Supporting Information

Transcription

1 Supporting Information Towards rational design of Cu/SSZ-13 selective catalytic reduction catalysts: implications from atomic-level understanding of hydrothermal stability James Song, a,c Yilin Wang, a Eric D. Walter, b Nancy M. Washton, b Donghai Mei, a, * Libor Kovarik, b Mark H. Engelhard, b Sebastian Prodinger, a Yong Wang, a,c Charles H.F. Peden, a Feng Gao a, * a Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99354, United States b Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99354, United States c The Gene & Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, P.O. Box , Pullman, WA 99164, United States *Corresponding authors: feng.gao@pnnl.gov (F. Gao); Donghai Mei (donghai.mei@pnnl.gov) S1

2 S1 H 2 Consumption (a.u.) H 2 -TPR Si/Al = 6, Cu/Al = 0.11 Si/Al = 12, Cu/Al = 0.13 Si/Al = 35, Cu/Al = Temperature ( C) Figure S1 Hydrogen temperature-programmed reduction (H 2 -TPR) profiles for three model Cu/SSZ-13 catalysts with varying Si/Al and Cu/Al ratios. Replotted from Reference [2]. H 2 -TPR profiles in Figure S1 demonstrate that Cu(II) ions reduce at different temperatures. Based on our previous studies, Cu 2+ -2Z sites reduce at ~400 C and [Cu(OH)] + -Z sites reduce at ~250 C. 1, 2 This graph demonstrates clearly that the nature of the isolated Cu-ion species can be controlled by varying Si/Al and Cu/Al ratios; in particular, at low Si/Al and low Cu/Al conditions, Cu 2+ -2Z sites predominate while at high Si/Al ratios, the primary Cu-ion species are [Cu(OH)] + -Z sites. At intermediate Si/Al and Cu/Al ratios, both types of Cu sites are present in the same catalyst. The assignment of H 2 -TPR features to specific Cu(II) sites is based on two factors: (1) Cu 2+ -2Z sites are thermodynamically more stable than [Cu(OH)] + -Z; 3 and (2) paired framework Al sites are rare at high Si/Al (e.g., 35) ratios. S2

3 S2 NO x /NH 3 Conversion (%) NO x, Fresh NH 3, Fresh NO x, HTA-800 NH 3, HTA Reaction Temperature ( C) (a) Figure S2 (a) NO x and NH 3 light-off curves for standard SCR on fresh and HTA-800 Cu/SSZ-13 samples (Si/Al = 12, Cu loading = 2.1%). (b) NO x light-off curve for standard SCR on a BASF commercial sample hydrothermally aged at 800 o C for 16 h. (replotted from Ref. [4]). NO x Conversion (%) BASF catalyst, hydrothermally aged at 800 C for 16 h GHSV ~ 30,000 h -1 (washcoated form) Reaction Temperature ( C) (b) Figure S2(a), collected at a GHSV of ~200, 000 h -1, shows that low temperature activity of the catalyst decreases after hydrothermal aging (800 C, 16 h), due largely to loss of active sites during aging. Furthermore, high temperature selectivity decreases after hydrothermal aging as a result of CuO x formaiton. Figure S2(b) presents standard SCR performance of a BASF commercial catalyst that experienced hydrothermal aging at 800 C for 16 h. 4 The detrimental effects of HTA on high-temperature selectivity is even more pronounced for this catalyst, indicating even more CuO x formation. S3

4 S3 (a) (b) Figure S3 NH 3 oxidation reaction results for the fresh and HTA samples under (a) high space velocity conditions of ~650,000 h -1, and (b) a relatively low space velocity of ~200,000 h -1. The feed gas contained 360 ppm NH 3, 14% O 2, ~2.5% H 2 O, and balanced with N 2. In the low temperature range (200 to 400 C), the general overall trend is that NH 3 oxidation activity decreases with increasing HTA temperature. The activity decrease is due to (1) the loss of active isolated Cu(II) sites during aging, and (2) a decrease in NH 3 storage capacity with aging, which lowers NH 3 residence time. Even though CuO x forms during hydrothermal aging, these moieties are only active in NH 3 oxidation above ~400 C. However, at reaction temperatures above ~400 C, the heavily aged HTA-800/900 samples display higher activity than mildly aged HTA-700/750 ones. This indicates that the nature of the CuO x species experienced a rather dramatic change when the aging temperature was raised to 800 C and above. From reaction results shown in Figure S3(a) and EPR quantification displayed in Figure 6, it can be concluded that [Cu(OH)] + -Z is much more active than Cu 2+ -2Z at reaction temperatures 325 C, and CuO x sites are largely inert. At 400 C and above, as shown in Figure S3(b), NH 3 oxidation activity for HTA-800 becomes much higher than HTA-750. Note that the quantities of CuO x in these two samples are very similar (Figure 6). This indicates that the nature of CuO x changes significantly when the aging temperature increases from 750 to 800 C. Based on NMR, EPR, and reaction results, we postulate here that EFAl primarily resides at CuO x surfaces at hydrothermal aging temperatures of 750 C and lower, and interacts with the latter through an S4

