Periodic DFT Characterization of NOx Adsorption in Cu-Exchanged

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Periodic DFT Characterization of NOx Adsorption in Cu-Exchanged SSZ-13 Zeolite Catalysts Trunojoyo Anggara,† Christopher Paolucci,† and William F. Schneider*,†,‡ †

Department of Chemical and Biomolecular Engineering and ‡Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: We report a density functional theory (generalized gradient approximation (GGA)) and hybrid-exchange (HSE06 and B3LYP) characterization of NOx adsorption on Cu-exchanged SSZ-13, in particular to compare predictions of computational models, to understand the relative stability of adsorbates under reaction conditions, and to aid in the interpretation of experiments. We consider Cu exchanged near a single Al tetrahedral site without and with a proximal Brønsted acid. Computed structures, vibrational spectra, and oxidation states are consistent with prior observations. The GGA generally overpredicts adsorbate binding energies to extents that are species specific. The tendency for a Cu site to coadsorb two species is in particular exaggerated within the GGA. Proximal acid sites generally have a small effect on adsorbate binding. Computed vibrational spectra support the assignment of various experimentally observed features, including the assignment of an experimentally observed feature near 2100 cm−1 to an Al site charge-compensated by NO. First-principles thermodynamic analysis shows that Cu-bound nitrite and nitrate are thermodynamically preferred under oxidizing conditions in the presence of NO or NO2.

1. INTRODUCTION Cu-exchanged zeolites have long been known to be active for both the catalytic decomposition and ammonia-selective catalytic reduction (NH3−SCR) of nitrogen oxides (NOx, x = 1,2) to dinitrogen (N2).1−7 Recently, small-pore Cu-exchanged chabazite (CHA) SSZ-13 zeolites have demonstrated both sufficiently high activity and hydrothermal stability8−10 to be practically viable for diesel NOx emission control.11 For these reasons, the nature of the Cu active sites and their interactions with NOx adsorbates has garnered much interest. In Cu-SSZ-13 zeolite catalysts, spectroscopic and chemical titration evidence indicates that isolated Cu-exchanged cations are the active site for NH3−SCR9,12−16 and dimers or higher aggregates the active site for dry NO oxidation.15 Cu exhibits both 1+ and 2+ oxidation states, and under operando SCR conditions both oxidation states are observed in X-ray spectroscopies.14,17 Recent operando X-ray absorption spectroscopy (XAS) experiments and density functional theory (DFT) calculations support a case for both reduced Cu(I) and oxidized Cu(II) under standard NH3− SCR conditions.17−20 Reactant cutoff experiments have been used to demonstrate that these observations reflect a redox catalysis in which Cu(II) is reduced to Cu(I) by NH3 and NO and Cu(I) reoxidized to Cu(II) by some combination of NO and O2.21 The adsorption and reaction of NOx on Cu(I) sites is thus central to the NOx decomposition and SCR chemistries. Cluster, embedded cluster, and periodic models have been employed in computational models of exchanged Cu in zeolites. © XXXX American Chemical Society

The structures, vibrational spectra, and energetics of NOx adsorption and reaction on an isolated Cu charge-compensated by a single Al (“ZCu”) were first characterized using local density functional theory (LDA-DFT) calculations of single tetrahedral (T)-site cluster (Cu+−Al(OH)−4 )22−25 and larger cluster26 models. NO and N2O decomposition reactions were proposed to occur by ZCu ↔ ZCuO redox cycles. Others have explored the same idea using embedded cluster models27,28 and with hybrid functionals.29 More recently, periodic DFT models have been used to explicitly treat the extended zeolite lattice. Pulido and Nachtigall30,31 used these models to develop an NO decomposition mechanism in FER zeolites. Because SSZ-13 can be represented with a modest-sized trigonal unit cell, periodic models are particularly manageable. Such models14,15,17,32−36 recover the experimentally observed preference9,13,37 of Cu cations for the six rings of the SSZ-13 lattice. Periodic models show good agreement with smaller clusters in predicting O2 side-on binding preference in Cu(I)-SSZ-13.38 Periodic models describe the adsorption of H2O, O2, and their decomposition products (OxHy, x,y = 1,2)15,17 on ZCu and Z2Cu in SSZ-13. The evidence for NO to adsorb on Cu(II) either as a mono- or dinitrosyl is less strong.32−35,39 Recently the computed electronic structure of various SCR adsorbates on a Cu ion Received: August 7, 2016 Revised: October 13, 2016 Published: November 4, 2016 A

