hide
Free keywords:
-
Abstract:
Differential heats of ammonia sorption onto and into heteropolyacids (HPA's) of tungsten and molybdenum (HPW, HSiW, HPMo and HSiMo) were measured microcalorimetrically to characterize the acidic properties of these metal oxygen cluster compounds (MOCC's). An effect of activation conditions was observed in that a lower activation temperature resulted in a higher differential heat of sorption but a lower number of acidic sites accessible for NH3. Complementary investigations, i.e., TG, XRD, TPD of water-containing species and ammonia as well as 1H MAS NMR experiments, were carried out. These provided evidence that structural and acid properties as well as site accessibility of the HPA's significantly depend on H2O (or H5O+2) and NH3 loading. A comparison of the results of microcalorimetry of NH3 sorption into and TPD of NH3 from HPA's suggests that the former technique is more reliable in order to elucidate acidic properties of HPA's.
