Stability and Structures of Silver Subnanometer Clusters in EMT

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Stability and Structures of Silver Sub-nm Clusters in EMT Zeolite with Maximum Aluminium Content Sandro Giuseppe Chiodo, and Tzonka Mineva J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00808 • Publication Date (Web): 10 Feb 2016 Downloaded from http://pubs.acs.org on February 16, 2016

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The Journal of Physical Chemistry

Stability and Structures of Silver sub-nm Clusters in EMT Zeolite with Maximum Aluminium Content

Sandro Giuseppe Chiodo, Tzonka Mineva*

Institut Charles Gerhardt Montpellier, CNRS/ENSCM/UM1/UM2, 8 rue de l'Ecole Normale, 34296 Montpellier Cédex 5, France

∗ Correspondence to: Tzonka Mineva; e-mail: [email protected]

Abstract The stability of small silver clusters, Agn with n=3, 4, 6, and 8, supported in a sodalite of EMT zeolite with a maximum possible aluminium content (Si/Al = 1) were studied with the Density Functional Theory based cluster approach. The results showed that various silver clusters, partially or compeltely reduced, can be stabilised in this zeolite with the maximum negative defects. Computed formation energies for two reduction reactions reveals that completely reduced Agclusters can be stabilised trough hydrogenation, giving some preference to the formation of hydrogenated Ag6 species. The hydrogenated neutral AgnHm/EMT clusters remain stable at T=300 K as obtained from the analysis of the Born-Oppenheimer Molecular Dynamics trajectories. Ion exchange of Na+ by Ag+ cations is obtained to be exothermic with energy gain of 0.5 eV per Ag cation. The formation of Ag-O ionic bonds, known to stabilise metal clusters in zeolitic frameworks, is found to be preponderant in the structures with less than six silver atoms, which leads to strong geometrical deformations especially for Ag3 and Ag4 species. The largest possibly stabilised in the sodalite completely reduced silver cluster is Ag8. Partially reduced intact structures are obtained stable for silver clusters with n ≥ 4. Formation energies predict a co-existence of silver exchanged cations and partially reduced silver clusters with several oxidation states in the sodalite. 1 ACS Paragon Plus Environment


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The density of state spectra (DOSS) of the supported completely and partially reduced Ag4 and Ag8 clusters are very similar despite the different cluster size, topology, and reduction degrees. DOSS of the host zeolite did not undergo any changes due to the interaction with silver clusters.

Key words: Ag-clusters, sodalite; EMT zeolite; DFT, Hydrogenation; Ion exchange; Formation energy, Density of state spectra

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I. Introduction Stabilization of silver aggregates inside the cavities in zeolites, alluminosilicate-based porous materials, has been extensively studied due to many applications in the chemical industry for their optical, redox and catalytic properties.1-16 Metal clusters can be stabilised because of the negative charges at the substitution sites of Si4+ cation by Al3+.1,17-19 Small metal clusters are usually stabilised in the zeolites by applying ion-exchange procedure followed by an intra-zeolitical reduction process. Ag cations are reduced by heating, by reaction with reducing agents

7,9,15

or via

radiolytic reduction.20 The Si/Al molar ratio in mordenite zeolites was reported to be the main parameter that permits to vary the size of formed metal species by reduction processes.9 Stable silver clusters with four and eight atoms were obtained in Rho, Pau and Beta zeolites with Si/Al molar ratio of 3.7 and 13 for Pau and Beta, respectively.16 This led to conclude that the surface acidity of Rho, Pau and Beta zeolites does not affect the stability of small silver clusters, which remained stable over a long time period. Silver sub-nm particles were obtained in various pores and on the surfaces of nanosized crystallite zeolites EMT, LTL, MFI, LTA, BEA with very different Si/Al ratio in each zeolite type.13 Despite the enormous amount of information gathered, especially, in the last years, on the synthesis routes and reduction protocols, significantly fewer works have been devoted to a nonroutine description of the topology, geometrical structures, electronic properties and type of interactions of silver clusters with the oxide frameworks. For an extensive review about the challenges and earlier achievements on the characterisation of ionic clusters, including silver species, in zeolites we refer to the work of Anderson.19 More recently, inter-atomic distances in Agspecies formed in Y and A-zeolites21 have been estimated from the extended X-ray absorption fine line structure (EXAFS) and the X-ray absorption near edge spectroscopy (XANES) measurements. In this study, the relatively short Ag-Ag distance of 2.75 Å were attributed to Ag4δ+ species in agreement with the Ag-Ag distances in ZSM-5.22 Applications of EXAFS, XANES and X-ray photoelectron spectroscopies to characterise Na-mordenite catalysts with 5, 10 and 15 wt% of silver 3 ACS Paragon Plus Environment


