transformation of the ammonium form of the zeolite into t responding protonic form during the thermal treatment describer above. After recording the background spectrum of the sample wafer at 77 K the cell was dosed with a fixed amount of co2 closed. and a series of IR spectra was recorded upon gradual warm ing up of the cell. while simultaneously registering temperatu and gas equilibrium pressure inside the cell. For that t purpose, cell was equipped with a platinum resistance thermometer (Tins ley) and a capacitance pressure gauge (MKS, Baratron). Precision of measurements was better than *10-2 mbar and +2 K for pre sure and temperature, respectively. Pressure correction for helium was determined from a calibration plot described elsewhere as 14 Transmission FTIR spectra were recorded, at 3 cm resolution, on a Bruker FS66 instrument, accumulating 64 for each spectrum scans A series of IR spectra obtained over relatively large tempera a ture range can be used to determine the corresponding standard adsorption enthalpy, AH0. and entropy, Aso. by t vriK using method described in detail elsewhere 136.48]. Briefly, for the present case, let Eq. (1) represent the temperature-dependent adsorption equilibrium of co2 on the adsorption sites, ZOH, rep resenting the Bronsted-acid centres of the zeolite: zoHs) co zoH...co 2(g) 2(ads) As shown in Section 3 below. formation of the hydrogen-bonded zoH CO2 adsorption complex brings about a bathochromic shift of the free zo-H stretching band, with its concomitant decrease of integrated intensity. Therefore, at any given temperature T, the intensity of that band should be inversely proportional to surfac coverage, 8. thus providing direct information on the activity in the thermodynamic sense) of both the adsorbed species and the empty ites (1-1): simultaneously, the equilibrium pressure. p monitors the activity of the gas phase. Hence, the equilibrium constant, for K. the adsorption process at that temperature can be determined. The variation of K with T is given by the van't Hoff equation: KT -exp(-AH0/RT)exp(ASO/R)