SSZ-13 NH3-SCR Catalysts: Nature of the

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Iron loading effects in Fe/SSZ-13 NH3-SCR catalysts: nature of the Fe-ions and structure-function relationships Feng Gao, Yang Zheng, Ravi K. Kukkadapu, Yilin Wang, Eric D. Walter, Birgit Schwenzer, Janos Szanyi, and Charles H. F. Peden ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00647 • Publication Date (Web): 30 Mar 2016 Downloaded from http://pubs.acs.org on April 4, 2016

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Iron Loading Effects in Fe/SSZ-13 NH3-SCR Catalysts: Nature of the Fe-Ions and Structure-Function Relationships Feng Gao a,*, Yang Zheng, Ravi K. Kukkadapu b, Yilin Wang a, Eric D. Walter b, Birgit Schwenzer a, János Szanyi a, Charles H. F. Peden a,* a

Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, United States

b

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, United States

Abstract Using a traditional aqueous solution ion-exchange method under a protecting atmosphere of N2, a series of Fe/SSZ-13 catalysts with various Fe loadings were synthesized. UV-Vis, EPR and Mössbauer spectroscopies, coupled with temperature programmed reduction and desorption techniques, were used to probe the nature of the Fe sites. The major Fe species are extra-framework Fe(III) species, [Fe(OH)2]+ (monomeric) and [HO-Fe-O-Fe-OH]2+ (dimeric). Larger oligomers with unknown nuclearity, poorly crystallized Fe-oxide particles, together with isolated Fe2+ ions, are minor Fe-containing moieties. Reaction rate and Fe loading correlations, and temperature and Fe loading effects on SCR selectivities, suggest that isolated Fe3+ ions are the active sites for low-temperature standard SCR and dimeric sites provide the majority of reactivity at higher temperatures. For NO oxidation, dimeric sites are the active centers. NH3 oxidation, on the other hand, is catalyzed by sites with higher nuclearity.

Keywords: selective catalytic reduction, Fe/SSZ-13, Mössbauer, UV-Vis, EPR, temperature-programmed desorption, reaction kinetics. _______________________________________ *Corresponding authors – email addresses: [email protected] (F. Gao); [email protected] (C.H.F. Peden).

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1. Introduction Cu and Fe-ion exchanged zeolites have been investigated for more than 3 decades as SCR catalysts.1-10 In the past few years, properly formulated Cu/SSZ-13 catalysts have been shown to meet the requirements for commercialization as diesel engine exhaust aftertreatment SCR catalysts.9-13 This is a significant achievement in environmental catalysis, in some sense comparable to the commercialization of three-way catalysts for the abatement of gasoline engine exhausts in the 1970s. Along with the success in practical applications, the fundamental understanding of Cu-zeolites, e.g., nature of the active centers, reaction mechanisms and structure-function correlations, has also achieved tremendous progress in recent years. Taking Cu/SSZ-13 as an example, it is now generally accepted that isolated Cu-ions are key active sites in these catalysts, and redox of Cu-ions play a significant role for standard NH3-SCR.14-18 Practically, even though Cu/zeolites offer advantages over Fe/zeolites in low-temperate (≤ 300 °C) SCR performance, high temperature (> 300 °C) performance can be quite important during vehicle accelerations. Under such conditions, properly formulated Fe/zeolites do offer advantages such as higher SCR selectivities and lower N2O formation. In part for this reason, Cu- and Fe-containing zeolites as a combined catalyst system are being explored because they potentially provide both lower (Cu) and higher (Fe) temperature performance. Therefore, continuing studies on the fundamentals of Fe/zeolite SCR catalysts are readily justified. However, fundamental understanding of Fe-zeolites is generally more difficult.5, 10, 19-23 This difficulty may arise from the following issues: (1) the presence of multiple Fe-species, e.g., isolated (mononuclear) Fe-ions, oligomeric FexOy species and Fe-oxide particles, within the same catalysts; (2) interconversion between different Fe species, especially under SCR reaction conditions; (3) the lack of in situ/in operando spectroscopic methods that precisely quantify


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different Fe species and to identify their catalytic roles. Due to extensive research efforts during the last decades, some key fundamental discoveries regarding Fe/zeolite SCR catalysts have been made: (1) in terms of the Fe active sites, it is generally believed that the majority of Fe-containing moieties, including isolated Fe-ions, FexOy oligomers and surface sites on Fe-oxide particles, can contribute to NH3-SCR.20-23 Since Fe-oxide particles only provide (limited) activity at rather elevated temperatures, the issue of the active sites can be reduced to whether monomeric ions or clustered species catalyze the SCR reaction. Brandenberger et al., based on SCR kinetics and estimation of different Fe site quantities from UV-Vis measurements (assuming a Poisson distribution), suggested a strong temperature dependence in relative activities of the various Fe sites for their Fe/ZSM-5 catalysts. In particular, isolated sites provide the majority of activity at temperatures below 300 °C; with increasing temperature, contributions from oligomeric sites dominate.23 Brückner and coworkers also suggested these differences in activity for different Fe sites in Fe/ZSM-5. Based on kinetics, ex situ characterizations and operando-EPR studies, these authors proposed that isolated Fe-ions in different locations have different reactivities for standard SCR, while all of them have lower activity as compared to oligomers.24, 25 On the other hand, Høj et al. proposed that isolated Feions are the major active sites in Fe/beta catalysts, although not all of them have the same activity.26 Clearly, discrepancies still exist even in ranking relative activity between monomers and oligomers; these discrepancies are not readily addressed, at least for the cases of Fe/ZSM-5 and Fe/beta, since monomers can have different locations (coordinations) and oligomers can have different nuclearity. It is generally agreed, however, that Fe-ion monomers are intrinsically more selective for SCR. This follows since multinuclear sites are also active in the main side reaction, non-selective NH3 oxidation.19,

21-23

Notably, according to Brandenberger et al., 3

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monomeric sites are completely inactive for NH3 oxidation up to 500 °C.23 (2) In standard NH3SCR on Fe/zeolite catalysts, the introduction of NO2 to the reaction feed has always been found to promote reaction rates at lower temperatures (≤ 300 °C).27-30 Some suggest that NO oxidation to NO2 is the rate-limiting step for standard SCR; that is, standard SCR is mechanistically identical but “slower” than fast SCR.4 A recent kinetic study by Ellmers et al. suggested otherwise: NO oxidation to NO2 and standard SCR were found to be uncorrelated over a wide range of catalysts and reaction conditions.31 However, these latter authors’ experimental procedures seem ambiguous for such a conclusion, even if correct, to be made; notably, the studies did not include H2O in their reactant feed. The fact that H2O is a product in SCR but not in NO oxidation likely causes dramatic differences in the nature of the catalytic active centers (i.e., hydrated during SCR but dehydrated during NO oxidation), making kinetic comparisons less meaningful. In a recent study by Ruggeri et al., NO oxidation was carried out on a physical mixture of Fe/ZSM-5 + BaO/Al2O3, where NO oxidation occurred on Fe/ZSM-5 but oxidation products can be trapped by BaO/Al2O3 and further analyzed. This led the authors to conclude that nitrites/HONO were the common intermediates during both standard SCR and NO oxidation to NO2.32 From these prior studies, a clear picture on the roles of NO2 is still lacking, although the notion that NO2 plays a promoting role in SCR and seems to be closely associated with other intermediates, appears well-accepted. (3) Standard NH3-SCR on Fe/zeolites follows a redox mechanism, where Fe-ion centers cycle between Fe(II) and Fe(III) oxidation states during reaction.4, 24 For multinuclear Fe centers, this redox can proceed readily via a Mars-van Krevelen mechanism in the steps where NO oxidizes to NO2. For example, reactions (1) and (2) can be used to describe this chemistry, [− − − −] + → [− − □ − −] + ()

(1)


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[− − □ − −] + → [− − − −]

(2)

where [-Fe-O-Fe-] represents a multinuclear Fe center with extra lattice oxygen (ELO), and “□” represents an oxygen vacancy. For isolated Fe-ions, the situation can be more complex. Using [FeO]+ as a representative isolated Fe-ion, NO oxidation to NO2(ads) can be understood, for example, from equation (3) shown below:33 [− − −] + → [− − − ] → [− −] + ()

(3)

while reoxidation of Fe(II) back to Fe(III) is not readily achievable in this case. A recent operando EPR and UV–Vis spectroscopy study by Brückner and coworkers24 has demonstrated that for low Fe-loaded Fe/ZSM-5 catalysts, isolated Fe-ions in β and γ positions are nonreversibly reduced to Fe(II) and provide no activity during standard SCR. Only under fast SCR conditions do Fe ions in these positions remain trivalent and contribute to SCR. Interestingly, DFT studies by Li et al. suggest that a prototypical Fe(II) site, [FeIIOH]+, can only be reoxidized by NO2 but not by O2 or O2/NO mixtures in the presence of NH3. 34 To better understand the fundamentals of standard NH3-SCR on Fe/zeolites, a viable strategy is to study Fe/zeolites with simple structures. Fe/ZSM-5 is not the best choice; as described above, even for Fe-ion monomers, there are three possible locations in this material. Fe/beta has received considerable attention in recent years due to its great potential for applications; however, this material is also not a good choice for fundamental studies because of its significant structural complexity (e.g., highly defective, containing two polymorph structures). In contrast, the Chabazite structure that SSZ-13 adopts is much simpler. The study of Cu/SSZ-13, in turn, has allowed rapid advances in our understanding of Cu/zeolite SCR catalysts in general. 8-10

Recently, we demonstrated that Fe/SSZ-13 is an active and hydrothermally stable SCR 5 ACS Paragon Plus Environment

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catalyst. Using CO/NO titration FTIR and Mössbauer spectroscopies, we also attempted to elucidate the nature of the Fe sites in this catalyst.35 However, this study only investigated catalysts with one Fe loading. In the present study, we systematically investigate Fe/SSZ-13 catalysts with a series of low Fe loadings, using kinetics and a few spectroscopic methods, trying to better understand this useful and structurally simple SCR catalyst, and hoping to shed some light to Fe/zeolite SCR catalysts in general.

