N. Essayem et al., AMMONIA ADSORPTION-DESORPTION OVER THE STRONG SOLID ACID CATALYST H3PW12O40 AND ITS CS- COMPARISON WITH SULFATED ZIRCONIA( AND NH4+ SALTS ), Journal of the Chemical Society. Faraday transactions, 93(17), 1997, pp. 3243-3248
Citations number
34
Categorie Soggetti
Chemistry Physical","Physics, Atomic, Molecular & Chemical
Ammonia thermodesorption using a thermal conductivity detector does no
t allow an unambiguous study of the acidic properties of strong solid
acids such as heteropolycompounds and sulfated zirconia. As a matter o
f fact, these strong acids release compounds other than NH3 during TPD
experiments which obviously involves side-reactions. In the present w
ork, in order to circumvent the preceding problems, NH3 adsorption and
TPD experiments were performed using a dual mass spectrometry (MS)-TC
D detection system. The equipment was applied to the characterization
of the acidic properties of porous heteropolycompounds. CsxH3-xPW12O40
and (NH4)(2.8)H0.2PW12O40, by comparison with the parent acid H3PW12O
40 and another strong solid acid, sulfated zirconia. It was observed t
hat gaseous NH3 reacted and formed ionic species, leading to (NH4)(3)P
W12O40 in the case of heteropolyacids (HPA) and probably (NH4)HSO4 in
the case of sulfated zirconia. Consequently, based on the amount of NH
3 irreversibly adsorbed at 373 K the number of acid sites has been ded
uced. For all HPA samples, TPD profiles of NH3 appeared quite similar
suggesting that the TPD curves were more connected to the stability of
these ammonium salts rather than to the acid strength of the solid.