Anhydrous and Water-Assisted Proton Mobility in Phosphotungstic Acid

Nonlocal gradient-corrected density functional theoretical calculations were used to determine
the energetics associated with proton migration in phosphotungstic acid. The activation energy for anhydrous
proton hopping between two oxygen atoms on the exterior of the molecular Keggin unit was calculated to
be 103.3 kJ mol-1. The quantum-tunneling effect on the rate of proton movement was determined using
semiclassical transition-state theory and was found to be a major contributor to the overall rate of proton
movement at temperatures below approximately 350 K. The adsorption of water on an acidic proton
decreases the activation barrier for hopping to 11.2 kJ mol-1 by facilitating proton transfer along hydrogen
bonds. The overall rate constant for proton hopping was determined as a function of temperature and
water partial pressure. Small amounts of water greatly enhance the overall rate of proton movement.