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`Contents lists available at Sciencebirect
`
` journal hoimepage: www.elsevier.r:orn/iocateioattod
`
`0 fCatalysis Today
`
`The role of pore size on the thermal stability of zeolite supported
`Cu SCR catalysts
`
`Philip G. Blakemana, Eric M. Burkholderii, Hai—Ying Chen“. Jillian E. Collier”,
`Joseph M. Fedeykoe, Hoi jobsonb, Raj R. Rajaramb
`Sjohnson Matthey Emission Control Technologies. Wayne, PA 19087, USA
`“Johnson Matthey Technology Centre, Reading, RG4 QNH, UK
`
`
`
`ARTICLE INFO
`
`ABSTRACT
`
`Article history:
`Received 3l August 2013
`Received in revised form 12 October 2013
`Accepted 15 October 2013
`Available online 12 November 2013
`
`Keywords:
`Cu/zeolite SCR catalysts
`Hydrothermal stability
`Zeolite pore size effect
`In situ XRD
`Cu,/Al;O3 interaction
`
`A comparison study on the hydrothermal stability of Cu SCR catalysts supported on an 8—ring small
`pore chabazite and a 12—ring large pore beta zeolite was performed to understand why small pore zeolite
`supported Cu catalysts are in general hydrothermally more stable than medium pore or large pore zeolite
`supported Cu catalysts and how different pore sizes affect the hydrothermal stability of the catalysts. In
`situ XRD hydrothermal experiments show that, even though the parent chabazite and beta zeolites have
`comparable hydrothermal stability and both can maintain their zeolite framework structure after a 9001C
`hydrothermal exposure. the presence of Cu in the Cu/beta catalyst accelerates the collapse of the BEA
`framework structure at temperatures above 800 " C whereas Cu in the Cu/chabazite catalyst has little effect
`on the CHA framework structure. The loss of beta zeolite framework structure is due to a detrimental
`Cu/N203 interaction. It is proposed that the constricting dimensions of the small openings in a small
`pore zeolite can hinder the destructive Cu/A1203 interaction as well as the zeolite dealumination process,
`hence significantly improving the hydrothermal stability of the small pore zeolite supported Cu catalyst.
`
`© 2013 Elsevier B.V. Alt rights reserved.
`
`1 . Introduction
`
`Zeolite supported Cu catalysts are highly active in the selec-
`tive catalytic reduction (SCR) of NOX (NO and N02) from lean—burn
`exhaust emissions when NH3 is available as a reductant. Under
`typical reaction conditions, the catalysts can convert a very low
`concentration of NO,‘ (~l 00 ppm) with the stoichiometric amount
`of NH3 at >90°/o efficiency in the presence of nearly 1000 times
`of excess of O2 (~l0%). This makes the catalysts highly attractive
`for diesel engine powered vehicles to meet the stringent emission
`regulations it —6}. In addition, the presence of ordered crystalline
`lattice structures with defined exchangeable sites of Cu ions in the
`zeolite supports makes it feasible to study the fundamental details
`about the nature of active sites and how those sites are influenced
`
`by the framework structure. Therefore, extensive studies have been
`devoted to Cu/zeolite SCR catalysts in the past several decades {'28 E.
`Both the zeolite support and the exchanged Cu ions in a
`Cu/zeolite SCR play important roles in SCR reactions. The intra-
`crystalline pores in a zeolite support generate a large surface area
`for low concentrations ofNOX and NH3 to adsorb and to concentrate
`
`* Corresponding author. Tel.: +1 610 341 3441.
`E mail address: (518:2i}@v}i?ZUS;Z.i‘t}lZ‘i (H.—Y. Chen).
`
`09205861/S v see front matter © 2013 Elsevier l3.V. All rights reserved.
`,§i‘>i.
`"fir":ii}0li§j;.(iaE$,(?xi?.Z{?1'i.3£3.{}-'37
`
`
`on the surface. The pore structure also provides spatial confinement
`for the molecules to react. More importantly, the Bronsted acid
`sites originated from the tetrahedral aluminum centers in the zeo-
`lite support activate the adsorbed NH; forming NH4“ ions, which
`is believed to be a key step in the SCR reaction mechanism $89}.
