On the Promoting Effect of Water during NOx ... - ACS Publications

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On the Promoting Effect of Water during NOx Removal over SingleSite Copper in Hydrophobic Silica APD-Aerogels Tina Kristiansen and Karina Mathisen* Department of Chemistry, The Norwegian University of Science and Technology, Høgskoleringen 5, 7491 Trondheim, Norway S Supporting Information *

ABSTRACT: Single-site copper cations incorporated into hydrophobic silica APD-aerogels (2−8 wt %) are highly active for the Selective Catalytic Reduction of NOx with C3H6 as reducing agent (SCR-HC-deNOx) in the range 300−450 °C, reaching conversions up to 67% at 450 °C. In contrast to reported behavior of zeolite type matrixes, water in the feed has a promoting effect on the deNOx activity of the hydrophobic Cuaerogels, making these systems promising commercial candidates in the 300−450 °C activity windows under realistic conditions. The Cu-aerogels were compared to Cu-xerogel, a hydrophilic denser gel analogue, and Cu-ZSM-5, an established deNOx catalyst featuring poor hydrothermal stability. This study aims to elucidate the origins of deNOx activity and the effect of water by correlating copper speciation with surface species and gel nature at different reaction stages in dry and wet feed. X-ray Absorption Spectroscopy (XAS) and Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) measurements have been performed in situ, while monitoring reactor effluents during the experiments. Reversibility of the Cu2+/Cu+ redox pair was confirmed in the Cu-aerogel during and after wet redox cycling. This was not the case for the Cu-xerogel, or the Cu-ZSM-5, where the Cu2+/Cu+ redox pair and surroundings were reversibly and irreversibly affected by wet feed, respectively. Exclusive to Cu-aerogels was the formation of Brønsted acidic silanol cluster surrounding copper by multihydroxyl interaction with water. These silanol clusters are capable of storing reaction components and key intermediates, which we believe is responsible for enhancing the catalytic removal of NOx in Cu-aerogels.

1.0. INTRODUCTION The current automotive exhaust catalyst constitutes an economical barrier for the fuel-efficient and environmentally friendly lean burn technology, by containing expensive noble metals like platinum. However, finding a cheaper alternative catalyst that can selectively reduce NOx under oxidizing (lean) and realistic conditions is a complicated task.1 Popular solutions to selective catalytic reduction of NOx include the use of exhaust hydrocarbon residuals (SCR-HC-deNOx) or ammonia added to the exhaust (SCR-NH3-deNOx) as the reductant; the former technology being suitable for light duty passenger vehicles, whereas the latter is applied to medium or heavy duty vehicles.2 Recent progress on suitable SCR-HC-deNOx catalyst has been made with perovskites containing rare-earth lanthanum, strontium, and palladium, the latter being a less expensive noble metal than platinum.3,4 Even more abundant and inexpensive is copper, which shows particularly high activity for SCR-HC-deNOx; however, the catalytic performance of copper under realistic conditions (H2O, 2−18%) is particularly dependent on the carrier material of choice and copper siting.5−8 In addition to diffusion limitations and coking, the commonly used zeolites are also subject to dealumination in wet conditions.9 Poor hydrothermal stability of the well-known Cu-ZSM-5, inhibiting its commerci© 2014 American Chemical Society

alization, has been associated with competitive adsorption of water and feed components,10,11 migration of well-distributed copper cations to inactive sites,12−14 and copper oxide clustering.15,16 In the search of more suitable systems, considerable emphasis is put on carrier materials where flexible Cu2+/Cu+ redox pairs are stabilized under realistic conditions, while maintaining an overall hydrothermal stability.9,11,16−18 Recent advances were reported for SCR-NH3-deNOx from industrial and academic research approaches, showing that the Cu-exchanged zeolites possessing the CHA structure are excellent catalyst materials for this reaction with sufficient hydrothermal stability.2,19,20 Striking improvements were reported for wet SCR-C3H6-deNOx as well, by incorporating transition metals into the aluminum-free MFI structure (ZSM-5 analogue). However, follow-ups and advances are scarce.21,22 Silica aerogels are a class of exceptionally open-structured materials with large surface areas and high chemical stability, recognized as new promising parent materials in heterogeneous catalysis.23−27 Commercialization of aerogels has previously been inhibited by costly and hazardous preparation routes; Received: July 4, 2013 Revised: January 9, 2014 Published: January 10, 2014 2439

dx.doi.org/10.1021/jp406610v | J. Phys. Chem. C 2014, 118, 2439−2453

The Journal of Physical Chemistry C

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2.2. X-ray Absorption Spectroscopy. 2.2.1. Data Collection. XAS data were collected in transmission mode at the Swiss−Norwegian Beamlines (SNBL; BM01B) at ESRF, Grenoble. The white beam is collected from the storage ring by a bending magnet to the SNBL. The beamline is equipped with a channel cut Si(111) monochromator and double-crystal monochromator to select the desired wavelength. The spectral range is 4−70 keV. The initial and transmitted intensities I0 and It (31 cm) were detected by ion chamber detectors with lengths of 17 and 31 cm, respectively. The gas compositions were 80% N2 + 20% Ar and 75% N2 + 25% Ar. The ESRF provides electron beam energy of 6 GeV and a maximum current of 200 mA. The in situ XAS data were collected by using a stainless steel cell uniformly heated by two cartridges where temperature was measured by a thermocouple. The samples were ground and loaded in the 1.5 mm thick reaction cell and were kept in place by glass wool on each side, thereby allowing gas to flow freely through the sample and cell. Due to different sample densities, the reaction cell filled 11−12 mg Cu-aerogel, and ca. 22−23 mg Cu-xerogel and Cu-ZSM-5. The cell was sealed with graphite windows of 0.2 mm thickness. The reaction gases were passed directly through the sample, controlling the flow rate by mass flow controllers. XAS spectra were measured in a QuickXAS mode with steps of 0.5 eV and counting time of 50 ms in the range 8.8−10 keV upon heating/cooling and steady state with a sample scan time of approximately 2 min. 2.2.2. Data Reduction. The XAS data were summed, normalized, and energy corrected relative to the metal foil (Cu foil K-edge = 8979 eV) using Athena, a program in the IFEFITT package.38 Data collected in Quick-XAS mode were rebinned with the pre-edge grid set to 10 eV, the edge region chosen from −30 to 50 eV, the XANES grid to 0.5 eV, and the EXAFS grid to 0.05 Å−1. The k-edge absorption energy E0 on the XAS spectra of unknown samples was consistently positioned halfway up the absorption edge jump. XANES and EXAFS scans were normalized from 30 to 150 eV above the edge and from 150 to the end of the scan, respectively. The data were carefully deglitched and truncated at the end of EXAFS scans when needed. The smooth background μ0(E) was checked and corrected to achieve the maximum overlap with total absorption μ(E). The reference compounds copper(II) hydroxide and copper(I) oxide were used to approximate the amplitude reduction parameter (AFAC) of the unknown samples. The valence state of the major component dictated the choice of reference to estimate the value of AFAC. The characteristic Cu+ pre-edge corresponding to the 1s−4p transition can be used as a fingerprint to identify and quantify the amount of such species. A peak-fitting procedure was performed in Athena to calculate the area under the 1s−4p preedge. The comparison of pre-edge areas dictates the amount of Cu+ species, limited to comparisons of each sample in different reaction stages and not between samples. A background function was used to model the step portion of the data with the centroid value set to the E0 value before refinements. The E0 value was determined as the first inflection point after the pre-edge in all cases. Also crucial to the fitting procedure was the fitting range, which was manually chosen, and then varied to give the optimal fit. The pre-edge peak centroid was determined manually and then refined. Both the Gaussian and Lorentzian functions were used, but only the former gave conclusive fit. The functions are unit normalized, thus the peak amplitudes directly give the areas.

however, the ambient pressure drying (APD) method constitutes an environmentally friendly and cost-effective alternative approach, which can easily be up-scaled.28−31 The APD method yields hydrophobic inner surfaces, which can be coupled with the aerogels flexibility and capacity to stabilize high amounts of single-site metal cations (2−11 wt %).32 A fundamental study on single-site Cu-aerogels (2−11 wt %) shows flexible redox interplay of the Cu species and the coordinating framework.33 Silanol clusters were created upon stepwise reduction of copper, resulting from the unsaturated coordination to the framework. Silanol clusters interact strongly through polyhydrogen bonding, and thus exhibit Brønsted acidity several magnitudes higher than monobonded or free silanols, which are normally present on hydrophilic silica aerogels and xerogels.34,35 We believe the flexible redox properties of single-site copper, coupled with the stability of being incorporated in hydrophobic silica-based aerogels, make this system a promising test candidate for the harsh conditions in the SCR-HC-deNOx reaction. In this work we report on an in situ XAS and in situ DRIFTS study to elucidate the origin of SCR-HC-deNOx activity measured for single-site Cu-aerogels in dry vs wet feed. With hydrophilic Cu-xerogel and the well-studied Cu-ZSM-5 as references, the copper speciation, surface species, and the gel surface chemistry were monitored for Cu-aerogel during dry and wet redox stages in four cycles. Cu-aerogel was further studied in situ with sequential treatments in dry and wet conditions. X-ray absorption spectroscopy (XAS) is the technique of choice for probing the local surroundings of a transition metal in amorphous carrier materials during catalytic reaction conditions. It offers valuable information about the target element, such as valence state, local symmetry, multiplicities (coordination numbers), and shell distances. Adding DRIFTS to this study also gives important complementary information regarding key intermediates, and the influence of H2O to the catalytic mechanism.36 Hence, coupling DRIFTS with XAS provides a tool for correlating speciation with the dynamics of the carrier system in different reaction stages.

