Mechanistic understanding of Cu-CHA catalyst as sensor for direct

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Energy, Environmental, and Catalysis Applications

Mechanistic understanding of Cu-CHA catalyst as sensor for direct NH-SCR monitoring: the role of Cu mobility 3

Peirong Chen, Valentina Rizzotto, Abhishek Khetan, Kunpeng Xie, Ralf Moos, Heinz Pitsch, Daiqi Ye, and Ulrich Simon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22104 • Publication Date (Web): 01 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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ACS Applied Materials & Interfaces

Mechanistic understanding of Cu-CHA catalyst as sensor for direct NH3SCR monitoring: the role of Cu mobility Peirong Chen,a,b,c,* Valentina Rizzotto,b,c Abhishek Khetan,c,d,e Kunpeng Xie,b,c Ralf Moos,f Heinz Pitsch,c,d Daiqi Ye,a Ulrich Simonb,c,* a

Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, School of Environment and Energy, South China University of Technology, Guangzhou 510006, China

b

Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, Aachen 52074, Germany

c

Center for Automotive Catalytic Systems Aachen, RWTH Aachen University, Aachen, Germany

d

Institute for Combustion Technology, RWTH Aachen University, Templergraben 64, Aachen 52056, Germany

e

Department of Mechanical Engineering, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States

f

Department of Functional Materials and Bayreuth Engine Research Center (BERC), University of Bayreuth, Bayreuth 95440, Germany

* Corresponding authors. [email protected] (P.C.); [email protected] (U.S.)

Abstract: The concept to utilize catalyst directly as sensor is fundamentally and technically attractive for a number of catalytic applications, in particular for the catalytic abatement of automotive emissions. Here, we explore the potential of microporous copper-exchanged chabazite (Cu-CHA, including Cu-SSZ-13 and Cu-SAPO-34) zeolite catalysts, which are used commercially in the selective catalytic reduction of automotive nitrogen oxide emissions by NH3 (NH3-SCR), as impedance sensor elements to monitor directly the NH3-SCR process. The NH3-SCR sensing behavior of commercial Cu-SSZ-13 and Cu-SAPO-34 catalysts at typical reaction temperatures (i.e. 200 °C and 350 °C) was evaluated according to the change of ionic conductivity, and was mechanistically investigated by complex impedance-based in situ

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modulus spectroscopy. Short-range (local) movement of Cu ions within the zeolite structure was found to determine largely the NH3-SCR sensing behavior of both catalysts. Formation of NH3-solvated, highly mobile CuI species showed a predominant influence on the ionic conductivity of both catalysts and, consequently, hindered NH3-SCR sensing at 200 °C. Density-functional theory calculations over model Cu-SAPO-34 system revealed that CuII reduction to CuI by co-adsorbed NH3 and NO weakened significantly the coordination of Cu site to the CHA framework, enabling high mobility of CuI species that influence substantially the NH3-SCR sensing. The in situ spectroscopic and theoretical investigations not only unveil the mechanisms of Cu-CHA catalyst as sensor element for direct NH3-SCR monitoring, but also allow to get insights into the speciation of Cu active sites in NH3-SCR under different reaction conditions with varied temperatures and gas compositions. Keywords: in situ impedance spectroscopy; Cu ion movement; DFT calculation; modulus; NH3 solvation; Cu redox

1. Introduction Metal-promoted zeolites, in particular copper ion-exchanged chabazite (Cu-CHA), are highly reactive in the selective catalytic reduction of nitrogen oxides (NOx) by NH3 (NH3-SCR) following the routes in Eqs. 1-3 4 NH3 + 4 NO + O2 → 4 N2 + 6 H2O

(standard SCR)

(1)

2 NH3 + NO + NO2 → 2 N2 + 3 H2O

(fast SCR)

(2)

(NO2 SCR)

(3)

8 NH3 + 6 NO2

→ 7 N2 + 12 H2O

and thus are widely explored for the control of automotive NOx emissions from lean-burn engines (especially diesel engines).1–5 In real applications, urea solutions serving as an external NH3 source are injected into the NH3-SCR unit from a storage tank via a delivery system and are hydrolyzed to generate NH3 at elevated temperatures (above 180 °C).6 The urea injection has to be adjusted precisely and instantly in order to achieve sufficiently high NOx conversion on the one 2


