Formation and Effect of NH4+ Intermediates in ... - ACS Publications

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Formation and Effect of NH4+ Intermediates in NH3−SCR over FeZSM‑5 Zeolite Catalysts Peirong Chen,*,†,‡ Magdalena Jabłońska,‡,§ Philipp Weide,¶ Tobias Caumanns,⊥ Thomas Weirich,‡,⊥ Martin Muhler,¶ Ralf Moos,# Regina Palkovits,‡,§ and Ulrich Simon*,†,‡ †

Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany Center for Automotive Catalytic Systems Aachen, RWTH Aachen University, Aachen, Germany § Chair of Heterogeneous Catalysis and Chemical Technology, RWTH Aachen University, Worringerweg 2, 52074 Aachen, Germany ¶ Laboratory of Industrial Chemistry, Ruhr-University Bochum, 44780 Bochum, Germany ⊥ Central Facility for Electron Microscopy, RWTH Aachen University, 52074 Aachen, Germany # Department of Functional Materials, Bayreuth Engine Research Center and Zentrum für Energietechnik, University of Bayreuth, Universitätsstraße 30, 95440 Bayreuth, Germany ‡

S Supporting Information *

ABSTRACT: With the help of a technique combining in situ electrical impedance spectroscopy and DRIFT spectroscopy, we observed directly the formation of ammonium ion (NH4+) intermediates resulting from the interaction of NO and NH3 on Fe-ZSM-5 catalysts for selective catalytic reduction by NH3 (NH3−SCR). The formed NH4+ intermediates, indicating the activation of NO in the presence of adsorbed NH3, were found to be strongly related to the NH3−SCR activity of Fe-ZSM-5 catalysts at low temperatures. These findings, which are not easily achievable by conventional methods, provide new and important perspectives to understand mechanistically the NH3−SCR reaction over Fe-zeolite catalysts.

KEYWORDS: ammonium ion intermediates, NO activation, proton transport, redox cycle, NH3−SCR mechanism

C

the formed Cu(I) then interacts with NO and O2 resulting in the reoxidation to Cu(II) and the completion of the catalytic cycle.13−15 As compared to that of Cu-zeolites, a fundamental understanding of NH3−SCR on Fe-zeolite catalysts is more difficult, mainly because of the presence and dynamic change (under SCR reaction conditions) of multiple Fe species, such as isolated Fe cations, oligomeric FexOy clusters, and FexOy particles, and so on.3,5,16−20 Extensive research efforts in the last decades, mostly focusing on Fe-ZSM-5, lead to a wellaccepted understanding that the isolated (monomeric) cations and/or clustered species (i.e., FexOy oligomers) contribute majorly to the NH3−SCR activity of Fe-zeolites.3,18,21−23 Nevertheless, the detailed contributions of these two kinds of active sites under different reaction conditions (e.g., temperature, NO/NO2 ratio) remain as strong debates. In standard NH3−SCR, the reduction of Cu(II) to Cu(I) leads to the generation of an intermediate proton on the adjacent Brønsted site, which subsequently interacts with NH3 on metal sites forming highly reactive NH4+ intermediates.13,24 In a recent investigation by in situ electrical impedance

opper- and iron-exchanged zeolites, because of their superior activity and hydrothermal stability, have been widely applied as catalysts for selective catalytic reduction of nitrogen oxide emissions by NH3 (DeNOx by NH3−SCR).1,2 Despite the successful utilization and even commercialization of Cu-CHA zeolites (e.g., Cu-SSZ-13 and Cu-SAPO-34) as NH3− SCR catalyst for diesel engine exhaust aftertreatment in recent years,3−6 Fe-zeolites (especially the zeolites with small pore diameters such as ZSM-5, BEA, MOR, FER, etc.) as NH3−SCR catalysts are still of high industrial interest because of their high activity and high N2 selectivity (low N2O formation) at exhaust temperatures (i.e., above 250 °C).7−9 Among others, Fe-ZSM-5 was found to provide an optimal combination of activity and stability in a wide temperature range,10,11 and thus was extensively investigated for the mechanistic understanding of Fe-zeolites as NH3−SCR catalysts in general.1 The comprehensive and in-depth investigation of Cu-CHA has promoted significantly the mechanistic understanding of Cu-zeolite NH3−SCR catalysts in general.4,12 It is well-accepted that Cu-zeolite catalysts, with isolated Cu(II) as active sites, undergo a Cu(II)↔Cu(I) redox cycle during SCR reactions.12−14 Taking standard NH3−SCR (i.e., 4NH3 + 4NO + O2 → 4N2 + 6H2O) as an example, the NO-NH3 coadsorption on Cu-zeolites leads to the reduction of Cu(II) to Cu(I), and © XXXX American Chemical Society

