A closer look at catalysts

Understanding and improving chemical reactions

It's almost redundant to talk about chemistry at the nanoscale, because that region of the physical world is where all chemical reactions occur. You could say that chemists have been working with nanotechnology for centuries without ever actually seeing what they were doing. However, new techniques allow researchers to watch reactions on the atomic level as they occur and provide new insights into chemical reactions in general, as well as into the particular role of catalysts as accelerators of these interactions.

ORNL postdoctoral associate Daniela Anjos prepares to study reactions at the liquid interface with catalytic nanomaterials. Photo: Jason Richards

Catalysts work by providing an environment that encourages specific chemical reactions. Usually this means that catalytic materials are broken down into nano-sized pieces to maximize their surface area and then placed in the mix with the other chemical reactants. The greater the surface area of the catalyst, the faster the reaction will proceed. ORNL chemist Steve Overbury explains that his research team is trying to understand, on a very basic level, how molecules interact with catalytic surfaces. "For example," he explains, "a catalyst could be used to remove oxygen from liquids in the biofuel production process or to promote a reaction between carbon monoxide and oxygen." In order for a catalytic reaction to happen, reactant molecules have to attach themselves to the surface of the catalyst; then a bond has to form between them; finally, the new molecule has to release itself from the catalyst's surface. "There are a number of steps in most catalytic reactions," Overbury says. "Knowing exactly how they occur allows researchers to apply, modify and improve them."

A better understanding

This kind of insight is illustrated by Overbury's ongoing work with cerium oxide particles. His team uses these particles to accelerate the process of removing hydrogen from ethanol—another biofuelrelated application of catalysts. To help understand the effect of the catalyst's structure on the interactions occurring on its surface, Overbury's group produces two kinds of cerium oxide nanoparticles: some cube-shaped and others in the form of octahedrons. Overbury wants to determine how differences in shapes and the structure of surfaces affects the catalytic activity of nanoparticles. "We have already seen some differences in the way the catalytic reaction occurs on the differently shaped particles," Overbury says. "Surface structure definitely plays a role in this process."

To gain a better appreciation of the nuances of catalytic surfaces, Overbury also collaborates with researchers at ORNL's Center for Nanophase Material Sciences. These experiments often involve using electron microscopy, as well as neutron and x-ray-based analytical tools, to study catalytic nanoparticles attached to supports or substrates. "Surprisingly," Overbury says, "one of the most interesting catalysts we have been working with is gold." Scientists have found that gold's ability to catalyze reactions is heavily dependent on the size and shape of the gold particle and the composition of the material supporting the particle. "If the particle is very small," he says, "it can be very catalytically active. Larger particles are much less active—even if you take into account differences in surface area." There has been a huge amount of interest in understanding why this happens and whether the effect is related to the structure of the particle's surface or its electrical charge or something else entirely. This question is interesting not only in terms of basic research, but also because it could help determine whether gold could be used to catalyze certain types of chemical reactions.

"We ask the same basic questions in each experiment," Overbury says. "Does the reaction occur? Does it occur on the surface of the nanoparticle? Does it occur on the support? Does it occur at the interface between the two? Are there differences in reaction based on the type of support or on the size of the particles?" Ultimately, Overbury and his colleagues will use the answers to these questions to advance their understanding of how molecules react on the surfaces of these catalysts. However, this research has considerable practical value as well. "If someone wants to create a new catalyst to decrease exhaust emissions, or a photocatalyst to produce hydrogen from water," he says, "they will need to know, at a fundamental level, what is going on in these reactions. The chemical industry is based on creating catalysts that can make a particular reaction occur. Understanding these properties and applying them to technological challenges is what the nanoscale revolution is about."

Accelerating energy technologies

Overbury expects that catalytic techniques will be in increasingly high demand for their ability to enhance energy technologies.

One of these technologies is producing hydrogen for fuel cells and other applications. "ORNL is home to the Fluid Interfaces Reactions Structure to Transport (FIRST) center," Overbury says. "So our group is particularly interested in reactions that occur at the fluid– solid interface in processes like water splitting—extracting hydrogen from water." He notes that the main stumbling block in the way of creating a hydrogen-based economy is the lack of an efficient means of producing hydrogen. "People want to be able to use photons from the sun to split water and create hydrogen," he says, "but we can't do this economically. Before hydrogen power can be made practical, we have to understand how water interacts with other molecules at semiconductor surfaces that harvest photons." Overbury anticipates that understanding the fundamentals of the water splitting process will increase the practicality of both applying solar energy to hydrogen production and using hydrogen fuel to meet our energy needs.

Another promising prospect for the application of catalytic technologies is the process of converting biomass to biofuel. The biomass fermentation process creates a number of different types of molecules that are heavily loaded with oxygen. "These aren't very good for fuel," Overbury says. "However, if we convert them catalytically, stripping out the oxygen and putting the small molecules together to make long-chain molecules, we can make something that's a lot like gasoline." There are already technologies available to do this, but they are relatively inefficient. By developing a deeper scientific understanding of these catalytic transformations, Overbury hopes to develop a more elegant and efficient means of converting biomass to fuel.

All of the catalysis research in Overbury's group has the same basic aim: applying an improved understanding of the underpinnings of chemical reactions to the job of streamlining catalytic processes. "We're always looking for ways to reduce the energy needed to run a catalytic system," he says. "This can involve increasing the speed or efficiency of the catalyst or increasing the selectivity of the reaction. Process improvements like these are what catalysis is all about."— Jim Pearce