Tuning the energetics and tailoring the optical properties of silver clusters confined in zeolites

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1 Tuning the energetics and tailoring the optical properties of silver clusters confined in zeolites Oliver Fenwick 1 Ϯ, Eduardo Coutiño-Gonzalez 2Ϯ, Didier Grandjean 3, Wouter Baekelant 2, Fanny Richard 1, Sara Bonacchi 1, Dirk De Vos 4, Peter Lievens 3, Maarten Roeffaers 4 *, Johan Hofkens 2 *, Paolo Samorì 1 * 1 ISIS & icfrc, Université de Strasbourg & CNRS, 8, allée Gaspard Monge, Strasbourg, France. samori@unistra.fr 2 Department of Chemistry, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium. E- mail: Johan.Hofkens@chem.kuleuven.be 3 Laboratory of Solid State Physics and Magnetism, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium 4 Department of of Microbial and Molecular Systems, KU Leuven, Celestijnenlaan 200F, B Leuven, Belgium. Maarten.Roeffaers@biw.kuleuven.be Current address: School of Engineering and Materials Science, Queen Mary University of London, Mile End Road, London E1 4NS, United Kingdom. Ϯ These authors contributed equally to the work. Contents 1. X-ray photoelectron spectroscopy (XPS) data 2. Photoelectron spectroscopy in air (PESA) data 3. Excitation-emission spectra of the silver cluster-containing zeolites 4. Electron spin resonance (ESR) experiments 5. Extended X-ray absorption fine structure (EXAFS) NATURE MATERIALS Macmillan Publishers S-1 Limited. All rights reserved.

2 1. X-ray photoelectron spectroscopy (XPS) data 3A FAUX Supplementary Figure 1 Atomic composition of zeolites 3A and FAUX across the full range of silver loadings (as indicated by number of atoms per normalised unit cell). Supplementary Figure 2 Actual degree of ion exchange determined by XPS plotted against those supposed from the concentrations of the starting solutions. In the surface region of the zeolite probed by XPS, it is clear that the cationic exchange is more effective in the FAUX zeolite than in the 3A. When excess silver is included in the exchange solution, the nominal and measured degree of silver exchange agree (overlapping blue and red stars). Supplementary Figure 3 Amount of potassium (blue, left axis) and sodium (red, right axis) as a proportion of the total number of non-framework metal atoms in the 3A zeolites detected by XPS plotted against the nominal number of Ag + ions exchanged per normalised unit cell. The stars represent the proportions detected by XPS if an excess of silver is used in the exchange solution. 2 NATURE MATERIALS

3 SUPPLEMENTARY INFORMATION Supplementary Figure 4 Background-corrected Ag M 4N 4,5N 4,5 Auger spectra for a number of the zeolites studied. Supplementary Table 1 Binding energies and Auger parameters of silver clusters in the zeolites. Material or framework Silver content, x Binding energy, Ag 3d 5/2 (ev) Kinetic energy, Ag M 4N 4,5N 4,5 (ev) Modified Auger parameter 3d 5/2 M 4N 4,5N 4,5 (ev) Ag metal Ag 2O powder Silver nanoparticles A * Excess FAUY FAUX Where x denotes the number of silver atoms per normalised unit cell 10 nm diameter nanoparticles with citrate as stabiliser (Sigma Aldrich ML) measured as a dropcast film. * Unable to determine peak energy accurately due to low signal level. NATURE MATERIALS 3

4 2. Photoelectron spectroscopy in air (PESA) data Supplementary Figure 5 Schematic of the determination of ionisation potential. The ionisation potential is determined from the intersection of the baseline counts with a straight line fit to (photoelectron yield) 1/3 above threshold. Supplementary Figure 6 Raw PESA data for the four zeolites across all silver loadings. Red arrows indicate the shift in the onset of photoelectrons with increasing silver loading in the zeolite. 4 NATURE MATERIALS

5 SUPPLEMENTARY INFORMATION Supplementary Figure 7 Normalised gradient of the photoelectron yield (derived from data in Supplementary Figure 6) which serves as an approximation to the density of states. Data have been smoothed prior to calculating the derivative to reduce noise. Levels of noise after calculating the derivative were too high in the case of FAUY. Red arrows indicated the reduction of the density of states at low energies with increasing silver content. Supplementary Figure 8 Ionisation potentials of silver-exchanged, calcined zeolites (a) FAUX and (b) 3A plotted as a function of the silver content rated as a proportion of silver atoms relative to all nonframework metal atoms (measured by XPS). NATURE MATERIALS 5