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hand, and to avoid ammonia slip on the other hand.6 Currently, closed-loop feedback control systems, which are based on measured inlet and/or outlet NOx concentrations and the calculated NH3 storage in the NH3-SCR catalyst, are utilized to optimize the injection parameters.6 The efficiency and accuracy of such an NH3 management approach is often hampered by the crosssensitivity (to NH3 or other exhaust components) of the NOx sensor placed downstream or/and upstream of the NH3-SCR unit.6,7 Thus, a series of novel sensing materials or sensor architectures have been developed in the last years to lower the cross-sensitivity and, in turn, to improve the NOx sensing performance.8–10 Nevertheless, more advanced strategies capable of monitoring directly the NH3 storage and even the NH3-SCR conversion level are still highly desirable, in order to achieve a more sophisticated control of urea injection and, in turn, a higher efficiency of the NH3-SCR system.11,12 Among others, a direct utilization of the catalyst as active sensing element is highly promising and has already been explored by different research groups from both academia and industry.13–16 Our previous in situ investigations by impedance spectroscopy (IS) revealed that NH3 adsorption increases the mobility of protons or proton carriers (e.g. NH4+) within zeolite catalysts, resulting in an increased ionic conductivity.17–24 The ionic conductivity decreases when the adsorbed NH3 is depleted by either thermal desorption or SCR conversion.20,22,24,25 Such NH3-supported ion transport/conduction in zeolites enables to determine in situ or operando the loading, desorption, and SCR conversion of NH3 in a variety of metal-promoted zeolite catalysts (e.g. Cu-SAPO-34, Cu-SSZ-13, Fe-ZSM-5, Cu-ZSM-5), by means of IS at either low or high frequencies.13,18,23 Recent theoretical and operando spectroscopy studies disclosed that the isolated single-atom Cu active sites in Cu-CHA catalysts are solvated and mobilized by adsorbed NH3 species in NH3-SCR reactions at low temperatures (< 250 °C).22,26– 29

Ab initio molecular dynamics (AIMD) simulations indicate that NH3 solvation leads to an

enhancement of the mobility of dehydrated Cu species by a factor of 9.4 or more at 298 K.27 More interestingly, NH3-solvated CuI species, i.e. CuI(NH3)2, are even mobile enough to travel 3



through the eight-member-ring (8MR) window to the neighboring cages in CHA zeolite.26 Hence, these nominally single-site catalysts exhibit a behavior that lies outside the canonical definition of a single-site heterogeneous catalyst.26 While our previous proof-of-concept investigations already confirm that the Cu-zeolite catalysts can be utilized as impedance sensors for the direct monitoring of NH3-SCR reactions,18,19,23 it has not been known yet, whether the high mobility of NH3-solvated Cu species influences the NH3-SCR sensing performance of Cu-zeolite catalysts, in particular at low reaction temperatures. Here, commercial Cu-CHA zeolite catalysts, including Cu-SSZ-13 (Cu-exchanged aluminosilicate) and Cu-SAPO-34 (Cu-exchanged silicoaluminophosphate), were examined and compared in the impedance sensing of NH3 at varied concentrations and of the NH3-SCR reaction in atmospheres with varied NH3/NO ratios. Impedance-based in situ modulus spectroscopy investigations were carried out under NH3-SCR reaction conditions, in order to differentiate the contribution of different ion conduction or movement processes, including the long-range (i.e. low-frequency) translational transport of protons/proton carriers and the short-range or local (i.e. high-frequency) movement of NH3-solvated Cu ions.19,22 Periodic density-functional theory (DFT) calculations over model Cu-SAPO-34 system as an example were conducted to verify and supplement the in situ IS observations.

2. Experimental section 2.1 Materials and characterization Cu-SAPO-34 zeolite powders (from Clariant AG) were calcined at 500 °C (in air) before use. Cu-SSZ-13 powders were scraped from a monolithic catalyst coat provided by a major manufacturer in the market, and were calcined at 500 °C (in air) as well. Surface morphologies of the two zeolite catalysts were studied by scanning electron microscopy (SEM) with a Zeiss DSM 982 Gemini microscope, while the chemical compositions, namely the Si/Al and Cu/Al ratios for Cu-SSZ-13, and the (Al+P)/Si and Cu/Si ratios for Cu-SAPO-34, were analyzed by 4


