Molecular Level Understanding of How Oxygen and Carbon Monoxide

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Molecular Level Understanding of How Oxygen and Carbon Monoxide Improve NOx Storage in Palladium/SSZ-13 Passive NOx Adsorbers: The Role of NO and Pd(II)(CO)(NO) Species +

Konstantin Khivantsev, Feng Gao, Libor Kovarik, Yong Wang, and János Szanyi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01007 • Publication Date (Web): 01 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018

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The Journal of Physical Chemistry

Molecular Level Understanding of How Oxygen and Carbon Monoxide Improve NOx Storage in Palladium/SSZ-13 Passive NOx Adsorbers (PNA): The Role of NO+ and Pd(II)(CO)(NO) Species

Konstantin Khivantsev1, Feng Gao1, Libor Kovarik1, Yong Wang1,2, and János Szanyi1* 1

Institute for Integrated Catalysis, Pacific Northwest National Laboratory Richland, WA 99352 USA

2

Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99163, USA *corresponding author: email address: [email protected]

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

ABSTRACT: Model Pd/SSZ-13 with high dispersion of Pd ions (0.1 and 1 wt% Pd) was synthesized. The material was characterized with FTIR and cryo-STEM. Adsorption of NO leads to the formation of Pd(II)-NO and Pd(I)-NO complexes as well as NO+ species that replace residual H+ (extra framework) sites. These nitrosyl species have notable thermal stability, with resistance to decomposition under high vacuum at 200 ºC. Addition of molecular oxygen to NO-containing stream improves NOx storage of the Pd/H-SSZ-13. In particular, addition of O2 to NO slightly increases the amount of Pd(II)-NO complex with νNO at ~1865 cm-1 whereas the low frequency νNO band at 1805 cm-1, assigned to Pd(I)-NO, decreases in intensity. Simultaneously, polydentate nitrate species appear in small amounts, contributing to the high temperature NOx release stage during a PNA (passive NOx adsorber) cycle. The concentration of NO+ (characterized by the broad IR band centered at 2170 cm-1), in the presence of O2, increases in intensity profoundly and contributes to the increased capacity of Pd/SSZ-13 to store NOx and release it at temperatures >140 ºC. In the presence of H2O/O2, Pd/SSZ-13 does not perform satisfactorily as PNA, but the addition of CO to the stream improves the PNA storage capacity and shifts the NOx release peak temperature to >320 ºC where SCR catalysts are the most effective. With the aid of FTIR spectroscopy, we reveal the selective formation of a mixed carbonyl-nitrosyl complex Pd(II)(NO)(CO) in the presence of CO. Due to shielding of the Pd(II) ion from excess water and selective formation of such stable coordinatively saturated Pd(II)(NO)(CO) complexes, the PNA performance is improved by CO. Therefore, we demonstrate that besides NO species adsorbed on Pd, nitrosyl ions (NO+) in extra framework positions of chabazite are important for PNA storage. Furthermore, the important role of CO in promoting PNA performance is elucidated, thus highlighting the utility of the combined spectroscopic approach (in addition to materials performance testing) to derive structure/PNA performance relationships and identify new avenues to improve the PNA performance.


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1. INTRODUCTION Removal of criteria pollutant NOx species from diesel engine exhaust is a challenge for current lean NOx control technologies 1. Substantial decrease in the amount of released NOx has been achieved by the successful development and commercialization of NH3 selective catalytic reduction (SCR) technology

2,3

. Although, this process is very effective in reducing

NOx in a complex exhaust gas stream, the performance of the SCR catalyst is highly temperature dependent. Indeed, it is well-established that at temperatures below 150 ºC, these materials do not carry out NOx conversion into N2 effectively

2,3

which, in turn, leads to a

non-negligible fraction of produced NOx escaping into the air. Lean NOx traps can adsorb NOx but not continuously: they require periodic regeneration under hydrocarbon-rich (reducing) flow 4-7. Therefore, there is great interest to develop materials to store NOx at low temperatures (350 ºC). Ceria and doped ceria-based PNA materials have also been developed with Pd, Pt with their characteristic ability to store NOx at 120 °C and release them above 350 °C 14. The main drawback of the ceria-based formulations is their well-known susceptibility to sulfur poisoning. The most effective known PNA formulations were reported in 2016 by JonsonMatthey

11,12

. These materials contained, according to the authors, isolated Pd ions dispersed

on/in acidic zeolites with MFI, CHA, and BEA topology with considerable NOx storage at 100 °C and NOx release above 200 °C. Later on, Pd/SSZ-13 with Si/Al ratio 30

18

were

studied before and after hydrothermal aging. It was concluded that 2 wt% Pd/SSZ-13 after hydrothermal aging showed the best performance due to re-dispersion of Pd at elevated temperature under aging conditions. Recently, we performed a comparative study of Pd-containing PNAs based on zeolites (MFI, CHA (SSZ-13) and BEA) with similar Si/Al ratios