Received: August 31, 2016 Revised: September 28, 2016

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the isomorphously incorporated, tetrahedrally coordinated Fe3+ in Fe-silicalite and the isolated octahedral Fe3+ sites in Al2O3, respectively, were observed in all the three Fe-ZSM-5 catalysts.18,29,30 The bands at 328 and 388 nm can be assigned to dimeric Fe species and small oligomeric Fe species as FexOy inside the zeolite pores, respectively. The broad band centered at 479 nm is known to result from relatively large Fe2O3 particles outside the zeolite pores.30 Quantitative UV−vis analysis (Table 2) indicates that, while the Fe species in FeZ-1 are mainly in isolated states, a considerable amount of Fe dimers and oligomers appeared in FeZ-2 and FeZ-3 due to the agglomeration of isolated Fe species at high Fe loadings.3,18,30 XPS analysis confirmed that the Fe species are mainly in the isolated state, and suggested a coexistence of Fe(II) and Fe(III) species in the synthesized Fe-ZSM-5 catalysts (see Figure S3 and related text). NH3−SCR reaction (up to 500 °C) did not lead to aggressive agglomeration of Fe species in the Fe-ZSM-5 catalysts (see UV−vis results in Figure S4 for the catalysts after NH3−SCR reaction). Despite the presence of relatively large Fe2O3 particles, a homogeneous distribution of Fe species in the ZSM-5 zeolite lattice was observed even in FeZ-3 after NH3−SCR reaction (see the Fe map in Figure S5). In situ IS-DRIFTS (see the measurement configuration in Figure S6) studies were performed over Fe-ZSM-5 zeolite films deposited on interdigital electrodes (IDEs) in order to analyze simultaneously the proton transport properties of the zeolite catalysts and the associated surface processes.8,31,32 In a typical measurement, the zeolite catalyst was first loaded with NH3 until saturation, and then it was exposed to NO/O2, NO and NO/O2 in sequence, or N2 and NO/O2 in sequence for different surface processes. Typical time-courses of the proton conductivity of Fe-ZSM-5 are shown in Figure S7. In general, the loading of NH3 increased the proton conductivity of FeZSM-5 catalysts as a result of NH3-supported proton transport, while the consumption of stored NH3 by desorption or SCR reaction led to a decreased conductivity.8,31−33 Figure 2a shows the time-resolved DRIFT spectra after Kubelka−Munk (KM) transformation for NH3-loaded FeZSM-5 (FeZ-1) exposed to NO/O2 mixture at 175 °C (see Figure S8 for the full spectra and Table S2 for detailed assignments). Characteristic bands at 1610 cm−1, 1457 and 1266 cm−1, which result from NH3 species on Lewis sites, NH4+ ions on Brønsted acid sites, and NH3 species on Fe sites, respectively,22,34 decreased continuously reaching a NH3-free state within a period of ca. 25 min (Figure 2b). Consequently, the proton conductivity IIS (IIS = |Y*|; Y* is the reciprocal of the complex impedance Z*, i.e. Y* = 1/Z*) of the Fe-ZSM-5 catalyst decayed and reached an NH3-free state as soon as the complete absence of surface NH3 species, which is in good agreement with our previous observations.31 Interestingly, the decay rate of the IIS signal varied apparently during the exposure in NO/O2. Specifically, after a rapid decay in the first 5 min, the IIS signal decreased at a clearly lower rate in the period of 5−20 min, and afterward underwent a rapid decay again (Figure 2b). While the initial decay of IIS is apparently determined by the NH3 species on Fe sites and Lewis sites, the fast decay of IIS after 20 min is more related to the NH4+ ions on Brønsted sites.31 In a comparative measurement shown in Figure 2c, the NH3loaded Fe-ZSM-5 was first exposed to N2 for a partial desorption of adsorbed NH3 species, in particular on Fe sites. In the subsequent exposure to NO/O2, the IIS experienced an unexpected further increase, before it decreased to a NH3-free

