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Summer 2018

When Less Is More

Researchers achieve single- or few-atom metal clusters that act as more stable catalysts

Bor-Rong Chen

“The more, the merrier” is true for most occasions, but for catalysts, sometimes having less -- or even being single -- could be more beneficial.

What is a catalyst? Catalysts are substances added to a chemical reaction to increase the rate of the reaction and aren’t consumed in the process. When a chemical reaction proceeds from reactants to products, the reactants have to overcome the activation energy needed for this reaction to happen. If this activation energy barrier is high, this reaction does not happen easily, and the reaction rate is slower. Catalysts increase the rate of reaction by providing an alternative reaction pathway with a lower activation energy. Without catalysts, some crucial chemical reactions related to our everyday life, such as processing petroleum, cleaning car exhausts, and producing fertilizers, cannot happen as easily. Among many types of catalysts, platinum group metals, such as platinum, palladium, and rhodium, are the most active elements for accelerating chemical reactions.

The progress of a chemical reaction with (dashed black line) and without (solid black line) the presence of a catalyst. The catalyst provides an alternative reaction pathway with lower activation energy (Ea).

Although these noble metals are highly active, a major challenge of catalysts based on platinum group metals is their high cost and limited supply. The average prices of platinum and palladium are approximately $1500 an ounce and $600 an ounce, respectively, which are significantly higher than more common but less active metals, such as copper and nickel, both are around 30 cents an ounce. How can we lower the costs of catalysts without sacrificing their activity? Scientists from Inorganometallic Catalyst Design Center (ICDC) and Integrated Mesoscale Architectures for Sustainable Catalysis (IMASC), two of the Energy Frontier Research Centers (EFRCs), provide potential solutions by introducing two different types of catalysts.

Shrinking down and exposing more active sites. Metal nanoparticle catalysts are functional because of the active sites on their surfaces. On the surface of nanoparticles, the atoms are undercoordinated, meaning that they are not completely bonded to their neighbors. Undercoordinated atoms are found at the surfaces, edges, and corners of particles. These undercoordinated spots are usually where catalytic reactions happen, and for this reason, metal nanoparticle catalysts are usually made small (usually less than 10 nanometers in diameter) and are well dispersed on the surface of supporting materials to maximize the amount of active surfaces.

Few-atom cluster catalysts have to be sinter-resistant. What if we made metal particles into small clusters consisting of only a few atoms? In this way, the whole nanoparticle is a “surface site," which might lead to new behaviors, such as increased activity compared to a larger cluster. Further, smaller nanoparticles reduce the catalyst’s total metal consumption. Well, not so easy here. Extremely small metal clusters are not stable during chemical reactions, which are usually happening at elevated temperatures. The metal clusters tend to migrate and aggregate into larger nanoparticles and eventually lose the high surface area and activity. To address this challenge, ICDC scientists use a process called AIM (ALD in MOFs) to install well-dispersed, sinter-resistant platinum clusters by using a combination of atomic layer deposition (ALD) and metal-organic-framework (MOF) supports.

The MOFs are compounds consisting of metal ion nodes and organic linkers forming 3D channel-like networks. The MOF used in this study, NU-1000, is a structure with microporous channels (approximately 3 nanometers, or 30 times thinner than human hair) and zirconium-based nodes (see Figure 2). The multiple channels inside the MOF structure create a huge surface area of approximately 2000 square meters per gram (equal to five times of a standard basketball field), allowing the platinum clusters to be well dispersed. The platinum clusters are created by using ALD, which is a method depositing platinum onto the MOF surface through pulsed sequences of a platinum precursor.

Structure of NU-1000, highlighting the pores and zirconia nodes of the nanosponge. Image courtesy of Kim et al. (see “More Information”)

It turns out that the size of these platinum clusters is extremely small -- about three atoms. These platinum trimers are uniformly distributed where the zirconium resides in the MOF, with average loadings of 0.15 to 2.5 platinum atoms per zirconium node. Will these tiny platinum clusters survive at elevated temperatures and in reaction conditions? To answer that question, the ICDC research team conducted real-time X-ray analysis to observe the size change of the platinum clusters under hydrogen gas flow at around 200 degrees Celsius (slightly under 400 degrees Fahrenheit), as a proxy for practical reaction conditions. Although the platinum clusters aggregated a bit, they stopped growing at a size of approximately 34 atoms (1 nanometer). This sinter resistance could be attributed to the presence of the MOF support. Because the platinum clusters are sitting at the zirconium nodes, which are spatially separated by the organic linkers, the migration of platinum atoms between the nodes is constrained and only minimum aggregation happens.

These exciting results successfully demonstrate the possibility of using high surface area MOFs with spatial and dimensional uniformity to synthesize sinter-resistant, few-atom cluster catalysts. In the future, AIM can be a versatile approach to further understand the mechanism of sinter-resistant noble metal catalysts and can serve as an important design principle of highly active and noble-metal-saving catalysts.