DOI: 10.1021/acs.jpcc.6b07972 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

dispersion. For our initial NxOy adsorbate configurations, we drew on available literature and tested different geometric isomers for each species.24,27,47 We compared two approaches to improve upon the GGA energies. Hybrid-exchange B3LYP48−50 energies were evaluated using a cluster correction model.15,51−56 A cluster containing the reaction center of interest was extracted from the GGAoptimized periodic structure, terminated with hydrogen atoms, and single-point GGA and B3LYP energies were evaluated using the Amsterdam density functional (ADF) code,57,58 an all-electron triple-ζ polarized basis, and an integration precision of 10−8. The hybrid DFT supercell energy was approximated according to

charge-compensated by a single Al has been used to rationalize observed X-ray absorption near-edge spectra.36 The supercell computations to date have generally been based on the generalized gradient approximation (GGA). This approximation may be problematic for the reactions on Cu ions that involve charge separation.21,34 In this work, we use a periodic supercell and hybrid-exchange DFT to compute and analyze the adsorption characteristics of NxOy species on a nominally Cu(I)-exchanged ion in SSZ-13. We consider a Cu ion exchanged near an isolated Al T-site (ZCu) as well as a Cu ion near both an Al T-site and a proximal Brønsted site (ZCu/ ZH) like that proposed to be formed during the SCR cycle.21 We find that hybrid exchange is important in overcoming the deficiencies of the GGA in describing molecular NOx energetics, including the interaction of NO with Brønsted sites. A first-principles phase diagram based on these results shows that Cu2+(NO−2 ) and Cu2+(NO−3 ) are the most stable surface species under oxidative conditions.

B3LYP GGA B3LYP GGA Esupercell ≈ Esupercell + Ecluster − Ecluster

(1) 59,60

In addition, Heyd−Scuseria−Ernzerhof (HSE06) screenedexchange single-point energies were calculated within VASP using parameters as described for GGA above. These calculations allow us to compare the performance of two popular alternative methods for incorporating hybrid exchange into computed adsorption energies. To provide some calibration of these methods, Table SI-1 (SI = Supporting Information) summarizes computed gasphase geometries of O2 and various NOx molecules. The GGA tends to overestimate experimental bond lengths by less than 2%; hybrid B3LYP and HSE06 calculations decrease the discrepancies to less than 1%. Table 1 compares computed energies

2. COMPUTATIONAL DETAILS We performed plane-wave, supercell DFT calculations using the Vienna ab-initio simulation package (VASP)40,41 version 5.2.12 following an approach similar to that reported earlier.14,15,17 We used the projector augmented wave (PAW)42,43 method and a plane wave cut off of 400 eV for core and valence states, respectively. Initial calculations were performed using generalized gradient approximation (GGA) of Perdew et al.44 to describe electron exchange and correlation. PW9144 and PBE45 produce similar results;34 we favor the PW91 formulation of GGA for consistency with our prior work.14−17,21 All calculations were performed spin-polarized and tested to identify the ground magnetic spin states reported here. We converged self-consistent-field (SCF) electronic energies to 10−8 eV and atomic forces for geometric relaxation using the conjugategradient algorithm to less than 0.01 eV/Å. Harmonic vibrational frequencies were also calculated at the GGA level by numerical differentiation of atomic forces with 0.01 Å differential displacements. Harmonic GGA frequencies tend to correspond well with observed frequencies.34,46 The approximately trigonal SSZ-13 supercell, including 12 T-sites, is shown in Figure 1. Periodic replication of the

Table 1. Computed NxOy Gas-Phase Reaction Energies at 0 Ka

a

reactions

PW91

HSE06

B3LYP

exptlb

2NO + O2 → 2NO2 2NO → N2 + O2 N2O → N2 + 1/2O2 mean absolute error

−2.42 −1.94 −0.13 0.71

−1.67 −1.87 −0.43 0.34

−1.36 −1.88 −0.69 0.15

−1.11 −1.88 −0.88

All molecules at optimized geometries. bRef 61.

for several relevant NOx reactions. Unlike the geometries, reaction energies are quite sensitive to method. The GGA overpredicts NO oxidation exothermicity by a factor of 2; the hybrid methods improve considerably on this performance. NO decomposition is described well by GGA and hybrid methods. N2O decomposition is the most problematic; only B3LYP provides values within 0.3 eV of experiment. Overall, the hybrid energies are superior to the GGA. We used HSE06-computed Bader charges to estimate Cu oxidation states, taking computed charges for Cu chargecompensated by one (“ZCu”) and two Al (“Z2Cu”) as 1+ and 2+ references, respectively (Table SI-2). Normalized HSE06computed Bader charges are consistent with B3LYP Mulliken charges (Table SI-3). The normalization process is described in our previous papers.16,21

Figure 1. SSZ-13 periodic supercell. Yellow, red, gray, and green spheres are Si, O, Cu, and Al, respectively.

