In Situ Spectroscopic Studies of Proton Transport in Zeolite Catalysts for NH 3-SCR

Proton transport is an elementary process in the selective catalytic reduction of nitrogen oxides by ammonia (DeNOx by NH3-SCR) using metal-exchanged zeolites as catalysts. This review summarizes recent advancements in the study of proton transport in zeolite catalysts using in situ electrical impedance spectroscopy (IS) under NH3-SCR reaction conditions. Different factors, such as the metal cation type, metal exchange level, zeolite framework type, or formation of intermediates, were found to influence the proton transport properties of zeolite NH3-SCR catalysts. A combination of IS with diffuse reflection infrared Fourier transformation spectroscopy in situ (in situ IS-DRIFTS) allowed to achieve a molecular understanding of the proton transport processes. Several mechanistic aspects, such as the NH3-zeolite interaction, NO-zeolite interaction in the presence of adsorbed NH3, or formation of NH4 intermediates, have been revealed. These achievements indicate that IS-based in situ methods as complementary tools for conventional techniques (e.g., in situ X-ray absorption spectroscopy) are able to provide new perspectives for the understanding of NH3-SCR on zeolite catalysts.


Introduction
Selective catalytic reduction (SCR) is one of the key technologies to reduce nitrogen oxide emissions (NO x ) from "lean-burn" engines and power plants [1][2][3].Because of their superior activity and hydrothermal stability, Cu-or Fe-exchanged zeolites are widely applied as SCR catalysts, especially in diesel-powered automobiles [3,4].To meet the continuously tightening NO x emission legislation, it is necessary to further improve the performance of the metal-exchanged zeolite catalysts in SCR, which requires understanding more deeply both the reaction mechanisms and the real-time physico-chemical properties of the zeolite catalysts under operational conditions [2][3][4][5][6].
For SCR reactions using NH 3 as a reductant (NH 3 -SCR; see Equations ( 1)- (3) for different reaction routes depending on the NO x composition): One of the fundamental issues is to understand the NH 3 -zeolite interaction.This interaction has been known to largely determine the storage capability, the uptake and release energetics, and the reactivity of NH 3 within zeolite catalysts [3,7], and eventually the catalytic performance of the zeolite catalysts in NH 3 -SCR.Considerable advancements have been achieved by means of, for example, temperature-programmed desorption (TPD), infrared spectroscopy, and X-ray based methods such as Extended X-Ray Absorption Fine Structure, X-ray Absorption Near Edge Structure, X-ray Emission Spectroscopy (all using NH 3 as a probe molecule) [7][8][9][10][11][12][13]. Nevertheless, more elementary processes associated with the NH 3 storage and conversion, in particular, the proton transport, are not fully understood.
It is known that proton transport, which can take place either from the bridging hydroxyl groups (Brønsted acid sites) to reactant molecules or between the reaction intermediates, plays an important role in a series of catalytic reactions such as methylation [14], cracking and methanol-to-olefin [15,16], as well as abatement of NO x emissions [3,4,17].Comprehensive experimental and theoretical investigations had been performed over zeolites with different framework types (FAU, BEA, MFI, FER, CHA, etc.), in order to understand and take advantage of the proton transport processes for further improving the catalytic performance of zeolites [18][19][20][21][22]. Density functional theory (DFT) calculations revealed that proton transfer takes place in several elementary processes in NH 3 -SCR reactions over zeolite catalysts, including NO oxidation, fast SCR, NO 2 -SCR, NH 3 oxidation and N 2 O decomposition [12,20,22,23].For Cu-ZSM-5 catalyzed NO decomposition, the presence of protons was found to significantly lower the energy barrier for the NO activation on Cu sites [24].Although the proton transport processes can be probed by 1 H MAS NMR spectroscopy under well-controlled conditions [12,19,21], studies under technically relevant reaction conditions are practically challenging.In the last years, we applied electrical impedance spectroscopy (IS) to study the proton transport in various zeolites (such as H-ZSM-5, Fe-ZSM-5, Cu-ZSM-5, Cu-SSZ-13, Cu-SAPO-34, etc.) as NH 3 -SCR catalysts under in situ or operando conditions [17,[25][26][27][28][29][30][31].In NH 3 -SCR over zeolite catalysts, the adsorption of NH 3 molecules on Brønsted acid sites leads to the formation of ammonium ions (NH 4 + ), which interact further with NH 3 molecules forming NH 4 + •(NH 3 ) n complexes at low temperatures [2].The formed NH 4 + and NH 4 + •(NH 3 ) n complexes, which can provide additional paths or carriers for proton transport [26,[32][33][34], lead to increased proton conductivities which can be monitored by IS in a broad frequency range (mHz-GHz) [27,[32][33][34][35][36].The consumption of adsorbed NH 3 , either by desorption or SCR conversion, leads to decreased conductivity due to a loss of proton carriers [17,26].A further combination of IS and diffuse reflection infrared Fourier transformation spectroscopy (in situ IS-DRIFTS) allowed us to achieve a molecular understanding of the proton transport processes and their impact in NH 3 -SCR catalysis [17,[27][28][29].
In this review, we will briefly introduce the physical background and instrumentation of in situ IS and in situ IS-DRIFTS (Section 2).In Section 3, we will summarize the mechanisms and influential factors of NH 3 -supported proton transport in zeolite catalysts, and the impact of proton transport in NH 3 -SCR catalysis.The future perspectives, which arise from the achieved understanding, will be discussed at the end (Section 4).

