B Kerkeni

D Berthout

D Berthomieu

D E Doronkin
email: dmitry.doronkin@kit.edu

M Casapu

J.-D Grunwaldt

C Chizallet
email: celine.chizallet@ifpen.fr

Copper Coordination to Water and Ammonia in Cu II -Exchanged SSZ-13: Atomistic Insights from DFT Calculations and In Situ XAS Experiments

In this study the coordination sphere of copper in Cu-SSZ-13 as catalyst for the selective catalytic reduction (SCR) of NO x by ammonia is analyzed as a function of the environmental parameters: temperature, partial pressure of water P(H 2 O) and partial pressure of ammonia P(NH 3 ). By periodic Density Functional Theory (DFT) calculations, we obtain stability domains for variable loadings of water and ammonia (nH 2 O+mNH 3 with (m+n ≤ 6)) close to Cu II ions, which are located at 6MR or 8MR of the zeolitic structure. Ab initio calculations and thermodynamic investigations were performed to build phase diagrams, with vibrational analysis, so as to provide Gibbs free energy G values. Copper located in the 8MR appears to be more reactive towards H 2 O and NH 3 adsorption than the one in the 6MR, due to a lower coordination number of copper at 8MR in the absence of adsorbates. Depending on the operating conditions, structures containing adsorbed water and ammonia as ligands at the metal site can simultaneously be stabilized. The most widespread coordination number of Cu II is 4 even at m+n>4. The theoretical predictions were validated by in situ XAS, in two gas atmospheres: dry He with P(NH 3 )=10 -3 bar (1000 ppm), as well as He with P(NH 3 )=10 -3 bar and P(H 2 O)=10 -2 bar. Trends in terms of ammonia desorption temperature as well as coordination numbers are well reproduced. Experimentally determined behaviors of Cu I and Cu II open new perspectives for the systematic computational investigation of the behavior of Cu I in H 2 O/NH 3 atmosphere.

INTRODUCTION

The emission of nitrogen oxides (NOx) into the earth's atmosphere, produced through the combustion of fossil fuels, is a major concern for today's society. [START_REF] Von Schneidemesser | Chemistry and the Linkages between Air Quality and Climate Change[END_REF] As a result, stricter legislation requires removal of most of NOx emitted from artificial sources. Diesel engine after-treatment systems for passenger cars have to face several challenges regarding the level of emitted nitrogen oxides (NOx) required for the current and upcoming emission regulations (California LEV III, US EPA Tier 3, Euro 6c/6d and Real Driving Emissions legislation) as well as too high NOx concentration levels in certain urban areas. This has led researchers to design and develop new catalysts with a wide temperature range of activity and high selectivity towards N 2 . In particular, Selective Catalytic Reduction with ammonia (NH 3 -SCR) has emerged as an efficient technology to remove environmentally harmful nitrogen oxides (NOx, x=1,2) from oxygen-rich exhausts, typical of diesel engines, to levels required by emission regulations. [START_REF] Brandenberger | The State of the Art in Selective Catalytic Reduction of NOx by Ammonia Using Metal-Exchanged Zeolite Catalysts[END_REF][START_REF] Deka | Local Environment and Nature of Cu Active Sites in Zeolite-Based Catalysts for the Selective Catalytic Reduction of NOx[END_REF][START_REF] Beale | Recent Advances in Automotive Catalysis for NOx Emission Control by Small-Pore Microporous Materials[END_REF] NH 3 involved in the SCR process is typically generated via decomposition of aqueous urea solution, and produces N 2 and H 2 O upon reaction with NO x .