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energy-dispersive X-ray spectroscopy (EDX) with an Oxford INCA Energy 200 EDX system (Oxford Instruments). Temperature-programmed desorption studies using NH3 as a probe molecule (NH3-TPD) were performed over Cu-SSZ-13 and Cu-SAPO-34 to understand the surface acidities of and NH3-zeolite interactions for the two catalysts. The measurements were carried out in a quartz micro-reactor (with an outer diameter of 6 mm) which was placed in a horizontal tubular furnace (Carbolite TZF). Gas dosing was performed using mass flow controllers (MKS Instruments), and a total flow rate of 200 mL min-1 was maintained to achieve a weight hourly space velocity (WHSV) of 120 000 mL h-1 g-1. Before each measurement, the zeolite samples (100 mg) were heated to 500 °C (at a ramping rate of 15 °C min-1) and maintained at the temperature for 0.5 h in 6.5 % O2 (N2 balance). After the pretreatment, the reactor was cooled down to 100 °C under N2 flow. NH3 adsorption was carried out at 100 °C by exposing the samples to 1000 ppm NH3 (N2 balance) for 2 h. After that, the zeolite samples were purged with N2 for 2 h to remove physically adsorbed NH3 species. The desorption of NH3 was carried out in N2 at temperatures ranging from 100 °C to 700 °C (with a ramping rate of 5 °C min-1). Concentration of NH3 leaving the reactor was measured continuously with an FTIR gas analyzer (MKS MultiGasTM 2030) placed downstream. 2.2 Impedance spectroscopy measurements 2.2.1 Theory and instrumentation Impedance spectroscopy is an electric perturbation-based measuring technique. The application of an alternating voltage U (with amplitude U0 and angular frequency ω) to a system in thermodynamic equilibrium induces the motion of ions (e.g. cations exchanged into zeolites) and, consequently, a macroscopically measurable current I(ω).30 The complex impedance Z(ω) can be derived according to

5



𝑍(𝜔) =

𝑈(𝜔) 𝐼(𝜔)

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(4)

(5)

𝑍(𝜔) = 𝑍′(𝜔) + 𝑗𝑍′′(𝜔)

In the typical Argand plot (or Nyquist plot; imaginary part Z″ versus real part Z′), low-frequency processes (such as the polarization of sample/electrode interface) dominate, thus hindering the visualization of high-frequency processes such as the local motion of ions.20,25 Modulus spectroscopy, which presents the imaginary part of modulus M, i.e. M” (6)

𝑀′′ = 2π𝜈𝐶0𝑍′

where ν = ω/2π is the perturbing frequency, C0 is the capacitance of the empty capacitor, and Z′ the real part of the complex impedance, was found to enable the resolving of multiple ionconducting processes in complex systems such as zeolite catalysts.20,30,31 As revealed in our previous studies,20,25 for zeolites, a local maximum in the modulus plot results from an ion conduction-induced dielectric relaxation phenomenon. Relaxation time (τ) for the respective ion conduction process can be calculated based on the resonance frequency (νres) via τ = 1/νres

(7)

Our previous investigations revealed that, usually, two distinct resonance peaks can be resolved in the modulus spectrum of a zeolite catalyst under reaction conditions, namely the low-frequency (LF; 1–100 Hz) one resulting from long-range translational conduction or motion of ions, and the high-frequency (HF; 104–106 Hz) one for shortrange or local motion of ions.20,22,25 Both the LF and HF processes were found to be sensitive to temperature,31–33 and the activation energy (Ea) for the respective ionconducting process can be obtained from the Arrhenius equation ln(Y’resT) ~ ln(σT) = A - Ea /kBT

6


(8)

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where Y’res is the real part of admittance Y (Y = 1/Z) at the resonance frequency νres (determined by the resonance peak position in the corresponding M” plot), σ is the specific conductivity of the zeolite, A is a pre-exponential factor which depends on the involved mobile ions, T is the temperature and kb is the Boltzmann constant.32 For in situ IS measurements, the zeolite powders were deposited as thick films (a thickness of ca. 50 µm) on inter-digital electrodes (IDEs; with an electrode spacing of 125 µm) by screen printing.24 The IDEs are composed of an Al2O3 substrate with Au electrodes on the front side and integrated heater on the back side.24 Our previous experiments already proved that the screen-printing method renders reproducible preparation of zeolite films with similar thickness and packing density, and improved electrical contact between the zeolite sample and the electrode.23 Configuration of the in situ IS instrument is detailed elsewhere.20,24,25 Briefly, a home-made stainless-steel chamber, which is equipped with a zinc selenide window for contactless regulation of sensor temperature by a pyrometer (Heitronics), a gas dosing system (MKS Instruments), and a digital multimeter (Keithley) for the power supply of the integrated heater, was used.24 The total volume of the chamber is about 30 cm3. 2.2.2 Sensing of NH3 and NH3-SCR reaction Sensing behavior of the two commercial zeolite catalysts was evaluated by analyzing their complex impedance response in NH3 atmosphere or SCR-related mixtures at selected temperatures (200 °C, 350 °C, or 450 °C), following the experimental procedures and data presentation methods reported previously.23 Sensing performance of the zeolite catalysts were assessed according to the change of admittance |Y| (|Y| = 1/|Z|; |Z| is the absolute value of Z) with the variation of gas condition, and the measuring frequency was fixed at 10 kHz for an improved time resolution.24 Flowing NH3 with varied concentrations (0-100 ppm) was applied in the NH3 sensing experiments. For NH3-SCR sensing, NO and NH3 concentrations in the gas mixture of NH3, NO, O2 and N2 were varied stepwise to achieve different conversion levels.23 The measured |Y| values were normalized to [0, 1], and are denoted as IIS throughout the text.24 7



