Deactivation mechanism of Ce/TiO2selective catalytic reduction catalysts by the loading of sodium and calcium salts

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Abstract
In this paper, the poisoning effect of alkali and alkaline earth metal on Ce/TiO2 catalysts was investigated for the first time and a deactivation mechanism was proposed. The Ce/TiO2 catalyst was observed to be deactivated seriously by the loading of Na+, K+ or Ca2+ ions. When the Na/Ce, K/Ce or Ca/Ce molar ratio exceeded 0.25, NO conversion of the Ce/TiO2 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/TiO2 sample, and would change to ceria crystals with the loading of Na+ or Ca2+ ions, leading to a poor dispersion of ceria on the surface of TiO2 as well as the transformation of Ce3+ to Ce4+. 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 Ce4+/Ce3+ 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/TiO2 catalyst after loading with Na+ or Ca2+ ions.

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