2. Experimental 2.1 Catalyst Synthesis Five Fe/SSZ-13 samples with different Fe loadings were prepared using a solution ionexchange method.35 The detailed procedures are as follows: (1) measure 2g of ambient NH4SSZ-13 powder (Si/Al = 12) and disperse in 200 mL of deionized water; (2) add a few drops of 1.0 M HNO3 to adjust the pH of the suspension to ~3.0; (3) with stirring, heat the suspension to 80 °C under the protection of flowing N2; (4) add various amounts of FeSO4⋅7H2O into the suspension (during which N2 flow is interrupted), and resume N2 protection as soon as possible; (5) maintain the suspension at 80 °C for 1 hour (pH of the suspension is not adjusted in the period); (6) separate the solid by centrifuging, wash with deionized water for at least 3 times, and dry the wet solid at 120 °C under flowing N2; (7) in static air, heat the powder to 550 °C at 2 °C/min and calcine at 550 °C for 5 hours. Table 1 lists the amount of FeSO4⋅7H2O used for each

synthesis and the Fe loadings (wt.%) in the final products measured via ICP. Fe/Al ratios for the 5 samples are also listed in Table 1. Note that the Fe loadings are purposely maintained at low levels (Fe/Al < 0.2) to maintain structural simplicity of these materials. To facilitate Mössbauer spectroscopic measurements, another series of 5 samples were synthesized using Fe-57 labelled


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FeSO4, prepared by dissolving Fe-57 metal powder (98%, Cambridge Isotope) with stoichiometric amounts of 0.1 M H2SO4 solutions at ambient temperature. Note that the dissolution process can be greatly facilitated by sonicating the Fe/H2SO4 mixture. The two series of catalysts have the same Fe loadings and catalytic properties. 2.2 Catalyst Characterization Routine characterization methods, e.g., elemental analysis, powder X-ray Diffraction (XRD), surface area/pore volume measurements, are described elsewhere.14,

35-37

XRD

measurements of the catalysts (not shown) revealed typical Chabazite patterns; no iron oxides of any kind were detectable, even for the highest Fe-loaded sample.35 Surface area/pore volume values are displayed in Table 1. Within experimental error, these values can be judged to be identical, suggesting no Fe oxide blocking of pores/channels. UV/Vis spectra of the ambient samples (samples stored in air and saturated with moisture) were recorded using a Cary 5000 absorption spectrometer from Agilent Technologies with an internal integrating sphere accessory (DRA 2500). The data were recorded as total reflectance spectra and subsequently converted to absorption spectra using the instrument’s software (Varian Cary WinUV). All spectra were recorded in double beam mode at a scan rate of 600 nm/min. Any artifacts visible in the spectra at 350 nm are due to a light source changeover. A baseline for subtraction was recorded from a pre-packed polytetrafluoroethylene (PTFE) cell and used for all measurements. In all cases the powder sample was packed into a sample holder with a springloaded plunger to accommodate different sample quantities. Both the PTFE cell and the sample holder contain a quartz window through which the spectra were recorded. Temperature-programmed reduction (TPR) experiments were performed on a Micromeritics AutoChem II analyzer. TPR was carried out on ambient samples and, typically,


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~50 mg of sample was used for each measurement. TPR was carried out in 5% H2/Ar at a flow rate of 30 mL/min. The temperature was ramped linearly from ambient to 700 °C at 10 °C/min and H2 consumption was monitored with a TCD detector. Electron paramagnetic resonance (EPR) experiments were conducted on a Bruker E580 X-band spectrometer equipped with a SHQE resonator and a continuous flow cryostat.14 Powder samples (∼10 mg) were contained in 4 mm OD quartz tubes. Microwave power was 20 milliwatts, and the frequency was 9.34 GHz. The field was swept by 6000 G in 84 s and modulated at 100 kHz with 10 G amplitude. A time constant of 41 ms was used. Details about Mössbauer measurements of hydrated

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Fe/SSZ-13 catalysts can be found

elsewhere.35 To increase recoilless-fraction of weakly adsorbed Fe species, measurement was performed at 8 K in order to immobilize all Fe species. Spectra were analyzed by the Voigtbased fitting model,38 where elemental mutiplet Lorentzian half widths at half maximum (HWHM) were fixed at 0.097 mm/sec. NH3 temperature programmed desorption (NH3-TPD) was used to probe “residual” Brønsted acid sites in the samples. 60 mg samples (60-80 mesh) were used for these measurements, carried out using a plug-flow SCR reaction system. NH3 desorption was monitored with an MKS 2030 FTIR analyzer. To completely eliminate NH3 adsorbed on weak acidic and Fe-ion sites, a protocol recently developed by Gounder and coworkers was followed. Specifically, after the samples were saturated with NH3, they were purged with a wet carrier gas before TPD.39 In their original method, purging was carried out at 160 °C overnight. In the course of this study, it was found that using a wet purging N2 flow (containing ~2.5% H2O) of 300 sccm and a temperature of 200 °C, weakly-bound NH3 can be completely removed within 2


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h. Samples were then ramped from 200 to 600 °C at 5 °C/min, and maintained at 600 °C until NH3 desorption was complete. 2.3 SCR and NO/NH3 oxidation reaction tests NH3-SCR and NO/NH3 oxidation kinetics measurements were carried out in the plugflow reaction system described above. Experimental details and mathematic equations used for data analysis are given elsewhere.14, 36 Before reaction measurements, the samples (60 mg) were treated in a 300 sccm N2 flow containing 10% O2 and ~2.5% H2O at 550 °C for 2 h. This pretreatment was necessary to stabilize the catalysts. After one cycle of reaction tests (~40 h of use in the presence of ~2.5% H2O, with reaction temperatures never exceeding 550 °C), all reactions were repeated using the used catalysts to check catalyst stability and data reproducibility. Following which, the same reaction measurements were carried out one more time on new catalysts, again to check data reproducibility. As will be shown below, all reactions were highly reproducible; and, unless otherwise specified, all data were reported by averaging the three measurements (error bars represent standard deviations). Specifically, all kinetic data for detailed analyses were acquired under low conversion (< 20%) conditions, and were judged kinetically benign using the Koros-Nowak criterion. 16, 36

3 Results 3.1 Catalyst characterizations Figure 1 presents UV-Vis absorbance spectra of the fresh catalysts stored under ambient conditions. The lineshapes of the spectra (arising from O → Fe3+ charge transfer) are rather similar and their intensity increases with increasing Fe loading. The insert of Figure 1 presents Gaussian peak fitting of the sample with the highest Fe loading (1.2 wt%). Two strong bands are found below 300 nm that can be assigned to isolated Fe3+ ions. Presumably, the ~220 nm band is 9 ACS Paragon Plus Environment

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attributed to charge transfer to tetrahedrally coordinated Fe3+ while the ~270 nm band to Fe3+ with higher coordinations.19,

20, 40

A sizable feature is found at ~350 nm, which is due to

octahedral Fe3+ in small oligomeric FexOy clusters.40 The much weaker band centered at ~470 nm can be assigned to larger Fe-oxide particles.19, 20, 40 Before a more detailed analysis on Fe loading dependences of the UV-Vis data, it is imperative to know the stability of various Fe moieties during SCR and related chemistry. In other words, if Fe species interconvert readily during such reactions, it becomes less meaningful to quantify the various Fe species of the fresh samples. Due to the lack of in situ/in operando methods, a comparison is made between ex situ UV-Vis spectra of fresh catalysts and their used counterparts, where each used catalyst has been exposed to SCR and NO/NH3 oxidation reaction environments (detailed below) for ~40 h, calcined again at 550 °C in static air for 5 h (to remove trapped NH3 and other adsorbed species), and maintained at ambient prior to UV-Vis measurements. As displayed in Figure 2, all three samples (Fe loadings at 0.27, 0.62 and 1.20 wt.%) display small decreases in intensity of isolated Fe3+ sites after use suggesting agglomeration of some isolated sites during use. For samples with Fe loadings at 0.62 and 1.20 %, this decrease is accompanied with an expected slight increase in the oligomer and larger particle signal intensities. However this trend is less obvious for the 0.27% Fe loaded sample. Note that calcination treatment of the used catalysts may revert the agglomeration process. This, coupled with the qualitative nature of the UV-Vis technique, makes quantification of the extent of agglomeration during catalysis a challenging task. However, the fact that activity of the samples after ~40 h of use does not vary (as will be shown below) strongly suggests that interconversions between different Fe species (mainly agglomeration) do not proceed extensively,


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and concentrations of the various Fe species in the fresh samples can be used as a good approximation of the Fe species under reaction conditions. After establishing the stability of Fe species, spectra shown in Figure 1 are further analyzed (each spectrum peak fitted), and peak areas of the monomer (the ~270 nm band), oligomer and Fe2O3 particle are plotted as a function of Fe loading in Figure 3. The monomer band centered at ~220 nm is not included here since the peak areas cannot be reliably determined. From this graph, concentrations of each Fe3+ species increase approximately linearly as a function of increasing Fe loading. It is important to note that the sampling cross sections for different Fe species are typically not known for this method; therefore, Figure 3 is at best semiquantitative. Nevertheless, it is readily affirmed that in all samples the majority sites are extraframework Fe-ion monomers and oligomers that play charge-balancing roles. Especially for the lowest Fe-loaded samples, Fe2O3 particles are essentially absent. To further understand the nature of the Fe moieties, for example coordinations of different Fe-ion monomers, degree of oligomerization for FexOy clusters and whether some Fe species stay as Fe(II), additional methods are used to characterize the catalysts. Figure 4 presents NH3-TPD of the Fe/SSZ-13 samples. Note that following purging with wet N2 at 200 °C to remove weakly-bound NH3, desorption should occur only from Brønsted acid sites.39 Clearly, TPD areas decrease with increasing Fe loading. Desorption curves share very similar leading edge temperatures while samples with lower Fe loadings have higher trailing edge temperatures. This is expected since NH3 molecules undergo multiple desorption-readsorption events prior to final releasing to the carrier gas; therefore, higher residual Brønsted acid site density corresponds to higher trailing edge temperatures. It is seen from the insert that the integral NH3 peak areas (normalized to the NH3 peak area extrapolated to zero Fe loading) decrease linearly with


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increasing Fe/Al ratios. The slope of the linear fit (β = −0.91±0.05) indicates that each Fe-ion charge-balances one negative framework charge. Note that this finding is consistent with our recent study,35 and studies on Fe/ZSM-5 catalysts also synthesized from solution ion exchange.41, 42