3. RESULTS AND DISCUSSION 3.1. Adsorbate Structures and Energies. The chabazite lattice contains one symmetry-distinct tetrahedral site (T-site) that assembles into a secondary building unit of interconnected 4-, 6-, and 8-rings (Figure 2). Figure 3a shows the preferred 6-ring location of an unligated Cu(I) ion (“ZCu”) near an Al.17,35 We first consider the adsorption of formally neutral NOx species, including even-electron N2 and N2O and odd-electron

supercell generates the familiar double six-ring (D6R) structure of the CHA lattice.17 Supercell parameters were taken from previous work.17 Calculations were performed with either one or two Si sites substituted with Al, to obtain Si/Al ratios of 11:1 and 5:1, respectively. The first Brillouin zone was sampled at the Γ point as appropriate for an insulator with minimal band B


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Figure 2. SSZ-13 zeolite structure with 4-Si-membered ring, double 6-membered ring (d6r), and 8-membered ring. Bridge oxygens are hidden for clarity.

NO at this site. GGA-computed adsorption geometries are shown in Figure 3; binding energies and normalized charges are reported in Table SI-2. In general, adsorption brings Cu out of the 6-ring plane and closer to the Al atom and two framework oxygens. N2, N2O, and NO all preferentially bind N-down on ZCu, the first two in linear and the last in a bent conformation. As shown in Figure 4, adsorption induces modest charge transfer from Cu to adsorbate in the order N2 < NO. The partial oxidation of Cu by NO is consistent with the observed NO bond lengthening. Both NO and N2O also exhibit higher energy, metastable O-down, and bent binding isomers (Figure 3c and f, respectively). Figure 5(a) plots the HSE06-computed vs the GGA-computed binding energies. In general the inclusion of hybrid exchange diminishes binding energies by several 100 meV. N2 and N2O hybrid-corrected binding energies are a modest ≈−0.5 eV. NO binds approximately 0.3 eV more strongly than these two, at an

Figure 4. Normalized Cu Bader charges based on HSE06 calculations.

energy consistent with available literature.34,46 Unfortunately there are no experimental reports of corresponding heats of adsorption on Cu-SSZ-13. The heat of adsorption of NO on Cu-exchanged MFI has been determined by microcalorimetry to be 1.03 eV,62 intermediate between the GGA and hybrid adsorption energies. As shown in Figure 5(c), the fully periodic HSE06 model and cluster-corrected B3LYP model yield binding energies that differ by 0.3 eV more stable within the HSE06 model. In analogy to the dinitrosyl described above, a variety of other coadsorbate structures can be located within the GGA (Figure SI-2), including notably coadsorbed NO and O2, a potential intermediate in the production of adsorbed nitrate.20 Other examples include combinations of NO with NO2 or NO3 and diadsorbed NO2, among others. While the GGA results suggest all of these to be bound with respect to desorption of either coadsorbate, the hybrid-exchange models always predict the coadsorbed structures to be unstable except in cases involving secondary bonding between the two coadsorbates (Table SI-4).

Figure 5. (a) HSE06 vs GGA adsorption energies of formally neutral adsorbates. Blue circles and red squares indicate HSE06 energies at GGA and HSE06-optimized geometries, respectively. (b) HSE06 vs GGA adsorption energies of formally oxidizing adsorbates. (c) Clusterbased B3LYP vs fully periodic HSE06 adsorption energies. Circles and squares represent formally neutral and oxidizing adsorbates, respectively.

The NO bind bent inward and create a pseudotetrahedral coordination environment about Cu. While the GGA predicts this second NO to be bound by −0.93 eV, single-point hybrid calculations predict it to be nearly unbound (ZCu(NO)−NO in Figure 5(a)). To ensure that this difference was not an artifact of the GGA geometries, we reoptimized the NO monomer and dimer at the HSE06 level (Figure SI-1). The structures are nearly unchanged, and the first and second NO binding energies each change in energy by at most about 0.06 eV (Table SI-2). The GGA thus appears to systematically exaggerate the binding of NO to Cu(I). An analogous effect has been observed for NO binding to Cu(II) and has D


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Figure 6. GGA-optimized structures of various formally oxidizing NxOy adsorbates at a ZCu site. Yellow, red, gray, green, and blue spheres are Si, O, Cu, Al, and N, respectively.

