Mechanism for Solid-State Ion Exchange of Cu+ ... - ACS Publications

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Mechanism for Solid-State Ion Exchange of Cu+ into Zeolites Lin Chen,*,†,‡ Jonas Jansson,§ Magnus Skoglundh,‡,∥ and Henrik Grönbeck*,†,‡ †

Department of Physics, ‡Competence Centre for Catalysis, and ∥Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-412 96 Göteborg, Sweden § Volvo Group Trucks Technology, SE-405 08 Göteborg, Sweden S Supporting Information *

ABSTRACT: Density functional theory calculations are used to investigate solid-state ion exchange of copper into zeolites. In particular, the energetic conditions for functionalization of chabazite (CHA) with copper ions from Cu2O(111) via formation of Cu(NH3)2+ are explored. It is found that the diamine complexes form easily on Cu2O(111) and diffuse with low barriers over the surface and in the CHA framework. The charge neutrality of the systems is maintained via counterdiffusion of H+ in the form of NH4+ from the zeolite to the Cu2O surface where water can be formed. The efficient solvation of Cu+ and H+ by ammonia renders the ion-exchange process exothermic. The present results highlight the dynamic character of the Cu ion sites and provide means to understand zeolite functionalization.

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al.15 studied the effect of copper loading on the NOx reduction activity and the catalysts were found to be active also at low copper loadings, which suggests that the energetically preferred positions for Cu ions are active in the reaction. Using firstprinciples calculations, Göltl and co-workers16 determined the distribution of Cu ions at different metal loadings. The stable positions for both CuI and CuII ions were found to be in the sixmembered ring. It was, moreover, suggested that the structural separation of the six-membered rings in the zeolite is an important property of SSZ-13 and one reason for the high catalytic efficiency.16 Recently, Paolucci et al.13 reported that the Cu sites respond sensitively to the operating conditions and that Cu species are mobilized by NH3 during SCR conditions. This finding implies that modification of the reaction conditions could be an alternative way to tune the catalytic activity of functionalized zeolites. Although direct (one-pot) synthesis of ion-exchanged zeolites is possible,17−19 the materials are generally synthesized via an aqueous route where metal salts are dissolved in water and mixed with the zeolite. This procedure has, however, several drawbacks as it is time-consuming and requires consecutive wash-and-dry steps to achieve the desired ionexchange level.10,20−22 Additionally, ion exchange into smallpore zeolites, including SSZ-13, may be hindered by bulky aqueous metal ion complexes3 which cannot enter the zeolite cages. An alternative route for ion exchange of zeolites is the, so-called, solid-state ion exchange (SSIE) method.23−25 Traditionally, the SSIE method has been used with high temper-

INTRODUCTION Single-site catalysis is a growing field within heterogeneous catalysis which may provide both high activity and selectivity.1 One route for stabilization of single sites is functionalization of zeolites by ion exchange, where counterions in the zeolite are replaced by, for example, copper and iron ions. Such catalysts are presently used for selective catalytic reduction (SCR) of nitrogen oxides (NOx) in automotive catalysis2−4 and hold promise for efficient partial oxidation of methane to methanol in one step.5−7 Zeolites are porous aluminosilicates that exist naturally or can be synthesized with a range of structures and pore sizes.8 The basic building block of the zeolite is a tetrahedral TO4 unit, where T is either silicon or aluminum. The difference in oxidation states between silicon and aluminum (Si4+ versus Al3+) requires one counterion per Al3+, for example H+, to maintain the charge neutrality. Functionalization of the zeolites generally includes exchange of the counterion with metal ions. SSZ-13 is a synthetic small pore zeolite with chabazite (CHA) structure9 that over the past years has emerged as an efficient catalyst for selective reduction of NOx with ammonia when ion exchanged with copper ions. This zeolite is composed of large and small cages with 36 and 12 tetrahedral sites, respectively. The large cages are constituted by four-, six-, and eight-membered rings, whereas the small cages are made up of four- and six-membered rings. The common Si/Al ratio in SSZ13 is between 5 and 20.10−13 As the activity and stability of the catalyst are determined by the location of the Cu ions in the microporous structure of the zeolite, considerable work has been directed to characterization of the active sites. Copper may exist as either CuI or CuII in the zeolite, and both oxidation states are involved in the NOx reduction cycle.13,14 Kwak et © XXXX American Chemical Society