Considering the fact that Fe2O3 particles (which do not play a charge-balancing role) are

minority species in all samples, the most likely monomeric and oligomeric ferric ions that satisfy such a 1/1 ratio are [Fe(OH)2]+ and [HO-Fe-O-Fe-OH]2+ in hydrated forms. Upon dehydration, these should convert to [FeO]+ and [-Fe-O-Fe-]2+ as dehydrated forms.35 Figure 5(a) presents H2-TPR results of the ambient fresh samples. For each sample, a single broad reduction feature is found between ~200 and ~500 °C. Based on prior H2-TPR measurements on various other Fe/zeolites,42-45 this is readily assigned to reduction of the extraframework Fe3+ to Fe2+. Note that crystallized Fe2O3 typically reduces at temperatures slightly above 500 °C.45 The lack of apparent reduction from 500 to 700 °C suggests that the Fe2O3 species in samples with Fe loadings ≥ 0.62% (Figure 3) are poorly crystallized, a notion fully consistent with XRD measurements. To gain more quantitative information, Figure 5(b) plots H2 consumption as a function of Fe loading. Also included in this plot is H2 consumption versus Cu loading for four low Cu-loaded Cu/SSZ-13 reference samples.46 For low Cu-loaded Cu/SSZ-13, we have established previously that reduction below ~500 °C is due entirely to reduction of the extra-framework Cu2+ to Cu+.14, 46 Therefore, the slope of the linear fit from H2 consumption versus Cu loading provides a good reference for the determination on the extent of Fe3+ reduction. Indeed, for Fe/SSZ-13 samples, a linear fit from the H2 consumption versus Fe loading data gives a very similar (slightly lower) slope, confirming that reduction shown in Figure 5(a) is due predominately to Fe3+ to Fe2+. However, this linear fit does not extrapolate to


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the origin, demonstrating the presence of ~0.1 wt% non-reducible Fe species in the Fe/SSZ-13 samples. EPR spectra of the ambient samples are depicted in Figure 6. For all of the samples, low field signals are found at effective g values of g’ = 8.8, 6 and 4.3 while at high field, g’ = 2.3 and 2 are observed. These features are frequently observed in other Fe/zeolites, yet their assignments are still controversal.20, 40, 47-50 We tentatively assign g’ = 8.8, 6, 4.3 and 2.0 to isolated Fe3+ species, and signals at g’ = 2.3 to small oligomeric FexOy clusters. Our reasoning will be shown below. In spite of the possible uncertainties in the assignments, it is important to note that the lineshapes of the EPR spectra are strikingly similar, indicating similar distributions of EPR active Fe-ion species for samples with different Fe loadings. It is worthwhile noting that the UV-Vis and EPR spectroscopies used above are only sensitive to Fe3+ species (furthermore, none of them is quantitative); however, H2-TPR results indicate the presence of non-reducible Fe species in the Fe/SSZ-13 catalysts. In principle, nonreducible Fe may include framework Fe3+ and extra-framework Fe2+. However, formation of framework Fe3+ is not expected via solution ion exchange.20 In the following, Mössbauer spectroscopy is applied to quantify all possible Fe species within our Fe/SSZ-13 samples. To avoid non-detection of weakly adsorbed species (if any), measurements were performed at 8 K. Figure 7(a) presents unmodeled spectra of the 5

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Fe/SSZ-13 samples. Despite the loading

differences, the spectra are qualitatively similar to each other; they mainly differ in the relative contents of doublet (paramagnetic, P) and sextet (magnetic, M) features. Figure 7(b) presents a fitted spectrum of the 0.27% Fe loaded sample, using the Voigt-based fitting model.38 For the sake of simplicity, modeling results for other samples are not shown. For each sample, a decent fitting is achieved that includes 3 Fe components: 2 doublets, and a sextet. The modeled


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parameters are presented in Table 2. The component with a center shift (CS) of ~1.4 mm/s is readily assigned to Fe(II) while the other two, with CS ≈ 0.5 mm/s, can be assigned to Fe(III).35, 51, 52

Based also on their magnetic properties (P or M), the three components are designated as

Fe(II)-P, Fe(III)-P and Fe(III)-M, respectively, in Table 2 and in the following text. Figure 8(a) presents weight percentages of these three components as a function of total Fe loading. It is interesting that the amount of Fe(II) is practically independent of the Fe loading, whereas the amounts of Fe(III) components increase linearly with increasing Fe loading. Note that the presence of Fe(II) species in the Fe/SSZ-13 samples provides an excellent explanation to the observation of non-reducible Fe species from H2-TPR (Figure 5). It is also worthwhile noting that the Fe(II)-P component is barely detectable at room temperature (data not shown) indicating weak interaction with the zeolite lattice. Therefore, we suggest that Fe(II)-P are hydrated isolated Fe2+ ions charge-balanced by the CHA framework, similar to isolated Cu2+ ions in hydrated Cu/SSZ-13. The assignment for Fe(III)-P is also fairly straightforward. From literature, high QS values ranging from 0.76 to 2.4 mm/s are typical for dinuclear Fe3+ complexes.52 Therefore, Fe(III)-P is assigned to [HO-Fe-O-Fe-OH]2+, a species frequently suggested to be present in Fe/zeolites.44, 52-55 Note also that this assignment is fully consistent with the NH3-TPD results (Figure 4), which indicated that [HO-Fe-O-Fe-OH]2+ is a major oligomeric species in our samples. However, the assignment for Fe(III)-M is more difficult, and cannot be done without aid from other characterization techniques. Fe-oxide particles, irrespective of the type of oxide, particle size, and crystallinity display magnetic ordering (i.e., sextet features) at low temperatures.56 UV-Vis results shown in Figures 1 and 3, however, indicate that samples with 0.27 and 0.45% Fe contain essentially no Fe-oxide particles. Considering also the detection of strong isolated Fe3+ signals via EPR from these samples (Figure 6), it is almost certain that


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Fe(III)-M comprises primarily isolated Fe3+ species. Therefore, we suggest herein that Fe(III)-M contains isolated Fe3+ species as the major species, FexOy oligomers larger than dimers, as well as Fe-oxide particles of different particle size domains (only in samples with Fe loadings ≥0.62%), as the minor species. To corroborate this argument, a refined fitting of the Fe(III)-M component in samples with 0.27 and 1.20% Fe was performed, where 3 Fe(III)-M sextets were used during the simulation. Table 3 presents modeled Mössbauer parameters of the spectra fitted with 3 Fe(III)-M sextets. Figure 8(b) presents the fitted spectrum of the 0.27% Fe sample containing 3 Fe(III)-M sextets. Compared with Figure 7(b), this refined fitting indeed simulates the experimental result more accurately. However, these results further illustrate the difficulty in quantifying all possible Fe species in zeolites, even with cryogenic temperature Mössbauer spectroscopy, the best choice at the current state-of-the-art. More discussion of presence of multiple Fe-phases, and how these complicate the understanding of SCR and related chemistry, will be given below. 3.2 Reaction kinetics 3.2.1 Standard NH3-SCR (4NO + 4NH3 + O2 = 4N2 + 6H2O) Figure 9(a) presents steady-state NO light-off (i.e., NO conversion vs. temperature) curves for the 5 Fe/SSZ-13 catalysts, at a GHSV of 200,000 h-1. NO conversions are reported as the average of three measurements as described above. Except for measurements in the light-off regime (300 and 350 °C, during which it can take longer times for steady-state reactivity to be realized), NO conversions are highly reproducible between measurements, including measurements on samples after ~40 h of use. At relatively low reaction temperatures (< 300 °C), NO conversions increase with increasing Fe loading. Above ~350 °C, reaction reaches external mass-transfer limitations and NO conversions become relatively unchanged with temperature.


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For selected catalysts, Figure S-1 plots a comparison between NO and NH3 conversions to gain insights into catalyst selectivities. At low temperatures, all catalysts are highly selective for SCR as manifested by the essentially identical NO and NH3 conversions. At temperatures ≥ 300 °C, however, NO conversions become lower (up to ~20%) than the corresponding NH3 conversions. At reaction temperatures of 300 and 350 °C, for example, the NO and NH3 conversion differences increase with increasing Fe loadings (i.e., SCR selectivity decreases with increasing Fe loading). This phenomenon is rather common for Fe/zeolites, sometimes known as parasitic NH3 oxidation since it occurs at temperatures far below NH3 oxidation by O2 alone.57 More discussion of this will be given below, together with results from NH3 oxidation kinetics. Figure 9(b) displays normalized reaction rates (mol NO per gram of catalyst per second) versus Fe loading at three temperatures (220, 240 and 260 °C), where NO conversions are ≤ 20%. At such differential conversions, normalized rates increase approximately linearly with increasing Fe loading at all three temperatures indicating fulfillment of the Koros-Nowak criterion.16, 36, 58, 59 In other words, mass and heat transfer limitations do not exist under these conditions. An exception appears to be the 0.27% Fe sample where reaction rates are consistently lower than expected from the linear expressions (see dashed oval in Figure 9(b)). More detailed discussion of this abnormal behavior will be given below. Figure 9(c) plots normalized NO conversion rates collected between 220 and 280 °C in the form of Arrhenius plots. Clearly for all 5 samples, SCR falls in a linear Arrhenius regime with similar apparent activation energies. Using these differential NO conversion data points, apparent activation energies are calculated and shown in Table 4. For all of the 5 samples, Ea = ~52 kJ/mol suggesting identical reaction mechanisms/active centers in all samples. Note specifically that, although the 0.27% Fe sample displays some abnormity in reaction rates (Figure 9(b)), the nature of the active centers do not


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seem to differ from other samples because of the identical apparent activation energies measured on this sample. 3.2.2 NO oxidation (2NO + O2 = 2NO2) Unlike standard SCR and NH3 oxidation reactions where H2O only has a weak influence on their kinetics, H2O has profound effects on NO oxidation to NO2. Therefore, NO oxidation was carried out under both “dry” and “wet” reaction conditions where, in the latter, ~2.5% H2O was added to the reactant feed. As shown in Figure 10(a), the catalysts are very active under water-free conditions. Even for the 0.27% Fe sample, NO conversions reach > 70% at 250 °C. NO2 formation rolls over above ~250 °C, due to thermodynamic limitations.60 Note that these results are reported from single measurements; however, high reproducibility was confirmed from repeated measurements on selected samples (data not shown). Using normalized rates collected at low temperatures (≤ 140 °C, at which the reaction is largely under kinetic control and effects from thermodynamic limitations are negligible), Arrhenius plots are displayed in Figure 10(b), with apparent activation energies (~35 kJ/mol) given in Table 4. These values are expected when there is no NO2 co-feeding in the reactant,61 the situation chosen here. The similarity in activation energies suggests common active sites in different samples. In order to verify whether the Koros-Nowak criterion is obeyed, Figure 10(c) plots normalized reaction rates (after subtraction of “background” NO2 formation measured using the H/SSZ-13 parent material) as a function of Fe loading at 100, 120 and 140 °C. Reaction rates increase linearly with increasing Fe loading at all temperatures (linearity at 140 °C is not quite ideal, presumably because NO conversions are somewhat higher than differential conversions at this temperature).