2.0. EXPERIMENTAL SECTION 2.1. Synthesis, Porosity, and DRIFTS Measurements. Copper cations were incorporated into the silica aerogel framework by adding copper(II) nitrate (Cu(NO3)2·2.5 H2O) prior to the sol−gel stage. The wet gel inner surfaces were modified to become hydrophobic by grafting of silyl groups, followed by drying at ambient pressure. The detailed synthesis and characterization of the copper-containing silica aerogels (3.5 wt %, Cuag-3.5 and 8 wt %, Cuag-8), Cu-xerogel (11 wt %),32 and ion-exchanged Cu-ZSM-5 (Si:Al = 30) is described in detail elsewhere.37 BET measurements applying the BJH method for average pore size and porosity determinations were performed as described previously.32 DRIFTS were carried out by using Bruker Vertex 80 with a LN-MCT detector and a hightemperature cell from Pike Technologies. The samples (5−22 mg) were loaded into a ceramic porous cup. Data were collected in situ in the range 4000−800 cm−1 with a 12 s delay between measurements. The sample scan time was 8 s, and the background scan time was 64 s. Difference spectra were calculated by subtracting the contribution from the last spectra in the previous treatment. 2440



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were cooled to 250 °C to start the reaction. After the last injection at 450 °C, water was added to the feed and the procedure reversed. To account for possible temperature influence, the wet feed measurements were repeated by heating fresh samples in wet feed to ensure reproducibility. Second, keeping the temperature at 350 °C, cycles of alternating dry and wet reaction feed were conducted. After steady state was obtained (30 min), four injections were taken during the next 30 min. 2.3.2. Part 2: In Situ Spectroscopic Studies. The following experiments were performed at 350 °C, monitored in situ by XAS and DRIFTS applying the same conditions in regards to both sample and flow as specified above. Each treatment lasted at least 30 min, allowing the samples to reach stabilization (steady state). A. Redox experiments were carried out for the Cu-aerogel (8 wt %), the Cu-xerogel, and the Cu-ZSM-5. The samples were treated in C3H6 and NO/O2 alternatingly in four cycles in dry− wet−dry feed, as described below. Blue indicates cycling in wet feed.

2.2.3. EXAFS Analysis. The EXAFS least-squares refinements were carried out by using EXCURV98, which conducts the curve fitting of the theoretical χth(k) to the experimental χexp(k) using the curved wave theory. The calculation of ab initio phase shifts for the expected neighboring elements also took place in EXCURV98. The least-squares refinements were carried out over the fit window Δk = 2−13 Å−1, using a k3 weighting scheme. To obtain a value for AFAC for the reference compounds, these refinements were carried out keeping shell multiplicities N fixed at literature values, while shell distances R, the correction parameter to E0 (EF), and the Debye−Waller factor 2σ2 were refined. During this process the value of AFAC was set to 1. Finally, AFAC was refined and transferred to the EXAFS data of the unknown samples. EXAFS analyses of the copper-containing samples were carried out by introducing and refining the nearest absorber− backscatterer pair until stable fits were obtained. The nextnearest and also a third absorber−backscatterer shell were introduced, following the same procedure. The parameter validity of introducing several shells to the analysis was justified by calculating the reduced χ2 in EXCURV98. Note that, for all reported parameters, errors quoted on parameters are statistical and the true experimental errors on refined distances are ±0.01 Å. For Debye−Waller factor and multiplicities, the true error is ±10% and ±20% for nearest and distant shells, respectively. To verify the type of backscattering atoms in the third shell, Fourier filtering was used to extract the contribution from the third shell from 2.8 to 3.2 Å in R-space. It was transformed back to k-space and refined with the following absorber−backscatterer pairs Cu···Cu, Cu···Si, and Cu···O, where the best quality of fit and k value of the amplitude maximum in the k3χexp(k) revealed the most probable backscattering element. 2.3. Catalytic Measurements and Spectroscopic Studies. For catalytic measurements (part 1) and in situ spectroscopic studies using DRIFTS and XAS (part 2), the following conditions applied: The samples were pretreated in in O2 (6%) with a 10 deg/min heating rate to 450 °C, followed by 30 min dwell time before cooling to the desired reaction temperature. The gas compositions for SCR-HC-deNOx testing, in molar percentage, was 0.2% NO, 2% O2, and 1.3% C3H6 and helium as a balance gas to give a feed rate of WHSV = 700 h−1. However, due to the significant differences in sample density for the three copper-containing systems, the in situ XAS and DRIFTS studies were carried out with WHSV = 350 h−1 for the Cu-xerogel and Cu-ZSM-5, and WHSV = 700 h−1 for the Cu-aerogel. H2O was added to the feed by using a vapor saturator bubbler cooled to give the precise partial pressure of 15% H2O. The reaction gas mixture was checked with and without H2O in a bypass line prior to every experiment to ensure no leaks were present, particularly in the saturator. 2.3.1. Part 1: Catalytic Activity Measurements. The catalytic reactions were carried out by using a 30 cm glass tube reactor with an inner diameter of 7 mm. Cu-ZSM-5 and Cu xerogel powders were pressed to wafers that were crushed and sieved to obtain particles in the size range 0.25−0.42 mm. The reactor was loaded with appropriate sample amounts (22− 23 mg) supported by silica wool. The samples were treated in reaction gas mixture for 30 min until steady state before every injection. Catalytic activity measurements were performed 2-fold: First, temperature-programmed reactions were carried out in the temperature range 250 to 450 °C with 50 °C intervals using a 10 deg/min ramp rate. After the pretreatment, the samples

B. Sequential treatments for the Cu-aerogel were carried out in dry (1) and wet (2) parallels. The study started with adsorption of NO, and subsequently NO/O2, C3H6, C3H6/O2, and complete reaction feed SCR-HC-deNOx, illustrated below. Blue indicates cycling in wet feed.

C. Sequential treatments for the Cu-aerogel and the CuZSM-5 were carried out with prereduction in C3H6, and subsequently C3H6/O2, before switching to dry and wet cycling of complete reaction feed SCR-HC-deNOx (Supporting Information).

2.3.3. Reactor Effluent Analysis. The reactor effluent was analyzed by gas chromatography (Agilent 6890A GC) connected to a mass spectrometer (5975C MDS) and a flame-ionized detector (FID), using a SPB-5 column (60 m, 3 μm film thickness, 0.53 mm i.d.). The analysis was carried out by using oven temperature at 35 °C to separate the retention times for the products N2, O2, N2O, and NO2. Formation of NO2 is accounted for by empty runs, and copper-free model systems. Calculation of conversion is consequently carried out by taking into account both NO and NO2 from empty runs as reactants. The yield of N2 is a product of conversion of NO and selectivity to N2, which are calculated in eqs 1 and 2 as follows: conversion of NO =

selectivity to N2 = 2441

[NO]bypass − [NO]reactor [NO]bypass

100%

[N2] 100% [NO]bypass − [NO]reactor

(1)

(2)



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copper (11 wt %), the maximum conversion at 450 °C did not exceed 59% in dry feed. By contrast to the copper-containing gel systems, Cu-ZSM-5 exhibits peak conversion at 350 °C. The activity window in dry feed of copper-containing silica APD-aerogels under these specific conditions is 300−450 °C for Cuag-8 and 350−450 °C for Cuag-3.5, showing clear dependence on copper loading. Wet feed greatly affects the catalytic performance in all three copper-containing carrier materials. Cu-aerogels show significant improvements in performance by increasing conversion of NO and selectivity to N2, particularly around 350 °C in both Cu-aerogels. Hence, wet feed causes the activity window to shift toward lower temperature (300−450 °C) for both Cu-aerogels. The promoting effect of wet conditions is gradually canceled upon further heating. By contrast, loss of catalytic performance for the Cu-xerogel and Cu-ZSM-5 occurs from 350 °C. As the effect of wet conditions was most prominent at 350 °C for both Cu-aerogels and the references, we chose 350 °C for further studies of the effect of wet SCR-HC-deNOx on these coppercontaining carrier materials. 3.2.2. Reversibility: Cycling Dry and Wet SCR-HC-DeNOx. Figure 2 shows the conversion of NO, yield of N2, and

3. RESULTS 3.1. Porosity and Copper Loading. Table 1 lists the results from the porosity measurements, including specific Table 1. Copper Loading, Specific Surface Area (SSA), Average Pore Diameter (Pd), and Cumulative Pore Volume (Vc) for Cu-aerogels, the Cu-xerogel, and the Cu-ZSM-5a sample

wt %

SSA (m2/g)

Pd (nm)

Vc (cm3/g)

Cuag-8 Cuag-3.5 Cu-xerogel Cu-ZSM-5

8 3.5 11.3 4.2

655 544 515 354

4.9 8.3 2.9

1.05 1.58 0.44

a

SSA = specific surface area. Pd = pore diameter calculated by using the BJH method based on the nitrogen desorption branch. Vc = cumulative pore volume, calculated at P/P0 = 0.98 from the desorption branch. Copper loading is obtained from ICP-MS elemental analysis.

surface area, average pore sizes, and cumulative pore volume from nitrogen adsorption−desorption isotherms with use of the BET and BJH method, and elemental analysis by ICP-MS for the Cu-containing silica aerogels, the Cu-xerogel, and the CuZSM-5.32,37 The results show low-range mesoporosity for the Cu-aerogels, with pore sizes ranging from 5 to 8 nm, while the Cu-xerogel exhibits an average pore size at 2.9 nm. The pore volume is in the range 1−1.6 cm3/g showing high porosity compared to that of the Cu-xerogel (0.44 cm3/g). Clearly, the pore characteristics of aerogels are significantly affected by the copper loading. The copper sites are available on the gel inner surfaces and can be assumed to remain hydrophilic during APD surface modification; hence the copper sites lower the relative coverage of silyl groups, and comprise a significant amount of extra-framework species and associated silanol defects attracting water and ammonia, which lead to gel shrinkage during drying.32 3.2. Part 1: Catalytic Activity for SCR-HC-DeNOx. 3.2.1. Temperature-Programmed SCR-HC-DeNOX. Figure 1

Figure 2. Conversion of NO (%), yield of N2, and selectivity to N2 (%) in four cycles of dry and wet (H2O 15%) SCR-HC-deNOx feed at 350 °C for Cuag-8 and Cuag-3.5.

