Deactivation of a Cu- CHA NH 3 -SCR catalyst by SO 2 and SO 3

Transcription

1 Deactivation of a Cu- CHA NH 3 -SCR catalyst by SO 2 and SO CLEERS workshop October 217 Peter S. Hammershøi, Yasser Jangjou, William S. Epling, Anker D. Jensen, Ton V.W. Janssens Submitted to Appl. Catal. B (217).

2 Thanks to collaborators! University of Houston/University of Virginia Prof. William S. Epling PhD student Yasser Jangjou Technical University of Denmark Prof. Anker D. Jensen Haldor Topsøe A/S Research Scientist Ton V.W. Janssens Research Scientist Peter N.R. Vennestrøm Innovation Fund Denmark 2

3 The Diesel Exhaust System and NH 3 -SCR Diesel exhaust gas contains NO x (NO and NO 2 ) and SO 2 (1-2 ppm) NO x is removed by selective catalytic reduction (SCR) with NH 3 Std. SCR: Fast SCR: 4NO + 4NH 3 + O 2 4N 2 + 6H 2 O NO + NO 2 + 2NH 3 2N 2 + 3H 2 O Catalysts for automotive NH 3 -SCR: Vanadia-based, Fe- or Cu-Zeolites 3 Different sensitivity to SO 2 -poisoning!

4 Motivation and aim SO 2 is always present in the exhaust gas. Therefore, SO 2 -poisoning must be controlled either by catalyst engineering or regeneration strategies! This is improved by a better understanding of the SO 2 -poisoning mechanism(s) of Cu- CHA catalysts. We aim to elucidate on the SO 2 deactivation mechanism(s) by testing the impact of three relevant parameters on the deactivation: Temperature 2 C and 55 C Presence/absence of H 2 O Presence/absence of SO 3 4

5 Cu-CHA/Cordierite monolith catalyst and measurements Cu-CHA/Cordierite monolith: Si/Al = 16.6, 2.5 wt% Cu 3 cpsi Cylindrical 21x27 mm (DxL) measurements: Feed gas: 5 ppmv NO 53 ppmv NH 3 1 % O 2 5 % H 2 O N 2 balance to 8 SLPM GHSV: 5 kh -1 Temperature range: C SO X exposure conditions: 1 % O 2 N 2 balance to 8 SLPM SO 2 /SO 3 content: 1 ppmv SO 2 7 ppmv SO ppmv SO 3 H 2 O: 5 % H 2 O No H 2 O Temperature: 2 C 55 C 5

6 Evaluation of activity and deactivation Measure NO x conversion Assume SCR reaction is 1 st order in NO x Calculate rate constant as: k = F W ln 1 X Deactivation is defined as: 1 k treated k fresh 6

7 Typical SO 2 -poisoning behaviour of Cu-CHA - powder Fresh SO 2 exposure Sulfated Regeneration 55 C Regenerated NO x conversion [%] Fresh Sulfated Regenerated Two forms of deactivation below 55 C Total = Reversible + Irreversible Temperature [ C] 7

8 Decoupling of reversible and irreversible deactivation and temperature range for kinetic evaluation Cu-CHA powder NO x conversion [%] Temperature [ C] Fresh Sulfated Regenerated 1. 4 h of regeneration at 55 C is required to efficiently decouple reversible and irreversible deactivation. 2. Below 3 C SCR kinetics are not influenced by regeneration Relative 2 C NO x 2 C [%] Regeneration time [h] Regeneration Temperature [ C]

9 Deactivation by SO 2 and SO 3 Experiments Fresh SO X exposure Sulfated Heating: SO 2 -TPD 55 C Regeneration 55 C Regenerated SO 2 -TPD 9 C 1 C/min SCR gas 4 h SCR gas 1 C/min N 2 1 ppmv SO 2 7 ppmv SO ppmv SO 3 5 % H 2 O No H 2 O 2 C 55 C 9

10 Deactivation from measurements of fresh, sulfated and regenerated samples Fresh SO X exposure Sulfated S-exposure: SO 2 +H 2 O@2 C Temperature [ C] Fresh SO 2 -TPD 55 C Regeneration Deac. = 1 k treated k fresh = 1 Regenerated SO 2 -TPD 9 C A treated exp E a,treated R A fresh exp E a,fresh R 1 ln(k) Sulfated Regenerated /T [K -1 ] Different E a s of Sulfated and Fresh Different SCR mechanism over sulfated sample? Or diffusion limitations? Similar E a s of Regenerated and Fresh Consistent with irreversible deac. due to loss of sites Consequently: Reversible deac. dependent on temperature of activity measurement Irreversible deac. independent on temperature of activity measurement

11 Estimation of reversible S content SO 2 -TPD Fresh SO X exposure Sulfated SO 2 -TPD 55 C Regeneration Regenerated SO 2 -TPD 9 C SO 2 concentration [ppm] Temperature SO3-H2O-2 SO3-2 Reversible S-species Temperature [ C] Samples exposed to SO 3 at 2 C stand out: High SO 2 desorption at 4 C decomposition of SO 3 /sulfates not consistent with CuSO 4 decomposition Time [min]