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loading revealed the co-existence of Ag+, AgxO and Agnδ+ (n Ag4H2/EMT 14 ACS Paragon Plus Environment

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> (Ag4)2+/EMT correlates with the HOMO-LUMO energy gap tendency ∆EHOMO-LUMO ((Ag4)4+/EMT) > ∆EHOMO-LUMO (Ag4H2/EMT) > ∆EHOMO-LUMO ((Ag4)2+/EMT). The bands are narrow if clusters are not formed in (Ag4)4+/EMT, thus increasing the peak intensities. The (4d,5s) orbital contributions of the Ag4 cluster in gas-phase are in the range of -9.16 ˗ -7.25 eV. When supported, the Ag(4d,5s) orbitals overlap with O2p and give rise to the energy band in the interval 16.5 ˗ -6.8 eV. There is therefore a shift of the Ag4d band to the lower energies when supported in the zeolite. This down-shift can be associated with the partial positive charge at the silver clusters even in the completely reduced hydrogenated structures. The O2p band in the zeolite does not undergo any significant change due to the O-Ag ionic interactions as follows from the comparison of DOSS in Figure 8a for the large EMT model. H1s orbitals, including those of the adsorbed on the silver cluster hydrogens or the bridging Ag-H-O hydrogens, lie in the same band. The highest occupied orbital, or the Fermi level, is composed predominantly by Ag5s and O2p orbitals and the plot of HOMO is shown in Figure 9a. Nevertheless, the three supported Ag4 clusters are characterised by different geometrical parameters, charges, and chemical environment (hydrogen adsorption), their DOS spectra are very similar. The only difference of the electronic structures in the four types of Ag4-species is the HOMO-LUMO energy gap and the relative intensities. There is also a negligible difference with DOSS of Ag8-species in Figure 8c, where DOSS of Ag8H3/EMT, (Ag8H)2+/EMT, (Ag8H)4+/EMT, (Ag8)6+/EMT and (Ag8)8+/EMT are presented. The 4d contributions of the Ag8 cluster in gas-phase are in the range from -9.18 to -7.14 eV and in the case of the supported eight-atomic clusters the Ag4d,5s, O2p and H1s orbitals form the band between -16.8 and - 6.5 eV. As already observed for the (Ag4)q/EMT species, the intensity of the peaks are, in general, related to the HOMO-LUMO energy gap. The most peaked DOSS is obviously given by the symmetrical (Ag8)8+/EMT structure to which is assigned the largest HOMOLUMO gap of 3.20 eV (see Figure 8c). The isolated and supported completely reduced Ag8 species are characterised with similar width of the HOMO-LUMO energy gap, whereas a large difference is found for the isolated and supported completely reduced Ag4 cluster. We attribute this difference to 15 ACS Paragon Plus Environment


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the strong geometrical deformations caused by the interaction of Ag4 cluster with the sodalite (see Figure 2(a)). The structural features of Ag8 in EMT were found to resemble those of the gas-phase counterpart, which is also reflected by the computed similar energies of the HOMO-LUMO gap. The frontier orbitals of gas-phase Ag8 and Ag8H3/EMT are shown in Figure 9b. Again, the major contributions to HOMO are those of Ag5s and O2p orbitals. In summary, the DOSS of the small silver clusters supported in the sodalite is very little affected by the number of Ag atoms, cluster charges and particular geometries. The highest occupied band is dominated by the O2p and silver 4d and 5s orbitals. The support of silver clusters in the zeolite does not modify its O2p or other bands as follows from the comparison with DOSS of the solely EMT models in Figure 8a. This can be expected, because the stabilisation of silver clusters occurs via ionic bond formations with the framework oxygens.

Conclusion We have studied the effect of the support of EMT zeolite with Si/Al=1 on the geometrical and electronic features of small Agn clusters, n = 3, 4, 6, and 8. Completely and partially reduced species were considered in the sodalite cavity. Completely reduced silver sub-nm particles can be stabilized due to hydrogenation. Adsorbed hydrogens withdraw electron density from the silver atoms, causing a partial oxidation of the metal cluster and therefore enhancing the ionic Ag-O bonds. The hydrogenated structures were found stable also at T=300K as followed from the BOMD trajectories. Bare, completely reduced silver clusters were not found in the sodalite cage to be energetically favored. The Ag-O ionic bonds dominate the Ag-Ag bonds in the completely reduced clusters with n ≤ 6 causing therefore strong geometrical deformations. The Ag-Ag interactions are least perturbed and dominate in the hydrogenated Ag8 species hosted in the sodalite, whose topology remains similar to that in their gas-phase counterparts. Hydrogenation is also found energetically favorable for partially reduced (Ag8)2+ and (Ag8)4+. Charged silver particles interact strongly with the zeolite than the hydrogenated completely 16 ACS Paragon Plus Environment

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reduced ones. Computed formation energies, including those of the non-reduced (Agn)q for q = n that were taken as reference values, predict that partial silver reduction can occur nearly spontaneously and explains the co-existence of silver exchanged cations and small clusters with several oxidation states in sodalites. DOS spectra of all the considered here Ag4 and Ag8 clusters in gas-phase, and supported in the sodalite do not vary with the size, charge and topology of the clusters. The Ag4d orbitals overlap with O2p in the energy interval of -17 – -6.5 eV. The electronic structure of the host sodalite is found unaffected by the guest silver clusters.

