In this post, I want to describe the three ideas we’re investigating to synthesize nanoreactors right now and the general methodology of nanoreactor design.
The key characteristics of a nanoreactor are:
- A closed vessel that can confine metal nanoparticles
- Allows limited diffusion of gas through their walls
- A scale on the order of tens to thousands of nanometers
- Robust enough to withstand harsh reaction conditions
- Uniform geometry – all reactors should be a similar size and shape
The first three requirements are necessary for the dynamics we aim to create within the reactors. The fourth requirement ensures that the catalysts we make will not degrade as we perform tests, and will be useful under industrial conditions. The last requirement is not as important but allows us to more easily characterize the performance of nanoreactors, understand them mechanistically, and ensure that multiple batches of nanoreactors will behave similarly.
Mesoporous metal oxides satisfy the first, second, and fourth requirements right off the bat. They are fairly strong materials, resistant to both physical stress and high temperature, are frequently used as inert supports for metal catalysts, and have a complex network of interconnected pores that allow gasses to slowly diffuse through them.
The question then is how to manufacture metal oxide nano-geometries consistently. There is a well-known method to produce uniformly sized beads from silicon dioxide, known as the Stöber process. If a solution of a silicon-containing molecule (tetraethylorthosilicate) is reacted with a solution containing water under basic conditions, it forms silicon dioxide. If this hydrolysis is performed in a well-mixed solution of carefully balanced ethanol and water, it is possible to produce silica spheres of a controlled size, typically on the scale of hundreds of nanometers.
From here, the final challenge is to produce a cavity and insert the catalyst particles that we intend to use. While there are some methods to selectively etch metal oxides, it is only possible under very tightly controlled conditions, and even then is often only possible when the silica is crystalline, rather than amorphous (as is the result of this process). Instead, it is easier to build a new layer on top of the silica sphere, then dissolve the sphere.
This is the general approach we are taking: by adhering the catalyst particles to the surface of the silica, then using a similar approach of hydrolyzing an organo-metallic precursor molecule, we can form a thin film of zirconium or titanium oxide on the surface of the silica spheres, coating the spheres and catalyst. We can then use sodium hydroxide to remove the silica from the inside. Right now, we’re just beginning to test how reliable this coating process is, and whether it is possible to control the thickness, porosity, and other properties of the coating.