Nanostructured Oxides Synthesised via scCO2-Assisted Sol-Gel Methods and Their Application in Catalysis

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 CO 2 (scCO 2) in the sol-gel process can be functional to the formation of nanostructured materials. The physical properties of the scCO 2 medium can be controlled by adjusting the processing temperature and the pressure of CO 2 , thus enabling the synthesis conditions to be tuned. This paper provides a review of the studies on the synthesis of oxide nanomaterials via scCO 2-assisted sol-gel methods and their catalytic applications. The advantages brought about by scCO 2 in the synthesis of oxides are described, and the performance of oxide-based catalysts prepared by scCO 2 routes is compared to their counterparts prepared via non-scCO 2-assisted methods.

Role of supercritical carbon dioxide (scCO2) in fabrication of inorganic-based materials: a green and unique route

“…Compared to RESS, several efforts have been dedicated to applying the SAS approach extensively towards the fabrication of various inorganic-based nanoarchitectures. In a case, Miedziak and coworkers fabricated the nanocrystalline cerium oxide (CeO 2 ) supported gold-palladium alloy catalysts utilizing the widely applied SAS method [ 47 ]. Compared to the products obtained from the conventional preparation techniques, this unconventional SCF-assisted SAS process resulted in the lesser polluting components with higher active oxidation catalytic performance.…”

“…Indeed, scCO 2 offers comprehensively distinctive properties, which can be explicitly used to synthesize and activate new types of materials, such as aerogels (metal or metal oxide aerogel, some polymer aerogels, aerogels particles) [ 47 , 69 , 76 , 98 , 99 ]. The process of sol-gel-based synthesis generally begins with the formation of a pseudo-solution through a hydrolysis and condensation reaction by blending some chemically active precursors.…”

Recent Advances in Structured Catalysts Preparation and Use in Water-Gas Shift Reaction

“…The sol-gel preparations are based on the formation of a colloidal solution (sol), obtained by hydrolysis and condensation of the soluble precursors, such as metallic, organometallic or alkoxides precursors [26]. Upon further condensation, the sol gives rise to an integrated network (gel), containing solvent molecules trapped, finally the gel can be converted to solid material by removing the solvent through drying and/or thermal treatments [27,28]. Films of variable thicknesses can be realized by dip-coating or spin-coating of the structures, making this technique effective and not expensive.…”

Niobium oxide prepared through a novel supercritical-CO2-assisted method as a highly active heterogeneous catalyst for the synthesis of azoxybenzene from aniline

“…In addition, scCO 2 has high diffusivity and extremely low surface tension, which are very important for maximum preservation of the formed nanostructures when removing CO 2 from the system, which can be achieved simply by depressurisation. 35 It has been extensively reported that the formation of nanomaterials typically proceeds differently with the assistance of scCO 2 . [36][37][38] Further advantages of the use of scCO 2 as a reaction medium include the low toxicity and low cost of carbon dioxide and the relatively mild conditions needed to achieve its supercritical point (T c = 31.1°C, p c = 73.9 bar).…”