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In search of selective catalysts, ORNL researchers have synthesized solid materials with nanosized pores and shown that catalytic metallic nanoparticles can be confined in these tiny pores.

Nanoporous Materials for Novel Catalysts

From clothes to drugs to gasoline, almost half the products that fuel our economy could not have been made without catalysts—substances that initiate key chemical reactions and enable them to proceed at lower temperatures or pressures than would be possible otherwise. The problem is that unlike the body’s enzymes—nature’s catalysts—which yield specific products, most industrial catalysts are not very selective. Industrial catalysts stimulate reactions that produce many chemicals, not just the desired product; these by-products in turn require separations, treatment of pollutants, and costly disposal of wastes. The holy grail in catalysis research is a highly selective catalyst that makes one specific product, reducing energy use and the generation of wastes and pollutants. “Ultraselective” catalysts could significantly cut costs for many American industries, making them more competitive.

Haoguo Zhu, a postdoctoral scientist in ORNL’s Chemical Sciences Division
Haoguo Zhu, a postdoctoral scientist in ORNL’s Chemical Sciences Division, (foreground) prepares to examine materials containing catalytic gold particles under an optical microscope, and (background) operates a catalyst characterization system to learn about processes occurring on a catalyst’s surface, such as a reaction between the catalyst and a gas at a certain temperature. (Photo by Curtis Boles; enhanced by Jamie Payne)

“We are learning how to prepare meso-porous nanostructured material that could be used as a tool for understanding how catalysts work and how to make them more selective,” says Steve Overbury of the Surface Chemistry and Heterogeneous Catalysis Group in ORNL’s Chemical Sciences Division (CSD). “We hope to find out how the size of a metal or oxide particle affects its catalytic reactivity. Also, we want to know whether confining particles makes them more effective catalysts.”

Overbury and his CSD colleagues Sheng Dai, David Mullins, Phil Britt, Ed Hagaman, and Jun Xu, as well as Steve Pennycook and Steve Spooner, both of ORNL’s Solid State Division, hope to provide new insights into catalysis through studies of nanocatalysts—metallic particles as small as a few billionths of a meter. In 2001 they were one of two ORNL groups that received funding from the Nano-science, Engineering, and Technology (NSET) Program of the Department of Energy’s Office of Science—a program that is part of the multi-agency National Nanoscience Initiative.

Sheng Dai and postdoctoral scientist Haoguo Zhu can synthesize mesoporous solid materials in which the size of each pore ranges from 2 nanometers (nm) to 15 nm in diameter. They have shown that these mesopores, also called nanopores, can be used to confine metallic nanoparticles (e.g., gold, uranium oxide, and palladium) in solid pieces the size of confetti.

In their early experiments, they made a solution of water, silica (silicon dioxide), uranium oxide, and organic molecules (e.g., cetyl trimethyl amino bromide). They began by adding protons to the silica molecules, giving them a positive charge. The metallic nanocatalyst ions (in this instance uranium oxide, but gold chloride could also be used) are negatively charged so they will stick to the silica molecules. Organic molecules glob together in solution, like detergent in water, forming a “micelle.” The outer surface of an organic micelle likes water (it is “hydrophilic”); thus, silica condenses by hydrolysis on the micelle in the solution. The micelle becomes sandwiched between walls of silica molecules. The solution is then placed in an oven, where the water is evaporated and the organic micelle is burned off. If the experiment is successful, the result is a crusty silica structure filled with pores in which the metallic nanoparticles are confined. “We expect the metal ions to aggregate and stick to the walls of the pores,” says Dai.

This bright-field scanning transmission electron micrograph shows catalytic gold particles on a silica support that has highly ordered mesopores.
This bright-field scanning transmission electron micrograph shows catalytic gold particles on a silica support that has highly ordered mesopores. Various synthetic techniques are being applied to control location and size of the gold particles and to confine them within the pores of the mesoporous support.

“After we get metal nanoparticles in the pores, we will study their catalytic properties,” says Overbury. “We will try to vary pore size and particle size to see how these changes affect the catalytic properties of metallic nanoparticles, such as gold.”

One of the ORNL group’s interests is to see if gold nanoparticles can do what a foil of the catalyst platinum does—that is, cause oxygen to react with carbon monoxide (CO) to form carbon dioxide. Normally, gold foils cannot catalyze the oxidation of CO, but evidence indicates that as nanoparticles trapped in pores, gold can perform this function.

Using neutron scattering, Spooner will determine the size distribution of the metal particles in the mesoporous structure—that is, the location and number of particles of various sizes. Mullins will study the effect of temperature on the formation and early growth of metal nanoparticles confined in pores. Pennycook will use Z-contrast electron microscopy to determine the crystalline structure of the particles relative to the size of the pores.

“The particles might grow big enough to break the walls of the pores,” Overbury says. “Or the pores might confine the nanoparticles in some way to control their growth.”

This research should lead to new insights into how catalysts work. The information obtained may result in the design of a selective catalyst of high stability and unprecedented uniformity.

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