spectroscopy (IS), we observed directly the formation of NH4+ intermediates resulting from the interaction of NH3 and NO on Cu-ZSM-5.8 As compared to NH4+ on residual Brønsted sites after ion exchange, these in situ formed NH4+ species are more mobile and, therefore, significantly enhance the overall proton conductivity,8 in line with their higher reactivity as suggested previously.13 For standard NH3−SCR over Fe-zeolite catalysts, although the active Fe sites are generally suggested to undergo a Fe(III)↔Fe(II) redox cycle similar as the Cu(II)↔Cu(I) redox cycle during SCR reaction,11,25,26 it is not clear whether intermediate proton or NH4+ species form and, consequently, favor the NH3−SCR reaction. In this work, a relatively novel technique combining both in situ IS and diffuse reflectance infrared Fourier transform spectroscopy (in situ IS-DRIFTS) was applied to investigate simultaneously the proton transport properties of Fe-ZSM-5 catalysts and the associated surface processes under SCRrelated reaction conditions. Thereby, we revealed the formation of NH4+ intermediates on Fe-ZSM-5 as a result of the interaction of coadsorbed NH3 and NO, as well as the beneficial effect of NH4+ intermediates in NH3−SCR reactions catalyzed by Fe-ZSM-5 catalysts. Iron cations were introduced into the proton-form ZSM-5 zeolite (H-ZSM-5; with a MFI-type framework and a SiO2/ Al2O3 ratio of 27) by ion exchange (IE) in aqueous phase following the protocol described in the Supporting Information. By repeating the whole IE process, three Fe-ZSM-5 catalysts, namely, FeZ-1 (after one cycle of IE), FeZ-2 (after two cycles of IE), and FeZ-3 (after three cycles of IE), were obtained. The synthesized Fe-ZSM-5 catalysts were thoroughly characterized by inductively coupled plasma optical emission spectroscopy (ICP-OES), temperature-programmed desorption using NH3 as a probe molecule (NH3-TPD), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), diffuse reflection ultraviolet−visible spectroscopy (DR UV−vis), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). As revealed by ICP-OES and quantitative XPS analysis (Table 1), the repeated IE process led to increased Fe loading Table 1. Chemical Compositions by ICP-OES (Fe wt%) and Quantitative XPS Analysis (Fe at%) Fe/Al ratio FeZ-1 FeZ-2 FeZ-3

Fe wt%

Fe at%

ICP-OES

XPS

0.70 1.17 1.47

0.40 0.65 0.93

0.11 0.18 0.23

0.12 0.18 0.27

in the Fe-ZSM-5 catalyst. According to NH3-TPD (Figure 1a), the increase of Fe loading led to increased Lewis acidity (characterized by the NH3 desorption peak at ca. 100−130 °C) and decreased Brønsted acidity (characterized by the NH3 desorption peak at ca. 300−350 °C), in line with previous observations.3,27 SEM did not indicate any significant morphology changes of the original ZSM-5 material after ion exchange (see Figure S1). XRD patterns emphasize typical MFI crystal structures for all three Fe-ZSM-5 catalysts.28 Reflections from iron oxides were not detected by XRD, indicating the absence of larger crystalline domains formed potentially due to severe agglomeration of metal species (Figure 1b). In the deconvoluted DR UV−vis spectra (Figure 1c), strong charge transfer (CT) bands at 234 and 275 nm, resulting from 7697

