The gasification of carbonaceous feedstocks is one method of producing syngas; however, the impurities present in the feedstock require a purification of the syngas prior to the ultimate use, with sub-ppm levels of H2S required. Significant efficiency gains are obtained by performing this gas cleaning step at temperatures between the gasification temperature and the ultimate application temperature (500 - 1200 K). In this thesis, the desulfurization performance of La/γ-Al2O3 is investigated for the removal of 100 ppm H2S in H2.
Supporting La2O3 on γ-Al2O3 has yielded a highly dispersed La2O3 phase. Due to the complications of H2S, CO2 was used as a probe molecule for characterization. At higher lanthanum weight loadings, CO2 adsorbed on La2O3 sites was observable in DRIFTS spectra. Also, volumetric chemisorption and TPD data indicate an increased capacity for CO2 adsorption, due to La2O3. The volumetric chemisorption data suggest that an increased number of high energy (> 40 kJ/mol) sites are available for strong CO2 adsorption with increased lanthanum content. With this increase in strongly bound CO2, there is an increase in weakly bound CO2, which may compete with H2S for adsorption sites.
The use of low weight loadings of La/γ-Al2O3 has yielded materials with sulfur capacities of 10-20 times more sulfur at breakthrough than the unsupported lanthanum oxide on a per gram of lanthanum basis. Regeneration of these materials in dilute air resulted in the rapid poisoning with sulfate, and the capacity decreased to that of the γ-Al2O3. Regeneration with humidified argon yielded a consistent capacity that was more than double that of the γ-Al2O3. While successful at removing H2S from H2, La/γ-Al2O3 was unsuccessful when CO, CO2 & H2O were added to the feed. The enhanced adsorption of CO2 found with DRIFTS and volumetric chemisorption acts to compete with H2S for adsorption sites, while H2O present resulted in an inhibition of the sulfidation. On the basis of characterization, an ideal sorbent would bind CO2 less strongly than alumina. From this, alumina is an unsuitable support for hot gas desulfurization with lanthana when significant quantities of CO2 or H2O are present.