Copper-exchanged zeolites and zeotypes are widely used catalysts for NOx removal from mobile sources via NH 3 -SCR, and have been studied in numerous works. [START_REF] Brandenberger | The State of the Art in Selective Catalytic Reduction of NOx by Ammonia Using Metal-Exchanged Zeolite Catalysts[END_REF][START_REF] Deka | Local Environment and Nature of Cu Active Sites in Zeolite-Based Catalysts for the Selective Catalytic Reduction of NOx[END_REF][START_REF] Beale | Recent Advances in Automotive Catalysis for NOx Emission Control by Small-Pore Microporous Materials[END_REF][START_REF] Doronkin | Operando Spatially-and Time-Resolved XAS Study on Zeolite Catalysts for Selective Catalytic Reduction of NOx by NH 3[END_REF][START_REF] Ganemi | Conversion and In Situ FTIR Studies of Direct NO Decomposition over Cu-ZSM5[END_REF][START_REF] Gao | Current Understanding of Cu-Exchanged Chabazite Molecular Sieves for Use as Commercial Diesel Engine DeNOx Catalysts[END_REF][START_REF] Iwasaki | In Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts[END_REF][START_REF] Liu | Recent Advances in Catalytic DeNOx Science and Technology[END_REF][START_REF] Palomino | Oxidation States of Copper Ions in ZSM-5 Zeolites. A Multitechnique Investigation[END_REF][START_REF] Schoonheydt | UV-vis-NIR Spectroscopy and Microscopy of Heterogeneous Catalysts[END_REF][START_REF] Wang | In Situ-DRIFTS Study of Selective Catalytic Reduction of NOx by NH 3 over Cu-Exchanged SAPO-34[END_REF][START_REF] Yu | Recent NH 3 -SCR Mechanism Research over Cu/SAPO-34 Catalyst[END_REF][START_REF] Kwak | Effects of Hydrothermal Aging on NH 3 -SCR Reaction over Cu/Zeolites[END_REF][START_REF] Wang | Recent Advances in the Selective Catalytic Reduction of NOx with NH 3 on Cu-Chabazite Catalysts[END_REF][START_REF] Borfecchia | Revisiting the Nature of Cu Sites in the Activated Cu-SSZ-13 Catalyst for SCR Reaction[END_REF][START_REF] Paolucci | Dynamic Multinuclear Sites Formed by Mobilized Copper Ions in NOx Selective Catalytic Reduction[END_REF][START_REF] Andersen | Location of Cu 2+ in CHA Zeolite Investigated by X-ray Diffraction Using the Rietveld/Maximum Entropy Method[END_REF][START_REF] Godiksen | Coordination Environment of Copper Sites in Cu-CHA Zeolite Investigated by Electron Paramagnetic Resonance[END_REF][START_REF] Lomachenko | The Cu-CHA deNOx Catalyst in Action: Temperature-Dependent NH 3 -Assisted Selective Catalytic Reduction Monitored by Operando XAS and XES[END_REF] Cu-SSZ-13 catalysts (framework type CHA) have been found to be one of the most active and selective materials in NH 3 -SCR, [START_REF] Beale | Recent Advances in Automotive Catalysis for NOx Emission Control by Small-Pore Microporous Materials[END_REF][START_REF] Kwak | Excellent Activity and Selectivity of Cu-SSZ-13 in the Selective Catalytic Reduction of NOx with NH 3[END_REF][START_REF] Moliner | Synthesis Strategies for Preparing Useful Small Pore Zeolites and Zeotypes for Gas Separations and Catalysis[END_REF] less prone to deactivation by hydrocarbon inhibition or thermal degradation than other metal-exchanged zeolites. [START_REF] Deka | Local Environment and Nature of Cu Active Sites in Zeolite-Based Catalysts for the Selective Catalytic Reduction of NOx[END_REF][START_REF] Kwak | Effects of Hydrothermal Aging on NH 3 -SCR Reaction over Cu/Zeolites[END_REF] They show exceptional activity for the NH 3 -SCR in the emission control systems of diesel vehicles, and are now in commercial use. [START_REF] Zhang | NO Chemisorption on Cu/SSZ-13: A Comparative Study from Infrared Spectroscopy and DFT Calculations[END_REF] The CHA framework contains interconnected 6-, and 8-membered rings (6MR, 8MR) and relatively small pore radius (∼3.8 Å) in an 8MR. 24 Depending on the preparation conditions, [START_REF] Wang | Recent Advances in the Selective Catalytic Reduction of NOx with NH 3 on Cu-Chabazite Catalysts[END_REF] Cu-SSZ-13 exhibits different catalytic activity. Typical Si/Al~10 and Cu/Al~0.3 [START_REF] Luo | New Insights into Cu/SSZ-13 SCR Catalyst Acidity. Part I: Nature of Acidic Sites Probed by NH 3 Titration[END_REF] are reported for Cu-SSZ-13 materials used in the NH 3 -SCR, both to ensure hydrothermal stability and to reduce copper species clustering. [START_REF] Deka | Local Environment and Nature of Cu Active Sites in Zeolite-Based Catalysts for the Selective Catalytic Reduction of NOx[END_REF] According to the literature, the valence state of copper active sites under ambient conditions is +II. [START_REF] Wang | Recent Advances in the Selective Catalytic Reduction of NOx with NH 3 on Cu-Chabazite Catalysts[END_REF] These sites were reported to be present as isolated Cu II and Cu II -OH (depending on the hydration conditions) [START_REF] Wang | Recent Advances in the Selective Catalytic Reduction of NOx with NH 3 on Cu-Chabazite Catalysts[END_REF][START_REF] Bendrich | Unified Mechanistic Model for Standard SCR, Fast SCR, and NO 2 SCR over a Copper Chabazite Catalyst[END_REF] with a preferential location in the plane of the 6MR, according to several spectroscopic studies employing X-ray diffraction (XRD), X-ray absorption (XAS), and UV-Vis spectroscopy. [START_REF] Deka | Local Environment and Nature of Cu Active Sites in Zeolite-Based Catalysts for the Selective Catalytic Reduction of NOx[END_REF][START_REF] Fickel | Copper Coordination in Cu-SSZ-13 and Cu-SSZ-16 Investigated by Variable-Temperature XRD[END_REF][START_REF] Gao | Structure-Activity Relationships in NH 3 -SCR over Cu-SSZ-13 as Probed by Reaction Kinetics and EPR Studies[END_REF][START_REF] Korhonen | Isolated Cu 2+ Ions: Active Sites for Selective Catalytic Reduction of NO[END_REF][START_REF] Kwak | Two Different Cationic Positions in Cu-SSZ-13?[END_REF][START_REF] Deka | Confirmation of Isolated Cu 2+ Ions in SSZ-13 Zeolite as Active Sites in NH 3 -Selective Catalytic Reduction[END_REF] However, X-ray based techniques and Density Functional Theory (DFT) calculations provide contrasted results, with copper located not only in 6MR [START_REF] Fickel | Copper Coordination in Cu-SSZ-13 and Cu-SSZ-16 Investigated by Variable-Temperature XRD[END_REF] but also in 8MR units of the SSZ-13 matrix. [START_REF] Borfecchia | Revisiting the Nature of Cu Sites in the Activated Cu-SSZ-13 Catalyst for SCR Reaction[END_REF][START_REF] Andersen | Location of Cu 2+ in CHA Zeolite Investigated by X-ray Diffraction Using the Rietveld/Maximum Entropy Method[END_REF][START_REF] Godiksen | Coordination Environment of Copper Sites in Cu-CHA Zeolite Investigated by Electron Paramagnetic Resonance[END_REF][START_REF] Lomachenko | The Cu-CHA deNOx Catalyst in Action: Temperature-Dependent NH 3 -Assisted Selective Catalytic Reduction Monitored by Operando XAS and XES[END_REF] Finally, it was proposed that monomeric and / or dimeric Cu II are the active species in the SCR of NOx with ammonia on Cu-SSZ-13 catalysts. [START_REF] Beale | Recent Advances in Automotive Catalysis for NOx Emission Control by Small-Pore Microporous Materials[END_REF][START_REF] Gao | Understanding Ammonia Selective Catalytic Reduction Kinetics over Cu/SSZ-13 from Motion of the Cu Ions[END_REF] It was recently demonstrated that the formation of dimeric Cu II sites result from copper mobility occurring during the SCR with the ammonia addition process. [START_REF] Paolucci | Dynamic Multinuclear Sites Formed by Mobilized Copper Ions in NOx Selective Catalytic Reduction[END_REF] From DFT calculations using cluster models and periodic models, mobility of copper in its reduced Cu I exchanged zeolite valence state in the presence of NH 3 or H 2 O is predicted. [START_REF] Paolucci | Dynamic Multinuclear Sites Formed by Mobilized Copper Ions in NOx Selective Catalytic Reduction[END_REF][START_REF] Berthomieu | Molecular Dynamics Simulations of H 2 O with Sites of Cu I -FAU and Cu II -FAU[END_REF] Despite significant efforts in the past decade, in particular in terms of operando catalysts characterization, [START_REF] Deka | Local Environment and Nature of Cu Active Sites in Zeolite-Based Catalysts for the Selective Catalytic Reduction of NOx[END_REF][START_REF] Doronkin | Operando Spatially-and Time-Resolved XAS Study on Zeolite Catalysts for Selective Catalytic Reduction of NOx by NH 3[END_REF][START_REF] Borfecchia | Revisiting the Nature of Cu Sites in the Activated Cu-SSZ-13 Catalyst for SCR Reaction[END_REF][START_REF] Paolucci | Dynamic Multinuclear Sites Formed by Mobilized Copper Ions in NOx Selective Catalytic Reduction[END_REF][START_REF] Godiksen | Coordination Environment of Copper Sites in Cu-CHA Zeolite Investigated by Electron Paramagnetic Resonance[END_REF][START_REF] Lomachenko | The Cu-CHA deNOx Catalyst in Action: Temperature-Dependent NH 3 -Assisted Selective Catalytic Reduction Monitored by Operando XAS and XES[END_REF][START_REF] Paolucci | Isolation of the Copper Redox Steps in the Standard Selective Catalytic Reduction on Cu-SSZ-13[END_REF][START_REF] Gunter | Structural Snapshots of the SCR Reaction Mechanism on Cu-SSZ-13[END_REF] the relationships between zeolite composition, reaction conditions and active site location, as well as some aspects of the proposed SCR reaction mechanisms remain to be elucidated. [START_REF] Deka | Local Environment and Nature of Cu Active Sites in Zeolite-Based Catalysts for the Selective Catalytic Reduction of NOx[END_REF] The first aspect to be unraveled in this context is the structure of copper during SCR reactions. Ammonia as well as water are present in the SCR gas mixture at various partial pressures. The oxidative (due to the presence of water and O 2 ) or reductive (due to the presence of ammonia or hydrocarbons) nature of the atmosphere, depending on the operating conditions, also influences the oxidation degree of copper (+I or +II). [START_REF] Doronkin | Operando Spatially-and Time-Resolved XAS Study on Zeolite Catalysts for Selective Catalytic Reduction of NOx by NH 3[END_REF][START_REF] Giordanino | Interaction of NH 3 with Cu-SSZ-13 Catalyst: A Complementary FTIR, XANES, and XES Study[END_REF][START_REF] Mcewen | Integrated Operando X-Ray Absorption and DFT Characterization of Cu-SSZ-13 Exchange Sites during the Selective Catalytic Reduction of NOx with NH 3[END_REF] Since the late 90's (first with small clusters, then with periodic models), DFT based calculations were brought to determine copper active site structures in metal cation-exchanged zeolites, and recently for copper-exchanged chabazite. [START_REF] Beale | Recent Advances in Automotive Catalysis for NOx Emission Control by Small-Pore Microporous Materials[END_REF][START_REF] Borfecchia | Revisiting the Nature of Cu Sites in the Activated Cu-SSZ-13 Catalyst for SCR Reaction[END_REF][START_REF] Paolucci | Dynamic Multinuclear Sites Formed by Mobilized Copper Ions in NOx Selective Catalytic Reduction[END_REF][START_REF] Zhang | NO Chemisorption on Cu/SSZ-13: A Comparative Study from Infrared Spectroscopy and DFT Calculations[END_REF][START_REF] Paolucci | Isolation of the Copper Redox Steps in the Standard Selective Catalytic Reduction on Cu-SSZ-13[END_REF][START_REF] Schneider | Density Functional Theory Study of Transformations of Nitrogen Oxides Catalyzed by Cu-Exchanged Zeolites[END_REF][START_REF] Mao | Theoretical Investigation of NH 3 -SCR Processes over Zeolites: A Review[END_REF][START_REF] Crandell | Computational and Spectroscopic Characterization of Key Intermediates of the Selective Catalytic Reduction Cycle of NO on Zeolite-Supported Cu Catalyst[END_REF][START_REF] Berthomieu | Recent Advances in Cu I/II Y: Experiments and Modeling[END_REF][START_REF] Bates | Identification of the Active Cu site in Standard Selective Catalytic Reduction with Ammonia on Cu-SSZ-13[END_REF][START_REF] Paolucci | Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites[END_REF][START_REF] Göltl | What Makes Copper-Exchanged SSZ-13 Zeolite Efficient at Cleaning Car Exhaust Gases?[END_REF][START_REF] Göltl | The Impact of Finite Temperature on the Coordination of Cu Cations in the Zeolite SSZ-13[END_REF][START_REF] Göltl | Developing a Thermodynamic Model for the Interactions between Water and Cu in the Zeolite SSZ-13[END_REF][START_REF] Janssens | A Consistent Reaction Scheme for the Selective Catalytic Reduction of Nitrogen Oxides with Ammonia[END_REF][START_REF] Siahrostami | Exploring Scaling Relations for Chemisorption Energies on Transition-Metal-Exchanged Zeolites ZSM-22 and ZSM-5[END_REF][START_REF] Lezcano-Gonzalez | Determining the Storage, Availability and Reactivity of NH 3 within Cu-Chabazite-Based Ammonia Selective Catalytic Reduction Systems[END_REF][START_REF] Moreno-González | Ammonia-Containing Species Formed in Cu-Chabazite As Per In Situ EPR, Solid-State NMR, and DFT Calculations[END_REF][START_REF] Concepción | Identification of Distinct Copper Species in Cu-CHA Samples Using NO as Probe Molecule. A Combined IR Spectroscopic and DFT Study[END_REF] A large set of studies concluded (or assumed) the better stability of Cu II located at the 6MR, with respect to 8MR, in the absence of adsorbates. [START_REF] Paolucci | Isolation of the Copper Redox Steps in the Standard Selective Catalytic Reduction on Cu-SSZ-13[END_REF][START_REF] Bates | Identification of the Active Cu site in Standard Selective Catalytic Reduction with Ammonia on Cu-SSZ-13[END_REF][START_REF] Paolucci | Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites[END_REF][START_REF] Göltl | What Makes Copper-Exchanged SSZ-13 Zeolite Efficient at Cleaning Car Exhaust Gases?[END_REF] X-ray absorption spectroscopy (XAS) is ideally suited to study transition metal based catalysts under realistic or operating conditions (gas composition, temperature, pressure etc.) due to high penetrating power of X-rays which allows using sophisticated in situ cells. [START_REF] Grunwaldt | X-Ray Absorption Spectroscopy under Reaction Conditions: Suitability of Different Reaction Cells for Combined Catalyst Characterization and Time-Resolved Studies[END_REF] X-ray absorption near edge structure (XANES) is sensitive to oxidation state and coordination environment (coordination numbers, nature of neighboring atoms, and coordination geometry), while extended X-ray absorption fine structure (EXAFS) allows assigning coordination number and distance to nearest neighbor atoms as well as approximating their atomic numbers. XAS, unlike X-ray diffraction, can be applied to study systems without longrange order and is one of the techniques of choice to characterize Cu sites in zeolite catalysts.