Theory of Impedance Spectroscopy
Impedance spectroscopy is an electric perturbation technique, and can be employed to analyze the mobility of ions in solid materials [37,38].In a typical IS measurement over zeolite, an alternating voltage U(ω) with angular frequency ω and amplitude U 0 is applied to form an electric field over a zeolite in thermodynamic equilibrium.A response of the system, i.e., a movement of the mobile cations either via translation motion or a local displacement, is induced by the electric perturbation, and can be macroscopically measured as a current I(ω).The complex impedance Z(ω) is defined as and can be described by a real part Z and an imaginary part Z".Both depend on the angular frequency ω: (5) In the analysis of complex impedance Z(ω), low-frequency phenomena such as the sample/electrode interface polarization can be identified from the dominating low-frequency tail of the traditional Argand representation (also known as Nyquist plot; the real part Z is plotted against the imaginary part Z"; see Figure 1a for an example) [39].The high-frequency processes are more visible in the Modulus plot, which shows the imaginary part of the Modulus M, i.e., M"(ω), against the frequency f (Figure 1b).The modulus M"(ω) is defined as wherein C 0 is the capacity of the empty capacitor, i.e., the geometric capacitance.Thereby, two distinct relaxation processes, i.e., the local dipolar relaxation (as visualized by the maximum at high-frequency range) and the long-range proton transport (as visualized by the maximum at low-frequency range) can be clearly distinguished in one spectral representation. (5) In the analysis of complex impedance Z(ω), low-frequency phenomena such as the sample/electrode interface polarization can be identified from the dominating low-frequency tail of the traditional Argand representation (also known as Nyquist plot; the real part Z′ is plotted against the imaginary part Z″; see Figure 1a for an example) [39].The high-frequency processes are more visible in the Modulus plot, which shows the imaginary part of the Modulus M, i.e., M″(ω), against the frequency f (Figure 1b).The modulus M″(ω) is defined as wherein C0 is the capacity of the empty capacitor, i.e., the geometric capacitance.Thereby, two distinct relaxation processes, i.e., the local dipolar relaxation (as visualized by the maximum at highfrequency range) and the long-range proton transport (as visualized by the maximum at lowfrequency range) can be clearly distinguished in one spectral representation.The long-range proton transport within the zeolites is temperature-dependent, and can be represented using the Arrhenius equation: where Y is the real part of the admittance, i.e., Y(ω) = 1/Z(ω), at the resonance frequency f res (determined according to the low-frequency maximum in the Modulus plot at the respective temperature), A is the pre-exponential factor (which depends on the charge and number of the mobile species, its on-site oscillation frequency and the hopping distance [38]), E a is the activation energy of the proton transport process, σ is the specific conductivity of the zeolite, k B is the Boltzmann constant, and T is the temperature.An example is shown in Figure 1c for an Arrhenius-like representation of IS results over the zeolite H-ZSM-5.

Instruments for In Situ IS and In Situ IS-DRIFTS
The measurement configurations for in situ IS and in situ IS-DRIFTS are schematically displayed in Figure 2a,b, respectively.For both methods, the zeolite catalysts were deposited as a thick film on screen-printed interdigital electrodes (IDEs) comprised of an alumina substrate with gold electrodes on the front side and an integrated heater on the reverse side.In this way, an excellent electrical contact between the zeolite film and the IDE structure can be achieved [30].An external power supply is used for temperature control via resistive heating.Temperature calibration was performed for each sample with a pyrometer for the remote monitoring of temperature on the surface of zeolite film.The gas composition is controlled by mass flow controllers (MFCs) dosing different gases such as NO, O 2 , NH 3 and N 2 (carrier gas).Prior to each measurement, the zeolite sample was pretreated at high temperatures (usually at 400 • C in 10 vol.% O 2 for 1 h) to remove any adsorbed water or hydrocarbon contaminants.The electrical impedance of the sample is measured with an impedance analyzer range up to 10 14 Ω (±1%).The voltage is set to 0.1 V (rms) for all measurements to stay in the linear response regime.The long-range proton transport within the zeolites is temperature-dependent, and can be represented using the Arrhenius equation: where Y′ is the real part of the admittance, i.e., Y(ω) = 1/Z(ω), at the resonance frequency fres (determined according to the low-frequency maximum in the Modulus plot at the respective temperature), A is the pre-exponential factor (which depends on the charge and number of the mobile species, its on-site oscillation frequency and the hopping distance [38]), Ea is the activation energy of the proton transport process, σ is the specific conductivity of the zeolite, kB is the Boltzmann constant, and T is the temperature.An example is shown in Figure 1c for an Arrhenius-like representation of IS results over the zeolite H-ZSM-5.

Instruments for In Situ IS and In Situ IS-DRIFTS
The measurement configurations for in situ IS and in situ IS-DRIFTS are schematically displayed in Figure 2a,b, respectively.For both methods, the zeolite catalysts were deposited as a thick film on screen-printed interdigital electrodes (IDEs) comprised of an alumina substrate with gold electrodes on the front side and an integrated heater on the reverse side.In this way, an excellent electrical contact between the zeolite film and the IDE structure can be achieved [30].An external power supply is used for temperature control via resistive heating.Temperature calibration was performed for each sample with a pyrometer for the remote monitoring of temperature on the surface of zeolite film.The gas composition is controlled by mass flow controllers (MFCs) dosing different gases such as NO, O2, NH3 and N2 (carrier gas).Prior to each measurement, the zeolite sample was pretreated at high temperatures (usually at 400 °C in 10 vol.% O2 for 1 h) to remove any adsorbed water or hydrocarbon contaminants.The electrical impedance of the sample is measured with an impedance analyzer range up to 10 14 Ω (±1%).The voltage is set to 0.1 V (rms) for all measurements to stay in the linear response regime.The in situ IS measurements were carried out using a homemade reaction chamber (Figure 2a).For in situ IS-DRIFTS measurements, a commercial high-temperature reaction chamber (Harrick Scientific Products, Pleasantville, NY, USA) was modified to allow the introduction of IDE chips with zeolite catalyst film (Figure 2b).A specially designed holder with electrical contacts was employed to keep the sensor chip inside the reaction chamber in a way that the zeolite film is in the focal point of the infrared beam of the DRIFTS mirror design.Simultaneous IS and DRIFTS measurements were carried out using the same catalyst film, allowing simultaneous monitoring of both the proton conductivity of zeolite catalysts and the vibration modes of the molecules on zeolite catalysts [26,28].The in situ IS measurements were carried out using a homemade reaction chamber (Figure 2a).For in situ IS-DRIFTS measurements, a commercial high-temperature reaction chamber (Harrick Scientific Products, Pleasantville, NY, USA) was modified to allow the introduction of IDE chips with zeolite catalyst film (Figure 2b).A specially designed holder with electrical contacts was employed to keep the sensor chip inside the reaction chamber in a way that the zeolite film is in the focal point of the infrared beam of the DRIFTS mirror design.Simultaneous IS and DRIFTS measurements were carried out using the same catalyst film, allowing simultaneous monitoring of both the proton conductivity of zeolite catalysts and the vibration modes of the molecules on zeolite catalysts [26,28].

