(513ey) Optical Trapping of Aerosol Sol-Gel Catalysts with in-Situ Raman Analysis

Aerosol-assisted heterogeneous catalyst synthesis is emerging as key manufacturing technology for tailored materials. The process combines sol-gel processing and evaporation induced self-assembly and can produce homogeneous materials with specific porosity characteristics (Debecker, 2018) (Figure 1). While the process has numerous advantages, it is challenging to interrogate the mechanism of catalyst synthesis as the synthesis reactions take place in individual droplets rather than within a bulk solution. To overcome this, we have taken advantage of optical trapping techniques coupled with Raman spectroscopy in order to conduct operando studies of the reactive synthesis processes occurring within a single aerosol droplet. This represents a significant extension of previous work, e.g. characterising surface reactions on solid particles (Rkiouak, 2018).

The catalyst studied is Ni/Al₂O₃, synthesised from a mixture of nickel nitrate hexahydrate, aluminium isopropoxide, ethanol, water, nitric acid and pluronic P123. Spectroscopic interrogation of droplets containing each component and mixtures of the components, up to and including the full synthesis mixture has been conducted (Figure 2). In addition, the effect of heating the droplets – a key synthesis step – has been investigated.

This work has successfully demonstrated that a single droplet comprising the reactive precursors can be successfully held in the optical trap, replicating the environment of aerosol-assisted synthesis. Raman spectroscopy shows several characteristic peaks for ethanol, aluminium isopropoxide, and nickel nitrate. Aluminium isopropoxide was observed to stabilise the droplet as compared to the droplet containing only ethanol, water, HNO₃ and P123. This is attributed to a self-assembly process forming a macro structure; no hydrolysis reactions were observed. Upon inclusion of nickel nitrate hydrolysis and condensation reactions were inferred from the spectra obtained. As expected, droplet heating intensified the rate of evaporation and reaction within the droplets. This provides the first operando study of this process and furthers understanding of the synthesis mechanism.