Mesoporous SBA-15 with mesopore diameter up to 10.1 nm was prepared by a hydrothermal method, and was further functionalized to obtain different surface properties. Thus-prepared SBA-15 was employed as a template to synthesize rhombohedrally crystallized mesoporous La0.8Sr0.2CoO3
perovskite via a nanocasting method. The surface properties of the SBA-15 were adjusted by treatment with concentrated hydrochloric acid, trimethylchlorosilane (TMCS), and 3-aminopropyltriethoxysilane (APTES). A series of characterization techniques verified that all the synthesized
templates possessed ordered two-dimensional hexagonal mesoporous structure, and the surface was successfully modified with methyl and amino groups. The mesoporous perovskite structure was formed in the samples and the surface properties of SBA-15 significantly influenced the structure and
properties of La0.8Sr0.2CoO3 perovskite oxides. Wide-angle X-ray diffraction patterns suggested that the modified silica templates were conducive to the formation of pure perovskite frameworks with good crystallinity. The catalysts also possessed mesoporous
structure, as confirmed by small-angle XRD patterns, high-resolution transmission electron microscopy images, and nitrogen adsorption analysis. Moreover, the La0.8Sr0.2CoO3 materials synthesized using surface-functionalized templates exhibited relatively higher
catalytic activity and stability in CO oxidation. Complete CO conversion could be achieved at 140°C #xb0;Cusing the thus-prepared La0.8Sr0.2CoO3 materials, and no significant loss in catalytic activity was observed after 100 h of on-stream reaction experiments.
X-ray photoelectron spectroscopy, H2 temperature-programmed reduction, and O2 temperature-programmed desorption experiments revealed that the existence of Co4+, Sr enrichment in the perovskite structure, and high content of adsorbed oxygen species play a critical
role in the enhanced catalytic activity of the catalysts. We also proposed the possible reasons for the effect of surface properties of the silica templates on the structure and properties of the La0.8Sr0.2CoO3 nanomaterials.
Changzheng Xie,Zhijie Chen,Xinghua Wang,Ying Meng,Yihao Wang
The perovskite LaCoO3and La0.8Sr0.2CoO3thin films were synthesized successfully by a polymer-assisted deposition method. The structural characterization and thermal decomposition measurements indicate that appropriately increasing the annealing temperature is helpful in improving the film crystallinity. Compared with LaCoO3films, the Curie temperature is enhanced to a higher temperature in La0.8Sr0.2CoO3films, which is attributed to the changes in both the valence state and spin state induced by the chemical doping. In addition, a broad transition temperature region is observed in La0.8Sr0.2CoO3films, revealing the existence of inhomogeneous ground states in this system.Keywords:ferromagnetic perovskite;chemical doping;spin-state transition;lattice relaxation
An depth understanding of the fundamental interactions between surface termination and catalytic activity is crucial to prompt the properties of functional perovskite materials. The elastic energy due to size mismatch and electrostatic attraction of the charged Sr dopant by positively charged oxygen vacancies induced inert A-site surface enrichment rearrangement for perovskites. Lower temperatures could reduce A-site enrichment, but it is difficult to form perovskite crystals. La 0.8 Sr 0.2 CoO 3-δ (LSCO) as a model perovskite oxide was modified with additive urea to reduce the crystallization temperature , and suppress Sr segregation. The LSCO catalysts with 600 °C annealing temperature (LSCO-600) exhibited a 19.4-fold reaction reactivity of toluene oxidation than that with 800 °C annealing temperature (LSCO-800). Combined surface-sensitive and depth-resolved techniques for surface and sub-surface analysis, surface Sr enrichment was effectively suppressed due to decreased oxygen vacancy concentration and smaller electrostatic driving force. DFT calculations and in-situ DRIFTs spectra well revealed that tuning the surface composition/termination affected the intrinsic reactivity. The catalyst surface with lower Sr enrichment could easily adsorb toluene, cleave, and decompose benzene rings , thus contributing to toluene degradation to CO 2 . This work demonstrates a green and efficient way to control surface composition and termination at the atomic scale for higher catalytic activity.
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