Abstract
M.Sc. (Chemistry)
The usefulness of vibrational spectroscopy in identifying surface species, determining adsorbate structures and studying surface reactions has been widely demonstrated. Most of the infrared work on surface species is currently performed using the transmission technique with very thin pressed discs of self supported catalysts. This technique has several disadvantages of which the limited transmission of many catalysts and the loss of available surface area during sample preparation, are but a
few. Because of these limitations, information obtained from conventional transmission studies has limited application in terms of understanding and/or improving commercial catalysts. An alternative method which does not suffer from these limitations is Diffuse Reflectance Infrared Fourier Transform Spectroscopy or DRIFTS as it is commonly known. This spectroscopic technique has only recently been extended into the infrared region because of the progress in FTIR instrumentation. Because this is a reflectance technique, the sample is most appropriately a powder,
so a high surface area catalyst in its normal powder form can be examined directly without altering its state. A spectroscopic facility which is capable of obtaining DRIFT spectra of adsorbed species at high sensitivity and in situ operating conditions has
been established. This facility consists of an FTIR instrument fitted with a diffuse reflectance unit, a heatable high pressure cell and the necessary attachments for gas flow, pressure and temperature control. The project work consisted of an investigation into CO-adsorption on supported rhodium catalysts. By using different combinations of the three parameters: reduction temperature, metal loading and support material, several different species of surface bonded CO have been identified. The three rnein species being: geminal dicarbonyl, linear and bridging CO. As each of these species is associated with a specific Rh-site, conclusions concerning the oxidation state and dispersion of the Rh on the surface could be made. The thermal stability of the different
CO-species was studied by increasing the catalyst temperature in a linear fashion. It was found that the geminal dicarbonyl species was the most stable in an oxidising atmosphere. The interconversion of chemisorbed . CO-species at higher temperatures has been spectroscopically verified. A mechanism for CO-dissociation on Rh-catalysts was proposed. In additional experiments the sensitivity of DRIFTS for adsorbed hydrocarbons has been demonstrated. It is concluded that this spectroscopic
technique has been proven to be of great significance in the study of surface species on heterogeneous catalysts.