Journal of Catalysis 331 (2015) 25–38
`
`Contents lists available at ScienceDirect
`
`Journal of Catalysis
`
`j o u r n a l h o m e p a g e : w w w . e l s ev i e r . c o m / l o c a t e / j c a t
`
`Effects of Si/Al ratio on Cu/SSZ-13 NH3-SCR catalysts: Implications
`for the active Cu species and the roles of Brønsted acidity
`⇑
`, Nancy M. Washton, Yilin Wang, Márton Kollár, János Szanyi, Charles H.F. Peden
`
`Feng Gao
`
`⇑
`
`Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, United States
`
`a r t i c l e
`
`i n f o
`
`a b s t r a c t
`
`Article history:
`Received 23 April 2015
`Revised 27 July 2015
`Accepted 11 August 2015
`Available online 3 September 2015
`
`Keywords:
`Selective catalytic reduction
`Cu/SSZ-13
`Si/Al ratio
`NMR
`Temperature-programmed desorption
`Reaction kinetics
`
`Cu/SSZ-13 catalysts with three Si/Al ratios of 6, 12 and 35 were synthesized with Cu incorporation via
`solution ion exchange. The implications of varying Si/Al ratios on the nature of the multiple Cu species
`that can be present in the SSZ-13 zeolite are a major focus of this work, as highlighted by the results
`of a variety of catalyst characterization and reaction kinetics measurements. Specifically, catalysts were
`characterized with surface area/pore volume measurements, temperature programmed reduction by H2
`(H2-TPR), NH3 temperature programmed desorption (NH3-TPD), and DRIFTS and solid-state nuclear mag-
`netic resonance (NMR) spectroscopies. Catalytic properties were examined using NO oxidation, ammonia
`oxidation, and standard ammonia selective catalytic reduction (NH3-SCR) reactions on selected catalysts
`under differential conditions. Besides indicating the possibility of multiple active Cu species for these
`reactions, the measurements are also used to untangle some of the complexities caused by the interplay
`between redox of Cu ion centers and Brønsted acidity. All three reactions appear to follow a redox
`reaction mechanism, yet the roles of Brønsted acidity are quite different. For NO oxidation, increasing
`Si/Al ratio lowers Cu redox barriers, thus enhancing reaction rates. Brønsted acidity appears to play
`essentially no role for this reaction. For standard NH3-SCR, residual Brønsted acidity plays a significant
`beneficial role at both low- and high-temperature regimes. For NH3 oxidation, no clear trend is observed
`suggesting both Cu ion center redox and Brønsted acidity play important and perhaps competing roles.
`Ó 2015 Elsevier Inc. All rights reserved. Agreement signed 2015
`
`1. Introduction
`
`The mechanism for ammonia selective catalytic reduction
`(NH3-SCR) over Cu ion exchanged zeolite catalysts is still widely
`debated. Some key points of disagreement are as follows. (1)
`Whether the catalytically relevant Cu species are monomeric or
`dimeric (even, perhaps, very small Cu ion clusters) [1–3]. (2) Which
`NOx and NHx species are most relevant in the formation of key
`reaction intermediates? Specifically, whether NO oxidation to
`NO2 is an indispensable step in standard SCR [2,4–6], and which
`
`NHx species, NH4+, molecular NH3, or even NH2(a), is the most rele-
`vant reductant? (3) Which reaction mechanism best describes
`NH3-SCR? In particular, Langmuir–Hinshelwood, Eley–Rideal, and
`Mars van Krevelen (redox) mechanisms have been previously
`proposed for NH3-SCR [7]. Note specifically that most prior
`mechanistic understanding for NH3-SCR has been derived from
`the exhaustively studied VOx/TiO2 system. However, the applica-
`bility of such knowledge to the zeolite system is questionable since
`
`⇑ Corresponding authors.
`
`addresses:
`E-mail
`(C.H.F. Peden) .
`
`feng.gao@pnnl.gov
`
`(F. Gao),
`
`chuck.peden@pnnl.gov
`
`http://dx.doi.org/10.1016/j.jcat.2015.08.004
`0021-9517/Ó 2015 Elsevier Inc. All rights reserved. Agreement signed 2015
`
`the latter has different catalytically active centers, much higher
`NH3 storage capabilities and unique structural characteristics (con-
`finement at a nanometer scale that also gives rise to electrostatic
`field effects). Most recently, the involvement of redox of metal ions
`as part of the reaction mechanism, seems to gain common agree-
`ment for zeolite-based catalysts [8–10]. (4) Whether Cu/CHA are
`dual functional in SCR; that is, whether both Cu ion sites and
`Brønsted acid sites collectively provide the catalytic functionality.