NH 3 -Supported Proton Transport
Zeolites are a class of crystalline, microporous solids consisting of tetrahedral TO 4 (T denotes as Si, Al, Ti, etc.) units.The TO 4 units serve as primary building blocks forming three-dimensional frameworks with interconnected cages and channels of distinct sizes and shapes.The Brønsted acidity of zeolites, which results from the non-equivalent substitution of T-atoms (e.g., the substitution of Si by Al in TO 4 units as shown in Figure 3) and the subsequent charge-balancing by external, exchangeable cations (Na + , NH 4 + , H + , etc.) at the adjacent oxygen sites within the pore space, enables several characteristic functions, such as ion-exchange capacity, proton donating ability and ionic conductivity [3,[40][41][42].These properties allow zeolites to be used as adsorbents, separators, ionic conductors, sensors, or catalysts [3,[40][41][42].

NH3-Supported Proton Transport
Zeolites are a class of crystalline, microporous solids consisting of tetrahedral TO4 (T denotes as Si, Al, Ti, etc.) units.The TO4 units serve as primary building blocks forming three-dimensional frameworks with interconnected cages and channels of distinct sizes and shapes.The Brønsted acidity of zeolites, which results from the non-equivalent substitution of T-atoms (e.g., the substitution of Si by Al in TO4 units as shown in Figure 3) and the subsequent charge-balancing by external, exchangeable cations (Na + , NH4 + , H + , etc.) at the adjacent oxygen sites within the pore space, enables several characteristic functions, such as ion-exchange capacity, proton donating ability and ionic conductivity [3,[40][41][42].These properties allow zeolites to be used as adsorbents, separators, ionic conductors, sensors, or catalysts [3,[40][41][42].In NH3-SCR catalysis, protons on Brønsted acid sites of zeolite catalysts could transfer to adsorbed NH3 forming ammonium ions (NH4 + ), which interact further with NH3 molecules leading to the formation of NH4 + •(NH3)n complexes at low temperatures [2].Both NH4 + and NH4 + •(NH3)n complexes can provide additional paths or carriers for proton transport, which consequently increase the proton conductivity of zeolite catalysts [26,[32][33][34].The physico-chemical features of NH3supported proton transport were revealed by in situ studies over NH3-loaded zeolites using techniques combining IS with TPD or quantum chemical calculations [26][27][28][29]34,43], and are schematically illustrated in Figure 3 for proton-form zeolites.Four distinct temperature-dependent mechanisms can be distinguished, specifically (i) the Grothuss-like transport along condensed NH4 + •(NH3)n chains at low temperatures, i.e., below the desorption temperature of NH3; (ii) proton hopping along partially disintegrated chains of NH3 molecules (i.e., in the temperature range, where weakly bound solvent molecules desorb); (iii) vehicle-supported transfer of protons between the neighboring Brønsted sites with NH4 + carriers as "proton vehicles", and (iv) thermally activated proton hopping along the electron density located at the oxygen atoms of the zeolite lattice in the absence of solvate molecules (above 340 °C) [26,32,33].

Factors Influencing the Proton Transport in Zeolite Catalysts
As can be seen in Figure 3, the proton transport in zeolites is largely determined by the abundance of Brønsted sites serving as the primary sites for the adsorption of NH3.This was already confirmed by our previous studies over ZSM-5 zeolites with different Si/Al ratios [32,33].For metal- In NH 3 -SCR catalysis, protons on Brønsted acid sites of zeolite catalysts could transfer to adsorbed NH 3 forming ammonium ions (NH 4 + ), which interact further with NH 3 molecules leading to the formation of NH 4 + •(NH 3 ) n complexes at low temperatures [2].Both NH 4 + and NH 4 + •(NH 3 ) n complexes can provide additional paths or carriers for proton transport, which consequently increase the proton conductivity of zeolite catalysts [26,[32][33][34].The physico-chemical features of NH 3 -supported proton transport were revealed by in situ studies over NH 3 -loaded zeolites using techniques combining IS with TPD or quantum chemical calculations [26][27][28][29]34,43], and are schematically illustrated in Figure 3 for proton-form zeolites.Four distinct temperature-dependent mechanisms can be distinguished, specifically (i) the Grothuss-like transport along condensed NH 4 + •(NH 3 ) n chains at low temperatures, i.e., below the desorption temperature of NH 3 ; (ii) proton hopping along partially disintegrated chains of NH 3 molecules (i.e., in the temperature range, where weakly bound solvent molecules desorb); (iii) vehicle-supported transfer of protons between the neighboring Brønsted sites with NH 4 + carriers as "proton vehicles"; and (iv) thermally activated proton hopping along the electron density located at the oxygen atoms of the zeolite lattice in the absence of solvate molecules (above 340 • C) [26,32,33].

Factors Influencing the Proton Transport in Zeolite Catalysts
As can be seen in Figure 3, the proton transport in zeolites is largely determined by the abundance of Brønsted sites serving as the primary sites for the adsorption of NH 3 .This was already confirmed by our previous studies over ZSM-5 zeolites with different Si/Al ratios [32,33].For metal-exchanged zeolites used as catalysts in NH 3 -SCR, several structural or chemical parameters such as framework topology, metal cation type, and metal exchange level also influence considerably the NH 3 -supported proton transport by affecting the formation of NH 4 + •(NH 3 ) n complexes and/or the affinity between the NH 3 species and the zeolite catalysts.

Metal Cation Type
Based on the NH 3 -supported proton transport, the loading and desorption of NH 3 in Fe-and Cu-ZSM-5 can be effectively monitored by means of in situ IS (Figure 4a).Comparative studies revealed that, as compared to Fe-ZSM-5, the Cu-ZSM-5 demonstrated a stronger retention ability against thermal desorption for the adsorbed NH 3 species (i.e., NH 4 + on Brønsted as indicated by the IR band at 1457 cm −1 , and the NH 3 on metal sites as indicated by the IR band at 1276/1266 cm −1 ), which is due to a stronger NH 3 -zeolite interaction (according to the higher activation energy E a for proton transport by multi-frequency IS experiments; see Figure 4b).During exposure in NO/O 2 mixture for the SCR conversion of stored NH 3 (Figure 5), while the proton conductivity of NH 3 -saturated Fe-ZSM-5 decreased rapidly, that of NH 3 -saturated Cu-ZSM-5 increased further significantly (Figure 5a).Such unexpected increase of proton conductivity during NO/O 2 exposure was observed in the temperature range of 100-250 • C over NH 3 -saturated Cu-ZSM-5 (Figure 6a).In situ IS-DRIFTS studies revealed that the increased proton conductivity results mainly from NH 4 + intermediates (Figure 6b), which formed via the following route: (i) interaction of NO and adsorbed NH 3 on Cu 2+ sites; (ii) reduction of Cu 2+ to Cu + and release of a proton on the adjacent Brønsted site (i.e., Cu 2+ → Cu + + H + ); and (iii) interaction of the released proton and adsorbed NH 3 on Cu sites (i.e., H + + NH 3 → NH 4 + ) [27,29,44,45].The enhanced proton transport of NH 3 -saturated zeolite catalysts by NO exposure is thus considered to manifest the NO activation ability in the presence of adsorbed NH 3 [29].
Catalysts 2016, 6, 204 6 of 15 exchanged zeolites used as catalysts in NH3-SCR, several structural or chemical parameters such as framework topology, metal cation type, and metal exchange level also influence considerably the NH3-supported proton transport by affecting the formation of NH4 + •(NH3)n complexes and/or the affinity between the NH3 species and the zeolite catalysts.