`The tetrahedral aluminum centers can also anchor Cu cations so
`that they are atomically dispersed inside the pores of the zeolite
`structure. It is generally accepted that the highly dispersed Cu ions
`activate NO through a redox cycle and that the oxidation of NO
`and the subsequent formation ofNOx surface adsorption complexes
`on the catalyst is another key step in the SCR reaction mechanism
`§‘lO,t ll. The presence of both exchanged Cu sites and the Bronsted
`acid sites in a Cu/zeolite catalyst enables adsorbed NOX surface
`complexes to further react with adsorbed NH4‘ producing N2 at
`a very high selectivity $8,? 2,'l 3}.
`Since a Cu/zeolite SCR catalyst is a bi-functional catalyst which
`provides both acidic and redox functions for SCR reactions. it is vital
`that the two functions are well balanced for the catalyst to achieve
`the optimum SCR activity and selectivity. For practical applications,
`such as mobile diesel NOX emission control, the catalyst also must
`maintain both functions during the useful life ofthe vehicle in order
`for the vehicle to meet the emission regulations. Typically, this
`would require the Cu/zeolite catalyst to be durable after hydro-
`thermal exposure at a high temperature (for example 670‘C) for
`
` Exhibit 2017.001
`
`Exhibit 2017.001
`
`

`
`RC. Blakemcm et :11. / Catalysis Today 231 (2014) 56-63
`
`57
`
`Table 1
`Examples of Cu/zeolite SCR catalysts and their N0, conversion {%) at 250C before
`and after different hydrothermal treatment mg.
`
`Catalysts
`Cu/Beta
`
`Structure
`BEA
`
`Cu/ZSM—5
`
`MFI
`
`Cu/SAPO-34
`Cu/Chabazite
`Cu/Nu—3
`Cu,/ZSM-34
`Cu/Sigma-1
`
`CHA
`CHA
`LEV
`ERl
`DDR
`
`Pore size
`Large pore
`12-ring
`Medium pore
`10-ring
`Small pore
`8—ring
`8-ring
`8-ring
`8—ring
`8-ring
`
`Fresh
`
`750 “C/24 h
`
`900‘ C/1 h
`
`98%
`
`98%
`
`95%
`100%
`97%
`100%
`88%
`
`59%
`
`19%
`
`99%
`99%
`90%
`98%
`
`58%
`
`28%
`
`97%
`99%
`98%
`
`85%
`
`stability of a Cu/zeolite catalyst is strongly influenced by the pore
`size of the zeolite support.
`In this study, a detailed comparison of the hydrothermal stabil-
`ity ofCu SCR catalysts supported on a large pore 12-ring beta zeolite
`and on a small pore 8—ring high silica chabazite was carried out to
`understand how different pore sizes affect the hydrothermal sta-
`bility ofthe two types of catalysts. in situ XRD measurements were
`applied to monitor the framework structure change of the cata-
`lysts under hydrothermal treatment conditions. The results clearly
`demonstrate that the pore size of the zeolite structure significantly
`affects the Cu/zeolite interaction with small pores preventing Cu
`from degrading the zeolite framework structure.
`
`a long time periods (for example 64 h). As extremely high temper-
`ature excursions may also happen in real world applications, the
`Cu/zeolite catalyst also has to be durable after short time expo-
`sure to those extreme conditions, such as 900 ‘C/1 h hydrothermal
`exposure lltil.
`On the contrary, zeolites are metastable materials because of
`the nature oftheir porous framework structures. When a zeolite is
`exposed to high temperatures its framework structure tends to col-
`lapse forming denser crystalline phases 315;. Under hydrothermal
`conditions, water accelerates this phase transition by attacking
`the tetrahedral aluminum sites through a dealumination process
`E1637}. The presence of Cu ions at the exchange sites ofa zeolite
`can affect the dealumination process in two different ways depend-
`ing on the temperature [t ‘l. At a relatively low temperature, Cu
`ions at the exchange sites may shield water away from attacking
`the adjacent tetrahedral aluminum centers and hinder the dealin-
`mination process. At a relatively high temperature, Cu ions at the
`exchange sites can react with alumina forming stable copper alumi-
`nate (Cu/K1204 ) spinel, which accelerates the dealumination process
`and affects not only the zeolite function but also the Cu function of
`the Cu/zeolite catalysts, causing severe deactivation of the cata-
`lyst. ln its early stage, even though the dealumination process may
`not cause an obvious structure change of the zeolite, it reduces
`the number of acidic and exchangeable sites, which can signifi-
`cantly deactivate a Cu/zeolite SCR catalyst. In fact, dealumination
`has been identified as one of the main deactivation mechanisms
`of Cu/zeolite SCR catalysts after high temperature hydrothermal
`exposure {T9-22}.