15

. It was found that under PNA

conditions for materials prepared via conventional ion exchange with loadings of ~ 1 wt% Pd, a multitude of species exist in these materials, including exchanged isolated Pd ions, nanosized PdOx domains and large PdOx clusters deposited on the external surface. Pd/SSZ-13 was shown to be an effective material with an optimal NOx release temperature window under realistic conditions, ideal for further conversion to N2 on SCR catalysts. Considering the 3 ACS Paragon Plus Environment


resistance of the small-pore CHA framework to structural collapse during even under the most strenuous conditions

19-21

, Pd/SSZ-13 materials thus seem to represent one of the most

promising PNA materials. Since the PNA concept is currently in the early stage of development, basic scientific understanding has to be developed with regards to the Pd/chabazite interaction with gas species: starting from NOx and (NOx+O2) to more realistic industrially relevant and complex gas mixtures (NO+CO). However, this link is clearly still missing since there have been no studies on Pd interaction with NOx and (NOx+O2) in order to establish structure/property relationships on the basic level before moving on to elucidation of more complex structure/function relationships under complex gas environments. Some evidence in recent reports suggests that well-dispersed Pd ions are the active species for PNA during NOx adsorption 18. Additional data suggest 17 that some reduced Pd phase could also be active for NOx storage in the presence of CO. Therefore, our goal was to prepare well-defined Pd/H-SSZ-13 materials, characterize them under NO, (NO+O2) and mixed gas conditions using FTIR, and establish structure/performance relationships. We find that O2 promotes NOx storage via formation of nitrosyl ions (NO+) in the cationic positions of SSZ-13. NO+ species thus formed are thermally stable under high vacuum even above 200 ºC. A fraction of the Pd(II)-NO species is converted into nitrates in the presence of oxygen, contributing to high temperature NOx desorption. CO-mediated improvement of PNA performance in the presence of H2O is related to the formation of mixed Pd(II)(CO)(NO) complexes. These coordinatively saturated complexes can be selectively formed from Pd(II)(NO) in the presence of CO.

2. EXPERIMENTAL METHODS Na-SSZ-13 with Si/Al = 6 was hydrothermally synthesized following a method described elsewhere

19

and ion-exchanged twice with 1 M NH4NO3 aqueous solution at 80 °C for 3

hours yielding the ammonium forms of SSZ-13. NH4-SSZ-13 was subsequently dried under ambient conditions and then at 80 ºC for 24 h. Samples with 0.1 and 1 wt% Pd loadings were prepared by pore volume impregnation with Pd(NO3)2 dihydrate (Sigma-Aldrich,≥ 99.9%) solution. More specifically, Pd(NO3)2 was dissolved in the minimum amount of water approximately equivalent to the total pore volume of the zeolite. The thick paste was mixed and stirred vigorously for 30 minutes, followed by calcination in air at 650 °C for 5 h (ramping rate 2 °C/min).


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The in situ static transmission IR experiments were conducted in a home-built cell housed in the sample compartment of a Bruker Vertex 80 spectrometer, equipped with an MCT detector and operated at 4 cm-1 resolution. The powder sample was pressed onto a tungsten mesh which, in turn, was mounted onto a copper heating assembly attached to a ceramic feedthrough. The sample could be resistively heated, and the sample temperature was monitored by a thermocouple spot welded onto the top center of the W grid. The cold finger on the glass bulb containing CO was cooled with liquid nitrogen to eliminate any contamination originating from metal carbonyls. The NO was purified via multiple freeze– pump–thaw cycles. Prior to spectrum collection, a background with the activated (annealed, reduced or oxidized) sample in the IR beam was collected. Each spectrum reported is obtained by averaging 256 scans. Cryo-HAADF-STEM at 77 K was used to probe the dispersion of Pd in the freshly prepared samples. The analysis was performed with a FEI Titan 80-300 microscope operated at 300 kV (with liquid nitrogen cooling to 77 K). We chose to perform measurements under cryo conditions in order to avoid beam damage of the Al-rich zeolite crystals under the electron beam