2.2.3 Modulus spectroscopy based on multi-frequency in situ IS measurements Resonance frequencies of the zeolite catalyst at selected temperatures were determined using modulus spectroscopy, which is based on multi-frequency in situ IS measurements in a broad range from 10-1~106 Hz.20,22 The reaction atmospheres were carefully selected, aiming at resolving the contributions of long-range (low-frequency) and short-range (high-frequency) ion conducting processes.19,22 For each modulus spectrum, a total of 106 points were measured in the frequency range of 10-1~106 Hz at equidistant logarithmic steps, and the measuring duration is about 30 min. To ensure that a steady state was achieved, three spectra were collected in each reaction atmosphere and the last spectrum was used for comparison. The zeolite catalyst was pretreated at 450 °C in pure O2 for 1.5 h prior to any measurement, in order to minimize unintended effects by solvate molecules. 2.3 DFT calculations In order to verify and supplement the in situ IS observations, periodic DFT calculations were performed using the GPAW code.34 Cu-SAPO-34 was selected as an example of the Cu-CHA catalysts. Further computational details were described elsewhere.35 Briefly, The Bayesian error corrected BEEF-vdW functional was used for a better estimation of van der Waals interactions.36,37 A SAPO-34 supercell containing 108 atoms and with a size of 14.0482 Å × 14.0482 Å × 15.3318 Å was established and optimized according to our previous work.35 CuSAPO-34 configuration was set up by introducing a Cu atom into the SAPO-34 supercell structure.

3. Results and discussion 3.1 Physicochemical characterization Cu-exchanged microporous CHA zeolites, including SSZ-13 and SAPO-34 (see the supercell structure in Figure S1a), are utilized commercially for the abatement of NOx emissions from vehicles equipped with lean-burn engines (including diesel engines).1,38 The small-pore feature, 8


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i.e. with eight or less oxygen atoms in a window (corresponding to a pore diameter of 4.2 Å; see Figure S1b for the pore system of CHA zeolite),39 excludes the entrance of unburnt large hydrocarbon molecules into the inner CHA cage, and consequently prevents structural damage of zeolite catalysts by high temperatures resulting from the exothermic hydrocarbon combustion within the CHA framework.2 Chemical compositions of the used Cu-SSZ-13 and Cu-SAPO-34 zeolites were analyzed by EDX and are summarized in Table 1. The quality of screen-printed zeolite films on IDE chips was examined by SEM, and representative images are displayed in Figure 1. It can be seen that dense and uniform packing of zeolite particles was achieved in both Cu-SSZ-13 (Figure 1a and b) and Cu-SAPO-34 (Figure 1c and d) films. The high quality of the film ensures a similar accessibility to the gas molecules for the zeolite particles.23 Although the diverse particle sizes and shapes (see the SEM images in Figure 1b and d) may result in a grain boundary effect influencing slightly the response time of the IDE sensor, chemical properties (for example the type and amount of cations) of the zeolite are known to play a more significant or even predominant role in gas sensing.23,25,40

Table 1 Chemical compositions of the used Cu-SSZ-13 and Cu-SAPO-34 catalysts by EDX analyses

a

Si/Al or (Al+P)/Sia

Cu/Al or Cu/Sib

Cu-SSZ-13

15.7

0.30

Cu-SAPO-34

11.2

0.22

Atomic ratio: Si/Al for Cu-SSZ-13, (Al+P)/Si for Cu-SAPO-34; b Cu/Al for Cu-SSZ-13, Cu/Si for

Cu-SAPO-34.

9



Figure 1 Representative SEM images for Cu-SSZ-13 (a and b) and Cu-SAPO-34 (c and d) films on IDE sensor chips.

Surface acidity and NH3-zeolite interactions for the two zeolite catalysts were examined by NH3-TPD, and the NH3 desorption profiles are displayed in Figure 2. Three contribution can be identified for both zeolite catalysts, namely the low-temperature peak centered at 180 ~ 190 °C for desorption of NH3 weakly bound on Lewis sites (such as the extra-framework Al sites), the medium-temperature peak at 265 ~ 280 °C for NH3 desorption from Cu sites, and the high-temperature peak at 410 ~ 420 °C corresponding to NH4+ strongly bounded to Brønsted sites.2,35,41 Clearly, while the abundance of Lewis sites and Cu sites is similar on the two catalysts, Cu-SSZ-13 possesses a significantly higher amount of Brønsted sites than Cu-SAPO34. As well understood in previous studies, NH4+ species on Brønsted sites are crucial for the formation of NH4+·(NH3)n chains that serve as ion conduction paths at low temperatures and as NH3 reservoirs in NH3-SCR reactions,4,19,42 and determine largely the tethering and consequently the mobility of NH3-solvated Cu species during NH3-SCR reactions.26 Therefore, different catalytic and sensing behaviors are anticipated between the two zeolite catalysts. 10


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Figure 2 NH3-TPD profiles for the Cu-SSZ-13 (red) and Cu-SAPO-34 (blue) zeolites.