`Recently it was reported that Cu SCR catalysts supported on
`zeolites with interconnected 8—ring small pores exhibited improve
`hydrothermal stability compared to the catalysts supported on the
`zeolites with 10-ring medium or 12~ring large pores 123,24}. As
`examples. Table 1 summarizes the NOX conversions at 250 "C on a
`series of Cu/zeolite SCR catalysts reported by Andersen et al. £233.
`In table, the “750’rC/24 h" and the “900 C‘;C/1 h” each represents
`that the catalysts have been hydrothermally treated in a flow of
`10% H20/air mixture at 750 “C for 24h, or in a flow of4,5% H20/air
`mixture at 900 “C for 1 h. respectively. The two hydrothermal treat-
`ment conditions were used to assess the long—term hydrothermal
`durability and the upper temperature limit of these catalysts.
`Both the large pore Cu/beta and the medium pore Cu/ZSM—5 cat-
`alysts showed very good SCR activity when they were fresh, but
`were severely deactivated after either “750 DC/24 h" or “900 1C/'1 h"
`hydrothermal treatment. In contrast, all the small pore zeolite sup-
`ported Cu SCR catalysts showed high NOX conversion efficiency
`and maintained their activity after the hydrothermal treatments.
`The small pore zeolites in Tahie 1 cover a variety of framework
`structures, such as CHA, LEV, ERI, and DDR. The trend of improved
`hydrothermal durability was also reported on other small pore 8-
`ring zeolites with different framework structures, such as AFX, AEI,
`KFI, and SAV $25-30;. Therefore, it appears that the hydrothermal
`
`2. Experimental
`
`2.1. Catalyst preparation
`
`Powder samples of Cu/beta and Cu/chabazite, both with SiO2
`to Al2O3 ratios (SAR) of approximately 25, were prepared by ion
`exchange of Cu into the corresponding zeolites. The Cu loading was
`maintained at 3 wt.% for both samples. After ion exchange, the pow-
`der samples were calcined at 550 C for 2h. The power samples
`together with the parent zeolites samples were used for the in situ
`XRD measurements.
`For SCR activity evaluation, the powder samples were subse-
`quently washcoated on monolithic substrates with a cell density of
`62 cell/cmz and 0.11 mm wall thickness. Cores of25.4 mm diame-
`ter and 50.8 mm length were taken from the monoliths for catalytic
`activity testing.Hydrothermal treatment and catalytic activity eval-
`uation
`
`To achieve reproducible results, a set of core samples were pre-
`treated at 650 ‘C for 2 h in air. This set of samples will be labeled
`as “degre-ened" catalysts. To assess the long—term hydrothermal
`stability of the catalysts, a separated set of core samples were
`hydrothermally treated at 670 3C for 64h in a flow reactor with
`a gas containing 4.5% H20 in air at a flow rate 0f3 L/min. A third set
`of core samples were hydrothermally treated at 900 ‘C for 1 h with
`the same feed gas to determine the hydrothermal stability of the
`catalysts after an extremely high temperature exposure.
`SCR activity of the hydrothermally treated samples was eval-
`uated in a separate flow reactor following a 4-step protocol as
`reported by Kamasamudram et al. E31 1. The feed gas composition is
`regulated by the mass flow controllers of each individual gas line.
`The typical gas feed composition was 350 ppm NO, 350 ppm NH3,
`14% 02, 4.6% H20. 5% CO2 and balance N2. The flow rate was set
`at 12.9 L/min, corresponding to a gas hourly space velocity (GHSV)
`of 30,000 h’ ‘ over the catalysts. Steady state NOX and NH3 conver-
`sions, together with NH3 storage capacity of the catalysts at 200,
`250, 300, 350, 450, and 550 *C points were measured.
`Separate HC inhibition tests were carried out at 300C. Once
`the steady state NO, conversion was reached, 1000 ppm (as C1) of
`propene (C3H5) or n-octane (H'CgH]g) was added to the feed. NOX
`conversion efficiency before and after HC addition was monitored
`to illustrate the HC inhibition effect on the catalysts.
`
`2.2.
`
`In situ XRD measurements
`
`In situ XRD measurements were conducted on a Bruker AXS D8
`in parallel beam mode with an Anton Paar XRK 900 sample cham-
`ber adapted for hydrothermal conditions. Each powder sample was
`pressed into a 13-mm diameter pellet and loaded into the sample
`chamber under flowing atmosphere of 5% H20, 19% O2, and 76%
`N2 . A dataset was collected at 50 ‘C when the system was stabilized.