Received: September 21, 2016 Revised: November 21, 2016 Published: November 30, 2016 A

DOI: 10.1021/acs.jpcc.6b09553 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Table 1. Lattice Parameters (a0), Bulk Modulus (B0), Band Gap (Ebg), and Formation Energy (ΔHf) for Cu2O Calculated with the PBE and HSE06 Functionals, Respectivelya PBE HSE06 theory exp51,52 a

a0 (Å)

B0 (GPa)

Ebg (eV)

ΔHf (eV)

4.31 4.30 4.32 (PBE)48 4.29 (HSE)50 4.27

111 112 104 (PBE)48 114 (HSE)50 112

0.26 2.18 0.64 (PBE)48 2.04 (HSE)50 2.17

−1.26 −1.60 −1.28 (PBE)49 −1.62 (HSE)49 −1.75

The results are compared with previous computational reports (theory) and experimental data (exp).

over the Brillouin zone is approximated by finite sampling using the Monkhorst−Pack44 scheme. The calculation of the Cu2O bulk is performed using a six-atom cell (Cu4O2) with 35 irreducible k-points. As SSZ-13 is an insulator and the unit cell is large (the side of the cell is 9.42 Å), calculations of the zeolite are performed using the Gamma point approximation. Cu2O(111) is calculated using either p(1 × 2) or p(2 × 2) surface cells. For the PBE calculations, the k-point samplings of the surface cells are (6 × 3 × 1) and (3 × 3 × 1), respectively. For the HSE06 calculations, a (3 × 1 × 1) k-point sampling is used for the p(1 × 2) surface. The number of layers required to describe the surface is found to be five O−Cu−O trilayers, as determined from the convergence of the surface energy within 0.01 J/m2. Repeated slabs are separated by 15 Å of vacuum. Structural optimization is performed with the two bottom layers fixed to the corresponding bulk structure. The geometries are considered to be converged when the energy differences are less than 1 × 10−5 eV and the largest force is smaller than 1 × 10−2 eV/Å. Gaussian smearing45 of the Fermi discontinuity is applied with a smearing width of 50 meV. Reaction barriers are calculated using the nudged elastic band (NEB)46,47 technique as implemented in transition state tools in VASP. The NEB calculations are performed with the PBE functional. However, the barriers are recalculated with HSE06 using the PBE structure. The applied method is evaluated by calculations of the relevant bulk systems. The results for Cu2O are collected in Table 1. The lattice constant, bulk modulus, and enthalpy of formation for bulk Cu2O are within PBE calculated to be 4.31 Å, 111 GPa, and −1.26 eV, respectively. The lattice constant and bulk modulus are in fair agreement with experimental values.51 However, the energy of formation is overestimated and clearly better described within HSE06. This has implications for the phase diagram as described in the Supporting Information. The environmental conditions for the stability of Cu2O are in good agreement with experiments only if the HSE06 functional is applied. The importance of the hybrid functional is, furthermore, crucial for a proper description of the band gap. The calculated band gap is 0.26 eV in PBE, whereas it is 2.18 eV in HSE06, which is close to the experimental value. For the calculations of SSZ-13, the experimental cell parameters for the silicalite in the chabazite structure are considered, i.e. a rhombohedral cell with a cell size and angle of 9.42 Å and 94°, respectively.53 The cell is fixed during the calculations, while the positions of the ions are optimized. The band gap of SSZ-13 is calculated to be 5.62 (6.95) eV with PBE (HSE06), which is in good agreement with a previous study54 reporting a direct band gap of 5.5 eV with PBE and 7.0−7.3 eV, with HSE03 and HSE06 for pure silica chabazite.

atures where a metal salt or an oxide is mixed with the zeolite powder and heated to 700−800 °C.26−29 Metal ions are formed during the heating and diffuse into the porous structure of the zeolite. Interestingly, it was recently demonstrated that SSZ-13 could be functionalized with copper at low temperatures in the presence of ammonia.14,20,30 In particular, facile ion exchange could be reached at 450 °C. This SSIE method is straightforward and could offer an easy way to control the metal loading in the zeolite and obtain samples with Cu/Al ratios up to one. Despite the successful use of the low-temperature SSIE method for synthesis of active and selective catalysts, the mechanisms governing the ion-exchange process are largely unknown. However, on the basis of Cu K-edge XANES spectroscopy it has been suggested31,32 that Cu+ diffuses in the form of a diamine complex Cu(NH3)2+, which furthermore has been measured33 to be a dominating Cu species during lowtemperature SCR. Importantly, the structure and function of copper-exchanged SSZ-13 appear to be the same for samples prepared with either the aqueous or the solid-state ion-exchange method.20,31 In the present work, we use density functional theory calculations to explore the solid-state ion exchange of copper into SSZ-13. We find that diamine complexes form easily over Cu2O(111) and that the diffusion in the zeolite proceeds with low barriers. The charge neutrality of the systems is maintained via counterdiffusion of H+ in the form of NH4+. The present results highlight the dynamic character of the Cu ion sites and provide means to understand zeolite functionalization.