Previous studies have shown that Cu species in Cu-SSZ-13 zeolites are very mobile and can be located at 8MR, 32 6MR 36 or both at the same time, [START_REF] Borfecchia | Revisiting the Nature of Cu Sites in the Activated Cu-SSZ-13 Catalyst for SCR Reaction[END_REF] while coordination numbers vary between 5 and 2 depending on temperature and gas atmosphere [START_REF] Giordanino | Interaction of NH 3 with Cu-SSZ-13 Catalyst: A Complementary FTIR, XANES, and XES Study[END_REF][START_REF] Doronkin | Lithographically Fabricated Silicon Microreactor for Operando QEXAFS Studies in Exhaust Gas Catalysis during Simulation of a Standard Driving Cycle[END_REF] with the most strongly coordinating ligands being NH 3 and H 2 O. [START_REF] Gunter | Structural Snapshots of the SCR Reaction Mechanism on Cu-SSZ-13[END_REF] Efficient application of Cu-SSZ-13 catalysts for NOx removal from mobile sources requires precise NH 3 dosage which, in turn, is possible only if the amount of NH 3 interacting with Cu sites is known. Previously, XANES spectroscopy demonstrated rapid changes in the coordination sphere of Cu-zeolite automotive catalysts during driving cycles [START_REF] Doronkin | Lithographically Fabricated Silicon Microreactor for Operando QEXAFS Studies in Exhaust Gas Catalysis during Simulation of a Standard Driving Cycle[END_REF] but the comprehensive mapping of the structure of Cu-sites under realistic conditions has not been performed so far.

The aim of the present study is to investigate the possible adsorption sites, to identify the most stable structures during experimental conditions and to quantify the capacity of Cu centers to adsorb variable loads of gaseous water and ammonia (nH 2 O+mNH 3 with (m+n≤ 6)). The present study considers the isolated Cu II ions exchanged-SSZ-13 as the active site. In our computational modeling we included the two principal locations 6MR and 8MR for the Cu ions in the CHA framework in order to determine coordination numbers in combination with stability domains. The predictions are benchmarked against results obtained by in situ XAS performed on a Cu-SSZ-13 catalyst under reaction conditions relevant for the real-world applications and comparable to the ones studied by DFT. We consider these investigations as a first step to be tackled prior to deriving catalytic reaction mechanisms.

Experimental and methods

DFT calculations

Periodic Density Functional Theory (DFT) calculations were performed using a planewave method as implemented in the Vienna Ab initio Simulation Package (version 5.4.1). [START_REF] Kresse | Ab Initio Molecular-Dynamics Simulation of the Liquid-Metal-Amorphous-Semiconductor Transition in Germanium[END_REF][START_REF] Kresse | Efficiency of Ab-Initio Total Energy Calculations for Metals and Semiconductors using a Plane-Wave Basis Set[END_REF] The exchange-correlation functional was treated within the generalized gradient approximation (GGA) parameterized by Perdew, Burke and Ernzerhof [START_REF] Perdew | Generalized Gradient Approximation Made Simple[END_REF] (PBE) and the electron-ion interaction was described by the projector augmented wave (PAW) scheme. [START_REF] Kresse | From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method[END_REF] Van der Waals corrections as proposed within the Grimme formalism (D2) were applied. [START_REF] Grimme | Semiempirical GGA-Type Density Functional Constructed with a Long-Range Dispersion Correction[END_REF] We also carried out test calculations with the opt-PBE [START_REF] Klimes | Chemical Accuracy for the van der Waals Density Functional[END_REF] non-local exchange-correlation functional, with deviation on adsorption energies between 5 and 20 kJ•mol -1 with respect to PBE-D2. Note also that Göltl et al. [START_REF] Göltl | Developing a Thermodynamic Model for the Interactions between Water and Cu in the Zeolite SSZ-13[END_REF] showed that for water adsorption issues, the choice of PBE versus a hybrid functional (HSE06 in their case) has no significant effect on the thermodynamic diagrams, whereas the introduction of dispersion corrections has a significant effect. For these reasons, we chose to perform routine calculations at the PBE-D2 level.

The energy cutoff was set to 800 eV for the initial lattice parameters optimizations, then to 400 eV for all other investigations. A 1×2×2 k-point mesh was employed for the double rhombohedral cell modeled (see below). Gaussian smearing with σ = 0.02 eV was used. Spinpolarized calculations were performed. The criterion for the convergence of the self-consistent cycles was set to 10 -5 eV. Bader charges [START_REF] Sanville | Improved Grid-Based Algorithm for Bader Charge Allocation[END_REF][START_REF] Henkelman | A Fast and Robust Algorithm for Bader Decomposition of Charge Density[END_REF] were calculated at the same level of theory. Geometry optimizations were run until forces on relaxed atoms were lower than 2.10 -2 eV•Å -1 .

