Comparison of alkali-promoted ZrO2 catalysts towards carbon black oxidation

The effect of alkali metals deposition on zirconia has been studied in the oxidation of carbon black, considered as a model of diesel soot. The study of the influence of alkali content and alkali precursor was undertaken for K/ZrO₂, evidencing a better activity for a catalyst prepared with an atomic ratio K/Zr = 0.14 and from a nitrate precursor. Using the latter preparation conditions, alkali/ZrO₂ are found to be active in the oxidation of carbon black according the sequence ZrO₂ < Li/ZrO₂ < Na/ZrO₂ < K/ZrO₂ < Rb/ZrO₂ < Cs/ZrO₂. Alkali metals have an influence on the tetragonal–monoclinic crystalline modification. Alkali metals ions with low size tend to stabilize the tetragonal ZrO₂ phase whereas those with higher ionic radius favour the tetragonal–monoclinic modification. Fourier transform-infrared spectroscopy (FTIR) and temperature programmed reduction (TPR) measurements show that the catalytic activity partially depends on the presence of nitrate species stabilized in alkali/ZrO₂ even after calcination treatment at 600 °C. Nitrate species are more stable in the presence of alkali with high ionic radius than those of low size.