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Under “wet” conditions, as displayed in Figure 11(a), strong H2O inhibition of NO oxidation is observed. For example, even the 1.20% Fe sample merely gives an NO conversion of ~7.5% at 250 °C. Interestingly, NO2 formation rolls over only above ~400 °C, suggesting mitigation of H2O inhibition with increasing temperature. At reaction temperatures higher than ~200 °C, the reverse reaction (i.e., 2NO2 → 2NO + O2) cannot be neglected for detailed kinetics analysis. Using the equation = (1 − ), where represents the approach to equilibrium,61 forward reaction rates between 200 and 350 °C were calculated and used for further analysis, and Figure 11(b) presents the resultant Arrhenius plots. Based on these, apparent activation energies for the forward “wet” reaction were calculated and the results are also presented in Table 4. For all of the 5 samples, Ea ≈ 32 kJ/mol, again very similar among samples, suggesting identical active centers in all samples. It is somewhat puzzling that apparent activation energies measured under “dry” and “wet” conditions are similar. Note, however, that data used for calculating the activation energies were collected at very different temperatures for the two cases. Since the extent of H2O inhibition is expected to decrease with increasing temperature, it appears that even though H2O competes with NO/O2 for active sites, it does not seem to alter their nature in a significant way at temperatures where the “wet” kinetic data used for activation energy calculations (200-350 °C) were collected. Figure 11(c) plots correlations between forward rates (again, after “background” subtraction) and Fe loadings at 250 and 300 °C. Similar to the results shown in Figure 10(c), reaction rates increase roughly linearly with Fe loading. Note, however, that data collected at 300 °C do not extrapolate exactly to the origin. The exact cause for this is not entirely clear, but could be due to errors introduced during background reaction subtraction and forward reaction calculations. This is not a significant point worthwhile addressing in the following. 18 ACS Paragon Plus Environment

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3.2.3 NH3 oxidation (4NH3 + 3O2 = 2N2 + 6H2O) Figure 12(a) depicts NH3 oxidation conversion-temperature curves for the 5 Fe/SSZ-13 catalysts. No NH3 conversion is found below 300 °C. For each catalyst, NH3 conversions increase gradually with increasing temperature and no apparent “light-off” behavior is found. This is quite different from standard SCR where clear “light-off” is observed at temperatures where the catalysts are essentially inactive for NH3 oxidation (Figure 9(a)). Figure 12(b) presents Arrhenius plots using data points at differential conversions shown in Figure 12(a). Apparent reaction activation energies are calculated and the results are also displayed in Table 4. For the 3 lower Fe-loaded samples, these values are similar. However for the two higher loaded samples, apparent activation energies are clearly higher. Note that this behavior is very different from standard SCR and NO oxidation reactions in which, as shown in Table 4, apparent activation energies remain relatively unchanged with Fe loading. This suggests that the nature of the active sites for NH3 oxidation changes with Fe loading, as will be discussed in more detail in the next section. Figure 12(c) plots normalized rates versus Fe loading curves at three selected temperatures (350, 375 and 400 °C) where differential NH3 conversions are largely maintained. Different from standard SCR and NO oxidation, a linear relationship between normalized rates and Fe loading is not found in this case. Interestingly, linear relationships are found that can be described as

!

∝ ( #$%&'())* . Figure 12(d) gives an example for the case of α = 1.5,

where excellent linear relationships are obtained for catalysts with Fe loadings ≤ 0.83%. In fact, for α = 1.5±0.2, decent linear relationships are always maintained with adjusted R-squared values > 99%. Note, however, that rates obtained from the 1.20% Fe sample deviate substantially from the linear expressions.


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4. Discussion 4.1 Nature of the Fe species Fe/zeolite catalysts have been extensively studied for hydrocarbon and ammonia SCR applications.1-10 Among which, Fe/ZSM-5 and Fe/beta have received the most attention. Early studies had a clear focus on their hydrothermal stabilities; quite a few of which demonstrated the importance of high Fe loading in benefiting hydrothermal stabilities.43, 44, 53 While a high Feloaded catalyst may be needed for hydrocarbon SCR, low to moderate Fe loadings appear to be a better choice for NH3-SCR, especially considering the fact that SCR selectivities may decrease at high Fe loadings (due to competition from NH3 oxidation).20,

23, 62-64

As has been described

earlier, the nature of Fe species in Fe/zeolites can be rather complicated, ranging from isolated Fe ions with different oxidation states and coordinations, to oligomeric FexOy clusters of varying nuclearity, and Fe-oxide particles with different sizes and crystallinities. The origin for this complexity, as suggested by Brandenberger et al., is the similar energy of formation (per Fe) for different Fe-containing species.65 At the current state-of-the-art, it is nearly impossible to precisely quantify all of them. In this regard, it is expected that lowering Fe loading and using structurally simpler zeolites could considerably simplify the nature and location of Fe species. In the present study, SSZ-13 is chosen since its CHA structure is one of the simplest zeolite structures. We also only focus on low Fe-loaded catalysts with Fe/Al ratios no higher than ~0.2. Still, for fair comparisons to be made among samples, it is important that these have similar structural characteristics (e.g., surface areas and pore characters) and uniform dispersion of Fe sites. As shown in Table 1, all five samples have similar surface areas and pore volumes, expected since these were prepared from the same batch of SSZ-13 material. For the dispersion


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of Fe sites, it is most desirable that all Fe ions stay in cationic sites, and that these are uniformly dispersed within the zeolite particles. For our solution ion-exchange method, recommendations from literature (i.e., using conditions with relatively low pH and a temperature of 80 °C to reduce the size of the Fe2+ hydration sphere for better ion transport, as well as under protection of an inert gas to avoid hematite formation)65-67 have been carefully followed. The ion-exchange duration chosen here (1 h), however, is less than literature recommendations (> 5 h),67 the tradeoff being that, while longer ion-exchange times may lead to more uniform dispersion of Fe ions, it also increases the possibility for zeolite dealumination. In our case, post ion-exchange ICP analysis indicates no apparent dealumination in our samples as compared to the parent SSZ-13 material. As displayed in Figure 1, small amounts of Fe2O3 particles (UV-Vis features > 400 nm) do form in our samples, a sign of non-uniform dispersion. Unfortunately, UV-Vis and Mössbauer spectroscopies cannot make precise quantification of this species as will be detailed below. Qualitatively, it is safe to state however, that Fe2O3 only comprises a very small fraction of Fe present even in the 1.20% Fe sample. From NO titration FTIR measurements, we can further corroborate this statement (Figure S-2). For the portion of Fe-ions that are exchanged into SSZ13 particles (monomers and oligomers), it is difficult to determine whether or not these are uniformly dispersed. With the aid of atom probe tomography (Figure S-3), it can be argued that Fe-ions are largely uniformly dispersed in our catalysts. This notion, however, has to be treated with caution since the sampling volume (~105 nm3) is only a very small portion of an average SSZ-13 particle (~109 nm3). Overall, as far as catalyst synthesis is concerned, we believe that our catalysts were prepared with as few variables as manageable for spectroscopic and kinetics studies.


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There are two objectives for the present study: understanding the nature of Fe species, and establishing structure-activity relationships in SCR and related chemistry catalyzed by Fe/SSZ-13. The second objective is best done using operando methods; these, unfortunately, were not available in the course of this study. Therefore, a more traditional strategy is applied where catalysts are examined prior to and after reaction tests to infer changes that might have occurred to the active sites during reaction. From UV-Vis measurements shown in Figure 2, clearly there is some degree of Fe-ion monomer oligomerization occurring during use. For the 0.27% Fe-loaded sample, the monomer signal drop is even quite dramatic (~18%). However, since the sampling cross sections for different Fe species are not known, the extent of oligomerization is not readily estimated. Importantly, samples after ~40 h of use only display insignificant variations in activity (Figures 9-11), suggesting instead that interconversion between different Fe species is a slow process, and concentrations of the various Fe species in the fresh samples can be used as a good approximation of the Fe species under reaction conditions. This conclusion serves as the basis for why the majority of characterizations here were done on fresh samples, and structure-function relationships are based largely on such characterizations. As displayed in Figures 1 and 3, for the samples with Fe loadings at 0.27 and 0.45%, isolated and oligomeric Fe species dominate. Even for the highest Fe-loaded sample, Fe-oxide particles are still minority species. For isolated Fe sites, it is important to elucidate their locations and coordinations. For oligomeric Fe species, knowing their nuclearity is also important. NH3TPD results shown in Figure 4, especially the slope of the linear fit displayed in the insert, suggest that each ligated Fe-ion balances one negative framework charge. This leads naturally to a speculation that the most abundant isolated and oligomeric Fe(III) species are [Fe(OH)2]+ and


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[HO-Fe-O-Fe-OH]2+ in hydrated samples. Mössbauer and H2-TPR measurements also reveal the presence of Fe(II) species in all of the samples. The nature of this species (i.e., they are nonrecoil free at room temperature, therefore, not readily detectable by Mössbauer spectroscopy) indicates that they are hydrated isolated Fe2+ ions. In principle, each Fe2+ ion should balance 2 framework charges. However, Fe2+ ions are not sufficiently abundant in any of the Fe/SSZ-13 samples to cause a deviation of the linear expression shown in the insert of Figure 4. The assignment of the dominant oligomeric Fe(III) species as dinuclear [HO-Fe-O-Fe-OH]2+ also receives strong support from Mössbauer measurements: in particular, high QS values for Fe(III)P (Table 2) are typical for dinuclear Fe3+ complexes.52 Furthermore, the “oligomer” species shown in Figure 3 and the “Fe(III)-P” species shown in Figure 8 share a high degree of similarity: both increase linearly with Fe loading and both are the second most abundant Fe3+ moieties, indicating that they are the same species. Therefore, we assign the major oligomeric species as [HO-Fe-O-Fe-OH]2+. However, this conclusion does not exclude the presence of other minor oligomeric Fe species, and this will be addressed further below when discussing NH3 oxidation kinetics. Moreover, the precise locations for [HO-Fe-O-Fe-OH]2+ cannot be determined from our characterizations. More details on isolated Fe3+ sites can be derived from UV-Vis, EPR and Mössbauer measurements. As shown from Figure 1, UV-Vis features for monomeric Fe3+ ions are readily fit using two components at ~220 and ~270 nm, respectively. In Fe-Silicalite synthesized using a “one-pot” method where isomorphous substitution of Si4+ with Fe3+ occurs, Bordiga et al. assigned CT bands at 215 and 241 nm to framework tetrahedral Fe3+ sites, and the band at 278 nm to isolated Fe3+ in octahedral complexes.40 In Fe/ZSM-5 catalysts formed using a few different ion-exchange procedures, Kumar et al. assigned the CT band at 228 nm to isolated