Figure 1. Conversion of NO (%) and selectivity to N2 (%) in the dry and wet (H2O 15%) SCR-HC-deNOx feed for Cuag-8, Cuag-3.5, Cuxerogel, and Cu-ZSM-5.

selectivity to N2 in four cycles of SCR-HC-deNOx and SCRHC-deNOx/H2O for the Cuag-3.5 and Cuag-8 at 350 °C. The conversion of NO was higher for Cuag-8 than for Cuag-3.5, correlating with copper loading. The conversion was further enhanced 10−15% in both Cu-aerogels in wet feed. When resuming dry feed the conversion resumed approximately the initial level of conversion, which demonstrates reversibility. However, we observe increasing conversions of NO and yield of N2 in dry feed over the Cu-aerogels with time on stream and

shows the conversion of NO and selectivity to N2 in the temperature range 250−450 °C for Cu-containing aerogels Cuag-3.5 and Cuag-8, Cu-xerogel, and Cu-ZSM-5 in dry and wet feed (15% H2O). The conversion increased with temperature up to 450 °C for the Cuag-3.5 and Cuag-8 up to 48% and 67%, correlated with increasing copper loadings. Although the Cu-xerogel contains even higher amounts of 2442



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NO/O2/H2O than in dry NO/O2, while the amount of NO2 increased gradually in NO/O2 from cycle 1 to cycle 3. Cu-xerogel. In C3H6/H2O significant amounts of NO, N2O, and NO2 were present in the exhaust, indicating a competitive adsorption of −NxOy species and H2O. In NO/O2/H2O less N2 was produced and more O2 was present in the exhaust than in NO/O2 (cycle 1). Upon the switch back to dry feed, the selectivity toward N2 resumed to the initial level. Cu ZSM-5. Upon the switch to C3H6/H2O, desorption of NO and N2O from the sample was observed. An abrupt drop in concentration of O2 was observed. Lower levels of N2, NO2, and N2O/CO2 in NO/O2/H2O correlated with the higher concentration of NO in the exhaust. Tails of NO and N2O/ CO2 were observed in the following treatment in C3H6/H2O. Back in dry feed, N2, NO2, and N2O/CO2 appear to resume the levels in the first cycle. 2. In Situ XAS. XANES. Figure 4 shows a plot of the 1s−4p pre-edge areas from XANES, characteristic of Cu+, during redox

after two cycles in wet feed. Even if conversion increases with increased copper loading, the selectivity toward N2 is higher for the Cuag-3.5 than Cuag-8, which could reflect the higher extent of hydrophobicity in Cuag-3.5. 3.3. Part 2: Spectroscopic Studies. 3.3.1. Experiment A: The Redox Study. For Cu-aerogel, Cu-xerogel, and the CuZSM-5, the Cu2+/Cu+ redox behavior, surface species, and the correlating nature of copper local surroundings were monitored in situ by XAS and DRIFTS while measuring reaction effluents during cycling in C3H6 and NO/O2 in alternating dry−wet−dry feed at 350 °C. The aerogel chosen for study was Cuag-8, and is further denoted as the Cu-aerogel in part 2 of this work. 1. Effluent Analysis. Figure 3 shows the reactor effluents from the redox study for the Cu-aerogel (Cuag-8), Cu-xerogel,

Figure 4. Cu+ 1s−4p pre-edge area during redox cycling at 350 °C for the Cu-aerogel, the Cu-xerogel, and the Cu-ZSM-5 upon stabilization (steady state).

cycling in C3H6 and NO/O2 in alternating dry−wet−dry feed at 350 °C for the Cu-aerogel, Cu-xerogel, and Cu-ZSM-5. Figure S2 in the Supporting Information shows selected normalized XANES in O2 at 350 °C prior to the redox cycling, in C3H6 and NO/O2 (dry and wet feed).32,39 The XANES spectra in these figures were recorded after stabilization (steady state). Copper species in all three systems were initially in the divalent state in O2 at 350 °C, prior to redox cycling, with shoulder features indicating tetragonally distorted octahedral surroundings. For the Cu-aerogel, XANES show complete reversibility of the Cu+/Cu2+ redox pair during cycling in C3H6 and NO/O2 in both dry and wet feed. The Cu species were mostly unaffected by the switch to wet feed; however, completion of the Cu2+/ Cu+ redox cycle for the Cu-aerogel in wet feed took twice the time of the same process in the dry feed. NO/O2 treatments led to a mixture of Cu2+ and Cu+, where the fraction of Cu2+ was somewhat higher in NO/O2/H2O than in the dry NO/O2 treatments. When resuming dry C3H6 feed, Cu+ species mostly remained unaffected; however, a slight increase of Cu+ species was observed in dry NO/O2 (cycle 3) and C3H6 (cycle 4). For the Cu-xerogel, XANES show complete reversibility of the Cu+/Cu2+ redox pair during cycling in dry C3H6 and NO/ O2 feed. The switch from dry C3H6 to wet C3H6/H2O feed resulted in complete reoxidation from Cu+ to Cu2+. The effect

Figure 3. Effluent concentration for the Cu-aerogel (a), Cu-xerogel (b), and Cu-ZSM-5 (c) during redox cycling at 350 °C.

and Cu-ZSM-5 during redox cycling in C3H6 and NO/O2 in alternating dry−wet−dry feed at 350 °C. Mutual for the Cuaerogel, Cu-xerogel, and the Cu-ZSM-5 was the high amounts of N2, NO2, and N2O (and CO2) observed in the NO/O2 stages after a pretreatment in C3H6. “Tails” of N2O and N2 in the subsequent treatments in C3H6 were observed, likely due to reaction with stored NxOy species. Cu-aerogel. The wet redox cycle followed the trend seen in the dry cycles. However, the levels of N2 and N2O are higher in 2443



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of water on Cu species was reversible, seen by rereduction to the initial state Cu+ after resuming dry C3H6 feed. For Cu-ZSM-5, XANES show complete reversibility of the Cu+/Cu2+ redox pair during cycling in dry C3H6 and NO/O2 feeds. Immediate and irreversible changes occurred in C3H6/ H2O, seen by a decreasing Cu+ pre-edge area, with an energy shift from 8984 to 8983.6 eV for the pre-edge centroid. Irreversible changes was also observed in NO/O2/H2O, seen by a significant increase of Cu+. EXAFS. The results from EXAFS least-squares refinements from the redox cycling in C3H6 and NO/O2 in alternating dry− wet−dry feed at 350 °C for the Cu-aerogel, Cu-xerogel, and Cu-ZSM-5 are listed in Tables S1−S3, respectively (Supporting Information). Table S5 (Supporting Information) compares data from EXAFS least-squares refinements on model compounds with crystallographic data. Figure 5 illustrates the

