Adsorption, Co-Adsorption and Catalytic Reactions on Rh(111) and Ru(001) Surfaces

Citation

Thiel, Patricia Ann (1981) Adsorption, Co-Adsorption and Catalytic Reactions on Rh(111) and Ru(001) Surfaces. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/8m9w-p414. https://resolver.caltech.edu/CaltechTHESIS:03222018-134525550

Abstract

The adsorption of oxygen, hydrogen, carbon monoxide , nitric oxide and water on Rh(111) and Ru(001) surfaces has been studied using the techniques of high-resolution electron energy loss spectroscopy, low-energy electron diffraction, Auger electron spectroscopy, ultraviolet photoelectron spectroscopy and thermal desorption mass spectrometry. In many cases co-adsorption experiments have provided insight into the nature of interaction between different adsorbates, with potential implications for heteroqeneous catalytic reaction mechanisms.

The interaction of oxygen with Rh(111) consists of adsorption, irreversible thermally induced ordering and disordering phenomena, dissolution into the subsurface region and desorption. The thermodynamic parameters which describe these phenomena have been investigated. The forms of the kinetic rate expressions for the catalytic reaction of adsorbed oxygen with hydrogen are different for ordered and disordered arrays of oxygen adatoms. In agreement with co-adsorption studies of hydrogen and oxygen, and supported by studies of hydrogen chemisorption on clean Rh(111), this implies that the rate of adsorption of hydrogen is sensitive to the structure of the adsorbed oxygen lattice.

Two forms of molecularly adsorbed NO are readily distinguished on a Ru(001) surface by the frequencies of the nitrogen-oxygen stretching vibrations, which can then be used to observe the influence of co-adsorbates. Oxygen, nitrogen and hydrogen adatoms compete selectively for the adsites which are occupied by multiply-coordinated NO, whereas CO competes for adsites occupied by the singly-coordinated molecular NO. Carbon monoxide can even induce conversion of adsorbed molecular NO from sites of single coordination to sites of multiple coordination with the metal substrate.

Water interacts with the Ru(001) surface to form chemisorption bonds, but it also forms intermolecular hydrogen bonds which are of comparable strength. This leads to formation of layered, hydrogen bonded aggregates at all coverages. The properties of the first two layers are distinct from those of the subsequent ice multilayers, and a specific structural model for the hydrogen bonded lattices is proposed. A thermally induced ordering effect is observed which is analogous to the vitreous-to-cubic phase transformation of bulk ice.

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