FTIR study of low-temperature CO adsorption on Mn-ZSM-5 and MnY zeolites. Effect of the zeolite matrix on the formation of Mn 2þ ðCOÞ x geminal species Konstantin Hadjiivanov a, * , Elena Ivanova a , Margarita Kantcheva b , Erkan Z. Ciftlikli b , Dimitar Klissurski a , Lubomir Dimitrov c , Helmut Kn€ ozinger d a Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria b Department of Chemistry, Faculty of Science, Bilkent University, 06533 Bilkent, Ankara, Turkey c Institute of Catalysis, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria d Department Chemie, Physikalische Chemie, Ludwig Maximilians Universit€ atM€ unchen, Butenandtstrasse 5-13 (Haus E), D-81377 M€ unchen, Germany Received 24 April 2002; received in revised form 7 June 2002; accepted 7 June 2002 Abstract Adsorption of CO on Mn-ZSM-5 zeolite at 85 K results in formation of physically adsorbed CO, several kinds of H- bonded CO and Mn 2þ ðCOÞ x geminal species ð2202 cm 1 Þ. Decreasing the coverage during evacuation results in dis- appearance of the physically adsorbed CO and the H-bonded forms and in conversion of the dicarbonyls to linear Mn 2þ –CO species ð2214 cm 1 Þ. The latter are quite stable at 85 K. Coadsorption 12 CO and 13 CO reveals that the CO molecules in the geminal polycarbonyls behave as independent oscillators. In contrast, CO adsorption at 85 K on MnNaY zeolite only leads to formation of linear Mn 2þ –CO species (2210 cm 1 ) and mono- and di-carbonyls associated with residual sodium cations. The results are interpreted as evidence that site-specified geminal carbonyls are formed with cations possessing an ionic radius bigger than a critical value. This value is different for different positions in various zeolites and is bigger for cations in S II positions in Y zeolites than is the case of cations in a ZSM-5 ma- trix. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Adsorption; Carbon monoxide; Geminal carbonyls; MnY; Mn-ZSM-5 1. Introduction The possibility of bonding of more than one molecule to one surface site is of great importance for the heterogeneous catalysis because this facili- tates the interaction between the adsorbed mole- cules. Therefore, it is of definite interest to know the factors determining the formation of geminal species. Simultaneous coordination of two or three molecules to one site is a well-known phenome- non. Typical examples are the di- and tri-carbonyls Catalysis Communications 3 (2002) 313–319 www.elsevier.com/locate/catcom * Corresponding author. E-mail address: kih@svr.igic.bas.bg (K. Hadjiivanov). 1566-7367/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S1566-7367(02)00140-1