Total oxidation of acetic acid in aqueous solutions over noble metal catalysts

Ruthenium catalysts (1-5 wt%) were prepared by exchange or impregnation of different supports: CeO2, TiO2, or ZrO2. Their activity for acetic acid oxidation in aqueous solutions was investigated in a batch reactor (20 bar O-2, 0.083 M CH3COOH, 200 degrees C). Ru catalysts supported on a high-surface area ceria have an excellent activity for the title reaction. The following order of activity was obtained: Ru > Ir > Pd approximate to Fe approximate to Cu > Ag approximate to Ni approximate to Co approximate to Cr approximate to unpromoted CeO2, which can be correlated with the tendency of the metals to remain in the metallic form. The operating Ru/CeO2 catalyst would be composed of large Ru-o particles with surface Run+ species: the prereduced catalysts are more active than the preoxidized ones and turnover frequencies are higher on the larger Ru particles which are less oxidizable. The catalysts were examined by XRD and TEM before and after the oxidation reaction. Only metallic Ru could be seen by these techniques which confirmed besides that Ru is remarkably stable (no apparent sintering). A kinetic study was performed on a catalyst with Ru particles of about 20-30 nn. The activation energy was close to 100 kJ mol(-1) while the kinetic order was negative in acetic acid (n approximate to -0.5) and positive in oxygen (n approximate to 0.5). The reaction rate decreased significantly at higher pH values; acetate anion is much less reactive than acetic acid. A mechanism including a homolytic scission of the O-H bond is proposed and discussed in the light of the present results and of the literature data. (C) 1998 Academic Press.