Nanotechnology filters

Researchers from Rice University (Rice) in the USA, and their colleagues from Finland and Hungary, found a way to produce filters from carbon nanotubes that could be used in a variety of applications such as extremely fine air filters and a substrate for catalysts that accelerate chemical reactions.

The research findings, published in the scientific journal ACS Nano, demonstrate how these filters can remove up to ninety-nine percent of particles with a diameter of less than a micrometer.

Using the chemical vapor deposition (CVD) method, a research team from Rice University, led by Robert Vajtai, a researcher in the Department of Mechanical Engineering and Materials Science, prepared devices that at first glance look like tiny shower heads. After about thirty minutes in the VCD facility, the nozzles, which were created by the laser, and are found in the nitrogen dioxide molds, are filled with carbon nanotubes and then only nanometer particles can pass through them.

When chemical catalysts are connected to these tubes, then particles enter through one side of the filter and exit from the other side as a different substance, after undergoing a chemical reaction. The process is similar to what happens in a car's catalytic converter, where carbon monoxide is converted into a less toxic mixture of carbon dioxide, nitrogen compounds and water.

"Even when the nozzles are larger than the particle itself, the filter remains highly effective," says the lead researcher. "The main idea is that you have a 'forest' of carbon nanotubes. The gas flows through it, and due to the very small distance between the tubes, the gas atoms bump into many of them before coming out the other side.

"This strong reaction, compared to other macroscopic and even microscopic materials, provides an excellent method for preparing filters or a catalyst template that are much more efficient than those available in stores today," says the researcher.

The degree of permeability of the filters depends closely on the length of time it took for the tubes to form, which determines their length and density. The team tested the filter's ability to function as a catalyst by connecting palladium to the inside of the nanotubes and using them to convert propene to propane, a test that is an accepted benchmark for the existence of catalytic ability. They found that the activated membranes "showed excellent and stable activity," according to the article.

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