(158e) Mesoporous Mixed Rare Earth Oxides for Hot Gas Desulfurization, Tar Cracking and Nanoparticle-Aided Combustion

Mesoporous
rare-earth mixed oxides (REOs, often supporting a small amount of a transition
metal) are among the most difficult to design and optimize for catalytic
applications. In particular, controlling the mesopore structure of even binary
REOs is difficult, while for practical application ? e.g., as hot gas
desulfurization adsorbents ? ternary REOs are often needed.  We have studied
synthesis of mixed REOs for hot gas desulfurization and a variety of other
applications (e.g., nanoparticle-aided combustion) and here we report
comparisons of nanostructured, mesoporous binary and ternary REOs prepared by
templated sol-gel and microemulsion techniques.

Relevance

Mixed REOs (e.g., CeO₂/La₂O₃/Tb₂O₃)
can simultaneously adsorb H₂S (to give M₂O₂S),
crack tars and reform slip methane. The oxide can be regenerated with dilute O₂
or alternative media such as SO₂.  While CeO₂/La₂O₃
by itself is an initially effective H₂S sorbent, it rapidly loses
surface area and so sulfur adsorption capacity (>80% after 3 redox cycles);¹
Another problem is the formation of sulfate during oxidative regeneration.

La, Pr,
Sm, and Nd are all more stable oxysulfides than Ce₂O₂S,²
and therefore more effective for H₂S removal. This has been proven
for La₂O₃, where intimate mixing with CeO₂ greatly
improves regeneration to the oxides under either oxidizing or reducing
conditions.^3-4  The third oxide (ZrO₂, Tb₂O₃,
Gd₂O₃, Al₂O₃ can all be effective)
is chosen to retard sintering, and is present either as a support for the
binary REO (ZrO₂, Al₂O₃) or in small quantity
(Tb₂O₃, Gd₂O₃).  For ternary oxides
the optimal Ce/La ratios differ from that of the simpler mixed Ce/La oxides.

Results

The above REOs or REOs with Al₂O₃
or ZrO₂ have been synthesized as high surface area mesoporous
materials at >3.0 nm pore diameter by a variety of techniques, ranging from
surfactant-templated to reverse microemulsion. The resulting materials have
been characterized by high-temperature steaming (sintering) tests with before
and after surface area measurements and porosimetry, H₂S
adsorption-desorption, XRD, DSC-TGA to measure crystallization exotherms, and
SAXS. 

To date
we have found surfactant-templated methods^5-6  to be reliable for
the production of high surface area (sometimes >200 m²/g)
mesoporous REOs, regardless of the precursor salts used. Homogeneity looks
good, based on DSC and XRD characterizations. Reverse microemulsion methods
seem to work only for certain combinations of salts or alkoxides. We did modify
a reverse microemulsion method for ZrO₂ films^7-8 to prepare
larger mesoporous (>6 nm) CeO₂/La₂O₃
composites using block EO-PO copolymer (Pluronic) templates.

Using
DSC we found weaker crystallization exotherms for intermediate Ce/La atomic
ratios of  0.9-3, stronger for either lower or higher ratios. This suggests
better sintering resistance for the intermediate oxides; steaming tests with
before-and-after porosimetry mostly confirmed this. Using simulated combustion
effluent (5% water, 30% CO₂, 60% H₂, 5% N₂/H₂S)
we noted that the CeO_x in intermediate Ce/La ternary oxides was only
partly reduced at 1030 K, while pure CeO₂ would be almost completely
reduced under these conditions.⁹  The partial reduction goes hand in
hand with the enhanced hydrothermal stability.

References

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3.  Flytzani-Stephanopoulos,
M.; Sakbodin, M.; Wang, Z.  Regenerative adsorption and removal of H₂S
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Zhang, X.L.; Zhao, Q.Z.; Li, J.  Study on simultaneous catalytic reduction of
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7.  Crepaldi, E.L; Soler-Ilia,
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8.  Crepaldi, E.L.;
Soler-Ilia, G.J. de A.A.; Bouchara, A.; Grosso, D.; Durand, D.; Sanchez, C., 
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