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COMPUTATIONAL METHOD AND REFERENCE CALCULATIONS The calculations are performed within density functional theory (DFT) as implemented in the Vienna Ab-Initio Simulation Package (VASP).34−37 The Kohn−Sham orbitals are expanded on a plane wave basis, with an energy cutoff of 480 eV, and the interaction between the valence electrons and the core is described with the plane augmented wave (PAW) method.38,39 The number of electrons treated in the valence for H, N, O, Al, Si, and Cu is 1, 5, 6, 3, 4, and 11 electrons, respectively. Geometry optimizations are performed within the generalized gradient approximation according to Perdew−Burke−Ernzerhof (PBE).40 The electronic self-interaction error present in semilocal functionals makes accurate descriptions of oxides difficult. The energetics are, therefore, also evaluated with a hybrid functional using a fraction of Hartree−Fock exchange, namely HSE0641−43 after geometry optimization using the PBE functional. The computational expense of the hybrid functional makes complete structural relaxation at this level of theory demanding. The calculations of the bulk and surface systems are performed with periodic boundary conditions, and integration B


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The Journal of Physical Chemistry C The adsorption energy of an adsorbate (A) with zero point energy (ZPE) corrections on Cu2O(111) is calculated according to Eads = EA/Cu 2O − EA − ECu 2O + ΔZPE

(1)

where EA/Cu2O is the total energy of the adsorbate on the Cu2O(111) slab, EA is the total energy of the adsorbate in the gas phase, and EA/Cu2O is the total energy of the clean Cu2O(111) slab.

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RESULTS AND DISCUSSION In the present work, the energetics of Cu exchange into SSZ-13 from cuprous oxide in the presence of ammonia is investigated. Experimentally20 it has been shown that CuO is reduced to Cu2O in the presence of NO and NH3 during the SSIE process which proceeds via cuprous oxide (Cu2O) despite the fact that cupric oxide (CuO) is the stable phase during SCR conditions; see Supporting Information. Consequently we study the case where CuI ions are transported. Key steps in the exchange process are discussed below, namely (i) ammonia adsorption and formation of the Cu(NH3)2+ complex over Cu2O, (ii) diffusion of the complex and H+ in SSZ-13, and (iii) formation of bare CuI ions in the zeolite, and water on the Cu2O surface. NH3 Adsorption on Cu2O(111). We consider ammonia adsorption on Cu2O(111) as this is the cuprous oxide surface with the lowest surface energy.55 The nonpolar stoichiometric O-terminated Cu2O(111) has been the subject of several adsorption studies, including H256 and CO2.57 Side and top views of the stoichiometric Cu2O(111)(1 × 2) (henceforth denoted as (111)(1 × 2) surface) are shown in Figure 1a. Each trilayer consists of a copper layer sandwiched between two oxygen layers. This surface has two distinctly different copper atoms, namely, the coordinately unsaturated copper atom (Cucus) which is bonded to only one oxygen atom and the coordinately saturated copper atom (Cucs) which is bonded to two oxygen atoms. In agreement with the previous work by Bendavid and Carter, we find that the Cucus atoms are displaced with respect to the bulk termination.55 The original Cu−O bond length in Cu2O is 1.87 Å, and the Cucus−O bond length is calculated to be 1.92 Å. The elongation of the bond can be understood from the formation of a Cu−Cu bond with a distance of 2.40 Å. A Bader charge analysis58 reveals that the Cucus atoms are charged by +0.34 electron. This is clearly lower than the value of the Cucs atoms which have a charge of about +0.50 electron. For Cu2O in the bulk, the copper atoms are charged by +0.51 electron and the oxygen atoms by −1.02 electron. Following the previous work by Bendavid and Carter55 we have also considered the case where the Cucus atoms have been removed (Figure 1b). This surface, denoted (111)(1 × 2)-V, is thermodynamically preferred over a wide range of oxygen chemical potentials (see the Supporting Information). The effect on the structure to remove the Cucus atoms from the surface is small. However, the Bader charges of the coordinated saturated copper atoms in the first layer have increased to +0.62 electron. The influence on copper atoms in the subsurface layers is minor (within 0.05 electrons). Sequential ammonia adsorption and Cu(NH3)2+ formation is investigated on the two types of Cu2O(111) surfaces. Figure 1c−e shows the structures for adsorption on the stoichiometric Cu2O(111), whereas Figure 1f−h shows the corresponding results for the (111)(1 × 2)-V surface. The calculated