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Figure 1. (a) NH3-TPD profiles, (b) XRD patterns, and (c) deconvoluted DR UV−vis spectra for the synthesized Fe-ZSM-5 catalysts.

similar evolutionary trend in NO/O2 after exposure to N2, that is, increased further first before it decreased to a NH3-free state. In the comparative study by exposing NH3-loaded Fe-ZSM-5 first to NO, such unexpected increase of the impedance-derived IIS signal and the DRIFTS signal at 1457 cm−1 was not observed during the subsequent exposure in NO/O2 (Figure 2d). However, comparing the measurements shown in Figures 2c,d, we notice that both the IIS and the DRIFTS signal at 1457 cm−1 were considerably retained during the exposure in NO as compared to the counterparts during the exposure in N2. Such phenomenon was also observed at a higher temperature of 250 °C (Figure S7). In both cases in Figures 2c,d, the decay of IIS and the decrease of DRIFTS signal at 1457 cm−1 were strongly correlated during the exposure in NO/O2, suggesting that the proton conductivity of the Fe-ZSM-5 catalyst is largely determined by the NH4+ ions on Brønsted sites. Therefore, the higher IIS of NH3-loaded Fe-ZSM-5 in NO than in N2 can be attributed to a higher amount of NH4+ ions on Brønsted sites. It is already known that the coadsorption and interaction of NO and NH3 on a Fe(III) site in Fe-ZSM-5 led to partial reduction of the site to Fe(II) and the release of N2 and H2O as products.5,11 Such reduction of Fe(III) to Fe(II) is likely to generate a proton on the adjacent Brønsted sites,8,32 similar to the redox cycle in Cu-zeolites.13−15,35 The in situ generated proton interacts with adsorbed NH3 species on Lewis sites or Fe sites leading to the formation of an intermediate NH4+ on Brønsted sites, which can be detected by DRIFTS at 1457 cm−1. As a result, considerably retained IIS and DRIFTS signal at 1457 cm−1 were observed over Fe-ZSM-5 during the exposure in NO after NH3 loading (Figure 2d). A proposed mechanism including the above-mentioned elementary steps is summarized in Figure 3a. It is noteworthy that the adsorption and activation of NO on NH3-loaded Fe-ZSM-5 need to overcome the well-known NH3-inhibition effect.9,11,36 Therefore, after a partial desorption of adsorbed NH3, the NH3−NO interaction on Fe-ZSM-5 was facilitated leading to a rapid formation of NH4+ intermediates, as for the case shown in Figure 2c. The formed NH4+ intermediates are highly mobile proton carriers,8,32 thus leading to increased proton conductivity, as illustrated in Figure 2c,d. Similar phenomena (i.e., formation of NH4+ intermediates and increase of IIS) were also observed at different temperatures (Figures S9a,b) or over FeZSM-5 with a different Fe loading (Figure S9c), in the case of a partial desorption of adsorbed NH3. The formation of NH4+ intermediates, indicating the activation of NO in the presence of adsorbed NH3, was

Table 2. Relative Concentrations of Different Fe Species by Quantitative UV−vis Analysis (%)

FeZ-1 FeZ-2 FeZ-3

I (234 nm)

II (275 nm)

III (328 nm)

IV (388 nm)

V (479 nm)

26.4 23.1 20.7

42.5 31.6 31.0

23.6 28.3 22.9

7.5 6.5 14.3

0 10.5 11.1

Figure 2. (a) Time-resolved in situ DRIFT spectra and (b) normalized proton conductivity (IIS; green line) and DRIFTS signals (red symbols) at characteristic wavenumbers of NH3-loaded Fe-ZSM-5 exposed to NO/O2 mixture. Normalized IIS (green line) and DRIFTS signals (red symbols) at characteristic wavenumbers of NH3-loaded Fe-ZSM-5 (c) exposed to N2 and NO/O2 mixture in sequence and (d) exposed to NO and NO/O2 mixture in sequence. IIS: absolute value of complex admittance |Y*| (Y* is the reciprocal of the complex impedance Z*, i.e. Y* = 1/Z*) at 10 kHz. 1457 cm−1: NH4+ ions on Brønsted acid sites; 1610 cm−1: NH3 species on metal Lewis sites; 1266 cm−1: NH3 species on Fe sites. Fe-ZSM-5: FeZ-1 with a Fe/Al ratio of 0.11 (by ICP-OES). The catalyst was pretreated at 450 °C in 10% O2 for 1 h before each measurement.

