Photocatalyzed degradations on a TiO2-coated quartz crystal microbalance. I. Adsorption/desorption processes in the degradation of phenol and catechol

Abstract

The photocatalyzed degradation of phenol and catechol was re-examined to assess their adsorption/desorption behavior on a TiO2-coated (sol–gel technique) quartz crystal microbalance (QCM) sensor. The QCM senses changes in mass occurring during the photodegradation, which of necessity implicates photoinduced adsorption and desorption of molecules of the original substrate and its degradation intermediates. The temporal loss of spectral features of the benzene ring with irradiation time was monitored by fluorescence spectroscopy. The increase in the quantity of TiO2 deposited on the QCM sensor scaled linearly with the number of dip-coatings (five). Water adsorption on QCM in the dark was relatively independent of pH. Adsorption of the organic substrates was pH dependent since the surface charge of TiO2 varies with pH. Thus, both phenol and catechol tended to be adsorbed more at neutral pH 7 than at the other pHs examined and within the concentration range of substrate 0 to 0.012 mM. For phenol, adsorption reached near saturation around 0.007 mM in the order pH 7 > pH 9 > pH 5. Catechol showed a similar behavior except at the higher concentrations (around 0.010 mM). Mass changes occurring during the photodegradation of phenol and catechol on the TiO2-coated QCM sensor surface are reported.