`There have been numerous recent studies on the nature of Cu
`catalytic centers in Cu/SSZ-13 [9–20]. Cu has been suggested to
`stay predominately as isolated Cu2+ ions in SSZ-13; under dehy-
`drated conditions they prefer windows of 6MR as noted above.
`However, depending on the Al content/distribution and Cu loading,
`even when only isolated Cu2+ ions are considered, there is still a
`high degree of complexity regarding their locations [10,13–15].
`For example, at relatively low Si/Al ratios and low Cu loadings,
`one expects that (again, in dehydrated form) the majority of iso-
`lated Cu2+ ions reside near the windows of 6MR and bind with 2
`Al T-sites [11,16]. In contrast, at very high Si/Al ratios where the
`possibility for finding 2 Al sites in one 6MR is highly unlikely, it
`is possible that isolated Cu2+ ions will interact with 2 Al sites that
`are farther apart (e.g.,. 2 Al sites in one 8MR). Alternatively, Cu ions
`
`Exhibit 2020.001
`
`

`
`26
`
`F. Gao et al. / Journal of Catalysis 331 (2015) 25–38
`
`at high Si/Al ratios may charge-balance only one Al site. In this lat-
`ter case, [CuII(OH)]+ or Cu+ species are required in order to appro-
`priately balance the negative framework charge [21]. Recently,
`Andersen et al. [22] studied the location of Cu2+ in CHA zeolite
`by X-ray diffraction using the Rietveld/maximum entropy method.
`These authors confirmed the presence of [CuII(OH)]+ located in
`windows of 8MRs for high Cu-loaded Cu/SSZ-13, a notion fully
`consistent with theoretical studies [23].
`Very recently, we conducted a detailed kinetic study of NH3-SCR
`and two relevant reactions – i.e., NO and NH3 oxidation – on a ser-
`ies of Cu/SSZ-13 catalysts with Si/Al = 6 and various Cu/Al ratios
`[10]. In terms of the nature of catalytically active Cu species, our
`results suggested the following key findings. (1) Below a reaction
`temperature of 300 °C, strong solvation effects from H2O and
`NH3 render high mobility for isolated Cu2+ ions such that transient
`Cu-ion dimers form (in equilibrium with monomers) at low to
`intermediate Cu loadings. These transient dimeric species are
`catalytically relevant for standard NH3-SCR and NH3 oxidation
`reactions. (2) For standard NH3-SCR, Cu-ion monomers are also
`catalytically active and even become the dominant catalytic
`centers at intermediate Cu loadings below 300 °C. However,
`low-temperature active monomers are not located at windows of
`6-membered rings (6MR). (3) Cu-ion monomers migrate to win-
`dows of 6MR at elevated reaction temperatures (>350 °C) and
`become the high-temperature catalytically relevant species.
`Because of the high redox barriers for Cu ion monomers in these
`locations, NH3-SCR is characterized by high apparent reaction acti-
`vation energies (140 kJ/mol), much higher than typical apparent
`activation energies obtained on Cu/SSZ-13 zeolites at lower tem-
`peratures [9,15]. (4) At high Cu loadings (more than one Cu2+ ion
`per unit cell), permanent Cu-ion dimers form under SCR reaction
`conditions. While these moieties are active in NO oxidation to
`NO2 and are believed to be (even highly) active for NH3-SCR, they
`do not appear to improve catalyst performance. This can be
`rationalized by internal mass transfer limitations induced by these
`species since they both block pore openings and occupy space
`within CHA cages.
`Overall, the interplay between Cu ion loading, Cu ion mobility
`and reaction temperature makes the entire NH3-SCR reaction net-
`work quite complicated. Indeed, SCR catalysis is further influenced
`by CHA zeolite Si/Al ratios, and the effects are at least twofold:
`Si/Al ratios affect Cu ion locations as briefly discussed above, as
`well as significantly altering Brønsted acidity and, therefore, NH3
`storage of the catalysts. With regard to the roles of Brønsted acidity
`in SCR, the literature has been rather controversial. For V2O5-based
`+ has been frequently
`SCR catalysts, reactive NH3 in the form of NH4
`proposed [7,24–27]. In explaining the beneficial effects of low Si/Al
`ratio to SCR on zeolite-based SCR catalysts, Yang and coworkers
`adapted the same argument and proposed that this is because
`+ species are formed at lower Si/Al ratios (i.e., more
`more NH4
`Brønsted acid sites) [28,29]. However, titration experiments in a
`+ spe-
`few recent studies on Cu/CHA catalysts demonstrate that NH4
`cies are far less reactive toward NOx than molecular NH3 adsorbed
`+
`on Cu sites [30–32]. Interestingly, in V2O5-based catalysts, NH4
`binds to the catalysts more weakly than NH3 adsorbed on Lewis
`acid sites [7], while it is the opposite for zeolite-based catalysts.
