Ultra-High Control: Catalysis in a Vacuum
Inside a tightly sealed stainless steel sphere with an internal pressure so low it approaches the vacuum of space, rests a smooth platinum crystal, the surface of which displays atoms neatly arranged in a hexagonal pattern. Nearby, a valve opens and molecules whiz around, bumping into this highly ordered surface, which is cooled well below room temperature using liquid nitrogen. Some of the molecules stick to the face of the platinum crystal, while others bounce off and collide with the walls of the chamber and are sucked into vacuum pumps. This process is one step in a set of surface science techniques that are conducted under ultra-high vacuum conditions.
Researchers at the Catalysis Center for Energy Innovation (CCEI) are using ultra-high vacuum techniques to determine which metal catalyst surfaces are most active for generating fuels and important chemicals from plant matter, or biomass. One major challenge is that most of the starting materials derived from biomass, such as glucose, have high oxygen content in addition to the carbon atoms required in fuels. Therefore, it is necessary to develop methods for selectively removing oxygen from these precursor molecules, which are called biomass-derived oxygenates, and ideally replacing it with hydrogen. Currently, on platinum catalysts, attempts to convert biomass-derived molecules to high-value products, such as engine fuels, often lead to decomposition and the formation of low-value products, such as carbon monoxide.
One way to achieve high selectivity to a desired product in catalysis is to add a second metal to the catalyst, creating an alloy. This second metal can dramatically change how molecules interact with the surface, and thus affect which products are formed.
In their work reported in Catalysis Today, Jesse McManus and his co-workers Eddie Martono and John Vohs used a single-crystal platinum disk, made up of precisely ordered platinum atoms, as a model catalyst and added small amounts of zinc to the platinum surface. They studied the effects of the added zinc on the binding and reaction of biomass-derived oxygenates on the surface. The oxygenates are introduced to the crystal surface under ultra-high vacuum conditions so that there are few background molecules and the researchers can precisely control what is put on the model catalyst.
Once the biomass-derived molecules have stuck to the model catalyst, the alloy is carefully heated so that the molecules on the surface gain energy and react. During heating, physical chemistry techniques, such as mass spectrometry and high-resolution electron energy loss spectroscopy (HREELS), are employed to detect starting molecules, intermediate reaction products, and final products. Mass spectrometry records the masses of molecules that come off the surface during heating—a technique known as temperature-programmed desorption. Using HREELS, the researchers study the unique vibrations that serve as "fingerprints" for identifying specific molecules on the surface.
With these two pieces of the puzzle, the CCEI scientists determined exactly what orientation the biomass molecules take when they attach to a surface with a given mixture of platinum and zinc—whether they bind through an oxygen atom, or a carbon atom, or some combination of the two. These bonding configurations have a critical influence on what products are formed following reaction on the surface. This is somewhat analogous to a game of Twister, where different people can have one or both hands and/or feet on the game board, and their position relative to the board and to other individuals determines how they interact with other players. By uncovering the combinations and arrangements of platinum and zinc atoms that lead to desired bonding configurations of the biomass-derived oxygenates, the researchers pinpointed the specific catalytic sites that are active for selectively removing oxygen without causing decomposition.
By identifying the important role of zinc in facilitating the selective removal of oxygen from biomass-derived oxygenates, these studies on model catalysts represent a key step toward the use of plant materials as a sustainable feedstock for the production of fuel components and consumer products, such as synthetic fabrics and plastics. The principles derived from these model surfaces can then be used to direct the design of catalyst materials that are optimized for industrial reaction conditions.
This work was supported as part of the Catalysis Center for Energy Innovation, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.
McManus JR, E Martono, and JM Vohs. 2014. "Reaction of Glyceraldehyde and Glucose on Zn-Modified Pt(111) Surfaces." Catalysis Today 237:157-165. DOI: 10.1016/j.cattod.2013.11.010