RESUMEN
Adsorption of NO and coadsorption of NO and O2 on H-ZSM-5 have been studied at low and room temperature by means of FTIR spectroscopy. For better interpretation of the spectra, experiments involving isotopic labeled molecules have been performed. Low temperature adsorption of NO on H-ZSM-5 results initially in formation of NO which is H-bonded to the zeolite acidic hydroxyls. A second NO molecule is inserted into the OH-NO species at higher coverages, thus forming OH(NO)2 complexes. Different kinds of NO dimers are also formed. Negligible amounts of oxygenated compounds have been detected. In the presence of oxygen, the (di)nitrosyl species are oxidized very fast even at 100 K to N2O3, NO+, NO2, and N2O4. Different kinds of adsorbed N2O3 species have been evidenced. With increasing temperature, NO+ migrates and occupies cationic positions. The latter species interacts with NO at low temperature to give an [ONNO]+ complex. This reaction is used to prove that the different bands in the 2206-2180 cm(-1) region are also due to NO+ species.
RESUMEN
By using a supersaturation gradient along a protein solution contained in a glass capillary tube, we modified the classical double pulse technique, thus substantially accelerating the procedure of measurement of nucleation parameters. Data for the number of crystal nuclei, n vs nucleation time, t, were obtained for hen-egg-white lysozyme, chosen as a model because of the availability of reliable solubility data in the literature. The stationary nucleation rate and the nucleation time lag have been measured. Quantitative data for the work required for nucleus formation (A(k) = 4.3 x 10 (-1)3 erg) and the size of the critical cluster (three molecules) were also obtained. Besides, it was observed that Ostwald ripening seems to play an important role for nucleation times longer than 150 min. Using the same technique, semi-quantitative investigations were performed with porcine pancreatic trypsin.