Browsing by Subject "Platinum"
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Item Activity of methanol electro-oxidation at PtRu materials at temperatures in the range of 23°C to 70°C(Texas Tech University, 2004-05) Xu, ShanhongThe electrochemical oxidation of 0.5 M methanol in 0.1 M HCIO4 on catalyst materials comprised of platinum and ruthenium (PtRu) was investigated. Cyclic voltammetry and constant potential amperometry were used to characterize the catalyst materials and study the methanol reaction kinetics. Measurements were performed at temperature in the range of 23°C to 70°C. The following catalyst materials were employed: PtRu black containing 50 at. % Ru supplied by Johnson Matthey of Ward Hill. MA (JM PtRu black); sonochemically prepared nanoparticles of PtRu containing either 50 at. % Ru (SC PtRu(50)) or 25 at. % Ru (SC PtRu(25)); and Pt black (supplied by Johnson Matthey) modified by spontaneous deposition of Ru via either two (JM Pt-Ru(2)) or four deposition cycles (JM Pt-Ru(4)). The rate of methanol oxidation was assessed through constant potential amperometry measurements. Current was recorded 20 min after stepping to the reaction potential. Mechanistic information was derived from Tafel plots (plot of the logarithm of the current versus the reaction potential).Item Cathode catalysts for low-temperature fuel cells : analysis of surface phenomena(2013-12) Mathew, Preethi; Manthiram, Arumugam; Goodenough, John B.The electrochemical oxygen reduction reaction (ORR) steps on a noble metal catalyst in an acidic aqueous electrolyte depend on the nature of the catalytic surface with which the O₂ molecule interacts. It has been assumed that the O₂ molecules interact directly with a bare noble-metal surface. By studying the nature of chemisorbed species on the surface of a metal catalyst as a function of the voltage on the anodic and cathodic sweeps, it is shown here that the O₂ reacts with a surface covered with oxide species extracted from the aqueous electrolyte and not from the O₂ molecules; the ORR is more active when the surface species are OH rather than O. Moreover, the strength of the chemical bond of the adsorbed species was shown to depend on the relative strengths of the metal-metal versus metal-oxide bonds. The Pt-Pt bonds are stronger than the Pd-Pd bonds, and the relative Pd-O bonds are stronger than the relative Pt-O bonds. As a result, the chemisorbed O species is stable to lower anodic potentials on Pd. CO oxidation to CO₂ occurs at a higher potential on Pd than on Pt, which is why Pd (not Pt) is tolerant to methanol. Experiments with alloys show the following: (1) methanol tolerance decreases with the increase of Pt in the Pd-Pt alloys with Pd₃Pt/C showing an initial tolerance that decreases with cycling; (2) OH is formed on Pt₃Co/C and core-shell Pt-Cu/C, which results in a higher activity and durability for the ORR on these catalysts; (3) a 300°C anneal is needed to stabilize the Pd₃Au/C catalyst that forms an O adsorbate; and (4) OH is formed on Pd₃Co/C and Pd₃CoNi/C. These studies provide a perspective on possible pathways of the ORR on oxide-coated noble-metal alloy catalysts.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 Luminescent and magnetic materials based on conducting metallopolymers(2011-08) Chen, Xiaoyan; Holliday, Bradley J.; Campion, Alan; Cowley, Alan H.; Ellison, Christopher J.; Jones, Richard A.Conducting metallopolymers are a new and fascinating class of materials that incorporate metals into conducting polymer systems. These new materials combine the processing advantages of polymers with the electronic, optical and catalytic properties provided by the presence of metal centers. A large number of conducting metallopolymers have been synthesized and studied and have found applications in areas such as sensors, memory and light-emitting devices, solar cells, and catalysis. Among the various applications, conducting metallopolymers as emitting layers in high-efficiency polymer light-emitting diodes (PLEDs) attract great research interest. In order to get PLEDs with long lifetime, high quantum efficiency, and excellent color purity, we have developed an approach to synthesize well-defined conducting metallopolymers that incorporate lanthanide complexes in an inner sphere fashion. As such, we aim to take full advantage of the properties of both organic and inorganic components with high efficiency due to the direct electronic interface this configuration creates. Lanthanide complexes with polymerizable groups have been synthesized, characterized and utilized as precursors for conducting metallopolymers. These lanthanide monomers and corresponding metallopolymers display visible and near-infrared luminescence at room temperature that is consistent with efficient energy transfer from the organic polymer matrix to the lanthanide metal ion followed by lanthanide luminescence. As a second but closely related area, electrogenerated chemiluminescence (ECL) of polymers is attractive for light-emitting devices. Up to now, there are limited studies dealing with ECL from pure active materials deposited as solid films on electrodes. The operation theory and degradation mechanism are still under investigation. To advance the development of ECL of conducting metallopolymers, we prepared cyclometalated Pt(II) complexes with polythiophene system. Conducting metallopolymer films are prepared through controlled electropolymerization. ECL of the Pt(II) containing conducting polymers are observed for the first time. Finally, a preliminary study of