back in dry feed. We observed no significant changes in the Cu···Si shell by the presence of H2O, and only minor changes from ca. 2 to 1 Cu···Si pairs were observed when switching between oxidizing and reducing conditions. Cu-xerogel. In O2 at 350 °C, prior to the redox cycles, we obtained 3.2 Cu−O shell distances at 1.95 Å, and 2.3 Cu···Si shells at 3.15 Å, verified by Fourier filtering. Similar to the Cuaerogel, Cu species in the xerogel exhibit tetragonally distorted octahedral environments and are incorporated into the silica gel framework. The second axial Cu−O shell could not be fitted due to antiphase behavior typical for this environment.40 In dry C3H6 feed the nearest surroundings was altered in accordance to the Cu2+ to Cu+ reduction, seen by 1.5 nearest Cu−O shells at 1.95 Å. After the subsequent reoxidation in NO/O2, the 3.3 Cu−O shell distances expanded to 1.97 Å. Major alterations in the nearest surroundings were observed upon the switch to wet C3H6 feed. Upon the reoxidation from Cu+ to Cu2+, the nearest Cu−O shell distances re-expanded from 1.95 to 1.97 Å, while the shell multiplicity increased from 1.4 to 3.4 in C3H6/H2O. In contrast, we obtained similar nearest surroundings of copper cations in NO/O2 and NO/O2/H2O. The Cu species resumed the initial Cu+ state and surroundings upon the switch back to dry C3H6 feed, seen by 1.4 Cu−O shell distances at 1.95 Å. Cu-ZSM-5. For the initial copper cation surroundings, prior to the redox cycles (O2 at 350 °C), we obtained 4.0 Cu−O shell distances at 1.96 Å, consistent with either a tetragonal or tetragonally distorted octahedral environment, as a second oxygen shell could not be confirmed.40 In C3H6 we obtained 1.6−1.8 Cu−O shell distances at 2 Å after reduction of Cu2+ to Cu+. The elongated Cu+−O distances in Cu-ZSM-5 contradict the shortened Cu+−O distances seen in the Cu-aerogel and the Cu-xerogel after reduction in C3H6. For copper systems there is a reported inconsistency as far as the Cu−O distance is regarded for the monovalent state. Long oxygen distances for Cu-ZSM-5 were previously reported by Mathisen et al.37 and Lamberti et al.,41 while short Cu+−O distances are reported for CuAlPOs and Cu:SAPO-n systems,37,42 as well as known monovalent references copper(I) oxide (1.84 Å) and copper(I) diammine (1.88 Å).43,44 Clearly the reported discrepancies in Cu+−O shell distance must originate from other factors, such as possible interactions with the matrix of choice, site coordination/geometry, and type of ligand. In NO/O2, 4 Cu−O shell distances are found at 1.96 Å after the reoxidation to Cu2+. Redox cycling in wet feed led to significant alterations of the nearest surroundings of copper. The nearest 1.6 Cu−O shell distances contracted irreversibly from ∼2 Å in C3H6 to 1.97 Å in C3H6/H2O. In NO/O2/H2O the local surroundings of copper is altered noteworthy compared to that in NO/O2, seen by considerable decrease in shell multiplicity from 3.8 to 1.8, coinciding with higher amounts of Cu+. The effect was, however, mostly reversible, seen by the nearest Cu−O shell multiplicity at 3.6 when resuming dry NO/O2 feed. The contribution of framework neighbors in the third Cu···T shell was in general low during the redox cycling, and detecting differences in dry feed vs wet feed was a challenging task; hence we cannot indicate a possible dealumination process previously encountered in wet feed for Cu-ZSM-5.45 3. In Situ DRIFTS. Cu-aerogel. Figure 6 shows, in detail, the spectral regions 3770−1500 cm−1 of Cu-aerogel covering the infrared υOH, υNH, silanol-bonded υR‑N, υNxOy, and Cu-bonded υNCO, υCO, υCC/CO, and υNO modes (difference spectra) showing

Figure 5. The nearest Cu−O/N shell distances and multiplicities during redox cycling at 350 °C for Cu-aerogel, Cu-xerogel, and CuZSM-5 upon stabilization (steady state).

changes of first Cu−O/N shell multiplicities and distances during the redox cycling after stabilization (steady state). EXAFS cannot distinguish between oxygen and nitrogen neighboring atoms, which are both expected when studying copper in SCR-HC-deNOx conditions. For simplicity, without excluding the presence of nitrogen-containing adsorbates on Cu species, we hereafter denote the nearest absorber−backscatterer pair Cu−O rather than Cu−O/N. Cu-aerogel. For the initial copper cation surroundings, prior to the redox cycles (O2 at 350 °C), we obtained 3.3 + 1 nearest Cu−O shell distances at 1.95 and 2.2 Å, indicative of a tetragonally distorted octahedral environment.32 Copper cations are also partly incorporated into the silica gel framework, seen by 2.2 Cu···Si third shell at 3.11 Å, verified by Fourier filtering.32 In C3H6 the nearest 2.1 Cu−O shell distances contract to 1.92−1.90 Å upon the reduction of Cu2+ to Cu+. In C3H6/H2O the nearest Cu−O shell distances contracted slightly to 1.90 Å with multiplicities increasing from 2.1 to 2.3. In both dry and wet NO/O2, we obtained nearest 3.8−3.9 Cu−O shell distances at 1.95 Å. A slight expansion of 3.6 nearest Cu−O shell distances to 1.96 Å was seen in NO/O2 2444



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species”.6,36,47 Recommencing C3H6 feed, these spillover NxOy/ R−CN and Cu-bound NCO/CO species (2285, 2220−2130 cm−1) resume a fraction of their initial intensity throughout the treatment, upon gradual removal of stored Cu2+−NO2/NO3 species (1610 cm−1) and formation of oxygenates (1600−1500 cm−1), coinciding with the perturbation of vicinal/geminal silanols at 3680 cm−1 and lone silanols at 3700 cm−1.51The main organic byproducts 2-propenal and 2-propenenitrile were detected shortly after switching to C3H6. Switching from dry to wet C3H6 feed resulted in removal of N-containing spillover and Cu-bound species, upon the appearance of −NH2 species, seen by their two coupled bands at 3363 and 3285 cm−1. −NH2 is formed by hydrolysis of CN-containing species and is considered a highly reactive intermediate.10 The adsorption of −CxHy species appeared to occur in larger extent in C3H6/H2O (C2, C3), compared to dry C3H6 feed (C1, C2, C4), concurring with the formation of Cu2+/+−NCO, R−CN, and Cu+−CO species (2220−2130 cm−1). In NO/O2/H2O silanol clusters of two kinds appeared at 3210 and 3150 cm−1, coinciding with perturbation of the geminal/vicinal silanols (3680 cm−1) to a large extent.34 Isolated tetrasilanols appear at 3180 cm−1; however, due to interactions with a variety of absorbents (spillover species) less polar than the OH group, the steric hindurance and overall lower polarity result in a blue shift. The high-frequency part is assigned trisilanols, which exhibit higher Brønsted acidity, due to close interaction of the hydroxyls and weakening of the O− H bonding to a larger extent.34,35 Stored −CxHy species (2967 and 2907 cm−1) were removed to a larger extent in NO/O2/H2O than seen in the dry feed analogue, concurring with increased adsorption of −CxHy species in wet C3H6 feed. The Cu2+/+−NCO/R−CN maxima at 2195 cm−1 clearly exhibited lower intensity than in NO/O2/ H2O dry NO/O2 feed, and the combination of at least two bands is more distinguishable. Replacing these species was spillover NxOy (∼2155 cm−1).47 The transient formation of [NOδ+−Cu+δ+−(NO2−)] species, seen by two coupled bands at 1634 and 1910 cm−1, was observed exclusively in NO/O2/H2O during the redox experiment.47 A gradual buildup of Cu2+− NO2/NO3 species (1610 cm−1) was observed throughout the NO/O2/H2O treatment to a higher extent than in the dry feed analogue.6,10 However, due to spectral noise and varying assignments in the literature of these species, we choose not to specify the differences without further DRIFTS studies and optimization of the setup.47 Recommencing C3H6/H2O led to disappearance of the band at 3210 cm−1, assigned tetrasilanols interacting with adsorbents, upon an increase of the band at 3150 cm−1, associated with trisilanols.35 Cu-xerogel. Figure 7 shows, in detail, the spectral regions 3770−1500 cm−1 of Cu-xerogel covering the infrared υOH, υNH, silanol-bonded υR‑N, υNxOy, and Cu-bonded υNCO, υCO, υCC/CO, and υNO modes (difference spectra) at interesting stages during redox cycling in C3H6 and NO/O2 in alternating dry−wet−dry feed at 350 °C. Table 2 lists the bands and their assignments in this study. Initiating the redox cycling by C3H6 feed resulted in adsorption of −CxHy species seen at 2963 and 2907 cm−1 and associated species at 1850 and 1620 cm−1, which can include the presence of oxygenates, due to residual oxygen from the pretreatment. Switching to NO/O2 led to removal of the latter species, followed by the appearance of 2974 cm−1, indicating nitrogen-containing organic species, formation of RCH210 or −CN−OH52 at 3125 cm−1, silanol clusters interacting with adsorbents at 3210−3210 cm−1, and perturbation of geminal/

Figure 6. DRIFTS difference spectra for the Cu-aerogel during redox cycling at 350 °C. SS = steady state (30 min), trans = 5−10 min into the treatment. C1−C4 = cycle 1−4.

transient changes 5−10 min into each treatment and after stabilization (steady state, SS) during redox cycling in C3H6 and NO/O2 in alternating dry−wet−dry feed at 350 °C. Figure S9 (Supporting Information) shows complementing GC-FID chromatograms and mass spectra of the main organic byproducts during transient state (5−10 min). The silica aerogel is prepared via the ambient-pressure drying (APD) method, and exhibit surfaces grafted with silyl species, and already contain the bands at 2967, 3065, and 2907 cm−1 corresponding to υC−H modes. These bands remain in intensity during the pretreatment in oxygen to 450 °C, and thus the aerogel remained hydrophobic for the redox cycling at 350 °C. Table 2 lists the bands and their assignments in this study. Initiating the redox cycling by C3H6 feed resulted in adsorption of −CxHy species, (2967 and 2907 cm−1). Residual oxygen from the pretreatment is present, resulting in partial oxidation of CxHy species to Cu+−CO (2200−2130 cm−1) of various kinds.46 The Cu+−CO species are probably also present in proceeding treatment in NO/O2, partly overlapping in a broad band (2220−2130 cm−1) with Cu2+/+−NCO or R−CN (2220−2180 cm−1) species observed within 5−10 min.6,47,48 Concurrently, the appearance of a band at 1610 cm−1 is gradually increasing throughout the treatment. This band is removed in all following C3H6 treatments, apart from the treatment in C3H6 following the pretreatment in O2. Hence we can exclude oxygenates and assign the band at 1610 cm−1 to Cu2+−NO2/NO3− species, which is in accordance with literature.10,47,49,50 Replacing the −NCO and −CO intermediate species from 10 min onward were various NxOy and/or R− CN species, seen by two low-intensity broad bands at 2200− 2130 and 2285 cm−1, respectively. Such species are reported to adsorb at hydrogen-bonded sites (Si−OH), coinciding with the perturbation of the geminal/vicinal silanols (3680 cm−1). We hereafter refer to silanol-bonded species as “spillover 2445



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Figure 8. DRIFTS difference spectra for Cu-ZSM-5 during redox at 350 °C. SS = steady state (30 min), trans = 5−10 min into the treatment.