ZCu + ZH/ZH → ZH + ZCu/ZH

For completeness, Figure 6h and i report geometries of atomic O and N on ZCu. O oxidizes Cu more strongly than N (Figure 4), and the binding energies of both species are positive with respect to O2 and N2, respectively, within the GGA and HSE06 model. A Cu(I) proximal to a Brønsted site has been proposed as the Cu(I) form of the SCR active site.16,21 We placed a Brønsted acid site on the oxygen framework next to one of the Al atoms in the 6-ring of SSZ-13 and reoptimized; Figure SI-3a shows that the presence of the proximal proton has a modest influence on Cu location and Cu−O separations. To test the relative stability of these two types of Cu(I) sites, we constructed a 12 T-site supercell containing two Al on one side of the d6r and one Al on another and computed the energy of the exchange reaction:

(2)

We find the two sites to be essentially isoenergetic and to place Cu in the same 1+ oxidation state. Figure SI-3 and Table SI-2 report results for adsorbates at the ZCu/ZH sites, and Figure 7 plots the ZCu/ZH binding energies against the ZCu ones. For the majority of adsorbates considered, a proximal proton has negligible impact on adsorption geometry and energy. The only exceptions are the higher energy isomers of adsorbed NO2, which are stabilized by secondary hydrogen bonds to the proximal protons. This stabilization decreases the energy separation but does not alter the ordering of the three NO2 isomers. 3.2. Vibrational Spectra. The GGA-computed harmonic vibrational frequencies in the range of experimental interest are E


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at 1826 and 1728 cm−1 with increasing NO dosing,65 consistent with observation on other zeolites.66,67,69 These two correspond with the symmetric and asymmetric N−O stretch modes of a Cu(I)-dinitrosyl.24 The GGA-computed N−N stretch band of N2 also agrees quite well with experimental observations.64,66,70 The consistent features observed across zeolites suggest that the exchanged Cu(I) environment is similar in all. As noted above, NO2 and NO3 can adsorb in a variety of conformations, all of which produce vibrational features in the 1550−1650 cm−1 range characteristic of nitrite (NO−2 ) and nitrate (NO−3 ).66 We can compare the computed results to recent experiments by Szanyi et al., who observed the appearance of multiple vibrational features after dosing NO2 at 300 K to annealed Cu-SSZ-13 (Si/Al = 6, Cu/Al = 0.4).64 The assignment of a band observed at 1295−1299 cm−1 to nitrite is consistent with computation for the ZCuO2N symmetric stretch. Bands observed at 1573, 1596, and 1618 cm−1 were assigned to nitrate. The DFT calculations predict only a ZCuNO3 stretch mode in the center of this range (1601 cm−1). The N−O modes from ZCuNO2 and ZCuONO isomers are in the low end of the observed range and could account for some of the observed features. The presence of proximal Brønsted sites shifts the modes out of this range. Precise assignments are complicated by uncertainties in the computed frequencies and by the likely presence of Cu dimers and larger aggregates in these relatively high Cu density samples. In addition to the vibrational features attributable to Cu-bound NO, NO2, and NO3, bands in the 2100−2217 cm−1 region are often observed over acid and metal-exchanged zeolites dosed with NO or NO2.64,66,71−73 None of the Cu-bound NOx species reported above exhibit features at frequencies as high as 2100 cm−1. Rather, these high frequency modes are suggestive of oxidation to NO+ and/or NO2+. Szanyi et al. assigned bands in this region to NO+ formed via charge transfer with a Cu2+:64

Figure 7. Parity plot of HSE06 adsorption energies at ZCu sites proximal to a Brønsted site vs an isolated ZCu site.

summarized in Table 2; complete results can be found in Table SI-5. In general, frequencies are red-shifted from the gas (Table SI-1) to the adsorbed state, consistent with the observed general lengthening of bonds. The blue shift of the asymmetric NNO stretch is the one notable exception. Table 2 also summarizes experimental vibrational assignments following NOx dosing to Cu-exchanged SSZ-13. We first Table 2. Selected GGA-Computed Vibrational Features of NxOy Adsorbates and Corresponding Experimentally Observed Features from NOx Adsorbates on Cu-SSZ-13a structure 2

ZCuNO ZCuN2 1 ZCuN2O 1 ZCu(NO)2 1

1

ZCu(ONNO)