5 acid-base type of reaction; i.e., bonding between Lewis acidic Al 3+ and Lewis basic center bridging O from Cu-O-Cu in CuO x. Such an interaction conceivably can passivate the NH 3 oxidization reactivity of the CuO x clusters. At higher aging temperatures, this passivation is removed in some way (e.g., changes in coordination environments for Cu due to changes in CuO x -EFAl interactions at such high temperatures), resulting in increased oxidation reactivities. Without further (spectroscopic) evidence, however, no more details on the (Al containing) CuO x structures can be given at this time. S5

6 S4 HTA-900 Intensity (a.u.) HTA-800 HTA-750 HTA-700 HTA-650 HTA θ (deg) Figure S4 Powder X-ray diffraction (XRD) patterns of the HTA samples obtained with a Philips PW3040/00 X Pert powder X-ray diffractometer with Cu K α radiation. Table S1 BET surface areas (m 2 /g) and t-plot micro pore volumes (cm 3 /g) of the fresh and HTA samples measured with a Quantachrome AutoSorb-6 via N 2 absorption. Sample Surface Area (m 2 /g) Micro Pore Volume (cm 3 /g) Fresh HTA HTA HTA HTA HTA HTA HTA S6

7 S V ads (cm 3 /g) Parent 700 ºC HTA 800 ºC HTA p/p 0 Figure S5 Adsorption isotherms for selected samples (fresh, HTA-700, and HTA-800) obtained using Ar on a Micromeritics ASAP XRD as a textural property characterization method is not sensitive enough to reveal any noteworthy differences caused by HTA. BET surface areas and pore size distributions also show relatively insignificant differences between the fresh and harshly aged samples. However, utilizing Ar as a probe for obtaining absorption isotherms does reveal clear differences in mesoporosity and microporosity. Utilizing Ar instead of N 2 allows for more sensitive and accurate determination of micro- and meso-pore size distributions. The isotherm results show mesopore formation during HTA, but to different extents depending on aging temperature. Secondary mesopore formation of the HTA-700 sample does not appear to affect primary micropore volumes while this is not the case for HTA-800. These results can most likely be attributed to the differences in the size of the CuO x agglomerates that diffuse through the pore windows; the CuO x clusters formed during HTA at 800 C are large enough to create mesoporosity while, correspondingly, destroying the microporous structure. S7

8 S6 Table S2 Calculated Cu species distributions for the fresh and HTA samples from results of EPR experiments on hydrated and dehydrated samples, and, for CuOx wt%, comparisons with ICP elemental analyses. Sample EPR active Cu (wt%), hydrated EPR active Cu (wt%), dehydrated Cu 2+ -2Z (wt.%) [Cu(OH)] + -Z (wt.%) CuOx (wt.%) Fresh HTA HTA HTA HTA HTA HTA HTA In fully hydrated ambient Cu/SSZ-13 samples, isolated Cu ions maintain a +2 oxidation state making them EPR visible. CuO x is EPR silent and, thus, wt% of CuO x can be estimated from the difference between Cu wt% of the fresh sample as obtained by ICP, and the EPR active Cu wt% as obtained by EPR for the range of hydrated samples. Under dehydrated conditions, Cu 2+ -2Z remains EPR active. On the other hand, [Cu(OH)] + becomes EPR silent for various possible reasons, including a pseudo Jahn-Teller effect (as described in main text). Thus, the difference between the wt% of EPR active Cu in the hydrated state and dehydrated state gives the wt% of [Cu(OH)] + species in the samples. It is evident that Cu 2+ -2Z content reaches a maximum with the HTA-700 sample, and [Cu(OH)] + -Z content gradually decreases while CuO x content gradually increases with increasing HTA severity. S8

