Angle-Resolved Thermal Desorption Spectroscopy as a Surface Probe

In the past fifteen years it has become apparent that the desorption process is not as well understood as it appeared when Knudsen formulated his cosine law of desorption in the early 1900's. Directed desorption, where the flux is focused along the surface normal, has been observed in several systems, notably hydrogen desorbing from nickel. This has been explained by invoking a flat barrier to adsorption that selects particles with some minimum normal component of kinetic energy to adsorb. However, all of the previous work has been done without directly measuring the activation barrier to adsorption. By using angle-resolved thermal desorption spectroscopy one can observe directed desorption under conditions where the activation barrier to adsorption can be measured independently. For hydrogen on polycrystalline nickel, the desorption distribution observed is approximately cos ('5)(theta), which is similar to distributions observed at much higher temperatures. The activation barrier to adsorption is found to be negligible. A model is proposed in which the desorption flux distribution is intimately related to the desorption site structure. Data for carbon monoxide on the (100) face of tungsten are also presented. Progressive reconstruction of the tungsten surface with CO desorption is indicated. Adsorption and desorption have long been regarded as reverse processes even under nonequilibrium conditions. The present results call this assumption into question. The work here demonstrates that angle-resolved thermal desorption spectroscopy can be used to provide a probe which is sensitive only to desorption site structure, and not to the host of other sites present, making it a useful tool to study active sites on catalysts.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Chemical Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2014

Author dc:creator

• Horton, Dorothy Roy

• Engineering, Chemical

Identifier

(UMI)AAI8302884

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/69727