Metal Cation Type
Based on the NH3-supported proton transport, the loading and desorption of NH3 in Fe-and Cu-ZSM-5 can be effectively monitored by means of in situ IS (Figure 4a).Comparative studies revealed that, as compared to Fe-ZSM-5, the Cu-ZSM-5 demonstrated a stronger retention ability against thermal desorption for the adsorbed NH3 species (i.e., NH4 + on Brønsted as indicated by the IR band at 1457 cm −1 , and the NH3 on metal sites as indicated by the IR band at 1276/1266 cm −1 ), which is due to a stronger NH3-zeolite interaction (according to the higher activation energy Ea for proton transport by multi-frequency IS experiments; see Figure 4b).During exposure in NO/O2 mixture for the SCR conversion of stored NH3 (Figure 5), while the proton conductivity of NH3-saturated Fe-ZSM-5 decreased rapidly, that of NH3-saturated Cu-ZSM-5 increased further significantly (Figure 5a).Such unexpected increase of proton conductivity during NO/O2 exposure was observed in the temperature range of 100-250 °C over NH3-saturated Cu-ZSM-5 (Figure 6a).In situ IS-DRIFTS studies revealed that the increased proton conductivity results mainly from NH4 + intermediates (Figure 6b), which formed via the following route: (i) interaction of NO and adsorbed NH3 on Cu 2+ sites; (ii) reduction of Cu 2+ to Cu + and release of a proton on the adjacent Brønsted site (i.e., Cu 2+ → Cu + + H + ); and (iii) interaction of the released proton and adsorbed NH3 on Cu sites (i.e., H + + NH3 → NH4 + ) [27,29,44,45].The enhanced proton transport of NH3-saturated zeolite catalysts by NO exposure is thus considered to manifest the NO activation ability in the presence of adsorbed NH3 [29].It has to be noted that the redox cycle of active metal sites consists of a reduction half-cycle (Cu 2+ → Cu + or Fe 3+ → Fe 2+ ) and a re-oxidation half-cycle (Cu + → Cu 2+ or Fe 2+ → Fe 3+ ), and the latter one is usually considered to be the rate-determining step in the whole redox processes [10,11].Therefore, further IS studies on the re-oxidation half-cycle are needed to understand in more detail the very different low-temperature NH3-SCR activities of Cu-and Fe-exchanged zeolite catalysts (Figure 5b) [29].

Metal Exchange Level
The metal exchange level (estimated according to, for example, the metal to Al ratio in metalexchange aluminosilicate zeolites) of zeolite catalyst is known to have a strong impact in NH3-SCR catalysis [46][47][48][49][50].In the case of ZSM-5 with a Si/Al ratio of 13.5, while the Cu species in Cu-ZSM-5  It has to be noted that the redox cycle of active metal sites consists of a reduction half-cycle (Cu 2+ → Cu + or Fe 3+ → Fe 2+ ) and a re-oxidation half-cycle (Cu + → Cu 2+ or Fe 2+ → Fe 3+ ), and the latter one is usually considered to be the rate-determining step in the whole redox processes [10,11].Therefore, further IS studies on the re-oxidation half-cycle are needed to understand in more detail the very different low-temperature NH3-SCR activities of Cu-and Fe-exchanged zeolite catalysts (Figure 5b) [29].

Metal Exchange Level
The metal exchange level (estimated according to, for example, the metal to Al ratio in metalexchange aluminosilicate zeolites) of zeolite catalyst is known to have a strong impact in NH3-SCR catalysis [46][47][48][49][50].In the case of ZSM-5 with a Si/Al ratio of 13.5, while the Cu species in Cu-ZSM-5 It has to be noted that the redox cycle of active metal sites consists of a reduction half-cycle (Cu 2+ → Cu + or Fe 3+ → Fe 2+ ) and a re-oxidation half-cycle (Cu + → Cu 2+ or Fe 2+ → Fe 3+ ), and the latter one is usually considered to be the rate-determining step in the whole redox processes [10,11].Therefore, further IS studies on the re-oxidation half-cycle are needed to understand in more detail the very different low-temperature NH 3 -SCR activities of Cu-and Fe-exchanged zeolite catalysts (Figure 5b) [29].

