A reality show called “Density Functional Theory” or “DFT” is not going to be challenging “American Idol” in the television ratings anytime soon. After all, DFT does not involve snarky judges and viewer voting; it is a quantum mechanical computational method used to calculate quantities such as the binding energy of molecules. But in the right hands, DFT has something in common with the popular talent contest—the ability to sort through a mob of “wannabes” and find a small number of candidates with unusual talents.

In a series of three recent papers, 12 chemical engineers Jeff Greeley and Manos Mavrikakis 13 of the University of Wisconsin-Madison reported the promising results of their talent search using DFT calculations—a search for better catalysts.

Catalysts reduce the amount of energy needed to start a chemical reaction, enabling the reaction to happen faster or at a lower temperature. They play a major role in the chemical industry for the production and processing of materials such as plastics, fuel, and pesticides, and in the pharmaceutical industry for drug synthesis.

Hydrogen catalysts are particularly important to the DOE Hydrogen Program, whose mission is to research and develop fuel cell and hydrogen production, delivery, and storage technologies, thus ensuring an abundant and affordable supply of clean energy. One of the Hydrogen Program’s priority research areas is design of catalysts at the nanoscale, where reduced size often maximizes catalytic properties. Catalysis is vital to the success of the program because of its role in producing hydrogen from water or from carbon-containing fuels such as coal and biomass, and its role in producing electricity from hydrogen in fuel cells.

Figure 18. Illustration of bimetallic near surface alloys (NSAs) that bind H as weakly as noble metals but activate H₂ much more easily. Niels Bohr (1885–1962, left) and Paul Sabatier (1854–1941, right) represent the founders of quantum mechanics and catalysis theory.

“Anything that would reduce the cost of materials in low-temperature fuel cells would certainly make the technology more economically competitive and thus bring fuel cells closer to being implemented in our everyday lives,” Mavrikakis said. “A major fraction of that cost is the cost of noble metals, such as platinum, that are used as catalysts.” Combining platinum with less expensive metals is one possible way of reducing the price of fuel cells, but only if the alloys are efficient catalysts—and even today’s expensive catalysts are not efficient enough to be economically competitive.

So the research goal is clear: find less expensive, more efficient catalysts for hydrogen-related reactions. However, the discovery of catalysts by experimental methods is an expensive process of trial and error, often involving hundreds or even thousands of candidate materials. But a new theoretical approach—rational design of catalysts from first principles, based on the reactions we want them to facilitate—can speed up the discovery process by eliminating most of the candidates that are unlikely to succeed.

Using this approach, Greeley and Mavrikakis found a new class of near-surface alloys (NSAs) that exhibit superior catalytic behavior for hydrogen-related reactions (Figure 18). NSAs are alloys in which the composition near the surface differs from the bulk composition. Two idealized NSA structures (Figure 19) were used in the study: an overlayer of the solute metal on the host metal, and a subsurface alloy, in which a layer of solute lies just below the surface of the host.

Figure 19. Two types of near-surface alloys were studied: overlayers (left) and subsurface alloys (right).

Earlier studies had discovered a few NSAs that had unusual catalytic properties, so Greeley and Mavrikakis developed a novel and systematic DFT framework for screening a large number of NSA structures and compositions to determine their stability in hydrogen-rich environments. The stable NSAs were then examined for their hydrogen binding and dissociation energies. (In fuel cells, dissociation is the separation of the hydrogen atoms’ protons and electrons, which allows a flow of electrical energy to be created.)

After screening 46 different NSAs, the researchers found a group of five that bind atomic hydrogen (H) as weakly as noble metals, such as gold, copper, and silver, while at the same time dissociating molecular hydrogen (H₂) much more easily. This is an unusual combination of properties: most materials with weak H binding are inactive for H₂ dissociation. “This unique set of properties may permit these alloys to serve as low-temperature, highly selective catalysts for pharmaceuticals production and as robust fuel-cell anodes,” Greeley and Mavrikakis wrote in their Nature Materials article.

Figure 20. Schematic representation of an NSA defect site in cross-section and top view. The gold circles denote host metal atoms, while the blue circles indicate the subsurface solute impurity.

Another important consideration for fuel cell anodes is resistance to carbon monoxide (CO) poisoning. Small amounts of CO are always present in hydrogen fuel streams, and those traces can bind to a platinum anode, ruining its ability to act as a catalyst. However, the NSAs with weak H binding also exhibit weak CO binding, making them ideal candidates for fuel cell anodes. The DFT results identified the following subsurface alloys as having all of these desirable properties: vanadium in platinum, tantalum in palladium, tungsten in platinum, molybdenum in platinum, and tantalum in platinum.

How relevant are these computational results, based on idealized structures, to NSAs in the real world? In their Catalysis Today paper, Greeley and Mavrikakis admitted, “In reality, it is fairly uncommon for NSAs to have a pure solute monolayer in either the first or second metal layers.” But, they argued, “Calculations have shown … that modest deviations … do not qualitatively change the unusual catalytic properties of NSAs.” In fact, they suggest that minor impurities, or “defect sites” (Figure 20), may have unusual catalytic properties in their own right, combining fast kinetics for bond-breaking or bond-making events with resistance to poisoning by the reactants or products of those reactions.

In any case, deviations from the ideal structure would produce quantitative changes in NSA properties. “Thus, to fully benefit from the unique catalytic properties of NSAs, it will be necessary to achieve more precise control of metal catalyst surface structure, perhaps by implementing improved catalyst nanofabrication techniques,” the researchers wrote. Several promising nanofabrication techniques are already being developed, and perfecting such techniques is part of the mission of DOE’s new Nanoscale Science Research Centers, established under the National Nanotechnology Initiative.14

One additional advantage of the “talented” NSAs is that they allow easy diffusion of atomic hydrogen into the bulk of the metal, raising the possibility of using them as light hydrogen-storage media. This characteristic will be studied in more detail in future research.

 

Research funding: BES, NSF
Computational resources: NERSC, NPACI, MSCF
This article written by: John Hules