Rare Earth Oxysulfides: High-Temperature Regenerable Sulfur Sorbents and Catalysts for the Water-Gas Shift Reaction.
Valsamakis, Ioannis.
2012
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Abstract:
Substantial effort in the scientific literature has been focused on advanced power
generation technologies that can play a significant role in mitigating global climate
change. Fuel cell technologies based on hydrogen have been proposed, but their
commercialization has been delayed due to issues of durability and cost, and the lack of
infrastructure for hydrogen distribution and ... read morestorage. Alternatively, there is great
interest in using liquid fuels (such as diesel and jet fuels) or coal as hydrogen
sources. In addition to greenhouse gas emission concerns, a big challenge in using these
fuels is the elimination of sulfur compounds. Lanthanide (rare earth) oxides are
suitable sulfur sorbents at high temperatures, and can readily achieve the strict
requirements for sub-ppm levels of H2S in the fuel gases even at temperatures as high as
650 to 700°C. 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 La2O2SO4
(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.
Thesis (Ph.D.)--Tufts University, 2012.
Submitted to the Dept. of Chemical and Biological Engineering.
Advisor: Maria Flytzani-Stephanopoulos.
Committee: Howard Saltsburg, Terry Haas, Christos Georgakis, and Zhijiang Li.
Keyword: Chemical engineering.read less - ID:
- zg64tz38z
- Component ID:
- tufts:21046
- To Cite:
- TARC Citation Guide EndNote