3

ZCuO2 ZCuO2N 2 ZCuNO2 2 ZH/ZCuNO2 2

2

ZCuONO ZH/ZCuONO

2

2

ZCu(η2-NO3)

a

mode

harmonic frequency

νN−O νN−N νas‑NNO νs‑N‑O νas‑N−O νN−N νas‑ONNO νO−O νs‑ONO νN−O νas‑ONO νOH νN−O νas‑ONO νOH νN−O

1797 2297 2367 1826 1705 1707 978 1209 1270 1583 1487 2391 1578 1396 2565 1601

experimental frequency 1808,b 1809c 2300c 2249b 1826c 1728c

Cu 2 + + NO ⇋ Cu+ − NO+

(3)

However, hybrid DFT calculations do not support this assignment.21,74 Lobo et al.75 attributed this feature to an NO+ charge compensated by an Al T-site, “ZNO”. To test this hypothesis, we computed the structures of NO, NO2, and O2 near a proton-free Al T-site; structures are shown in Figure SI-4. Absolute binding energies decrease from NO (≈2 eV) to NO2 (≈1 eV) to O2 (≈0 eV) in both the GGA and HSE06 models; NO is the most likely adsorbate at such a site. While the binding energies are consistent, computed harmonic frequencies are sensitive to the computational method (Table SI-6). To obtain a more robust estimate, we compared the HSE06-computed vibrational frequencies of various small molecules to the known experimental anharmonic frequencies; the two are linearly correlated with a scale factor of 0.9507 (Figure SI-5). This scale factor adjusts the HSE06 ZNO vibrational frequency to 2121 cm−1, close to the values reported by Szanyi et al. in Cu-SSZ-13 and by Lobo in the H-form SSZ-13. These results support the Lobo assignment. A variety of other local minima can be found for ZNO corresponding to different orientations of NO relative to and distance from the T-site (Figure SI-6). The range of computed, scaled HSE06 frequencies is consistent with experimental observation for NO over acid zeolites, supporting the assignment of these bands to NO chargecompensating an Al T-site.73 3.3. ZCuNxOy Phase Diagram. The computational results provide the information necessary to construct a thermodynamic phase diagram17,76 for NOx adsorbates at a Cu site

1295−1299b

1573, 1596, 1618b

Frequencies in cm−1. bRef 64. cRef 65.

consider the non-oxidizing adsorbates. The computed Cu(I)− NO stretch band at 1797 cm−1 corresponds well with the band observed at 1808 cm−1 in NO dosed to annealed, reduced, or oxidized Cu-SSZ-13 (Si/Al = 6, Cu/Al = 0.4).64 This assignment is consistent with prior computations and experimental observation of NO dosed to Cu-exchanged SSZ-13 and other zeolites where samples were pretreated by vacuum or thermal treatments to reduce a fraction of Cu(II) to Cu(I).25,65−68 In the same experiment,64 Szanyi et al. attribute a small band at 2247−2249 cm−1 to N2O adsorbed on Cu(I), as previously assigned in other zeolites.66,67 The harmonic GGA frequencies overestimate this blue shift of the N−N stretch whether N2O is bound N- or O-down. In a similar experiment performed at 100 K, the Cu(I)−NO band at 1810 cm−1 splits into two bands F


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The Journal of Physical Chemistry C charge-compensated by a single Al. In the absence of any adsorbates, a Cu at this site has a 1+ oxidation state, and as noted above, relevant NOx adsorbates oxidize the Cu to varying extents. Experimental and computational evidence indicates that a key step in NOx SCR is oxidation of Cu(I) to Cu(II), and a thermodynamic analysis can give insight into the species that likely participates in this oxidation. We write the free energy of some adsorbate combination NxOy in terms of the chemical potentials of NO and O2: ΔGx , y(T , Δμ NO , ΔμO ) 2

= ΔErxn + ΔG NxOy(T ) − xΔμ NO −

(y − x ) ΔμO 2 2 (4)