`Then the cell temperature was increased stepwise from 50 to 900 ‘C
`at a ramping rate of 10"C/min and 50 C/step. The holding time at
`
`VExhibit 2017.002
`
`Exhibit 2017.002
`
`

`
`58
`
`100
`
`RC. Blolwman ()1 ol. / Catalysis Today 231 (2014) 56- 63
`
`2.5
`
`(Z!O
`
`8
`
`N0xConversion
`
`‘T
`
`‘*2
`;;;;.;e;;...;.;;i.a.......i,
`;
`.»-cuichaisame (em c/my
`E -rculchabazite (900 CI1l1)
`-—9—-Cu/Beta (Degreensd)
`~+ ~CuIBeta(670 Cl64h)
`
`(“/a) 8 20
`
`0
`
`100
`
`200
`
`E »-~>~~CulBatz(B00 cm.)
`400
`500
`300
`Tam perature (C)
`
`$00
`
`Fig. 1. NO, conversion efficiency as a function of reaction temperature on Culbeta
`and Cuichabazite SCR catalysts before and after different hydrothermal treatment.
`(SCR reaction conditions: 350 ppm NO, 350 ppm NH3, 14% 0;. 4.6% H20, 5% CO2. and
`balance N2. GHSV = 30,000 h"‘)
`
`each step was about 20 min while an XRD scan was performed.
`The holding time was increased to 1 hour when the temperature
`reached 900 ‘C. Three sets of XRD data were recorded during this
`period. The sample was subsequently cooled to 100°C stepwise
`and data were collected at 100 “C increments. A final dataset was
`collected at S0“C at the end of the hydrothermal experiment. All
`datasets were collected as follows: 3-77” 20 range with a step size
`of 0.029’ and a collection time of 0.42 s/step. Each dataset took
`about 20 min to record. Cu KOL radiation with .3 Ni filter was used as
`the X—ray source.
`
`3. Results
`
`3.]. Catalytic activity after hydrothermal treatments
`
`The steady state NOX conversion as a function of temperature
`over the Cu/beta and Cu/chabazite catalysts before and after dif-
`ferent hydrothermal treatment is shown in Fig. 1. After a mild
`650 ’C/2 h air treatment, both catalysts show high NO, conversion
`in a wide temperature window. In the 200—350”C region which
`is the typical temperature measured in the diesel exhaust, both
`catalysts achieve above 90% NOX reduction efficiency. At tempera-
`tures above 300 i C the NOX conversion on the Cu/chabazite catalyst
`is ~l5% higher. suggesting that the Cu/chabazite catalyst is more
`selective in utilizing NH3 for NOX reduction at those high tem-
`peratures. After hydrothermal treatment at 670’C for 64 h, ~lO%
`decrease of NOX conversion in the entire temperature window is
`observed on the Cu/beta catalyst, but only 10% decrease ofNOx con-
`version at the 200 “C and 550 “C points is seen on the Cu/chabazite
`catalyst. With the clear trend of deactivation, the Cu/beta zeolite
`barely meets 90% NO,, reduction efficiency. When the two cata-
`lysts are subject to hydrothermal treatment at 900“C for 1 h, the
`Cu/beta catalyst shows a significant drop of the NOX conversion
`whereas the Cu/chabazite catalyst still maintains >90% NOX conver-
`sion in the 250——350"C temperature range even though a decline
`of NOX conversion in the entire temperature region is notice-
`able. Clearly, the Cu/chabazite catalyst is more durable after the
`long-term hydrothermal treatment and can toierate much higher
`temperature excursions.
`The NH3 storage capacity of the degreened and the 670‘C/64 h
`hydrothermal treated catalysts is plotted in Fig. 2. Separate NH;
`TPD experiments on the parent zeolites indicate that they both
`have similar NH3 storage prior to the addition of Cu. This is as
`expected for materials with similar SAR. Following Cu addition and
`degreening, the NH3 storage between the two samples diverged
`
`1.5
`
`lilcliiéiiauaziie i5駣;é}{$&T‘=
`, -9..-Culchahazite (670 C:'5dh)
`0
`- -4-CuIBeta (Degrnened)
`g
`
`‘ ~ —CuIBeta (670 c/(uh;
`
`3
`
`catalyst) 0.5
`NH3storagecapacity(gll.
`
`
`
`
`100
`
`200
`
`360
`Temperature (C)
`
`400
`
`560
`
`Fig. 2. NH; storage capacity ofthe Cu/beta and the Cu/chabazite SCR catalysts before
`and after 670"C/64h hydrothermal treatment.