Metal Exchange Level
The metal exchange level (estimated according to, for example, the metal to Al ratio in metal-exchange aluminosilicate zeolites) of zeolite catalyst is known to have a strong impact in NH 3 -SCR catalysis [46][47][48][49][50].In the case of ZSM-5 with a Si/Al ratio of 13.5, while the Cu species in Cu-ZSM-5 remain mainly in isolated state at a Cu/Al ratio of ca.0.2, a considerable amount of Fe dimers or oligomers form in Fe-ZSM-5 with a Fe/Al ratio of ca.0.2 and above [31].The introduced metal species can adsorb NH 3 at an intermediate strength (stronger than the adsorption on Lewis sites, but weaker than that on Brønsted sites), as characterized by the NH 3 desorption at temperatures between 130 and 250 • C (Figure 7a) [7].The influence of metal exchange level on the NH 3 -zeolite interaction can be examined by analyzing the mobility of adsorbed NH 3 -species as proton carriers under thermal desorption conditions by means of in situ IS [31].As indicated by the activation energies for proton transport (i.e., the strength of NH 3 adsorption on zeolites), while the increase of Fe loading weakened slightly the NH 3 -zeolite interaction, a higher Cu loading enhanced significantly the NH 3 -zeolite interaction (Figure 7b).The weakening interaction between NH 3 and Fe-ZSM-5 with increasing Fe/Al ratio can be clearly visualized in DRIFTS (Figure 7c), according to the decreasing band intensity at 1266 cm −1 originating from the NH 3 species on Fe sites [17,31].In NH 3 -SCR catalysis, for the zeolite catalysts shown in Figure 7b, while the low-temperature (below 250 • C) NH 3 -SCR activity of Fe-ZSM-5 decreased with Fe/Al ratio, that of Cu-ZSM-5 increased with Cu/Al ratio [17,31].
Catalysts 2016, 6, 204 8 of 15 remain mainly in isolated state at a Cu/Al ratio of ca.0.2, a considerable amount of Fe dimers or oligomers form in Fe-ZSM-5 with a Fe/Al ratio of ca.0.2 and above [31].The introduced metal species can adsorb NH3 at an intermediate strength (stronger than the adsorption on Lewis sites, but weaker than that on Brønsted sites), as characterized by the NH3 desorption at temperatures between 130 and 250 °C (Figure 7a) [7].The influence of metal exchange level on the NH3-zeolite interaction can be examined by analyzing the mobility of adsorbed NH3-species as proton carriers under thermal desorption conditions by means of in situ IS [31].As indicated by the activation energies for proton transport (i.e., the strength of NH3 adsorption on zeolites), while the increase of Fe loading weakened slightly the NH3-zeolite interaction, a higher Cu loading enhanced significantly the NH3-zeolite interaction (Figure 7b).The weakening interaction between NH3 and Fe-ZSM-5 with increasing Fe/Al ratio can be clearly visualized in DRIFTS (Figure 7c), according to the decreasing band intensity at 1266 cm −1 originating from the NH3 species on Fe sites [17,31].In NH3-SCR catalysis, for the zeolite catalysts shown in Figure 7b, while the low-temperature (below 250 °C) NH3-SCR activity of Fe-ZSM-5 decreased with Fe/Al ratio, that of Cu-ZSM-5 increased with Cu/Al ratio [17,31].At high Fe loadings or under harsh conditions (e.g., hydrothermal aging), the Fe species in Fe-zeolites may aggregate forming Fe x O y dimers or small clusters within the zeolite pores, or even relatively large Fe 2 O 3 particles outside the zeolite pores [31].At low temperatures, these Fe x O y species or Fe 2 O 3 particles could provide additional acidic sites for the adsorption of NH 3 species, favoring the formation of NH 4 + •(NH 3 ) n chains (as indicated by the broad band centered at ca. 2520 cm −1 in the DRIFT spectra in Figure 7d) and consequently the proton transport within the zeolite lattice [28,29].At high temperatures (175 • C and above), however, the weakly bound NH 3 species cannot be retained on the Fe x O y species or Fe 2 O 3 particles (as indicated by the disappearance of the broad band centered at ca. 2520 cm −1 ) and thus have no (or just negligible) contribution to the proton transport [17,29].

Zeolite Framework Type
Among different zeolite framework types, those with medium or small pore diameters (such as CHA, MFI, MOR, FER, etc.) after metal ion exchange were found to be especially advantageous for NH 3 -SCR catalysis [2][3][4].A comparison of Cu-ZSM-5 (MFI type) and Cu-SAPO-34 (CHA type) zeolites using in situ IS revealed that the framework type influenced significantly the proton transport properties of Cu-zeolite catalysts under NH 3 -SCR related conditions [27].At low temperatures, formation of highly proton-conducting NH 4 + intermediates was observed in both zeolites (Figures 6b   and 8), which is due to the reduction of Cu 2+ to Cu + as a result of the interaction of NH 3 and NO [17,25,29,44].In the monitoring of NH 3 -SCR using the zeolite catalysts directly as sensors, while the performance of Cu-SAPO-34 can be improved by increasing the temperature to 200 • C and above, that of Cu-ZSM-5 was impeded by the formation of NH 4 + intermediates even at high temperatures as 350 • C (Figure 9).Both zeolites performed similarly in the direct monitoring of NH 3 -SCR at temperatures above 350 • C. At 200 • C and below, Cu-SAPO-34 also showed higher NH 3 -SCR activity than Cu-ZSM-5 [27].The difference in proton transport, NH 3 -SCR reaction monitoring and NH 3 -SCR catalysis can be attributed to the different coordinative nature of the metal sites in the two catalysts [4].Systematic investigations are required to gain more insights into this issue.
At high Fe loadings or under harsh conditions (e.g., hydrothermal aging), the Fe species in Fezeolites may aggregate forming FexOy dimers or small clusters within the zeolite pores, or even relatively large Fe2O3 particles outside the zeolite pores [31].At low temperatures, these FexOy species or Fe2O3 particles could provide additional acidic sites for the adsorption of NH3 species, favoring the formation of NH4 + •(NH3)n chains (as indicated by the broad band centered at ca. 2520 cm −1 in the DRIFT spectra in Figure 7d) and consequently the proton transport within the zeolite lattice [28,29].At high temperatures (175 °C and above), however, the weakly bound NH3 species cannot be retained on the FexOy species or Fe2O3 particles (as indicated by the disappearance of the broad band centered at ca. 2520 cm −1 ) and thus have no (or just negligible) contribution to the proton transport [17,29].

Zeolite Framework Type
Among different zeolite framework types, those with medium or small pore diameters (such as CHA, MFI, MOR, FER, etc.) after metal ion exchange were found to be especially advantageous for NH3-SCR catalysis [2][3][4].A comparison of Cu-ZSM-5 (MFI type) and Cu-SAPO-34 (CHA type) zeolites using in situ IS revealed that the framework type influenced significantly the proton transport properties of Cu-zeolite catalysts under NH3-SCR related conditions [27].At low temperatures, formation of highly proton-conducting NH4 + intermediates was observed in both zeolites (Figures 6b and 8), which is due to the reduction of Cu 2+ to Cu + as a result of the interaction of NH3 and NO [17,25,29,44].In the monitoring of NH3-SCR using the zeolite catalysts directly as sensors, while the performance of Cu-SAPO-34 can be improved by increasing the temperature to 200 °C and above, that of Cu-ZSM-5 was impeded by the formation of NH4 + intermediates even at high temperatures as 350 °C (Figure 9).Both zeolites performed similarly in the direct monitoring of NH3-SCR at temperatures above 350 °C.At 200 °C and below, Cu-SAPO-34 also showed higher NH3-SCR activity than Cu-ZSM-5 [27].The difference in proton transport, NH3-SCR reaction monitoring and NH3-SCR catalysis can be attributed to the different coordinative nature of the metal sites in the two catalysts [4].Systematic investigations are required to gain more insights into this issue.(VI) 100 ppm NO, N2 balance.Cu-SAPO-34: 1 wt % Cu, (P + Al)/Si = 12.9.1457 cm −1 : NH4 + ions on Brønsted acid sites; 1273 cm −1 : NH3 species on Cu sites.Reproduced with permission from [25].wt % Cu, (P + Al)/Si = 12.9.Cu-ZSM-5: 1 wt % Cu, Si/Al = 13.5.Reproduced with permission from [27].

