Novel metal modified Mn/TiO2 catalyst for the reduction of NOx with NH3 at low temperatures

다국어 초록 (Multilingual Abstract)

Many DeNOx technologies, which can reduce nitrogen oxides (NO, NO2 and N2O) from the exhausts, have been developed for more than three decades in order to abate environmental pollution, such as PM, smog, ozone depletion, and acid rain, caused by NOx in the atmosphere. Among the several existing DeNOx technologies, the selective catalytic reduction (SCR) is one of the most efficient technologies used for the reduction of NOx from the exhausts. Generally, nitrogen oxides are generated through the combustion of fossil fuels and these gases are mainly emitted from the exhausts of automobiles and stationary sources.
The commercial TiO2 based catalysts for the NH3-SCR systems consist of WO3-V2O₅ and MoO3-V2O₅, which are effective in the temperature range of 573 – 673 K. However, recent studies on the NH3-SCR technology have been focused on finding catalysts that can be used even at lower temperatures. Finding catalysts that are effective at lower temperatures is important because the SCR system might be relocated from downstream to upstream of the exhaust line past the desulfurizer or diesel particulate filter in the automobile configuration for example. Having an effective catalyst at lower temperatures would eliminate the need to reheat the stack gas entering the SCR system in the power plant system. Therefore, it is necessary to synthesize low temperature active NH3-SCR catalysts for the replacement of the existing commercial catalysts.
Several Mn based catalysts have recently been reported to exhibit excellent activity for NH3-SCR at low temperatures. However, Mn based catalysts are barely commercialized for several reasons such as poor N2 selectivity and deactivation by H2O through the blockage of active sites. Therefore, in order to solve these problems, many researches carried out study of enhancement of the NH3-SCR activity of Mn based catalysts with the addition of different metals. Among a variety of promoters, the promoter associated with ceria was published most widely due to its excellent synergetic performance with manganese catalysts. However, even though the catalyst using Ce is widely known to be excellent in its NH3-SCR ability, its poor N2 selectivity is yet to be resolved. Therefore, finding new promoters is an important task in improving the NH3-SCR system.
Our previous studies have shown that the addition of Sb can help enhance the activity and increase the tolerance to H2O in vanadium based catalysts and a manganese based SCR catalyst with antimony has never been reported. Thus, the effect of antimony in manganese catalysts supported on TiO2 was investigated. Mn/TiO2 and Fe-Mn/TiO2 were used to analyze the ability of Sb on manganese based SCR catalysts. In the study, the novel metal Sb was modified into Mn/TiO2 and Fe-Mn/TiO2, and the NOx conversion was compared to those of Mn/TiO2 and Fe-Mn/TiO2 catalysts to investigate the effect of Sb. The NOx reduction activities of the catalysts were evaluated in the temperature range of 100–250ºC at a space velocity of 60,000 h-1. The physicochemical properties of all the catalysts were characterized by Brunauer-Emmett-Teller (BET) surface area, temperature programmed desorption of ammonia (NH3-TPD), temperature programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and high resolution transmission electron microscopy (TEM). Interestingly, the Sb promoted Mn based catalysts showed significantly higher NOx conversion than the other catalysts with or without 6 vol.% of H2O. The high performance of Sb modified catalysts could be related to the increase of acid sites and redox properties.

목차 (Table of Contents)

• Abstract----------------------------------------------------------------------------------i
• Abstract (in Korean; 국문요약) --------------------------------------------------- iv Contents--------------------------------------------------------------------------------- vi
• List of Figures----------------------------------------------------------------------------x
• List of Tables-------------------------------------------------------------------------xiii
• Chapter 1. Introduction-----------------------------------------------------------------1
• 1.1. Backgrounds ---------------------------------------------------------------------- 1
• 1.2. NOx Properties---------------------------------------------------------------------7
• 1.3. Formation of nitrogen oxides----------------------------------------------------9
• 1.5.2.1. Thermal NOx------------------------------------------------------------9
• 1.5.2.2. Prompt NOx-----------------------------------------------------------10
• 1.5.2.3. Fuel NOx---------------------------------------------------------------11
• 1.4. Lean gasoline and diesel conditions-------------------------------------------14
• 1.5. Emission Limitations------------------------------------------------------------16
• 1.5.1. Mobile applications---------------------------------------------------- 16
• 1.5.2. Stationary power plant applications-----------------------------------17
• 1.6. Technological methods for NOx removal-------------------------------------18
• 1.6.1. Primary methods--------------------------------------------------------18
• 1.6.2. Secondary methods-----------------------------------------------------21
• 1.6.2.1. Three way catalysts (TWCs) -------------------------------22
• 1.6.2.2. Hydrocarbon-SCR-------------------------------------------24
• 1.6.2.3. Ammonia-SCR-----------------------------------------------25
• 1.7. NH3-SCR system ----------------------------------------------------------------30
• 1.7.1. Catalysts for NH3-SCR------------------------------------------------32
• 1.7.1.2. Vanadia (V2O5) ----------------------------------------------33
• 1.7.1.2. Manganese (MnOx) -----------------------------------------35
• Chapter 2. Sb modified Fe-Mn/TiO2 Catalyst for the Reduction of NOx with NH3 at Low Temperatures ------------------------------------------- 38
• 2.1. Introduction -------------------------------------------------------------------38
• 2.2. Experimental ------------------------------------------------------------------41
• 2.2.1. Preparation of the catalysts-----------------------------------------41
• 2.2.2. Characterization of the catalysts-----------------------------------42
• 2.2.3. Activity measurements----------------------------------------------43
• 2.3. Results and discussion -------------------------------------------------------44
• 2.3.1. Activity Studies ------------------------------------------------------44
• 2.3.2. Characterization of the catalysts -----------------------------------55
• 2.4. Conclusion ---------------------------------------------------------------------70
• Chapter 3. Influence of the Niobium on H2O Resistance and N2O Suppression for NH3-SCR at Low Temperatures-----------71
• 3.1. Introduction -------------------------------------------------------------------71
• 3.2. Experimental ------------------------------------------------------------------74
• 3.2.1. Preparation of the catalysts-----------------------------------------74
• 3.2.2. Characterization of the catalysts-----------------------------------74
• 3.2.3. Activity measurements----------------------------------------------75
• 3.3. Results and discussion -------------------------------------------------------77
• 3.3.1. Activity Studies ------------------------------------------------------77
• 3.3.2. Characterization of the catalysts -----------------------------------82
• 3.4. Conclusion ---------------------------------------------------------------97
• References------------------------------------------------------------------------------98