Structure and redox properties of MnOx/yttrium-stabilized zirconia (YSZ) catalyst and its used in CO and CH4 oxidation

The complex structure, the redox properties, and the catalytic activity for the MnOx/yttrium-stabilized zirconia (YSZ) catalytic system were explored and reported here. The MnOx/YSZ material (10% by wt. as MnO2) was characterized by temperature-programmed reduction (TPR), X-ray diffraction (XRD) and FITR spectroscopy of low-temperature CO adsorption. The redox properties were explored by exposing the material to H-2-rich/O-2-rich environment cycles, at various temperatures (up to 1173 K), followed by material characterization. The catalytic activity of the MnOx/YSZ system in CO and CH4 oxidation (fuel rich/lean) was investigated and correlated with the observed structure and redox properties. XRD data indicated that MnOx is well dispersed on the YSZ support, with crystallites below 2-3 nm (close to the XRD detection limit). TPR data show that most of the Mn is present as Mn3+ and Mn2+. Low-temperature CO adsorption on MnOx/YSZ shows the formation of Mn3+-CO species (2180cm(-1)) which are easily desorbed by evacuation at 85 K. Heating the sample (120 K and up) in CO atmosphere shows the formation Of CO2 and bridged or bidentate carbonates which block the CO adsorption sites. At higher temperatures, the CO32- species are mostly converted into HCO3- species. When CO is adsorbed at 85 K on a sample exposed to a H-2-rich atmosphere, Mn2+-CO species are formed. Heating the sample in CO atmosphere (O-rich) leads again to oxidation of CO. In this case no CO2 is formed, but at similar to 130 K monodentate carbonates start to emerge. Catalytic activity results indicate that the MnOx presence favored the CO oxidation process but had an inhibiting effect on the CH4 oxidation. The kinetic experiments showed that for CO oxidation, the rate is first order in O-2 on MnOs/YSZ catalyst and zero order on the YSZ support. (C) 2002 Published by Elsevier Science B.V.