Figure 1. Side and top views of (a) stoichiometric Cu2O(111)(1 × 2) surface and (b) Cu2O(111)(1 × 2) surface with two Cu atom vacancies [denoted (111)(1 × 2)-V)]. The surface cell is indicated by solid white lines. Cucus and Cucs denote coordinatively unsaturated and coordinatively saturated copper ions, respectively. (c−e) Diamine complex formation over Cu2O(111): (c) adsorption of one ammonia, (b) adsorption of two ammonias, and (e) structure of Cu(NH3)2+ on Cu2O(111). (f−h) Diamine complex formation over Cu2O(111)-V: (f) adsorption of one ammonia, (g) adsorption of two ammonias, and (h) structure of Cu(NH3)2+ on Cu2O(111). Atomic color codes: copper (orange), oxygen (red), nitrogen (blue), and hydrogen (white). C


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endothermic with respect to the two adsorbed ammonia molecules. The Bader charge of the Cucs atom changes from +0.62 electron for the bare surface to +0.76 electron with one ammonia to +0.86 electron for two ammonias and +0.71 electron for the complex. Note that the trend in the charge variation of Cucs atom is same as for the case of the Cucus atom. The higher charges in the case of the (111)(1 × 2)-V surface are a consequence of that this surface is copper deficient. The coverage dependence of the ammonia adsorption and Cu(NH3)2+ formation was explored by considering a (2 × 2) surface cell (Table 2). These calculations were performed only with the PBE functional. Comparison with the results for the (1 × 2) surface cell shows that the adsorption energies only have a slight coverage dependence. Cu(NH3)2+ Adsorption in SSZ-13. In this work, we consider SSZ-13 with an Si/Al ratio of 11, which corresponds to substitution of one Si by Al per unit cell. The Al atoms are consequently regarded to be evenly distributed in the zeolite. The substitution of Si by Al in the zeolite framework requires a counterion for charge neutrality, and we have here considered a proton. By evaluating all different oxygen adsorption sites in the unit cell, we find that the proton preferably is located adjacent to the Al atom in a four-membered ring of the smallest cage; see Figure 2a,b. This is in agreement with previous work by

adsorption energies using either PBE or HSE06 are summarized in Table 2. Table 2. Zero-Point Corrected Adsorption Energies (eV) for One NH3, Two NH3 Molecules, and the Cu(NH3)2+ Adsorbed on the Stoichiometric Cu2O(111) [Denoted (111)] and the Surface with Cucus Vacancies (denoted (111)V)a PBE surface

adsorbate

HSE06 (1 × 2)

(1 × 2)

(2 × 2)

(111)

NH3 2NH3 barrier complex NH3 2NH3 barrier complex

−1.39 −0.97 0.40 −1.31 −0.26 −0.77 1.01 −0.06

−1.30 −0.97 0.34 −1.30 −0.22 −0.59 0.85 −0.10

−1.40 −1.19

(111)-V

−1.58 −0.26 −0.69 −0.18

Two surface cells, i.e., (1 × 2) and (2 × 2), are considered with the PBE functional, whereas only the (1 × 2) case was calculated with the HSE06 functional.

