Air pollution has been a global environmental issue for decades and efforts to mitigate the global threat are a huge challenge to many environmental researchers. As the industries have been developing fast, desires for clean environment and better lif...
Air pollution has been a global environmental issue for decades and efforts to mitigate the global threat are a huge challenge to many environmental researchers. As the industries have been developing fast, desires for clean environment and better life have also increased for past years. The major air pollutions that have caught much attention are nitrogen gases (NOx), sulfurous gases(SOx), carbon dioxides(CO2), and other greenhouse gases(GHG). For recent years, the harmfulness of these emissions has been highlighted as cause of climate change. Hence, the global goals for clean environment have implemented to reduce the environmental impacts for public health.
Among these air pollutants, NOx emission from stationary sources like coal power plants that uses fossil fuels as energy sources is a huge challenge because it can produce the secondary product when it emits to air. It also plays a critical role in increasing ozone and smog. Various technologies have been developed to suppress NOx emissions including selective catalytic reduction with NH3, lean burn engines and so on. Although selective catalytic reduction with NH3 as a reducing agent (NH3-SCR) has been applied to industries as an efficient technology, especially to after treatment process, to reduce NOx emission. The injected ammonia(NH3) into exhaust gas stream, passing through a catalyst bed where the NOx is converted to nitrogen (N2) and water (H2O), which are harmless compounds in air. The NH3-SCR technology are advantageous over other technologies because of its lower operating costs with high efficiency. However, new technology on NH3-SCR system must be introduced to fulfill the NOx emission regulations placed by governments and SOx are usually present in after treatment process of power plants. During the NH3-SCR process, the present of SOx must be considered as well because it can affect the lifetime of catalyst and efficiency.
Various types of metal oxide-based catalysts and zeolite-based catalysts have been utilized in NH3-SCR systems depending on their properties and operating factors like temperature and composition of exhaust gas. Among various DeNOx catalysts, vanadium oxide-based catalysts are most used in after treatment process of stationary sources for its high thermal stability and affordable prices. Although the vanadium oxide-based catalysts (V2O5/TiO2 or V2O5/WO3-TiO2) are world widely used for its advantages, there are practical issues to meet the recent obligations. First, the high NOx conversion of the conventional vanadium oxide-based catalysts can only achieve only at high temperature above 300 oC while the recent operating temperature in actual industries are mostly below 300 oC. In addition, since SO2 and water exist during the after-treatment process. When SO2 are oxidized to SO3 and react with NH3 and H2O, ammonium bisulfate (ABS) is produced and it causes sulfur poisoning on catalysts, leading to reduce efficiency and lifetime of catalysts. Hence, innovative NH3-SCR technology that acquires high sulfur resistance with less energy consumptions and higher NOx removal efficiency at low temperature simultaneously are required as for environment-friendly industrial applications. Therefore, this research provides innovative insights to improve sulfur resistance of vanadium oxide-based catalysts via simple synthesis method that can be advantageous applying to practical industrial fields.
In details, this study discussed that a series of V2O5/WO3-TiO2 and alumina calcined at different temperatures are prepared by physical mixing to enhance sulfur resistance and regenerability at low temperatures. Among the mechanically alumina mixed catalysts (V2O5/WO3-TiO2 + Al), the V2O5/WO3-TiO2 mixed with alumina calcined at 900 oC achieved the highest sulfur resistance due to increase of strongly adsorbed acid sites. This research also demonstrated that ABS formed on vanadia sites migrated to the mixed alumina sites and vanadia active sites were protected from sulfur poisoning, resulted in superior sulfur resistance at low temperature. The physical mixed V2O5/WO3-TiO2 catalyst with alumina can enhance sulfur resistance of V2O5/WO3-TiO2 catalyst and accomplish regenerability at low temperature.
This study also discussed the physically mixed vanadia catalyst with surface modified zeolite that can resolve physical and chemical deactivation simultaneously. When V2O5/WO3-TiO2 catalysts and Al-rich zeolite Y (Si:Al2=5.1) were mechanically mixed are designed for sulfur-resistant deNOx catalysts, degradation of activity was observed with improved sulfur resistance. The main cause of degradation was the chemical interaction between VOx and mobile AlOx species, most likely extra-framework Al species, on zeolite surface during the mechanical mixing, which were confirmed by various characterizations including H2- temperature-programmed reduction (H2-TPR) and line energy dispersive X-ray spectroscopy (line-EDS). To resolve the problem, octadecyltrichlorosilnae (OTS) was coated on zeolite surface first and mechanically mixed with V2O5/WO3-TiO2 catalysts. In summary, the developed catalyst V2O5/WO3-TiO2 + OTSY catalyst obtained high NOx conversion and enhanced sulfur resistance by suppressing the physical and chemical poisoning simultaneously.
The present study also investigated ball milling effect over hybrid catalyst composed of V2O5/WO3-TiO2 + zeolite Y catalysts. It included how the ball milling process affected on catalytic properties and activity depending on synthesis method.