Date of Award

12-14-2015

Document Type

Open Access Dissertation

Department

Chemical Engineering

First Advisor

Jochen Lauterbach

Abstract

The reduction of automotive exhaust emissions has been one of the hottest topics in catalysis community since 1970s. Thanks to the development of three-way catalysts, main auto-exhaust pollutants, such as carbon monoxide, nitrogen oxides and unburned hydrocarbons, can be removed simultaneously. Nowadays, diesel engines and lean-engines become more and more popular due to their high fuel-efficiency and low CO2 emissions. Unfortunately, conventional three-way catalysts are not effective at reducing the NOx emissions from these engines due to the excess oxygen in the exhaust stream. Platinum catalysts were found to be highly active for selective NO reduction with hydrocarbons in excess oxygen and has good thermal stability and sulfur resistance. However, discrepancies still exist in the reaction mechanism of NO reduction with hydrocarbons on platinum catalysts. In this thesis, fundamental surface science studies were performed with in situ IR spectroscopic techniques combined with mass spectrometry, aiming to gain some new insight into the catalysis related to auto-exhaust emission control. Three catalysts system were explored, including model catalysts Pt(100) single crystal and Ru/Al2O3 thin film supported nanoparticles, and practical catalyst Pt/Al2O3 with various particle sizes. On Pt(100) single crystal surface, CO adsorption was investigated from ultra-high vacuum condition to elevated pressures with mass spectrometry and polarization vii modulation infrared reflection absorption spectroscopy (PM-IRAS). Different adsorption behaviors were observed for CO adsorption on Pt(100) at different pressures. Heating the sample at high pressure (>2 Torr) could cause carbon island formation on the surface, based on the IR spectra and temperature programmed desorption results. It was demonstrated that PM-IRAS is highly effective at detecting surface species under reaction conditions and can be applied to bridge the “pressure gap” between surface science and industrial catalysis. To bridge the “materials gap”, model thin film supported catalysts are desired and were prepared via wet deposition method in this work. Thin film of aluminum oxide supported Ru nanoparticles were synthesized and characterized with different techniques. Thermal oxidation of aluminum substrate could form γ-alumina thin film, and it was suitable for IR spectroscopic studies. Magnetron sputter deposition was also attempted for epitaxial alumina thin film synthesis and more research would be required to get ideal results. Colloidal method was successfully applied to synthesize Ru nanoparticle of different sizes with polyvinylpyrrolidone as the protective agent. After ethanol cleaning, extracted Ru nanoparticles were deposited onto alumina support followed by UVO cleaning. X-ray photoelectron spectroscopy characterization indicated that Ru nanoparticles were deposited successfully onto the support with about 0.67% weight loading. More tests are to be carried out on the model supported catalyst before it is loaded into the vacuum chamber for systematic studies. On Pt/Al2O3 catalysts with different particle sizes, selective reduction of NO with propene were investigated at various reaction conditions using diffuse reflectance infrared fourier transform spectroscopy combined with mass spectrometry. It was found that higher NO conversions with much higher TOFs were obtained on the catalysts with larger Pt particles, on which a relative higher NCO/CN ratio was observed in the meantime. From the systematic studies on Pt catalysts with different pretreatments, it was proposed that better activity on catalysts with larger Pt particles were attributed to the more abundant terrace sites rather than corner or edge sites on larger Pt particles. These terrace sites have better activity for NO dissociation, thus can enhance the formation of reactive intermediates such as isocyanate during the reaction. In addition, it was found that the activation of the hydrocarbon species by oxygen was a crucial step toward the formation of the isocyanate species, thus could influence the catalyst activity for NO reduction.

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