22

which largely leads to fast agglomeration of metal into nanoparticles. The

instrument is equipped with a CEOS GmbH double-hexapole aberration corrector for the probe-forming lens, which allows for imaging with 0.1 nm resolution in scanning transmission electron microscopy mode (STEM). The images were acquired with a high angle annular dark field (HAADF) detector with inner collection angle set to 52 mrad. Prior to imaging the sample was exposed to saturated water steam for 10 minutes at 100 ºC in order to check whether Pd noticeably agglomerates under such conditions. Standard NOx adsorption tests were conducted in a plug-flow reactor system with powder samples (120 mg, 60–80 mesh) loaded in a quartz tube, using a synthetic gas mixture that contained 200 ppm of NOx or (200 ppm of NOx and 16% O2) balanced with N2 at a flow rate of 210 sccm (corresponding to 70,000 h–1). In the experiments with CO and water vapor, 200 ppm of CO were added to the stream whereas water vapor was introduced by flowing the gas through a stainless steel vessel filled with water. All the gas lines were heated to over 100 °C. Concentrations of reactants and products were measured by an online MKS MultiGas 2030 FTIR gas analyzer with a gas cell maintained at 191 °C. Even though it is preferable to use a pure NO feed for the study, the actual NOx feed contained ∼185 ppm of NO and ∼15 ppm of NO2, where the latter came from the NO source and background NO oxidation by the heated gas lines. Since realistic engine exhausts do contain ∼5% of NO2 in the total NOx, no effort 5 ACS Paragon Plus Environment


was made to remove NO2 from our feed. Two four-way valves were used for gas switching between the reactor and the bypass. Prior to storage testing at 100 °C, the sample was pretreated in 14% O2 balanced in N2 flow for 1 h at 550 °C and cooled to the target temperature in the same feed. The gas mixture was then switched from the reactor to the bypass, and 200 ppm of NOx was added to the mixture. Upon stabilization, the gas mixture was switched back from bypass to the reactor for storage testing for 10 min. The sample was then heated to 600 °C at a rate of 10 °C/min to record the desorption profiles of gases in the effluent. Anaerobic NO titration experiments were also performed. Specifically, instead of switching the premixed gas from bypass to the reactor, the oxidized sample was first exposed to a flow of N2 at a given temperature for 25 min before introducing NOx in the absence of O2. 3. RESULTS AND DISCUSSION 3.1 Performance of 0.1 wt% Pd/SSZ-13 for NOx storage in the absence of O2. Curve (a) in Figure 1 shows NOx trapping followed by temperature-programmed desorption (TPD) of NOx for the 0.1 wt% Pd/SSZ-13 material with Si/Al = 6. During the first 10 min of data recording, the NOx/N2 feed ran through the bypass line while the pre-oxidized sample (held at 100 °C) was purged with dry N2. At t=10 min, the feed was switched to the reactor and kept flowing through the material bed for rest of data recording. At t=20 min, the sample temperature was ramped up to 600 °C at a linear ramp rate of 10 °C/min to desorb the trapped NOx. The measurement was stopped shortly above 500 °C. Since the NOx inlet concentration was ~200 ppm, in the 10–20 min time-on-stream interval, the negative peaks below 200 ppm represented NOx storage capacity, and beyond 20 min, the positive peaks above 200 ppm corresponded to NOx released. The material is evidently capable of adsorbing NOx at 100 °C. The NOx release is evident immediately upon start of the temperature ramp with a maximum amount of NOx desorbed around 120-130 °C (~ 20 ppm above the NOx flat level). Then it levels off and another, very broad NOx desorption peak can be observed with a maximum between 170-420 °C ~10 ppm above the flat NOx level. The stored NO/Pd adsorption ratio is ~1.1, indicating the binding of more than 1 NO molecules per Pd atom, suggesting that most (if not all) Pd is largely dispersed in a way that exposes all Pd atoms to NO molecules. However, if all of the NO adsorption were to come from NO-Pd interactions, then we would expect this ratio to be ~1 based on the results of FTIR measurements (vide infra). Most likely, more complex chemistry than simple Pd+n-NO interactions take place.


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b

250

NOx concentration, ppm

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


a 200

500 οC 150

100

100 οC 50 20

40

60

Time, minutes

Figure 1. (a) NOx adsorption at 100 °C for 10 min followed with TPD (10 °C/min up to 500 °C). The feed gas mixture contains 200 ppm of NOx (195 ppm of NO and 5 ppm of NO2) balanced with N2 at a flow rate of 210 sccm; oxygen is not present. (b) NOx adsorption at 100 °C for 10 min followed with TPD (10 °C/min up to 500 °C). The feed gas mixture contains 200 ppm of NOx (187 ppm of NO and 13 ppm of NO2) and 16% O2 balanced with N2 at a flow rate of 210 sccm. Note that during the first 10 min of data recording, the feed gas runs through a bypass line.