`Therefore, if one makes the reasonable argument that more weakly
`bound NHx species are more reactive, consistency still maintains
`for the two classes of catalysts. In any case, for zeolite-based SCR
`catalysts, Brønsted acidity may be expected to influence the reac-
`tion by affecting Cu ion location and NH3 storage (the latter plays
`a more significant role under transient rather than steady-state
`+.
`reaction conditions), rather than the formation of reactive NH4
`Indeed, in recent studies by Bates et al. [9,33] on a series of Cu/
`SSZ-13 SCR catalysts with Si/Al = 4.5 and a narrow range of Cu/Al
`ratios, the authors found an apparent zero-order dependence of
`
`SCR rates on the number of residual Brønsted acid sites measured
`from NH3 titration. In our recent study, TOFs were also found to be
`largely insensitive to Cu/Al ratios under certain reaction conditions
`[10]. Note, however, that this zero-order dependence on Brønsted
`acidity may not be general: using samples with the same Si/Al ratio
`and a narrow range of low Cu/Al ratios (<0.2), the numbers of
`residual Brønsted acid sites in different samples are not expected
`to vary dramatically to allow reliable correlations between SCR
`rate and Brønsted acid density.
`Importantly, correlation between SCR rates and residual
`Brønsted acid density requires the latter to be quantified in situ
`(i.e., under SCR reaction conditions). This is experimentally
`challenging. Recently, Gounder and coworkers developed protocols
`for ex situ quantification of Brønsted acid sites using NH3-TPD
`[33,34]. These authors showed that, for Cu/SSZ-13 samples with
`high Al content and low Cu/Al ratios, Brønsted acid sites are
`exchanged by Cu2+ ions stoichiometrically (i.e., 2H+ per Cu2+).
`However, this trend does not extend to high Cu loadings and is
`suggested to be due to the formation of CuxOy clusters that do
`not play a charge-balancing role [34]. Recent studies by others also
`revealed the presence of substantial residual Brønsted acid sites in
`Cu/SSZ-13 at high Cu loadings. For example, one study by
`Giordanino et al. [35] determined that Brønsted sites are still
`present even if the Cu/Al ratio is not far from the stoichiometric
`exchange level. They suggested that some of the Brønsted sites
`are exchanged by monovalent copper complexes [CuOH]+.
`In
`another study, Lezcano-Gonzalez et al. [31] showed that the inten-
`1 assigned to
`sity of mOH vibrations (bands at 3605 and 3585 cm
`Brønsted acid sites) in their Cu/SSZ-13 sample at a 100% ion
`exchange level (i.e., Cu/Al = 0.5), was still comparable to their
`parent H/SSZ-13.
`Clearly, from the recent publications described above, there is
`still much to learn about the nature of Cu ion species and the roles
`of Brønsted acidity in Cu/SSZ-13 SCR catalysts. For example, a cat-
`alytic property comparison between Cu2+ and [Cu(OH)]+ active
`sites has not yet been established and SCR performance between
`catalysts with vastly different Brønsted acid site densities has not
`yet been studied. In the present study, we aim to obtain a more
`general picture on the roles of Cu ion location and Brønsted acidity
`in NH3-SCR by using a relatively large number of samples with var-
`ious Si/Al and Cu/Al ratios.
`
`2. Experimental
`
`2.1. Catalyst synthesis
`
`Based on the above considerations, three SSZ-13 substrates
`with different Si/Al ratios were synthesized and used in the present
`study. The synthesis for SSZ-13 with Si/Al = 6 via a static
`hydrothermal method has been described in detail elsewhere
`[11,12] and will not be repeated here. For the two higher Si/Al ratio
`samples, synthesis was conducted hydrothermally under stirring
`using a method modified from a protocol developed recently by
`Deka et al. [16]. Composition of the gel is as follows: 10SDA:10-
`NaOH:xAl2O3:100SiO2:2200H2O, where x varies to allow prepara-
`tion of samples with different Si/Al ratios. The gel is prepared
`first by dissolving NaOH (99.95%, Aldrich) in water and adding
`the SDA (TMAda-OH, Sachem ZeoGen 2825). Following which, Al
`(OH)3 (contains 54% Al2O3, Aldrich) and fumed silica (0.007 lm
`average particle size, Aldrich) were added sequentially under vig-
`orous stirring until the gel was homogenized. The gel was then
`sealed into a 125 ml Teflon-lined stainless steel autoclave with a
`stir bar in. Thereafter, the autoclave was placed in a salt bath on
`top of a hot plate stirrer to carry out hydrothermal synthesis at
`160 °C for 96 h under stirring. Note that continuous stirring during
`
`Exhibit 2020.002
`
`

`
`F. Gao et al. / Journal of Catalysis 331 (2015) 25–38
`
`27
`
`hydrothermal synthesis was very critical for the formation of uni-
`form and highly crystallized SSZ-13 in this method. After synthesis,
`Na/SSZ-13 was separated from the mother liquid via centrifugation
`and washed with deionized water 3 times. Finally, the zeolite pow-
`ders were dried at 120 °C under flowing N2, and calcined in air for
`8 h to remove the SDA (calcination temperatures detailed below).
