Catalysts— Human and Otherwise
When asked about catalysts, most people probably recall a simple definition copied from the blackboard in chemistry class: a substance that accelerates or modifies a chemical reaction without itself being affected. Or certain personalities may spring to mind; the term is routinely borrowed from chemistry to refer to a person or team whose energetic, efficient work quickly creates change in a given field. Or the first thought may be of the car in one's driveway and its catalytic converter, which chemically grabs exhaust pollutants and makes them harmless before they reach the tailpipe.
In a way, continuing work by scientists at the Environmental Molecular Sciences Laboratory (EMSL), and Pacific Northwest National Laboratory embodies all three of these notions. Because chemical transformations are at the heart of numerous areas of energy production and use—and because catalysts are essential ingredients for these transformations—the two institutions have recognized the critical priority to dig beyond that simple definition for a deeper fundamental understanding of how catalysts aid reactions.
As for the “human catalysts,” EMSL provides transformational tools and staff expertise, while collaborating with interdisciplinary teams to tackle challenges in catalysis. These teams include high-profile scientific users from across the globe and just across the Richland, Wash., campus at PNNL’s Institute for Integrated Catalysis (IIC). And sustained success has enabled many market-deployed applications—including the catalytic converters in driveways all over the world.
EMSL’s chief scientist Don Baer said, “If you pick the right problem and form the right team, then you accelerate science.” Like a good catalyst, this enables a better output: real solutions to energy and environmental challenges.
In 1990, Baer organized a workshop that brought laboratory leaders together to discuss one question: exactly what is needed in the molecular sciences to support solutions to energy and environmental challenges?
“We walked away from that meeting with a clear charter for one area of EMSL,” Baer said. “To strive for a deeper fundamental understanding of the surface chemistry of oxides and minerals. Discoveries in this area are critically important for catalysis, energy materials, and environmental cleanup.”
With this direction confirmed for the EMSL user facility, the growing number of surface chemistry and catalysis experts at PNNL would prove to be a perfect fit over the decade that followed.
“In a way, it was a perfect marriage,” PNNL catalysis scientist Chuck Peden recalls. “With EMSL bringing new capabilities and expertise on the fundamental side, and PNNL continuing its leadership in several applied areas, we realized we had the critical mass to form a catalysis effort that was, and is, unmatched anywhere in the nation.”
The IIC was born in the early 2000s and began carrying out programmatic research within PNNL. It developed to become the nation's largest non-industrial catalysis research and development effort.
As they soon discovered, if you’ve formed the right team, sometimes a problem picks you. One specific example came as the IIC was forming, when engine manufacturer Cummins Inc., along with its catalyst provider Johnson Matthey Catalysts, approached Peden and his colleagues with a problem. To improve fuel efficiency by more than 25 percent over standard gasoline engines, Cummins was developing a “lean-burn” diesel engine for use in passenger vehicles such as the Dodge Ram pickup truck. “Lean” refers to using more air and less fuel during combustion in the engine (higher air-fuel ratios). While providing incredible gains in fuel savings, the lean-burn engine couldn’t meet strict emissions standards because of its catalytic converter’s poor performance. New and improved catalyst technologies were a necessity.
To solve the problem, engineers from both companies had been studying a recently invented material that was the foundation for a new catalyst technology, known as the Lean-NOx Trap (LNT), which could effectively remove harmful nitrogen oxide emissions from exhaust of fuel-efficient engines. But they faced serious barriers with these catalysts’ durability. The answers to why they failed too early would require unique fundamental studies, which are typically beyond the scope of companies whose expertise lies in product engineering.
This is where DOE national scientific user facilities and national laboratories play an important role. In 2003, Cummins and PNNL began a CRADA—to explore these problems using techniques and capabilities available at EMSL. Scientist Ja Hun Kwak led a team of researchers that included Peden, Do Heui Kim and Janos Szanyi, in studies to aid development of LNT catalysts.
The team integrated several EMSL capabilities, including various microscopy, diffraction, and spectroscopy instruments, as well as three generations of EMSL supercomputers. Because many autocatalysts involve precious metals like platinum (Pt), very small particles of the metal are spread out across a support material to maximize the surface area available to the reactants while minimizing costs. However, the research team found that typical operation of this catalyst caused these small Pt particles to grow, decreasing performance. To remedy this problem, it was important to fundamentally understand chemical and physical interactions between the catalytic Pt and BaO materials and alumina support. Unfortunately, such interactions are incredibly difficult to characterize.
Of all the tools and techniques the team used, ultrahigh-field, solid-state nuclear magnetic resonance (NMR) spectroscopy led to the most surprising and important results. NMR, which is like an MRI for molecules, is a very valuable tool for understanding catalysts. But it has generally failed to live up to its potential for studying catalytic materials such as alumina—until now. PNNL’s 900-MHz instrument—one of the largest NMR magnets in the world—generated the highest-resolution spectra ever obtained for an alumina support, and the first quantitative, atomic-scale observation of how it interacts with a catalytic material such as Pt.
These results challenged some very fundamental notions of how catalysts function, calling into question the standard blackboard descriptions of their preparation and operation. Peden explained why.
“Part of what makes a catalyst a catalyst is that it is supposedly unchanged by the chemical reaction it’s involved with. But as it turns out, at least in this case, the catalyst changes greatly. As the barium oxide additive was storing and converting NOx, the catalyst was growing and shrinking on macroscopic scales, morphing from nanoparticle sizes to orders-of-magnitude bigger.”
Armed with this information, the researchers then applied the same multiple experimental characterizations—and integration with supercomputer models—to study the catalyst’s shortfalls: it was suffering from both sulfur poisoning and degradation at high temperatures—problems that prevented Cummins and Johnson Matthey from commercializing the engine technology. The BaO additive wasn’t only hungry for NOx, but for sulfur dioxide (SO2) as well, which was also found in the lean exhaust. In fact, the BaO preferred to fill up on SO2, leaving little or no room for NOx. To remove sulfur, the catalyst must be treated at high temperatures. But the team also showed this causes many types of damage, including growth in Pt particle size.
By decoupling these two failures and showing how and why they happen at a molecular level, the researchers provided their industry partners with new, essential design parameters for the materials and operation of the LNT catalyst technology. Later designs were able to largely circumvent these issues, and the 2007 Heavy-Duty Dodge Ram became the first commercial implementation of LNT technology in the United States.
While supporting Cummins’s lean engines during a key crossroads in the technology’s development, these discoveries quickly extended benefits beyond the corporations directly involved. Among vehicle manufacturers, similar LNT catalysts are being put to work in new fuel-efficient models for the U.S. market, including the Volkswagen Jetta TDI.
Ross Carper is a writer at the Pacific Northwest National Laboratory.