12 Estimation of irreversible S content SO 2 -TPD Fresh SO X exposure Sulfated SO 2 -TPD 55 C Regeneration Regenerated SO 2 -TPD 9 C Irreversible S-species SO 2 concentration [ppm] SO3-H2O-2 SO3-2 Temperature Temperature [ C] Desorption peaks at 65-7 C Same irreversible S-species consistent with CuSO 4 decomposition! Samples exposed to SO 3 at 2 C show highest SO 2 desorption ICP: No residual S after heating at 9 C Time [min]

13 Total deactivation (18 C) [%] Deactivation mechanisms Deactivation vs SO 2 /Cu ratio Reversible deactivation SO Total molar S/Cu [%] SO3-H2O % S/Cu leads to surprisingly high degrees of deactivation! Irreversible deactivation [%] SO2-2 SO2-55 SO3-55 Irreversible deactivation SO2-H2O-2 SO3-H2O-55 SO2-H2O-55 SO3-H2O-2 SO Irreversible molar S/Cu [%] Better 1:1 correlation between irreversible S-species and irreversible deactivation Formation of CuSO 4 -like species? 13

14 Reversible deactivation internal diffusion limitations? E a,sulf /E a,fresh.5 can be indication of internal diffusion limitations Does the introduction of SO x lead to internal diffusion limitations for SCR? SO x treatment E a,sulf /E a,fresh SO2+H2O-2C SO2-2C SO2+H2O-55C SO2-55C SO3+H2O-2C SO3-2C Effectiveness factor ( ) (D eff = m 2 /s) SO3+H2O-55C SO3-55C D e [m 2 /s] η <.8 requires that the effective diffusion coefficient drops by about a factor 1 14

15 Reversible deactivation internal diffusion limitations? Deactivation vs SO 2 /Cu ratio Reversible deactivation Total deactivation (18 C) [%] SO3-2 SO3-H2O-2 Effectiveness factor ( ) (D eff = m 2 /s) Total molar S/Cu [%] D e [m 2 /s] If the reversible deactivation is due to internal diffusion limitations, only 1-2 % S/Cu has to infer a drop in the effective diffusion coefficient by a factor 1! 15

16 Reversible deactivation internal diffusion limitations? Sulfur distribution in zeolite crystals Si/Al = 16.6 and Cu/Al =.5 corresponds to approx. 1 Cu / unit cell If S/Cu = 2 % and S is homogeneously distributed: 1 S / 5 unit cells Situation with S predominantly in surface layers Not likely that this will affect the effective diffusion coefficient so dramatically If S is predominantly present in the surface layers it could result in a higher impact on the effective diffusion coefficient 16

17 Reversible deactivation internal diffusion limitations? STEM-EDS results from similar Cu-CHA with S/Cu = 28 % Elemental quantification shows that S is homogeneously distributed in zeolite crystals S Therefore, we believe it is unlikely that the reversible deactivation is due to internal diffusion limitations. 17

18 Impact of H 2 O, SO 3 and temperature on the deactivation Impact of H 2 O Reversible (18 C), Irreversible 1 SO 2-2 C 1 SO 2-55 C 1 SO 3-2 C 1 SO 3-55 C Wet Dry Wet Dry Wet Dry Wet Dry H 2 O always results in higher irreversible deactivation Z-CuOH + SO 3 Z-CuHSO 4 18

19 Impact of H 2 O, SO 3 and temperature on the deactivation Impact of SO 3 Reversible (18 C), Irreversible 1 H 2 O-2 C 1 Dry-2 C 1 H 2 O-55 C 1 Dry-55 C SO 2 SO 3 SO 2 SO 3 SO 2 SO 3 SO 2 SO 3 SO 3 presence at 2 C enhances deactivation significantly At 55 C deactivation by SO 2 and SO 3 are more alike SO 3 decomposition around 4 C 19

20 Impact of H 2 O, SO 3 and temperature on the deactivation Impact of temperature Reversible (18 C), Irreversible 1 SO 2 +H 2 O 1 SO 3 +H 2 O 1 SO 2 -Dry 1 SO 3 -Dry C 55 C 2 C 55 C 2 C 55 C 2 C 55 C Irreversible deactivation is always highest at 2 C compared to 55 C Higher SO x coverage at 2 C? 2

21 Conclusions 1-2 % S/Cu leads to high degrees of reversible deactivation Reversible deactivation is not likely to arise from internal diffusion limitations Irreversible deactivation by CuSO 4 -like species H 2 O always results in higher irreversible deactivation SO 3 presence at 2 C enhances deactivation significantly At 55 C deactivation by SO 2 and SO 3 are more alike Irreversible deactivation is always highest at 2 C compared to 55 C 21

22 22 Thank you for your kind attention! - questions are welcome