Real-Time Observation of Catalyst Activity Could Lead to More Tailored, Smarter Catalysts

Jose Michael

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Rhodium-palladium nanoparticles (left) and platinum-palladium nanoparticles (right), as revealed by transmission electron microscopy, morph as their surroundings change. Click to enlarge. Source: Berkeley Lab

Using a state-of-the-art spectroscopy system at Berkeley Lab’s Advanced Light Source, a team of scientists from the US Department of Energy’s Lawrence Berkeley National Laboratory observed as bi-metallic nanoparticle catalysts changed their composition in the presence of different reactants. Until now, scientists have had to rely on snapshots of catalysts taken before and after a reaction. A paper describing their work was published online 9 October in the journal Science.

The ability to observe nanoscale catalysts in action could lead to less-expensive and more efficient catalysts for applications from improved pollution control, fuel refining and hydrogen fuel cells, according to Berkeley Lab’s Miquel Salmeron and Gabor Somorjai, who led the work. It could also expedite the development of catalysts that mop up all the substances in a reaction except the desired product, the hallmark of “green chemistry” in which waste byproducts are minimized.

Gabor Somorjai is a surface science and catalysis expert who holds joint appointments with Berkeley Lab’s Materials Sciences Division and UC Berkeley’s department of chemistry. He conducted the research with Miquel Salmeron, a pioneer in a field of spectroscopy that enabled this work. Salmeron also holds joint appointments with Berkeley Lab’s Materials Sciences Division and UC Berkeley’s department of materials sciences and engineering.

By watching catalysts change in real time, we can possibly design smart catalysts that optimally change as a reaction evolves.

—Gabor Somorjai
Berkeley Lab’s Miquel Salmeron and Gabor Somorjai discuss how the capability to observe fundamental chemistry could lead to more efficient catalysts, cleaner skies, and less industrial waste. Source: Berkeley Lab

Because of their importance, researchers all over the world are working to better understand how catalysts work, and how to improve them. Until now, however, nanoscale catalysts could only be observed before and after a reaction. The crucial segment—how a catalyst morphs during a reaction—remained unobserved.

That barrier has been an obstacle. As Somorjai explains, it’s like trying to understand someone’s life by observing the person as a newborn baby, then fast-forwarding to old age. What transpires in between is incredibly important, but also incredibly difficult to decipher by observing two widely disparate stages.

It’s difficult to tune a catalyst to do exactly what you want unless you know how it adapts during a reaction. With our work, we can for the first time see what the catalyst is doing during the reaction, not before and after.

—Miquel Salmeron

Somorjai synthesized core-shell nanoparticles (NPs) composed of rhodium and palladium and platinum and palladium. To see how these bi-metallic catalysts change in the presence of reactants, they turned to one of the few spectroscopy instruments in the world that enables scientists to study catalytic and biological phenomena in their natural environment—i.e., at almost normal pressures and in the presence of different chemicals.

The instrument, the first of its kind, was developed by Salmeron and colleagues and is located at the Advanced Light Source. Like all spectroscopy systems, it identifies elements by detecting their unique spectral signals. But unlike most, the ambient pressure photoelectron spectroscopy system works under similar pressures and environments faced by everyday phenomena, instead of requiring a carefully controlled vacuum.

Using this system, the scientists watched, in real time, as the bi-metallic nanoparticles restructured themselves when exposed to different gases, such as nitrogen oxide, carbon monoxide, and hydrogen. In the presence of some reactants, rhodium rose to a particle’s surface. While in the presence of other reactants, palladium rose to the surface.

Heterogeneous catalysts that contain bimetallic nanoparticles may undergo segregation of the metals driven by oxidizing and reducing environments. The structure and composition of core-shell Rh0.5Pd0.5 and Pt0.5Pd nanoparticle catalysts were studied in situ, during oxidizing, reducing, and catalytic reactions involving NO, O2, CO, and H2 using x-ray photoelectron spectroscopy in near ambient pressure.

The Rh0.5Pd0.5 nanoparticles underwent dramatic and reversible changes in composition and chemical state in response to oxidizing or reducing conditions. In contrast, no significant segregation of Pd or Pt atoms was found in Pt0.5Pd0.5 nanoparticles. The distinct behavior in restructuring and chemical response of Rh0.5Pd0.5 and Pt0.5Pd0.5 nanoparticle catalysts under the same reaction conditions illustrates the flexibility and tunability of the structure of bimetallic nanoparticle catalysts during catalytic reactions.

—Tao et al. (2008)

From one gas to another, we observed different metals segregating to a catalyst’s surface, which is the part of the catalyst that drives chemical reactions. And this makes all the difference in establishing how the catalyst participates in the chemistry.

—Gabor Somorjai

With this information, scientists can develop nanoparticle catalysts and reactants that are tailored to most efficiently yield a product, whether it’s gasoline or cleaner emissions. For example, researchers can engineer bimetallic nanoparticle catalysts in which one metal rises to the surface during an initial stage of a reaction, and a different metal rises to the surface in a latter stage. The goal is to ensure that the most active metal is on the catalyst’s surface precisely when it’s needed most. In this way, the final product can be developed as quickly and cheaply as possible.

Somorjai and Salmeron hope to next observe how catalysts change shape during a reaction, which could be as equally important as compositional change in driving chemical reactions.

The restructuring phenomenon observed on the bimetallic NPs induced by changes in reactive gas offers an interesting way of engineering the nanostructure of nanoparticles for catalysis and other applications. One goal could be the synthesis of “smart” catalysts whose structure changes advantageously depending on the reaction environment. Our results suggest that the combination of a tunable colloid chemistry-based synthesis followed by the controllable engineering of the structure of nanoparticles using reactive gases opens a new door for designing new catalysts and shaping catalytic properties of nanomaterials by structural engineering in reactive environment.

—Tao et al. (2008)

This work was supported by the Department of Energy.

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