Browsing by Subject "CO oxidation"
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Item Characterization and Reaction Studies of Silica Supported Platinum and Rhodium Model Catalysts(2012-02-14) Lundwall, Matthew JamesThe physical and catalytic properties of silica supported platinum or rhodium model catalysts are studied under both ultra high vacuum (UHV) and elevated pressure reaction conditions (>1torr). Platinum or rhodium nanoparticles are vapor deposited onto a SiO2/Mo(112) surface and characterized using various surface analytical methods. CO chemisorption is utilized as a surface probe to estimate the concentration of various sites on the nanoparticles through thermal desorption spectroscopy (TDS) and infrared reflection absorption spectroscopy (IRAS) along with microscopy techniques to estimate particle size. The results are compared with hard sphere models of face centered cubic metals described as truncated cubo-octahedron. Results demonstrate the excellent agreement between chemisorption and hard sphere models in estimating the concentration of undercoordinated atoms on the nanoparticle surface. Surfaces are then subjected to high pressure reaction conditions to test the efficacy of utilizing the rate of a chemical reaction to obtain structural information about the surface. The surfaces are translated in-situ to a high pressure reaction cell where both structure insensitive and sensitive reactions are performed. Structure insensitive reactions (e.g. CO oxidation) allow a method to calculate the total active area on a per atom basis for silica supported platinum and rhodium model catalysts under reaction conditions. While structure sensitive reactions allow an estimate of the types of reaction sites, such as step sites (?C7) under reaction conditions (e.g. n-heptane dehydrocyclization). High pressure structure sensitive reactions (e.g. ethylene hydroformylation) are also shown to drastically alter the morphology of the surface by dispersing nanoparticles leading to inhibition of catalytic pathways. Moreover, the relationships between high index single crystals, oxide supported nanoparticles, and high surface area technical catalysts are established. Overall, the results demonstrate the utility of model catalysts in understanding the structure-activity relationships in heterogeneous catalytic reactions and the usefulness of high pressure reactions as an analytical probe of surface morphology.Item Electrochemistry of carbon monoxide on platinum single-crystal surfaces(Texas Tech University, 2008-08) Inkaew, Prachak; Korzeniewski, Carol; Casadonte Jr., Dominick J.; Bartsch, Richard A.Improving understanding of carbon monoxide (CO) adsorption and electrochemical oxidation on Pt type materials is important for the development of low temperature fuel cells and electro-synthetic reactions. CO is present as an impurity in H2 or forms as a stable intermediate in the conversion of small oxygenated organic fuels and is an intermediate in the electro-synthesis of fuels starting from CO2 feedstock. In fuel cells, Pt is the most effective and widely employed catalyst. CO adsorbs strongly to Pt and reduces its catalytic activity. Moreover, CO is a relatively simple molecule and ideally suited for model studies of molecular adsorption at metal/electrolyte interfaces. To deepen understanding of elementary steps involved in electrochemical reactions of CO, the single crystal electrodes Pt(111), Pt(100) and Pt(335) which is Pt(s)-[4(111) x (100)] in step-terrace notation were employed in this project to investigate CO oxidation in acidic media. Single crystal electrodes have surfaces that are structurally well-defined on the atomic level and provide insight into effects of surface structure on electrocatalytic reaction mechanisms. Methods for the fabrication of single crystal electrodes are discussed. The responses of in-house prepared Pt(111) and Pt(100) electrodes in comparison to literature benchmarks is demonstrated. Application of the electrodes as standards to monitor the stability and cleanliness of electrochemical experiments involving CO oxidation and oxygen reduction is shown. The oxidation of CO adsorbed near saturation coverage on Pt(111) and Pt(335) electrodes in 0.5 M H2SO4 over the range of potentials between 0.75 V and 0.9 V (versus a reversible hydrogen electrode (RHE) reference) was shown to adhere to a Langmuir-Hinshelwood (LH) model for adsorbed CO electrochemical oxidation. The model assumes fast CO transport and the reaction of homogeneously adsorbed CO and OH on the surface to form CO2 For Pt(100) electrodes, the responses were somewhat more complicated than those predicted by the LH model. Furthermore, introducing defects into Pt(100) by cooling the electrode in an Ar atmosphere (without added H2) following annealing resulted in responses similar to those for CO oxidation over nanometer-scale (< 10 nm diameter) Pt catalyst particles. In particular, current-time transients recorded in potential step measurements showed