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In this paper, the poisoning effect of alkali and alkaline earth metal on Ce/TiO 2 catalysts was investigated for the first time and a deactivation mechanism was proposed. The Ce/TiO 2 catalyst was observed to be deactivated seriously by the loading of Na + , K + or Ca 2+ ions. When the Na/Ce, K/Ce or Ca/Ce molar ratio exceeded 0.25, NO conversion of the Ce/TiO 2 catalyst at 380 °C decreased from 78% to negligibly low. After subjecting it to a range of analytical techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), Raman spectra etc. , it was found that amorphous ceria was dominant in the Ce/TiO 2 sample, and would change to ceria crystals with the loading of Na + or Ca 2+ ions, leading to a poor dispersion of ceria on the surface of TiO 2 as well as the transformation of Ce 3+ to Ce 4+ . Both of these led directly to the disappearance of oxygen vacancies in the ceria particles, slowing down the reduction rate of ceria and hence decreasing its rate of oxidation/reduction recycle. We proposed that the enlargement of ceria nanoparticles, the reduced Ce 4+ /Ce 3+ redox cycle rate and the change in the surface acidity could be the three major factors contributing to the decline of selective catalytic reduction (SCR) activity of the Ce/TiO 2 catalyst after loading with Na + or Ca 2+ ions.
Catalysis Science & Technology – Royal Society of Chemistry
Published: Oct 24, 2012
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