state. Surprisingly, the DRIFTS signal at 1457 cm−1, which results from NH4+ ions on Brønsted acid sites, displayed a 7698

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Figure 3. (a) Proposed pathway for the formation of NH4+ intermediate in NH3−SCR over Fe-ZSM-5 catalysts. (b) Correlation between the NH4+ intermediate formation and the normalized NO reduction rates at low temperatures; the NH4+ intermediate formation (ΔIIS) was evaluated according to the proton conductivity enhancement of NH3-loaded Fe-ZSM-5 after exposure in NO for 30 min in comparison to exposure in N2 for 30 min (Figure S5).

found to determine largely the activity of Fe-ZSM-5 in NH3− SCR, especially at low temperatures (see the temperatureconversion curves of three Fe-ZSM-5 catalysts in Figure S10). It can be seen in Figure 3b that the normalized NO reduction rate of Fe-ZSM-5 is correlated to the proton conductivity enhancement (ΔIIS) when NH3-loaded Fe-ZSM-5 was exposed to NO in comparison with N2 (for 30 min). It has been revealed by our previous in situ IS studies that the NH4+ intermediates are highly mobile within the zeolite catalysts.8 Such high mobility of NH4+ intermediates likely renders a high reactivity in NH3−SCR reaction, as suggested previously for Cu-SSZ-13 catalyzed NH3−SCR reactions.13 As a result, the NH3−SCR activity of Fe-ZSM-5 catalysts shows a strong dependence on the formation of NH4+ intermediates (Figure 3b). The dominant presence of isolated or dimeric Fe species in FeZ-1 with a low Fe loading (see the UV−vis results in Figure 1c and S4) likely renders a higher reducibility of Fe species by the NH3−NO interaction and, consequently, the formation of a higher amount of NH4+ intermediates favoring the NH3−SCR reaction.5,11,13,35 The observations by in situ IS-DRIFTS suggest that the formation of NH4+ intermediates may serve as a potential “descriptor” for the design of active Fe-zeolite catalyst for NH3−SCR, especially at low temperatures. In summary, by performing in situ IS-DRIFTS studies under well-controlled, SCR-related reaction conditions, we were able to observe the formation of NH4+ intermediates as a result of the interaction between coadsorbed NO and NH3 on Fe-ZSM5 catalysts. The low-temperature (175 and 250 °C) NH3−SCR activity of Fe-ZSM-5 catalysts was strongly related to the formation of highly mobile and reactive NH4+ intermediates. These findings, which are not easily achievable by conventional methods, provide new and important perspectives for the mechanistic understanding of the NH3−SCR reaction over Fezeolite catalysts and for the future design of active Fe-zeolite NH3−SCR catalysts.

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SCR reactions, TEM results, in situ IS-DRIFTS measurement configuration, normalized IIS of Fe-ZSM-5 under SCR-related conditions, comparison of in situ DRIFT spectra (for different catalysts and/or at different temperatures), detailed assignments of IR bands in DRIFTS, additional IS-DRIFTS data, and NO conversion-temperature curves (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the German Research Foundation (DFG) under grants MO 1060/19-1 and SI 609/14-1, and the Exploratory Research Space of RWTH Aachen University financed by the Excellence Initiative of the German federal and state governments to promote science and research at German universities. We thank D. Rauch for the preparation of IDE sensor chips, P. Kangvansura for experimental support, and W. Grünert for scientific discussions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b02496. SEM images of H-ZSM-5 and Fe-ZSM-5 zeolites, XPS results (survey, quantitative analysis, and spectra fitting), DR UV−vis spectra for Fe-ZSM-5 catalysts after NH3− 7699

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