6 3. Excitation-emission spectra of the silver cluster-containing zeolites Supplementary Figure 9 Excitation-emission two-dimensional plots of calcined FAUX zeolites with a full range of silver loadings showing emission from silver clusters. 6 NATURE MATERIALS

7 SUPPLEMENTARY INFORMATION Supplementary Figure 10 Excitation-emission two-dimensional plots of calcined FAUY zeolites with a full range of silver loadings showing emission from silver clusters. 4. Electron spin resonance (ESR) experiments In Supplementary Figure 11, the ESR spectra of the parent FAUX zeolite (a) and the heat-treated FAUX[Ag 11] (b) are shown after the cavity background subtraction. No significant signals were observed in the ESR of FAUX[Ag 1-11] samples, which suggests that heat-treated FAUX[Ag n] zeolites are ESR silent. Similar results were obtained for the whole series of FAUY[Ag n] samples. ESR properties of LTA zeolites are reported in the main text. NATURE MATERIALS 7

8 Supplementary Figure 11 ESR spectra of a) silver-free FAUX[Ag 0] zeolite and b) heat-treated FAUX[Ag 11], inset; zoom of the region of interest where signals due to paramagnetic silver species are expected. The broad peak around g = 2 and the sharp peak at g = 4.3 are assigned to iron impurities, which are typically present in commercial zeolites. The broad peak at g = 2 is typically assigned to iron in a spherical environment, while the sharp signal at g = 4.3 represents iron in an asymmetric environment, possibly intra-framework X-ray absorption measurements (EXAFS) Supplementary Figure 12 Schematic representation of FAU topology indicating the different sites where extra framework cations are typically found. FAU zeolites belong to the family of sodalite cagecontaining topologies. The sodalite cages (highlighted in blue) in FAU are connected through hexagonal rings (highlighted in yellow), such connections give rise to the so-called super cages (8 ring window delimited by green). In the Faujasite topology extra framework cations are mainly located in a few well-defined sites conventionally designated as site I (the centre of the hexagonal prism), site II (single six-ring -S6R- in the supercage) and the sites I' and II', opposite the sites I and II, but inside the sodalite cage (Supplementary Figure 12). 2-4 Cations at site I are surrounded by six oxygens and 12 Si/Al while cations at the other sites are surrounded by three oxygens and 6 Si/Al. 8 NATURE MATERIALS

