Investigation of surface structures at catalytically important metals by UHV - electron spectroscopic and electrochemical methods
Wasberg, Mikael Paul
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https://hdl.handle.net/2142/19932
Description
Title
Investigation of surface structures at catalytically important metals by UHV - electron spectroscopic and electrochemical methods
Author(s)
Wasberg, Mikael Paul
Issue Date
1990
Doctoral Committee Chair(s)
Wieckowski, Andrzej
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Analytical
Language
eng
Abstract
This thesis describes the work and results of an investigation of adsorbates at Pt(111) and Rh(100) single crystal electrodes and the basic electrochemical behavior of Rh(100) in perchlorate electrolyte. The measurements were made in a UHV (ultra high vacuum) - electrochemistry transfer system which allowed rapid contamination free UHV - electrolyte - UHV transfers. In UHV, low energy electron diffraction and Auger electron spectroscopic measurements were made and electrochemical characterization was made by linear sweep cyclic voltammetry. The target adsorbate systems were carbon monoxide and iodine. Electrosorption of carbon monoxide on Pt(111) produced a ($\sqrt{3}\ \times$ 3)rect over-layer structure stable at room temperature which survived UHV transfer, careful LEED characterization and back immersion without changes in electrooxidation behavior. As a result of iodide exposure of submonolayers of CO created by electrooxidative stripping a second, lower cover $c(\sqrt{3}\ \times$ 5)rect structure was found. The electrochemical behavior of Rh(100) in 0.1 M perchloric acid was also established. An asymmetric hydrogen region was found and a reversible surface reaction at the positive side of the double layer region was obtained. The general voltammetric evolution during multiple cycling was explained by perchlorate reduction and adsorption-desorption of the chlorides produced in the process. The reduction of perchlorates occurs when entering and leaving the hydrogen adsorption-desorption region. Electrooxidation of chemisorbed carbon monoxide indicated a coverage of one CO molecule per surface atom. After emersion of saturation CO coverages a ($\sqrt{2}\ \times$ 4$\sqrt{2}$)R45$\sp\circ$ adlayer was detected. Back immersion of this structure gave a electrochemical coverage estimation in correspondence with the LEED data. In contrast to Pt(111), CO is spontaneously removed from the Rh(100) surface if immersed at open circuit. Immersion at a potential at the hydrogen evolution edge preserved the CO. The amount of CO lost due to UHV exposure corresponds approximately to the difference between unit coverage and the gas phase saturation coverage. Both gas phase and electolytic iodine produced a c(2 $\times$ 2) saturation coverage. Silver underpotential deposition onto this adlayer is distinct and qualitatively similar to the same behavior on iodine covered Rh(111). Chloride interference from reduced perchlorate was noticeable for both iodine and CO adlayers.
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