ACKNOWLEDGEMENTS The financial support from ANR TAR-G-ED is acknowledged. Part of the calculations was performed at the HPC resources of TGCC-CURIE under the allocation x2015087369 made by GENCI (Grand Equipement National de Calcul Intensif). We are grateful to Dr. Vincent De Waele and Prof. Svetlana Mintova for the fruitful discussions.

Supporting Information Available: Optimized Ag3 and Ag6 completely reduced structures hosted in EMT sodalite. This material is available free of charge via the Internet at http://pubs.acs.org.

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Table 1: Formation energies in eV computed for Agn (n= 3, 4, 6 and 8) neutral and charged clusters in the sodalite cage of the EMT model as a difference between the energies of the products and the reagents in two reduction reactions R1 and R2 (see text). The capital letters A and B differentiate structures with protons interacting with different six-rings of the sodalite cavity (see Figure 6). Structures (a) refer to the hydrogenated clusters.

Species

Structure

R1

R2

Ag3/EMT

(a)

-0.19

-0.22

(b)

0.51

0.48

(c)

0.68

0.65

1+

(Ag3) /EMT

(a)

-0.98

-1.00

2+

(Ag3) /EMT

(a)

-0.47

-0.48

3+

(Ag3) /EMT

(a)

Ag4/EMT

(a)

-1.36

-1.39

(b)

0.22

0.19

(c)

0.88

0.85

(a)-(A)

-2.15

-2.17

(a)-(B)

-1.84

-1.85

(b)-(A)

-2.17

-2.19

(b)-(B)

-2.02

-2.03

(c)-(A)

-1.89

-1.91

(c)-(B)

-1.61

-1.63

(Ag4)2+/EMT

-1.64

4+

(Ag4) /EMT

(a)

Ag6/EMT

(a)

-0.86

-0.91

(b)

1.14

1.09

(c)

1.53

1.48

*Ag6/EMT

(a)

-1.87

-1.92

Ag8/EMT

(a)

-1.31

-1.38

(b)

0.27

0.21

(c)

1.33

1.26

(Ag8)2+/EMT

(a)

-2.58

-2.63

4+

(Ag8) /EMT

(a)

-4.12

-4.15

6+

(a)

-4.21

-4.23

(Ag8) /EMT

-2.14

8+

(Ag8) /EMT (a) -4.29 *Structure obtained from an initial model with two separated Ag3 clusters in the sodalite, Figure5. 21 ACS Paragon Plus Environment


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Figure Captions Figure 1. The EMT model structures: (a) large and (b) small. The (b) structure was derived from the optimized geometry of the large model (a) by fixing the positions of the atoms of the sodalite cage together with the four nearest Na+ to the sodalite cage. These cations are necessary to keep the structure neutral. In structure (b) only the coordinates of the hydrogens were relaxed.

Figure 2. Completely reduced Ag4Na32Al36Si36H52O168 and Ag8Na28Al36Si36H56O168 structures. Only sodalite is shown for clarity. The initial structures are reported left to the arrows and the optimized ones - right to the arrows. The structures are (a) fully hydrogenated silver clusters, and (b), and (c) partially hydrogenated silver clusters with protons forming bridging O-H-Ag groups. In the (c) structures some protons are not involved in the O-H-Ag bridge, pointing outside the sodalite cavity. The relative energies in eV are reported.

Figure 3. Selected Ag-Ag, Ag-O, and Ag-H bond distances in Å of the ground-state

(a)

Ag4H2/EMT (top) and Ag4 in gas-phase (bottom); and (b) Ag8H3/EMT (top) and Ag8 in gas-phase (bottom).

Figure 4: Atomic net charges in e obtained from Mulliken population analysis of the ground state (a) Ag4H2/EMT (top) and gas-phase Ag4 (bottom); and (b) Ag8H3/EMT (top) and gas-phase Ag8 (bottom).

