Fabrication of Nafion- and heteropoly acid-containing thin catalytic films without failure

It has been shown that homogeneous thin films of silica heavily loaded with phosphotungstic acid (30 wt.%) or Nafion (50 wt.%) can be uniformly deposited on various substrates by a dip-coating to give effective structured catalysts with strong acidic properties. Experiments indicate that the use of well condensed silica sols, with over 75% of Qn population in the form of Q3 sites, is a precondition to obtain crack-free films and that the withdrawal speed plays a second order role, especially at lower viscosities. Based on the quality maps of the films prepared under various conditions we conclude that defect-free coatings can be obtained provided that sol viscosity is above the specific threshold value and still within a linear part of the viscosity dependence upon time.