Advanced Materials for Air Pollutants Removal in A Combustion System

Air pollutants directly or indirectly impact human health and the environment. Large quantities of CO2, volatile organic compounds (VOCs), and particulate matter are emitted from combustion systems, and cause climate change, smog formation, and pose serious health risks. The increasing demand for the remediation of air pollutants at the source has drawn much attention to the use of advanced materials due to their high reactivities and special properties. In order to achieve the successful application of advanced materials for the remediation of problematic emissions, three aspects, (1) synthesis method, (2) characterization of materials’ structural properties, and (3) evaluation of materials’ kinetic characteristics, should be deeply understood. This dissertation pursued an understanding of these three aspects to develop efficient nanomaterials for air pollutant removal. Depending on the types of air pollutants and corresponding techniques, this dissertation is separated into two major parts. Part 1 Development of active nanomaterials for O2 removal for the oxy combustion system: The oxy combustion system has been proposed as an effective methodology for capture of CO2 in large power plants. The captured CO2 stream can be further utilized for enhanced oil recovery or sequestration. However, the O2 concentration in the stream (typically ~3%) should be lowered to 100 ppmv to avoid corrosion inside pipelines. Recently, catalytic reduction of O2 with CH4 has been applied as a promising solution and has shown a high removal efficiency of O2 with noble metal doped metal oxide nanomaterials. Even though a high O2 conversion was demonstrated, studies to reveal the fate of the nanoparticle catalysts and its correlation to the active sites of materials during the reaction have not been elucidated. There is also insufficient information about the effect of initial gas composition on O2 removal (i.e. excess CO2 in the initial gas stream). To elucidate and remove these limitations, a preliminary study on the flame synthesis of Pd-TiO2 nanoparticles for O2 removal from the CO2-concentrated stream has been first conducted. Kinetic and structural properties of the synthesized Pd-TiO2 nanoparticles, such as the size and oxidation state of Pd, were carefully evaluated. Then, the fractions of the total surface areas of three different Pd species (metallic Pd, intermediate PdOx, and PdO) were evaluated. By interpreting both structural and kinetic characteristics, a strong correlation between the apparent reaction rate constants and the fraction of the total surface area of metallic or/and reduced form of Pd was obtained. This finding indicates the important role of metallic and/or reduced form of Pd as an active phase for O2 removal. The effect of the excessive CO2 on O2 removal was also studied by using both experimental and theoretical approaches. A linear dependency between the CO2 generation rate and its concentration in the feed stream was experimentally observed. This observation was supported by density functional (DFT) calculations which demonstrated the positive effect of surface CO2 coverage on O2 removal by reducing activation energies for the overall reaction. Although Pd-TiO2 was successfully developed for O2 removal, it has a limitation due to its high costs. As a final step, an efficient and cost effective cobalt oxide nanomaterial was synthesized via a furnace aerosol reactor. Its superior performance for O2 removal was demonstrated and attributed to the enhanced oxygen vacancy defects in the structure. Part 2. Nanomaterial-integrated technique for VOCs and particulates removal: Electrostatic precipitator (ESP), one of the non-thermal plasma (coronas) reactors, has been widely used for the removal of particulate matter in large power plants. ESPs generally have an electric field generated by using a high voltage source. Corona discharge is obtained in this high voltage system, and reactive species, such as ions and electrons, are effectively generated. The target pollutants, such as VOCs and submicrometer particles, could be removed via the oxidation and the charging/capture processes. However, the system itself still has a limitation, such as lower charging efficiency for small particles. TiO2 film and conductive fabric liner were inserted in the ESP in the present study and were examined for the removal of VOCs and submicrometer particles. In both cases, enhanced removal efficiencies were achieved, which were attributed to the increased amount of reactive species, such as ions and radicals, generated on the materials’ surfaces. This dissertation elucidates the importance of the remediation of several air pollutants and successful application of advanced materials. Synthesis, characterization, and evaluation of their performances were performed independently, but they were closely correlated to reveal an active phase of nanomaterials. In addition, the synergistic effect of nanomaterial-integrated technique was also examined.