Browsing by Subject "Model Catalyst"
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Item Catalytic chemistry of Pd−Au bimetallic surfaces(2015-08) Yu, Wen-Yueh; Mullins, C. B.; Henkelman, Graeme; Hwang, Gyeong S.; Korgel, Brian A.; Sitz, Greg O.Catalyst development is important to the contemporary world as suitable catalysts can allow chemical processes to proceed with reduced energy consumption and waste production. In order to design catalysts with improved performance, the fundamental studies that correlate catalytic properties with surface structures are essential as they can provide mechanistic insights into the reaction mechanism. Pd−Au bimetallic catalysts have shown exceptional performance for a number of chemical reactions, however, the interplay between the reactive species and surface properties are still unclear at the molecular level. In this dissertation, the catalytic chemistry of Pd−Au surfaces was investigated via model catalyst studies under ultrahigh vacuum conditions. A range of Pd−Au model surfaces were generated by annealing Pd/Au(111) surfaces and characterized/tested by surface science techniques. The findings in this dissertation may prove useful to enhance the fundamental understanding of structure-reactivity relation of Pd−Au catalysts in associated reactions.Item Chemical and Physical Properties of Nanomaterials for Model Catalytic Systems and Smart Polymer Membranes(2014-10-24) Skiles, Stephanie LynThe increased development of surface science in the last half century has opened up new fields for exploration. Surfaces from the pristine to the complex can now be studied with relative ease. These developments along with the industrial society?s desire for improvement have led to the study of smart materials and model systems. Smart materials are designed to have a significant property change in response to a stimulus. Smart polymers can be synthesized that respond to a variety of stimuli including temperature, pH or light. The polymer responds to the stimulus by undergoing a transition that can affect its color, conductivity, shape, etc. Even slight changes in environment can induce large changes in the polymer. This work focuses on covalent layer-by-layer assembly grafts of the thermoresponsive polymer poly(N-isopropylacrylamide) and silica nanoparticles. When grafted to a surface, the system response to external stimuli inducing changes in topography and wettability. Utilizing nanoindentation the polymer graft?s switching elastic modulus was probed as it was exposed to varying external stimuli. It was found that the modulus of the polymer graft changed an order of magnitude based on the polymer?s history and current environment. Covalent layer-by-layer assembly additionally was used to functionalize porous substrates. The polymer?s conformational change was leveraged in the development of an oil and water separation membrane capable of demulsification. The polymer?s transition to a non-soluble configuration blocked pore passageways, preventing the oil from permeating the substrate leading to a pure water filtrate. Advances in surface science have pushed ahead the development of cheaper and better performing catalyst systems. These systems can be developed and tested using model catalyst systems. Herein, two model systems were investigated: a supported cobalt nanoparticle catalyst and a bimetallic palladium-copper system. In the cobalt system, the smallest particles are oxidized and deactivated during the Fischer-Tropsch reaction. In the bimetallic system, the electronic effect of metal alloying was investigated using X-ray photoelectron spectroscopy. The stable alloy was surface enriched with copper. The promotion effect of copper on palladium for the acetylene hydrogenation reaction was investigated. These model systems allow for the study of fundamental phenomena on a controlled surface.Item Synthesis of vinyl acetate on palladium-based catalysts(2009-06-02) Kumar, DheerajVinyl acetate (VA) is an important monomer used in the production of paints, surface coatings and adhesives. Synthesis of VA is usually carried out over supported Pd alloy catalysts with a selectivity as high as 96% and described as C2H4 + CH3COOH + ? O2 -> C2H3OOCCH3 + H2O Although the VA synthesis reaction has been industrially carried out for many years, the nature of the active sites and the reaction mechanism is still unclear. The goal of this study was to acquire a fundamental understanding of the VA reaction mechanism by carrying out detailed kinetic and spectroscopic investigations on single crystals and supported Pd catalysts, and to detail the role of alloying in optimizing the selectivity of this important industrial reaction. A combination of surface science techniques and kinetic measurements has been used to address the mechanism. Supported catalysts, 1 wt% Pd/SiO2 and 5 wt% Pd/SiO2, and 1 wt% Pd-0.5 wt% Au/SiO2, were prepared by an incipient wet-impregnation method and characterized using XRD and TEM. On Pd-only catalysts the reaction rates were found to be: Pd(100) < 5 wt% Pd/SiO2 (dpd = 4.2 nm) < 1 wt% Pd/SiO2 (dpd = 2.5 nm). Particle size-dependence of the reaction rates is evident for the Pd-only catalysts, which suggests a degree of structure sensitivity of the reaction. There is an increased availability of uncoordinated, edge atoms on small particles. With a Pd single crystal, fewer less-coordinated surface sites are present compared to a comparable area on a small Pd particle on a supported Pd catalyst. The formation of Pd carbide (PdCx) during the synthesis of VA was investigated over Pd/SiO2 catalysts with two different Pd particle sizes, as well as over a Pd-Au/SiO2 mixed-metal catalyst. XRD data indicate that smaller Pd particles show greater resistance to the formation of PdCx. The alloying of Au with Pd is apparently very effective in preventing PdCx formation in Pd-based catalysts for VA synthesis. Addition of Au to Pd/SiO2 catalysts significantly enhances the VA formation rate and selectivity. Infrared reflection absorption spectroscopy (IRAS) of CO on Pd/Au(100) and Pd/Au(111) confirms the presence of Pd as isolated monomers on a Au-rich surface. A pair of Pd monomers is the most favorable active site for the formation of VA. The spacing between the two active isolated Pd atoms is critical and is demonstrated by the relative rates of VA formation on Pd/Au model catalysts, i.e. Pd/Au(111) < Pd/Au(100). The role of Au is to isolate the surface Pd atoms and thus suppress the formation of by products, CO and CO2. A pair of Pd monomers required for VA synthesis is further confirmed by the results from model studies of Sn-Pd.