Formation of NH4 + Intermediates
In zeolite catalyzed NH3-SCR reactions, depending on the used catalysts and reaction conditions, different intermediate species, such as NO + [51], NO3 − [45], NO2 − [52], H + [11], NH4 + [44], have been observed forming.As shown in Figures 6b and 8, highly mobile NH4 + intermediates formed on Cu-ZSM-5 and Cu-SAPO-34 as a result of the NH3-NO interaction [44].In case of Fe-ZSM-5, due to the well-known NH3-inhibition effect [10,53], formation of NH4 + intermediates (resulting from the reduction of Fe 3+ to Fe 2+ similar as the Cu redox cycle) at low reaction temperatures can only be clearly observed by in situ IS-DRIFTS after a partial desorption of adsorbed NH3 (Figure 10a) [17].Nevertheless, adsorption and activation of NO did take place on NH3-satured Fe-ZSM-5, leading to the formation of NH4 + intermediates, clearly enhancing the proton conductivity of NH3-loaded Fe-ZSM-5 in NO (see the higher IIS values during exposure in NO than in N2; Figure 10b).These observations indicate a Fe 3+ ↔Fe 2+ redox cycle in Fe-ZSM-5 catalysts similar as the widely accepted Cu 2+ ↔Cu + redox cycle in Cu-SSZ-13 catalysts (Figure 11a) [3,10,11,44,45,54].More interestingly, the formed NH4 + intermediates were found to largely determine the NH3-SCR activity of Fe-ZSM-5 catalysts at low temperatures (Figure 11b).Therefore, the formation of NH4 + intermediates, indicating the activation of NO in the presence of adsorbed NH3, may potentially serve as a 'descriptor' of the activity of Fe-zeolite catalyst for NH3-SCR, especially at low temperatures.NH4 + ions on Brønsted acid sites; 1266 cm −1 : NH3 species on Fe sites.The catalyst was pre-treated at 450 °C in 10% O2 for 1 h before each measurement.Reprinted with permission from [17].Copyright American Chemical Society, 2016. 1 wt % Cu, (P + Al)/Si = 12.9.Cu-ZSM-5: 1 wt % Cu, Si/Al = 13.5.Reproduced with permission from [27].Copyright Elsevier, 2016.Cu-ZSM-5 and Cu-SAPO-34 as a result of the NH 3 -NO interaction [44].In case of Fe-ZSM-5, due to the well-known NH 3 -inhibition effect [10,53], formation of NH 4 + intermediates (resulting from the reduction of Fe 3+ to Fe 2+ similar as the Cu redox cycle) at low reaction temperatures can only be clearly observed by in situ IS-DRIFTS after a partial desorption of adsorbed NH 3 (Figure 10a) [17].Nevertheless, adsorption and activation of NO did take place on NH 3 -satured Fe-ZSM-5, leading to the formation of NH 4 + intermediates, clearly enhancing the proton conductivity of NH 3 -loaded Fe-ZSM-5 in NO (see the higher I IS values during exposure in NO than in N 2 ; Figure 10b).These observations indicate a Fe 3+ ↔Fe 2+ redox cycle in Fe-ZSM-5 catalysts similar as the widely accepted Cu 2+ ↔Cu + redox cycle in Cu-SSZ-13 catalysts (Figure 11a) [3,10,11,44,45,54].More interestingly, the formed NH 4 + intermediates were found to largely determine the NH 3 -SCR activity of Fe-ZSM-5 catalysts at low temperatures (Figure 11b).Therefore, the formation of NH 4 + intermediates, indicating the activation of NO in the presence of adsorbed NH 3 , may potentially serve as a 'descriptor' of the activity of Fe-zeolite catalyst for NH 3 -SCR, especially at low temperatures.6b and 8, highly mobile NH4 + intermediates formed on Cu-ZSM-5 and Cu-SAPO-34 as a result of the NH3-NO interaction [44].In case of Fe-ZSM-5, due to the well-known NH3-inhibition effect [10,53], formation of NH4 + intermediates (resulting from the reduction of Fe 3+ to Fe 2+ similar as the Cu redox cycle) at low reaction temperatures can only be clearly observed by in situ IS-DRIFTS after a partial desorption of adsorbed NH3 (Figure 10a) [17].Nevertheless, adsorption and activation of NO did take place on NH3-satured Fe-ZSM-5, leading to the formation of NH4 + intermediates, clearly enhancing the proton conductivity of NH3-loaded Fe-ZSM-5 in NO (see the higher IIS values during exposure in NO than in N2; Figure 10b).These observations indicate a Fe 3+ ↔Fe 2+ redox cycle in Fe-ZSM-5 catalysts similar as the widely accepted Cu 2+ ↔Cu + redox cycle in Cu-SSZ-13 catalysts (Figure 11a) [3,10,11,44,45,54].More interestingly, the formed NH4 + intermediates were found to largely determine the NH3-SCR activity of Fe-ZSM-5 catalysts at low temperatures (Figure 11b).Therefore, the formation of NH4 + intermediates, indicating the activation of NO in the presence of adsorbed NH3, may potentially serve as a 'descriptor' of the activity of Fe-zeolite catalyst for NH3-SCR, especially at low temperatures.NH4 + ions on Brønsted acid sites; 1266 cm −1 : NH3 species on Fe sites.The catalyst was pre-treated at 450 °C in 10% O2 for 1 h before each measurement.Reprinted with permission from [17].Copyright American Chemical Society, 2016.