The initial cell and atomic positions were taken from the International Zeolite Association database (CHA framework). [START_REF] Baerlocher | Database of Zeolite Structures[END_REF] We constructed a double cell (with respect to the primitive one, containing 73 atoms) by doubling the a cell parameter, so as to keep a rather high Si/Al = 11 while exchanging two aluminum atoms per double cell, and adding one copper atom per double cell (Cu/Al = 0.5), so that copper is initially in +2 oxidation state. Cu II species can also be modeled by a cell where only aluminum per copper exists, but compensating the charge by an extra HO -ion coordinated to copper (Cu II -OH species). The latter species were characterized by Borfecchia et al. [START_REF] Borfecchia | Revisiting the Nature of Cu Sites in the Activated Cu-SSZ-13 Catalyst for SCR Reaction[END_REF] For Si/Al and Cu/Al such as the one involved in the experimental part of our study (see section 2.2.), equal amounts of Cu II and Cu II -OH are expected. [START_REF] Paolucci | Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites[END_REF] Focusing on water or ammonia independent adsorptions at the 6MR site, Paolucci et al. compared the behavior of Cu II and Cu II -OH (called [Z 2 Cu II ] and [ZCu II OH] in their study) and found similar behaviors in terms of cation mobility. Similar trends in adsorption energies were also observed. Hence, for the co-adsorption investigation, which requires the calculation of many more species, we restricted ourselves to the Cu II case. Moreover, Paolucci et al. [START_REF] Paolucci | Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites[END_REF] demonstrated that Cu I nearly does not appear in the 6MR stability diagram in the water rich conditions, hence, Cu I systems were omitted to simplify calculations. We sampled several aluminum and copper locations (see section 3.1). To improve the sampling of stable configurations for water and/or ammonia adsorption, we performed short simulated annealing runs, based on first-principles NVT (constant number of atoms, volume and temperature) velocity scaled molecular dynamics at 673 K (400°C), followed by quenches at 0 K (with the same parameters as other geometry optimizations). The time for each step was set to 1 fs, for a total run of 1 -5 ps. Such an approach proved to be efficient for structure sampling of several kinds of systems. [START_REF] Mager-Maury | H 2 -induced reconstruction of supported Pt clusters: metal-support interaction versus surface hydride[END_REF][START_REF] Futschek | Structural and magnetic isomers of small Pd and Rh clusters: an ab initio density functional study[END_REF] For the most stable optimized structures for a given stoichiometry (n adsorbed water molecules and m adsorbed ammonia molecules), and for a given initial configuration (copper in 6MR or 8MR), the cumulated adsorption energy Δ ads U (n,m) is given by equation 1, function of the energy of the zeolitic cells U zeo(n,m) containing n water molecules and m ammonia molecules, U water and U ammonia being the energies of the isolated water and ammonia molecules, respectively. Δ ads U (n,m) = U zeo(n,m) -U zeo(0,0) -n U water -m U ammonia Equation 1For each of these species, the Gibbs free energy was calculated according to the approach detailed in ref. [START_REF] Rey | Ab Initio Simulation of the Acid Sites at the External Surface of Zeolite Beta[END_REF][START_REF] Larmier | Influence of Coadsorbed Water and Alcohol Molecules on Isopropyl Alcohol Dehydration on γ-Alumina: Multiscale Modeling of Experimental Kinetic Profile[END_REF] by considering the rotational, translational, and vibrational degrees of freedom for gas-phase water and ammonia, and the vibrational degrees of freedom only for the zeolite models. Harmonic frequency calculations were performed on the optimized structures with the lowest energy values, using a finite-difference approximation, with a displacement of ± 0.02 Å around the equilibrium atomic positions. All atoms within the zeolite cell were then allowed to relax. These calculations lead to the evaluation of the Gibbs free energy of adsorption Δ ads G (n,m) .

2.2.Catalyst synthesis

Cu-SSZ-13 zeolite (Si:Al=16:1, Cu:Al=1:5, 1.2 wt.% Cu) was prepared via hydrothermal synthesis reported earlier. [START_REF] Gunter | Structural Snapshots of the SCR Reaction Mechanism on Cu-SSZ-13[END_REF] First, 0.67 g sodium hydroxide, 41.1 g deionised water, 14.8 g N,N,N-trimethyladamantylammonium hydroxide (TMAdOH, 25 wt.%, Sachem) and 0.43 g aluminium hydroxide were mixed together and stirred for 30 min. Then 13.0 g colloidal silica (Ludox® AS-40) was added to the mixture and for stirred for another 10 min. The as prepared gel was transferred into a 200 mL Teflon-lined autoclave and aged at room temperature for 2 h after that it was heated statically for 4 days at 160 °C. Next the autoclave was cooled down and depressurized, the resulting slurry was filtered, washed with 1 L deionised water, dried at 80 °C and calcined at 550 °C for 2 h. The as prepared Na-SSZ-13 was ion exchanged with aqueous 1 M NH 4 NO 3 solution (20 mL/g zeolite) for 2 h at 75 °C, washed with deionised water, dried at 80 °C and calcined at 550 °C for 2 h. These steps were repeated for further two times without calcination at the final step to receive NH 4 -SSZ-13. Cu II was introduced by liquid ion exchange with 0.005 M Cu(OAc) 2 aqueous solution (100 mL/g zeolite) at room temperature for 24 h. The resulting material was filtered, washed with 1 L deionised water, dried at 80 °C and calcined at 550 °C for 8 h.

2.3.X-ray absorption spectroscopic studies

In situ XAS measurements around the Cu K edge (8979 eV) were performed at the SuperXAS beamline (SLS, Villigen, Switzerland) using a fast oscillating Si (111) channel-cut monochromator. [START_REF] Frahm | The dedicated QEXAFS facility at the SLS: Performance and Scientific Opportunities[END_REF] Quick XAS (QEXAFS) spectra were measured at a frequency of 10 Hz, all spectra collected in 1 minute were aligned and averaged for further analysis. A setup for transmission XAS with a heated quartz capillary microreactor (plug flow geometry, diameter 1.5 mm, wall thickness 0.02 mm, heated with an air blower, beam size of about 0.2 x 0.2 mm 2 , monitoring middle of the catalyst bed) was used. [START_REF] Grunwaldt | X-Ray Absorption Spectroscopy under Reaction Conditions: Suitability of Different Reaction Cells for Combined Catalyst Characterization and Time-Resolved Studies[END_REF] 6.2 mg of Cu-SSZ-13 (sieve fraction 0.1-0.2 mm, supported by two quartz wool plugs) was used, which resulted in 7 mm bed length. Prior to the measurements the catalyst was pretreated (dehydrated) in a flow of 10%O 2 in He (rest H 2 O content in the gas mixture 5-10 ppm, P(H 2 O)~10 -5 bar) at 823 K (10 K/min ramp rate, 10 min dwell time at 823 K) and cooled to room temperature in the same gas mixture. The dehydration experiment was monitored by QEXAFS. For the in situ measurements with NH 3 the catalyst was heated from 313 K to 823 K (10 K/min ramp rate) in two gas atmospheres: dry He with P(NH 3 )=10 -3 bar (1000 ppm), as well as He with P(NH 3 )=10 -3 bar and approx. P(H 2 O)=10 -2 bar (1000 ppm NH 3 and approx. 1% H 2 O). The gas flow through the catalyst was 50 mL/min which results in a Gas Hourly Space Velocity (GHSV) of approx. 200 000 h -1 . Gases were dosed via mass flow controllers, whereas water was fed via a saturator. The gas composition was monitored by an MKS MultiGas 2030 FTIR analyser.

The amount of Cu II , Cu I •xNH 3 , and Cu I was determined via linear combination analysis (LCA) of XANES spectra between 8976 eV and 9004 eV using ATHENA software from the IFFEFIT package. [START_REF] Ravel | HEPHAESTUS: Data Analysis for X-Ray Absorption Spectroscopy using IFEFFIT[END_REF] As references representing Cu II species a set of Cu-SSZ-13 spectra obtained in 1000 ppm NO, 10% O 2 , 1% H 2 O at 343, 473, 623, and 773 K were used to account for changes of the shape of Cu II XANES spectra with coordination number and geometry [START_REF] Doronkin | Lithographically Fabricated Silicon Microreactor for Operando QEXAFS Studies in Exhaust Gas Catalysis during Simulation of a Standard Driving Cycle[END_REF] . Previously, spectra measured under NO and O 2 at different temperatures were attributed to Cu II species using High Energy Resolution Fluorescence Detected (HERFD)