3.2 Performance of 0.1 wt% Pd/SSZ-13 for NOx storage in the presence of O2. Curve (b) in Figure 1 shows NOx trapping followed by TPD for the same material in the presence of 16% O2 in the gas stream. The most striking feature of the adsorption profile is the significant increase of the amount of NOx adsorbed via a new adsorption mechanism that is clearly slower than that without the presence of O2. More specifically, it takes about 10 minutes for this sample to recover 90% of NOx concentration in the effluent, whereas the same 90% recovery of NOx in the effluent stream is achieved within 2 minutes in the absence of O2 in the gas stream. The NOx concentration at the point of maximum uptake in the adsorption profile is quite similar (~40 ppm) for the two samples. The NOx desorption profiles recorded in the absence (Fig. 1, curve (a)) and in the presence of O2 (Fig. 1, curve (b)) display similar desorption stages. However, the absolute amount of NOx desorbed during the first desorption stage is much higher when the NOx uptake is carried out with O2 in the gas stream; the temperature of maximum NOx desorption rate (~50 ppm above the flat NOx level) is shifted to ~150 °C and extends to 210 °C where it levels off to approximately 10 ppm above the flat NOx level until the temperature reaches 7 ACS Paragon Plus Environment


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~420 °C (similarly with curve (a) in Fig. 1). In the presence of O2 a high temperature desorption peak is clearly observed with a maximum at about 420-440 °C. This peak is not well-pronounced in the absence of O2, although it is discernible. NO2 constitutes the major desorption product at this high temperature, while NO is the major desorption product at lower temperature. This observation is reminiscent of the decomposition of transition metal nitrates for which NO2 is the main nitrate decomposition product 23 according to the following process:

(1)

( ) → + 2 +

Stored NOx/Pd ratio in the presence of O2 is ~2.4 vs. 1.1 in the absence of O2. Clearly, O2 promotes NOx adsorption on Pd/SSZ-13. This phenomenon has not been discussed in detail before. We note, however, that we previously reported

15

NOx/Pd ratios >1 for samples with

1% wt Pd/zeolite in a NO/O2/N2 gas stream. Furthermore, this serves as a confirmation that a different avenue exists for NO storage on Pd/SSZ-13, other than NO interacting solely with Pdn+ sites. Additionally, O2 also promotes the formation of nitrates that decompose at higher temperature and are released predominantly in the form of NO2. Therefore, in order to understand the NOx adsorption process and how it is influenced by O2 on a molecular level, FTIR was used next to probe NOx adsorption on Pd/SSZ-13: infra-red signature of chemisorbed NO, due to its high molar absorption coefficient 24-26, is a commonly used probe to assess the state and distribution of metal and other species present in transition metal complexes/structures dispersed on inorganic oxides. 3.3 FTIR investigation of NO and (NO+O2) interaction with 0.1 wt% Pd/SSZ-13 at 100 ºC. Pd/SSZ-13 sample was pre-oxidized at 500 ºC, then cooled down in O2 to 250 ºC and evacuated afterwards. Evolution of the infrared spectrum upon adsorption of NO is presented in Fig 2. The three main IR features observed are a broad 2170 cm-1 feature (with a wide maximum between 2180 and 2165), and two bands below 2000 cm-1: 1865 and 1805 cm-1. The 1865 and 1805 cm-1 bands have been observed before

12, 15, 18

and assigned to the ν(NO)

vibrations in NO adsorbed on cationic Pd(II) sites. However, the assignment for the 1805 cm-1 band is still a matter of debate. The evolution of this band has been attributed to in the formation of Pd2+-NO species 18. Alternatively, the 1805 cm-1 band could belong to Pd(I)-NO species produced upon electron transfer from free radical NO to Pd(II): this process has been


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observed and characterized in detail for Cu(II)/SSZ-13 systems

27

where NO transferred its

+

electron to zeolite-tethered Cu(II), forming NO and interacting with reduced 1805 0.010

1865

0.008

Absorbance

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


0.006

2170

0.004

0.002

0.000 2300

2200

2100

2000

1900

1800

1700

1600

Wavenumbers, cm-1

Figure 2. Series of FTIR spectra recorded during NO adsorption (1 Torr, 298 K) on 0.1 wt% Pd/HSSZ-13 (Si/Al = 6).

Cu(I) cation side-on (eq. 2). One study has assigned the 1805 cm-1 band to ν(NO) in a Pd(NO)(H2O) complex

12

. Since our samples were treated under high vacuum at elevated

temperatures prior to dry NO admission, this latter assignment can be ruled out. Additional experiments (Figs S1 and S2) with introduction of H2O onto the sample further reinforce this conclusion. Therefore, we suggest that this band belongs to some other Pdn+-NO complex: either Pd(II)-NO located in a different zeolite position (6-membered-ring vs. 8-membered ring)

28

or, alternatively, a Pd(I)-NO complex with Zeofr-O-Pd(I)-NO structure. The latter

explanation is more likely since the 60 cm-1 split between the 1865 and 1805 bands is too large to be explained by the different position of the Pd cations of the same charge located in different cationic positions. The broad feature with a maximum around 2170 cm-1 belongs to the nitrosyl (NO+) species in cationic position in the zeolitic micropore