9 SUPPLEMENTARY INFORMATION Although Ag-exchanged LTA zeolites have been extensively studied by EXAFS in literature, only a few studies have been reported for Ag -FAUX and FAUY zeolites. 5, 6 Among several possible EXAFS models that have been applied to study Ag-exchanged LTA zeolites in literature, the model based on a threeshell structure (Ag-O, Ag-Ag, and Ag-Al/Si) 7 displayed the best agreement with the results obtained from our EXAFS analysis. In this model we have used a virtual mixed Al/Si site corresponding to a 0.7 Si:0.3 Al occupancy that better reflects the 2.7 Si/Al ratio of the FAUY zeolite. In order to support the EXAFS analysis and obtain information related to the nuclearity of these clusters from the fitting results, we have used one of the few crystallographic data available corresponding to a fully exchanged FAU[Ag 6.5] zeolite. 8 Details of the curve-fitting and a table of the fitting parameters are given in Supplementary Table 2. Figure 5 in the main text shows the χ(k) k 3 -weighted EXAFS data and the corresponding phasecorrected Fourier transform best fits of heat-treated FAUY[Ag 0.5]. In a first stage we fitted the data both in k 2 and k 3 spaces. However k 3 weighting amplifies the high k-range of the spectra that is more sensitive to heavy atoms such as silver. Silver atoms especially those forming Ag 4 clusters do not always occupy fixed crystallographic positions. They are therefore associated with relatively large static disorder making them difficult to detect. Fitting the EXAFS data with k 3 weighting allows more useful information on the silver shells to be obtained and gave the most reliable results. The first peak in the FT could be fitted with two O contributions. The first shell (i=1) consists of 2.0 O at 2.41 Å corresponding to a combination of the framework O for Ag cations occupying the center of the S6Rs (position II and II ) as well as to extra-framework H 2O ligands (see below). The (i=2) shell is composed of 0.9 O at a longer distance of 2.60 Å corresponding to the fraction of Ag located at the center of the hexagonal prisms (site I). The second peak in the FT is a multipeak composed of two Ag-Si/Al contributions. The first one (i=4) consists of 1.1 Si/Al atoms at a distance of 3.17 Å. This Si/Al contribution corresponds to a fraction of silver ions located close to the center of the S6Rs forming the supercages (sites II and II ). 9 Silver cations occupying this position (referred as Ag R) are strongly bonded to three framework oxygens (i=1) pointing towards the center of the S6R. In the crystallographic model of Ag 13-FAUY zeolite, 8 Ag R located directly in the plane of the S6R show a Ag R-SA distance of 3.20 Å in line with the distance we obtained. The corresponding Ag R - O is 2.30 Å confirming that the (i=1) distance of 2.40 Å is a combination of framework O and extra framework water molecules located at longer distances. The second Ag-Si/Al contribution (i=5) consists of 1.8 Si/Al atoms at a longer distance of 3.42 Å corresponding to silver atoms located in the center of the hexagonal prisms (position I) and referred as Ag P. Ag P are also coordinated to 6 oxygen atoms connecting the two rings that correspond to the (i=6) shell (1 O at 3.71 Å). Ag R are then coordinated to 6 Al/Si (i=4) and 3 O implying a 2:1 ratio of the Al/Si and O coordination numbers. A direct calculation indicates that the Ag R fraction (1.1 /6 Al/Si = 0.18) corresponds to ca. 18 % of the total number of the silver cations originally exchanged in the zeolite structure. Similarly these isolated Ag R cations are coordinated to 3*0.18 = 0.5 O of the S6R implying that the remaining part 1.5 ( ) of the oxygen in the first Ag-O shell corresponds to extra framework water molecules. Similarly Ag P are coordinated to 12 Al/Si (i=5) and 6 O (i=2) with a 2:1 ratio of the Al/Si and O coordination numbers. A direct calculation indicates that the Ag P fraction (1.8 /12 Al/Si = 0.15) corresponds to ca. 15 % of the total number of the silver cations originally exchanged in the zeolite structure. In addition to the large Al/Si contribution, a weaker Ag-Ag contribution (i=3) of 1.9 Ag at unusually long distances of 2.95 Å can also be detected in the second FT multipeak. This contribution corresponds to the remaining part of the silver atoms Ag C (ca. 67%) forming oligomeric clusters located inside the sodalite cage. A simple calculation shows that the Ag coordination around Ag C atoms is 2.84 (1.9/0.67) suggesting that the clusters nuclearity is very close to 4. In order to complete the EXAFS structural model each Ag C are coordinated to 2.2 O (1.5/0.67) from extra framework water molecules. NATURE MATERIALS 9