3.2 NH3 and NH3-SCR sensing performance Figure 3 demonstrates the NH3 sensing performance (i.e. the change of IIS with the variation of NH3 concentration) of Cu-SSZ-13 and Cu-SAPO-34 at two typical reaction temperatures, namely 200 °C and 350 °C. Due to NH3-supported ion conduction,17,20,23,25 IIS increased immediately upon NH3 feeding (100 ppm) and reached an equilibrium state within ca. 10 min at both temperatures. The decrease of IIS signal in the presence of co-fed O2 (10 vol.%), which was observed over both zeolite catalysts and got more pronounced at higher temperatures (Figures 3, S2 and S3), can be attributed to the catalytic oxidation of ammonia by the Cuzeolites.43 At 350 °C , IIS of Cu-SAPO-34 decreased more significantly upon O2 feeding (in 100 ppm NH3), which is likely due to a higher amount of small CuOx clusters actively catalyzing NH3 oxidation.44 In general, the stepwise decrease of NH3 concentration led to a corresponding signal decrease of IIS for both catalysts at 200 °C. Even though, Cu-SAPO-34 demonstrated a more sensitive response towards NH3 than Cu-SSZ-13, which may originate from different interactions between NH3 and the Cu sites in the zeolite catalysts at this specific temperature.20,22 At a higher temperature of 450 °C, along with the stepwise change of NH3 11



concentration, clearly stepwise decrease of IIS was observed in both zeolite catalysts without noticeable difference (Figure S2).

Figure 3 NH3 sensing performance (in terms of IIS change with the NH3 concentration) over the CuSSZ-13 (red traces) and Cu-SAPO-34 (blue traces) catalysts at 200 °C and 350 °C.

The clear electrical response of the Cu-CHA catalysts towards the change of NH3 concentrations enables monitoring directly the NH3-SCR reaction, in which Cu-zeolites catalyze the reaction between NH3 and NOx forming N2 and H2O (Eq. 1).1,2 Figure 4 shows the normalized IIS signals for Cu-SSZ-13 and Cu-SAPO-34 in NH3-SCR reaction-related atmospheres (i.e. NO/NH3/O2 mixtures) with varying NO/NH3 ratios. Interestingly, at 200 °C, the IIS signals for the two catalysts increased, rather than decreased, upon the co-feeding of NO with NH3/O2, and were largely stabilized without noticeable variation in gas mixtures with a NO/NH3 ratio between 30/100 and 100/100 (Figure 4). The IIS for Cu-SAPO-34 decreased stepwise with the NO/NH3 ratio increasing from 100/70 to 100/30, and gradually in NO/O2 (without NH3) approaching NH3-free state. As for Cu-SSZ-13, a gradual and slow decrease of IIS already took place in gas mixture with a NO/NH3 ratio of 100/70, and approached NH3-free state more rapidly than Cu-SAPO-34. At 350 °C, while the unexpected increase of IIS in gas 12


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mixtures with lower NO/NH3 ratio was observed over Cu-SAPO-34 as well, the IIS signal for Cu-SSZ-13 decreased stepwise corresponding to the increasing NO/NH3 ratio (Figure 4). At a higher temperature of 450 °C, a stepwise decrease of IIS with the increase of NO/NH3 ratio was observed over both zeolites (Figures S3). 10% O2 200 °C

90 60

0.5

30

Cu-SSZ-13 Cu-SAPO-34 1.0

350 °C

90 60

0.5

NH3

NO

0

50

100

150

200

30

NO/NH3 concentration / ppm

1.0

Normalized IIS

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


250

Time / min Figure 4 Sensing behavior (in terms of IIS change with NO/NH3 ratio in the fed gas mixture) under NH3SCR conditions over Cu-SSZ-13 (red traces) and Cu-SAPO-34 (blue traces) commercial catalysts at 200 °C and 350 °C.

In real engine exhausts, there is always the presence of H2O vapor (often in percentage level) from fuel combustion, hydrocarbon/CO oxidation and NH3-SCR reactions. As disclosed in our previous studies, H2O serving as solvate molecule and proton carrier could increase the ionic conductivity of zeolites in a broad temperature range.20,42,45 To examine whether H2O interferes drastically the sensing performances of Cu-CHA zeolite catalysts, NH3 and NH3-SCR sensing tests were conducted in the presence of H2O vapor at 350 °C. As displayed in Figure 5, both Cu-SSZ-13 and Cu-SAPO-34 demonstrated similar sensing behaviors as those observed under respective ‘dry’ conditions (i.e. without H2O; see Figures 3 and 4), implying that the Cu13



CHA zeolites as sensors are not interfered significantly by H2O vapor and thus are viable for application in real exhaust aftertreatment systems.

