Surface science studies of adsorption and hydrogenation reactions on single crystal metal surfaces

Abstract

Knowledge about the interactions of incident energetic hydrogen species on metal surfaces is important for our understanding of many processes such as those occurring in plasma-enhanced catalysis. A unique feature of plasma-enhanced catalysis compared to thermal catalysis is the presence of reactive H atoms and ions in the gas phase above the catalyst surface. These energetic species can penetrate the adsorbate-covered surface and produce subsurface hydrogen, which can carry out hydrogenation reactions more effectively than surface-bound hydrogen. This thesis describes use of a surface science approach to study such hydrogenation reactions on a single crystal Ni(110) model catalyst.

Subsurface hydrogen binding sites on Ni(110) are readily populated by incident D atoms and D2+ ions. These subsurface D atoms recombine to desorb as D2 in temperature programmed desorption measurements to create characteristic subsurface-derived D2 thermal desorption peaks. Subsurface D can hydrogenate postadsorbed adsorbates, carbon monoxide and ethylene, in subsequent TPD measurements to form hydrogenated products in characteristic reaction rate-limited thermal desorption peaks.

Additionally, research discussed in this thesis uses surface science studies to elucidate the complicated chemistry of the adsorption and decomposition of bio-oil compounds on single crystal surfaces that are model catalysts. An improved fundamental understanding of how these molecules interact with catalysts will aid in the development of hydrodeoxygenation catalysts for bio-oil upgrading.

The adsorption and decomposition of guaiacol (C6H4(OH)(OCH3)) was studied on a Pt(100) surface. At temperatures above 225 K, after the desorption of physisorbed layers, a dissociatively adsorbed species, C6H4O(OCH3), and coadsorbed H was observed. The adsorbed intermediate species was stable up to 337 K when O-C bonds breaking occurred. Molecularly adsorbed guaiacol in a horizontal configuration bound through its benzene ring was not observed.

Acetic acid (CH3COOH) adsorption and reaction was studied on Ni(110) and Pt(100) surfaces. Acetic acid dissociatively adsorbs on Ni(110) at 90 K, forming an η1(O)-acetate (CH3COO) species. Upon heating above 200 K, η1(O)-acetate species either decompose or form a η2(O,O)-acetate species. Decomposition of η2(O,O)-acetate species leads to a “surface explosion,” desorbing in an autocatalytic reaction H2 and CO2 products simultaneously at 425 K. Such a surface explosion was not observed on Pt(100).