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C: Surfaces, Interfaces, Porous Materials, and Catalysis +
Chemically Induced Formation of Monovalent Cd Ions and Reversible O Activation in Cadmium Loaded ZSM-5 Zeolite 2
Elena Morra, and Mario Chiesa J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01744 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018
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The Journal of Physical Chemistry
Chemically Induced Formation of Monovalent Cd+ Ions and Reversible O2 Activation in Cadmium Loaded ZSM-5 Zeolite Elena Morra and Mario Chiesa*
Department of Chemistry, NIS Centre, University of Turin, Turin, Via Giuria 7, 10125 Italy. Abstract We have studied the nature of monovalent Cd+ (4d10 5s1) species generated in a Cd loaded HZSM-5 zeolite by reaction with molecular O2. The open-shell species formed in the different steps of the reversible - reaction are characterized by Electron Paramagnetic Resonance (EPR) and hyperfine sublevel correlation (HYSCORE) spectroscopies at X- and Q- band frequencies. The same Cd+ species are obtained via a photochemical pathway, through UV irradiation of the Cd loaded zeolite. Unambiguous evidence for the formation of mononuclear Cd+ species is obtained by detection of the hyperfine interaction associated to naturally abundant 111Cd and 113Cd magnetic isotopes (I = ½ natural abundance 12.8 % and 12.22 % respectively). The full 111Cd A tensor (Ax=10620 Ay=10639, Az=10790) MHz is resolved, indicating that 84% of the unpaired electron spin density is localized on the Cd ion. The small spin density delocalization on the zeolite framework is observed through detection of 27Al hyperfine interactions by means of 6 pulse HYSCORE experiments, allowing for a detailed description of the geometric and electronic structure of the monovalent cadmium species and the zeolite stabilizing sites.
1. Introduction Metal-loaded zeolites are still attracting persistent interest because of the unusual ions, clusters, and filamentary structures, which can be stabilized and the unique physical and chemical properties associated to these exotic chemical species. Spatially and chemically stabilized metal species ranging from single ions to small clusters of a few metal atoms (Mnδ+) are known, which are of considerable interest in advancing our understanding of the size dependency of metal properties between the atomic and nanoparticle regime. In this size regime the electronic configuration and discrete energy levels of metals manifest some remarkable electronic and optical properties, such as strong photoluminescence,1 and high catalytic activity.2 In the field of catalysis, it is well known that the Al distribution inside zeolites determines the chemical properties of countercations 3-5, priming single cations or small ionic clusters for high activity towards many relevant reactions.6 Many examples of molecular metal clusters involving alkali metals 7 and silver 8 have been reported, while only few reports are available for heavy metals of group 12 (Zn, Cd and Hg).9-13 Cadmium cluster formation within zeolites by Cd metal vapor sorption has been reported by Seff and co-workers,11,14 which were able to identify different sorption complexes including Cd+ ,Cd22+ and Cd34+ species in Cd(II) exchanged zeolites. Recently, a weak and poorly resolved EPR signal resonating at g=2.003, observed on a UV irradiated Cd-ZSM-5 system, was attributed to Cd+.13 Cd+ is a paramagnetic ion with 4d10 5s1 electron configuration with a very high chemical reactivity, which prevents its observation in aqueous solutions. Cd+ species have been observed in 1 ACS Paragon Plus Environment
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the past as the result of high energy radiolysis experiments involving the use of gamma rays15,16 or atom beams at cryogenic temperatures 17 and detailed X-band EPR studies are available for CdH, CdOH and CdCN radical molecules isolated in rare gas matrices.18-22 Given its paramagnetic nature, the most direct evidence for the formation of this unusual oxidation state is Electron Paramagnetic resonance (EPR) spectroscopy. However, unambiguous identification of this species by EPR can be obtained only by the detection of the hyperfine interaction associated to 111Cd and 113Cd magnetic isotopes (I = ½ natural abundance 12.8 % and 12.22 % respectively). This is non trivial due to the low natural abundance of the magnetically active isotopes and very large hyperfine splitting, which make the low field component undetectable at standard operating frequency (9.5 GHz). As part of a general study of the chemistry of Group 12 metals in zeolites, we report in the following on the reversible chemical formation of monovalent Cd ions induced by the reaction of molecular O2 with a Cd loaded HZSM-5 zeolite. For the first time the full set of hyperfine interactions relative to 111Cd and 113Cd cadmium isotopes and the magnetic nuclei of the zeolite framework (27Al, 29Si and 1H) are resolved, providing direct evidence for the formation and localization of monovalent cadmium ions, the nature of the zeolite framework trapping site and the reversible chemical reactivity of Cd+ towards molecular oxygen. 2 Experimental Section 2.1 Sample Preparation Cd-loaded ZSM-5 was prepared by in situ sublimation of metallic cadmium on H-ZSM-5. The HZSM-5 zeolite (Si/Al = 15) was dehydrated by thermal treatment at 673 K under dynamic vacuum (residual pressure
