Or is this intrinsically correlated with changes in the nature and/or location of the catalytically active centers? FTIR, Synchrotron XRD, and EPR studies 13

14 Two Perturbed Zeolite Framework Vibrations also Suggest Multiple Cu Sites Absorbance Cu/% A ~9 cm ~9 cm Wavenumbers/cm -1 These spectra were for catalysts that had been oxidized and cooled in O 2 /N 2 gas mixtures. What do the spectra look like for an 8% Cu-exchanged catalyst when when water is also present? JH Kwak, H Zhu, JH Lee, CHF Peden, J Szanyi, Chemical Communications 48 (12)

15 FTIR Clearly Shows Loss of Water During Calcination to ºC 36 2 cm cm -1 Absorbance/a.u a T calc. / o C Absorbance/a.u b. 163 T calc. / o C Wavenumbers/cm Wavenumbers/cm -1 Molecular water fully desorbed above 2 ºC Loss of water brings about significant changes to the zeolite T-O-T vibrations. JH Kwak, T Varga, CHF Peden, F Gao, JC Hanson, J Szanyi, Journal of Catalysis (14) in press. 15

16 FTIR and Synchrotron XRD document Cu Movement During Dehydration c. ~9 cm ~9 cm Absorbance/a.u T calc. / o C Wavenumbers/cm -1 Changes in the framework vibrations caused by Cu ion relocation towards the zeolite framework JH Kwak, T Varga, CHF Peden, F Gao, JC Hanson, J Szanyi, Journal of Catalysis (14) in press. a/å a a Rietveld c c Rietveld Temperature/ o C CHA unit cell parameter change caused by dehydration and subsequent Cu ion movement EPR data also evidence these changes c/å

17 Behind the Seagull Shape Key questions: 1. Is this caused by mass transfer limitations, or reactant or product inhibitions? NO 2. Is this caused by changes in reaction mechanisms as a function of reaction temperature? NO 3. Or is this intrinsically correlated with changes in the nature and/or location of the catalytically active centers? Likely YES So, what is the nature and structure of the active and hydrated Cu species? 17

18 Results for low Cu-SSZ-13 catalysts indicate reactivity very sensitive to loading BG Cu Loading (wt%) Temperature ( o C) Convert NOx conversions to TOFs based on total Cu. Drop in TOFs at higher loadings due to the intra-particle diffusion limitations Why do TOFs increase for the very low Cu loaded catalysts? F Gao, ED Walter, M Kollár, Y Wang, J Szanyi, CHF Peden, Journal of Catalysis, submitted. TOF (mole NO/mole Cu 2+ /s) Cu-SSZ-13 (Si/Al = 6) Standard NH 3 -SCR, 423 K Cu Loading (wt%) 18

19 Experimental TOFs difficult to explain by only considering Cu ion monomers o C o C 225 o C 25 o C o C o C 225 o C 25 o C 1 st 2 nd Cu Loading (wt%) o C o C 225 o C 25 o C (Cu Loading) 2 3 rd 4 th (Cu Loading) 3 (Cu Loading) o C o C 225 o C 25 o C Low T NOx conversions versus Cu loading best fit by using the square of the Cu loading! F Gao, ED Walter, M Kollár, Y Wang, J Szanyi, CHF Peden, Journal of Catalysis, submitted. 19

20 u2reaction Temperature ( C)CCumodel for active Cu structures as a function of temperature +O2+Cu <25oresults suggest the following 2+Cu2+[O]>35oCCuCuCOur O F Gao, ED Walter, M Kollár, Y Wang, J Szanyi, CHF Peden, Journal of Catalysis, submitted. At temperatures < 25 ºC, water and/or ammonia mobilizes Cu leading to formation of highly active dimeric species. At higher temperatures, coordinated water and/or ammonia desorbs, Cu dimers dissociate, and monomeric Cu ion migrate back to zeolitic ion exchange sites.

21 Summary and Conclusions Cu/SSZ-13 catalysts display outstanding performance for NH 3 SCR relative to other Cu/zeolite-based catalysts, including considerably lower sensitivity to high-temperature hydrothermal aging. The nature of the active Cu site in CHA zeolites is an active area of research. Two different Cu species within Cu/SSZ-13 were identified by H 2 TPR, EPR and FTIR. The relative distribution of Cu species is dependent on Cu loading levels and reaction conditions. Dimeric Cu species appear to be the active Cu species at lower (<25 ºC) reaction temperatures where significant amounts of adsorbed water are present. The mechanism of NH 3 SCR may involve the facile reduction of reactive Cu +2 species by NO. 21

22 Acknowledgments U. S. DOE, Office of Energy Efficiency and Renewable Energy/Vehicle Technologies Program Ja Hun Kwak, Nancy Washton and Haiyang Zhu, Institute for Integrated Catalysis, PNNL Experiments performed in DOE/BER s Environmental Molecular Sciences Laboratory located at PNNL 22