Quantification of Silver Sites in Zeolites: Carbon Monoxide Sorption Monitored by IR Spectroscopy Karolina Tarach,* Kinga Gó ra-Marek,* Magdalena Chrzan, and Stanislaw Walas Faculty of Chemistry, Jagiellonian University in Krakó w, Ingardena 3, 30-060 Krakó w, Poland * S Supporting Information ABSTRACT: This work was aimed to provide a well-established approach for the quantification of silver sites in zeolites. The experimental procedure based on carbon monoxide sorption in silver exchanged zeolites ZSM-5, Y, and X was monitored by IR spectroscopy. With regard to the Lambert-Beer law the values of the absorption coefficients of the IR bands of CO interacting with exchangeable cations (Ag + ) and metallic species (Ag 0 ) were attained. Subsequently, the concentrations of silver species of different kinds were calculated. The values of the absorption coefficients are valid for zeolites of FAU and MFI topologies; thus, they are argued to be used for zeolites of other structures. 1. INTRODUCTION Zeolites are of the greatest interest by reason of their microporous structure of channels and cavities, which offers the possibility of stabilizing the small metal clusters and other coordination complexes. The high reactivity of cations hosted in zeolites is explained in terms of a considerably high coordinative unsaturation. Ag-exchanged zeolites are consid- ered as effective catalysts in several catalytic and photocatalytic processes. In particular, zeolites hosting highly dispersed Ag + cations show high activity in the selective catalytic reduction of NO by ethylene, 1 photocatalytic decomposition of NO, 2 the aromatization of alkanes, alkenes, and methanol, 3 and the photochemical of H 2 O into H 2 and O 2 . 4 Also the formation of the active Ag clusters and their detailed structures have been discussed extensively 5-12 and some reports demonstrated a specific catalytic activity of Ag clusters in zeolites in a photocatalytic degradation of malathion, 5 photodimerization of alkane, 6 and methane conversion into propylene in the presence of ethylene. 13 Silver ions introduced to zeolite can be reduced by heating, radiation by ultraviolet rays, or the reaction with reducing molecules. Reduction of the Ag + ion to clusters Ag 0 can be realized by the treatment with CO, alcohols, and alkylbenzenes above 350 °C. 12 In turn, a very important role of the thermal treatment of the Ag-zeolites has been reported. Highly dispersed isolated Ag + cations were found to be the most favored at temperatures of treatment below 400 °C. Higher temperature of the treatment facilitates the Ag + ion aggregation and the Ag n clusters formation. 13 Carbon monoxide is the probe molecule widely applied for the studies of the electron donor/acceptor properties of transition metal cations. 14-16 A characteristic feature of CO bonded to unsaturated transition metal cation is that these species usually possess higher frequency than the CO in the gas phase (2143 cm -1 ). In contrast, the lower frequencies are observed for CO engaged in interaction with metals or transition metal cations of the lowest oxidation states. This phenomenon points to the vital competition between the relative extent of σ-donation from CO to the cation which can increase the CO bond strength, and thus its frequency, and π-back-donation from cation to CO molecules resulting in the opposite effect, weakening of the CO bond. Thus, the up- or downshift of metal cation monocarbonyl band with respect to the position of gaseous CO resulting from the electron flow between probe and cation can be used as a measure of the redox properties of Ag species. IR studies have shown that CO sorption in Ag-exchanged zeolites led to the creation of Ag + (CO) monocarbonyls band at ∼2190 cm -1 , which at low Received: July 9, 2014 Revised: September 23, 2014 Published: September 23, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 23751 dx.doi.org/10.1021/jp506820v | J. Phys. Chem. C 2014, 118, 23751-23760