H2O Vapor
In real diesel exhausts, a considerable amount of H2O vapor resulting from the fuel combustion processes and the NH3-SCR reactions is always present.As revealed previously, H2O as a solvate molecule could serve as proton carrier as well and consequently enhance the proton conductivity of zeolites in a broad temperature range [33].As compared to NH3, H2O demonstrates a significantly weaker supporting effect for the proton transport in zeolites [16], specifically, 1 vol.% H2O vapor only has the same effect as 6 ppm of NH3 in terms of the conductivity change of H-ZSM-5 (at 420 °C) [55].In the above-mentioned transient IS measurements (in the absence of H2O), a significant influence of H2O as the product of NH3-SCR reaction can thus be ruled out because of the very low concentration.Nevertheless, minor contribution of H2O to the overall proton conductivity of the respective zeolite catalyst cannot be fully excluded.Further comparative studies (with or without H2O) are required to achieve a more complete understanding of the influence of H2O.

Summary and Perspectives
In summary, by analyzing the proton transport properties of zeolite catalysts under SCR-related reaction conditions using in situ IS, the NH3-zeolite interaction, NO-zeolite interaction (in the presence of adsorbed NH3), and formation of proton-conducting intermediates can be probed.A combination of IS with DRIFTS allows for understanding molecularly the proton transport properties of zeolite NH3-SCR catalysts.Several structural or chemical parameters, such as framework topology, metal cation type and metal exchange level, influenced the proton transport to different degrees by affecting the reactant-zeolite interactions.On the one hand, the mobility of adsorbed NH3-species as proton carriers, determined by the NH3-zeolite interaction, was influenced differently by the type,

H 2 O Vapor
In real diesel exhausts, a considerable amount of H 2 O vapor resulting from the fuel combustion processes and the NH 3 -SCR reactions is always present.As revealed previously, H 2 O as a solvate molecule could serve as proton carrier as well and consequently enhance the proton conductivity of zeolites in a broad temperature range [33].As compared to NH 3 , H 2 O demonstrates a significantly weaker supporting effect for the proton transport in zeolites [16], specifically, 1 vol.% H 2 O vapor only has the same effect as 6 ppm of NH 3 in terms of the conductivity change of H-ZSM-5 (at 420 • C) [55].In the above-mentioned transient IS measurements (in the absence of H O), a significant influence of H 2 O as the product of NH 3 -SCR reaction can thus be ruled out because of the very low concentration.Nevertheless, minor contribution of H 2 O to the overall proton conductivity of the respective zeolite catalyst cannot be fully excluded.Further comparative studies (with or without H 2 O) are required to achieve a more complete understanding of the influence of H 2 O.

Summary and Perspectives
In summary, by analyzing the proton transport properties of zeolite catalysts under SCR-related reaction conditions using in situ IS, the NH 3 -zeolite interaction, NO-zeolite interaction (in the presence of adsorbed NH 3 ), and formation of proton-conducting intermediates can be probed.A combination of IS with DRIFTS allows for understanding molecularly the proton transport properties of zeolite NH 3 -SCR catalysts.Several structural or chemical parameters, such as framework topology, metal cation type and metal exchange level, influenced the proton transport to different degrees by affecting the reactant-zeolite interactions.On the one hand, the mobility of adsorbed NH 3 -species as proton carriers, determined by the NH 3 -zeolite interaction, was influenced differently by the type, loading and coordinative environment of the exchanged Fe or Cu species in zeolite catalysts.On the other hand, highly mobile NH 4 + intermediates, as identified by IS combined with DRIFTS, can form on zeolite catalysts as general intermediate species resulting from the interaction of co-adsorbed NH 3 and NO on metal active sites.The formed NH 4 + intermediates not only significantly influenced the proton transport properties, and consequently the reaction monitoring performance of zeolite catalysts due to the highly proton-conducting nature, but also largely determined the low-temperature NH 3 -SCR activity because of their high mobility and reactivity.These findings, which are not easily achievable by conventional methods, thus provide new perspectives to understand mechanistically the NH 3 -SCR reaction over zeolite catalysts.
To understand further the role of proton transport in NH 3 -SCR catalysis, both chemical nature (e.g., surface acidity, chemical composition) and structural properties (e.g., size, shape, or porosity) of the zeolite catalysts should be taken into account.Substantial improvements, for example the synthesis of zeolites with well-controlled crystal sizes or porosity, have already been achieved using delicately designed bottom-up (i.e., controlling the chemical and structural properties by adjusting the synthetic procedure) or top-down (i.e., post-synthetic modifications) approaches [60,61].Although the electrical and catalytic properties of zeolites can be correlated to certain specific chemical or structural features in a collective manner, it is still practically challenging to discriminate intrinsic and interfacial effects in the mentioned applications.In recent years, several advanced techniques, such as X-ray based micro-spectroscopy or local-probe measurement, have been developed to analyze in situ the chemical nature, the three-dimensional structure, the electrical properties, and eventually the structure-activity relationship at single-particle levels without the interference of boundary effects [15,62,63].It is expected that a combination of these new techniques will promote achieving more reliable guidelines for the rational development of zeolite catalysts in the future.

Figure 1 .
Figure 1.(a) Argand diagram (plot of the imaginary part of the impedance −Z″ versus the real part of the impedance Z′ in the complex plane) of dehydrated H-ZSM-5 (Si/Al 13.5) at 250 °C; the gray arrow indicates the increase of frequency; (b) modulus spectra of the imaginary part M″ versus frequency f of H-ZSM-5 at temperatures 200-450 °C; the gray arrow indicates the increase of temperature; (c) Arrhenius-like plot of logarithmic proton conductivity at resonance frequencies (in low-frequency range) derived from the Modulus spectra in (b).Reproduced with permission from [29].The Royal Society of Chemistry, 2016.

Figure 1 .
Figure 1.(a) Argand diagram (plot of the imaginary part of the impedance −Z" versus the real part of the impedance Z in the complex plane) of dehydrated H-ZSM-5 (Si/Al 13.5) at 250 • C; the gray arrow indicates the increase of frequency; (b) modulus spectra of the imaginary part M" versus frequency f of H-ZSM-5 at temperatures 200-450 • C; the gray arrow indicates the increase of temperature; (c) Arrhenius-like plot of logarithmic proton conductivity at resonance frequencies (in low-frequency range) derived from the Modulus spectra in (b).Reproduced with permission from [29].The Royal Society of Chemistry, 2016.

