Journal of Catalysis 236 (2005) 168–171 www.elsevier.com/locate/jcat Research Note New type of rhodium gem-dicarbonyls formed in Rh-ZSM-5: An FTIR spectroscopy study Elena Ivanova a , Mihail Mihaylov a , Frederic Thibault-Starzyk b , Marco Daturi b , Konstantin Hadjiivanov a,∗ a Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria b Laboratoire Catalyse et Spectrochimie, CNRS-ISMRA, Université de Caen, 6, bd. Maréchal Juin, 14050, Caen cedex, France Received 12 July 2005; revised 13 September 2005; accepted 14 September 2005 Available online 13 October 2005 Abstract CO adsorption on Rh-ZSM-5 leads to the formation of both the well-known Rh + (CO) 2 gem-dicarbonyls and another kind of dicarbonyl species as yet unknown. The latter are most probably formed with the participation of Rh n+ (n> 1) ions in cationic positions in the zeolite. These species are characterized by ν s (CO) at 2176 cm −1 and ν as (CO) at 2142 cm −1 . The dicarbonyl structure is proven by 12 CO– 13 CO coadsorption. In excellent agreement with the theoretically expected values, the mixed complexes, Rh n+ ( 12 CO)( 13 CO), were characterized by ν ( 12 CO) at 2164 cm −1 and ν ( 13 CO) at 2106 cm −1 . In contrast to the classical gem-dicarbonyls, the new species are destroyed in the presence of water. This is consistent with the proposed higher oxidation state of rhodium in this case.  2005 Elsevier Inc. All rights reserved. Keywords: Adsorption; Carbon monoxide; FTIR spectroscopy; Rhodium; Dicarbonyls; Zeolites 1. Introduction The carbonyl complexes of supported rhodium have been a subject of steady interest [1–26]. It is well established that CO adsorption of rhodium-containing catalysts results in the for- mation of Rh + (CO) 2 species characterized by ν s (CO) at 2120– 2075 and ν as (CO) at 2053–1990 cm −1 [1–25]. Most of the investigations of CO adsorption on supported rhodium catalysts have been performed with prereduced samples. Because of the high stability of the gem-dicarbonyls of Rh + , they are produced via oxidation of metallic rhodium, and most authors believe that the support surface hydroxyl groups are involved in the process [20–25]. There have been few studies of nonreduced rhodium- containing samples [21,26]. Several authors have reported car- bonyl bands at relatively higher frequencies and assigned them to Rh n+ –CO species (n> 1) [7–9,19,20,26]. In this paper we report the formation of new types of rhodium dicarbonyl com- * Corresponding author. Fax: +359 2 8705024. E-mail address: kih@svr.igic.bas.bg (K. Hadjiivanov). plexes formed with rhodium cations in an oxidation state higher than 1+. These species are produced in an Rh-ZSM-5 sample, and presumably the high coordinative unsaturation of cations in zeolites contributes to their formation. Indeed, some authors have reported [26] that Rh + cations in Rh-ZSM-5 are low co- ordinated and can accommodate up to four CO molecules. In this paper we report on the formation of the as-yet unknown rhodium dicarbonyl species. The starting H-ZSM-5 material, supplied by Degussa, had a Si/Al ratio of 26.8. Rh-ZSM-5 was prepared by solid-state ion exchange; 1.5 g of H-ZSM-5 was mixed with 0.15 g RhCl 3 · nH 2 O, the mixture was ground in an agate mortar, and then placed in a quartz reactor and heated to 773 K in a nitrogen flow for 1 h. The nominal rhodium concentration in the sample was 3.7 wt% Rh and corresponded to an exchange degree of ca. 60%. The IR spectra were recorded on a Nicolet Avatar 360 spec- trometer at a spectral resolution of 2 cm −1 and accumulation of 128 scans. A self-supporting pellet (ca. 15 mg cm −2 ) was pre- pared from the sample powder and treated directly in a purpose- made IR cell. The latter was connected to a vacuum-adsorption 0021-9517/$ – see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2005.09.017