a

On the stoichiometric (111)(1 × 2) surface, NH3 adsorbs atop the Cucus atom. The adsorption removes the displacement of the Cucus atom, and the O−Cu−N angle is close to 180°. The O−Cu bond length is slightly reduced upon ammonia adsorption. Ammonia is adsorbed by −1.39 eV (HSE06), and the chemical bond is characterized by a mixing between the ammonia lone pair and the Cu 3dz2 orbital. The structure is rearranged upon adsorption of a second ammonia molecule. In this case, the lone pairs of the molecules hybridize with the Cu 3dxz orbital. The N−Cu−N angle is close to 90°, and the Cu− N bond distance increases with respect to the case of one ammonia molecule. The O−Cu distance is clearly elongated from 1.87 to 1.98 Å. The adsorption energy of an additional NH3 molecule is endothermic by 0.42 eV with respect to the situation with one ammonia adsorbed. The adsorption energies have a coverage dependence and the endothermicity is reduced in the p(2 × 2) cell, which was considered only with the PBE functional. Formation of the Cu(NH3)2+ complex is exothermic as compared with the situation of two adsorbed ammonia molecules, with a slight barrier of 0.40 eV. The complex is close to linear with N−Cu distances of about 1.90 Å. This structure is close to the gas-phase case where the complex is linear and the N−Cu distance is 1.91 Å. In the preferred adsorption site, Cu(NH3)2+ is coordinated to oxygen atoms via hydrogen bonds. The Bader charge of the Cucus atom changes from +0.53 electron with one adsorbed ammonia to +0.59 electron with two ammonias and +0.52 electron when the complex has formed; thus the changes are small. Ammonia adsorbs weaker on the (111)(1 × 2)-V surface, with an adsorption energy of −0.26 eV (HSE06). The Cu−N bond length is 2.20 Å, and the molecule is slightly tilted with respect to the surface normal. In contrast to the stoichiometric surface, adsorption of a second ammonia is clearly exothermic with the formation of a structure where the N−Cu−N axis is parallel to the surface. This structure is rationalized by the hybridization of the ammonia lone pairs and the Cu 3dxy orbital. The ammonia molecules hybridize with the lobes of one phase, whereas the oxygen atoms in the surface hybridize with the lobes of the other phase. The complex is in this case formed with a barrier of 1.01 eV, and the adsorbed complex is slightly

Figure 2. Different molecular species in SSZ-13: (a and b) two views of a proton, (c and d) two views of the complex, (e) NH4+, and (f) CuI ion. Atom color codes: copper (orange), oxygen (red), nitrogen (blue), hydrogen (white), silicon (yellow), and aluminum (purple). D


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The Journal of Physical Chemistry C Göltl et al.16 The difference in stability between the oxygen sites close to aluminum is within 0.1 eV, while sites further away are clearly less stable; see Supporting Information. Figure 2c,d shows the most favorable position for the complex in SSZ-13. The complex is located over the sixmembered ring with an O−Cu distance of 3.65 Å. The structure is consistent with a previous report by Giordanino et al.31 H atoms in the complex coordinate to oxygen atoms in the ring of the framework. The two Cu−N distances in the complex are the same, namely 1.90 Å, and the N−Cu−N bond angle is 176°. The close resemblance between the structure of the complex in the zeolite and that in the gas phase indicates that the interaction between the complex and the zeolite is of electrostatic character. The Bader charge of the complex in the zeolite is calculated to be +0.91 electron. Formation of CuI ions in the zeolite requires decomposition of the complex. We find that the bare Cu ion is bridging two oxygen atoms in the six-membered ring, in agreement with refs 16, 59, and 60. This position is clearly preferred with respect to other configurations in the zeolite; see the Supporting Information. However, it should be noticed that direct complex decomposition is strongly endothermic. The structure in Figure 2f together with two ammonias in the gas phase is 2.68 eV (HSE06) higher in energy than having the complex in the zeolite, which is in good agreement with previous calculated results.13,61 This is important as it demonstrates that Cu preferably is solvated by adsorbates and that direct bonds to the zeolite framework cannot compensate for the loss of Cu−NH3 bonds. The formation of unsolvated Cu ions could instead be formed through thermodynamic conditions that favor gas-phase ammonia or strongly exothermic reactions that involve ammonia. The presence of NH3 in the zeolite also has consequences for the charge compensating protons. We find that ammonium (NH4+) is formed without any barrier when NH3 is allowed to structurally relax close to the proton in SSZ-13. Ammonium is preferably adsorbed close to the Al site; see Figure 2e. We also considered hydronium (H3O+) as an alternative proton carrier. However, hydronium is found to decompose without barrier into H2O and H+ in SSZ-13 upon relaxation. Consequently, water facilitated proton transfer would require a high water concentration. Energy Landscape of SSIE. The energy landscapes of the SSIE process from the two cuprous oxide surfaces are shown in Figure 3. As already described, the Cu(NH3)2+ complex may form on both surfaces. The process is exothermic on the stoichiometric surface with a barrier of 0.40 eV (HSE06), whereas the barrier is 1.01 eV on the (111)(1 × 2)-V surface. The step from 3 to 4 in Figure 3 describes the ion-exchange process, thus the step where the complex diffuses to the zeolite and the proton diffuses to the cuprous oxide surface forming an OH group. The SSIE process is exothermic by −1.05 eV for (111)(1 × 2) and by −1.94 eV for (111)(1 × 2)-V. The considerably higher energy gain for the surface with vacancies is rationalized by a stronger (1.27 eV) O−H bond on this surface. We note that the (111)(1 × 2)-V surface is the stable cuprous oxide surface within a wide range of oxygen chemical potentials. Thus, the formation of the complex over this surface can be expected to be an upper bound of the barrier and energy gain. The exchange of the proton from the zeolite to the oxide surface retains a surface where the proton takes the role of Cu+. The exchange process is further stabilized by the formation of gas phase water. The ammonium ion dissociates exothermically