`
`with the Cu/beta sample showing significantly lower degreened
`storage which is a clear indication oflow acid site stability for the
`Cu/beta sample. Additional NH3 storage capacity is lost on both the
`Cu/beta and the Cu/chabazite catalyst after hydrothermal treat-
`ment, and the Cu/chabazite catalyst still maintains higher NH3
`storage compared to the Cu/beta catalyst.
`
`32. Pore size effect on HC inhibition
`
`The beta zeolite support used in the Cu/beta catalyst is a 12-
`ring large pore zeolite with a BEA framework structure with the
`dimension of the openings being about 0.66-0.72 nm in diameter.
`The chabazite zeolite support used in the Cu/chabazite catalyst is an
`8-ring small pore zeolite with a CHA structure with the dimension
`of the openings being about 0.38 nm in diameter. Both types of
`pores are large enough for small gas molecules, such as 02, NO,,
`and NH3, to freely enter the pores to participate in SCR reactions.
`They can respond differently if larger molecules or compounds are
`involved in reactions due to the spatial restriction.
`To illustrate this molecular sieving effect, NO, conversions at
`300 "C in response to the addition of propene or n—octane on the
`Cu/beta and Cu/chabazite catalysts are monitored as a function of
`time and shown in Fig. 3. In these experiments. when steady state
`NO,‘ conversion is achieved, l00Oppm C1 of C3H5 or l’l-CgH}g is
`added into the feed (at time=8 min in Fig. 3). When C3H5, a short
`chain HC, is added into the gas mixture, a decline of the NOX con-
`version is observed on both catalysts. This is due to HC entering the
`pores of the zeolite support and inhibiting the SCR reaction. The
`
`100
`
`ca6
`
`
`
`
`
`NO:conversion(‘/4 so
`
`-Culaeta with C3H6
`
`; -wcwchabazite with C3H6
`
`V ~.c-—CuIchabazi'e with n-cams
`
`N I
`
`M -<-1-CulBs!a with n-cams
`
`O
`
`2
`
`4
`
`6
`
`3
`Time {minute}
`
`10
`
`1 2
`
`14
`
`16
`
`Fig. 3. N0. conversion efficiency at 300' C before and after HC addition.
`(SCR reaction conditions: 350 ppm NO, 35{)ppm NH3, 14% 0;, 4.6% H20, 5% CO2,
`lUUO ppm C, (when added) and balance N2, GHSV=3O,0OOh ‘)
`
`Exhibit 2017.003
`
`Exhibit 2017.003
`
`

`
`PC. Blukemon el :11. / Catalysis Today 231 (2014) 56 6?
`
`59
`
`decline of NOX conversion on the Cu/beta zeolite is slightly more
`pronounced indicating a slightly more severe inhibition effect.
`When n-Cgll13,a long chain HC, is added into the gas mixture, the
`NO, conversion on the Cu/beta zeolite decreases to ~3tJ%, showing
`an even more severe HC inhibition effect. In contrast, no change
`of NOX conversion is observed on the Cu/chabazite catalysts. At the
`temperature evaluated here, the HC inhibition effect is attributed to
`hydrocarbon masking of the active sites within the zeolite. indeed
`the performance of catalyst recovers after a purge of the hydrocar-
`bon species. Therefore, the results clearly demonstrate that large
`molecules, such as Tl-CgH1g, which are able to enter the pores of
`a large pore zeolite can be excluded from entering the pores of a
`small pore zeolite.
`
`3.3.
`
`In situ XRD measurement
`
`To verify if the chahazite zeolite sample itselfis hydrothermally
`more stable than the beta zeolite sample used in the study, in situ
`XRD measurements were carried out on the two parent zeolite
`samples. The XRD scans and the corresponding contour plot of
`the parent beta zeolite are compiled in Fig. 4. On heating from 50
`to 200 C, the intensity of the strong low angle peak at 2H=7.9"
`increases slightly. This is likely due to dehydration of the sample
`which leads to increased ordering. Above 200“C, the intensity of
`this low angle reflection peak gradually decreases until it is not
`evident when the temperature reaches 900 The sharper peak at
`29:22.5‘ shows a more subtle reduction in intensity, reaching a
`minimum during the 900 ’C holding period. On cooling from 900
`to 50~C, both peaks at 2l9=7.9" and 22.5‘ gradually return. This
`suggests that the decline of the intensities at high temperature is
`
`caused by temperature induced disordering rather than complete
`zeolite framework structure collapse. indeed, a comparison of the
`XRD patterns recorded at 50‘”'C before and after the in situ hydro-
`thermal treatment reveals that the sample still maintains the BEA
`structure although a reduction of the peak intensity by about 1/3 is
`observable at 2l9= 7.9“.
