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

摘要

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 Ca~(2+) 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 Ca~(2+) ions, leading to a poor dispersion of ceria on the surface of TiO2 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/TiO2 catalyst after loading with Na~+ or Ca~(2+) ions.