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tetrahedral Fe3+ and the band at 290 nm to isolated Fe3+ with higher coordinations.20 In Fe/beta formed via aqueous ion-exchange, bands at 235 and 275 nm were found and assigned to isolated tetrahedral and octahedral Fe3+ ions, respectively, by Kim, et al.19 It is important to emphasize again that formation of framework Fe3+ is unlikely from solution ion-exchange.20 The strongest evidence supporting this argument from the present study comes from H2-TPR and Mössbauer quantifications. As shown in Figure 5(b), the linear fitting of H2 consumption versus Fe loading indicates that ~0.1 wt% of Fe is non-reducible up to 700 °C. Mössbauer quantification results from Figure 8 demonstrate similar amounts of Fe(II) in all of the Fe/SSZ-13 samples. Therefore, we assign the ~220 nm band in Fe/SSZ-13 to isolated extra-framework Fe3+ in tetrahedral coordination, and the ~270 nm band to isolated extra-framework Fe3+ in higher coordinations. Such assignments find support from both EPR and Mössbauer measurements. From EPR results shown in Figure 6, signals with g’ ≈ 8.8, 6.0, 4.3 and 2.0 are assigned to isolated extraframework Fe3+ and signals at g’≈ 2.3 to small oligomeric FexOy clusters. In Fe/zeolites containing isomorphous substituted Fe3+, a g’ ≈ 4.3 feature has frequently been assigned to framework Fe3+.47-50 However, we once again emphasize the almost certain absence of framework Fe3+ in our samples. Since Fe3+ ions giving rise to g’ ≈ 4.3 are more distorted than the ones giving rise to g’ ≈ 8.8, 6.0,20 it is reasonable to assign the g’ ≈ 4.3 feature to extraframework Fe3+ in tetrahedral coordination (i.e., the ~220 nm band from UV-Vis measurements), and the g’ ≈ 8.8, 6.0 features to extra-framework Fe3+ in higher coordinations (i.e., the ~270 nm band from UV-Vis measurements). The assignment for the g’≈ 2.0 signal is more difficult. Note that this signal arises either from isolated Fe3+ ions in perfect tetrahedral coordination, or from FexOy clusters in which magnetic interactions between the Fe3+ ions average out the zero field splitting. However, a dinuclear species is not expected to give rise to this feature.20, 40 Note from 24 ACS Paragon Plus Environment

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Figure 6 that the g’≈ 2.0 signal is rather sizable, and formation of large amounts of oligomers other than dimers are not expected (at least for the low Fe-loaded samples). Therefore, the g’≈ 2.0 signal is best assigned to isolated extra-framework Fe3+ in tetrahedral coordination, yet in local environments different from the ones giving rise to the g’ ≈ 4.3 feature. The assignment for the g’≈ 2.3 signal is quite straightforward; it is generally attributed to oligomeric FexOy clusters.47-50 Previous Mössbauer studies on Fe/zeolites have suggested that CS < 0.3 mm/s at room temperature can be assigned to Fe3+ ions in tetrahedral coordination.52 It is also known that CS values increase up to 0.15 mm/s when the measurement temperature drops from room temperature to 77 K.68 Since our Mössbauer measurements were performed at 8 K, a further increase in CS compared to room temperature values is anticipated. Therefore, components with CS < 0.4 shown in Table 3 can be safely assigned to isolated Fe3+ ions in tetrahedral coordination. Components with higher CS values are assigned to isolated Fe3+ ions in higher coordinations and multinuclear Fe-oxides. Overall, semi-quantification of the various Fe species in our Fe/SSZ-13 catalysts is possible via the combination of three spectroscopic methods, coupled with H2-TPR and NH3TPD titrations. In all samples, isolated (a large portion remaining tetrahedrally coordinated) and oligomeric (mainly dimeric) Fe3+ species dominate while the rest are isolated Fe2+ sites and Fe2O3 particles (in the higher Fe-loaded samples). It is interesting that large amounts of isolated Fe2+ sites (~0.1 wt%) remain for all of the samples calcined in air (Figure 8). Presumably, these are stabilized by residing in 6-membered rings of the CHA structure with 2 Al sites, locations known to stabilize Cu2+ ions in Cu/SSZ-13.9, 15, 17, 18, 69 As noted above, experimentally it is difficult to deduce precise locations of the isolated and oligomeric Fe3+ sites in SSZ-13.


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Theoretical studies by our group and collaborators are currently being carried out trying to provide a more complete picture about Fe/SSZ-13 catalyst structures. 4.2 Structure-function relationships in SCR and related chemistry As shown in the introduction, perhaps the biggest debate for the standard SCR mechanism is how NO is activated. Brandenberger et al. firmly stated in a review article that the uniqueness of the standard SCR process over zeolites is that NO first has to be oxidized to NO2, which is the rate-determining step of the mechanism.4 They further stated that the NO2 is consumed immediately after production in the SCR process (as NO2,ads), so it does not show up as gas-phase NO2. The latter statement clearly defines NO2’s role as a low coverage reaction intermediate, a situation different from fast SCR (NO + NO2 + 2NH3 = 2N2 + 3H2O) where NO2 is an abundant reactant, and, therefore, high surface coverages are expected. If we consider N2O3,ads, N2O4,ads and NO+ species as possible reaction intermediates as suggested previously,4 while reaction between NO and NO2 (eq. 4) is expected to occur in both cases, N2O4 formation and disproportionation (eq. 5) is not expected during standard SCR because of the low

, concentrations.

+ , →

,,

(4)

+ →

-, → . + ,/

(5)

As has been well-documented from the literature, ,/ formation correlates directly with NH4NO3 deposition and N2O formation. Indeed, it is well-known that N2O formation is typically facilitated by increasing the NO2 content of the NOx reactants.30, 54, 70 As shown in equations (1) – (3), while NO oxidation by a dimeric site via a redox mechanism can be readily understood, continuous turnover on a monomeric site is questionable since reoxidation of isolated Fe2+ to Fe3+ moieties by O2 is difficult.34 Having established [HO-


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Fe-O-Fe-OH]2+ and [Fe(OH)2]+ being the dominant dimeric and monomeric Fe3+ sites in our Fe/SSZ-13 catalysts, we propose here that NO oxidation is catalyzed predominately by [HO-FeO-Fe-OH]2+ in Fe/SSZ-13, a species suggested to be responsible for NO oxidation by Sachtler and coworkers years ago.44 This proposal is supported by the linear rate versus Fe loading behavior shown in Figures 10(c) and 11(c), and the quantifications of these sites as displayed in Figure 8(a), also showing a linear increase with Fe loading. The recent proposal by Grünert and coworkers that NO oxidation is catalyzed by monomeric minority sites,25 does not appear to be consistent with our results, or predictions made by theoretical calculations (i.e., monomers have high redox barriers).34 The strong H2O inhibition for the NO oxidation reaction, as clearly shown via a comparison between Figures 10(a) and 11(a), can be understood from competitive chemisorption of H2O with NO onto the [HO-Fe-O-Fe-OH]2+ sites, and/or from competitive chemisorption of H2O with O2 onto the reduced [HO-Fe-□-Fe-OH]2+ sites. These competitive processes do not appear to vary the dimeric nature of the active sites, as indicated by the similar apparent activation energies measured under both “dry” and “wet” reaction conditions (Table 4). Another possible route for H2O inhibition is via hydrolysis of active dimeric Fe sites to inert monomeric sites. The reversible dimer hydrolysis has long been suggested and confirmed via Mössbauer spectroscopy.71 It is important to note from a few recent studies, however, that NO oxidation and standard SCR kinetics are not closely correlated.25, 32, 60 For the standard SCR reaction, the same proposal (i.e., dimeric sites provide the majority of reactivity) may be made. As a redox reaction, Fe-ions that can readily cycle between Fe(III) and Fe(II) oxidation states seem highly likely to be most active,24 and facile redox cycling between Fe(III) and Fe(II) in oligomeric sites is readily understood. However, this assignment is


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in contrast to recent proposals that isolated (monomeric) Fe3+ ions are active sites for NH3-SCR on a few Fe/zeolites.20, 24, 26 As noted earlier, the biggest uncertainty for assigning isolated Fe3+ ions as active centers lies in the uncertainty for Fe2+ reoxidation by O2 that has frequently been suggested. DFT studies by Li et al. suggest that a prototypical Fe(II) site, [FeIIOH]+, can only be reoxidized by NO2 but not by O2 or O2/NO mixtures in the presence of NH3.34 Note specifically that for a [Fe(OH)2]+ active center, one of the likely reduced forms is [FeIIOH]+ as will be shown below. In the present study, all of our Fe/SSZ-13 samples have been calcined in air prior to Mössbauer analysis, yet the quantification results shown in Table 2 demonstrate the presence of substantial amounts of Fe2+ ions. This corroborates the theoretical study addressing the difficulty for reoxidation of isolated Fe2+ by O2. However, there exists rather strong evidence not to exclude isolated Fe3+ ions as active centers, especially at relatively low reaction temperatures. Therefore, alternative oxidation pathways for isolated Fe2+ ions are needed. This will be discussed with more detail below. As displayed in Figure S-1, the Fe/SSZ-13 catalysts maintain excellent SCR selectivities at temperatures ≤ 260 °C. At higher temperatures, NH3 conversions become higher than NO conversions, and the extent of these differences increases with increasing Fe loading. At temperatures where NH3 oxidation by O2 alone is insignificant (e.g., ≤ 350 °C, Figure 12(a)), NH3 overconsumption during SCR must be due to reactions between NH3 and NOx species with oxidation states for N higher than +3. Note that in both standard and fast SCR, NH3 and NOx are consumed at a stoichiometric ratio of unity (i.e., no NH3 overconsumption). NO2-SCR (6NO2 + 8NH3 = 7N2 + 12H2O), on the other hand, does proceed with considerable NH3 overconsumption. The results displayed in Figure S-1, then may indicate dramatic differences in SCR reaction pathways below and above ~260 °C. This, in turn, suggests differences in active centers below