24,25

. As described elsewhere

24,25

, the NO+ cation

on H-ZSM-5 is characterized by ν(NO) at 2133 cm-1. Since NO+ cannot be formed directly from NO without charge redistribution, it is most likely formed either via the direct electron transfer from NO to cationic Pd(II) species (eq. 2) or with intermediacy of N2O3 (which is formed from NO+NO2 reaction (eq. 3,4), NO2 in turn is formed on Pd sites containing active forms of oxygen (e.g., PdxOy) [15]: ( ) + → ( ) + + → + 9 ACS Paragon Plus Environment

(2) (3)


+ + 2 → 2 +

(4)

Addition of O2 to NO-Pd/SSZ-13 system leads to profound changes in the IR spectra (Fig. 3). c

2170

0.10

Absorbance

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

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b 0.05

1865 1805

1650-1570

a 0.00 2300

2200

2100

2000

1900

1800

1700

1600

1500

-1

Wavenumbers, cm

Figure 3. FTIR spectra during O2 addition (15 Torr in total, 298 K) to NO-species present on 0.1 Pd/H-SSZ-13 after NO adsorption: (a) spectrum after NO adsorption, (b)-(c) species evolution during subsequent O2 addition (spectra were taken after equilibration). The most striking change is the significant increase in the intensity of the IR band representing NO+ species (~2170 cm-1). Its integrated intensity increases ~ 50 times compared with that observed after exposure to NO only (spectrum a). The intensity of the 1865 cm-1 IR feature slightly increases, concomitantly, that of the 1805 cm-1 Pdn+-NO band slightly decreases. This provides further evidence that the 1805 cm-1 band belongs to Pd+-NO since O2 can oxidize Pd(I) to Pd(II). Moreover, polydentate nitrate bands appear in the 1670-1570 cm-1 region and their intensities increase with increasing amount of O2 introduced. Polydentate nitrite NO2¯ band is initially observed at ~1550 cm-1 after the first pulse of O2 (spectrum b); it then disappears as nitrite gets converted to polydentate nitrate NO3¯ in O2 (spectrum c). Some water formed via reaction (3) and (4) may also contribute to the band at about 1630 cm-1. These observations may help to explain the NOx uptake profiles discussed above which demonstrated that the addition of O2 to the NOx-containing stream significantly promoted NOx storage capacity of the Pd/H-SSZ-13. The NO/Pd ratio between 1 and 2 could hypothetically be explained by the formation some amount of Pd(NO)2 species, for example. There is, however, no evidence for the formation of such species under the conditions of this study. Indeed, if Pd(NO)2 were to form, two bands in the ν(NO) region would be observable and they would change in concert, which is apparently not the case (Fig. 3). Thus, clearly, the 10 ACS Paragon Plus Environment

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observed increase in NOx storage capacity in the presence of O2 in the gas stream should directly be related to the presence of NO+ ions. In order to substantiate this phenomenon, we studied the thermal stability of NO+ species in the micropores of an H/SSZ-13 zeolite (Fig. 4) by increasing the temperature of the sample from 25 to 200 ºC. Notably, the NO+ species show surprisingly high thermally stability. Although their decomposition starts above ambient temperate, at 100 ºC a significant population of this species is still present. This corresponds well with the NO release feature with a maximum at ~200 ºC that trails off into 230-250 ºC region (Fig. 1b).

0.20

0.15

a Absorbance

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


0.10

0.05

b c

0.00

d -0.05

2400

2250

2100

1950

1800

1650

1500

Wavenumbers, cm-1

Figure 4. FTIR spectra recorded during heating of the NO+/H-SSZ-13 sample from 25 ºC (a) to 100 ºC (a)-(b) at 10 ºC/min ramp rate and then to 150 ºC (c) 200 ºC (d) in vacuum. NO+ was produced on H-SSZ-13 sample according to [24].

Unlike NO+/ZSM-5, which has lower thermal stability, the NO+ in H-SSZ-13 is unexpectedly thermally stable. Investigation of the factors governing the formation and stability of NO+/HSSZ-13 is currently underway. The current hypothesis is that the small cage of SSZ-13 is more ideal for stabilization of such species – this effect manifests itself electronically with NO stretching frequency blue shifted by almost 40 cm-1 (2170 cm-1 in H-SSZ-13 vs. 2133 cm1

in H-ZSM-5), suggesting a “tighter” chemical interaction with SSZ-13 than ZSM-5 and

significantly higher positive charge of NO+/SSZ-13. Another high-temperature desorption feature with significantly lower intensity can be seen at a higher temperature (~430 ºC). The major constituent of this high temperature release band is NO2. Indeed, the observed promotion of nitrate (NO3-) formation explains the increase in this high-temperature release peak well: nitrates typically decompose with the formation of NO2 at elevated temperatures. With the aid of infrared spectroscopy, the observed effect of O2 on NOx storage could be 11 ACS Paragon Plus Environment