9 S7 In this experiment, a NH 4 /SSZ-13 sample (Si/Al = 12) was first ion exchanged with 0.1M CuSO 4 solution to generate a Cu/SSZ-13 sample with a high Cu loading (3.3% via ICP). Then this sample was hydrothermally aged at 600 C for 10, 20, 30, and 60 h. Based on NMR studies shown in the main text, negligible dealumination occurs at this temperature. Thereafter, the fresh and HTA samples were ion-exchanged with a 0.15M Co(NO 3 ) 2 solution at room temperature while stirring for 24 h and repeated once to make sure all Cu 2+ -2Z sites are replaced by Co 2+ -2Z. Then ICP was conducted to quantify Co exchanged into the samples. The results are displayed in Table S7. Table S3 Co contents in fresh and HTA Cu/SSZ-13 fully exchanged with Co(NO 3 ) 2. Sample Co wt% via ICP Fresh 1.88 HTA, 10 h 1.64 HTA, 20 h 1.59 HTA, 30 h 1.49 HTA, 60 h 1.47 Since Co 2+ can only be present as Co 2+ -2Z but not [Co(OH)] + -Z, 3 were there Al redistribution and pairing during Cu/SSZ-13 hydrothermal aging at 600 C (a temperature low enough so that dealumination essentially does not occur), then the aged samples should accommodate more Co 2+ ions during the subsequent exchange with Co(NO 3 ) 2. Table S3 clearly demonstrates that the fresh sample accommodates more Co than aged ones, and Co loading decreases with aging duration. This titration experiment, therefore, confirms that framework Al redistribution does not occur during hydrothermal aging. This in turn indicates that [Cu(OH)] + -Z conversion to Cu 2+ -2Z is not due to Al redistribution during hydrothermal aging, but rather to Cu redistribution. S9

10 S8 (a) (b) S10

11 (c) (d) Figure S6 (a-d) Additional STEM images (HAADF and elemental mapping images for Cu, O, Al, Si, and Al+Si) of the fresh Cu/SSZ-13 catalyst. All images depict a high degree of homogeneity for all elements (e.g. Cu, Al, etc.) in the fresh sample, which is consistent with an assumption that all framework T sites and Cu are randomly distributed within the particles. Images in the main text are representative of all images acquired. S11

12 S9 (a) (b) S12

13 (c) Figure S7 (a-c) Additional STEM images (HAADF and elemental mapping images for Cu, O, Al, and Si) of the HTA-800 sample. CuO x particles were observed quite infrequently. All elemental species remain relatively uniformly dispersed. Even though CuO x particles are occasionally observed, for the vast majority of particles examined, no Cu and Al aggregation or surface segregation is found. It remains possible that CuO x clusters formed during hydrothermal aging (with Al incorporation) are in the sub-nanometer range in size, and that they reside inside CHA pores and channels. S13

14 S10 (a) (b) (c) (d) Figure S8 (a-d) XPS spectra for the fresh (blue) and HTA-800 (red) samples. Three measurements were performed at three different locations for each of the four XPS regions Cu 2p, Al 2p, Si 2P, and O 1s. S14

15 S11 (c) (A) (B) (C) (D) (E) (F) S15

16 (i) (G) (H) (I) (J) (K) Figure S9 (A K) The optimized relaxed intermediate structures during zeolite dealumination, via Al-O(H)-Si bond hydrolysis in sequential order. S16

17 S12 (A) (B) (C) (D) Figure S10 (A-D) The optimized intermediate structures in the hydrolysis pathway for the removal of [Cu(OH)] + -Z from cationic positions within the 8MRs. S17

18 S13 (A) (B) (C) (D) (E) (F) (G) (H) Figure S11 (A-H) The optimized intermediate structures in the hydrolysis pathway for the removal of Cu 2+ -2Z from cationic positions within the 6MRs. S18

19 S TOF (mol NO mol Cu -1 s -1 ) C Fresh HTA-550 HTA-600 HTA-650 HTA-700 HTA-750 HTA-800 HTA-900 Figure S12 TOFs plotted for standard SCR carried out at 150 C over the fresh and all HTA samples (normalized per isolated Cu detected by EPR). References: 1. Gao, F.; Walter, E. D.; Kollar, M.; Wang, Y. L.; Szanyi, J.; Peden, C. H. F. J. Catal. 2014, 319, Gao, F.; Washton, N. M.; Wang, Y. L.; Kollar, M.; Szanyi, J.; Peden, C. H. F. J. Catal. 2015, 331, Paolucci, C.; Parekh, A. A.; Khurana, I.; Di Iorio, J. R.; Li, H.; Caballero, J. D. A.; Shih, A. J.; Anggara, T.; Delgass, W. N.; Miller, J. T.; Ribeiro, F. H.; Gounder, R.; Schneider, W. F. J. Am. Chem. Soc. 2016, 138, Schmieg, S. J.; Oh, S. H.; Kim, C. H.; Brown, D. B.; Lee, J. H.; Peden, C. H. F.; Kim, D. H. Catal. Today 2012, 184, S19