Figure 5 NH3 (a) and NH3-SCR (b) sensing behaviors of Cu-SSZ-13 (red traces) and Cu-SAPO-34 (blue traces) commercial catalysts at 350 °C in the presence of H2O vapor. 3% H2O and 10% O2 were maintained throughout the whole sensing experiments.

3.3 Mechanistic studies 3.3.1 Modulus spectroscopy Modulus spectroscopy is capable of distinguishing multiple (often overlapping) ion conduction/transport processes within one complex system.30,35 According to our previous in situ IS and theoretical investigations,42,46 NH3 solvation of zeolites results in two diverse ion conduction/movement processes, namely the short-range ion motion and the long-range ion transport, and leads to increased ionic conductivity which can be measured by IS.20,25,35 Dielectric relaxations appearing at clearly diverse timescales are induced by the two ion movement processes, and can be visualized by modulus spectroscopy based on resonance peaks 14


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at different frequencies.20,25,30,35 As shown in Figure 6, well-resolved resonance peaks were observed in neither of the zeolite catalysts in N2 due to the absence of NH3-solvation effect.35 In NH3-containing atmospheres, both HF and LF peaks appeared in the modulus spectra of CuSSZ-13 (Figure 6a) indicating the existence of both short- and long-range conduction of ions in the catalyst. On the contrary, only a HF peak was clearly observed over Cu-SAPO-34 (Figure 6b) pointing to the fact that the short-range ion conduction prevails in the catalyst. As revealed in our previous investigations, NH4+ ions that are formed via proton transfer from Brønsted sites to the adsorbed NH3 molecules serve as vehicle-like carriers for the long-range ion conduction across zeolite lattice at temperatures 200~300 °C.19,42 Due to a lower Brønsted acidity (see NH3-TPD in Figure 2), long-range ion conduction was not favored on Cu-SAPO34, leading to the absence of LF peak in the modulus spectrum at 200 °C (Figure 6b). This conclusion was also supported by a higher activation energy for the hopping of protons between neighboring Brønsted sites within Cu-SAPO-34 than within Cu-SSZ-13 (see Figure S5 for the Arrhenius plots of IS results collected in N2), and a clearly lower specific conductance (i.e. 1/|Z|) of Cu-SAPO-34 than Cu-SSZ-13 under similar conditions (Figure S6). A subsequent exposure of NH3-loaded Cu-CHA zeolite catalysts in NO/O2 atmosphere eliminated all the pre-adsorbed NH3 via NH3-SCR conversion23,24 leading to the disappearance of all the evolved resonance peaks for both catalysts (Figure 6).

15



a)

b)

Figure 6 Modulus spectra of Cu-SSZ-13 (a) and Cu-SAPO-34 (b) collected during exposure in N2, NH3 and NO/O2 in sequence at 200 °C.

Further multi-frequency in situ IS studies were carried out in NO/NH3/O2 gas mixtures with varied NH3/NO ratios to understand the abnormal sensing behavior of the two Cu-CHA zeolite catalysts (Figure 4). The obtained modulus spectra are illustrated in Figures 7 and 8 for Cu-SSZ-13 and Cu-SAPO-34, respectively. At 200 °C, the co-feeding of 50 ppm or 100 ppm NO with NH3/O2 (100 ppm/10%) to Cu-SSZ-13 resulted in two identical modulus spectra (Figure 7a). In comparison with that obtained in NH3, the LF peak in NO/NH3/O2 shifted to a lower frequency pointing to a less pronounced long-range ion transport (i.e. translational movement of protons or proton carriers) as a consequence of the SCR conversion of surface NH3 species.22 Unexpectedly, the HF peak for Cu-SSZ-13 zeolite was shifted to a higher frequency (Figure 7a) indicating an enhanced short-range (local) ion movement within the catalyst.22 A further increase of NO/NH3 ratio to 100/50 led to the shift of both LF and HF peaks to lower frequencies, which can be attributed to the SCR conversion of surface NH3 species diminishing both the long- and short-range ion movement processes.19 At a higher temperature of 350 °C, both the LF and HF peaks of Cu-SSZ-13 shifted to lower frequencies

16


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with increasing NO/NH3 ratio in the gas mixture (Figure 7b) corresponding to the weakening of both long- and short-range ion movement processes.19,22,23

Figure 7 Modulus spectra of Cu-SSZ-13 collected during exposure in NH3/NO/O2 mixtures with different NH3/NO ratios at 200 °C (a) and 350 °C (b). The spectrum of Cu-SSZ-13 in 100 ppm NH3 (in N2 without O2) is shown as reference (empty spheres).