XANES and X-ray Emission Spectroscopy (XES). [START_REF] Lomachenko | The Cu-CHA deNOx Catalyst in Action: Temperature-Dependent NH 3 -Assisted Selective Catalytic Reduction Monitored by Operando XAS and XES[END_REF][START_REF] Gunter | Structural Snapshots of the SCR Reaction Mechanism on Cu-SSZ-13[END_REF] Two other reference spectra for Cu I •xNH 3 and Cu I species with and without ammonia in the coordination sphere, were extracted from the experimental dataset measured at P(NH 3 )=10 -3 bar using Multivariate Curve Resolution -Alternating Least Squares method (MCR-ALS). [START_REF] De Juan | Multivariate Curve Resolution (MCR). Solving the Mixture Analysis Problem[END_REF][START_REF] Jaumot | MCR-ALS GUI 2.0: New Features and Applications[END_REF] This mathematical procedure used in chemometrics allows extraction of a priori unknown reference spectra from a set of spectra of mixtures with changing concentrations. Non-negativity constraint was applied to the matrix of reference spectra and unimodality constraint was applied to concentration profiles. The obtained reference spectra indeed demonstrated all the features of Cu I with and without directly adsorbed ammonia as previously described in refs. [START_REF] Borfecchia | Revisiting the Nature of Cu Sites in the Activated Cu-SSZ-13 Catalyst for SCR Reaction[END_REF][START_REF] Gunter | Structural Snapshots of the SCR Reaction Mechanism on Cu-SSZ-13[END_REF] and were assigned correspondingly. MCR-ALS was required because high energy resolution data available in the literature cannot be used for fitting conventional XANES spectra due to differences in spectral resolution (the comparison between the used Cu I •xNH 3 and Cu I reference spectra and the previously attributed HERFD-XANES spectra is available in the Supporting Information S1). Furthermore, the strategy of using the catalyst sample in question to obtain reference XANES spectra allows obtaining the best fits as was demonstrated by

Lamberti et al. [START_REF] Lamberti | Determination of the Oxidation and Coordination State of Copper on Different Cu-Based Catalysts by XANES Spectroscopy In Situ or In Operando Conditions[END_REF] The total coordination number in the first shell around Cu sites (sum of N+O neighbors)

was estimated by analysis of EXAFS spectra. EXAFS spectra were background subtracted, normalized, k 1 -, k 2 -, and k 3 -weighted and Fourier transformed in the k range 2 -10 Å -1 using ATHENA. Finally, fitting in R-space was performed using ARTEMIS. [START_REF] Ravel | HEPHAESTUS: Data Analysis for X-Ray Absorption Spectroscopy using IFEFFIT[END_REF] E 0 was selected at 8989 eV at a normalized absorbance of 0.5, and not taking into account the shoulder at 8983 eV related to different Cu I species. Theoretical backscattering amplitudes and phases were calculated by FEFF 6.0 [START_REF] Rehr | Theoretical Approaches to X-Ray Absorption Fine Structure[END_REF] and adjusted to the experimental spectra by a least square method in R-space between 1 and 1.9 Å (corresponding to the first Cu-O/N shell). From the fit of a reference CuO spectrum amplitude reduction factor S 2 0 = 0.8 was obtained and then the coordination numbers, interatomic distances, energy shift (δE 0 ) and mean square deviation of interatomic distances (σ 2 ) were refined. The absolute misfit between theory and experiment was expressed by ρ.

RESULTS AND DISCUSSION

Models for Cu 2+ exchanged SSZ-13

In the present work, we choose to simulate Cu-SSZ-13 by cells containing only Cu II as charge-compensating cations, thus, with a constant Cu/Al ratio of 0.5. Several positions of aluminum and copper were investigated, with results provided in Supporting Information S2.

Two configurations were selected for copper in 6MR and 8MR, being the most stable configurations found for each MR (Figure 1). In the following, regarding the computational results, copper-ligand distances higher than 2.3 Å are not considered in the numbering of the bonds in which Cu II is involved.

For copper in the 6MR, a distorted planar square environment was found (Figure 1-(a)), in agreement with previous computational findings, [START_REF] Paolucci | Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites[END_REF][START_REF] Göltl | What Makes Copper-Exchanged SSZ-13 Zeolite Efficient at Cleaning Car Exhaust Gases?[END_REF] with the two aluminum in diametrically opposed positions within the same 6MR. For copper in 8MR, the most favorable configuration is found if aluminum substitutes second neighbors in terms of T sites. The copper ion is then three-fold coordinated, which is likely the origin of an enhanced reactivity, as it will be shown in the following. The most stable configuration corresponds to copper at 6MR, with an energy difference of 97 kJ•mol -1 with respect to the selected configuration where copper is at the 8MR, in agreement with many computational and experimental studies. [START_REF] Deka | Local Environment and Nature of Cu Active Sites in Zeolite-Based Catalysts for the Selective Catalytic Reduction of NOx[END_REF][START_REF] Fickel | Copper Coordination in Cu-SSZ-13 and Cu-SSZ-16 Investigated by Variable-Temperature XRD[END_REF][START_REF] Gao | Structure-Activity Relationships in NH 3 -SCR over Cu-SSZ-13 as Probed by Reaction Kinetics and EPR Studies[END_REF][START_REF] Korhonen | Isolated Cu 2+ Ions: Active Sites for Selective Catalytic Reduction of NO[END_REF][START_REF] Kwak | Two Different Cationic Positions in Cu-SSZ-13?[END_REF][START_REF] Deka | Confirmation of Isolated Cu 2+ Ions in SSZ-13 Zeolite as Active Sites in NH 3 -Selective Catalytic Reduction[END_REF][START_REF] Göltl | What Makes Copper-Exchanged SSZ-13 Zeolite Efficient at Cleaning Car Exhaust Gases?[END_REF] For a larger sampling of aluminum locations in the presence of water, the reader is referred to the work of Göltl et al. [START_REF] Göltl | Developing a Thermodynamic Model for the Interactions between Water and Cu in the Zeolite SSZ-13[END_REF] In the present work, we do not aim at probing all theoretically possible aluminum positions. We rather put our effort on the investigation of the adsorption of variable amount of water and/or ammonia on the two representative sites, sampling all possible combinations in terms of amount of water and / or ammonia in the coordination shell of copper.

Independent water or ammonia adsorption from DFT calculations

Next, the coordination of n water molecules (1 ≤ n ≤ 6) or m ammonia molecules (1 ≤ m ≤ 6) to copper was investigated (the most stable structures are reported in the Supporting Information S3). Regarding the 6MR site, the first adsorbed water / ammonia molecule coordinates on top of the copper atom. The coordination number is increased to 5, whereas the position of the copper atom is shifted upward, above the plane of the 6MR. Higher (in absolute value) adsorption energy (Figure 2) is calculated for adsorption of ammonia than for water (-127 and -60 kJ•mol -1 respectively), in line with previous gas phase calculations. [START_REF] Pavelka | Theoretical Description of Copper Cu(I)/Cu(II) Complexes in Mixed Ammine-Aqua Environment. DFT and Ab Initio Quantum Chemical Study[END_REF] This trend is followed and becomes more pronounced upon the increase of the number of water or ammonia molecules coordinated to copper in 6MR. The orders of magnitude of adsorption energies for water and ammonia are in agreement with the literature. [START_REF] Paolucci | Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites[END_REF] Starting from n = 6 or m = 3, the copper ion is strongly shifted above the plane of the 6MR with final loss of coordination to the framework. The ligands are in fact coordinated to copper up to n or m = 5, the 6 th molecule is ejected from the coordination sphere in the course of the optimization and is linked by H bonds to the first shell.

Note that we tried to optimize dissociated form of the water molecule at the 6MR site,

for the adsorption of one water molecule, and all calculations led to the reformation of the dissociated O-H bond, and went back to a non-dissociated water molecule. We cannot exclude that such configurations can exist for other (n,m) values however.

Regarding Cu II initially close to 8MR, the adsorption energies are more negative than on the 6MR (-198 and -241 kJ•mol -1 for water and ammonia on 8MR, respectively), as expected from the lower initial coordination number of copper (3 instead of 4). Cu II then adopts a distorted square planar geometry, then the coordination number increases before disconnecting from the framework from n= 5 or m=4. Again, the interaction with ammonia is stronger than with water.

Bader charges on Cu II were calculated for each site and water/ammonia content (Figure 2-(b)). Without water nor ammonia, the charge of Cu II at the 8MR site is higher than at the 6MR site, and in both cases lower than two, consistent with the previous findings. [START_REF] Berthomieu | Theoretical Modeling of a Copper Site in a Cu(II)-Y Zeolite[END_REF] This difference between the two sites is again likely due to the lower coordination number of copper at the 8MR, leading to lower electronic density donated from the oxygen ligands. This strong difference is compensated right from the adsorption of the first ammonia molecule or the second water molecule. The charge is also higher in the presence of water rather than in the presence of ammonia, in line with previous results on the 6MR site. [START_REF] Paolucci | Isolation of the Copper Redox Steps in the Standard Selective Catalytic Reduction on Cu-SSZ-13[END_REF] This may be related to the respective positions of H 2 O and NH 3 in the spectrochemical series, with higher donor nature of ammonia with respect to water. Finally, increasing the water content leads to an increase of the charge, whereas increasing the ammonia content decreases it. In this way the framework oxygen can be localized in the spectrochemical series between water and ammonia. Note, however, that in all cases the Bader charge is substantially lower than +2.

