Rare Earth Oxysulfides: High-Temperature Regenerable Sulfur Sorbents and Catalysts for the Water-Gas Shift Reaction

The first part of this thesis is focused on the preparation and evaluation of lanthanide oxide sorbents of enhanced H2S absorption properties. Of the formulations examined, lanthana and 30% praseodymium-doped lanthana, both prepared by the urea gelation-coprecipitation, are highly efficient sulfur sorbents, and they are stable and suitable as high-temperature filters for once-through operation; at 800°C, the sulfur capacity at ∼1 ppmv breakthrough of H2S exceeds 50 mg S/gsorbent for Pr-doped lanthana. Oxycarbonate formation, due to the preparation method or due to sorbent pretreatment in air-rich gas streams, would decrease the sulfur removal efficiency of the sorbents. However, in this work, it was found that even if formed, the lanthanide oxycarbonates are not stable in the reformate fuel gas at 800oC. These sorbents are stable in both reducing and oxidizing atmospheres and in shutdown/restart operation. The presulfided samples obtained in this fashion can reversibly adsorb H2S on their surface. By preparing them from high surface- area mesoporous lanthanide oxysulfates, it was found that the higher surface area was reflected in a proportionately higher sulfur capacity.;This dissertation presents for the first time the potential of lanthanide oxysulfides as sulfur-tolerant catalysts for the high temperature water-gas shift (WGS) and reverse water-gas shift (RWGS) reactions. Preparation of such type of WGS catalyst, able to maintain activity both in the presence and absence of sulfur in the feed gas, would constitute a major breakthrough and a key step in the effort to combine and integrate processing units of the fuel cell system in power generation.;The activity of these catalysts is linked to their enhanced oxygen storage capacity, which stems from the redox of the sulfur ion during interconversion of the oxysulfide to the oxysufate phase. X-Ray diffraction and time-resolved X-Ray diffraction with in-situ H2-TPR have confirmed the reversibility of the phases. The light-off temperature for the WGS reaction is around 400°C and at temperature above 750°C equilibrium conversions are reached. No loss of sulfur from the catalyst takes place with time-on-stream, indicative of a stable catalyst composition. The redox reaction mechanism is inferred by the comparable reaction rates and rates of reduction or oxidation of the oxysulfate or the oxysulfide lanthanides, respectively. Comparison of lanthanum oxysulfide with a commercial Fe-Cr based catalyst, in a product-free gas mixture with 700 ppm of H2S in the feed, clearly shows the superiority of the former in terms of sulfur resistance.;In the last part of this thesis work, lanthanum oxysulfate was used as support of atomically dispersed gold catalysts for the low-temperature WGS reaction. The study of this non-reducible (at temperature < 400°C) support surface to disperse gold could provide information on the active site for the low temperature water gas shift reaction and lead to a better understanding of the reaction mechanism. Catalyst preparation via the anion adsorption preparation method has led to strong gold-support interaction and high gold dispersion as confirmed by high-resolution TEM and NaCN leaching. XPS of the fresh catalyst found the presence of only cationic gold in the fresh material. The catalysts prepared this way are active for the low temperature WGS reaction, whereas the support itself does not light off until about 400°C. Complete CO conversion is achieved at 400°C, but catalytic activity gradually decreased to about ∼ 1/3 of the original value at this temperature; and remained the same for many hours. XANES and XPS of the used catalyst suggest, that deactivation during reaction is due to reduction of ionic gold. When compared to gold-based catalysts previously studied, these materials are less active. The lower reaction rate is due to lower surface area and lower degree of reducibility of La2 O2SO4 (SDS) at temperatures below 400°C. However, when the rates are normalized by the surface area or the amount of surface hydroxyls, the rates of all catalysts are of the same order of magnitude. These results suggest that a similar gold species [Au-Ox(OH)] is the active site for the WGS reaction on all these supports. (Abstract shortened by UMI.).