`Si and Al contents of the product powders were measured with
`Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-
`AES) at Galbraith Laboratories (Knoxville, TN, USA). Prior to these
`composition measurements, the samples were dehydrated at
`150 °C for 2 h in vacuum to remove adsorbed moisture. The Si
`and Al weight contents, as well as the Si/Al molar ratios thus
`derived, are listed in Table 1. For the Si/Al = 6 and 12 samples, a cal-
`cination temperature of 550 °C is sufficient to completely burn off
`the SDA. However, for the Si/Al = 35 sample, a calcination temper-
`ature of 650 °C is required. One likely explanation is that sodium
`promotes SDA combustion, and this sample contains much less
`Na than the lower Si/Al samples. XRD patterns and SEM images
`of the calcined samples are shown in Supplementary Information
`(SI-1 and SI-2, respectively). Solid-state nuclear magnetic reso-
`nance (NMR) analysis of the calcined samples was conducted on
`a Varian VNMRS system. Experimental details can be found else-
`where [36]. Cu/SSZ-13 samples were prepared using a standard
`two-step solution ion exchange protocol reported previously, first
`with NH4NO3 followed by CuSO4 solutions at 80 °C [15].
`
`2.2. Catalyst characterization
`
`Cu contents of the Cu/SSZ-13 samples were also determined
`with ICP-AES. Table 2 presents Cu weight percentages, and the cor-
`responding Cu/Al molar ratios and ion exchange levels for all
`Cu/SSZ-13 samples used in the present study. In the following text,
`in cases where only one measure of Cu loading is provided, the
`others can be readily found in this table. BET surface areas and
`micropore volumes of the samples were measured with a Quan-
`tachrome Autosorb-6 analyzer. Prior to analysis, the samples were
`
`Table 1
`Si and Al contents and corresponding Si/Al ratios for the 3 SSZ-13 samples.
`
`Sample
`
`Si content (wt.%)
`
`Al content (wt.%)
`
`Si/Al ratio
`
`1
`2
`3
`
`30.4
`40.3
`43.4
`
`5.03
`3.23
`1.20
`
`6
`12
`35
`
`Table 2
`Cu contents, and corresponding Cu/Al ratios and ion exchange levels for all of the Cu/
`SSZ-13 samples (in dehydrated form) used in this study.
`
`dehydrated under vacuum overnight at 250 °C. For all of the
`samples studied here, BET surface areas are 500–550 m2/g and
`micropore volumes are 0.25 cm3/g. Temperature-programmed
`reduction (TPR) was performed on a Micromeritics AutoChem II
`analyzer. TPR was carried out on both hydrated samples (samples
`stored in air and saturated with moisture) and fully dehydrated
`samples (samples calcined at 550 °C in 5% O2/He flow for 30 min
`and cooled to ambient temperature in the same flow). Typically
`50 mg of sample was used for each measurement. TPR was car-
`ried out in 5% H2/Ar at a flow rate of 30 ml/min. Temperature
`was ramped linearly from ambient to 650 °C at 10 °C/min and H2
`consumption was monitored with a TCD detector. A water con-
`denser was applied in front of the TCD detector to trap H2O within
`the outlet stream. Powder X-ray diffraction (XRD) measurements
`were performed on a Philips PW3040/00 X’Pert powder X-ray
`diffractometer with Cu Ka radiation (k = 1.5406 Å). Data were col-
`lected with 2h ranging from 5° to 50° using a step size of 0.02°.
`Scanning Electron Microscopy (SEM) was conducted on a FEI Helios
`600 FIB-SEM instrument. Samples were mounted on a carbon tape
`and 5 nm of carbon was deposited onto the samples for conductiv-
`ity. Imaging was done at 5 keV.
