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Fall 2020

Unlocking dilute alloy catalysts: the key lies just beneath the surface

Nicholas Marcella

Figure 1. A graphic that summarizes the findings of this work. The activity of the dilute alloy catalyst is dependent on the number of palladium atoms that are present at the surface of the nanoparticle. The IMASC team found gas and temperature regimes that increased, or decreased, the number of palladium atoms available for reaction.

Scientists at the Integrated Mesoscale Architecture for Sustainable Catalysis (IMASC) Energy Frontier Research Center have demonstrated that specific environmental conditions drive their so called “dilute alloy” catalyst to restructure, resulting in a surface enriched by the active species. The restructuring event, or the movement of active palladium atoms to and from the surface of the catalyst, can be used to control and enhance catalytic activity.

A catalyst is a material that, when added to a chemical reaction, modifies the outcome of the reaction while not being consumed in the process. Approximately 90% of all commercially produced chemical products involve catalysts in their manufacture. An estimated 20–30% of the entire world’s GDP is directly or indirectly related to catalysis and catalytic processes. The chemical industry, the heaviest user of catalysts, accounts for more than 25% of global energy use today, which corresponds to at least 20 billion tons of CO2 emitted each year. With an ever-increasing demand for synthetic products (fuel, cosmetics, plastics, fabrics, etc.), there is a major effort to increase the efficiency and reduce the negative aspects of catalytic processes. One promising step in that direction is the development of dilute alloy catalysts. These catalysts have been shown to be highly stable, show great catalytic performance, and they use very small amounts of precious, rare, and hence expensive, metals.

That’s where the IMASC team comes in. The dilute metal alloy catalyst that they have been working on involves two parts. There is the scaffolding, a silicon dioxide matrix that holds the catalyst in place, and the catalyst itself, a nanoparticle made of palladium and gold, where gold is the “host” majority and palladium is the “active” minority. The spherical nanoparticles are engineered unimaginably small, on the order of 1/75,000th of the diameter of a human hair, and millions of them are scattered within the scaffold to make sure plenty of surfaces are available for reactions.

The surface is where catalysis happens. The team looked at how they could improve the catalyst’s ability to add hydrogen atoms to a hydrocarbon, in what is known as a hydrogenation reaction. Key to this reaction is the palladium. If palladium atoms are located on the surface of the particles, where they can bump into hydrogen molecules, the palladium can then split those molecules into hydrogen atoms. The hydrogen atoms can then be added to the hydrocarbon. No palladium, no reaction.

It turns out that after the team’s particles were made, in this case using only 4% palladium, the minority palladium atoms were completely buried under the surface in a sea of gold atoms. With no surface palladium atoms available to break the bonds in hydrogen molecules, the reaction could not be started. However, the researchers found a way to get the palladium atoms to the surface using a relatively simple method.

Contrary to what their name would lead one to believe, solid materials are not as “solid” as one might think. In fact, they can be quite dynamic. Diffusion is the process by which atoms move through their environment and it is controlled by the laws of thermodynamics. Diffusion occurs when a thermodynamic “driving force” exists. In the dilute alloy particles, palladium atoms are diffusing all the time, mostly through the gold-rich core, and sometimes, they traverse the surface. The IMASC researchers found that, under specific temperature and gas conditions, they could trap the traversing palladium atoms at the surface, thereby increasing the population of active palladium atoms to use in reactions.

Figure 1 summarizes the specific conditions that the team used to turn the catalyst “on” and “off.” When the team exposed the catalyst to oxygen gas at high temperature, the palladium atoms moved to the surface and were trapped by the formation of palladium oxide, allowing for the subsequent hydrogenation reaction to proceed at a high rate. When the catalyst was exposed to hydrogen gas at high temperature, the palladium atoms diffused away from the surface, and because there was no driving force to keep the atoms at the surface, no catalytic activity was observed. Finally, the team found that exposing the catalyst to carbon monoxide at low temperature resulted in some palladium atoms being able to diffuse and then be trapped at the surface, enabling a small amount of hydrogenation.

Dr. Mathilde Luneau, the first author of the work that was published in Nature Communications Chemistry, was impressed by the tunability of the dilute catalysts: "There is a major opportunity for improving the selectivity in hydrogenation reactions by using dilute alloy catalysts. By combining advanced characterization under operating conditions and theoretical modeling, we have shown that this promising class of catalysts can be tuned and maintained in an active state."

And in conclusion, she added: “the future is dilute!”

The insights gained from this work have inspired IMASC researchers to dig deeper into dilute alloy catalysts. Future plans are in the works to investigate the atomic rearrangement of these catalysts, and to see if they can use this effect to enhance outcomes of other reactions alongside hydrogenation. Understanding the dynamic character of dilute alloy catalysts, and how they can be activated and controlled, will lead to more extensive design principles, which will enable new catalytic systems. While not an end-all solution, these new systems will certainly be stepping stones on the path to better, cheaper, efficient catalysts, and a future that includes sustainable chemical production.

More Information

Luneau, M., Guan, E., Chen, W. et al. Enhancing catalytic performance of dilute metal alloy nanomaterials. Commun Chem 3, 46 (2020). https://doi.org/10.1038/s42004-020-0293-2

Acknowledgments

This work was supported as part of the Integrated Mesoscale Architectures for Sustainable Catalysis (IMASC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award #DE-SC0012573. ML gratefully acknowledges support from TOTAL. This research used 8-ID (ISS) beamline of the National Synchrotron Light Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory (BNL) under Contract No. DE-SC0012704. Work was also carried out in part at the Singh Center for Nanotechnology at the University of Pennsylvania which is supported by the National Science Foundation (NSF) National Nanotechnology Coordinated Infrastructure Program grant NNCI-1542153. Additional support to the Nanoscale Characterization Facility at the Singh Center has been provided by the Laboratory for Research on the Structure of Matter (MRSEC) supported by the National Science Foundation (DMR-1720530).

About the author(s):

  • Nicholas Marcella is a Ph.D. Candidate at Stony Brook University under Dr. Anatoly I. Frenkel. Nick works closely with his colleagues in the Structure and Dynamics of Applied Nanomaterials (SDAN) group, and in the Integrated Mesoscale Architecture for Sustainable Catalysis (IMASC) EFRC, to develop new methods for characterizing nanomaterials.

More Information

Luneau, M., Guan, E., Chen, W. et al. Enhancing catalytic performance of dilute metal alloy nanomaterials. Commun Chem 3, 46 (2020). https://doi.org/10.1038/s42004-020-0293-2

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.