Figure 2 .
Figure 2. Schematic illustrations of the measurement configurations for (a) in situ impedance spectroscopy (IS) and (b) in situ IS and diffuse reflection infrared Fourier transformation spectroscopy (in situ IS-DRIFTS).(a) Reproduced with permission from [30]; (b) adapted with permission from [28], Elsevier, 2016.

Figure 2 .
Figure 2. Schematic illustrations of the measurement configurations for (a) in situ impedance spectroscopy (IS) and (b) in situ IS and diffuse reflection infrared Fourier transformation spectroscopy (in situ IS-DRIFTS).(a) Reproduced with permission from [30]; (b) adapted with permission from [28], Elsevier, 2016.

Figure 4 .
Figure 4. (a) simultaneously measured proton conductivity (IIS; IS signal at 10 kHz; solid lines) and DRIFTS signals after Kubelka-Munk (KM) transformation (IDRIFTS; symbols) during the loading and thermal desorption of NH3 over zeolites at 100 °C.The colorful background indicates the period with NH3 supply to the system.IDRIFTS at 1457 cm −1 (triangles) and 1276/1266 cm −1 (circles) are attributed to the bending vibrations of NH4 + ions on Brønsted acid sites and bending vibrations of NH3 species on metal sites, respectively; and (b) Arrhenius-like representations for the IS results obtained in flowing N2 (empty symbols) and NH3 (100 ppm in N2; half-filled symbols) over H-ZSM-5 (squares), Fe-ZSM-5 (circles) and Cu-ZSM-5 (triangles).Reproduced with permission from[29].Copyright The Royal Society of Chemistry, 2016.

Figure 4 .
Figure 4. (a) simultaneously measured proton conductivity (I IS ; IS signal at 10 kHz; solid lines) and DRIFTS signals after Kubelka-Munk (KM) transformation (I DRIFTS ; symbols) during the loading and thermal desorption of NH 3 over zeolites at 100 • C. The colorful background indicates the period with NH 3 supply to the system.I DRIFTS at 1457 cm −1 (triangles) and 1276/1266 cm −1 (circles) are attributed to the bending vibrations of NH 4 + ions on Brønsted acid sites and bending vibrations of NH 3 species on metal sites, respectively; and (b) Arrhenius-like representations for the IS results obtained in flowing N 2 (empty symbols) and NH 3 (100 ppm in N 2 ; half-filled symbols) over H-ZSM-5 (squares), Fe-ZSM-5 (circles) and Cu-ZSM-5 (triangles).Reproduced with permission from [29].Copyright The Royal Society of Chemistry, 2016.

Figure 6 .
Figure 6.(a) time-courses of normalized proton conductivity I IS (at 10 kHz) for Fe-ZSM-5 and Cu-ZSM-5 during exposure to the indicated atmospheres at 100 • C and 250 • C; (b) normalized I IS (line) and I DRIFTS signals (symbols) for Cu-ZSM-5 under SCR-related atmospheres at 250 • C. (a) Reproduced with permission from [29].Copyright The Royal Society of Chemistry, 2016; (b) Reprinted with permission from [31].Copyright American Chemical Society, 2016.

Figure 7 .Figure 7 .
Figure 7. (a) NH3-TPD (temperature-programmed desorption using NH3 as a probe molecule) profiles showing the desorption of NH3 species on Lewis sites, metal sites and Brønsted sites in different temperature ranges; (b) activation energy (Ea) for proton transport as a function of metal exchange level.The Ea values were derived from the Arrhenius plots of the in situ IS results over NH3-loaded zeolite catalysts under thermal desorption conditions (in N2) at temperatures 130-250 °C; (c) in situ DRIFT spectra for NH3-saturated Fe-ZSM-5 zeolite catalysts with different Fe/Al ratios at 175 °C; (d) in situ DRIFT spectra for a NH3-saturated Fe-ZSM-5 zeolite catalyst (Fe/Al ratio of 0.11) at different temperatures.(a) Reproduced with permission from [29].Copyright The Royal Society of Chemistry,Figure 7. (a) NH 3 -TPD (temperature-programmed desorption using NH 3 as a probe molecule) profiles showing the desorption of NH 3 species on Lewis sites, metal sites and Brønsted sites in different temperature ranges; (b) activation energy (E a ) for proton transport as a function of metal exchange level.The E a values were derived from the Arrhenius plots of the in situ IS results over NH 3 -loaded zeolite catalysts under thermal desorption conditions (in N 2 ) at temperatures 130-250 • C; (c) in situ DRIFT spectra for NH 3 -saturated Fe-ZSM-5 zeolite catalysts with different Fe/Al ratios at 175 • C; (d) in situ DRIFT spectra for a NH 3 -saturated Fe-ZSM-5 zeolite catalyst (Fe/Al ratio of 0.11) at different temperatures.(a) Reproduced with permission from [29].Copyright The Royal Society of Chemistry, 2016; (b) Reprinted with permission from[31], American Chemical Society, 2016; (c,d) Reprinted with permission from[17].Copyright American Chemical Society, 2016.

Figure 10 .
Figure 10.Normalized I IS (green line) and DRIFTS signals (red symbols) at characteristic wavenumbers of NH 3 -loaded Fe-ZSM-5 (Si/Al 13.5, Fe/Al 0.11) exposed N 2 and NO/O 2 mixture in sequence (a) and exposed to NO and NO/O 2 mixture in sequence (b).I IS : 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 : NH 4 + ions on Brønsted acid sites; 1266 cm −1 : NH 3 species on Fe sites.The catalyst was pre-treated at 450 • C in 10% O 2 for 1 h before each measurement.Reprinted with permission from [17].Copyright American Chemical Society, 2016.

Figure 11 .
Figure 11.(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.Reprinted with permission from [17].Copyright American Chemical Society, 2016.

Figure 11 .
Figure 11.(a) proposed pathway for the formation of NH 4 + intermediate in NH 3 -SCR over Fe-ZSM-5 catalysts; (b) correlation between the NH 4 + intermediate formation and the normalized NO reduction rates at low temperatures; the NH 4 + intermediate formation (∆I IS ) was evaluated according to the proton conductivity enhancement of NH 3 -loaded Fe-ZSM-5 after exposure in NO for 30 min in comparison to exposure in N 2 for 30 min.Reprinted with permission from [17].Copyright American Chemical Society, 2016.