Adsorption studies of simple molecules on metallic, bimetallic and semi-conductor interfaces

Rowe, Simon John (1988) Adsorption studies of simple molecules on metallic, bimetallic and semi-conductor interfaces. University of Southampton, Doctoral Thesis.

Abstract

The UHV spectroscopic techniques of AES, TDS, HREELS and LEED were used to examine a variety of adsorbate systems.

Trimethyl aluminium (TMA) adsorption and decomposition on Si was investigated over the temperature range 77-1500K. At 77K, simple multilayer physisorption was observed, with TMA desorbing at 200K. Adsorption at 300K occurred at a slower rate as there was an activation barrier to chemisorption. This barrier was thought to be that for the formation of HjAl(CH3)i_s intermediates (1< x< 3), which were shown to be on the surface by desorption of CH4 and C2H2. As the surface temperature rose above 600K, diffusion of Al into the bulk (dominated > 800K) competed with Al deposition (dominated > 600K & < 770K). Pre-evaporation of Al onto the Si surface lowered the activation barrier and the process of film deposition was auto-catalytic.

Acetic acid and ethanol adsorption were investigated on Rh (111). Ethanol formed ethoxide, water and CO on adsorption at 180K. The ethoxide decomposed in the range 220-270K, to give acetaldehyde, ethanol and H2. Above 500K, only CH residues remained on the surface. Acetic acid adsorbed at 200K to form multilayers. The first monolayer contained some acetate and acetic anhydride; lateral interactions between these groups gave rise to unusual off-specular EELS signals and desorption temperature variations with coverage. On heating to 400K, the excess acid desorbed, leaving large quantities of acetic anhydride. Above 550K only CH residues remained on the surface. The similarity in M-M interatomic distances of Rh and the carbonyl-carbonyl distance in the anhydride was discussed.

Adsorption, trimerisation and desorption/decomposition of acetylene was investigated on Pd, Pd-Cu and Pd-Pb surfaces.

Benzene production was clearly detected on Pd(331), benzene desorbing at 300K and 500K. On adsorption, acetylene blocked many of the benzene decomposition pathways, though extensive dehydrogenation did occur, with ethylidyne species being detected on the surface. Some acetylene chemisorbed reversibly when adsorbed at 200K.

Dehydrogenation processes were almost eliminated by small surface concentrations of Cu, which greatly increased reversible adsorption of acetylene and benzene production, as well as changing their desorption temperatures compared to those on clean Pd. Cu adsorption reduced the available numbers of large Pd ensembles, needed for decomposition, whilst the small Pd ensembles which produce benzene & desorb acetylene are unchanged.

Adsorption of acetylene was greatly limited by low concentrations of lead on the Pd surface. Above a composition of PdgPb, a surface alloy formed; the highest lead content achievable was PdPb2 owing to diffusion of Pd to the surface. On unheated alloy surfaces, there was detectable dehydrogenation but no reversible chemisorption. Heating a surface of composition Pd2Pb led to formation of Pd3Pb in which the lead was evenly distributed over the surface Pd triplet sites, such that almost all adsorption was blocked. CO desorption studies on the mixed surfaces were used to provide information on alloy structures formed. (DX 84802)

This record has no associated files available for download.

Contributors

Download statistics