Characterization of semiconducting catalysts by gas adsorption: Nickel oxide and magnesium ferrite
Allspach, Eugene Robert
Hightower, Joe W.
Master of Science
In previous work in this laboratory Murphy, Veerkamp, and Leland have derived a model that measures the fraction of a semiconductor catalyst participating in electronic surface effects due to gas adsorption. The parameter, xg, proposed in the model represents the fraction of the catalyst that has undergone significant changes in its electronic properties when compared to the properties that existed before gas adsorption. (The properties at the center of the catalyst particles normally remain unaltered so the comparison can be made between the surface and the center of a particle.) xg is calculated from an equation involving the thermoelectric power, Q, and an electronic heterogenity factor, f. The latter factor is determined using the linear slope of the lncrvs. Q plots obtained from gas adsorption on the catalyst, where a-is the electrical conductivity. In the model the quantitative value of xg for a catalyst is postulated as being a characteristic of that catalyst. If so, the x values for different catalysts can then be used to compare the electronic surface effects which occur an different catalysts. In this work the model was tested with two catalysts: nickel oxide and magnesium ferrite. With nickel oxide reasonable x values were calculated from the model for the carbon monoxide, hydrogen, and oxygen adsorptions which were conducted. The following xg values were found for the various adsorbates: .23 for carbon monoxide, .19 for hydrogen, and .37 and .44 for oxygen. For magnesium ferrite a reasonable xg value Df .65 for carbon monoxide adsorption was calculated from the model but the x value for oxygen adsorption was greater than unity which is meaningless. The failure during oxygen adsorption is attributed to the presence of significant numbers of holes compared to electrons. The number of holes present is assumed negligible in the theory. However, due to the small number of electrons available for conduction in magnesium ferrite, the loss of electrons to oxygen during adsorption significantly alters the relative proportions of electrons and holes and causes comparable contributions to the total conductivity by each.