`Fig. 5 shows the XRD scans and the corresponding contour plot
`of the parent chabazite zeolite. Similar to what observed on the
`beta zeolite sample, the intensity of the XRD peaks increases upon
`heating from S0 to 2S0‘"C. then gradually decreases and reaches
`a minimum during the 900 ‘C holding period. The intensity of the
`XRD peaks returns when the sample is cooled from 900 to 50
`A
`comparison of the XRD patterns recorded at 50‘C before and after
`the 900' C hydrothermal treatment indicates about 2/3 decrease
`in intensity for the peak at 26=9.4~ but no other obvious change
`of the reflection pattern. Therefore. both the beta zeolite and the
`chabazite support used in this study have reasonably good hydro-
`thermal stability. Although they both become less ordered, their
`framework structures remain intact after 900" hydrothermal expo-
`sure.
`
`Similar in situ XRD hydrothermal experiments were carried out
`on the corresponding Cufzeolite SCR catalysts.
`I-‘lg.
`ti shows the
`XRD scans and the corresponding contour plot of the Cu/beta SCR
`catalyst. The XRD pattern of the sample before the hydrothermal
`treatment shows only BEA structure with no Cu related bulk com-
`pounds identified, suggesting a very high Cu dispersion on the
`sample. Upon heating from 50 up to 800 ‘C, the diffraction pat-
`terns and the change of peak intensity of the sample follow the
`same trend as observed on the parent beta sample. At temperatures
`above 800 *C, however, a drastic reduction of the peak intensity
`
`
`
`
`
` g.
`
`
`
`
`Position [’2Theta] (Copper (Cu}}
`
` 900°C
`
`5
`
`10 H
`
`15
`
`so
`25
`20
`Position ["ZTheta} {Copper (Cm)
`
`35
`
`-so
`
`45
`
`50°C
`
`Fig. 4.
`
`ln situ XRD scans and the contour plot of a beta zeolite during hydrothermal treatment.
`
`Exhibit 2017.004
`
`-4W
`3'3
`‘3a.
`
`EE'
`
`3
`
`Exhibit 2017.004
`
`

`
`60
`
`PI}. Blakeman er al. / Catalysis Today 231 {Z014} 56—63
`
`
`
` 32:0
`Position [°2Theta] (Copper (Cut)
`
`900°C
`
`
`
`(3,)aingmoduial
`
`
`
`15
`
`38
`20
`25
`Position [‘2Theta} (Copper (Cm)
`
`35
`
`40
`
`56°C
`
`Fig. 5.
`
`in situ XRD scans and the contour plot ofa chabazite during hydrothermal treatment.
`
`(more obvious at 29:22.5“) becomes apparent. The diffraction
`peaks that can be attributed to a BEA structure are no longer evi—
`dent when the temperature reaches 900 SC and for the remainder
`of the hydrothermal experiment. This clearly indicates a complete
`collapse ofthe beta zeolite structure ofthe sample once it is exposed
`to above 800 C. A few minor XRD peaks are visible on the sample
`after the 900 :C hydrothermal treatment, which can be assigned
`to aluminum silicate and silica. Even though no crystalline copper
`oxides or copper aluminates are evident on the 900‘ C treated sam-
`ple, formation of these compounds in clusters cannot be ruled out
`because of the low concentration of Cu in the sample.
`The XRD scans and the corresponding contour plot of the
`Cu/chabazite SCR catalyst are shown in Fig. 7. The XRD patterns
`and the change ofthe peak intensity during the hydrothermal treat-
`ment of the Cu/chabazite sample are nearly identical to that of the
`chabazite parent zeolite. The CHA structure remains during the
`entire hydrothermal experiment. After the 900“C hydrothermal
`treatment, the Cu/chabazite sample still shows only CHA struc-
`ture although the intensity reduction of the peak at 2H=9.4’ is
`more pronounced (~3/4 decrease) than that observed on the parent
`chabazite [~2/3 decrease in intensity).
