Enthalpy and Entropy of Acetone Interacting with Degussa P25 TiO2 Determined by Chemical Ionization Mass Spectrometry

Abstract

We have applied chemical ionization mass spectrometry (CIMS) to study the temperature-dependent interaction of acetone with Degussa P25 TiO2, an inexpensive catalyst often used to mineralize volatile organic compounds. Acetone binding was examined between room temperature and 373 K, moderate temperatures likely to be employed for remediating indoor air pollution. Degussa P25 substrates were exposed to environmentally representative acetone partial pressures between 5 × 10-8 and 2 × 10-4 Torr, and the interaction is found to be largely reversible. The enthalpies of adsorption obtained from model-dependent fits to the adsorption isotherms were compared with the model-independent isosteric heats of adsorption and are found to range between −12(10) and −17(5) kJ mol-1. Standard adsorption entropies range between −40(34) and −57(17) J mol-1 K-1 when referenced to a half-saturated catalyst surface standard state. Statistical thermodynamic calculations involving local versus mobile adsorbates suggest that a model involving mobile surface species fits the reversible adsorption entropy data better than does a localized one-site Langmuir model.