Surface Characterization of Heterogeneous Catalysts Using Low Energy Ion Scattering Spectroscopy Combined with Electrochemistry

Fundamental studies of heterogeneous catalysis were performed and presented in this dissertation to gain a better understanding of heterogeneous catalytic reactions at a molecular level. Surface science techniques were employed in achieving the goal. Low energy ion scattering spectroscopy (LEISS) is the main surface science technique which will be used in all the studies discussed throughout this dissertation. The main objectives of LEISS measurements are to: 1) obtain the information of surface composition of heterogeneous catalysts from the topmost layer; 2) observe the effects of reaction conditions on the surface composition of heterogeneous catalysts. The surface composition and morphology of Au-Pd clusters bimetallic model catalysts supported on SiO2 were characterized using LEISS, infrared reflection absorption spectroscopy (IRAS), and temperature programmed desorption (TPD). It is observed that relative to the bulk, the surface of the clusters is enriched in Au. Ethylene adsorption and dehydrogenation show a clear structure-reactivity correlation with respect to the structure/composition of these Au-Pd model catalysts. Fundamental studies of heterogeneous catalysis were performed and presented in this dissertation to gain a better understanding of heterogeneous catalytic reactions at a molecular level. Surface science techniques were employed in achieving the goal. Low energy ion scattering spectroscopy (LEISS) is the main surface science technique which will be used in all the studies discussed throughout this dissertation. The main objectives of LEISS measurements are to: 1) obtain the information of surface composition of heterogeneous catalysts from the topmost layer; 2) observe the effects of reaction conditions on the surface composition of heterogeneous catalysts. The surface composition and morphology of Au-Pd clusters bimetallic model catalysts supported on SiO2 were characterized using LEISS, infrared reflection absorption spectroscopy (IRAS), and temperature programmed desorption (TPD). It is observed that relative to the bulk, the surface of the clusters is enriched in Au. Ethylene adsorption and dehydrogenation show a clear structure-reactivity correlation with respect to the structure/composition of these Au-Pd model catalysts

An Electrochemical, Microtopographical and Ambient Pressure X-Ray Photoelectron Spectroscopic Investigation of Si/TiO_2/Ni/Electrolyte Interfaces

The electrical and spectroscopic properties of the TiO_2/Ni protection layer system, which enables stabilization of otherwise corroding photoanodes, have been investigated in contact with electrolyte solutions by scanning-probe microscopy, electrochemistry and in-situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Specifically, the energy-band relations of the p+-Si/ALD-TiO_2/Ni interface have been determined for a selected range of Ni thicknesses. AP-XPS measurements using tender X-rays were performed in a three-electrode electrochemical arrangement under potentiostatic control to obtain information from the semiconductor near-surface region, the electrochemical double layer (ECDL) and the electrolyte beyond the ECDL. The degree of conductivity depended on the chemical state of the Ni on the TiO2surface. At low loadings of Ni, the Ni was present primarily as an oxide layer and the samples were not conductive, although the TiO_2 XPS core levels nonetheless displayed behavior indicative of a metal-electrolyte junction. In contrast, as the Ni thickness increased, the Ni phase was primarily metallic and the electrochemical behavior became highly conductive, with the AP-XPS data indicative of a metal-electrolyte junction. Electrochemical and microtopographical methods have been employed to better define the nature of the TiO_2/Ni electrodes and to contextualize the AP-XPS results

Direct observation of the energetics at a semiconductor/liquid junction by operando X-ray photoelectron spectroscopy

Photoelectrochemical (PEC) cells based on semiconductor/liquid interfaces provide a method of converting solar energy to electricity or fuels. Currently, the understanding of semiconductor/liquid interfaces is inferred from experiments and models. Operando ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) has been used herein to directly characterize the semiconductor/liquid junction at room temperature under real-time electrochemical control. X-ray synchrotron radiation in conjunction with AP-XPS has enabled simultaneous monitoring of the solid surface, the solid/electrolyte interface, and the bulk electrolyte of a PEC cell as a function of the applied potential, U. The observed shifts in binding energy with respect to the applied potential have directly revealed ohmic and rectifying junction behavior on metallized and semiconducting samples, respectively. Additionally, the non-linear response of the core level binding energies to changes in the applied electrode potential has revealed the influence of defect-derived electronic states on the Galvani potential across the complete cell

In situ investigation of dissociation and migration phenomena at the Pt/electrolyte interface of an electrochemical cell

