This article first appeared in the St. Louis Beacon: October 6, 2008 - Jingyue (Jimmy) Liu, Ph.D., director of the Center for Nanoscience at the University of Missouri - St. Louis, has focused much of his attention on making better chemical catalysts. His work embodies many of the principles important to nanoscience and nanotechnology
A catalyst increases the rate of a chemical reaction without itself becoming changed. For instance, enzymes are biological catalysts; that pepsin in your stomach causes the breakdown of lots of proteins, but doesn't break down itself in the process.
A catalyst is needed for hydrogen fuel cells to work. Hydrogen cells are seen as a key energy saving device that can be used in hybrid vehicles, laptops, cell phones and power plants. The catalytic process tears electrons away from the hydrogen gas molecule, creating electric current and hydrogen ions (H+) that later react with oxygen from the air to form water. The usual catalyst for that reaction, called the ionization of hydrogen gas, is platinum.
Platinum is expensive, and large particles are not suitable for use in fuel cells. To optimize a nano-scale catalyst, the scientist has to consider:
- Size
- Placement on a support
- Composition
- Shape
Size Matters: Chemical catalysis takes place on the surface. The smaller the particle, the more available surface area per gram of a catalyst. Obviously, dividing a given amount of an expensive material into the smallest possible units will maximize the surface area. The inset box has a calculation showing how much surface area increases when a cube is subdivided.
Placement on a support: For efficiency, nanoparticulate catalysts are usually placed on a support like carbon or silica. The scientist must consider particle-support interactions, as well as optimal spacing to allow the most reactions to occur in a given time.
Composition: Liu has found that in some reactions normally catalyzed by pure platinum, an alloy will work better than platinum itself. For example, in some of the reactions in a hydrogen fuel cell, properly fabricated platinum-nickel (Pt3Ni) alloy nanoparticles provide better catalytic performance than Pt nanoparticles themselves. The use of nickel also reduces the total cost of the fuel cell catalyst since nickel is much cheaper than platinum.
Shape: Furthermore, different shapes of the nanoparticles and different arrangements of the atoms will affect the rate of reaction greatly. For example, surfaces of certain shapes of platinum-nickel alloy nanoparticles work much better than other shapes of the same alloy nanoparticles. The activities of nanoparticles vary greatly depending upon the shapes used.
You have made a better catalyst. You know it is better because your chemical reaction is faster and cleaner. Now how do you visualize the structure of your catalyst?
Here is where the high-resolution equipment is used. The powerful microscopes check such properties as uniformity in shape and arrangement of atoms. A scanning electron microsope (SEM) gives pictures of the material's surface structure . Transmission electron microscopy uses ultrathin slices to see interior structure. Atomic force and scanning tunneling microscopy probe the molecular topology of conducting and non-conducting materials respectively. Instruments that measure nuclear magnetic resonance and X-ray diffraction patterns can give more structural information.
Jo Seltzer is a freelance journalist.