Figure 5. The (a) Ag3…Ag3/EMT input structure left to the arrow and the optimized Ag6H4 structure - right to the arrow, shown only in the sodalite for clarity; and (b) the optimized Ag14/EMT structure presented in the full model. The full model is used in all the optimized structures. Figure 6. (Ag4)2+/EMT and (Ag4)4+/EMT structures. Left to the arrow are shown the starting geometries and right to the arrow - the optimized geometries. Only sodalite is shown for clarity. The (Ag4)2+/EMT structures are (a) fully hydrogenated, and (b), and (c) partial hydrogenated with protons forming bridging O-H-Ag groups. In the (c) structures some protons are not involved in the O-H-Ag bridge, pointing outside the sodalite cavity. The capital letters A and B differentiate structures with protons interacting with different six-rings of the sodalite cavity. The relative energies in eV are reported.


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Figure 7. (Ag8)q / EMT (q = 2+, 4+, 6+ and 8+) minimum energy structures. Only sodalite is shown for clarity.

Figure 8. Density of state spectra of the ground state sructures: (a) EMT large (black) and EMT small (red) models; (b) gas-phase Ag4 cluster (black), Ag4H2/EMT (red), (Ag4)2+/EMT (cyan) and (Ag4)4+/EMT (yellow); (c) gas-phase Ag8 cluster (black), Ag8H3/EMT (green), (Ag8H)2+/EMT (blue), (Ag8H)4+/EMT (cyan), (Ag8)6+/EMT (red) and (Ag8)8+/EMT (yellow). Figure 9. Graphical representation of the HOMO and LUMO of the Ag4, Ag4H2/EMT, Ag8 and Ag8H3/EMT minimum energy structures.



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Figure 1. The EMT model structures: (a) large and (b) small. The (b) structure was derived from the optimized geometry of the large model (a) by fixing the positions of the atoms of the sodalite cage together with the four nearest Na+ to the sodalite cage. These cations are necessary to keep the structure neutral. In structure (b) only the coordinates of the hydrogens were relaxed.


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Figure 2. Completely reduced Ag4Na32Al36Si36H52O168 and Ag8Na28Al36Si36H56O168 structures. Only sodalite is shown for clarity. The initial structures are reported left to the arrows and the optimized ones - right to the arrows. The structures are (a) fully hydrogenated silver clusters, and (b), and (c) partially hydrogenated silver clusters with protons forming bridging O-H-Ag groups. In the (c) structures some protons are not involved in the O-H-Ag bridge, pointing outside the sodalite cavity. The relative energies in eV are reported.



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Figure 3. Selected Ag-Ag, Ag-O, and Ag-H bond distances in Å of the ground-state

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(a)

Ag4H2/EMT (top) and Ag4 in gas-phase (bottom); and (b) Ag8H3/EMT (top) and Ag8 in gas-phase (bottom).


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Figure 4: Atomic net charges in e obtained from Mulliken population analysis of the ground state (a) Ag4H2/EMT (top) and gas-phase Ag4 (bottom); and (b) Ag8H3/EMT (top) and gas-phase Ag8 (bottom).



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Figure 5. The (a) Ag3…Ag3/EMT input structure left to the arrow and the optimized Ag6H4 structure - right to the arrow, shown only in the sodalite for clarity; and (b) the optimized Ag14/EMT structure presented in the full model. The full model is used in all the optimized structures.


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Figure 6. (Ag4)2+/EMT and (Ag4)4+/EMT structures. Left to the arrow are shown the starting geometries and right to the arrow - the optimized geometries. Only sodalite is shown for clarity. The (Ag4)2+/EMT structures are (a) fully hydrogenated, and (b), and (c) partial hydrogenated with protons forming bridging O-H-Ag groups. In the (c) structures some protons are not involved in the O-H-Ag bridge, pointing outside the sodalite cavity. The capital letters A and B differentiate structures with protons interacting with different six-rings of the sodalite cavity. The relative energies in eV are reported.



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Figure 7. (Ag8)q / EMT (q = 2+, 4+, 6+ and 8+) minima structures. Only sodalite is shown for clarity.


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Figure 8. Density of the state spectra of the ground state sructures: (a) EMT large (black) and EMT small (red) models; (b) gas-phase Ag4 cluster (black), Ag4H2/EMT (red), (Ag4)2+/EMT (cyan) and (Ag4)4+/EMT (yellow); (c) gas-phase Ag8 cluster (black), Ag8H3/EMT (green), (Ag8H)2+/EMT (blue), (Ag8H)4+/EMT (cyan), (Ag8)6+/EMT (red) and (Ag8)8+/EMT (yellow). 31 ACS Paragon Plus Environment


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Figure 9. Graphical representation of the HOMO and LUMO of the Ag4, Ag4H2/EMT, Ag8 and Ag8H3/EMT minimum energy structures.


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TOC graphic


Stability and Structures of Silver Subnanometer Clusters in EMT

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