Abstract
Nanostructured metal oxides and silicates are increasingly applied in catalysis, either as supports or as active species in heterogeneous catalysts, owing to the physicochemical properties that typically distinguish them from bulk oxides, such as higher surface area and a larger fraction of coordinatively unsaturated sites at their surface. Among the different synthetic routes for preparing these oxides, sol-gel is a relatively facile and efficient method. The use of supercritical CO2 (scCO2) in the sol-gel process can be functional to the formation of nanostructured materials. The physical properties of the scCO2 medium can be controlled by adjusting the processing temperature and the pressure of CO2, thus enabling the synthesis conditions to be tuned. This paper provides a review of the studies on the synthesis of oxide nanomaterials via scCO2-assisted sol-gel methods and their catalytic applications. The advantages brought about by scCO2 in the synthesis of oxides are described, and the performance of oxide-based catalysts prepared by scCO2 routes is compared to their counterparts prepared via non-scCO2-assisted methods.
Highlights
It should be pointed out that the synthetic routes that will be discussed are not limited to the typical sol-gel synthesis in which macroscopic sol or gel intermediates are formed, and include other synthesis methods based on hydrolytic condensations in which metal oxide precipitates are formed but no obvious formation of sol or gel is observed
The advantages of supercritical CO2 (scCO2) discussed in Section 1.3 make it a promising solvent in sol-gel processes for the production of nanostructured metal oxides and silicates, as the starting precursors are directly dissolved in scCO2
Room temperature.b The number indicates the loading of TiO2 ; c This catalyst was synthesised by a sol-gel method in scCO2, and not by scCO2 drying as the other materials in this table; d Amount of P25 comparable with the amount of titania present in 40 TiO2 -SiO2
Summary
Metal oxides and silicates draw great research attention for their wide range of applications in catalysis [1]. A variety of methods have been developed for the synthesis of nanostructured metal oxides and silicates [1], including sol-gel methods, template techniques, hydrothermal or solvothermal routes, precipitation methods, chemical vapour deposition (CVD), laser ablation and electrochemical. SBA-15 silicates, sol-gel, template, hydrothermal and hydrothermal solvothermal methods are combined [13]. Compared to the and othergenerally methods,inexpensive sol-gel routes benefit from straightforward routes benefit straightforward synthesis conditions and a highand generally inexpensive synthesis conditions and a high degree of versatility [14,15].
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