FTIR study of low-temperature CO adsorption on Mn-ZSM-5 and MnY zeolites. Effect of the zeolite matrix on the formation of Mn2+(CO) geminal species

Abstract

Adsorption of CO on Mn-ZSM-5 zeolite at 85 K results in formation of physically adsorbed CO, several kinds of Hbonded CO and Mn 2þ ðCOÞ x geminal species ð2202 cm À1 Þ. Decreasing the coverage during evacuation results in disappearance of the physically adsorbed CO and the H-bonded forms and in conversion of the dicarbonyls to linear Mn 2þ -CO species ð2214 cm À1 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 À1 ) 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 matrix. Ó

Characterization of the Oxidation States of Supported Gold Species by IR Spectroscopy of Adsorbed CO

“…4, structure d). This conclusion is consistent with the earlier con- clusion [48,50] that a cation in an S II position in zeolite Y must have a sufficiently large radius to form a dicarbonyl species; thus, Mn 2+ ions (with a radius of 0.80 Å) are too small and only form a negligible fraction of dicarbonyls, but larger cations (such as Au 3+ with a radius of 0.85 Å) are expected to accommodate two CO molecules simultaneously.…”

“…However, in the case of weak adsorption (our case), there is a lack of vibrational interactions between the two CO oscillators, and no splitting of the CO stretching modes occurs [26, 43, 45 -49]. For instance, this is the case of simultaneous coordination of two or three CO or N 2 molecules to alkali or alkaline earth metal ions [45 -47], and also to Mn 2+ [48], Ni 2+ [49], etc., exchanged in zeolites.…”

Catalysts for selective propane oxidation in the presence of carbon monoxide: Mechanistic aspects

“…The band at 2100 cm À1 indicates that the CO molecule adsorbs on Cu + [20,21]. There are also bands at 3700 cm À1 (not shown here) where OH vibrations of metal oxides are postulated [13,17,20]. At 1678 and 1282 cm À1 , bands of chemisorbed CO were recorded.…”

“…There are the same CO species, whether there is propane in the feed or not. Both show intense bands at 1585 cm À1 and 1281 cm À1 assigned to carbonates [11,21,31] and a weak band at 2173 cm À1 , assigned to linearly adsorbed CO on Mn 2+ [17,32]. Apparently, the initially on Cu + -ions adsorbed CO starts changing its adsorption site with temperature to Mn 2+ -ions.…”

“…7a, the feed leads to a significant and narrow adsorption band at 2100 cm À1 typical for linearly adsorbed CO [15][16][17][18][19]. CO can adsorb on different copper species Cu 0 , Cu + , and Cu 2+ .…”