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and above ~260 °C; i.e., active sites below ~260 °C are less associated with NO2 formation while active sites above ~260 °C are more associated with NO2 formation. In line with the proposal by Brandenberger et al. that isolated Fe-ions provide the majority of SCR activity below ~300 °C for Fe/ZSM-5, together with the fact that an oligomeric site is responsible for NO oxidation, we herein propose the same for Fe/SSZ-13; i.e., low-temperature activity is provided by isolated Feions and high-temperature activity is governed by oligomeric sites. An important experimental finding that supports this proposal is shown in Figure 9(b), where the lower activity for the 0.27% Fe sample can be understood from the Mössbauer quantifications (Table 2). By subtracting the Fe(II) components from the total Fe loadings, Figure 9(b) is replotted displaying normalized SCR rates versus Fe(III) components of the samples, and the results are shown in Figure S-4. In this case, the “abnormal” behavior for the 0.27% Fe sample evidenced in Figure 9(b) is no longer obvious, demonstrating that the substantially higher Fe(II) percentage for isolated Fe-ions in this sample explains, to a large extent, its lower SCR activity. The key uncertainty in assigning isolated Fe(III) as low-temperature active centers for SCR, again, is redox for these sites. To summarize, among other things, there appear to be three key discoveries discussed earlier in literature regarding mechanisms/active sites for standard SCR on Fe/zeolites, that are highly relevant to the current study: (1) isolated Fe3+ ions appear to contribute significantly in low-temperature SCR activity, yet their own redox by O2, a key step for continuous turnover, appears questionable; (2) NO2 promotes low-temperature SCR rates but its formation kinetics clearly do not correlate with SCR kinetics;31 (3) HNO2/nitrite species are likely intermediates that lead to high SCR selectivity, while NO2 likely contributes to both SCR (i.e., promoting the fast SCR pathway) and parasitic NH3 oxidation (i.e., contributing the NO2-SCR pathway). To reconcile these (sometimes contradictory) discoveries, we propose in the following that the 29 ACS Paragon Plus Environment

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complex situation can be simplified to one primary issue: whether NO is activated by reacting with Fe–OH, or Fe–O–Fe active centers. In low Fe-loaded Fe/SSZ-13, it is further inferred that Fe(OH)2+ is the carrier of the Fe-OH active center and [HO-Fe-O-Fe-OH]2+ is the carrier of the Fe–O–Fe active center. For Fe-OH active centers, the following reactions comprise the major SCR pathway: − 0 + ⇌ + 0

(6)

0 + 0, → 0- →

+ 20

(7)

2 + 34 + 0 ⇌ 2 − 0

(8)

Reactions (6)-(8) provide a selective path to NO reduction to N2, where reaction (6) has been proven via an elegant trapping method32 and reaction (7) has long been recognized.4 The viability of this pathway relies on the efficiency of reaction (8), a process proven not to proceed without the participation of H2O.34 For this reaction to proceed efficiently, solvation effect from H2O, which renders high mobility of isolated Fe2+ ions (Fe(OH)+ in the case of Fe/SSZ-13), is expected to be key to the redox process since this requires the participation of two Fe2+ ions. This process, therefore, is destined to occur only at relatively low temperatures. As reaction temperatures rise, the loss of solvation water will force isolated Fe-ions to locate at zeolite framework sites and, correspondingly, will render redox barriers too high and this reaction pathway insignificant. For Fe-O-Fe active sites, NO activation is shown in reaction (1), with NO2 (from an ONO intermediate) as the immediate product. As described earlier, this species is a low concentration surface species, indicating that its disproportionation reaction leading to NO+ and NO3- (reaction (5) shown above) is unlikely to occur. The lack of this reaction directly links to the lack of N2O 30 ACS Paragon Plus Environment

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formation during standard SCR (as compared to fast SCR). Instead, reactions between NO2(ads) and the abundant reactants NO and NH3, are expected to occur upon NO2(ads) formation. On condition that NO2(ads) species interact first with NO, another high selective SCR pathway can be constructed using reactions (4) shown above, and (9) below.

, + 2 0, = 2

+ 30

(9)

This is essentially one of the suggested fast SCR pathways.4 However, the presence of parasitic NH3 oxidation, as demonstrated from numerous prior studies on Fe/zeolites, also suggests direct interactions between NO2(ads) and NH3. Although the detailed pathways might be rather complicated as evidenced from kinetic studies using isotope-labeled reactants,57 a simple global path can be written as follows:

+ 2 0, + → +

+ 30

(10)

Even though the exact nature of the NO2-NH3 complexes is not known, isotope-labeled experiments indicate scrambling of N atoms in such complexes.57 In any case, reaction (10) is much simpler and, therefore, more reasonable than a global NO2-SCR pathway (6NO2 + 8NH3 = 7N2 + 12H2O) in explaining parasitic NH3 oxidation. This is due, again, to the low NO2(ads) situation met in standard SCR so that reaction paths involving multiple NO2 molecules are unlikely. It is interesting to note that the [HO-Fe-O-Fe-OH]2+ species also has Fe-OH active centers and, in principle, should also catalyze SCR via reactions (6)-(8). While this certainly cannot be excluded, these species are expected to be much less mobile for reaction (8) to proceed efficiently. From kinetic reasons, they may not be key active sites at low temperatures. Following the above discussions, it appears that the complex situation regarding the relationships between NO2 formation and standard SCR can be rationalized such that, on isolated 31 ACS Paragon Plus Environment

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active sites, these two processes are uncorrelated while, on dimeric active sites, they are closely coupled. Depending on reaction temperature, Fe content and distribution, as well as other minor issues, reaction kinetics measured and the conclusions derived by different researchers are highly case specific resulting in the current complex situation. Finally, NH3 oxidation results shown in Fig 12 are discussed. Note first that NH3 oxidation is also a redox reaction. Considering the fact that monomeric Fe ions are difficult to be reoxidized to Fe3+ ions from Fe2+ by O2, it is reasonable to suggest only oligomeric Fe species and Fe2O3 particles are active for this reaction. According to Brandenberger et al., monomeric sites are completely inactive for NH3 oxidation up to 500 °C.23 Note first that [HO-Fe-O-FeOH]2+ ions are the major oligomeric species in our samples. Note also from Table 4 that the activation energies are sizable, especially for the two highest Fe-loaded samples. On the condition that active sites are abundant, “light-off” behavior similar to that shown in Figure 9(a) for SCR is expected. Therefore, the lack of “light-off” seen in Figure 12(a) for NH3 oxidation can only be explained from the low concentrations of active sites. This suggests that even [HOFe-O-Fe-OH]2+ ions are likely not active sites for NH3 oxidation, as supported by the results shown in Figure 12(c). Were these species the major active sites for NH3 oxidation, one would expect a linear rate versus Fe loading relationship, the same as in NO oxidation (Figures 10(c) and 11(c)). Apparently this is not the case yet, interestingly, a rather unusual linear rate versus (Fe loading)1.5±0.2 relationship is found as displayed in Figure 12(d). This unusual behavior suggests that the active species are oligomers larger than dimers, and perhaps also surface sites on Fe2O3 particles, sites that are relatively minor in our Fe/SSZ-13 samples. This assignment appears to be consistent with the popular hypothesis that NH3 oxidation occurs via a mechanism similar to standard SCR, where a portion of NH3 is oxidized to NO, which further reacts with the 32 ACS Paragon Plus Environment

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remaining NH3.72, 73 Thus, in order for NH3 to be oxidized to NO, active oligomeric sites with more than one extra lattice oxygen (ELO), or surface sites of Fe2O3 particles are needed.

5 Conclusions The nature of Fe species in a series of low Fe-loaded Fe/SSZ-13 catalysts synthesized with solution ion exchange are probed using UV-Vis, EPR and Mössbauer spectroscopies, coupled with temperature programmed reduction and desorption techniques. The major monomeric and dimeric Fe species are extra-framework [Fe(OH)2]+ and [HO-Fe-O-Fe-OH]2+. Larger oligomers with unknown nuclearity, poorly crystallized Fe2O3 particles, together with isolated Fe2+ ions are minor Fe-containing moieties. Based on these characterizations, semi-quantification of the various Fe species is achieved. In standard NH3-SCR and NO oxidation reactions, apparent reaction activation energies do not change with Fe loading, suggesting identical active centers in different samples. Reaction rate and Fe loading correlations suggest that isolated Fe3+ ions are the active sites for standard SCR at low temperatures (≤ 260 °C). Since redox cycling of these sites requires H2O solvation and ion mobility, these lose high temperature activities. On the other hand, the dimeric sites are the active centers for NO oxidation to NO2, and processes involving NO2 (fast SCR and parasitic NH3 oxidation). NH3 oxidation, on the other hand, is catalyzed by minority sites with higher nuclearity. Supporting Information Available: (1) NO/NH3 light-off curves on three Fe/SSZ-13 catalysts with Fe loadings of 0.27, 0.62 and 1.20%; (2) NO chemisorption from FTIR on Fe/SSZ-13 catalysts prepared with and without inert atmosphere protection; (3) atom probe tomography imaging on Fe/SSZ-13 as a way to probe Fe distribution. (4) Normalized SCR rates versus Fe(III) loadings. This material is available free of charge via the Internet at http://pubs.acs.org. 33 ACS Paragon Plus Environment

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Acknowledgements The authors gratefully acknowledge the US Department of Energy (DOE), Energy Efficiency and Renewable Energy, Vehicle Technologies Office for the support of this work. The research described in this paper was performed in the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for the US DOE by Battelle. Discussions with Drs. A. Yezerets, K. Kamasamudram, N. Currier, Y.H. Zha and J.Y. Luo from Cummins, Inc., and H.Y. Chen, H. Hess and Z.H. Wei from Johnson-Matthey are greatly appreciated. The authors thank Dr. Arun Devaraj (PNNL) for atom probe tomography measurements.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