magnetism and conductivity of conducting metallopolymers has been done. We incorporate Fe(II)/Fe(III) into our newly designed ligand systems with polymerizable thiophene derivatives. Three complexes show spin crossover (SCO) phenomena with the highest transition temperature at 265 K, which are further verified by variable temperature electron paramagnetic resonance spectra.Item Methods for modifying the physical and catalytic properties of surfaces(2010-05) Flaherty, David William, 1980-; Mullins, C. B.; Henkelman, Graeme; Hwang, Gyeong S.; Korgel, Brian A.; Sitz, Greg O.Catalysts can be significantly improved by modifying their structure or composition. Simple adaptations of the physical structure of a catalyst can give rise to changes in the chemical behavior, in part, due to alterations in the coordination of active sites. Modifications in the surface or bulk composition of a material have a profound impact on the chemistry that is promoted as a result of electronic and physical factors. Optimizing these qualities may enhance the catalyst’s activity, selectivity or stability. In this dissertation, we explore the application of two distinct approaches for modifying the chemical properties of catalytically active materials. Through the use of a broad array of techniques we quantify changes in critical properties such as physical-crystallographic structure; morphology, surface area and porosity; as well as catalytic activity, selectivity and stability. First, reactive ballistic deposition of metal atoms within a low pressure gas provides a unique opportunity for synthesizing thin films of a wide variety of materials. The morphology, structure, and porosity of the resulting material can be tailored through control of the deposition angle and substrate temperature. By conducting deposition perpendicular to the surface, a film can be grown with a dense, conformal structure. On the other hand, deposition at oblique angles results in high surface area, porous films comprised of regular arrays of nanocolumnar structures. Furthermore, variations in the deposition angle allow for the inclusion of under-coordinated sites which change the chemical activity of the surface. Improvements in the activity, selectivity and stability of transition metal catalysts can be made by alloying the catalyst with a second element. The formation of molybdenum carbide decreases the strength of chemisorption on the surface, with respect to molybdenum, and improves selectivity for the dehydrogenation of formic acid. Platinum is active for the water-gas shift reaction. However, this catalyst cannot operate at low temperatures due to CO poisoning and is susceptible to deactivation due to accumulation of carbonaceous deposits. The formation of a platinum-copper near-surface alloy dramatically modifies the interactions of the surface with CO, H₂O and H₂ which can enhance the performance of this catalyst for the water-gas shift reaction.Item Simulation tools for predicting the atomic configuration of bimetallic catalytic surfaces(2012-12) Stephens, John Adam; Hwang, Gyeong S.Transition metal alloys are an important class of materials in heterogeneous catalysis due in no small part to the often greatly enhanced activity and selectivity they exhibit compared to their monometallic constituents. A host of experimental and theoretical studies have demonstrated that, in many cases, these synergistic effects can be attributed to atomic-scale features of the catalyst surface. Realizing the goal of designing -- rather than serendipitously discovering -- new alloy catalysts thus depends on our ability to predict their atomic configuration under technologically relevant conditions. This dissertation presents original research into the development and use of computational tools to accomplish this objective. These tools are all based on a similar strategy: For each of the alloy systems examined, cluster expansion (CE) Hamiltonians were constructed from the results of density functional theory (DFT) calculations, and then used in Metropolis Monte Carlo (MC) simulations to predict properties of interest. Following a detailed description of the DFT+CE+MC simulation scheme, results for the AuPd/Pd(111) and AuPt/Pt(111) surface alloys are presented. These two systems exhibit considerably different trends in their atomic arrangement, which are explicable in terms of their interatomic interactions. In AuPd, a preference for heteronuclear, Au-Pd interactions results in the preferential formation of Pd monomers and other small ensembles, while in AuPt, a preference for homonuclear interactions results in the opposite. AuPd/Pd(100) and AuPt/Pt(100) were similarly examined, revealing not only the effects of the same heteronuclear/homonuclear preferences in this facet, but also a propensity for the formation of second nearest-neighbor pairs of Pd monomers, in close agreement with experiment. Subsequent simulations of the AuPd/Pd(100) surface suggest the application of biaxial compressive strain as a means increasing the population of this catalytically important ensemble of atoms. A method to incorporate the effects of subsurface atomic configuration is also presented, using AuPd as an example. This method represents several improvements over others previously reported in the literature, especially in terms of its simplicity. Finally, we introduce the dimensionless scaled pair interaction, whereby the finite-temperature atomic configuration of any bimetallic surface alloy may be predicted from a small number of relatively inexpensive calculations.