A study of the stability of tungstophosphoric acid, H3PW12O40, using synchrotron XPS, XANES, hexane cracking, XRD, and IR spectroscopy

Tungstophosphoric acid (HPW) has been investigated using different spectroscopic and chemical techniques. Bulk-sensitive techniques such as X-ray diffraction (XRD) and infrared (IR) spectroscopy indicate that the acid is stable at temperatures as high as 300 °C or higher. However, our work suggests that, besides the surface dehydration, HPW starts loosing stability at temperatures as low as 200 °C. For instance, P 2p peak was not detected in the synchrotron radiation XPS spectrum of HPW preheated at 100 °C, but was clearly observed after preheating the acid at 200 and 400 °C. This suggests the destruction of the molecules of the surface leading to the apparent enrichment of surface with phosphorous. These results may explain why HPW deactivates very fast, e.g., 8 min at 200 °C, in hexane-cracking experiments. This could limit the use of HPW in surface reactions that even require moderate temperatures. Detailed infrared spectroscopic investigation of the HPW as a function of temperature showed a gradual increase in absorbance of the WOW corner-shared vibration relative to the absorbance of the other bands. This indicates that the symmetry, and hence the stability, of the molecule was decreased upon heating.