Considering the differences in stability of the 6MR system and the 8MR one without adsorbates, revised cumulated adsorption energies were calculated. For this purpose, Cu at 6MR (the most stable one) without adsorbates was taken as a reference point. This shifts the data up by 97 kJ•mol -1 for 8MR (Supporting Information S4). Adsorption makes the two systems to be closer in energy than the initial systems without adsorbates, the 8MR system even becoming the most stable for some compositions (n = 3 for example). This confirms that the initial energy difference was mainly due to the threefold coordinated nature of Cu II in 8MR, which is increased to 4 as soon as it coordinates with the first water or ammonia ligand. This is also due to the fact that copper is dislodged from its exchange position for sufficiently high water or ammonia loading, rendering the considered systems quasi-equivalent. However, the two configurations need to be considered when copper is still attached to the framework. Based on the calculated free energies of each species, phase diagrams were constructed (Figure 3). They describe the nature of the most stable system as a function of the expected to be more abrupt (transition from (5,0) to (0,0) for water at P(H 2 O) < 10 -1 bar, from (0,4) to (0,0) for ammonia at P(NH 3 ) < 10 -4 bar) at the 6MR rather than on the 8MR.

Note that the trends in the (P(H 2 O)), T) diagram for water adsorption in 6MR are comparable to the trends obtained by Göltl et al. [START_REF] Göltl | Developing a Thermodynamic Model for the Interactions between Water and Cu in the Zeolite SSZ-13[END_REF] and by Paolucci et al. [START_REF] Paolucci | Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites[END_REF] In the latter work the authors did not consider 8MR sites, but solely 6MR sites with Cu I /Cu II species. It was shown that Cu I nearly does not appear in the diagram in the sampled water rich conditions [START_REF] Paolucci | Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites[END_REF] confirming that omitting Cu I is not critical under these conditions. In the conditions chosen for the representation of Figure 4, water initiates the dislodgment of copper from its exchange position for temperatures lower than about 340 K at the 6MR site, and below 300 K at the 8MR site. For ammonia, the dislodgment is more strongly dependent on the initial configuration of Cu II . For copper at the 6MR, mobility is to be expected below 600 K, but only below 400 K for Cu located at 8MR. However, for the last case, pronounced distortion relative to the 8MR plane is maintained up to about 600 K. coordination number is expected, first at the 6MR site (close to 340 K, from 6 to 5) and then at the 8MR site (close to 660 K, from 4 to 3). With ammonia in the feed (Figure 5-(b)), in the experimental operating conditions of interest (up to 823 K), the total coordination number of Cu II is always 4. At the lowest temperature, the coordination sphere is composed of nitrogen only, and a gradual nitrogen (from ammonia) exchange by oxygen (from the zeolitic framework) takes place during temperature increase, which induces ammonia desorption and re-coordination of copper to the framework site. Here again, a more abrupt transition from a nitrogen-rich to an oxygen-rich coordination sphere, is found for 6MR in comparison with 8MR. Under these environmental conditions, the sharpest transition takes place close to 600 K. We are aware that the approximations made in the present work for the evaluation of the adsorption free energy (in particular, the entropic contributions) lead to uncertainties in the positions of the frontiers in the stability diagrams. As suggested previously, [START_REF] Paolucci | Isolation of the Copper Redox Steps in the Standard Selective Catalytic Reduction on Cu-SSZ-13[END_REF][START_REF] Paolucci | Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites[END_REF][START_REF] Li | Zeolite Adsorption Free Energies from ab Initio Potentials of Mean Force[END_REF] increased mobility of copper with increasing adsorbate loading may lead to an increase of the residual rotational and translational entropy in the adsorbed state. The entropy in the adsorbed state may not be accurately rendered by the sole estimation of the vibrational degrees of freedom. The residual rotational and translational entropy in the adsorbed state however depends on the adsorption site and of the presence of co-adsorbates, [START_REF] Li | Zeolite Adsorption Free Energies from ab Initio Potentials of Mean Force[END_REF] making the full resolution of this question very challenging. Some improvements of these aspects should be achieved in future, e.g. by systematic molecular dynamics investigations, in the spirit of investigations performed for simpler cases in refs. [START_REF] Li | Zeolite Adsorption Free Energies from ab Initio Potentials of Mean Force[END_REF][START_REF] Chen | Activation of oxygen on (NH 3 -Cu-NH 3 ) + in NH 3 -SCR over Cu-CHA[END_REF] .

Water and ammonia co-adsorption considered by DFT calculations

The same approach was chosen to model the co-adsorption of water (n molecules) and ammonia (m molecules), resulting in systems denoted (n,m), for n + m ≤ 6, for copper at both 6MR and 8MR. To the best of our knowledge, such a systematic study is not reported in the literature yet. The (1,1) systems (co-adsorption of one water molecule plus one ammonia molecule) are represented in Figure 6 (all most stable structures found for each (n, m) values are reported in the Supporting Information S3). Basically, the same trends are observed as for the adsorption of water or ammonia alone, in terms of gradual loss of coordination between copper and the zeolite framework, which generally takes place for n + m ≥ 4 (with a few exceptions). The cumulated adsorption energy for each (n,m) value are plotted in Figure S4. Trends are rather constant upon increase of the number of coordinated molecules, with a systematically higher adsorption energy (in absolute value) for 8MR. This shift with respect to 6MR is due to the lower initial coordination number inducing higher adsorption energies for the first molecule (water or ammonia). 2D phase diagrams are represented in Figure 7 with the The interaction with ammonia dominates under most of the conditions, which can be clearly seen by the vertical domains occupying the largest part of the diagrams drawn at

constant P(NH 3 ). These domains correspond to systems where no water molecules are present in the coordination sphere of copper. Mixed systems are also encountered, such as the (1,1) systems, both on 6MR and 8MR, typically for temperatures around 400-600K, P(NH 3 ) < 10 representing the most stable structure if NH 3 only is present, is still valid in this case.

Adsorption of water and ammonia identified by XANES

In situ X-ray absorption spectroscopy in terms of XANES and EXAFS was used to probe the interaction of Cu species with ammonia and water under relevant operating conditions of Cu-SSZ-13 SCR catalysts, in order to estimate the validity of the theoretical calculations. We exploited that XANES at the Cu K edge is very sensitive to the oxidation state and geometry of Cu complexes, especially if HERFD-XANES is regarded. [START_REF] Lamberti | Determination of the Oxidation and Coordination State of Copper on Different Cu-Based Catalysts by XANES Spectroscopy In Situ or In Operando Conditions[END_REF][START_REF] Yamashita | In-Situ XAFS, Photoluminescence, and IR Investigations of Copper Ions Included within Various Kinds of Zeolites. Structure of Cu(I) Ions and Their Interaction with CO Molecules[END_REF] This allows distinguishing between Cu II species with different coordination numbers, Cu I •xNH 3 (species coordinated with ammonia), and Cu I species not coordinated with ammonia. The corresponding spectral features have been assigned in detail in the previous studies [START_REF] Gunter | Structural Snapshots of the SCR Reaction Mechanism on Cu-SSZ-13[END_REF][START_REF] Giordanino | Interaction of NH 3 with Cu-SSZ-13 Catalyst: A Complementary FTIR, XANES, and XES Study[END_REF] using HERFD-XANES and X-ray emission spectroscopy. The HERFD-XANES spectra reported earlier [START_REF] Gunter | Structural Snapshots of the SCR Reaction Mechanism on Cu-SSZ-13[END_REF] together with the corresponding conventional XANES spectra used for data analysis in this work can be found in the Supporting Information S1. Prior to the in situ X-ray absorption measurements involving NH 3 , the as-received hydrated Cu-SSZ-13 catalyst was dehydrated in oxidizing atmosphere (10% O 2 in He) during heating to 823 K, similar to the procedure reported by Borfecchia et al. [START_REF] Borfecchia | Revisiting the Nature of Cu Sites in the Activated Cu-SSZ-13 Catalyst for SCR Reaction[END_REF] The gas mixture was not additionally dried and contained approx. 10 ppm (10 [START_REF] Borfecchia | Revisiting the Nature of Cu Sites in the Activated Cu-SSZ-13 Catalyst for SCR Reaction[END_REF][START_REF] Gunter | Structural Snapshots of the SCR Reaction Mechanism on Cu-SSZ-13[END_REF][START_REF] Giordanino | Interaction of NH 3 with Cu-SSZ-13 Catalyst: A Complementary FTIR, XANES, and XES Study[END_REF] but linear combination analysis in this case was not successful, possibly due to need to account for a mixture of co-existing species (Cu II in 6MR and 8MR) [START_REF] Borfecchia | Revisiting the Nature of Cu Sites in the Activated Cu-SSZ-13 Catalyst for SCR Reaction[END_REF] each changing geometry at different temperatures. Hence, transformations of Cu species were followed qualitatively by following the energy shift of the rising edge at a normalized absorbance 0.2 (Figure 9-(a)) and a maximum of the white line (peak at approx. 9000 eV, Figure S6). Cu II site restructuring occurring at approx. 340 K and 673 K can be seen which can be attributed to water desorption from Cu II in 6MR (Figure 4-(a)). The outlier datapoint at 338 K is not an artifact but indeed lies on top of a volcano as confirmed by evaluation of spectra recorded at 313 K and 343 K (not shown).