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16. Bates, S. A.; Verma, A. A.; Paolucci, C.; Parekh, A. A.; Anggara, T.; Yezerets, A.; Schneider, W. F.; Miller, J. T.; Delgass, W. N.; Ribeiro, F. H. J Catal 2014, 312, 87-97. 17. McEwen, J. S.; Anggara, T.; Schneider, W. F.; Kispersky, V. F.; Miller, J. T.; Delgass, W. N.; Ribeiro, F. H. Catal Today 2012, 184, (1), 129-144. 18. Deka, U.; Juhin, A.; Eilertsen, E. A.; Emerich, H.; Green, M. A.; Korhonen, S. T.; Weckhuysen, B. M.; Beale, A. M. J Phys Chem C 2012, 116, (7), 4809-4818. 19. Kim, J.; Jentys, A.; Maier, S. M.; Lercher, J. A. J Phys Chem C 2013, 117, (2), 986-993. 20. Kumar, M. S.; Schwidder, M.; Grunert, W.; Bruckner, A. J Catal 2004, 227, (2), 384-397. 21. Schwidder, M.; Kumar, M. S.; Klementiev, K.; Pohl, M. M.; Bruckner, A.; Grunert, W. J Catal 2005, 231, (2), 314-330. 22. Schwidder, M.; Grunert, W.; Bentrup, U.; Bruckner, A. J Catal 2006, 239, (1), 173-186. 23. Brandenberger, S.; Krocher, O.; Tissler, A.; Althoff, R. Appl Catal B-Environ 2010, 95, (3-4), 348357. 24. Velez, R. P.; Ellmers, I.; Huang, H. M.; Bentrup, U.; Schunemann, V.; Grunert, W.; Bruckner, A. J Catal 2014, 316, 103-111. 25. Ellmers, I.; Velez, R. P.; Bentrup, U.; Schwieger, W.; Bruckner, A.; Grunert, W. Catal Today 2015, 258, 337-346. 26. Hoj, M.; Beier, M. J.; Grunwaldt, J. D.; Dahl, S. Appl Catal B-Environ 2009, 93, (1-2), 166-176. 27. Rahkamaa-Tolonen, K.; Maunula, T.; Lomma, M.; Huuhtanen, M.; Keiski, R. L. Catal Today 2005, 100, (3-4), 217-222. 28. Grossale, A.; Nova, I.; Tronconi, E.; Chatterjee, D.; Weibel, M. J Catal 2008, 256, (2), 312-322. 29. Grossale, A.; Nova, I.; Tronconi, E.; Chatterjee, D.; Weibel, M. Top Catal 2009, 52, (13-20), 18371841. 30. Iwasaki, M.; Shinjoh, H. Appl Catal a-Gen 2010, 390, (1-2), 71-77. 31. Ellmers, I.; Velez, R. P.; Bentrup, U.; Bruckner, A.; Grunert, W. J Catal 2014, 311, 199-211. 32. Ruggeri, M. P.; Selleri, T.; Colombo, M.; Nova, I.; Tronconi, E. J Catal 2014, 311, 266-270. 33. Delahay, G.; Valade, D.; Guzman-Vargas, A.; Coq, B. Appl Catal B-Environ 2005, 55, (2), 149-155. 34. Li, J.; Li, S. H. J Phys Chem C 2008, 112, (43), 16938-16944. 35. Gao, F.; Kollár, M.; Kukkadapu, R. K.; Washton, N. M.; Wang, Y. L.; Szanyi, J.; Peden, C. H. F. Appl Catal B-Environ 2015, 164, 407-419. 36. Gao, F.; Walter, E. D.; Kollar, M.; Wang, Y. L.; Szanyi, J.; Peden, C. H. F. J Catal 2014, 319, 1-14. 37. Gao, F.; Walter, E. D.; Washton, N. M.; Szanyi, J.; Peden, C. H. F. Acs Catal 2013, 3, (9), 20832093. 38. Rancourt, D. G.; Ping, J. Y. Nucl Instrum Meth B 1991, 58, (1), 85-97. 39. Di Iorio, J. R.; Bates, S. A.; Verma, A. A.; Delgass, W. N.; Ribeiro, F. H.; Miller, J. T.; Gounder, R. Top Catal 2015, 58, (7-9), 424-434. 40. Bordiga, S.; Buzzoni, R.; Geobaldo, F.; Lamberti, C.; Giamello, E.; Zecchina, A.; Leofanti, G.; Petrini, G.; Tozzola, G.; Vlaic, G. J Catal 1996, 158, (2), 486-501. 41. Brandenberger, S.; Krocher, O.; Wokaun, A.; Tissler, A.; Althoff, R. J Catal 2009, 268, (2), 297-306. 42. Lobree, L. J.; Hwang, I. C.; Reimer, J. A.; Bell, A. T. J Catal 1999, 186, (2), 242-253. 43. Chen, H. Y.; Sachtler, W. M. H. Catal Today 1998, 42, (1-2), 73-83. 44. Voskoboinikov, T. V.; Chen, H. Y.; Sachtler, W. M. H. Appl Catal B-Environ 1998, 19, (3-4), 279287. 45. Mauvezin, M.; Delahay, G.; Coq, B.; Kieger, S.; Jumas, J. C.; Olivier-Fourcade, J. J Phys Chem B 2001, 105, (5), 928-935. 46. Gao, F.; Washton, N. M.; Wang, Y. L.; Kollár, M.; Szanyi, J.; Peden, C. H. F. J Catal 2015, 331, 2538. 47. Mcnicol, B. D.; Pott, G. T. J Catal 1972, 25, (2), 223-&. 35 ACS Paragon Plus Environment

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Table 1: The amount of FeSO4⋅7H2O used during ion-exchange with 2 g of NH4/SSZ-13 (Si/Al = 12), the Fe loadings, Fe/Al ratios, and surface area/pore volumes of the calcined Fe/SSZ-13 catalysts. Sample

1

2

3

4

5

FeSO4⋅7H2O weight (g)

0.1

0.2

0.5

1.0

2.0

Fe loading (wt.%)

0.27

0.45

0.62

0.83

1.20

Fe/Al ratio

0.041

0.068

0.093

0.125

0.180

surface area (m2/g)

532

542

538

518

549

t-plot pore volume (cm3/g)

0.289

0.294

0.293

0.282

0.299

Table 2: Simulated key Mössbauer parameters and estimated percentages of various Fe components in the 57Fe/SSZ-13 samples measured at 8 K. Sample (Fe wt.%)

0.27

0.45

0.62

0.83

2 mm/s

3 mm/s

σQS4 mm/s

5 mm/s

6 Tesla (T)

σH 7 T

% phase8

Fe(II)-P

1.4

3.34

0.29

−

−

−

35.2

Fe(III)-P

0.54

0.89

0.46

−

−

−

15.4

Fe(III)-M

0.46

−

−

0.02

43.83

15.25

49.4

Fe(II)-P

1.4

3.31

0.29

−

−

−

22.6

Fe(III)-P

0.53

0.84

0.45

−

−

−

19.2

Fe(III)-M

0.5

−

−

-0.001

41.05

15.82

58.2

Fe(II)-P

1.39

3.31

0.27

−

−

−

18.1

Fe(III)-P

0.51

0.83

0.43

−

−

−

20.3

Fe(III)-M

0.49

−

−

-0.008

39.38

16.34

61.6

Fe(II)-P

1.4

3.34

0.29

−

−

−

13.9

Fe(III)-P

0.53

0.89

0.48

−

−

−

23.6

Fe(III)-M

0.5

−

−

0.006

37.2

17

62.5

Fe(II)-P

1.39

3.33

0.29

−

−

−

10.9

Fe(III)-P

0.52

0.88

0.5

−

−

−

25.5

Phase

1

1.20 Fe(III)-M 0.49 0.005 35.92 17.3 63.6 − − Spectral component (P = paramagnetic and M = Magnetic); 2average center shift (CS); 3average quadrupole splitting (QS), 4standard deviation of QS; 5average quadrupole shift parameter (ε); 6average magnetic hyperfine field (H); 7standard deviation of H; 8Fe atom% (assuming identical recoilless fraction for all the species). 1


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Table 3: Refined simulation of the Fe(III)-M component in 57Fe/SSZ-13 samples with Fe contents of 0.27 and 1.20 wt.% measured at 8 K. 2 mm/s

3 mm/s

4 Tesla (T)

σH 5 T

% phase6

Fe(III)-M1

0.45

0.027

55.5

2.61

17.6

Fe(III)-M2

0.1

-0.314

20.5

6.91

10.8

Fe(III)-M3

0.21

-1.12

42.1

11.4

22.0

Fe(III)-M1

0.48

0.04

55.3

2.6

11.2

Fe(III)-M2

0.44

-0.04

17.4

6.9

23.1

Sample (Fe wt.%)

0.27

Phase

1

1.20 Fe(III)-M3 0.31 -0.19 44.3 11.4 29.5 Spectral component (M = Magnetic); 2average center shift (CS); 3average quadrupole shift parameter (ε); 4average magnetic hyperfine field (H); 5standard deviation of H; 6Fe atom% (assuming identical recoilless fraction for all the species). 1

Table 4: apparent reaction activation energies for standard SCR, NO and NH3 oxidation catalyzed by the Fe/SSZ-13 catalysts. Fe loading (wt.%)

0.27

0.45

0.62

0.83

1.20

Ea (kJ/mol)1

54.8 ± 2.6

53.2 ± 3.7

52.0 ± 1.3

52.1 ± 1.9

51.1 ± 2.2

Ea (kJ/mol)2

35.8

37.8

35.4

34.7

35.2

Ea (kJ/mol)3

32.2 ± 1.6

33.2 ± 1.3

32.3 ± 1.8

31.9 ± 1.1

30.6 ± 0.7

Ea (kJ/mol)4

60.2 ± 2.2

58.6 ± 2.4

59.4 ± 0.6

76.5 ± 1.4

81.3 ± 0.7

1: standard SCR reaction, measured between 220-280 °C; 2: dry NO oxidation, measured between 100 and 140 °C; 3: wet NO oxidation, measured between 200 and 350 °C; 4: wet NH3 oxidation, measured between 300 and 375 °C.