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explained by identifying NO+ in the cationic extra-framework positions of zeolite as an intermediate for PNA, whereas contribution from nitrate species is less pronounced. 3.4 Effect of H2O on NOx uptake by Pd/SSZ-13. The effect of H2O on the NOx uptake was investigated on a 1 wt% Pd/H-SSZ-13 PNA material at 100 ºC. In our previous report

15

we observed that in the presence of H2O the

performance (NOx/Pd ratio) of Pd/zeolite-based PNA materials significantly decreased compared to the dry (NOx+O2) stream. Indeed, for a 0.88% Pd/SSZ-13, NO/Pd ratio drops from 1.08 to 0.14 in the presence of water. There are two possible explanations for this: firstly, in the presence of water steam Pd cations could agglomerate as was suggested for metal ions stabilized in extraframework positions of zeolites

29

. The other possible reason

might be water coordination to the Pd ions to form a stable 16-electron neutral [Pd(II)(OH)2(H2O)2] complex or, alternatively, hydrolysis with the formation of an anionic [Pd(OH)4]2- stabilized in the ring by two H3O+ ions and [Pd(OH)3(H2O)]- stabilized in the ring by one H3O+. In this case, no agglomeration of Pd is expected, however, the Pd center loses capacity to coordinate NOx. In order to explore the influence of water on Pd dispersion, we performed cryo-HAADF-STEM measurements on 1 wt% Pd/H-SSZ-13. Cryo-regime of HAADF-STEM was chosen to minimize the well-known damage of the zeolite structure accompanied by sintering of zeolite-confined metal species due to the high energy electron beam 22. No PdOx nanoparticles or clusters could be identified on the surface of non-steamed and water-steamed 1 wt% Pd/SSZ-13 with Si/Al = 6 (Fig. 5). Therefore, our assumption of close to 100% dispersion of the initial 0.1 and 1 wt% Pd/SSZ-13 is well-founded. Note that this does not necessarily mean that PdxOy clusters in the zeolite bulk are fully absent. However, even if they do exist, they must be small enough to satisfy an assumption of close to 100% dispersion of the initial 0.1 and 1 wt% Pd/SSZ-13.


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Figure 5. Cryo HAADF-STEM images of 1 wt% Pd/H-SSZ-13 (Si/Al = 6): (a) original sample. (b) original sample after exposure to steam for 10 minutes at 100 ºC in saturated H2O vapor.

Furthermore, steaming in the presence of water (although at the rather low temperature of 100 ºC) did not lead to agglomeration of the Pd species in SSZ-13: the observed decrease of PNA performance in the presence of water was probably not related to PdOx agglomeration upon presence water (100 ºC), and most likely, arises from H2O competition for the first coordination sphere of Pd(II) with the formation of stable Pd(H2O)2(O-Z)2 or [Pd(OH)4]2-/ [Pd(OH)3(H2O)]- complexes. 3.5 Promotion of NOx uptake by CO. The effect of CO on the NOx uptake was investigated on a 1 wt% Pd/H-SSZ-13 PNA material at 100 ºC. When CO is present in the stream in significant amounts (as is always true in a realistic lean NOx stream), the presence of water does not lead to such a profound PNA performance decrease with NOx/Pd ratio maintained at a higher level and the maximum NOx abatement maintained for longer times compared with the non-CO containing stream. The performance

and

NOx

adsorption/release

profiles

in

the

(NO+O2+H2O)/N2

and

(NO+O2+H2O+CO)/N2 streams are presented in Fig. 6. The second NOx adsorption peak for curve (b) in Fig. 6 is observed above 150 °C when H2O begins to desorb from Pd/SSZ-13 thus allowing for coordination of NO to Pd.



a b NOx concentration, ppm

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

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200

150

100

50

0 0

10

20

30

40

50

Time, minutes

Figure 6. NOx adsorption at 100 °C for 10 min (after 10 min bypass) followed with TPD (linear heating rate 10 °C/min starts at 20 minutes). The feed gas mixture contains 200 ppm of NOx, 16% O2, 2.7 % H2O with (a) and without 200 ppm CO (b).

