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.
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.
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 methods5-6 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 films7-8 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 CeOx 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.
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