`The difference of the hydrothermal stability between the
`Cu/beta and the Cu/chabazite SCR catalysts is apparent by compar-
`ing Figs. 8 and 7. To further illustrate the difference, Fig. 8 plots the
`relative intensity of the second most intensive XRD peak ofthe two
`
`samples and their parent zeolites as a function oftemperature dur-
`ing the temperature ramp-up portion of the in situ hydrothermal
`experiments. The most intense peak at the low angle is not used
`because the temperature broadening effect on the low angle peak
`is too severe. The second most intensive XRD peak is at 29= 22.5‘
`for the beta zeolite samples and at 26: 21.0’ for the chabazite
`samples. in Fig.
`both the beta and the chabazite parent zeolites
`show about 50% decrease of the relative intensity with tempera-
`ture increased from 50 to 900‘ C. A similar trend is observed for the
`Cu/chabazite sample. The Cu/beta zeolite also shows a similar trend
`as the other three samples at temperatures below 800“C. Above
`this temperature, however, a precipitous decline of the peak inten-
`sity is observed and the sample completely loses its BEA structure
`when temperature reaches 900 “C. The results clearly demonstrate
`that at high temperatures Cu accelerates the collapse of beta zeolite
`framework whereas such a Cu/zeolite negative interaction does not
`happen on the Cu/chabazite sample.
`
`4. Discussion
`
`The comparison ofthe SCR activity of Cu/chabazite and Cu/beta
`catalysts before and after different hydrothermal treatment shown
`in Fig.
`1 clearly demonstrates that the Cu/chabazite catalyst is
`more stable either after a long-term hydrothermal treatment or
`after a short time hydrothermal exposure to an extremely high
`
`Exhibit 2017.005
`
`Exhibit 2017.005
`
`

`
`l’.G. Blakeman er al./Catalysis Today 237 (2014) 56~63
`
`
`
`Position [°2Theta1 (Copper €Cu})
`
`
`
`
`O(3,)8lfl1€.ladUJ3,L
` 15
`
`25
`30
`20
`Position [‘2Theta] (Copper (Cu);
`
`35
`
`40
`
`50°C
`
`Fig. 6.
`
`In situ XRD scans and the contour plot of the Cu/bcta SCR catalyst during hydrothermal treatment.
`
`temperature. Since the in situ XRD hydrothermal experiments
`reveal that both the parent beta and chabazite zeolites have good
`hydrothermal stability and can maintain their zeolite framework
`structures even after the 900°C treatment, the difference in the
`hydrothermal stability of the two catalysts is not simply caused by
`the different stability ofthe parent zeolite materials used. in addi-
`tion, as reported by Andersen et al. and other recent publications
`$23--303, it appears that small pore zeolite supported Cu catalysts
`are in general hydrothermally more stable than their medium pore
`or large pore counterparts. Therefore, the results suggest that the
`pore size ofa zeolite support plays an important role in determining
`the hydrothermal stability of the zeolite support Cu catalyst.
`The presence of pore size effect, or molecular sieving effect, on
`the Cu/chabazite and the Cu/beta SCR catalysts is apparent by com-
`paring the difference between the two catalysts in their response to
`the HC inhibition (Fig. 3). When propene, a relatively small HC with
`a kinetic diameter ofO.45 nm, is added to the feed gas, both catalysts
`show a decrease of NOX conversion. As the kinetic diameter of the
`propene molecule is only slightly larger than the diameter of the
`chabazite pore, molecular motions of the propene molecule and of
`the zeolite framework at elevated temperatures can allow the gas
`molecule to enter the pore of the Cu/chabazite catalyst. Therefore,
`under the reaction conditions propene enters the pores of both zeo-
`lite supports and inhibits the SCR reaction on both catalysts. When
`n—octane, a long-chain HC, is added to the feed gas, no change of
`the SCR activity is observed on the Cu/chabazite catalyst whereas
`NOX conversion over the Cu/beta catalyst is significantly reduced.
`Even though the kinetic diameter ofan n—octane molecule is nearly
`
`the same as a propene molecule, the increased chain length in an
`n-octane is sufficient to prevent it from entering into the pores of
`the chabazite support, hence not affecting the SCR reaction on the
`Cu/chabazite catalyst. Then the question is how the pore size of a
`zeolite support affects the hydrothermal stability of the Cu/zeolite
`catalyst.
`Dealumination ofthe zeolite supports has been identified as one
`of the main deactivation mechanisms on Cu/zeolite catalysts upon
`hydrothermal treatment. Fickel et al. [26] suggested that, during
`the dealumination process at high temperature, Al(OH)3 needs to
`exit the framework and pore system ofa zeolite to introduce struc-
`tural defects. They estimated the kinetic diameter of an Al(OH)3
`as approximately 0.50 nm. This is smaller than the pore openings
`in a i0—ring medium pore or a 12-ring large pore zeolite so dealu—
`mination can readily occur on those zeolites under hydrothermal
`conditions. On the other hand, the kinetic diameter ofan Al(0H)3 is
`larger than the openings in most of the 8—ring small pore zeolites.
