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Project 3: Zirconium oxide model systems
Abstract:
Model systems of the alkane skeletal isomerisation catalyst sulfated zirconia were successfully produced via a range of different preparation techniques. The model systems were investigated with various techniques, including thermal desorption, photoelectron, X-ray absorption and IR spectroscopies. Electrically and thermally conducting thin films of sulfated zirconia were prepared on oxidised silicon wafers, in order to allow the application of surface science techniques. Thermal treatment of the films was optimised to chemically mimic the powder process, resulting in films possessing the essential features (including tetragonal phase, nanocrystallinity and sulfur content of ~3 atomic %) of active powder catalysts.
Two distinctly different chemisorption sites were detected on the sulfated zirconia thin films by both ammonia and n-butane adsorption studies. Strongly chemisorbed ammonia reacts with certain sulfate species leading to the evolution of SO2 above 473 K. Low temperature (300-100 K) n-butane adsorption-desorption equilibrium isobaric measurements showed adsorption to be promoted over the sulfated zirconia thin films, as compared with oxidised silicon wafers. Strong and weak n-butane chemisorption, releasing heats of between 59-40 and 47-34 kJ/mol, corresponds to 5 and 25% of a monolayer coverage, respectively. The total amount of chemisorbed n-butane coincides very well with the estimated number of surface sulfate groups. An increase in adsorption heat was observed between coverages of ~5-8% of a monolayer, indicating adsorbate-adsorbate interactions. A bimolecular isomerisation mechanism is thus considered plausible under such coverages. Physisorption on the films generates heats of ~28 kJ/mol, for coverages from 30% up to a complete monolayer. Multilayer adsorption results in the formation of an electrically insulating adsorbate structure.
Carbonaceous deposits were detected on the films after exposure to n-butane under reactive conditions ( 481 K), thus proving the films have reactive centres. Analysis has shown the deposits to contain unsaturated hydrocarbons, which have a * resonance typical of butenes; furthermore, sulfate groups are reduced during exposure, thus proving the oxidative dehydrogenating ability of sulfated zirconia. The deposits are also shown to be oxygenated, thus are consistent with the stabilised form of the reactive carbocation intermediates.
Powder sulfated zirconia catalysts were prepared from sulfating agents containing one and two pregrouped sulfur atoms, via a variety of different methods using various sulfur loadings, to test whether disulfate groups are responsible for the catalytic activity of the material. Sulfated zirconias synthesised from two pregrouped sulfur atoms were however found to be less active. Nevertheless, the presence of disulfate groups was found to be a prerequisite for catalytic activity and for materials prepared using the same sulfation method the more active were shown to have higher disulfate concentrations.
It is thus proposed that the more strongly chemisorbing sites, which react with ammonia, correspond to a minority disulfate species. These disulfate sites may oxidatively dehydrogenate n-butane, initiating the formation of catalytically active isomerisation centres. The chemical environment of these disulfate groups is envisioned to strongly influence the catalytic reactivity of the active sites they generate.
