Modification of the photocatalytic activity of Pd/TiO2 and Zn/TiO2 systems through different oxidative and reductive calcination treatments

Two different solids consisting of Pd or Zn-containing titania systems (metal/titanium nominal ratio of 1%) were submitted to diverse oxidative and reductive calcination treatments and tested for gas-phase selective photooxidation of 2-propanol. As regards the Pd system, reduction at low temperature (≤500 °C) resulted in a gradual increase in catalytic activity which was ascribed to the gradual reduction of bulk palladium to Pd0. Thermal treatment of the system at high temperature (850 °C) in static air, air flow or hydrogen flow led to a decrease in activity as the result of the sharp decrease in surface area. Nevertheless, those systems containing Pd0 only were more active than the one consisting of Pd + PdO. Finally, Pd-system overcame Pd migration to the surface on reduction at 850 °C which resulted in a significant increase in selectivity to acetone up to 97% for a time on stream of 5 h. As regards the Zn-containing system, none of the applied treatments resulted in improvement in photocatalytic activity. It seems that the most favourable situation for photocatalysis is that on which Zn atoms are substituting titanium ones in the lattice, whereas segregation of Zn to form small ZnO clusters is especially detrimental to activity.