Figure 7. DRIFTS difference spectra for the Cu-xerogel during redox cycling at 350 °C. SS = steady state (30 min), trans = 5−10 min into the treatment. C1−C4 = cycle 1−4.

Initiating the redox cycling by C3H6 feed resulted in adsorption of −CxHy species seen at 2970 and 2930 cm−1 and the coinciding perturbation of Brønsted acid sites (3600 cm−1), lone silanols (3745 cm−1), and aluminols (3560 cm−1). Further perturbation of acid sites is observed in NO/O2 (transient spectra) upon formation of oximes (RN−OH, 3130 cm−1),6 −NCO, and formation of CxHyOz species, such as Cu+−CO at 2157 cm−1, corresponding with removal of CxHy species (2970 + 2930 cm−1).10 Upon stabilization in NO/O2 (steady state) adsorbates are removed from silanols and acid sites, corresponding with decreasing amounts of oximes and Cu+−CO species.6 The following C3H6 treatment led to removal of Cu+−CO species, adsorption of −CxHy (3080, 2970, 2930 cm−1), −NH2 at 3380 cm−1,10 and perturbation of free silanols and acid sites. Minor amounts of −NCO (2253 cm−1) are also seen 5−10 min into the treatment. Switching to wet feed affects surface species in NO/O2, seen by significant loss of −NCO intermediate species, desorption of −CxHy species, higher levels of Cu+−CO (2157 cm−1), in particular in the transient spectra, and a shift of the band assigned oximes to 3140 cm−1, also observed by Kharas et al. upon Cu-ZSM-5 deactivation.16 The free silanols remain perturbed; however, it appears that adsorbates are removed from acid sites upon stabilization (steady state). Resuming C3H6/H2O led to decreased intensities of the −CxHy bands and there appears to be a buildup of Cu+−CO (2157 cm−1) from the previous NO/O2/H2O treatment. The most apparent irreversible changes when switching back to dry redox cycling was the strongly decreased intensity for the −NCO band (2250 cm−1) in NO/O2 (cycle 3) and −CxHy

vicinal silanols at 3680 cm−1. When resuming C3H6 feed, the above-mentioned bands were removed, showing redox reversibility. It is worth noticing the transient decrease in CxHy species early in C3H6 following NO/O2 treatments, which is reinstated to the initial level upon stabilization. Switching from dry to wet C3H6 feed did not result in significant changes other than removal of −CxHy species. However, in wet NO/O2/H2O feed the band at 2974 cm−1, associated with nitrogen-containing organic species did not reach its initial intensity. Comparing the other bands in NO/ O2/H2O vs NO/O2 in 3800−3000 cm−1 is not straightforward, as water especially affects the spectra in the OH and NH region in the highly hydrophilic Cu-xerogel systems. Switching back from NO/O2/H2O to C3H6/H2O feed led to desorption of the −CxHy species at 2963 and 2907 cm−1 from dry feed, and the band assigned oxygenates and/or nitrites/nitrates 1850 and 1620 cm−1 did not reach their initial intensities. The −CxHy species are, however, restored when switching to dry C3H6 feed again. The species observed in the initial dry redox cycle all reappeared upon the switch from wet to dry redox cycling, which demonstrates reversibility of the alterations in the wet redox cycle. Cu-ZSM-5. Figure 8 shows, in detail, the spectral regions 3770−1500 cm−1 of Cu-ZSM-5 covering the infrared υOH, υNH, silanol-bonded υR‑N, υNxOy, and Cu-bonded υNCO, υCO, υCC/CO, and υNO modes (difference spectra) at interesting stages during redox cycling in C3H6 and NO/O2 in alternating dry−wet−dry feed at 350 °C. Table 2 lists the bands and their assignments in this study. 2446



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species do not adsorb to the initial extent in C3H6 (cycle 4), Cu+−CO continue to build up in C3H6, showing an irreversible alteration of the reaction pathways. 3.3.2. Sequential Treatments of Cu-aerogel in Dry or Wet Feed (Experiment B). In two parallels runs (dry and wet feed), the Cu-aerogel was treated sequentially with NO, NO/O2, C3H6, C3H6/O2, and SCR-HC-deNOx feed at 350 °C, studied by XAS and DRIFTS in situ while monitoring the reaction effluents. 1. Effluent Analysis. Figure 9 shows the concentration of effluents from two parallel experiments (dry feed and wet feed)

Figure 10. The nearest Cu−O/N shell distances, multiplicities Cu+ pre-edge area for Cu-aerogel in dry (red) and wet feed (blue).

Information) lists the complete results from EXAFS leastsquares refinements. XANES. In dry and wet NO feed, partial reduction of Cu2+ to Cu+ is observed, corresponding to approximately 5% (dry NO) and 10% (wet NO) monovalent fractions. While the Cu+ species in dry NO were highly transient, seen by rapid reoxidation within a few minutes of exposure, the Cu+ species in wet NO are stable. Upon switching to C3H6, Cu2+ reduces to 70% Cu+ in dry feed and 55% Cu+ in wet feed. A further reduction is seen in wet feed upon switching to C3H6/O2, which did not occur in dry feed. In both dry and wet feed, a partial reoxidation to Cu2+ is observed in complete SCR-HCdeNOx feed. EXAFS. The initial nearest surroundings of the copper species in the Cu-aerogel, prior to the sequential treatment, are comprised of 3.3 + 1.1 nearest Cu−O shell distances at 1.95 and 2.20 Å. Formation of 10% Cu+ in NO/H2O corresponds to 3.1 nearest Cu−O shell distances at 1.93 Å. By comparison, a shell contraction was seen in NO (∼5% Cu+) to be 3.4 Cu−O shell distances at 1.94 Å. This contraction is canceled after 30 min where we observe a reoxidation to Cu2+. In NO/O2 the nearest shell multiplicity increased to ca. 4, independent of the presence of H2O. The shell distance in wet feed expands to 1.957 Å, similar to Cu-ZSM-5 in NO/O2 (experiment A), while in dry feed the shell distance expanded to 1.950 Å, which may indicate different adsorbents at this stage. Longer shell distances, increased multiplicities, and reoxidation behavior combined strongly indicate bidentate bonding environments, most likely to nitrate or nitrite species in NO/O2.47,50 In both dry and wet C3H6 feed, the shell distances are around 1.93 Å; however, the nearest Cu−O shell multiplicity decreased to 2.9 in C3H6/H2O and 2.5 in C3H6. In wet and dry C3H6/O2 feed we observe only minor differences in shell multiplicity and distance (2.7 Cu−O shell distances at 1.93/4 Å). In dry SCRHC-deNOx feed the Cu−O shell environment remained stable, while in SCR-HC-deNOx/H2O feed the shell distances contracted to 1.93 Å, similar to dry feed. 3. In Situ DRIFTS. Figures 11 (dry feed) and 12 (wet feed) show a DRIFTS difference spectral region 3770−2050 cm−1 for the Cu-aerogel during sequential treatments in NO, NO/O2, C3H6, C3H6/O2, and SCR-HC-deNOx feed at 350 °C, covering υOH, υNH, υCH, silanol-bonded υR‑N, υNxOy, Cu-bonded υNCO, and υCO modes. Additionally, the spectral region 2000−1500 cm −1 is available in Figures S3 and S4 (Supporting

Figure 9. Effluent concentration from dry and wet sequential treatment of the Cu-aerogel at 350 °C.

upon sequential treatment of the Cu-aerogel in NO, NO/O2, C3H6, C3H6/O2, and SCR-HC-deNOx feed at 350 °C. Cu species were active in the direct decomposition of NO to N2 almost exclusively in wet feed, seen by the formation of N2 and byproducts N2O and NO2 in NO/H2O. The stochiometrically too high concentration of N2 early in the NO/H2O treatment can indicate an initial buildup of NO on the Cuaerogel, which stabilizes below stociometric levels at steady state. The efficacy toward N2 and O2 formation decreased in NO/O2/H2O compared to dry NO/O2 feed, which can be caused by oxidation of adsorbed NO on the Cu-aerogel rather than direct decomposition.47 In C3H6/H2O the concentration of O2 slowly decreased compared to dry C3H6 feed, which can be interpreted as O2 being released or produced by remaining/ stored NxOy species reacting with C3H6 (higher levels of N2O and NO2). It is worth noticing the low level of O2 in C3H6/O2/ H2O feed, compared to dry C3H6/O2 feed, indicating either a consumption of O2 by reaction with gel adsorbates or combustion with C3H6. High amounts of NO2 were observed in this stage in wet feed. N2 was produced in dry C3H6/O2, showing that both components were necessary in order to produce N2, whereas in wet feed, only C3H6 was needed to form N2. In SCR-HC-deNOx feed, the level of N2 increased in both dry and wet feed. 2. In Situ XAS. Figure 10 shows the nearest Cu−O/N shell distances and multiplicities from EXAFS least-squares refinements for the Cu-aerogel in sequential treatment in NO, NO/ O2, C3H6, C3H6/O2, and SCR-HC-deNOx feed at 350 °C in dry or wet feed (H2O 15%), and the 1s−4p pre-edge area characteristic of Cu+ species. The nearest Cu−O/N shell is hereafter denoted Cu−O for simplicity. Table S4 (Supporting 2447



