A study of the stability of tungstophosphoric acid, H₃PW ₁₂O₄₀, 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.