A Study on Structure Modification of Mixed Metal Oxide Catalysts for Efficient Removal of Nitrogen Oxides

Abstract

Nitrogen Oxides (NOx), which are released from fossil fuel-based system such as power plant, vessels, automobiles, are a major contributor to the air pollution issues. In this scenario, the regulations of NOx emission are more rigorous worldwide and technologies that convert NOx into harmless compounds have received considerable attention. Selective catalytic reduction of NOx with NH3 (NH3-SCR) has been one of the most effective technologies that can satisfy the improved environmental restrictions. The NH3-SCR technology has several advantages, such as high NOx removal efficiency and stability. In this field, vanadium based catalysts have been commercialized because it was suitable for harsh operating condition of NH3-SCR systems. However, numerous studies of catalysts for NH3-SCR are proceeded to decrease operating temperature and improve SO2 resistance. The control of catalyst structures should be one of the strategies of catalyst development by the enhancement of the physicochemical properties such as specific surface area and dispersion of metal cations. The present study is aimed at improving the NOx removal efficiency of catalysts for NH3-SCR via structure modification of catalysts. Furthermore, for practical application of the NH3-SCRsystem, the catalytic activity of developed catalysts in this study were evaluated under H2O and SO2 containing conditions. In Chapter 2, I report characteristics of developed Ce doped Mn-Cr layered structure catalysts and its use as an effective catalyst for the NH3-SCR. This catalyst is simply prepared by co-precipitation method and revealed improved NOx removal efficiency and SO2 resistance than non-uniformed structure catalysts. The resulting Ce0.2-Mn2Cr1Ox-LDO achieved almost 100 % NOx conversion at 200 °C with a WHSV of 90,000 mL g cat-1 h-1. For an H2O-SO2 resistance and regeneration test, the catalyst showed improved SO2 resistance and NOx conversion was restored to 78% after H2O and SO2 were shut off. The improved texture properties, acidity, and reducibility and Mn-Cr redox cycle by structure distortion are responsible for the high catalytic activity. The suppression of chemical deactivation by Cr species could improve SO2 resistance and reusability through a regeneration process at 300 °C. The cerium added as a promoter improved the catalytic efficiency and SO2 resistance by further improving the catalytic properties decrease the ammonium sulfates decomposition temperature. In Chapter 3, discusses characteristics of Co-Cr based layered structure catalyst and it’s used as an effective catalyst for the NH3-SCR. This catalyst was also prepared by co-precipitation method followed by calcination at 400 °C. The Co-Cr based layered structure catalysts were formed successfully, and the properties of the catalyst were characterized using various physicochemical analyses. The resulting Co2Cr1Ox layered structure catalyst showed almost 95 % NOx conversion at 200 °C with a WHSV of 90,000 mL g cat-1 h-1. For resistance test, the layered structure catalyst showed SO2 resistance and NOx conversion was recovered to 89 % after switching off H2O and SO2 at 200 °C. The used catalyst, which was sulfated during the SO2 resistance test, showed nearly 100 % NOx conversion at temperatures near 200 °C with an improved SO2 resistance. The enhanced surface acidity, Cr reducibility and structure distortion from the unique texture properties of Co2Cr1Ox layered structure catalyst are responsible for the improved catalytic activity and the increase of acidity by formation of chromium sulfate is responsible for the catalytic performance after sulfation. The findings of this study indicate the possibility of application of sulfation effect of Cr based catalysts for the NH3-SCR containing SO2, emitted from industrial sites.