New research from UC Davis shows that particle size is much more important to chemical reactivity than previously thought. Understanding the differences between how large and small particles behave will have a wide array of applications, from cleaning up environmental pollutants and crumbling infrastructure to the biology of bacteria and the origins of life.
The research into particle size was led by Alexandra Navrotsky, professor of ceramic, ?earth and environmental materials chemistry at UC Davis. Navrotsky is also the director of the Nanomaterials in the Environment, Agriculture and Technology program on campus. Navrotsky studied the energy changes involved in oxidation and reduction reactions – a pair of reactions responsible for the transfer of electrons in compounds.
“Oxidation and reduction reactions are the energy source for most chemistry in nature,” she said.
Metals like iron, manganese, cobalt and nickel combine with different numbers of oxygen atoms. The resulting compounds form crystals with different “oxidation states” and thus, different properties. Metals occupy the lowest oxidation states and rock salt oxides occupy the highest.
Navrotsky and her team made very precise measurements of the energy changes resulting from changing from one oxidation state to another, using the data to make two major discoveries on the relationship between particle size and their behavior in oxidation and reduction reactions.
First, the properties of oxidation vary dramatically with particle size. Oxidation and reduction reactions take place under certain conditions of temperature and pressure, with a certain energy cost associated with starting the reaction.
“If you ask in what conditions – temperature, pressure and such – these compounds will oxidize, they will oxidize differently depending on their size,” Navrotsky said.
Second, the research team found that in general very small particles were formed with a lower energy cost for a given metal when the compound was in an intermediate structure called a spinel than compared with other states. Crystals that are in their spinel form have the lowest surface energy, which allows smaller particles to form, Navrotsky said.
Because metal oxides are used so widely in both industry and nature, the findings have a great many applications.
The process of rusting on crumbling infrastructure is also caused by oxidation-reduction reactions. As metal structures like bridges age, they are exposed to oxygen. The oxidation process causes rust and damages the integrity of metal structures. Navrotsky’s findings help to explain how exactly these processes occur and eventually how to help save the structures.
Metal oxides are also very important in biology. Bacteria that grow in anaerobic environments, such as underground, must get the oxygen essential for respiration from the oxidation-reduction reactions of metal oxides, Navrotsky said.
Early life on Earth had very limited access to oxygen as the atmospheric oxygen levels were much lower than they are today, so understanding how the metal oxides react differently depending on their size can help explain how life in different conditions were still able to get energy from the metal oxides around them.
“The authors are to be congratulated on demonstrating that the redox properties of metal oxide nanoparticles are dramatically different than their larger counterparts,” said Israel Wachs, professor of chemical engineering at Lehigh University in Pennsylvania. “This paper is a watershed contribution to metal oxide nanotechnology and is destined to become a classic publication.”
In 2009, Navrotsky was awarded the Mineralogical Society of America (MSA) Roebling Medal. The Roebling Medal is the highest award of the MSA for “outstanding original research in mineralogy.” The award was presented by Nancy Ross, the president of MSA.
“[Navrotsky’s] research is truly interdisciplinary and has successfully bridged many scientific divides, and that has enriched all of them,” she said.
Navrotsky’s co-authors on the study were graduate students Chengcheng Ma, Nancy Birkner and postdoctoral researcher Kristina Lilova. The work was supported by grants from the U.S. Department of Energy.
AMY STEWART can be reached at science@theaggie.org.