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Table 2. Assigned Bands Observed during in Situ DRIFTS for the Cu-aerogel, Cu-xerogel, and Cu-ZSM-5

Figure 11. DRIFTS difference spectra of the Cu-aerogel during dry sequential treatment at 350 °C. SS = steady state (30 min), trans = 5− 10 min into the treatment.

wavenumber/cm‑1

assignment

3720−3730 3680 3363 + 3285 3210/3180 3150 3125−3140 2967 + 2907 + 3065 2400−2340 2280−2250 2250−2210 2240 2200 2195 2157 2155 2133 2114 1910 + 1634 1733 1600−1634 1598 + 1575 1576 1507 1587 + 1547

SiOH (isolated)33 SiOH (Vicinal/geminal)51 −NH210 Six(OH)4 + ads/Six(OH)434 Six(OH)334 RN−OH, RCH252 C−H CO210 Si−OH to R−CN/NCO/NxOy47,36,6 N2O47 NCO (Cu-ZSM-5)10 SiOH−NO2−47 Cu2+/+−NCO6 Cu+−CO (Cu-ZSM-5)10 silanol-bonded NO2δ+47 Cu+−CO, SiOH−NO2+10,47 SiOH−NO+53 [NOδ+−Cu+δ+−(NO2−)]47 Cu+(NO+)247,49 Cu2+−NO2−/ NO3−49 Cu2+−NO3− unidendate50 carbonate6 H−COO10 CH3COO10

and 3730 cm−1) and CxHy species removed.6,10 There is no indication of the formation of polybonded silanols in the range 3400−3100 cm−1 (not shown). Wet Feed. Transient spillover NxOy species were observed after a few minutes in NO/H2O at 2220−2150 and 2133 cm−1, coinciding with the formation of Cu+−(NO)2 and [NOδ+− Cu+δ+−NO2−] species at 1730 cm−1 and the two coupled bands 1910 + 1634 cm−1, respectively.47,49,50 Species increasing in intensity during the treatment in NO/ H2O are spillover NO+ at 2114 cm−1 and spillover NxOy adsorbed on silanols of varying acidity, seen by a broad shoulder at 2280−2114 cm−1.47 Concurrently, silanol clusters was observed by a broad band in the polyhydroxyl region, centered at 3210 cm−1.34 The shift from 3180 to 3150 cm−1 is indicative of a significant change in polarity caused by adsorbents. These bands were previously observed exclusively in wet feed in the redox study (experiment A, Figure 6). The lone silanols were perturbed (3730 cm−1) within a few minutes, associated with spillover NxOy species or polyhydrogen bonding of 3−4 silanols (silanol clusters). The addition of oxygen to the feed caused reduction of spillover NO+ and [NOδ+−Cu+δ+−NO2−] species, seen by decreased intensity of the band at 2114 cm−1 and the two coupled bands at 1910 + 1634 cm−1. Concurrently, spillover NxOy and Cu2+−NO2−/NO3− formed, seen by increasing intensity of the band at 2280 cm−1 and several bands in the spectral region 1610−1590 and 1575−1540 cm−1, caused by varying bonding geometry to copper cations.50 The switch from NO/O2/H2O to C3H6/H2O resulted in adsorption of −CxHy species, seen by increasing bands at 2967 and 2907 cm−1. The silanol clusters were altered, seen by the shift from 3210 cm−1 to 3150 cm−1. Coinciding with removal of spillover NxOy and Cu+−(NO)2 species (1704−1735 cm−1) was the formation of Cu2+/+−NCO, Cu+−CO (2220−2150 and

Figure 12. DRIFTS difference spectra of the Cu-aerogel during wet sequential treatment at 350 °C. SS = steady state (30 min), trans = 5− 10 min into the treatment.

Information), covering Cu-bonded υCC/CO and υNO modes. Table 2 lists the bands and their assignments in this study. Dry Feed. Transient spillover NxOy species are present for a few minutes shortly after NO exposure, seen by a low intensity band at 2200 cm−1 These species are transient and disappear when reaching stabilization in NO. Upon the addition of oxygen, spillover NxOy species at 2200 and 2283 cm−1 form, coinciding with the perturbation of monobonded vicinal/ geminal silanols at 3680 cm−1.51 [Cu+−(NO)2]+ at 1730 cm−1 forms, while above-mentioned spillover NxOy species increased somewhat in intensity.36,47 Switching from NO/O2 to C3H6 resulted in adsorption of −CxHy species at 2967 and 2907 cm−1 and a slight increase of the band at 2200 cm−1, most likely due to −NCO/R−CN species which can form from preadsorbed NxOy species from the previous treatment. Correspondingly, the intensities of oxygenates (1600−1500 cm−1) are increasing. Adding oxygen to the feed led the intensity of these bands to increase somewhat, upon removal of CxHy species and −NCO/R−CN species, while the formation of Cu+−CO is observed at 2133 cm−1. The following treatment in complete SCR-HC-deNOx reaction feed resulted in the reappearance of spillover NxOy at 2285 cm−1 and copper-bound and spillover or Cu-bound R− CN/NCO/CO species in the range 2240−2130 cm−1. Coinciding with the appearance of spillover species, lone silanols and geminal/vicinal silanols became perturbed (3680 2448



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Figure 13. Illustration of the single-site copper in wet or dry feed (a) and the proposed mechanism for SCR-HC-deNOx (b−e) and direct decomposition of NO to N2 in wet feed (f).

2133 cm−1), CxHyOz (1580−1520 cm−1) and the reactive intermediate −NH2 (3285 and 3368 cm−1) formed.10 Adding oxygen to the feed led to removal of −CxHy species (2967 and 2907 cm−1) upon the formation of CxHyOz species around 1600−1500 cm−1 and Cu2+−NO2−/NO3− at 1610 cm−1. Spillover R−CN are also observed in the range 2270−2210 cm−1.6,47 In complete SCR-HC-deNOx/H2O reaction feed, the formation of Cu2+/+−NCO, R−CN, and Cu+−CO species was observed around 2195 cm−1. In addition −NH2 species reappeared (3285 and 3368 cm−1) as trisilanols (3150 cm−1) gained intensity.

therefore aid in distinguishing between the intermediate species, seen by DRIFTS. The experiments were conducted at 350 °C, a temperature at which a promoting effect of water was observed for the Cu-aerogels as well as deactivation for the Cu-xerogel and the Cu-ZSM-5. Figure 13a illustrates the local surroundings of single-site copper incorporated into the aerogel framework followed by elucidation of central parts of the reaction mechanism in dry and wet feed (Figure 13b−f). Mutual for the Cu-aerogel, Cuxerogel, and Cu-ZSM-5 was initial reduction of Cu2+ to Cu+ in C3H6 (XAS) upon adsorption of CxHy species (DRIFTS). In NO/O2 feed, CxHy species reacted to form intermediate Ncontaining organic species (R−CN/NCO) and oxygenates (CxOyHz) such as Cu+−CO (DRIFTS, eq 3).10 The formation of N2 and NO2/N2O byproducts (Figure 3) coincides with reoxidation of Cu+ to Cu2+, confirming the expected redox mechanism during experiment A for all three systems (Figures 4, S2 (Supporting Information), and 13b−e).37 These findings clearly state that the reaction mechanism for the Cu-aerogel is similar to the mechanism for Cu-ZSM-5, which is well interpreted and established in literature for SCR-C3H6deNOx.10,36,54 Thus, we are able to clarify important aspects regarding the reaction mechanism for the Cu-aerogel, based on literature for Cu-ZSM-5. N-containing organic species (R− CN/NCO) and oxygenates (CxOyHz) are also seen in C3H6 following NO/O2 treatments (Figure 6), which strongly indicates that C3H6 react with Cu2+−NO2−/NO3− to form N2 and N2O (eq 4 and Figure S9 in the Supporting Information).36 The removal of Cu2+−NO2−/NO3− is clearly seen in C3H6 except in the first C3H6 cycle following the pretreatment in O2

4.0. DISCUSSION For elucidation of the origin of deNOx activity and the promoting effect of water for silica Cu-aerogel and the destructive effect on Cu-xerogel and Cu-ZSM-5, we applied XAS coupled with DRIFTS in situ. The structurally powerful combination of these techniques proved effective in correlating the active Cu-speciation with the characteristics of the hydrophobic aerogel structure, also including the hydrophilic silica Cu-xerogel analogue and Cu-ZSM-5 as reference systems. This in-depth study is two-fold: (A) the copper redox behavior is monitored by alternating reductive (C3H6) and oxidative (NO/O2) flow in dry and wet redox cycles and (B) complementary sequential treatments of reaction components, now starting with NO. Central SCR-HC-deNOx intermediate species, such as Cu-bonded −NCO/−CO and silanol-bonded R−CN and NxOy species, exhibit overlapping band frequencies in the infrared spectral region, which complicates an elucidation of NOx removal mechanisms.6,36,47 These two approaches will 2449