Remarkably, CO promotes not only NOx uptake but it also shifts the NOx desorption peak to temperatures > 320 ºC where the performance of NH3 SCR catalysts is the highest. Epling et al. have recently reported a somewhat similar phenomenon on Pd/BEA zeolite

17

. Based on

the results of a combined FTIR and reactivity study they proposed that the reduction of cationic Pd in Pd/BEA materials was the origin of the observed PNA performance improvement in the presence of CO in the gas stream. In order to clarify this phenomenon, we investigated the adsorption of CO on an NO-saturated 1 wt% Pd/SSZ-13 by FTIR. FTIR spectra collected during the step-wise CO exposure of the NO/Pd/H-SSZ-13 sample are displayed in Fig. 7. Upon step-wise CO exposure, the intensity of the IR band centered at 1865 cm-1 gradually decreased and completely disappeared. Concomitantly, new features representing νC-O and νN-O vibrations developed at 2150 and 1800 cm-1, respectively. The narrow lineshape of the new bands most likely indicates that they belong to molecular-like species. Moreover, these two bands grow at the expense of the 1865 cm-1 band confirming the selective transformation of Pd(II)-NO species into another species.


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2150 0.20

1800 0.15

Absorbance

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0.10

1865

0.05

0.00 2200

2100

2000

1900

1800

1700

Wavenumbers, cm-1

Figure 7. FTIR spectra collected during step-wise addition of 0.5 Torr CO adsorption on an NO saturated 1wt% Pd/H-SSZ-13 sample. Final pressure 0.02 Torr.

The 2150 and 1800 cm-1 bands have different FWHMs (11 and 24 cm-1 respectively), and they belong to two different bond stretch: the 2150 cm-1 band is assigned to CO vibrational stretch while that at 1800 cm-1 - to NO stretch. It is known that the molecular Rh(I)(NO)2 (note that Pd(II) and Rh(I) are fully isoelectronic) species in HY zeolite are characterized by wider bands than Rh(CO)2 species, highlighting a different binding nature of NO as a ligand compared to CO

30,31

. Herein, we observe the selective transformation of Pd(II)-NO species

into a mixed carbonyl-nitrosyl Pd(II)(NO)(CO) complex:

( ) − + → − () −

(5)

Once the stoichiometric transformation was complete, the sample was evacuated in high vacuum in order to investigate the stability of the newly formed OC-Pd(II)-NO molecular complex (Fig. 8). Note, that under CO/NO gas environments the species thus formed are stable indefinitely. Evacuation restores the original (i.e., after NO saturation, prior to CO exposure) doublet with peak maxima at 1865 and 1805 cm-1 with intensities similar to those observed prior to CO introduction. It is evident that adsorbed NO represented by the 1805 cm1

IR band does not participate in the mixed ligand complex formation. In contrast, adsorbed

NO represented by the 1865 cm-1 band undergoes selective and complete transformation to produce the mixed ON-Pd(II)-CO species. Under vacuum this mixed ligand complex loses the less-strongly held CO ligand and restores the Pd(II)-NO complex. The loss of CO instead of NO ligand under vacuum is important for understanding the PNA chemistry of Pd(II) species in zeolites. If CO and NO binding strengths were similar, then one would expect loss of both



1800

2150

0.10

Absorbance

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Page 16 of 23

1865

0.05

0.00

2200

2000

1800

Wavenumbers, cm-1

Figure 8. FTIR spectra of Pd(II)(NO)(CO)/H-SSZ-13 under vacuum.

ligands at similar rates; the preferential loss of only CO clearly demonstrates stronger Pd(II)NO binding. The binding strength of CO in metal-carbonyl (M-CO) complexes can be readily compared based on C-O stretching frequencies of the complexes: the higher the frequency, the weaker the M-C bond is. This is because M-CO interactions can be conveniently described within the approximation of the Dewar-Chatt-Duncanson model with sigmabonding from CO and M, and π-backdonation from M to the antibonding orbital of CO. In comparison to an isoelectronic d8 Rh(I) complex (Rh(CO)2 symmetric band is 2117 cm-1 in HY zeolite 30), the 2150 cm-1 Pd-CO band found here obviously lies relatively high suggesting weaker binding. Indeed, Rh(I)(CO)2/HY(30) starts to lose CO ligands only above 270 ºC. As for the M-NO interactions, it is often assumed that NO binding to Mn+ on surfaces is similar to that of CO (i.e., linear binding in which NO is approximated as a NO+ quasi-linear fragment)

24,32

. This follows since experimentally observed M-NO frequencies on supported

materials are typically attributable to linear M-NO configurations: this assignment is based on the division of NO of M-NO organometallic complexes on linear and bent NO depending on the observed NO-frequencies 32. However, the experimental finding of stronger NO binding to Pd(II) site compared to CO in the present study clearly indicates a distinctly different binding mode of NO ligand to the Pd(II) site. It is in good agreement with a recent study 31 in which EXAFS, FTIR and DFT techniques were employed to show that NO exchanged with CO ligands of the 16-electron Rh(CO)2 anchored in the supercage of H-Y zeolite, leading to the unexpected formation of 14-electron Rh(NO)2 species with NO frequencies in the typical linear N-O vibrational range yet with a bent Rh-N-O geometry.