`As a result, an Al(OH}3, ifit even forms. might not be able to exit the
`pores ofa small pore zeolite and mightjust stay inside the pore in
`which the aluminum ion is originally located. This group also pro-
`posed that, when the temperature is low enough. this dislocated
`aluminum can be reattached back to the framework hence main-
`taining the integrity of the structure and chemical environment.
`Since the NH3 storage capacity of a Cu/zeolite SCR Catalyst
`is directly correlated to the numbers of acid sites on the zeo~
`lite support, the results in Fig. 2 seem to support the hypothesis
`proposed by Fickel et al. that Al(OH)3 mobility impacts mate»
`rial durability against dealumination. Loss ofNH3 storage capacity
`
`Exhibit 2017.006
`
`Exhibit 2017.006
`
`

`
`62
`
`RC. Blakeman et at’. / Catalysis Today 231 (ZDI4) 56 63
`
`
`
`
`Position [‘2Theta} (Copper (Coy)
`
`900°C
`
`5
`
`10
`
`15
`
`30
`25
`20
`Position ["2Theta] (Copper (Cu)}
`
`35
`
`40
`
`5
`
`Fig. 7. In situ XRD scans and the contour plot of the Cu/chabazite SCR catalysts during hydrothermal treatment.
`
`(or number of acid sites) is observed on both the Cu/chabazite
`and the Cu/beta catalysts after the hydrothermal treatment, but
`the Cu/chabazite catalyst maintains higher NH3 storage particu-
`larly since the chabazite and beta zeolites have similar acidity
`without Cu.
`As with the NH3 storage results. the in situ XRD hydrothermal
`experiments unambiguously demonstrate that the Cu/chabazite
`catalyst is structurally more stable against high temperature expo-
`sure compared to the Cu/beta catalyst. However, Al(OH)3 mobility
`alone cannot explain this difference as both parent zeolites show
`similar structural stability in the absence of Cu. This is not unex-
`pected as it is known that dealumination of a zeolite does not
`necessary lead to obvious framework structure change. Instead, it
`is more likely that the high temperature deactivation mechanism
`is related to the destructive Cu/A1203 interaction which induces
`zeolite framework structure collapse ofthe Cu/beta catalyst at tem-
`peratures above 800
`but has little effect on the CHA framework
`structure of the Cu/chabazite catalyst. Extending upon the hypoth-
`esis that I-"ickel et al. proposed for the dealumination process, we
`speculate that, for the detrimental Cu/A1203 interaction to cause
`a zeolite structure damage, the process may also involve some
`kinds of Cu— and Al—related moieties migrating out of the pores of
`
`the zeolite framework structure to form stable CuAl2O4—like com—
`pounds, Although there appears to be no steric restriction with
`l0-ring medium pore or 12-ring large pore zeolites, the Cu— and
`Al-related moieties may be too big to exit the small openings ofthe
`framework structure in an 8—ring small pore zeolite. Consequently,
`the destructive Cu/N203 interaction observed on medium or large
`pore zeolite supported Cu catalysts under hydrothermal conditions
`is diminished on small pore zeolite supported Cu catalysts.
`Recently Coltl et al. E32; calculated the energetics for Cu migra-
`tion between 8-ring (CHA) and 12-ring (FAU) zeolite structures and
`found that the diffusion barrier to be 100 and 40 kl/moi, respec-
`tively, for the two structures. The high diffusion barrier for CHA
`was attributed to the migration from preferred exchange sites at the
`double six membered rings through the 8 ring window. This high
`diffusion barrier limits the migration of Cu in the CHA framework
`structure, which can also explain why the destructive Cu/A1203
`interaction observed on medium or large pore zeolite supported
`Cu catalysts under hydrothermal conditions is diminished on small
`pore zeolite supported Cu catalysts.
`Therefore. the pore size of a zeolite support plays important
`roles in the zeolite supported Cu catalyst, not only does it govern
`the reactants entering to or exiting from the pores of the zeolite it
`
`Exhibit 2017.007
`
`Exhibit 2017.007
`
`

`
`RC. Blakeman el al./Ctztuly.si.s Today 231 (2014) 5&6?
`
`63
`
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`
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`
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`