Copper sites in zeolites - quantitative IR studies K. Góra-Marek a,⇑ , A.E. Palomares b , A. Glanowska a , K. Sadowska a , J. Datka a a Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland b Instituto de Tecnología Química, Universidad Politécnica de Valencia, Camino de Vera s.n., 46022 Valencia, Spain article info Article history: Received 7 May 2012 Received in revised form 19 June 2012 Accepted 20 June 2012 Available online 28 June 2012 Keywords: IR spectroscopy Cu-exchanged zeolites CO sorption abstract Quantitative IR studies were realized in order to determine the concentration of various kinds of Cu sites in zeolites CuZSM-5, CuITQ-2, CuTNU-9, and CuY. These zeolites contained Cu in the form of Cu 2+ , Cu + (in the form of exchange cations Cu + ex. and oxo-form Cu + ox ) and metallic Cu o (in reduced samples). The experimental conditions in which known amounts of CO (used as probe molecule) reacted with each kind of Cu sites in CuZSM-5 were elaborated and subsequently the extinction coefficients of IR band of CO in Cu n+ (CO) adducts were established. Following values of the extinction coefficients of monocarbonyl bands were obtained: 0.11 cm 2 /lmol for Cu 2+ (CO), 1.30 cm 2 /lmol for Cu + ex (CO), 0.91 cm 2 /lmol for Cu + ox (CO), and 0.79 cm 2 /lmol for Cu o (CO). The procedure elaborated for CuZSM-5 was also applied for other zeolites. Consequently, the concentrations of Cu 2+ , Cu + both in exchange positions and oxo-forms, as well as of Cu o were determined in CuZSM-5 treated at various conditions (reduced, oxidized and trea- ted with H 2 O) as well as in zeolites CuZSM-5, CuITQ-2, CuTNU-9, and CuY. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction The most widely applied methods of the controlling of NO x emis- sions are the selective catalytic reduction by NH 3 [1] or urea [2] and the selective catalytic reduction of NO x with hydrocarbons in the presence of oxygen [3–7]. Main attention has been paid to the sup- ported copper catalysts which have been widely studied as a conse- quence of their high catalytic activity in the SCR NO x and the low cost of the copper. On the other hand, Cu-exchanged zeolites do not present enough hydrothermal stability for commercial applica- tion [6], so that many efforts are concentrated on Fe [8], Co [9,10], and Ni [11,12] cations hosted in zeolites because of their high activ- ity, even in the presence of large amounts of water. The Cu + ions in zeolites were also found to activate multiple bonds in organic mol- ecules such as alkenes, benzene, and aldehydes [13,14]. The bond weakening was realized by p back donation of electrons from Cu + (enhanced by the interaction with zeolite framework) to p / anti- bonding orbital of molecule. In general, two types of transition me- tal ions (TMI) centers can be distinguished: isolated mononuclear TMI cations present in exchange positions, and oxo-like clusters M x O y of poorly defined stoichiometry. The concentration of both isolated and clustered centers can be adjusted by the modification of the zeolite type, its Si/Al, and the TMI concentration. To attain information about the oxidation state of the copper sites, IR spectroscopy with the use of the probe molecules such as CO and NO is usually applied. Many studies has been devoted to CO adsorption on copper-based catalysts but there are still many problems under debate, e.g. the discrimination between the vari- ous copper sites such as Cu 2+ , Cu + and Cu o . It is well known that Cu + cations in exchange positions, when interacted with carbon monoxide, produce highly stable monocarbonyls adducts Cu + (CO) that appear at 2150–2160 cm À1 [15,16]. When the equilibrium CO pressure increases the Cu + (CO) monocarbonyls are transformed into dicarbonyls Cu + (CO) 2 and further into tricarbonyls Cu + (CO) 3 when temperature reaches 170 K [15,16]. The IR bands of CO bonded to bulk Cu o and Cu + in the form of the oxide-species appear usually about 2130 cm À1 and they are characterized by low stabil- ity; they are decomposed by the evacuation at room temperature. The interaction of CO with Cu 2+ species results in the formation of Cu 2+ (CO) adducts, that are unstable and they are detectable only at 170 K after CO sorption at low temperature as a consequence of the electrostatic nature of the interaction of CO with Cu 2+ species [17,18]. The IR spectroscopy offers also the possibility of the quantita- tive determination of the concentration of various cationic sites [19,20] but the studies of the copper sites in this matter have been not delivered up to now. The aim of the present study was to create a tool for determina- tion the absolute values of the concentrations of various kinds of copper sites. The experimental conditions in which known amount of probe molecule (carbon monoxide) interacted with each type of copper sites were established. The values of the extinction coeffi- cients of the Cu 2+ (CO), Cu + ex (CO), Cu + ox (CO), and Cu o (CO) monocar- bonyls bands were determined. Subsequently, the concentration of Cu + both in the exchange positions and in the oxo-forms as well as 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.06.029 ⇑ Corresponding author. Tel.: +48 12 663 20 81; fax: +48 12 634 05 15. E-mail address: gorak@chemia.uj.edu.pl (K. Góra-Marek). Microporous and Mesoporous Materials 162 (2012) 175–180 Contents lists available at SciVerse ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso