Deactivation mechanism of Ce/TiO2 selective catalytic reduction catalysts by the loading of sodium and calcium salts

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.