Structure and redox properties of MnO x/Yttrium-stabilized zirconia (YSZ) catalyst and its used in CO and CH 4 oxidation

Abstract

The complex structure, the redox properties, and the catalytic activity for the MnO x /yttrium-stabilized zirconia (YSZ) catalytic system were explored and reported here. The MnO x /YSZ material (10% by wt. as MnO 2) 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 MnO x /YSZ system in CO and CH 4 oxidation (fuel rich/lean) was investigated and correlated with the observed structure and redox properties. XRD data indicated that MnO x 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 Mn 3+ and Mn 2+. Low-temperature CO adsorption on MnO x /YSZ shows the formation of Mn 3+–CO species (2180 cm −1) which are easily desorbed by evacuation at 85 K. Heating the sample (120 K and up) in CO atmosphere shows the formation of CO 2 and bridged or bidentate carbonates which block the CO adsorption sites. At higher temperatures, the CO 3 2− species are mostly converted into HCO 3 − species. When CO is adsorbed at 85 K on a sample exposed to a H 2-rich atmosphere, Mn 2+–CO species are formed. Heating the sample in CO atmosphere (O-rich) leads again to oxidation of CO. In this case no CO 2 is formed, but at ∼130 K monodentate carbonates start to emerge. Catalytic activity results indicate that the MnO x presence favored the CO oxidation process but had an inhibiting effect on the CH 4 oxidation. The kinetic experiments showed that for CO oxidation, the rate is first order in O 2 on MnO x /YSZ catalyst and zero order on the YSZ support.