Spectroscopic and Thermodynamic Characterization of Strontium Carbonyls Formed upon Carbon Monoxide Adsorption on the Zeolite Sr−Y

Abstract

The adsorption of CO on the faujasite-type zeolite Sr-Y generates four types of carbonyls: the Sr(CO) 2+ species, which has a characteristic IR absorption band at 2191 cm -1 ; the Sr(OC) 2+ species at 2095 cm -1 ; Sr(CO) 2 2+ , which has a single mode at 2187 cm -1 ; and Sr(CO, OC) 2+ , which has a mode at 2098 cm -1 . The use of a variable-temperature IR cell allowing the simultaneous measurements of temperature, pressure, and coverages (intensities) of CO species to be made has allowed the determination of the standard enthalpy and entropy of formation of each species, so obtaining a fairly thorough thermodynamic characterization of the CO/Sr-Y system. C-and O-bonded carbonyl species were found to be in a temperature-dependent isomerization equilibrium, which was also studied by means of variable-temperature IR spectroscopy.

Probing zeolites by vibrational spectroscopies

“…17,154,[274][275][276][277][278][279][280][281][283][284][285]288, In brief, the technique is based on displacement of the equilibrium between molecules in the gas phase and those adsorbed on a given type of surface site, which is carried out by changing the temperature T of the system, as shown in Fig. 17,154,[274][275][276][277][278][279][280][281][283][284][285]288, In brief, the technique is based on displacement of the equilibrium between molecules in the gas phase and those adsorbed on a given type of surface site, which is carried out by changing the temperature T of the system, as shown in Fig.…”

Variable‐Temperature Infrared Spectroscopy Studies on the Thermodynamics of CO Adsorption on the Zeolite Ca–Y

“…Figure 6 shows a plot of the left-hand side of Equation (5) against the reciprocal temperature for the six most intense spectra in Figure 3 (the others were discarded because of difficulty of obtaining A (OC) values with the required precision). This isomerization enthalpy value can be compared with that of DH 0 iso = + 9.8 kJ mol À1 reported in the literature [18] for isomerization between Sr 2 + ···CO and Sr 2 + ···OC species in the zeolite Sr-Y. This isomerization enthalpy value can be compared with that of DH 0 iso = + 9.8 kJ mol À1 reported in the literature [18] for isomerization between Sr 2 + ···CO and Sr 2 + ···OC species in the zeolite Sr-Y.…”

“…However, it seems pertinent to point out that such a positive correlation, also termed entropy-enthalpy compensation, [22][23][24] is widely reported to occur in a large variety of chemical processes involving weak interactions, including hydrogen bonding, [25,26] formation of weakly associated (van der Waals) molecular complexes [27] and hydrogen adsorption on zeolites. However, there is an intrinsic limit to DS 0 in the adsorption process, because the CO References : metal oxides [19][20][21] ; Na-ZSM-5, Na-FER, and K-FER [6] ; Sr-Y [18] ; Ca-Y (this work). In general terms, for the case of carbon monoxide adsorption entropy-enthalpy compensation reflects the fact that a stronger (enthalpy related) interaction between CO and the adsorbing centre leads to a greater decrease of its motion freedom (which is related to entropy).…”

“…As discussed in detail by Hadjiivanov and Knçzinger, [14] Ca 2 + (CO) 2 dicarbonyl species on Ca-Y give a characteristic IR absorption band at 2191 cm À1 , while that of Ca 2 + (CO) 3 tricarbonyl species appears at 2185 cm À1 . A temperature-dependent isomerization equilibrium between carbonyl and isocarbonyl species (linkage isomerism) is well known to occur when CO is adsorbed on zeolites containing extraframework alkali [3,16,17] or alkaline-earth [3,18] metal cations. Following previous reports, [14,15] the band at 2198 cm À1 is assigned to Ca 2 + ···CO monocarbonyls formed on Ca 2 + located at site S II .…”

“…The carbonyl species invariable shows the fundamental CÀO stretching mode at higher wavenumbers than free CO (2143 cm À1 ), while the isocarbonyl species shows the corresponding stretching mode below 2143 cm À1 . On the other hand, it is also relevant to add that for CO adsorption on the zeolite Sr-Y the corresponding value of standard adsorption enthalpy was reported to be [18] DH 0 = À45.3A C H T U N G T R E N N U N G (AE1.0) kJ mol À1 . Whatever its real nature is, which can be debatable, this shoulder does not affect the thermodynamic study discussed in this article, since (temperature-dependent) integrated absorbance of the IR absorption bands at 2198 cm À1 is obtained after excluding such a shoulder (by computer resolution).…”