Homogeneous Catalysts: Activity - Stability - Deactivation
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To gain a more complete picture of the mechanism of deactivation in these materials, and establish the distribution of both Cu and framework elements e. When the needles were reconstructed, compositional heterogeneities were found that spanned a range of length scales, which can account for the differences in deNO X reaction behavior. Two needles of fresh Cu-ZSM-5 and one of aged Cu-ZSM-5 were successfully reconstructed as this material was especially prone to failure, with images of the fresh and aged materials shown in Fig. The compositional histogram across this isoconcentration surface shows a rapid increase in the Cu content to nearly 15 times its bulk normalized concentration.
Interestingly, although the Cu content rises markedly across this region, the Al content only changes slightly, indicating that the heterogeneous Cu distribution is possibly caused by preferential material accessibility during the exchange. The Al NND for 1 also shows a small deviation from the normal distribution, which was studied using cluster analysis, with the identification of significant Al clusters shown in Supplementary Fig.
In our previous work using APT to study large, carefully prepared crystals of ZSM-5, the Al distribution was random in the fresh material, so finding clustered Al in the commercial material highlights the imperfections found in industrial catalysts compared to model systems This assertion is supported by the recent application of advanced microscale and nanoscale characterization techniques, which have revealed that small, industrial zeolite crystals can be quite heterogeneous, and that the results of bulk studies give an ensemble average from a diverse population of crystals 41 , 42 , 43 , 44 , This segregation was isolated using both a 2.
This needle had a homogeneous Al distribution, and the differences between the two needles from the same material reinforce the imperfect nature of industrial zeolite crystals, and it is possible that the heterogeneity of the material contributes to its lower activity and stability. Overview of atom probe tomography results for fresh and aged Cu-ZSM Strong Cu—Cu affinity is indicated, as well as much weaker, but still significant affinities between all other species. An Al—Cu affinity is expected due to Cu exchanging onto paired Al sites, as indicated in the schematic.
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Strong affinities are indicated between all species, pointing to the formation of copper aluminate domains after aging due to Cu—Al aggregation. All error bars were calculated from counting statistics using the method described in ref. After aging, significant changes occurred in both the Cu and Al distributions, which are immediately apparent from ion distribution maps in Fig.
Previously, it was only identified by bulk analysis, and here is shown as spatially isolated nanoscale features in 3D 14 , 24 , 31 , 32 , 33 , Therefore, we have been able to demonstrate a CuAl 2 O 4 stoichiometry in a nanoscale region and determine the proportion of material that has transformed into this phase after aging. Data were successfully collected from a second needle of aged Cu-ZSM-5, and the results are consistent with those of needle 3, which can be found in the Supplementary Methods and Supplementary Figs.
Two needles of fresh zeolite Cu-SSZ were measured, with all discussed results for samples 4 and 5 shown in Fig. The Al distribution was random as no deviation was observed from the NND; this contrasts with the Cu-ZSM-5 and may be due to the different preparation methods of the zeolites. Finding a random Al distribution suggests that a Cu exchange is unlikely to cause Al segregation, so the observation of a heterogeneous Al distribution in fresh Cu-ZSM-5 is most likely not caused by ion exchange.
It is known in small-pore zeolites that conventional aqueous ion exchange of Cu can be unpredictable and more challenging than larger pore materials, and this non-random Cu dispersion may be the result of this known difficulty 48 , This finding shows the Cu dispersion can be heterogeneous, though still as spectroscopically isolated Cu, giving a highly active catalyst.
In addition, it highlights the power of APT to probe these materials as no other techniques can resolve isolated Cu ions in 3D 28 , 30 , 33 , 34 , 37 , Overview of atom probe tomography results for fresh and aged Cu-SSZ Al—Cu affinity is indicated, which would be expected due to Cu exchanging onto paired Al sites, as indicated in the schematic. Strong affinities are indicated between all species, pointing to the migration and aggregation of Cu with aging, and a Cu aluminate species is shown, though it was not quantitatively identified. Complete APT characterization of samples 6 and 7 is in Fig.
The NNDs for Al show a small deviation from a random distribution, indicating that aging has caused the removal of some Al from framework positions, and aggregation into Al and Cu-rich regions is evident from the RDFs. Although this does indicate some material destruction, it is less significant than in Cu-ZSM-5, which is reflected in the better stability and activity of the catalyst.
Therefore, even though no significant formation of Cu aluminate domains is indicated in the aged material, a clear affinity of Cu for Al is demonstrated by APT. The Cu was significantly segregated such that Cu-rich regions could be easily identified using isoconcentration surface analysis and compositional histograms.
Although there is a less pronounced increase in Al in the Cu-rich regions compared with aged Cu-ZSM-5, it is still present, consistent with limited Cu and Al migration with aging. These observations agree with the known stabilizing influence of Cu in SSZ, which have been observed since the earliest reports on the material, and is thought to be caused by the Cu II exchanging onto paired Al sites, which stabilizes the material by preventing hydrolysis of the Al—O bonds Overall, this leads to limited material destruction and explains the remarkable stability of the material under demanding conditions.
