Duke Pratt School of Engineering scientists with colleagues at the University of Cambridge presented theoretical and experimental evidence of a viscous state for nanoparticles near their melting point in a ACS Nano published paper.

The state exists over a temperature range scaling inversely with the catalyst size, resulting in enhanced self-diffusion and fluidity across the solid to liquid transformation. The overall effect of this phenomenon on the growth of nanotubes is that, for a given temperature, smaller nanoparticles have a larger reaction rate than larger catalysts.

The viscous state when a material is at the range between being a solid and a liquid.  Generally viscous is thought as being thick or sticky in consistency.  A catalyst is an agent or chemical that facilitates a chemical reaction. It is estimated that more than 90 percent of chemical processes used by industry involve catalysts at some point.  Here the Duke/Cambridge effort is making the assertion that the surface-to-volume ratio of the catalyst particle – its size — is more important than generally appreciated.

Stefano Curtarolo, associate professor at Duke’s Department of Mechanical Engineering and Materials Sciences explains, “We found that the smaller size of a catalyst will lead to a faster reaction than if the bulk, or larger, version of the same catalyst is used. This is in addition to the usual excess of surface in the nanoparticles. This opens up a whole new area of study, since the thermo-kinetic state of the catalyst has not before been considered an important factor. It is on the face of it paradoxical. It’s like saying if a car uses less gas (a smaller particle), it will go faster and further.”

Curtarolo, came up with the theoretical basis of the findings three years ago and saw them confirmed by a series of intricate experiments conducted by Jie Liu, Duke professor of chemistry.  Thus, when matter is in this transitional state, a catalyst can achieve its utmost potential with the right combination of catalyst particle size and temperature.

The major assertion is based on their series of experiments conducted using carbon nanotubes, and the scientists believe that same principles they described in the paper apply to all catalyst-driven processes.  If so and very likely so, catalyst use will get much more efficient someday.

Liu proved Curtarolo’s hypothesis by developing a novel method for measuring not only the lengths of growing carbon nanotubes, but also their diameters. Nanotubes are microscopic “mesh-like” tubular structures that are used in hundreds of products, such as textiles, solar cells, transistors, pollution filters and body armor.

Viscous Catalyst Contructed Nanotubes Compared. Spagetti look left, parallel growth right. See text following.

Liu explains, “Normally, nanotubes grow from a flat surface in an unorganized manner and look like a plate of spaghetti, so it is impossible to measure any individual tube. We were able to grow them in individual parallel strands, which permitted us to measure the rate of growth as well as the length of growth.”

By growing these nanotubes using different catalyst particle sizes and at different temperatures, Liu was able to determine the “sweet spot” at which the nanotubes grew the fastest and longest. As it turned out, this happened when the particle was in its viscous state, and that smaller was better than larger, exactly as predicted by Curtarolo.

These measurements provided the experimental underpinning of Curtarolo’s hypothesis that given a particular temperature, smaller nanoparticles are more effective and efficient per unit area than larger catalysts of the same type when they reside in that dimension between solid and liquid.

Liu sums it up with, “Typically, in this field the experimental results come first, and the explanation comes later. In this case, which is unusual, we took the hypothesis and were able to develop a method to prove it correct in the laboratory.”  That speaks a lot about Curtarolo’s insight ability.

It might seem that the research has thrown a bit of a wrench into catalyst reactor designs.  Some catalysts are viscous at very high temperatures.  Then on the other side, catalysts that might not perform as needed in the current sate might be quite desirable working at the viscous state level.

This research should trigger a wave of catalyst research much of which could very well favorably impact the production of materials and fuels.
A last point, as Liu pointed out Curtarolo’s insight is worthy of special note.  Moreover, that others joined to prove up the hypothesis.

This is an extraordinary team earning more than the usual congratulations.  We’ll be watching for the other ideas they come up with.


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