After the dehydration experiment and the subsequent cooling down the gas feed was Without water in the feed the average CN was 3 already at the start of heating and it decreased to 2 at 523 K, after which it was stable (Figure 10-(d)). The initial CN of 3 agrees well with coexistence of 50% Cu II (CN = 4, in accordance with DFT analysis performed in this study (Figure 5) and earlier analyses [START_REF] Paolucci | Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites[END_REF][START_REF] Pavelka | Theoretical Description of Copper Cu(I)/Cu(II) Complexes in Mixed Ammine-Aqua Environment. DFT and Ab Initio Quantum Chemical Study[END_REF] ) and 50% Cu I (for which CN = 2 is the most preferable coordination number from DFT, [START_REF] Pavelka | Theoretical Description of Copper Cu(I)/Cu(II) Complexes in Mixed Ammine-Aqua Environment. DFT and Ab Initio Quantum Chemical Study[END_REF] and has been identified previously by XANES). [START_REF] Yamashita | In-Situ XAFS, Photoluminescence, and IR Investigations of Copper Ions Included within Various Kinds of Zeolites. Structure of Cu(I) Ions and Their Interaction with CO Molecules[END_REF] Hence, the coordination numbers retrieved from EXAFS during H 2 O/NH 3 desorption indicate an overall decrease of the number of NH 3 ligands, which is supported by the reported analysis of the XANES region (Figure 9). However, the EXAFS region is also strongly sensitive to the oxidation state of Cu, and points at the redox behavior simultaneously with the adsorptiondesorption behavior of NH 3 and H 2 O, as observed also by Paolucci et al. [START_REF] Paolucci | Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites[END_REF]

CONCLUSIONS

A theoretical mapping of the coordination sphere of Cu II in SSZ-13 in the presence of H 2 O and/or NH 3 was reached from DFT calculations for copper located at the 6MR or at the 8MR site. The 8MR site appears to significantly stabilize adsorbates in the coordination sphere of copper. The interaction of ammonia with Cu II ions is stronger than the one with water, which makes ammonia to dominate the phase diagrams even for higher water pressures. Oxygen atoms from the zeolitic framework are involved in the coordination sphere of Cu

ASSOCIATED CONTENT

Figure 1 .

 1 Figure 1. Models chosen to investigate reactivity of Cu II at 6MR (a) and 8MR (b). Top views of the rings and their location in the structure are shown.

Figure 2 .

 2 Figure 2. (a) Cumulated adsorption energies and (b) Bader charges at copper, for the adsorption of H 2 O (n molecules) or NH 3 (m molecules) independently, close to copper located in the 6MR or 8MR, from DFT calculations. Lines serve as guides to the eye.

  temperature and P(H 2 O) or P(NH 3 ). The investigated structures are called by (n,m), n being the number of water molecules, m the number of ammonia molecules per Cu II . The most stable structures at typical partial pressures (P(H 2 O) = 10 -2 bar, typical of the dehydration experiment, and P(H 2 O) = 10 -2 bar and P(NH 3 ) = 10 -3 bar) are shown in Figure4as a function of the temperature. The chosen conditions correspond to the X-ray dehydration and absorption experiment reported in sections 3.4 and 3.5. Not all calculated stoichiometries exhibit stability domains on the diagrams. The higher reactivity of Cu II at 8MR towards NH 3 and H 2 O adsorption is visible from the smaller stability domain of the structure without water and ammonia ligands ((0,0) structure, Figures3-(b),(d)) on the later site. The higher reactivity of ammonia with respect to water is also reflected by the smallest stability domain of the (0,0) structure. For both molecules, the loss of the adsorbed ligands upon thermal treatment is

Figure 3 .

 3 Figure 3. Phase diagrams from DFT calculations for (a) H 2 O adsorption at the 6MR, (b) H 2 O adsorption at the 8MR, (c) NH 3 adsorption at the 6MR, (d) NH 3 adsorption at the 8MR. The most stable systems in given conditions are called by (n,m), n being the number of water molecules, m the number of ammonia molecule per Cu II .

Figure 4 .Temperature

 4 Figure 4. The most stable structures of n (H 2 O) or m (NH 3 ) adsorbed on Cu II -SSZ-13 calculated using DFT based methods, as a function of the temperature, for (a) P(H 2 O) = 10 -5 bar, (b) P(NH 3 ) = 10 -3 bar. (b) is also valid when water and ammonia are both present in the atmosphere, at P(H 2 O) = 10 -2 bar and P(NH 3 ) = 10 -3 bar. The conditions corresponding to (b) are the ones chosen for the experimental study.

Figure 5 .

 5 Figure 5. Coordination numbers (CN) predicted from DFT calculations, as a function of the temperature, for (a) P(H 2 O) = 10 -5 bar, (b) P(NH 3 ) = 10 -3 bar (adsorption of ammonia alone), or for P(H 2 O) = 10 -2 bar and P(NH 3 ) = 10 -3 bar (co-adsorption of ammonia and water). 2.3 Å is the threshold copper-ligand distance value.

  the

Figure 6 .

 6 Figure 6. Structures of the (1,1) configurations (one water molecule, one ammonia molecule) at Cu II in 6MR (a) and 8MR (b).

Figure 7 .

 7 Figure 7. Phase diagrams obtained from DFT calculations for H 2 O and NH 3 co-adsorption at 6MR (a) and (c), and 8MR (b) and (d). Diagrams are drawn as a function of (a) and (b) P(H 2 O) (at constant P(NH 3 )= 10 -3 bar) or (c) and (d) P(NH 3 ) (at constant P(H 2 O)= 10 -2 bar). The most stable systems in the given conditions are called (n,m), n being the number of water molecules, m the number of ammonia molecule per Cu II .

- 4 bar

 4 for P(H 2 O) = 10 -2 bar (Figure 7-(c) and -(d)). If we come back to P(H 2 O) = 10 -2 bar and P(NH 3 ) = 10 -3 bar (the conditions selected for the sake of comparison with experimental results) it appears that the most stable systems under these conditions are almost the same as the one in pure ammonia at P(NH 3 ) = 10 -3 bar. Hence, the diagram of Figure 4-(b),

Figure 8 .

 8 Figure 8. Selected XANES spectra at Cu K edge obtained during heating Cu-SSZ-13 zeolite in a flow of (a) He with P(O 2 )= 10 -2 bar and P(H 2 O)= 10 -5 bar, (b) He with P(NH 3 )= 10 -3 bar, and (c) He with P(NH 3 )= 10 -3 bar and P(H 2 O)= 10 -2 bar. Temperature ramp rate 10 K/min.

