Characterization of zirconia-based gasification gas clean-up catalysts

Characterization of catalysts is an expanding field of catalysis and new techniques are adapted more and more from other disciplines of science. Catalyst characterization should answer at least to the following questions: 1) which properties of the catalyst correlate well with its activity, 2) what are the key intermediates and how they are adsorbed on the catalyst, 3) what kind of surface sites are involved, and 4) which is the mechanism for the studied reaction. In this thesis, zirconia-based catalysts were studied. Catalytic applications of zirconia often takes advantage of its acidic and basic surface properties although their strength is relatively weak. The catalytic properties and thermal stability of zirconia can be further enhanced by the addition of dopants. ZrO2 has been reported to be sulfur and water tolerant. These unique characteristics have led to study zirconia-based catalysts in gasification gas cleaning applications. Gasification of biomass is one potential and environmental benign way to produce energy, liquid biofuels and chemicals. Gasification is a thermo-chemical process where biomass is converted to gaseous products. The main components of gasification gas are carbon monoxide, hydrogen and carbon dioxide. Gas also contains impurities, such as ammonia and tar, and the gas has to be cleaned before use. Zirconia-based catalysts have shown to selectively oxidize tar molecules during hot gas cleaning at 600-900 °C when a small amount of oxygen is added into the gas. The catalysts selected for this thesis were ZrO2, Y2O3-doped ZrO2 and SiO2-doped ZrO2. The activity of the catalysts in gasification gas cleaning decreased in the order of ZrO2 > Y2O3-ZrO2 > SiO2-ZrO2. Relating the acidity and basicity of the catalysts to their activity suggested that acidity is not a desirable characteristic for gasification gas clean-up catalysts whereas basicity seems to be useful. Four types of toluene-derived surface species were discovered: molecularly adsorbed toluene, surface benzoate species, carbonaceous deposits and benzyl species, the latter being the key intermediate in toluene oxidation. Over all the catalysts, toluene was completely converted at temperatures above 550 °C to carbon dioxide, water, carbon monoxide and hydrogen. Of the main gasification gas components, water was shown to inhibit toluene oxidation activity over all these catalysts; the most over SiO2-ZrO2 and the least over pure ZrO2. The preferentiality, i.e. ability of the catalysts to protect the valuable gas components while oxidizing toluene, was addressed. The highest preferentiality of toluene over both CO and H2 was observed over pure ZrO2 at higher temperatures. Thus, pure ZrO2 was proven to manifest exceptional performance in preferential toluene oxidation. However, the tar oxidation activity of pure ZrO2 could be further improved and the gasification process could be further optimized in order to compete with fossil fuel based applications.