Cerium And Vanadium Supported On Sulfated Zirconia As A Solid Superacid Catalyst With Enhanced DeNO_x Activity And Poison Resistance

Nitrogen oxides （NOx） from combustion of fossil fuels are some of these major pollutants to the atmosphere. Selective catalytic reduction （SCR） with NH3 has been considered as the most developed and widely used technology to control NOx emissions from stationary sources. However, conventional catalysts suffer great deactivation in the presence of alkali metals （e.g., K, Na） and alkaline earth metals （e.g., Ca, Mg）, which are particularly high in flue gases of biomass （straw, wood, etc.） and waste incineration. Therefore, ceria and vanadia are supported on sulfated zirconia as solid superacid catalysts for SCR reaction. The resulting catalysts show enhanced deNOx activity and resistance towareds alkali metals, SO₂ and H2O.Firstly, synthesis methods of ceria-zirconia solid superacid catalysts were investigated, and the effects of sulfation treatment were studied. Experimental results showed that the sulfation of zirconia supports could greatly improve the SCR activity of the catalysts. Sulfated catalysts presente pure tetragonal zirconia phase, smaller particle size, higher surface area, enriched surface Ce3+and enhanced acidity. The NO conversion of optimized catalyst achieves 96%, and shows over 90%in the temperature range from 370 to 530 ℃.After synthesizing the ceria-zirconia solid superacid catalysts, their catalytic performance were investigated in the presence of alkali metals （potassium）. Catalysts after sulfation treatment exhibit a remarkable resistance towards alkali metal poisoning in deNOx application. When the mole ratio K to Ce was 0.4, the highest NO conversion of CeO₂/SO42--ZrO₂ could achieve 99.3%, while that of unsulfated catalyst was only ca.50%. Under the same condition, V2O5-WO3/TiO₂, has been completely deactivated （< 20%）. A long-term durability test revealed a stable activity for CeO₂/SO42--ZrO₂ within 168 h at 380 ℃ with no tendency to decrease.An unexpected result was that a small amount of potassium could somehow promote the activity of this ceria-zirconia solid superacid catalyst in the temperature range of 250-480 ℃. In present study, the roles of surface acid and base sites in SCR reaction were discussed, and a mechanism of potassium promotion effect on SCR catalysts was proposed. Moderate amount of potassium could enhance the adsorption and oxidation of NO, which has been proven to be efficiently involved in the SCR reaction, therefore promoted the catalytic activity within 250-480 ℃. On the other hand, adequate surface acidity was still necessary to achieve a high deNOx activity, while the types of acidity sites （Lewis or Bronsted） were not crucial for the catalytic process. Acidity ensured the adsorption and activation of ammonia on the catalyst surface, and also protected the active center （ceria） to be functional by preferentially capturing the potassium.Due to the poor SO₂ resistance of CeO₂/SO42--ZrO₂, vanadium was introduced in the catalysts to develop a novel deNOx catalyst with tolerance to both alkali metals and SO₂. The activity of optimized CeVSZ catalyst （Ce/V=1:1） could achieve over 95% within 400 min in the presence of potassium,600 ppm SO₂,5% H2O, and showed no tendency to decrease. CeO₂ （active center） would react with SO₂ to form Ce2（SO4）3, resulting in the deactivation of ceria based catalyst （from 93% to 63%）. After the introduction of vanadium, CeO₂ transformed to CeVO4, which was less reactive with SO₂ and O₂. Moreover, CeVSZ showed significantly decreased adsorption and oxidation of SO₂, thereby enhanced SO₂ resistance.