`NH3 temperature programmed desorption (NH3-TPD) was used
`to quantify Brønsted acid sites in the H/SSZ-13 samples, as well as
`to describe NH3 adsorption on Lewis and Brønsted acid sites in the
`Cu/SSZ-13 catalysts. NH3-TPD was carried out using our SCR reac-
`tion system, with NH3 detection via gas-phase FTIR. 60 mg catalyst
`(60–80 mesh) was used for each measurement and the following
`experimental steps were followed: (1) Heat the sample to 550 °C
`in O2/N2 (300 sccm, 14% O2) and keep at 550 °C for 30 min; (2) stop
`O2 flow, maintain N2 flow, and cool sample to NH3 adsorption
`temperatures (defined below); (3) adsorb NH3 (500 ppm in N2)
`until outlet NH3 concentration stays constant for 1 h; (4) turn off
`the NH3 flow and purge with N2 for 1 h at the adsorption temper-
`ature; and (5) ramp from the adsorption temperature to 600 °C at
`10 °C/min, and maintain at 600 °C until NH3 desorption is
`complete. For the quantification of Brønsted acid sites in the
`H/SSZ-13 samples, the NH3 adsorption and purging temperature
`were set at 200 °C to avoid weakly-bound NH3. For the Lewis/
`Brønsted site titrations in Cu/SSZ-13 catalysts, the NH3 adsorption
`and purging temperature were set at 100 °C. Stretching vibrations
`of Brønsted acid sites (mOH) were probed with DRIFT spectroscopy
`as described previously [30]. After the sample was loaded into the
`ceramic sample holder cup of a high temperature/high pressure
`it was dehydrated at 550 °C for 1 h in flowing helium
`cell,
`(10 ml/min). After that, the sample was cooled to 227 °C under
`flowing helium prior to spectrum acquisition. Spectra were ratioed
`to a background spectrum collected of KBr at
`the same
`temperature.
`
`Sample
`
`Cu/SSZ-13 (Si/Al = 6)
`
`Cu/SSZ-13 (Si/Al = 12)
`
`Cu content
`(wt.%)
`
`0.065
`0.095
`0.188
`0.378
`0.516
`1.31
`2.59
`3.43
`4.67
`5.15
`
`0.48
`1.00
`1.74
`
`Cu/Al
`ratio
`
`0.006
`0.008
`0.016
`0.032
`0.044
`0.11
`0.22
`0.29
`0.40
`0.44
`
`0.06
`0.13
`0.23
`
`Cu/SSZ-13 (Si/Al = 35)
`
`0.28
`0.64
`0.87
`
`a Ion exchange level ¼ mole content of Cu
`
`mole content of Al
`
`0.10
`0.23
`0.31
`
`  2  100%.
`
`Ion exchange level
`(%)a
`
`2.3. SCR and NO/NH3 oxidation reaction tests
`
`1.1
`1.6
`3.2
`6.4
`8.7
`22
`44
`58
`79
`87
`
`13
`26
`46
`
`20
`45
`62
`
`NH3-SCR and NO/NH3 oxidation kinetics measurements were
`carried out in a plug-flow reaction system. Experimental details
`and mathematic equations used for data analysis are given else-
`where [10,15]. Specifically, all kinetic data for detailed analyses
`were judged kinetically benign using the Koros–Nowak criterion
`[9,10], and the NH3-SCR data were further corrected assuming
`first-order kinetics.
`
`3. Results
`
`3.1. Solid-state nuclear magnetic resonance (NMR) results
`
`Fig. 1(a) presents 27Al NMR spectra for the calcined H/SSZ-13
`samples. For the sample with Si/Al = 6, the feature at 60 ppm is
`
`Exhibit 2020.003
`
`

`
`28
`
`F. Gao et al. / Journal of Catalysis 331 (2015) 25–38
`
`the appearance of a SiO2 phase is not unexpected. The XRD pattern
`for this sample shown in Fig. SI-1, however, indicates that the SiO2
`impurity does not possess a long range order.
`
`3.2. H2 temperature programmed reduction (H2-TPR) results
`
`Cu2+ ions located at different cationic sites have different bind-
`ing energies with the framework; H2-TPR provides a simple and
`direct measure of such differences. In our previous study, it was
`found that reduction of isolated Cu2+ to Cu+ in Cu/CHA catalysts
`occurs below 600 °C while Cu+ reduction to Cu0 occurs at much
`higher temperatures [15]. In the present study, H2-TPR experi-
`ments were conducted only up to 650 °C to gain insight into the
`reduction behavior of Cu2+ as a function of Si/Al and Cu/Al ratios,
`and the results are plotted in Fig. 2. Experiments were conducted
`on both hydrated and dehydrated samples. Note that for the fully
`hydrated samples (samples stored at ambient conditions long
`enough to saturate with moisture), no other treatment was applied
`prior to TPR, except purging with helium at ambient temperature.
`For fully dehydrated samples, on the other hand, the samples were
`heated to 550 °C in dry 5% O2/He, maintained at 550 °C for 30 min,
`and then cooled to ambient temperature in the same gas mixture
`prior to TPR.
