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Structure and Redox Properties of MnOx/Yttrium-stabilized Zirconia (YSZ) Catalyst and its Use in CO and CH4 Oxidation

Domenii publicaţii > Stiinte ingineresti + Tipuri publicaţii > Articol în revistã ştiinţificã

Autori: Radu Craciun, B. Nentwick, K. Hadjiivanov, and H. Knozinger

Editorial: Elsevier, Applied Catalysis A: General, 243 (1), p.67-79, 2003.

Rezumat:

The complex structure, the redox properties, and the catalytic activity in CH4 combustion for the MnOx-supported/ytrium-stabilized zirconia (YSZ) catalytic system were explored and reported here. The MnOx/YSZ material (at 10% by wt. MnO2 loading) was characterized by temperature-programmed reduction (TPR), X-ray diffraction (XRD) and low-temperature CO adsorption FTIR spectroscopy. The redox properties were evaluated by exposing the material to H2-rich/O2-rich environment cycles, at various temperature (up to 1173K), followed by material characterization. The catalytic property of the MnOx/YSZ system in CH4 oxidation (fuel rich/lean) was investigated and correlated with the observed structure and redox property data. XRD data indicate that MnOx is well dispersed on the YSZ support, with crystallites around 2-3 nm (close to the XRD detection limit). TPR data showed that most of the Mn is present as Mn3+ and Mn2+. The TPR results show the signals specific to Mn4+/Mn3+ and Mn3+/Mn2+ reduction. Low-temperature CO adsorption on MnOx/YSZ shows the formation of Mn3+-CO species (2180 cm﷓1) which are easily desorbed by evacuation at 85 K. Heating the sample (120K and up) in CO atmosphere shows the formation of CO2 and bridged or bi-dentate carbonates which block the CO adsorption sites. At higher temperatures the CO32+-like species are mostly converted into HCO3–like species. When CO is adsorbed at 85 K on a sample exposed to a H2-rich atmosphere, Mn2+-CO species (2163 cm﷓1) 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 ca. 130 K mono-dentate carbonates start to emerge. The latter enhance the surface acidity of nearby situated Mn2+ cations and as a result the Mn2+-CO carbonyls are detected at higher frequencies, namely 2191 cm﷓1. Catalytic activity in for MnOx/YSZ in CO and CH4 are reported.

Cuvinte cheie: CH4 oxidation, TPR, XRD, CO-adsorption FTIR spectroscopy; Mn/YSZ