FTIR spectroscopic study of CO and NO adsorption on Cr-ZSM-5 A. Penkova, K. Hadjiivanov * Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria Received 13 June 2003; accepted 14 July 2003 Published online: 12 August 2003 Abstract Low temperature adsorption of CO on activated Cr-ZSM-5 sample does not lead to formation of any chromium carbonyls. Very small amount of chromium nitrosyls were produced after NO adsorption at ambient temperature. When NO was adsorbed on a hydrogen-reduced sample (773 K) dinitrosyls of Cr 3þ (NO) 2 type were produced and evidenced by strong bands at 1908 and 1782 cm 1 . Surprisingly, CO fails to detect these Cr 3þ sites even at low tem- perature. The results are rationalized by assuming that Cr 3þ cations in Cr-ZSM-5 are coordinatively saturated and do not form chemical bonds with weak ligands. Since the Cr 3þ (NO) 2 are very stable (they decompose at 323 K), these species are formed with a simultaneous change in the original coordination of the Cr 3þ cations. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Adsorption; Carbon monoxide; FTIR spectroscopy; Nitrogen monoxide; Cr-ZSM-5 1. Introduction Metal-exchanged zeolites have found a wide application in catalysis. In particular, during the last years, the interest to the Me-ZSM-5 materials is immense [1–10]. This is mainly due to the promising properties of different Me-ZSM-5 sam- ples in the selective catalytic reduction of nitrogen oxides with hydrocarbons [1]. Analysis of sum- marized literature data shows that the bridged hydroxyls of H-ZSM-5 are among the most acidic hydroxyls not only in zeolites, but in different molecular sieves and also on oxides [11]. Metal cations exchanged in ZSM-5 are characterized by a low coordination number and are able to absorb up to four small molecules. Thus, studies of N 2 and CO adsorption on alkali-cation exchanged ZSM-5 revealed formation, at low temperature, of Me þ (L) 2 complexes [2–4]. Three CO molecules have been reported to be bound to one Cu þ [5] and Ni þ cation [6] in a ZSM-5 matrix. Formation of tetracarbonyls have been reported with Co þ [7] and Rh þ [8] cation, again in ZSM-5. The possibility of simultaneous coordination of more than one small molecule to one cation is re- ported for other zeolites and molecular sieves also, e.g. Y [12,13], EMT [14], and ETS-10 [15]. However, the formation of geminal species strongly depends Catalysis Communications 4 (2003) 485–491 www.elsevier.com/locate/catcom * Corresponding author. Fax: +359-2-8705024. E-mail address: kih@svr.igic.bas.bg (K. Hadjiivanov). 1566-7367/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S1566-7367(03)00129-8