`Fig. 2(a) presents results for the series of Cu/SSZ-13 samples
`with Si/Al = 6. As shown in the lower panel for the hydrated sam-
`ples, no reduction is found below 300 °C for samples with very
`low Cu loadings (Cu/Al 6 0.044), and a single, rather symmetric
`peak is found centered at 380 °C. As Cu loading rises, a lower-
`temperature reduction peak develops at 230 °C, and its intensity
`increases with increasing Cu loading. In the meantime, the high-
`temperature state shifts gradually to lower temperatures. This is
`especially the case for the two samples with the highest Cu load-
`ings. Note also that, for samples displaying two reduction states,
`the low-temperature reduction state starts commonly at 160 °C
`irrespective of Cu loadings. From the upper panel (for samples
`dehydrated in O2 prior to TPR), no noticeable difference is found
`as compared to their hydrated counterparts for samples with the
`lowest Cu loadings (Cu/Al 6 0.044). However for samples at inter-
`mediate Cu loadings (0.11 6 Cu/Al 6 0.29), the low-temperature
`state shifts to higher temperatures and partially converts to the
`high-temperature one. For the two highest Cu loading samples,
`one prominent finding is that the total H2 consumption (as deter-
`mined from the H2 consumption peak areas) becomes substantially
`less for the dehydrated samples. This indicates that some Cu2+
`species have already converted to Cu+ during dehydration via a
`so-called ‘‘autoreduction” even in flowing O2 atmospheres. This is
`important for our understanding of the relative stabilities of differ-
`ent Cu ion species as Si/Al ratios and Cu loading vary and more
`details will be given below. Note also that the onset of reduction
`for the dehydrated samples with the highest Cu loadings is as
`low as 100 °C, considerably lower than that for the hydrated
`samples (160 °C).
`Fig. 2(b) and (c) presents TPR results for samples with Si/Al = 12
`and 35, respectively. The hydrated Si/Al = 12 samples display two
`reduction states centered at 240 and 380 °C, resembling the
`Si/Al = 6 samples. However, the Si/Al = 35 samples only display
`one dominant reduction state at 240 °C. TPR for dehydrated sam-
`ples is more complicated. For the dehydrated Si/Al = 12 samples, as
`displayed in the upper panel of Fig. 2(b), the 380 °C reduction
`state is similar to the corresponding hydrated samples, while for
`the Cu/Al = 0.13 and 0.23 samples, H2 consumption at lower
`temperatures is apparently less than the corresponding hydrated
`samples. This again demonstrates that, during the dehydration
`process, a portion of Cu2+ converts to Cu+ even in the oxygen flow.
`Meanwhile, reduction peak temperatures also vary. For example,
`on the dehydrated sample with Cu/Al = 0.23, the predominant
`
`Fig. 1. Solid state (a) 27Al and (b) 31Si NMR spectra for the H/SSZ-13 samples with
`Si/Al ratio = 6, 12 and 35. Samples were prepared by ion-exchanging Na/SSZ-13
`samples to NH4/SSZ-13 followed by a calcination in air at 550 °C. Note that small
`features in the 27Al NMR spectra at 17 and 135 ppm, especially evident in the
`spectrum for the Si/Al = 6 sample, are spinning side bands.
`
`assigned to framework, tetrahedral Al (Alf) and the much weaker
`feature at 0 ppm is attributed to extra-framework, octahedral
`Al. A simple peak area analysis gives Alf/Altotal = 0.88. For the
`Si/Al = 12 sample, framework Al becomes more dominant with
`Alf/Altotal = 0.98. For the Si/Al = 35 sample, essentially all Al stays
`as Alf. Fig. 1(b) presents the corresponding 29Si NMR results. For
`the Si/Al = 6 sample, features at 110, 104 and 99 ppm are
`attributed to Si(4Si, 0Al), Si(3Si, 1Al) and Si(2Si, 2Al), respectively
`[37,38]. For the Si/Al = 12 sample, no –Al–O–Si–O–Al– linkages
`exist and tetrahedral Si in Si(4Si, 0Al) and Si(3Si, 1Al) is observed
`at 111 and 105 ppm, where the former is more dominant as
`expected from the Si/Al ratio of 12. For the Si/Al = 35 sample, fea-
`ture at 112 ppm assigned to Si(4Si, 0Al) becomes even more
`dominant, as compared to the Si(3Si, 1Al) feature at 105 ppm.