10 Two long distance shells of 0.6 Ag (i=7) and 1.7 Ag (i=8) at ca and 4.62 Å respectively were added to our model as they improved significantly the quality of the fit. Inspection of the sodalite model suggests that these long Ag-Ag distances correspond to distances from Ag C atoms to Ag R and Ag P cations located around the sodalite cage indicating that most of the silver cations are concentrated in a small number of sodalite cages. Although no Na shell has been clearly detected in the EXAFS model, it is likely that a small amount of Na cations is present in a limited number of S6Rs around the clusters. In summary, the Ag atoms are forming ca. 67 % of oligomeric Ag 4 clusters located in the sodalite cage in which each Ag atom is bonded in to 2.2 water molecules and surrounded mostly by ca. 33 % of Ag isolated cations (plus some additional Na cations) located at the center of the S6R rings of the sodalite cage (ca. 18 %) and in the center of the hexagonal prisms connecting the sodalite cages (ca. 15 %). Although the EXAFS analysis of the fully Ag-exchanged sample FAUY[Ag 6.5] indicates a very similar silver-based structure the cation distribution is slightly changed with (ca. 62%) of Ag 4 cluster surrounded by ca. 25 % of Ag R and ca. 13 % of Ag P. A significant increase of the Debye-Waller factors (A=2 2 ) associated with (i=3) Ag C-Ag C and (i=4) Ag R- Si/Al shells (0.026 to Å 2 and to Å 2 respectively) is observed in the fully Ag-exchanged sample compared to FAUY[Ag 0.5]. This suggests that Ag R cations located in the S6Rs and Ag C forming the Ag 4 clusters are more ordered in FAUY[Ag 0.5] than in fully loaded FAUY[Ag 6.5]. On the contrary Debye-Waller factors associated with Ag P-Si/Al (i=5) and Ag P-O (i=6) shells stay nearly constant suggesting that the change of Ag loading does not affect significantly the level of disorder of Ag P cations located in the hexagonal prisms. Supplementary Table 2 Summary of structural results of Ag K-edge EXAFS refinement of the FAUY[Ag 0.5] zeolite. FAUY[Ag 0.5] FAUX[Ag 1] FAUY[Ag 0.5] FAUX[Ag 1] E f (ev) -5.2 (7) -0.3 (6) N (2) Si/Al 1.7 (3) Si/Al AFAC R (3) 3.26 (3) k-range A (4) (3) N (3) O 1.1 (3) O N (8) Si/Al 1.7 (4) Si/Al R (2) 2.36 (4) R (2) 3.50 (3) A (6) (8) A (1) 0.03(2) N (1) O 0.9 (4) O N (2) O 3.4 (8) O R (3) 2.48 (3) R (3) 3.59 (3) A (3) (3) A (6) (2) N (6) Ag C 1.7 (5) Ag C N (3) Ag R 1.7(4) Ag R (2) 2.84 (3) R (3) 3.62 (2) A (4) (4) A (1) 0.06 (3) N (4) Na N (9) Ag P 2.1 (6) Ag R (3) R (4) 3.79 (2) A (4) A (1) (4) R (%) 31 % 40 % E f = contribution of the wave vector of the zero photoelectron relative to the origin of k [ev] AFAC = amplitude reduction due to many-electron processes N i = number of atoms in the i th shell R i = radial distance of atoms in the i th shell [Å] A i = Debye-Waller term of the i th shell (A=2 2 with = Debye-Waller factor)[å 2 ] R = R-factor of the fit in % 10 NATURE MATERIALS

11 SUPPLEMENTARY INFORMATION Supplementary Figure 13 - k 3 -weighted EXAFS signal a with the corresponding phase corrected Fourier transform b of heat-treated FAUX[Ag 1]. References 1. Grobet P. J. & Schoonheydt R. A. ESR on silver clusters in zeolite A. Surf. Sci. 156, (1985). 2. Gellens L. R., Mortier W. J. & Uytterhoeven J. B. On the nature of the charged silver clusters in zeolites of type A, type X and type Y. Zeolites 1, (1981). 3. Gellens L. R., Mortier W. J. & Uytterhoeven J. B. Oxidation and reduction of silver in zeolite Y - A structural study. Zeolites 1, (1981). 4. Smith J. V. Molecular Sieve Zeolites-I, vol American Chemical Society, Lamberti C., Prestipino C., Bordiga S., Fitch A. N. & Marra G. L. Characterization of isolated Ag cations in homoionic Ag-Y zeolites: A combined anomalous XRPD and EXAFS study. Nuclear Instruments & Methods in Physics Research Section B-Beam Interactions with Materials and Atoms 200, (2003). 6. Miyanaga T., et al. Formation of Ag clusters in zeolite X studied by in situ EXAFS and infrared spectroscopy. Microporous Mesoporous Mater. 168, (2013). 7. Yamamoto T., Takenaka S., Tanaka T. & Baba T. Stability of silver cluster in zeolite A and Y catalysts. 14th International Conference on X-Ray Absorption Fine Structure 190, (2009). 8. Gellens L. R., Mortier W. J. & Uytterhoeven J. B. Oxidation and Reduction of Silver in Zeolite-Y - a Structural Study. Zeolites 1, (1981). 9. Gellens L. R., Mortier W. J., Schoonheydt R. A. & Uytterhoeven J. B. The nature of the charged silver clusters in dehydrated zeolites of type A. J. Phys. Chem. 85, (1981). NATURE MATERIALS 11