| NOx is one of the key air pollutants from flue gas, and selective catalytic reduction with NH3was the most reliable method to reduce the emission of NOx from stationary sources. Although vanadia-based catalysts have been extensively employed in commercial SCR processes, the drawbacks associated with the toxicity of vanadium pentoxide to environment and the alkali&alkaline earth metal poisoning still remain. Aiming to develop non-vanadia deNOx catalysts with high activity and remarkable resistance to alkali&alkaline earth metal poisoning, the structure, SCR performance and resistance to poisons of ceria doped titanate nanotubes (Ce/TNTs) were systematically investigated in this paper.Firstly, the preparation process of highly active and selective Ce/TNTs was optimized. It was found that the Ce loading, the calcination temperature and the pH value of TNTs all significantly influenced the SCR performance of Ce/TNTs. The NO conversion over the optimal catalyst exceeded96%at270-500℃and was even close to100%at300-450℃.Secondly, the relationship between the structure of the titanium supports and the performance of ceria was studied. It was observed that the nature of titanium supports had significantly influenced the chemical state of Ce because of their special surface properties and unique structures and morphologies, leading to the presence of different catalytic performances for each catalyst. In comparison with the catalysts supported by TiO2nanoparticles, the Ce/TNTs showed a superiority in SCR of NO due to the improved redox potential and special adsorption of NH3.Thirdly, the deactivation mechanism of Ce/TiO2SCR catalysts by the deposition of Na+and Ca2+ions was proposed. Ce/TiO2catalyst using TiO2particle as the support was deactivated seriously by the deposition of Na+or Ca2+ions. It was found that amorphous ceria was dominant in the fresh Ce/TiO2catalyst, but the amorphous ceria would grow to ceria crystal during the calcination process with the deposition of Na-or Ca2+ions. Then, the dispersion of ceria on the surface of T1O2became worse and the surface Ce3+transformed to Ce4+.This transformation directly led to the disappearance of oxygen vacancy in ceria particles and slowed down the reduction rate of ceria. Thus, the rate of oxidation/reduction recycle was declined. Though the acidity of Ce/TiO2changed little, the enlargement of ceria nanoparticles and the restrained Ce4+/Ce3+redox recycle rate resulted in the decline of SCR activity of Ce/TiO2catalys.Finally, the resistance of Ce/TNTs to alkali poisoning was investigated. Ce/TNTs showed a remarkable resistance to alkali metal poisoning in deNOx application, where the NO conversion at350℃maintained at90%after wet-impregnation of Na (Na/Ce molar ratio at1) and kept at80%after solid Na mixing. The catalyst effectively shielded the main active phase, CeO2, from the poisons with the tubular channel of H2Ti12O25. Furthermore, the poisons (e.g., Na+) could also be stabilized in the interlayer of H2Ti12O25through ion exchange. This catalyst developed herein gives a new sight for the design of "shell protection" catalysts to improve their tolerance to poisons. In addition, the maintenance of the tubular morphology and the capacity of ion-exchange were the two key factors that determined the alkaline resistance. Treating the TNTs with ethanol could increase the amount of ion-exhangenable OH group and consequently improved the resistance. In such a case, the NO conversion at350℃still maintained at97%,88%and95%after Na+, K+and Ca2+adding. |
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