At 200 °C, Cu-SAPO-34 showed only the HF peak but no clear LF peak in the applied gas mixtures (Figure 8a). In general, the NO/NH3 ratio affected the HF peak of Cu-SAPO-34 in a similar (but less pronounced) manner as that for Cu-SSZ-13 (Figure 7a), i.e. a shift to a higher frequency in gas mixtures with a NO/NH3 ratio of 50/100 or 100/100 and to a lower frequency with the increase of NO/NH3 ratio to 100/50. At 350 °C, both the LF and HF peaks were visible in the applied gas mixtures (Figure 8b). Interestingly, with the increase of NO/NH3 ratio, while the LF peak position was largely unaffected, the HF peak shifted clearly to lower frequencies corresponding to a weakening of the short-range ion movement. Even though, largely identical modulus spectra were collected in gas mixtures with the NO/NH3 ratios of 100/50 and 100/100 pointing out that the short-range ion movement is insensitive to the change of NO/NH3 ratio in this range.

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Figure 8 Modulus spectra of Cu-SAPO-34 collected during exposure in NH3/NO/O2 mixtures with different NH3/NO ratios at 200 °C (a) and 350 °C (b). The spectrum of Cu-SAPO-34 in 100 ppm NH3 (in N2 without O2) is shown as reference (empty spheres).

Clearly, for both zeolite catalysts, the short-range or local ion movement dominates the overall ionic conductivity and, in turn, the NH3-SCR sensing performance at 200 °C. As shown in previous studies, isolated Cu sites in zeolite catalysts can be solvated by NH3 molecules at low temperatures (below 250 °C),22,27,28,47 generating mobile Cu-NH3 complex species which can travel rather freely within the CHA cage.26,27,29 Under similar solvating conditions (i.e. with or without NH3 solvation), the CuI species were found to be much more mobile than CuII species, and are even able to travel to neighboring CHA cages through the 8MR windows.26,27 In NH3SCR reaction atmospheres (NO/NH3/O2), a fraction of CuII can be reduced to CuI due to the CuII ↔ CuI redox cycle (Eqs. 8 and 9)3,4,17,23,26,27,48 CuII CuI

NH3 + NO

NO + O2

CuI

(8)

CuII

(9)

resulting in enhanced local ion movement within the CHA cage and, consequently, increased overall ionic conductivity of the Cu-CHA catalysts (Figure 4).17,23 For Cu-SSZ-13, the CuI → 18


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CuII re-oxidation half-cycle was largely prohibited in atmosphere with excess NH3, and the highly mobile Cu I-NH3 species prevail against the diminishing of long-range ion motion by NH3 loss from the zeolite surface (Figures 4 and 7a).19,21 In case of Cu-SAPO-34 catalyst, the local movement or conduction of NH3-solvated CuI species plays an even more predominant role because of the insignificant contribution of long-range ion motion to the overall ionic conductivity (Figures 6b and 8a). Therefore, a clear decrease of IIS was not observed in the presence of NH3, even at very small concentrations (i.e., NO/NH3 ratio of 100/30; Figure 4). At 350 °C, a better NH3-SCR sensing performance of Cu-SSZ-13 corresponds to a more pronounced weakening of the local ion conduction with increasing NO/NH3 ratio in the applied gas mixture. On the contrary, the IIS change for Cu-SAPO-34 is insensitive to the increasing NO/NH3 ratio from 30/100 to 100/100 (Figure 4), corresponding to the largely intact local ion conduction as disclosed by the modulus spectra in Figure 8b. The observations here agree well with our previous finding that the local mobility of Cu site is, whereas that of NH4+ species is not, reaction condition-dependent.35

3.3.2 DFT calculations To verify the in situ IS observations regarding the local movement of Cu ions within NH3solvated Cu-CHA, periodic DFT calculations were performed over the Cu-SAPO-34 system as an example. Three important reaction coordinates in the NH3-SCR cycle, namely bare CuII, CuII after NH3 solvation (CuII-NH3), and CuII-NH3 after interaction with co-adsorbed NO (CuINH2NO/H), were considered. It is known from previous investigations 26–28,35,48 that, depending on the temperature and gas composition, CuII(NH3)n complex species with different ligand number (normally with n = 1~4) may exist in Cu-CHA zeolite catalysts during NH3-SCR reaction.4,27,49,50 According to the phase diagram predicted by Paolucci et al.,27 singlet-ligated CuII-NH3 species could exist at temperatures ranging from ca. 250 °C to ca. 475 °C and thus are relevant for the sensing tests in this work. CuI-NH2NO/H was examined because it has been 19