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the exception being the copper cation and associated ligands which are considered as hydrophilic sites (Figure 13a), the xerogels are hydrophilic.32 Copper is initially incorporated into both the aerogel and xerogel (EXAFS); however, the hydrophobic surfaces in the previous may provide hydrothermal stability to the incorporated guest cation.32,33 Coinciding with the loss of deNOx activity in the Cu-ZSM-5, the XAS and DRIFTS study revealed irreversible changes of the Cu2+/Cu+ redox pair and nearest surroundings in wet feed. A major fraction of Cu species in Cu-ZSM-5 are irreversibly altered by water, seen by a lower Cu+ pre-edge intensity in C3H6/H2O (Figures 4 and 5). This effect has previously been attributed to migration to inert sites, and our findings clearly confirm the irreversible alterations of Cu species in Cu-ZSM-5 after being subjected to wet feed.13,14 We have evaluated the possibility of diffusion limitations in the samples during the experiments, by estimating Weisz modulus for the pelleted fractions, aggregates and primary particles for the Cu-aerogel, Cu-xerogel and Cu-ZSM-5 (Supporting Information). Although our calculations imply that no diffusion limitations are present in either of these systems, we cannot rule out the presence of diffusion limitations for the less porous systems CuZSM-5, or the Cu-xerogel. The increase in nearest Cu−O/N shell multiplicities up to ∼4 in NO/O2 for the Cu-aerogel and Cu-ZSM-5 (Figure 5) coincides with the formation of Cu2+−NO2−/NO3− species, seen by DRIFTS (Figures 6 and 8). Storage of nitrites and nitrates is central to the mechanism (eqs 3−5), and can be associated with higher catalytic performance in the Cu-aerogel and Cu-ZSM-5 vs Cu-xerogel (Figure 1).11,47 In contrast to the Cu-aerogel, the first Cu−O/N shell coordination in Cu-ZSM-5 decreases noteworthy from around 4 to 1.8 in wet feed upon partial reduction of Cu2+ to Cu+ (Figures 4 and 5). This is indicative of bidentate Cu2+−NO2−/NO3− complexes being unable to form in NO/O2/H2O, attributed to competitive adsorption of NO and H2O.10,11 Competitive adsorption by H2O coincides with significant desorption of NO when switching from C3H6 to C3H6/H2O for Cu-ZSM-5 (Figure 3). Cu2+−NO2−/NO3− complexes were reformed in dry NO/ O2 feed for Cu-ZSM-5, hence the effect is reversible. In the Cu-aerogel, spillover-NxOy species were observed on hydrogen-bonded sites (Si−OH) featuring varying acidity in NO or NO/O2. The nature and quantity of spillover NxOy species depends on the presence of water in the feed, thus exhibiting a relatively wide range of vibration frequencies (2300−2100 cm−1). The blue shift of acidic silanol clusters from 3180 cm−1 to 3210 cm−1 (Figure 13d,f) was observed in wet NO-containing feed, assigned to adsorption of spilloverNxOy and associated intermediate species (eqs 3 and 4).34 Reactor effluent analysis confirm that the Cu-aerogel is also active for direct decomposition exclusively in wet NO feed.9 Byproducts such as N2O and NO2 were also in remarkable high amounts (Figure 9). The presence of Cu+ is regarded as directly responsible for NO removal via direct decomposition to N2.50,47 In wet feed, Cu2+ species is partly reduced and stabilized as Cu+. Concurrently, a buildup of NO+ (2114 cm−1) and [Cu+−(NO)x]+ was observed, coinciding with the formation of acidic silanol clusters in the Cu-aerogel. By contrast, Cu+ and spillover NxOy species are present in limited amounts for the Cu-aerogel in the dry NO feed, excluding NO removal by direct decomposition under these conditions. Consequently we believe the acidic silanol clusters, forming in wet feed, are indirectly responsible for stabilizing Cu+ in the

(C3H6 C1, Figure 6). These intermediate species are regarded central for the selectivity to N2; R−CN or NCO form N2 by oxidation with O2, NO2, and NO, whereas CxOyHz species form N2 by reducing NxOy species (eqs 5 and 6).36,55 In the Cu-aerogel, the N-containing organic intermediates (R−CN/ NCO) were not completely consumed during the redox cycling until H2O was added to the feed, at which hydrolysis led to −NH2 species (DRIFTS). −NH2 is regarded extremely active toward N2 formation by reacting with NO or NO2 (eqs 7 and 8 and Figure 6), which explains the higher amounts of N2 in the subsequent NO/O2/H2O feed.54 Hydrolysis of organic Ncontaining intermediate species can clarify a promoting effect of water in SCR-HC-deNOx feed for the Cu-aerogel (Figure 13b− e).10 Cx Hy + O2 /NO/NO2 → Cx HyOz + R−CN + NCO (3)

Cu 2 +−NO2− /NO3− + C3H6(g) → Cx HyOz + R−CN + NCO

(4)

R−CN/NCO + O2 /NO/NO2 → N2 + O2 + N2O + CO2 + CO

Cx HyOz + NxOy → N2 + CO2 + H 2O + N2O

(5) (6)

In wet feed: CN/R−CN/NCO + H 2O → NH 2 + CO2 (7)

In wet feed: NH 2 + NO/O2 /NO2 → N2 + H 2O

(8)

The reversibility and reducibility of the Cu species in the aerogel are unaffected by the addition of water as seen by XAS, hence this material shows superior hydrothermal stability. It is worth noticing that completion of the Cu2+/Cu+ redox cycle for the Cu-aerogel in wet feed took twice the time as the same process in the dry feed. Exclusive events in wet conditions were the activation of Brønsted acidic silanol cluster entities by multihydroxyl interaction in the Cu-aerogel, seen by DRIFTS (Figure 13a). A third or fourth hydroxyl group is needed to create silanol clusters from monobonded silanols/siloxy groups, which in the Cu-aerogel are considered ligands and defects created during incorporation of copper. Hence, the silanol clusters will be in close proximity to copper cations (Cu···Si shell at ∼3.2 Å) in wet feeds. This hydroxyl component may be H2O, which exhibits the affinity to interact strongly with silanols up to ∼400 °C.32,51 Intermediate species, such as CxHy, are stored to a larger extent in wet feed (DRIFTS), which can be associated with the formation of acidic silanol clusters upon switching from C3H6 to C3H6/H2O feed (Figure 6). Thus we assign the slow reoxidation in NO/O2/H2O to the enhanced adsorption of CxHy species which react with NO/NO2 to form oxygenates and N-containing organic species. In contrast to the Cu-aerogel, the Cu2+/Cu+ redox pair and local surroundings in the Cu-xerogel were strongly affected by the wet feed, seen by irreversible changes in XAS, desorption of feed components, and a loss of deNOx activity. The desorption of −CxHy species in C3H6/H2O correlates with the reoxidation of Cu+ to Cu2+ and associated alterations in local surroundings of copper (Figures 4, 5, and 7). The Cu+ species were not restored until switching back to dry C3H6. It is as this point important to reflect on the differences in terms of surface chemistry of the gels; whereas APD-aerogels are hydrophobic, 2450



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acidity is also the clearly enhanced adsorption of −CxHy species, in comparison to dry C3H6 feed. The silanol clusters formed in the Cu-aerogel are not removed upon the switch to dry reducing feed again; rather they seem to increase, correlating with the increasing selectivity to N2 and N2O in the last treatment in NO/O2 (Figures 2 and 6). Water adsorbs strongly on the hydrophilic silanol sites even at 350 °C, but desorbs upon further heating according to the literature.32,51 This explains why the promoting effect of H2O is canceled upon further heating to 450 °C (Figure 1) over Cu-aerogels.

form of [Cu+−(NO)x]+. The low Cu−O/N multiplicities around 3.3 and contracted distances at 1.935 Å in wet feed vs dry feed (4 Cu−O/N distances at 1.95 Å) reflect the clearly distinguishable adsorption capabilities of NxOy species by copper (Figure 10). The [Cu+−(NO)x]+ intermediate is proposed to initialize the direct decomposition of NO to N2 (eq 9 and Figures 10 and 13f).36,47,49,50 As observed by DRIFTS and in the exhaust, N2O is formed and eliminated in the next stage, in which Cu+ reoxidizes while reattaching to an extra-lattice oxygen (ELO) [Cu2+−O−] (eq 10).47 In the proposed mechanism, in e.g. a review by Garin,47 the next step is readsorption of two NO molecules, which forms the active intermediate complex [NOδ+−Cu+δ+−(NO2−)], which was observed by DRIFTS in wet NO-containing feeds for the Cu-aerogel (Figures 6, S4 (Supporting Information), and S6 (Supporting Information) and eqs 11 and 12).47,49 The complex [NOδ+−Cu+δ+−(NO2−)] is suggested to form N2 and O2 upon reoxidation to [Cu2+− O−] (eq 13), and is established as a key intermediate for NO removal by a direct decomposition mechanism.50,10,53

5.0. CONCLUSION Single-site copper incorporated into hydrophobic silica APDaerogels (2−8 wt %) is highly active for the selective catalytic reduction of NOx with C3H6 as reducing agent (SCR-HCdeNOx) in the range 300−450 °C, reaching conversions up to 67% at 450 °C. Water in the feed (15%) has a promoting effect on the deNOx activity, which establishes the Cu-aerogels as promising commercial candidates for this reaction. The promoting effect is particularly apparent at 350 °C; whereas the dry activity window is established at 350−450 °C, the wet activity window is extended to 300−450 °C. The Cu-aerogel shows superior stability of the Cu2+/Cu+ redox pair and local surroundings through dry and wet redox cycles. In contrast, the dense hydrophilic Cu-xerogel and Cu-ZSM-5 references perform poorly in wet feed, attributed to reversible (Cuxerogel) or both reversible and irreversible (Cu-ZSM-5) alterations of the Cu2+/Cu+ redox pair. Exclusive to the Cuaerogel in wet conditions, acidic silanol clusters are created from silanol/siloxy groups in close proximity to copper cations. Water is most likely the responsible extra-framework component inducing multihydroxyl interaction, leading to increased Brønsted acidity. The silanol clusters appear central for expanding the storage capacity of key intermediates, such as NxOy species. Hence they display an indirect functionality in the removal of NOx. Treatments in wet NO feed reveal deNOx activity for the Cu-aerogel via direct decomposition to N2. DRIFTS also confirm the presence of key Cu+−NxOy species associated with mechanisms for direct decomposition in the literature. The SCR-HC-deNOx route may compete with removal via direct decomposition in wet feed for the Cuaerogel, thus being responsible for the promotion of deNOx activity in wet feed.

