Infrared studies of oxide catalysts and thin films.

Title: Infrared studies of oxide catalysts and thin films.
Authors: Molapo, David Theko.
Date: 1998
Abstract: This thesis focuses on the characterization of various oxide based materials, some of which are potential industrial catalysts. The major instrumental technique used in this work is infrared (IR) spectroscopy. To a lesser extent nuclear magnetic resonance (NMR) spectroscopy was used. Also, attempts were made to correlate spectroscopic results with those from a standard test reaction which would allow us to rank moderate strength catalysts, based on their acidity. This test involved the isomerization of 2-methyl-2-pentene (2M2P). The isomerization of 2M2P is a particularly useful reaction since the olefin can convert into a large number of products, the relative rates of formation of which reflect the acid strength of the catalyst. Halide containing solid acids are common catalysts, however the availability and use of halogen containing treatment gases in their preparation has come under increasing environmental scrutiny. As a result there is interest in developing alternative environmentally compatible halide free catalysts as replacements. Silica-aluminas are too acidic for this purpose. However, by adding weakly basic rare earth oxides to silica-alumina catalysts their overall acidity can be lowered to degrees comparable to halide containing acids. Yttria behaves in a manner similar to the rare earth oxides and was used for this purpose. Olefin model reactions, in particular the isomerization of 2M2P, show that the acid strength of silica-aluminas can be adjusted to that of halided, chlorided or fluorided, aluminas with the addition of rare earth oxides in the 4-20 wt. % range. In this work changes in the surface acidity of 4, 8, 12 and 16 wt. % $\rm Y\sb2O\sb3$ doped $\rm SiO\sb2$-$\rm Al\sb2O\sb3$ catalysts were investigated as well as those of fluorided aluminas using probe molecules. IR spectroscopy has been used for many years to characterize the acidity of oxides. This technique makes use of probe molecules. These molecules are expected to interact, i.e. H-bond, physically adsorb or chemically adsorb, with surface active sites. These active sites, commonly referred to as acid sites, contribute to acid catalysis in a host of reactions. The probe molecules which will be discussed in this thesis vary in size and basicity and include: ammonia (NH$\sb3$), pyridine ($\rm C\sb5H\sb5N$) and deuterated acetonitrile (CD$\sb3$CN). Absorption coefficients of probe molecules at gas-solid interfaces are generally not known hence absolute numbers for acid sites cannot be obtained using IR spectroscopy. The advantage of NMR over IR spectroscopy is that in the former the signal intensity is directly proportional to the number of probe nuclei present at a particular acid site. Trimethylphosphine, ($\rm CH\sb3)\sb3P$, was used as an NMR probe in order to obtain the concentrations of acid sites. Thin film (TF) experiments were performed on the pure oxides of silica, alumina, titania and zirconia. By performing TF experiments the samples used weighed about 0.1-0.5 mg/cm$\sp2$ as opposed to the 2.5-10 mg/cm$\sp2$ common for self supporting discs. This allowed us to probe the far IR region, down to 200 cm$\sp{-1}$, for the first time, as there was less interference from bulk modes.
CollectionTh├Ęses, 1910 - 2010 // Theses, 1910 - 2010
NQ32452.PDF5.35 MBAdobe PDFOpen