Fresh zeolite Cu-SSZ has a random Al distribution, but in fresh zeolite Cu-ZSM-5 the Al distribution is slightly heterogeneous, likely reflecting the different zeolite synthesis procedures, but highlighting the importance of understanding that real-life catalyst materials may be prepared under non-ideal conditions to satisfy economic requirements, leading to structural and compositional imperfections in the materials that are not encountered under more time consuming, small-scale laboratory preparation procedures, and this has been recently shown by several groups using a variety of techniques 41 , 42 , 43 , 44 , Both materials were Cu-exchanged using conventional aqueous procedures, followed by calcination.
Previous characterization studies have shown that this results in well-dispersed Cu ions, though it is known to be more difficult to exchange Cu in small-pore materials However, heterogeneities found in both materials from analysis of the APT data show that Cu-rich regions tend to form. It is possible that the lower activity of the fresh zeolite Cu-ZSM-5 compared to the fresh zeolite Cu-SSZ can be at least partially attributed to differences in the Cu dispersion, but we do not currently have sufficient data to clearly demonstrate this hypothesis.
A recent experimental and theoretical report has demonstrated that NO X SCR with Cu-SSZ falls outside the conventional boundaries of homogeneous or heterogeneous catalysis as the reaction exhibits a density-dependent interaction of multiple ionically tethered single sites Although a random Cu distribution is normally assumed, we have shown in fresh Cu-SSZ that this is not the case, as Cu-rich regions have been identified Fig.
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Selection of 3D atom probe tomography data for comparison. After aging according to an industrially relevant methodology, the contrasts between the zeolite frameworks are well-pronounced. This is not strongly reflected in the deNO X activity, and it is possible that this is due to the high conversions as well as the temperatures examined The APT isoconcentration surface analysis shows that both the Cu and Al migrate and segregate, and the formation of Cu-rich regions, Al-rich regions, as well as Cu aluminate domains, has been identified.
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This is an important observation, and one that is not trivial as the paramagnetic Cu complicates 27 Al NMR data collection and quantification to understand changes to the local environment of Al, which are too small to be detected by XRD. We have demonstrated using APT that as the Al is lost from framework positions it migrates into Al-rich regions, and the formation of Cu aluminate domains is indicated by the affinity of Cu for Al, and vice versa, after aging, leading to Cu—Al aggregation.
In aged zeolite Cu-ZSM-5 domains of CuAl 2 O 4 spinel have been mapped in 3D and their proportion relative to all Cu and Al species in the material has been estimated, giving insight into material deactivation. In the CHA framework the small 8 MR pores are very stable, preventing Al migration and subsequent deactivating Al—Cu clustering to the same extent as is possible in the MFI framework with its larger 10 MR pores, greatly limiting the degradation of this material, as is illustrated in Fig.
However, the CHA pores allow Cu migration, as recently demonstrated to be vital for low-temperature reactivity, though as there is limited migration of Al, the Cu remains electrostatically tethered and therefore catalytically active, explaining the retained performance of the material as inactive copper aluminate species are not significantly formed In summary, zeolite Cu-SSZ provides superior catalytic performance, and is importantly able to maintain that performance after a simulated , mile aging, whereas the deNO X performance of Cu-ZSM-5 degrades significantly.
This difference has been ascribed to framework destruction through the removal of Al, which serves as the exchange site for Cu, causing the formation of CuO and CuAl 2 O 4 species, which have been previously identified with bulk characterization methods. Both fresh catalyst materials contain a heterogeneous Cu distribution, though to a much greater extent in zeolite Cu-ZSM-5, which also contains a heterogeneous Al distribution.
The result of finding nanometer scale heterogeneities that nicely correlate with catalyst activity is only possible due to the application of APT as a powerful local characterization method. The findings of this study further reinforce the fundamental mechanisms behind the stability of the CHA framework under demanding tailpipe reaction conditions. The SSZ was prepared in our lab as a commercial version is not available. The gel was transferred into three Teflon lined autoclaves, in equal portions.
The resulting solid was a white powder.
Crystallinity was evaluated with XRD. The method used has been adapted from Lezcano-Gonzalez and colleagues 3. The Cu ion exchange was conducted following the method of ref. After drying, the material was recalcined in air using the same program. Aging was conducted based on the industry standard simulation for a , mile vehicle-aged catalyst The steaming was stopped prior to cooling the catalyst back to room temperature and was switched to dry air.
Catalytic activity tests were performed in a fixed bed plug flow set up. All SCR gases were provided by Linde. Equations 1 and 2 were used to calculate respectively the NO conversion and N 2 selectivity. After this the solutions were diluted to yield to appropriate concentrations.
Prior to TPD, 0. For Cu-ZSM-5 the zeolite crystal aggregates were too small for standard preparation, so crystals were attached to the lift-out needle by electrostatic force and transferred to Si micro tips where they were FIB milled into needle-shaped specimens. Data analysis is described in the supporting methods, following our previous publication Videos of the APT sample reconstructions are available in the supporting information to aid with visualizing the ion distributions, clusters and isoconcentration surfaces in 3D.
Granger, P. Catalytic NOx abatement systems for mobile sources: From three-way to lean burn after-treatment technologies. Oord, R.
Beale, A. Moliner, M. Synthesis strategies for preparing useful small pore zeolites and zeotypes for gas separations and catalysis. Go to our Instructions for using Copyright Clearance Center page for details. Authors contributing to RSC publications journal articles, books or book chapters do not need to formally request permission to reproduce material contained in this article provided that the correct acknowledgement is given with the reproduced material.
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Research and Markets: Homogeneous Catalysts: Activity - Stability - Deactivation
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