`Note that this sample also has a new feature at 101 ppm. This
`belongs to a Q3 SiO2 impurity phase [37]. At such a high Si/Al ratio,
`
`Exhibit 2020.004
`
`

`
`F. Gao et al. / Journal of Catalysis 331 (2015) 25–38
`
`29
`
`Cu/SSZ-13, Si/Al = 12
`Dehydrated
`Cu (wt%) Cu/Al Ratio
`0.48
`0.06
`1.00
`0.13
`1.74
`0.23
`
`Hydrated
`
`0123456
`
`0123456
`
`(b)
`
`H2 Consumption Signal Area (a.u.)
`
`Cu/SSZ-13, Si/Al = 6
`
`Dehydrated
`
`Hydrated
`Cu (wt%) / Cu/Al Ratio
` 0.065/0.006
` 0.198/0.016
` 0.516/0.044
` 1.31/0.11
` 2.59/0.22
` 3.43/0.29
` 4.67/0.40
` 5.15/0.44
`
`(a)
`
`16
`
`12
`
`048
`
`16
`
`12
`
`048
`
`H2 Consumption Signal Area (a.u.)
`
`0
`
`100
`
`200
`300
`400
`500
`Temperature (°C)
`
`600
`
`700
`
`0
`
`100
`
`200
`300
`400
`500
` Temperature (oC)
`
`600
`
`700
`
`Cu/SSZ-13, Si/Al = 35
`
`Dehydrated
`
`Cu (wt%) Cu/Al Ratio
`0.28
`0.10
`0.64
`0.23
`0.87
`0.31
`
`Hydrated
`
`0123
`
`0123
`
`(c)
`
`H2 Consumption Signal Area (a.u.)
`
`0
`
`100
`
`500
`400
`300
`200
` Temperature (oC)
`
`600
`
`700
`
`Fig. 2. Temperature-programmed reduction (TPR) data for Cu/SSZ-13 samples with various Cu loadings for (a) Si/Al = 6, (b) Si/Al = 12 and (c) Si/Al = 35. The upper panels
`show TPR for dehydrated samples while the lower panels show TPR for fully hydrated samples.
`low-temperature reduction state is found at 280 °C while a lower
`temperature state (seen as a shoulder) is also apparent at 180 °C.
`For the dehydrated Si/Al = 35 samples (Fig. 2(c), upper panel), the
`sample with the lowest Cu loading (Cu/Al = 0.10) yields one weak
`reduction peak at 180 °C. For the two samples with higher Cu
`loadings, the 180 °C reduction state also dominates. However,
`weaker but clear reduction states are also evident at 240 and
`300 °C. Furthermore, it is important to note that for the latter
`samples, reduction starts as low as 100 °C.
`Fig. 3 presents H2 consumption as a function of Cu loading for
`all hydrated samples (normalized to the same sample weight used
`in the TPR experiments). An excellent linear relationship is
`obtained in this case. Using CuO TPR as a calibration, H2
`consumption corresponds to H2/Cu = 0.5 for all of the hydrated
`samples demonstrating that reduction below 600 °C is due to
`the following reactions:
`
`Cu2þ þ 1=2H2 ! Cuþ þ H
`þ
`
`ފþ þ 1=2H2 ! Cuþ þ H2O
`½Cu OHð
`
`Importantly, the good linear relationship rules out the existence of
`appreciable amounts of CuO clusters in any of our samples even at
`the highest Cu loadings. This follows since CuO is reduced directly
`to Cu0 at 300 °C, consuming two times the quantity of H2
`compared to reactions (1) and (2). This is consistent with our previ-
`ous Electron Paramagnetic Resonance (EPR) spectroscopy analysis
`of these samples, where it was found that the majority of Cu species
`within our hydrated samples are EPR active (note that CuO clusters
`are EPR silent) [15]. For dehydrated samples, H2 consumption is also
`determined in order to quantify the amounts of Cu+ formed during
`dehydration via autoreduction. As shown in Table 3, for the Si/Al = 6
`samples, no apparent autoreduction is found for samples with Cu/
`Al 6 0.29. For the two higher Cu loading samples, more than 20%
`of Cu2+ is reduced to Cu+ during dehydration. For the Si/Al = 12 sam-
`ples, as with the Si/Al = 6 samples, the extent of autoreduction
`increases as Cu loading rises. Note that autoreduction becomes
`more facile for the Si/Al = 12 samples. For example, even the
`Cu/Al = 0.13 sample has 18% of Cu2+ autoreduced to Cu+ during
`dehydration. The Si/Al = 35 samples behave considerably different.