proposed repeatedly to be an intermediate for NO-assisted dissociation of CuII(NH3)n, which leads to relatively stable CuI(NH3)n (n = 1 or 2) species being spectroscopically detectable.4,27,47,48,50 As illustrated by the DFT-computed optimized geometries in Figure 9, the single-atom CuII was located at the 6MR plane interconnected with the CHA cage, in good agreement with literature reports.29,48,51 After solvation by adsorbed NH3, the Cu site was lifted clearly from its initial equilibrium position on the 6MR plane toward the CHA cage center, which is consistent with previous DFT calculation results.49,50 Interaction of CuII-NH3 with a co-adsorbed NO molecule reduced the site to CuI forming a CuI-NH2NO/H coordinate with the Cu site shifted even more pronouncedly to the CHA cage center.35,48 CuI re-oxidation by NO/O2 could regenerate the bare CuII site and complete a whole NH3-SCR cycle.35,48

Figure 9 Side- and top-views of DFT-computed geometries for CuII in SAPO-34 (a and d), NH3-solvated CuII in SAPO-34 (CuII-NH3; b and e) and CuII-NH3 after interaction with co-adsorbed NO (CuINH2NO/H; c and f). Purple, gray, blue, red and brown spheres indicate P, Al, Si, O and Cu atoms, respectively.

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The mobility of Cu sites within the CHA cage is known to depend on the electrostatic tethering to Si (for Cu-SAPO-34) or Al (for Cu-SSZ-13) centers on the framework.26 As shown in Figure 9, NH3 solvation released the Cu ion from its initial equilibrium position, leading to the formation of Cu-NH3 species (Figure 6) that are highly mobile in CHA cage.35 Consequently, ionic conductivities of the two Cu-CHA zeolites were significantly enhanced (Figure 3). The Cu mobility can be further enhanced after reduction by co-adsorbed NH3 and NO, because of a weaker electrostatic tethering of CuI to the Si or Al centers on the CHA framework (Figure 9).27 As a result, unexpected further increase of ionic conductivity was noticed after the feeding of NO to NH3-saturated Cu-CHA at 200 °C (Figure 4). It has to be noted that, at a higher temperature of 350 °C, while NH3-solvated Cu species are still stable and therefore contributing to the ionic conductivity, the CuI-NH2NO/H coordinate may be very short-lived.38 Nevertheless, CuII sites that are coordinated strongly to the CHA framework can be reduced by NH3 and NO as well, forming more mobile CuI sites in the CHA structure.27 These experimental and theoretical evidences not only unveil the mechanisms of Cu-CHA catalyst as sensor element for direct NH3-SCR monitoring, but also allow to get insights into the speciation of Cu active sites in NH3-SCR reactions.

4 Conclusions NH3-supported ion conduction allows the Cu-SSZ-13 and Cu-SAPO-34 zeolite catalysts to be used directly as sensors for NH3-SCR reaction monitoring. The sensing behavior of both catalysts, which was evaluated according to the change of ionic conductivity with the variation of gas conditions, was determined by the local (or short-range) Cu ion movement within the zeolite structure. In atmospheres with excess NH3, the formation and high mobility of NH3solvated CuI species contributed predominantly to the ionic conductivity and, consequently, hindered the sensing of NH3-SCR over both catalysts at 200 °C. DFT calculations over model Cu-SAPO-34 system revealed that CuII reduction to CuI by co-adsorbed NH3 and NO weakened 21



significantly the coordination of Cu site to the CHA framework, rendering high mobility of CuI species that influence substantially the NH3-SCR sensing. The in situ spectroscopic and theoretical investigations thus not only unveil the mechanisms of Cu-CHA catalyst as sensor element for direct NH3-SCR monitoring, but also allow to get insights into the speciation of Cu active sites in NH3-SCR under different reaction conditions with varied temperatures and gas compositions. We expect that based on this knowledge, advanced sensor materials taking advantage of the mobility of active sites may be designed and derived for a wide variety of gas sensing applications.

Associated content Supporting Information. Supercell structure of SAPO-34; channel and cage structures in a 1x1x1 CHA unit cell; NH3 and NH3-SCR sensing tests at 450 °C; modulus spectra collected at 350 °C; Arrhenius plots of IS results collected in flowing N2; specific conductance in N2 or NH3 at different temperatures.

Notes The authors declare no competing financial interest.

Acknowledgments This work was supported by the National Natural Science Foundation of China (21806039), the Natural Science Foundation of Guangdong Province (2018A030313302), the German Federal Ministry of Education and Research (BMBF) in the context of the DeNOx project (13XP5042A), and by the Excellence Initiative of the German federal and state governments to promote science and research at German universities. P. C. appreciates the funding from the

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Xinghua talent program of South China University of Technology. We thank Dr. D. Rauch for the preparation of IDE sensor chips.

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Graphic Abstract

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Mechanistic understanding of Cu-CHA catalyst as sensor for direct

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