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Figure Captions

Figure 1: UV-Vis spectra of the fresh, ambient Fe/SSZ-13 samples with different Fe loadings (loadings are marked adjacent to the spectra). Shown in the insert is the peak fitting results of the Fe loading 1.20% sample. Figure 2: UV-Vis spectra of selected fresh, ambient Fe/SSZ-13 samples and their used counterparts. The used catalysts are exposed to SCR and NO/NH3 oxidation reaction environments for ~40 h. Figure 3: quantification of the UV-Vis spectra shown in Figure 1 via peak fitting. Assignments of the different Fe species are marked adjacent to the linear plots. Figure 4: NH3-TPD curves for the parent H/SSZ-13 and the Fe/SSZ-13 samples. NH3 adsorption and purging was carried out at 100 °C prior to TPD. Shown in the insert is a linear fit between normalized NH3 desorption peak areas versus Fe/Al ratios. Figure 5: (a) H2-TPR curves for the ambient Fe/SSZ-13 samples. Fe loadings are marked adjacent to the spectra. (b) Integrated H2 consumption peak areas versus Fe loading linear fit. Shown for comparison is a linear fit of H2 consumption versus Cu loading for Cu/SSZ-13 reference samples. Figure 6: EPR spectra of the ambient Fe/SSZ-13 samples measured at 125 K. Samples with different Fe loadings are displayed with different colors. Effective g factors are marked adjacent to the spectra. Figure 7: (a) Mössbauer spectra of the ambient 57Fe/SSZ-13 samples measured at 8 K. Spectra are shifted vertically and displayed with different colors, and Fe loadings are marked adjacent to the spectra. (b). Peak fitting results of the Fe 0.27% 57Fe/SSZ-13 sample. Percentages of the three components, Fe(II)-P, Fe(III)-P and Fe(III)-M, are also displayed. Figure 8: (a) Loadings of the three Fe components determined from Mössbauer spectroscopy as a function of total Fe loading. (b). Peak fitting results of the Fe 0.27% 57Fe/SSZ-13 sample.


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 53

Percentages of the five components, Fe(II)-P, Fe(III)-P, Fe(III)-M1, Fe(III)-M2 and Fe(III)-M3 are also displayed. Figure 9: (a) Conversion-temperature curves for standard NH3-SCR on Fe/SSZ-13 catalysts. Only NO conversions are shown. Reactant feed contains 350 ppm NO, 350 ppm NH3, 14% O2, 2.5% H2O balanced with N2 at a GHSV of 200,000 h-1. (b) Normalized SCR rates (mol NO g−1 s−1) within the differential regime shown in (a) plotted as a function of the Fe loading at 220, 240 and 260 °C. Note highlighted SCR rates on the Fe 0.27% sample that are abnormally low. (c) Arrhenius plots for standard SCR in the low-temperature regime (≤ 280 °C) indicating similar apparent activation energies for different samples. Figure 10: (a) Conversion-temperature curves for “dry” NO oxidation on the Fe/SSZ-13 catalysts. Reactant feed contains 350 ppm NO, 14% O2 balanced with N2 at a GHSV of 200,000 h-1. (b) Arrhenius plots for dry NO oxidation in the low-temperature regime (≤ 140 °C) indicating similar apparent activation energies for different samples. (c) Normalized NO oxidation rates (mol NO g−1 s−1) shown in (a) plotted as a function of the Fe loading at 100, 120 and 140 °C. Figure 11: (a) Conversion-temperature curves for “wet” NO oxidation on the Fe/SSZ-13 catalysts. Reactant feed contains 350 ppm NO, 14% O2, 2.5% H2O balanced with N2 at a GHSV of 200,000 h-1. (b) Arrhenius plots for wet NO oxidation indicating similar apparent activation energies for different samples. (c) Normalized NO oxidation rates (mol NO g−1 s−1) shown in (a) plotted as a function of the Fe loading at 250 and 300 °C. Figure 12: (a) Conversion-temperature curves for NH3 oxidation on the Fe/SSZ-13 catalysts. Reactant feed contains 350 ppm NH3, 14% O2, 2.5% H2O balanced with N2 at a GHSV of 200,000 h-1. (b) Arrhenius plots for NH3 oxidation demonstrating variation of apparent activation energies with Fe loading. (c) Normalized NH3 oxidation rates (mol NH3 g−1 s−1) within the differential regime shown in (a) plotted as a function of the Fe loading at 350, 375 and 400 °C. (d) Normalized NH3 oxidation rates (mol NH3 g−1 s−1) within the differential regime shown in (a) plotted as a function of the (Fe loading)1.5 at 350, 375 and 400 °C.


Page 41 of 53

0.7

Fe/SSZ-13, Si/Al = 12

0.6

0.7

0.5

Fe Loading (wt%) 1.20 0.83 0.62 0.45 0.27

0.4 0.3 0.2

Absorbance

0.6

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

0.5 0.4 0.3 0.2 0.1 0.0 200 300 400 500 600 700 800

Wavelength (nm)

0.1 0.0 200

300

400

500

600

700

800

Wavelength (nm)

Gao, et al., Figure 1 41 ACS Paragon Plus Environment

ACS Catalysis

Fe ~ 1.20%

0.6 0.4

Fresh Used

0.2

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 42 of 53

0.0 0.6

Fe ~ 0.62%

0.4 Fresh Used

0.2 0.0 0.4 0.3 0.2 0.1 0.0 200

Fe ~ 0.27% Fresh Used

300

400

500

600

700

800

Wavelength (nm)

Gao, et al., Figure 2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Integral UV-Vis Signal (a.u.)

Page 43 of 53

40

30

Monomer@270 nm Oligomer

20

10

0 0.0

Fe2O3 particle

0.2

0.4

0.6

0.8

1.0

1.2

Fe Loading (wt%)

Gao, et al., Figure 3


Page 44 of 53

160 140 Fe Loading (%)

1.00

NH3 Peak Area (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

NH3 Desorption Signal (ppm)

ACS Catalysis

0.27 0.45 0.62 0.83 1.20

120 100 80

β = 0.91 ± 0.05

0.96 0.92 0.88 0.84

60

0.00 0.05 0.10 0.15 0.20

40

Fe/Al Ratio

20 0 200

300

400

500

600

isothermal

Temperature (°C)


Page 45 of 53

(a)

H2 Consumption (a.u.)

4

Fe/SSZ-13, Si/Al = 12

3 Fe Loading (wt%) 1.20 0.83 0.62 0.45 0.27

2

1

0 100

200

300

400

500

600

700

Temperature (°C) (b) H2 Consumption Peak Area (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

4

H2-TPR, Fe/SSZ-13, Si/Al = 12

3 Cu/SSZ-13

2 Fe/SSZ-13

1

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Cu/Fe Loading (wt%)


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 53

g'≈4.3

Fe Loading (wt%) 0.27 0.45 0.62 0.83 1.20

g'≈2.3 g'≈6 g'≈2

g'≈8.8

0

1000

2000

3000

4000

5000

6000

Field (G)


Page 47 of 53

(a) Mössbauer, 8 K

Fe Loading(%)

Intensity (a.u.)

1.20 0.83 0.62 0.45 0.27

-10

-5

0

5

10

Velocity (mm/s) (b)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Fe loading 0.27%

-15

Experiment Fe(III)-P; 15.4% Fe(II)-P; 35.2% Fe(III)-M; 49.4% Model

-10

-5

0

5

10

15

Velocity (mm/s) Gao, et al., Figure 7 47 ACS Paragon Plus Environment

(a)

Page 48 of 53

0.8 0.7 0.6

Fe(III)-M

0.5 0.4 Fe(III)-P

0.3 0.2 0.1 0.0 0.0

Fe(II)-P

0.2

0.4

0.6

0.8

1.0

1.2

Fe Loading (wt%) (b)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Loading of different Components (wt%)

ACS Catalysis

Fe loading 0.27% Experiment Fe(III)-P Fe(II)-P Fe(III)-M1; 17.6% Fe(III)-M2; 10.8% Fe(III)-M3; 22% Model

-15

-10

-5

0

5

10

15

Velocity (mm/s)


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(a)

NO Conversion (%)

100 80 60 40 Fe Loading (wt%) 0.27 0.45 0.62 0.83 1.20

20 0 200

250

300

350

400

450

500

550

600

Reaction Temperature (°C)

(c)

(b) 10

3.5

Fe Loading (wt%) 0.27 0.45 0.62 0.83 1.20

Normalized Rate (mol NO g−1 s−1) × 107

Standard SCR, GHSV = 200,000 h−1

Normalized Rate (mol NO g−1 s−1) × 107

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

3.0 2.5 2.0

Reaction Temperature (°C) 220 240 260

1.5 1.0 0.5

1

0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.1 1.80

Fe Loading (wt%)

1.85

1.90

1.95

2.00

2.05

−1

1000/T (K )


ACS Catalysis

(a)

(b) 10

Fe/SSZ-13, Si/Al = 12, GHSV = 200, 000 h−1 Fe Loading (wt%) 0.27 0.45 0.62 0.83 1.20

80 60 40 20

Normalized Rate (mol NO g−1s−1) × 107

100

100

200

300

400

Dry NO Oxidation, GHSV = 200,000 h−1

5

1

0.5

0

Fe Loading (wt%) 0.27 0.45 0.62 0.83 1.20

2.40

500

2.45

2.50

2.55

2.60

2.65

2.70

−1

1000/T (K )


(c) 5


NO Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Dry NO Oxidation, GHSV = 200,000 h−1

140 °C

4 3

120 °C

2 1 100 °C

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Fe Loading (wt%)


Page 51 of 53

(a)

(b)

NO Oxidation in ~2.5% H2O

Normalized Forward Rate (mol NO g−1 s−1) × 107

25

NO Conversion (%)

−1

GHSV ~ 200,000 h

20

15

10

Fe Loading (wt%) 0.27 0.45 0.62 0.83 1.20

5

0

3 2

1 Fe Loading (wt%)

0.5

0.27 0.45 0.62 0.83 1.20

0.2 1.6

200 250 300 350 400 450 500 550 600

1.7

1.8

1.9

2.0

2.1

2.2

-1

1000/T (K )


(c) 1.4


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Wet NO Oxidation, GHSV = 200,000 h−1

1.2 300 °C

1.0 0.8 0.6

250 °C

0.4 0.2 0.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Fe Loading (wt%)


ACS Catalysis

10

NH3 Oxidation, −1

GHSV = 200,000 h

Normalized Rate (mol NH3 g−1 s−1) × 107

NH3 Conversion (%)

100 80 60 40

Fe Loading (wt%) 0.27 0.45 0.62 0.83 1.20

20 0 300

350

400

450

500

550

Fe Loading (wt%) 0.27 0.45 0.62 0.83 1.20

1

0.1

0.01 1.55


1.60

1.65

1.70

1.75

−1

1000/T (K )

(b)

(a)

400 °C

4

3 375 °C

2 350 °C

1

0


400 °C


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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4

3 375 °C

2 350 °C

1

0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0.0

Fe Loading (wt%)

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.5

{Fe Loading (wt%)}

(d)

(c)


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ACS Catalysis

Graphic Abstract


SSZ-13 NH3-SCR Catalysts: Nature of the

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