Here ΔErxn is the DFT-computed 0 K adsorption energy referenced to x NO and (y − x)/2 O2; ΔGNxOy is the difference in internal free energy between the adsorbate-covered and adsorbatefree site at temperature T; and the Δμ are deviations in gas molecule free energies from their 0 K values. We report results with B3LYP energies because they reproduce most closely the experimental gas-phase reaction energies (Table 1); HSE06 energies yield qualitatively the same results. We estimate finite temperature contributions to the adsorbate free energies from the harmonic vibrational spectra in combination with a previously described correlation in which adsorbates retain 2/3 of their translational entropy and assume ideal gas behavior to relate gas potentials to temperature and pressure.16,21 Further details are in SI Section 1. Figure 8 reports a phase diagram using chemical potential references at 543 K chosen to match the conditions reported by Verma et al.15 We exclude adsorbed N2 and N2O from the analysis: while these species are lowest in free energy over a wide range of conditions, their generation is kinetically controlled. Rather, Figure 8 emphasizes species observed to form following NOx dosing and expected to be relevant to the oxidation half-cycle of SCR. Plotted are the lowest free energy species as functions of chemical potential differences, Δμ, on the bottom and right axes and corresponding partial pressures across the left and top axes. Adsorbate-free and reduced ZCu is most stable at the most negative chemical potentials (lower right). As NO chemical potential increases to the left, monomeric and dimeric NO drop in free energy and become successively the most stable adsorbed states. Reduced Cu sites exposed to NO either at high pressure or low temperature are expected to be NO covered. The appearance of dimeric NO at the limit of high NO and low O2 chemical potentials is also consistent with the splitting of the NO band observed during cryogenic NO dosing to Cu-SSZ-13.65 At even modest O2 pressures, adsorbed NO gives way to adsorbed nitrite (ZCuNO2) and then, over a broad range of conditions, nitrate (ZCuNO3) as the lowest free energy state of the site. The prevalence of nitrate is consistent with FT-IR observations of the intensification of nitrate bands upon NO and O2 dosing at 300 K to Cu-SSZ-13.64 The inset of Figure 8 reports the relative free energies of all adsorbates at 300 ppm of NO and 10% O2, the oxidizing conditions studied by Verma et al.15 ZCuNO3 is lowest in free energy at this condition, and adsorbed nitrite (ZCuNO2) and reduced ZCu are within 0.3 eV in free energy. At equilibrium the large majority of single sites is thus expected to be NO3covered, and fractions of a percent are expected to be either NO2-covered or free. Coadsorbed NO and O2, in contrast, is

Figure 8. B3LYP-derived phase diagram for ZCuNxOy intermediates referenced to NO and O2. Conditions equivalent to 300 ppm of NO and 10% O2 at 543 K indicated with the dotted lines at 543 K (bottom). Inset shows the relative free energy of all computed species at this condition.

1.55 eV higher in free energy than a free Cu site. Thus, an NO oxidation catalytic cycle that invokes as intermediates both states of a site must overcome a free energy barrier of at least 1.55 eV. While kinetics ultimately control the species observed in a given experiment, this free energy analysis is a useful guide for evaluating the energetic plausibility of different intermediates.

4. CONCLUSIONS The adsorption chemistry of NOx species on Cu-SSZ-13 catalysts is relevant to interpretation of spectroscopic characterizations and to understanding of likely reaction intermediates during NO oxidation, decomposition, and potentially SCR. Consistent with experimental observation, in this work we find that NOx species bind to isolated Cu in a variety of forms that either leave the oxidation state unchanged or oxidize Cu(I) to Cu(II). Both GGA and hybrid methods, based on either fully periodic or cluster-corrected methods, lead to qualitatively similar conclusions. However, the cluster-corrected and fully periodic hybrid-exchange calculations lead to equivalent conclusions. While the GGA computed structures are generally reliable, hybrid exchange is necessary to recover more accurate reaction energies involving NOx. The GGA generally tends to overestimate the hybrid-computed adsorption energies to extents that are species specific; the GGA in particular overestimates the tendency of Cu ions to coadsorb multiple NO or to simultaneously adsorb NO and O2. The presence of a proximal Brønsted site generally has little effect on computed results. The results highlight the importance of incorporating hybrid exchange into DFT-computed energies for this system.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07972. G


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■

Tables and figures provide additional information on coordinates, charge, and vibrational frequencies for the computed structures in the paper along with details on the thermodynamic phase diagram (PDF) CONTCARs and ADF logfile for the structures computed in the paper (ZIP)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

William F. Schneider: 0000-0003-0664-2138 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was provided by the U.S. Department of Energy (DoE) Vehicle Technology Program under Contract No. DE-EE0003977 and by the National Science Foundation GOALI program under award number 1258690-CBET. Computing resources were provided in part by the Notre Dame Center for Research Computing. TA thanks Drs. Florian Göltl and Kurt Frey for technical assistance.

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REFERENCES

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Periodic DFT Characterization of NOx Adsorption in Cu-Exchanged

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