tailing at long times characteristic of CO oxidation on Pt catalyst particles. The rate of CO oxidation over the Pt single crystal electrodes decreased in the order of Pt(335) > Pt(100) > Pt(111). Additionally, for CO adsorbed to full coverage at 0.1 V (versus RHE) on Pt (335) in 0.5 M H2SO4 at ambient temperature, oxidation of the layer gave 7.6 x 1014 CO/cm2 as the saturation CO coverage, just below the average value reported for CO on Pt(335) in ultra high vacuum (8.3 x 1014 CO/cm2). The oxidation of CO adsorbed to sub-saturation coverage on Pt(335) was also investigated. Responses for fractional CO monolayer (ML) coverages, determined relative to the Pt(335) surface atom density, were recorded for the range 0.08-0.52 ML, where 0.52 ML is the maximum attainable coverage. For reactions at 0.7 V (versus RHE) in 0.05 M H2SO4, numerically solving the rate equations to the LH model of adsorbed CO electrochemical oxidation reproduced the main features in current-time transients from potential step experiments. Above half saturation ( > 0.26 ML), the transients progressed through a current maximum, whereas below 0.26 ML a continuous decay were observed, in accordance with predictions of the LH model. Pt(111) and Pt(335) were intentionally contaminated with n-thiol and acetonitrile before adsorbing CO onto their surfaces. CO could displace acetonitrile from both surfaces. However, n-thiol adsorbed strongly to both surfaces and increased their coverages with potential cycles. On both surfaces the effect of contamination was to shift the CO oxidation stripping peaks to higher potential and decrease the CO oxidation rate. These responses are similar to those observed for CO oxidation on Pt nanoparticles, indicating contamination may play a role in CO oxidation on nanoparticle surfaces. Investigations were extended to determine the effects of adsorbed anions and crystalline defects, generated by applying a potential 1-2 V for 1 s to Pt(100), on CO oxidation. With more strongly adsorbing anions in the supporting electrolyte, CO oxidation became slower. The CO oxidation stripping peak shifted positively in the electrolytes with increasing concentration of more strongly adsorbing anion species. Applying higher anodic potentials to Pt(100) generated greater degrees of defects on the surface. When a small degree of defects was introduced to Pt(100), CO oxidation proceeded faster than on the well-ordered surfaces. However, at the highest levels of defects, the oxidation rate slowed. Based on the hard sphere model, the hydrogen adsorption charge indicates a transformation of Pt(100) to a structure of similar step density as Pt(11,1,1) = Pt(s)-[6(100) x (111)] with applied potential of 2 V for 1 s.Item Model catalytic studies of single crystal, polycrystalline metal, and supported catalysts(2009-05-15) Yan, ZhenThis dissertation is focused on understanding the structure-activity relationship in heterogeneous catalysis by studying model catalytic systems. The catalytic oxidation of CO was chosen as a model reaction for studies on a variety of catalysts. A series of Au/TiO2 catalysts were prepared from various metalorganic gold complexes. The catalytic activity and the particle size of the gold catalysts were strongly dependent on the gold complexes. The Au/TiO2 catalyst prepared from a tetranuclear gold complex showed the best performance for CO oxidation, and the average gold particle size of this catalyst was 3.1 nm. CO oxidation was also studied over Au/MgO catalysts, where the MgO supports were annealed to various temperatures between 900 and 1300 K prior to deposition of Au. A correlation was found between the activity of Au clusters for the catalytic oxidation of CO and the F-center concentration in the MgO support. In addition, the catalytic oxidation of CO was studied in a batch reactor over supported Pd/Al2O3 catalysts, a Pd(100) single crystal, as well as polycrystalline metals of rhodium, palladium, and platinum. A hyperactive state, corresponding to an oxygen covered surface, was observed at high O2/CO ratios at elevated pressures. The reaction rate at this state was significantly higher than that on CO-covered surfaces at stoichiometric conditions. The oxygen chemical potential required to achieve the hyperactive state depends on the intrinsic properties of the metal, the particle size, and the reaction temperature. A well-ordered ultra-thin titanium oxide film was synthesized on the Mo(112) surface as a model catalyst support. Two methods were used to prepare this Mo(112)- (8x2)-TiOx film, including direct growth on Mo(112) and indirect growth by deposition of Ti onto monolayer SiO2/Mo(112). The latter method was more reproducible with respect to film quality as determined by low-energy electron diffraction and scanning tunneling microscopy. The thickness of this TiOx film was one monolayer and the oxidation state of Ti was +3 as determined by Auger spectroscopy, high-resolution electron energy loss spectroscopy, and X-ray photoelectron spectroscopy.