Quantitative measurement of Bronsted acidity by TPD of ammonia on H sub(3)[PMo sub(12)O sub(40)] and its Cs1 salt, in bulk and supported on SBA-15

• Published in: Journal of thermal analysis and calorimetry Vol. 123; no. 1; pp. 557 - 569
• Main Authors: Sasca, Viorel Zoltan, Popa, Alexandru, Verdes, Orsina
• Format: Journal Article
• Language: English
• Published: 01.01.2016
• Subjects: Ammonia; Bonding strength; Calorimetry; Cations; Condensation; Desorption; Nitrous oxides; Reduction
• ISSN: 1388-6150; 1572-8943
• DOI: 10.1007/s10973-015-4896-2

The method for acidity measurement through temperature programmed desorption (TPD) of ammonia was adapted for simultaneous TG/DTG-DTA analysis of the H sub(3)[PMo sub(12)O sub(40)] and its Cs acid salts, in bulk and supported on SBA-15 support. The desorption of NH sub(3) consists of the NH sub(3) and H sub(2)O release in steps as a result of temperature increase. The last step corresponds to expelling of NH sub(3) by decomposition of species strong bonded like the ammonium cation, simultaneously with constitutive water, within the 300-600 degree C range. The MS analysis showed that the oxidation of ammonia to N sub(2)O and NO during desorption had occurred simultaneously with reduction of Mo super(6+). The reoxidation of samples with air in all cases was done finally. In order to separate the water release (water formed by condensation of hydroxyl groups belonging to SBA-15) from the ammonia desorption for supported samples, a TPD of H sub(2)O was done for every sample. In the case of pure compounds, the mass loss corresponding to the last stage of desorption (after correction with the mass loss as a result of reduction, equal with the mass increase owing to the reoxidation) gives the amounts of NH sub(3) and constitutive water, which correspond stoichiometrically with the Bronsted acidity. The subtraction of mass loss for TPD of water from the corrected mass loss for TPD of ammonia and water (because of the mass loss as a result of the reduction) gives the amount of NH sub(3) corresponding stoichiometrically with the Bronsted acidity. The acidic strength was estimated on the basis of the temperatures for the DTG peaks.