  switched to He with only NH 3 or NH 3 and H 2 O. The gas feed contained no oxygen to prevent NH 3 oxidation reaction. The catalyst was kept for 10 min at 313 K after which continuous heating to 823 K started. Already at 313 K interaction with ammonia in the dry feed led to appearance of a prominent peak at the rising edge (8983 eV) in the XANES spectra (Figure8-(b)). This edge feature has been previously assigned to 1s→4p transition in two-coordinated Cu I complexes[START_REF] Paolucci | Isolation of the Copper Redox Steps in the Standard Selective Catalytic Reduction on Cu-SSZ-13[END_REF] and suggests partial reduction of Cu II sites due to interaction with ammonia already at ambient conditions. During further heating the Cu I fingerprint becomes more intense and above 523 K the shape of the rising edge changes. This indicates a change in the structure of the Cu I complexes. If the same experiment is repeated in the presence of water vapor (P(H 2 O)=10 -2 bar, Figure8-(c)), different spectra are obtained below 423 K. The spectrum recorded at 313 K reveals a shoulder at 8988 eV ascribed to 1s→4p transition with ligand to metal charge transfer in Cu II complexes[START_REF] Giordanino | Interaction of NH 3 with Cu-SSZ-13 Catalyst: A Complementary FTIR, XANES, and XES Study[END_REF][START_REF] Kau | X-Ray Absorption Edge Determination of the Oxidation State and Coordination Number of Copper. Application to the Type 3 Site in Rhus Vernicifera Laccase and its Reaction with Oxygen[END_REF] and no Cu I fingerprint. The spectrum (Figure9). We could distinguish Cu II species (which may or may not have NH 3 in the coordination sphere) and two types of Cu I complexes: containing NH 3 ligands (Cu I •xNH 3 ) and Cu I without NH 3 in the coordination sphere. When gas feed without water was used, about 50% reduction of Cu II sites was observed already at 313 K, and all Cu I species were coordinated with NH 3 (Figure9-(b)). At elevated temperatures the fraction of Cu I •xNH 3 sites constantly increased reaching a maximum at 523 K, above which desorption of NH 3 started to occur. The fraction of Cu I species without NH 3 in the coordination sphere started to grow and became the most abundant one above 623 K. This is in agreement with the DFT calculations which predict under these conditions complete desorption of NH 3 from Cu sites at 6MR and partial desorption from Cu sites located at 8MR (Figure7-(c) and -(d)). If water vapor was present together with NH 3 , the Cu II reduction was inhibited in the low temperature region (Figure9-(c)). However, with temperature increase above 473 K, Cu I •xNH 3 species became dominating species. Above 523 K the influence of H 2 O could not be observed: the spectra measured at P(NH 3 )= 10 -3 bar were similar to the ones measured at P(NH 3 )= 10 -3 bar and P(H 2 O)= 10 -2 bar. Note that the thermodynamic stability of NH 3 adsorbed on Cu I and Cu II species located in 6MR was previously modeled by Paolucci et al.[START_REF] Paolucci | Catalysis in a Cage: Condition-Dependent Speciation and Dynamics of Exchanged Cu Cations in SSZ-13 Zeolites[END_REF] and was shown to hardly depend on the oxidation state of Cu sites. Hence, the predicted temperature of complete NH 3 desorption does not strongly depend on the choice of Cu I or Cu II as a model site for the DFT calculations, and both models agree with the experimental observations.

3. 5 .

 5 Coordination numbers in the first coordination sphere of Cu sites identified by EXAFSFourier transformed (FT) EXAFS spectra obtained during heating the Cu-SSZ-13 in the different gas feeds are reported in Figure10-(a),(c),(e). Analysis of the EXAFS data allows obtaining average coordination numbers and bond distances (detailed structural information is

Figure 10 .

 10 Figure 10. Fourier transformed k 2 -weighted EXAFS spectra (not corrected for the phase shift) at Cu K edge (a), (c), (e) and coordination numbers in the first shell (R = 1.9 -2 Å) derived from the analysis of the EXAFS spectra (b), (d), (f) obtained during heating Cu-SSZ-13 zeolite in a flow of He containing (a) anb (b) P(O 2 )= 10 -2 bar (with rest P(H 2 O)= 10 -5 bar, i.e. dehydration experiment); (c) and (d) P(NH 3 )= 10 -3 bar; (e) and (f) P(NH 3 )= 10 -3 bar and P(H 2 O)= 10 -2 bar. Temperature ramp rate 10 K/min.

  Even though NH 3 desorbs from Cu sites at T > 523 K, its place in the coordination sphere of Cu I is occupied by O atoms from the zeolite framework, and the resulting CN stays unchanged upon further heating. In the case of Cu-SSZ-13 heating at P(NH 3 )= 10 -3 bar and P(H 2 O)= 10 -2 bar, the original coordination is close to 4 in agreement with dominating Cu II species, but it decreases to 2 as Cu II species are reduced to Cu I variation suggests that water desorbs from Cu II sites at T > 300 K under experimental conditions (Figure 7-(c),(d)) and is completely desorbed at 523 K even if higher local concentration of water vapor is present (Figure 3-(a),(b)). Therefore, at low temperatures water acts as a stabilizer of Cu II species. As soon as water molecules desorb (approx. 300 -500 K, according to the calculations), Cu II sites undergo reduction to Cu I , resulting in the experimentally observed change of the coordination numbers from 4 to 2 (Figure 10-(d)-(f)).

  II for low P(NH 3 ) or P(H 2 O) values and high desorption temperatures, leading to almost constant coordination numbers (4 neighbors) over a wide range of conditions. For high ammonia concentrations and low temperatures Cu II disconnects from the framework and becomes mobile in the zeolite cavity. The theoretical predictions were validated by in situ XAS. Desorption of water ligands from Cu II , reduction of Cu II to Cu I in the NH 3 -containing atmosphere and high stability of NH 3 -containing complexes were observed by in situ X-ray absorption spectroscopy. The effect of water presence in the NH 3 -containing gas feed was visible below 423 K in form of inhibition of Cu II to Cu I reduction. Trends in terms of water and ammonia desorption temperatures were well reproduced by the analysis of the XANES region, with significant desorption of NH 3 from Cu sites at about 673 K at P(NH 3 )= 10 -3 bar in the presence or absence of H 2 O. The variation of the coordination numbers obtained from EXAFS analysis confirms stepwise H 2 O desorption from 6MR and 8MR Cu II sites near the predicted temperatures and supports NH 3 desorption tendency at higher temperatures. Coordination numbers obtained from EXAFS analysis also follow the reduction of Cu II to Cu I in the NH 3containing atmosphere. Hence, the convoluted behaviors of Cu I and Cu II open new perspectives for the computational investigation of the systematic behavior of Cu I in H 2 O/NH 3 atmosphere, and of the mechanism of Cu II to Cu I reduction in the presence of NH 3 .

  theoretical calculations. Hence, the stability of aqueous Cu II complexes in the zeolite matrix was evaluated in the dehydration gas mixture (without NH 3 ). Figure8-(a) reports XANES spectra of Cu II sites recorded during the dehydration experiment. The spectra can be attributed to Cu II sites with different geometries

			(a)
				1.6
			(a.u.)	1.2
			intensity Norm.	0.4 0.8				823 K 623 K 473 K 373 K 338 K
								303 K
				0
				8970	8990	9010	9030	9050
						Energy (eV)
	(b)				(c)
		1.2					1.2
	(a.u.)					(a.u.)
	Norm. intensity	0.8			823 K 723 K 623 K 523 K 423 K	intensity Norm.	0.4 0.8			823 K 723 K 623 K 523 K 423 K
					313 K					313 K
		0					0
		8970	9010	9030	9050	8970	8990	9010	9030	9050
			Energy (eV)					Energy (eV)
								-5 bar) H 2 O. As reported in the section
	3.3, under typical catalyst operating conditions (T> 300 K, P(H 2 O)≤ 10 -1 bar, P(NH 3 )> 10 -4
	bar) adsorbed ammonia dominates on Cu sites. Hence, observing the stability of H 2 O
	complexes in NH 3 -containing gas feeds or reaching NH 3 -H 2 O transition regions in
	experiments designed to study exhaust gas catalysis is often not possible, as shown by

measured at 423 K shows a mixture of Cu I and Cu II features whereas spectra measured at higher temperatures correspond to the ones recorded in the dry feed. Hence, interaction of Cu sites with water vapor is probably limited to low temperatures (< 423 K), where water inhibits Cu II reduction by ammonia. In order to quantify the states of Cu from the XANES spectra with NH 3 in the system linear combination analysis (LCA) using a priori known reference spectra was performed

reported in Supporting Information S9). As reported earlier for Fe-and Cu-zeolite SCR catalysts, the first shell consisting of O or N atoms can be clearly observed. [START_REF] Doronkin | Operando Spatially-and Time-Resolved XAS Study on Zeolite Catalysts for Selective Catalytic Reduction of NOx by NH 3[END_REF] Heating the Cu-SSZ-13 in all cases led to the decrease of the backscattering intensity due to decreasing coordination numbers. Results of EXAFS analysis are summarized in Tables S6-S8 and dynamics of the total coordination number (CN) in the first shell is shown in Figure 10

Acknowledgment

The

Notes

The authors declare no competing financial interest.