Figure 3. Zero-point corrected energies obtained with the HSE06 functional for the SSIE process over the two cuprous oxide surfaces with the p(1 × 2) surface cell. (a) Stoichiometric Cu2O(111) surface and (b) Cu2O(111) surface with two Cucus vacancies. The energy is reported with respect to two NH3 in the gas phase and the cuprous oxide surface together with the zeolite with one proton. The four different adsorption configurations correspond to 1, NH3/Cu2O; 2, 2NH3/Cu2O; 3, Cu(NH3)2+/Cu2O; and 4, Cu(NH3)2+@SSZ-13 and H/Cu2O(111). The barrier is calculated to form the complex on the cuprous oxide surface (2 → 3). Atom color codes as in Figures 1 and 2. The dashed circle indicates the copper vacancy.

over the cuprous oxide surfaces forming gas phase ammonia and an OH group. Water could be formed upon dissociation of two ammonium ions. The PBE values for stabilization of the exchange process via water formation are −2.20 and −3.14 eV for the (111)(1 × 2) and (111)(1 × 2)-V surfaces, respectively. The calculated ion-exchange energies are given in Table 3 using both PBE and HSE06. In order to investigate the Table 3. Copper Ion Exchange Energy with and without NH3 As Calculated with the PBE and HSE06 Functionals, Respectively ΔE (eV) SSIE strategy Cu+ ⇌ H+ Cu(NH3)2+ ⇌ H+

E

surface (111)(1 (111)(1 (111)(1 (111)(1

× × × ×

2) 2)-V 2) 2)-V

PBE

HSE06

0.39 0.69 −0.87 −1.78

0.09 0.46 −1.04 −1.93


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The Journal of Physical Chemistry C importance of ammonia for the process, also the case without ammonia is considered. Exchange by the formation of the complex is exothermic within both functionals with only a minor difference in the predicted energy gain. Interestingly, the exchange process is clearly endothermic in the absence of ammonia. This is in agreement with experiments which show that SSIE is facile at low temperatures with ammonia, whereas higher temperatures are required for the exchange process in inert atmosphere.20,31,32 We have calculated the exchange energy also for *BEA and MFI, and obtained similar exchange energies as compared to SSZ-13. In particular, the exchange energy with the diamine complex from the (111)(1 × 2) surface is calculated to be −0.72 and −0.78 eV for *BEA and MFI, respectively. Thus, it appears that the process outlined here for SSZ-13 could be of a general nature. It is generally assumed that isolated copper ions are the active site for selective catalytic reduction of NOx in copperexchanged zeolites.14,59,62 We find that the energy penalty to remove the two ammonia molecules from the complex and to form a CuI site is highly endothermic. Thus, it can be expected that isolated CuI sites cannot be formed in an atmosphere of only ammonia. However, it may be possible that additional reactions could facilitate the stabilization of such species. One option could be the standard ammonia SCR reaction: 4NH3 + 4NO + O2 → 4N2 + 6H 2O

(2)

Figure 4. Diffusion barrier across an eight-membered ring in SSZ-13 for (a) Cu(NH3)2+ and (b) ammonium. In (b), the bottom left structure shows the initial configuration. The TS1 structure is the transition state at the saddle point for the first barrier. The bottom right structure is the configuration for STRUC1, which is halfway through the eight-membered ring.