`First, autoreduction appears to be even more facile for these
`
`ð1Þ
`ð2Þ
`
`Exhibit 2020.005
`
`

`
`30
`
`F. Gao et al. / Journal of Catalysis 331 (2015) 25–38
`
`NH3 adsorption and purging at 200 °C
`
`H/SSZ-13
`
` Si/Al = 6
` Si/Al = 12
` Si/Al = 35
`
`350
`
`300
`
`250
`
`200
`
`150
`
`100
`
`50
`
`0
`
`NH3 Desorption Signal (ppm)
`
` Si/Al = 35
` Si/Al = 12
` Si/Al = 6
`
`Hydrated Samples
`
`0
`
`1
`
`4
`3
`2
`Cu Loading (wt%)
`
`5
`
`6
`
`200
`
`300
`
`400
`
`500
`600
`Temperature (°C)
`
`isothermal
`
`1.6
`
`1.4
`
`1.2
`
`1.0
`
`0.8
`
`0.6
`
`0.4
`
`0.2
`
`0.0
`
`H2 Consumption (a.u.)
`
`Fig. 3. H2 consumption peak areas as a function of Cu loading during TPR for
`hydrated samples shown in the lower panels of Fig. 2.
`
`Fig. 4. NH3-TPD curves for the three H/SSZ-13 samples. NH3 adsorption and
`purging were carried out at 200 °C prior to TPD.
`
`samples. Second, the percentage of autoreduction decreases as Cu
`loading rises perhaps suggesting a limitation for total Cu+ amounts.
`In any case, results shown in Table 3 appear to be consistent with an
`expected general picture that as the binding energy between Cu2+
`and the CHA framework decreases either by increasing Cu loading
`or by increasing the Si/Al ratio, Cu+ formation via autoreduction
`increases. The unusual behavior for the Si/Al = 35 samples will be
`discussed in more detail below.
`
`3.3. NH3 temperature programmed desorption (NH3-TPD) and DRIFTS
`results
`
`NH3-TPD was used to titrate Brønsted acid sites in H/SSZ-13,
`and both Lewis and Brønsted acid sites in Cu/SSZ-13 samples, in
`order to gain insights into the relationships between Alf and H+,
`+ and SCR kinetics. Note that
`as well as chemisorbed NH3/NH4
`NH3-TPD has been successfully used by Gounder and coworkers
`recently to quantify Brønsted acid sites in their MFI- and
`CHA-based SCR catalysts [9,33,34]. Fig. 4 depicts NH3-TPD profiles
`for the three H/SSZ-13 samples. To eliminate weakly-bound NH3
`species in these data, adsorption and purging were carried out at
`200 °C. Expectedly, the amounts of NH3 storage increase with
`decreasing Si/Al ratios. It is also found that the desorption peak
`maximum for the Si/Al = 35 sample (450 °C) is slightly lower
`than the lower Si/Al ratio samples (at 480 °C). Based on NH3
`desorption yields and 27Al NMR results shown in Fig. 1(a), Table 4
`
`Table 3
`Quantification of Cu+ formation during Cu/SSZ-13 sample dehydration in 5% O2/He at
`550 °C.
`
`Si/Al
`
`6
`
`12
`
`35
`
`Cu/Al
`
`0.006
`0.198
`0.044
`0.11
`0.22
`0.29
`0.40
`0.44
`0.06
`0.13
`0.23
`0.10
`0.23
`0.31
`
`Cu+/Cutotal
`0
`0
`0
`0.02
`0.03
`0.02
`0.21
`0.24
`0
`0.18
`0.34
`0.77
`0.47
`0.38
`
`presents the quantified titration results. For the Si/Al = 35 sample,
`an NH3/Alf ratio of unity is found. This demonstrates that our
`experimental procedure indeed allows all Brønsted acid sites to
`be probed by NH3. However, for the Si/Al = 6 and 12 samples,
`NH3/Alf ratios are only found to be 0.70 and 0.87, respectively. This
`could mean that not all Alf sites are associated with H+ as also evi-
`dent in the results of Bates et al. for their H/SSZ-13 samples [33].
`NH3-TPD was also performed on selected Cu/SSZ-13 samples
`with different Si/Al but similar Cu/Al ratios. In this case, NH3
`adsorption was conducted at 100 °C to allow saturation of both
`Brønsted acid and Lewis acid (including Cu ion and extra-
`framework Al) sites. Note that the relatively short purging time
`only drives off the most weakly bound NH3; therefore, the desorp-
`tion features contain adsorption from Lewis and Brønsted acid
`+ [35]. As shown in
`sites, even possibly NH3 complexed with NH4
`Fig. 5, each sample displays two NH3 desorption states: desorption
`above 400 °C assigned to desorption from Brønsted acid sites and
`desorption at lower temperature due to molecularly adsorbed NH3.
`The considerable overlap of the two states precludes reliable quan-
`tification. However, it is important to note that higher framework
`charge density (i.e., lower Si/Al ratios) plays a significant role in
`attracting weakly bound NH3. For example, low-temperature NH3
`desorption yields from the