Cu 2 + + NO → [Cu+−NO+] + NO → [Cu+(NO)2 ]+ (9)

[Cu+(NO)2 ] ⇔ [N2O−Cu 2 +−O−] ⇔ [Cu 2 +−O−] + N2O (10)

[Cu 2 +−O−] + NO → [Cu 2 + −( NO2−)]

(11)

[Cu 2 + −( NO2−)] + NO → [NOδ +−Cu+δ + −( NO2−)] (12)

[NOδ +−Cu+δ + −( NO2−)] → Cu 2 + + N2 + O2

(13)

+

Thus we propose that the storage of NO and other NxOy species on silanol clusters, adjacent to Cu sites in the aerogel system, leads to enhanced deNOx activity. Second, the SCRHC-deNOx route also competes with removal via direct decomposition in wet feed for the Cu-aerogel.45 In the presence of C3H6 in wet NO-containing feed, Cu+ may as well be reinstated by C3H6 as by NO.47,49 When switching to C3H6 in experiment B, the Cu+ pre-edge intensity reached only 70% (dry) and 55% (wet) of the maximum Cu+ pre-edge intensity yielded in C3H6 without NO/ O2 pretreatment (experiment A). The lower reducibility is, however, not a direct consequence of the wet conditions, as the Cu+ pre-edge remains unchanged from dry to wet C3H6 feed in the redox study (Figure 4). Rather, the limited reducibility seen in C3H6/H2O is more likely to result from the long NO and subsequent NO/O2 pretreatment causing the Cu2+−NO2−/ NO3− buildups observed by DRIFTS. We believe the buildup is caused by extra storage of spillover NO+/NxOy on silanol clusters before the treatment in C3H6 in wet feed (Figures S3 and S4 (Supporting Information) and Figures 12 and 13d,e). This is supported by the effluent analysis showing a sudden formation of N2, N2O, and NO2, which is particularly high in wet feed (Figure 9). For the Cu-aerogel, red shifts of the silanol cluster band from 3210 cm−1 to 3150 cm−1 were observed when switching from NO/O2/H2O to C3H6/H2O in both the redox study and the sequential treatments. The increased polarity of the trisilanols is indicative of NxOy adsorbent removal from the silanol clusters via R−CN/NCO intermediate species (Figure 6).34 Coinciding with the adsorbent removal from silanol clusters and increased

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ASSOCIATED CONTENT

S Supporting Information *

XANES collected in situ of Cuag-3.5 during heating in SCRHC-deNOx (150−500 °C), complemented with online MS monitoring effluents; data collected in situ from the redox cycling and sequential treatments: DRIFTS difference spectra (experiments A−C), XANES and complete results from EXAFS least-squares refinements for Cu-aerogel, (Cuag-8), Cu-xerogel, Cu-ZSM-5 and model compounds (experiments A and B); FID chromatograms and mass spectra of effluents for the Cu-aerogel in the redox cycling and sequential study (experiments A and C); and Weisz modulus calculations for evaluation of diffusion limitations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 2451



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Notes

(18) Quincoces, C. E.; Kikot, A.; Basaldella, E. I.; González, M. G. Effect of Hydrothermal Treatment on Cu-ZSM-5 Catalyst in the Selective Reduction of NO. Ind. Eng. Chem. Res. 1999, 38, 4236−4240. (19) Kispersky, V. F.; Kropf, A. J.; Ribeiro, F. H.; Miller, J. T. Low Absorption Vitreous Carbon Reactors for Operando XAS: A Case Study on Cu/Zeolites for Selective Catalytic Reduction of NOx by NH3. Phys. Chem. Chem. Phys. 2012, 14, 2229−2238. (20) Martínez-Franco, R.; Moliner, M.; Franch, C.; Kustov, A.; Corma, A. Rational Direct Synthesis Methodology of Very Active and Hydrothermally Stable Cu-SAPO-34 Molecular Sieves for the SCR of NOx. Appl. Catal., B 2012, 127, 273−280. (21) Iwamoto, S.; Kon, S.; Yoshida, S.; Inui, T. Sharp Contrast in Thermal Stability between MFI-type Metallosilicates and Metal-IonExchanged ZSM-5 and Their Catalytic Performances for NO Removal. In Studies in Surface Science and Catalysis; Hakze Chon, S.-K. I., Young Sun, U., Eds.; Elsevier: New York, NY, 1997; Vol. 105; pp 1587−1592. (22) Inui, T.; Iwamoto, S.; Kon, S.; Sakimon, T.; Kagawa, K. Evidently Advantageous Features of Metallosilicates as the Catalysts for Elimination of NO in the Exhaust Gases Containing a Large Excess of O2 and H2O. Catal. Today 1997, 38, 169−174. (23) Wang, C.-T.; Ro, S.-H. Nanocluster Iron Oxide-Silica Aerogel Catalysts for Methanol Partial Oxidation. Appl. Catal., A 2005, 285, 196−204. (24) Pajonk, G. M. Aerogel Catalysts. Appl. Catal. 1991, 72, 217− 266. (25) Pajonk, G. M. Catalytic Aerogels. Catal. Today 1997, 35, 319− 337. (26) Bali, S.; Huggins, F. E.; Huffman, G. P.; Ernst, R. D.; Pugmire, R. J.; Eyring, E. M. Iron Aerogel and Xerogel Catalysts for Fischer− Tropsch Synthesis of Diesel Fuel. Energy Fuels 2008, 23, 14−18. (27) Loche, D.; Casula, M.; Corrias, A.; Marras, S.; Moggi, P. Bimetallic FeCo Nanocrystals Supported on Highly Porous Silica Aerogels as Fischer−Tropsch Catalysts. Catal. Lett. 2012, 142, 1061− 1066. (28) Bhagat, S. D.; Kim, Y.-H.; Moon, M.-J.; Ahn, Y.-S.; Yeo, J.-G. A Cost-Effective and Fast Synthesis of Nanoporous SiO2 Aerogel Powders using Water-Glass via Ambient Pressure Drying Route. Solid State Sci. 2007, 9, 628−635. (29) Bhagat, S. D.; Kim, Y.-H.; Suh, K.-H.; Ahn, Y.-S.; Yeo, J.-G.; Han, J.-H. Superhydrophobic Silica Aerogel Powders with Simultaneous Surface Modification, Solvent Exchange and Sodium Ion Removal from Hydrogels. Microporous Mesoporous Mater. 2008, 112, 504−509. (30) Lee, C. J.; Kim, G. S.; Hyun, S. H. Synthesis of silica aerogels from waterglass via new modified ambient drying. J. Mater. Sci. 2002, 37, 2237−2241. (31) Prakash, S. S.; Brinker, C. J.; Hurd, A. J.; Rao, S. M. Silica Aerogel Films Prepared at Ambient Pressure by using Surface Derivatization to Induce Reversible Drying Shrinkage. Nature 1995, 374, 439−443. (32) Kristiansen, T.; Mathisen, K.; Einarsrud, M.-A.; Bjørgen, M.; Nicholson, D. G. Single-Site Copper by Incorporation in Ambient Pressure Dried Silica Aerogel and Xerogel Systems: An X-ray Absorption Spectroscopy Study. J. Phys. Chem. C 2011, 115, 19260− 19268. (33) Kristiansen, T.; Støvneng, J. A.; Einarsrud, M.-A.; Nicholson, D. G.; Mathisen, K. There and Back Again: The Unique Nature of Copper in Ambient Pressure Dried-Silica Aerogels. J. Phys. Chem. C 2012, 116, 20368−20379. (34) Dijkstra, T. W.; Duchateau, R.; van Santen, R. A.; Meetsma, A.; Yap, G. P. A. Silsesquioxane Models for Geminal Silica Surface Silanol Sites. A Spectroscopic Investigation of Different Types of Silanols. J. Am. Chem. Soc. 2002, 124, 9856−9864. (35) Chronister, C. W.; Drago, R. S. Determination of HydrogenBonding Acid Sites on Silica using the Cal-Ad Method. J. Am. Chem. Soc. 1993, 115, 4793−4798. (36) Pietrzyk, P.; Dujardin, C.; Gora-Marek, K.; Granger, P.; Sojka, Z. Spectroscopic IR, EPR, and Operando DRIFT Insights into Surface

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the Norwegian University of Science and Technology and the Norwegian Research Council for grants supporting the Swiss-Norwegian Beamlines (SNBL) and the assistance of the SNBL Project team (H. Emerich, W. van Beek, and P. Abdala) is very much appreciated. We thank the ESRF for beamtime. We are also grateful for the assistance of S. Forselv during in situ DRIFTS and catalytic measurements.

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