Confining catalysts in a particle's nanopores changes their behavior.
When a person is imprisoned, the loss of freedom can be life changing. In a similar way, when atoms or molecules are confined in a pore as small as a few billionths of a meter, the confinement can affect their reactivity and ability to catalyze chemical reactions by speeding up one reaction path relative to another to better control the yield of reaction products.
A. C. Buchanan III, leader of a physical organic chemistry group at Oak Ridge National Laboratory, group member Michelle Kidder and their colleagues have been studying the effect on the chemistry of organic molecules confined in porous media.
"Our focus is to use solid nanoporous materials, such as silicon dioxide, to confine organic molecules of interest to the Department of Energy," Buchanan says. "We were the first to show that confinement in tiny pores of silica can increase the molecules' conventional reaction rate in a gaseous or liquid environment." Silicon dioxide, or silica, is the chemical compound present in sand, quartz and glass.
For many years in the laboratory, organic chemists have run chemical reactions in a fluid inside a glass flask or beaker. "What we are trying to do, under extreme conditions, is observe an organic chemical reaction in pores in a solid silica particle," says Buchanan. "The pores are only two to three times the length of each trapped organic molecule. We chemically attach the molecules inside pores that range in diameter from almost 2 nanometers to under 3 nanometers and can be 1000 nanometers in length."
Porous silica surfaces end in silanol groups, which readily bind with an organic molecule called an aromatic phenol. The chemists attach the maximum number of molecules to the pore walls, achieving monolayer coverage. As the cylindrical pores become smaller, fewer molecules can be squeezed inside.
In several experiments, the chemists compared the reaction rates of the organic molecules in the gas phase, on the outside of a nonporous silica particle and inside the pores of mesoporous silica particles where pore sizes vary between 2 and 6 nanometers across.
"We found that, if the molecules are inside the pores, the effect of confinement is to increase the speed of the reaction," Buchanan says. "The molecules are closer together and bang into each other more frequently, increasing the probability of reactions."
Because each pore has a curved surface, the confined molecules tend to be more pointed to each other. If the reaction involves, say, a transfer of one hydrogen atom to another molecule, the desired result will happen more effectively on the inside surface of a pore than on the particle's exterior where the reactants can flop around. In some cases, the reaction rate for confined molecules is 400% to 500% faster than that of the same reactants on the outside particle surface.
One goal of the Department of Energy is to find ways to produce ethanol fuel and other products more economically from cellulosic biomass, such as fast-growing switchgrass and hybrid poplar trees. To make biorefineries profitable, chemists are seeking ways to generate useful products from lignin after separation from cellulose.
Buchanan's group is experimenting with a major structural unit of lignin based on phenethyl phenyl ether (PPE) molecules. "We are interested in gaining a fundamental understanding of functional groups in lignin, such as ether oxygen groups," Buchanan says.
When PPE molecules are heated to 375°C in the gas phase, the resulting reaction generates products from two competing pathways. The product yield in one path is three times the yield in the second parallel path.
"If we run the same reaction of PPE molecules confined inside the pores of a mesoporous silica particle, we can increase the chemical selectivity to as high as 45 to 1 versus 3 to 1 for the gas phase," Buchanan says. "We get a 15-fold change in the way the products of pore-confined PPE molecules come out by slowing down one reaction path relative to the other."
The relative product amounts depend upon the pore size, concentration of molecules in the pores and the rigidity of added inert molecules, which can serve as a "surface-confined solvent." If crowding occurs in a pore, the reaction along one pathway can be hindered relative to the other for PPE molecules. By adding rigid inert molecules to a pore, chemists can nearly eliminate a physical rearrangement of PPE molecules, dramatically changing the chemical selectivity of the reaction.
In these ways, chemists may be able to manipulate molecules and their reactions in confinement in mesoporous silica, thus controlling reaction rates and relative yields of reaction products. "Molecules in jail" (a phrase coined by Buchanan) could provide chemists with opportunities to produce larger amounts of desired products from lignin—thus turning a waste by-product of ethanol production into a valuable resource.
Buchanan credits Sheng Dai, leader of the Nanomaterials Chemistry group and staff scientist at ORNL's nanoscience center, who taught him and his colleagues how to synthesize silica particles dotted with pores of different sizes. They learned how to tailor the silica surface to control the chemistry of confined particles. Dai's group produces unique mesoporous carbon materials for various applications.
Mesoporous carbon and silica are made using a detergent-like surfactant whose molecules self assemble into spherical or cylindrical bubbles. A precursor of the carbon or silica particles is condensed around the cylindrical bubbles, which act as a template. The bubbles are burned off or removed in some other way, leaving a mesoporous material containing pores with diameters ranging from 1 to 10 nanometers.
One of Dai's interests is studying the confinement of platinum nanoparticles in the pores of mesoporous carbon to improve carbon cathodes in the fuel cell. Fuel cells supplied with hydrogen are used to produce electricity for electric vehicles.
ORNL researchers have developed a carbon cathode that contains pores 6 to 7 nanometers in width. Platinum nanoparticles 2 nm across can be entrapped inside these mesopores. "Platinum particles are separated by entrapment in cages," Dai says. "Only the external surfaces of the platinum particles serve as an electrocatalyst."
"If the particles come together, they expand and are less reactive," Buchanan explains. "If chemists can keep them separated using pores of the right size, the platinum particles are stable, and all the intrinsic reactivity of the metal particles is available. The reactivity of metal catalysts increases as the particle size decreases."
Dai's material is dominated by internal surface area because of the presence of so many tiny pores. The carbon cathode has 800 square meters of surface per gram of material.
For a fuel cell cathode a platinum catalyst can stimulate the oxygen reduction reaction. "Each oxygen molecule is taken up and chemically bound to a platinum nanoparticle surface," Dai says. "Each oxygen grabs four electrons and combines with a proton to form water—a key reaction in a fuel cell. By increasing stable platinum's effective surface area and the cathode's efficiency, we increase the fuel cell's lifetime."
Similar in some respects to successful prison rehabilitation, confined atoms and molecules can indeed exhibit beneficial changes in behavior.—Carolyn Krause
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