Investigation of the Cu-Zr-Y oxides activity in the carbon black catalytic oxidation by differential thermal analysis and temperature programmed reduction

Different copper/zirconium-yttrium catalysts have been tested in carbon black oxidation reaction. Supported mainly on differential thermal analysis and temperature programmed reduction, two different mechanisms have been proposed to explain the catalytic results. In the absence of copper, it has been shown that Zr³⁺ ions and associated anionic vacancies are responsible to the catalytic enhancement observed in the mixed oxides, oxygen species being activated on these sites. Among mixed zirconia-yttria solids, ZrO₂-5 mol%Y₂O₃ is the most active catalyst. Copper impregnation on these oxides leads to the formation of different copper species. Small particles of CuO in low interaction with the support, induce a catalytic improvement due to the highest reducibility of these species. Moreover, in order to be more efficient, CuO species should have some interactions with the support, since impregnated samples are more active than the simple mechanical mixtures.     

Total Oxidation of Propene and Toluene on Copper/Yttrium Doped Zirconia

The catalytic oxidation of propene and toluene has been investigated on pure ZrO₂, pure Y₂O₃, and ZrO₂ doped with 1, 5, and 10 mol % Y₂O₃ in the presence or absence of copper (0.5, 1, and 5 wt%). A synergetic effect has been detected since ZrO₂ and Y₂O₃ exhibit significantly lower activities than the mixed oxides. The higher surface areas, related to structural change from mononoclinic (ZrO₂) to tetragonal (ZrO₂–;;Y₂O₃), partly explained the higher activity of ZrO₂–;;Y₂O₃. However, it has been shown that the number of anionic vacancies, created by the substitution of Zr⁴⁺ by Y³⁺, in yttria-stabilized zirconia solids depends on the yttrium contents. Their effect on propene and toluene oxidation activity is significant. The anionic vacancies should induce better activity of the ZrO₂—5 mol % Y₂O₃ catalyst with or without copper, which presents the higher number of Zr³⁺ species. This support should favor the formation of CuO particles, which should be the most active catalytic sites in the studied reaction.     

Carbon black and propylene oxidation over Ru/CoxMgyAl2Oz catalysts

The effects of Co_xMg_yAl₂O_z mixed oxides composition and ruthenium addition on the oxidation of propylene and carbon black (CB) were investigated. Different reactive cobalt and ruthenium oxide species were formed following calcination at 600 °C. The addition of ruthenium was beneficial for the CB oxidation under “loose contact” conditions and for propylene oxidation when the cobalt content was intermediate to low. The calculated activation energy for CB oxidation was decreased from 151 kJ mol⁻¹ for the uncatalyzed reaction to 111 kJ mol⁻¹ over the best catalyst.     

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.