The development of efficient energy conversion systems requires precise engineering of electrochemical interfaces and thus asks for in situ techniques to probe the structure and the composition of the dynamic electrode/electrolyte interfacial region. This work demonstrates the potential of the near ambient pressure X-ray photoelectron spectroscopy (NAPXPS) for in situ studies of processes occurring at the interface between a metal electrode and a liquid electrolyte. By using a model membrane-electrode assembly of a high temperature phosphoric acid-imbibed proton exchange membrane fuel cell, and combining NAPXPS measurements with the density functional theory, it was possible to monitor such fundamental processes as dissociation and migration of the phosphoric acid within a nanostructured Pt electrode under polarization

Synthesis of carbon-supported PdSn–SnO2 nanoparticles with different degrees of interfacial contact and enhanced catalytic activities for formic acid oxidation

The conjunction of the PdSn alloy and SnO2 is of interest for improving catalytic activity in formic acid oxidation (FAO). Here, we report the synthesis of PdSn–SnO2 nanoparticles and a study of their catalytic FAO activity. Different degrees of interfacial contact between SnO2 and PdSn were obtained using two different stabilizers (sodium citrate and EDTA) during the reduction process in catalyst preparation. Compared to the PdSn alloy, PdSn–SnO2 supported on carbon black showed enhanced FAO catalytic activity due to the presence of SnO2 species. It was also found that interfacial contact between the PdSn alloy and the SnO2 phase has an impact on the activity towards CO oxidation and FAO.Web of Scienc

Investigation of the Si/TiO_2/Electrolyte Interface Using Operando Tender X-ray Photoelectron Spectroscopy

Semiconductor-electrolyte interfaces allow for the creation of photoactive semiconductor systems that have band bending and other characteristics analogous to semiconductor-metal junctions (Schottky junctions). We demonstrate herein that XPS measurements can be obtained on a full three-electrode electrochemical system under potentiostatic control by use of tender X-rays to provide photoelectrons with sufficient kinetic energy to penetrate through a thin electrolyte overlayer on a portion of the working electrode. The response of the photoelectron binding energies to variations in applied voltage demonstrates that the XPS investigation works in an operando manner to elucidate the energetics of such interfaces

Microemulsion-mediated syntheses of silicalite-1

Due to the character of the original source materials and the nature of batch digitization, quality control issues may be present in this document. Please report any quality issues you encounter to [email protected], referencing the URI of the item.Includes bibliographical references (leaves 83-88).Issued also on microfiche from Lange Micrographics.The cationic microemulsion-mediated synthesis of silicalite-1 has been studied. The focal point of this work is to determine if these microemulsions present a confined space in which zeolite particles of novel morphology can be formed. The phase behavior of the microemulsions has also been determined to help rationalize the results of the zeolite syntheses. The phase behavior studies of the microemulsions indicate that one-phase, optically transparent microemulsions can be formed at 368 K in the presence of linear and branched cationic surfactants, which can solubilize approximately ten weight percent zeolite synthesis mixture. The results from the phase behavior studies were used to guide the zeolite syntheses. The effects of synthesis parameters such as TPAOH content, microemulsion composition, surfactants and temperature on the morphology of silicalite-1 were analyzed. Using microemulsions with branched cationic surfactant at 95° C, silicalite-1 of novel morphology was synthesized. Disc like silicalite-1, around 400 nm in size, was synthesized from microemulsion-mediated syntheses, while spherical crystals of smaller size were synthesized in the absence of the microemulsion. However, when linear cationic surfactant is used, mesoporous silica is synthesized. At higher zeolite contents in the microemulsion, better crystallinity and shape of silicalite-1 were obtained, while at lower zeolite content (less than ten weight percent) amorphous materials were synthesized. By decreasing the TPAOH content, the shape of the crystals synthesized using microemulsion are more disc-like and slightly larger. Silicalite-1 of 400 nm in size was synthesized with zeolite mixture molar composition of 1 TEOS: 0.18 TPAOH: 20 H₂O, while smaller crystals, 200 nm, were observed at higher TPAOH content (above 0.30). Salt has deleterious effect on silicalite-1 formation at 95° C. Mesoporous silica is synthesized when NaCl is added into the synthesis mixture. Finally, our results indicate that the "nanoreactor concept" may not be valid, observed from the studies of mixtures before and after the solids sediment out of the synthesis mixture. Powder diffraction pattern of material extracted from microemulsion after the mixture phase separates indicates that silicalite-1 is synthesized and its FE-SEM image shows ordered shape. However, very poorly crystalline material is synthesized from one-phase synthesis mixtures