Although the exact

parameters of N-O binding to ion-exchange Pd(II) sites in SSZ-13 require further 16 ACS Paragon Plus Environment

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investigation, we hypothesize here that the particularly strong NO binding to Pd in comparison to CO is due to adoption of a bent configuration, with the formation of strong (short) Pd-N bond contrary to most previous reports that assume linear binding of NO to supported Pd 12,17,18,24. Finally, the Pd-NO species responsible for the 1805 cm-1 band does not complex (or ligand-exchange) with CO. Two explanations are suggested here: first, this is a Pd(II)-NO complex that is located in a more confined position within the SSZ-13 micropore. Second, this is a Pd(I)-NO complex. Formation of Pd(I)-NO complex can adequately explain this substitution inertness since, similarly to Cu(I)-NO type species, it assumes a linear or close to linear Oframework-Pd(I)-NO configuration with this complex being coordinatively saturated. Results shown in Figure 3 (i.e., intensity of this complex decreases upon O2 admission) also favor this latter assignment. We note finally that in recent studies, the role of CO in promoting NOx storage capacity has been suggested to a decrease of Pd oxidation states and therefore, an increase in NO binding 17

. This explanation may be reasonable for PdOx clusters present in some Pd/zeolite PNA

materials; for atomically dispersed Pd cations in ion-exchange positions, this explanation does not apply since these sites are not readily reduced by CO under typical low-temperature NOx storage conditions. Instead, we demonstrate here that the formation of coordinatively saturated stable ON-Pd(II)-CO complex explains the resistance of Pd/SSZ-13 PNA material to H2O poisoning when water is present in the PNA stream. Theis and Lambert recently suggested

16

that for Pd/CeO2-ZrO2 PNA materials, CO and NO form surface intermediates

that might play an important role in low-temperature NOx storage: the appearance of 2150 cm-1 band in the DRIFTS spectra in

16

upon CO adsorption on NO-Pd was attributed to the

formation of the isocyanate species. However, from the DRIFTS data in [16], the appearance of a broader spectroscopic feature around ~1800 cm-1 is also evident, concomitant with the 2150 cm-1 band. The frequencies observed in

16

for CO and NO match the CO and NO

frequencies of the Pd(II)(CO)(NO) complex observed in the present study quite well: thus, it is likely that in the

16

the formation of the Pd(NO)(CO) complex (and not NCO) species was

observed during NO-CO co-adsorption. Our work demonstrates how using model systems and spectroscopic tools helps uncover a layer of complexity for the industrially relevant PNA materials. 3.6 Conclusions. The promotion of O2 and CO on NOx storage capacity of model Pd/SSZ-13 PNA materials was investigated with the aid of TPD measurements and FTIR spectroscopy. Cryo HAADF17 ACS Paragon Plus Environment


STEM technique confirms high dispersion of Pd in the micropores of chabazite as well as its resistance to agglomeration in the presence of water vapor at 100 ºC. FTIR reveals that NO interaction with Pd/SSZ-13 yields various Pdn+-NO complexes as well as NO+ nitrosyl in cationic positions of H-SSZ-13. These nitrosyl species are characterized by notable thermal stability. O2 promotes NO+ formation, thereby increasing the total capacity of Pd/SSZ-13 material to store NOx and release it at temperatures >140 ºC. Water addition to the lean NOx PNA stream decreases Pd/SSZ-13 PNA performance due to competition between H2O and NO for the active Pd(II) site. On the other hand, when CO is simultaneously present in the PNA stream, the PNA storage capacity is improved, and the NOx release peak is shifted to >320 ºC which is the ideal temperature range for typical NH3 SCR commercial catalysts. The selective formation of mixed carbonyl-nitrosyl Pd complex Pd(II)(NO)(CO) was confirmed with FTIR, and is responsible for the observed phenomenon. Using well-defined materials and FTIR probe experiments on these materials, coupled with careful performance measurements, it becomes possible to determine specific factors contributing to the observed performance changes of PNA materials under various conditions, therefore establishing structure-storage property relationships without which the rational synthesis and/or search for better performing nanomaterials would be a challenge. Supporting Information Additional FTIR measurements of H2O adsorption on Pd-NO/SSZ-13 (Fig S1) and NO adsorption on Pd/SSZ-13 (Fig S2) which was pre-treated in vacuum at 70 °C, thus retaining H2O in the micropore. Acknowledgements: The authors gratefully acknowledge the US Department of Energy (DOE), Energy Efficiency and Renewable Energy, Vehicle Technologies Office for the support of this work. The research described in this paper was performed in the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for the US DOE by Battelle.

References.


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Molecular Level Understanding of How Oxygen and Carbon Monoxide

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