Nevertheless, the high stability of the complex indicates that such species should be important for reaction mechanisms in zeolites. In fact, the NH3-SCR reaction in SSZ-13 has been proposed to include Cu(NH3)2+.31 In fact, we find that the Cu(NH3)2+ complex is formed without barrier when two NH3 molecules adsorb on the bare CuI site in SSZ-13. The formation of the complex in the zeolite is exothermic by −2.68 eV. Diffusion Barriers in SSZ-13. Knowing that the SSIE process is facile in the presence of ammonia, it becomes important to investigate possible diffusion limitations both for the Cu(NH3)2+ complex and for ammonium in SSZ-13. Chabazite is composed of large cages with four-, six-, and eight-membered rings together with small cages with four- and six-membered rings. The diameter of the four-membered ring is too small to allow for molecular diffusion. The diameter of the six-membered ring is 5.26 Å, which is also too small for efficient molecular diffusion. Thus, diffusion between the large cages should proceed via the eight-membered ring. We consequently investigate the diffusion barrier for the complex passing through the eight-membered window; see Figure 4a. The insets show the initial, final, and transition states. The transition state calculation shows that the diffusion barrier for the complex passing through the eight-membered ring is only 0.29 eV, which is consistent with a previously reported value.13 The transition state is with the Cu ion roughly in the plane of the zeolite ring. The barrier originates mainly from the absence of hydrogen bonds in the transition state. In both the initial and final states, the complex is coordinated to the framework by hydrogen bonds. Also the diffusion of NH4+ in SSZ-13 proceeds with low barriers that arise from breaking hydrogen bonds. The diffusion can be characterized by three steps. Ammonium is initially adsorbed in an eight-membered ring, coordinated via a hydrogen bond to an oxygen atom adjacent to Al3+ in a fourmembered ring. In the first step, ammonium diffuses from this eight-membered ring to the neighboring eight-membered ring

with a barrier of 0.35 eV. Diffusion through this ring has a similar barrier. This is also the case for the third step, where ammonium again diffuses between two eight-membered rings. These results show that ammonium easily can facilitate proton transport in the zeolite.

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CONCLUSIONS We have used density functional theory calculations to investigate the energetic conditions for the SSIE of Cu into SSZ-13, starting from Cu2O(111). Adsorption of NH3 on Cu2O(111) leads to the formation of a linear Cu(NH3)2+ complex. The formation of the complex can occur both on a stoichiometric surface and on a surface with Cu vacancies. The ion exchange of the diamine complex into the SSZ-13 is found to be exothermic, whereas the exchange process is endothermic in the absence of NH3. We find that the complex can diffuse easily in SSZ-13 with a slight barrier when passing through the eight-membered ring. The charge neutrality of the systems is maintained via exchange of H+ from the zeolite to the Cu2O surface, where water can be formed. We suggest that the protons are transported from the zeolite in the form of NH4+. The high stability of the complex together with the dynamic character of this species in the zeolite could have important consequences for the understanding of both the SCR mechanism and possible deactivation routes. Our calculations show that the Cu ion preferably is solvated with ammonia which supports the assumption13 that the complex could be a part of the SCR mechanism. Moreover, as Cu(NH 3) 2+ spontaneously is formed from a bare CuI ion in an ammonia atmosphere, it is likely that the Cu ion distribution changes F


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during reaction conditions and that the diamine complex also could be a key component in catalyst deactivation mechanisms.

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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.6b09553. Theoretical phase diagram of Cu in the presence of oxygen, surface stabilities, and stability diagrams of H+ and Cu+ in SSZ-13 (PDF) Structure descriptions (ZIP)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Lin Chen: 0000-0002-7905-9587 Henrik Grönbeck: 0000-0002-8709-2889 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support is acknowledged from the Chalmers Area Advance Transport and the Competence Centre for Catalysis (KCK) at Chalmers University of Technology. KCK is financially supported by the Swedish Energy Agency and the member companies AB Volvo, ECAPS AB, Haldor Topsøe A/ S, Scania CV AB, Volvo Car Corp. AB, and Wärtsilä Finland Oy. The calculations were performed at C3SE (Göteborg) and PDC (Stockholm) via a SNIC grant.

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REFERENCES

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