More expensive isn’t always better. Do less active transition metals, like nickel and copper, have a chance to become a good catalyst? Yes -- just a little bit of cooperation is needed.

Copper nanoparticles are commercial catalysts used in ethanol dehydrogenation, a key reaction for synthesizing various chemicals. This reaction is quite important because ethanol is a renewable resource. However, the catalytic activity of copper has been found to decrease rapidly, even during the first hour of the reaction, due to sintering of copper nanoparticles at elevated temperatures. How do you make copper catalysts resist sintering? Scientists in IMASC are tackling this by incorporating isolated nickel atoms in the surfaces of copper nanoparticles. Although the addition of nickel is small in amount (adding one nickel atom in every 100 to 1000 copper atoms), the effect is significant -- the nickel improves the stability of the copper catalyst and also outperforms their noble metal counterparts in increasing the selectivity of the reaction!

This type of bi-metal catalyst is called a single-atom alloy. Usually, single-atom alloys are composed of singly dispersed active metal atoms embedded into a less-expensive (usually less active) host metal, such as embedding a small amount of platinum or palladium into copper. However, when the IMASC team compares the catalytic activity of ethanol dehydrogenation among palladium-copper, platinum-copper, and nickel-copper single-atom alloys, the nickel-copper counterintuitively defeats the noble metal counterparts in increasing the selectivity of the reaction! Why does that happen?

The IMASC scientists discovered that nickel is unique in the ethanol dehydrogenation reaction. Here, the nickel serves as a promoter of copper and lowers the energy barrier required to break off hydrogen in ethanol, which is the key step determining the rate of the whole reaction. Palladium and platinum, however, do not lower the energy barrier as much compared to nickel. Therefore, the addition of palladium and platinum does not improve the activity of copper for ethanol dehydrogenation.

Then, why don’t we just use nickel nanoparticles as the catalyst? As it turns out, metallic nickel is too active. Nickel not only breaks the carbon-hydrogen bond, but also the carbon-carbon bond in ethanol which will lead the reaction astray and produce unwanted side-products. Therefore, selectively breaking the carbon-hydrogen bonds but not the carbon-carbon bonds in ethanol dehydrogenation reaction is an important rule in designing catalysts. The nickel-copper single-atom alloys meet these criteria and are well-suited for this reaction. The unique catalytic activity and selectivity of nickel-copper single-atom alloy catalysts have the potential to be extended to other catalytic reactions where carbon-hydrogen bond scission is the rate-determining step.

Think small and act big. In the long term, the ICDC and IMASC researchers aim to develop these catalysts to the next step: bringing them from the lab to real industrial application. Once successful, these tiny metal-cluster catalysts can significantly reduce the use of precious metals and make a huge difference in making our life more environmentally sustainable.

Acknowledgments

Kim et al.: This work was supported as part of the Inorganometallic Catalysis Design Center, an Energy Frontier Research Center funded by the Department of Energy (DOE), Office of Science, Basic Energy Sciences. This work made use of the Advanced Photon Source, a DOE Office of Science user facility operated for the DOE Office of Science by Argonne National Laboratory. X-ray absorption spectroscopy measurements at Sector 20-BM and Sector 9-BM were further supported by the Canadian Light Source. The authors acknowledge the Minnesota Supercomputing Institute at the University of Minnesota for providing resources that contributed to the research results reported within the paper.

Shan et al.: This work was supported as part of the Integrated Mesoscale Architectures for Sustainable Catalysis, an Energy Frontier Research Center funded by the Department of Energy (DOE), Office of Science, Basic Energy Sciences. The X-ray absorption spectroscopy research used resources of the Advanced Photon Source, a DOE Office of Science user facility operated for the DOE Office of Science by Argonne National Laboratory.

More Information

Kim IS, Z Li, J Zheng, AE Platero-Prats, A Mavrandonakis, S Pellizzeri, M Ferrandon, A Vjunov, LC Gallington, TE Webber, NA Vermeulen, RL Penn, RB Getman, CJ Cramer, KW Chapman, DM Camaioni, JL Fulton, JA Lercher, OK Farha, JT Hupp, and ABF Martinson. 2018. “Sinter-Resistant Platinum Catalyst Supported by Metal-Organic Framework.” Angewandte Chemie International Edition 57(4):909. DOI: 10.1002/anie.201708092

Shan J, J Liua, M Li, S Lustig, S Lee, and M Flytzani-Stephanopoulos. 2018. “NiCu Single Atom Alloys Catalyze the C-H Bond Activation in the Selective Non-Oxidative Ethanol Dehydrogenation Reaction.” Applied Catalysis B: Environmental 226:534. DOI: 10.1016/j.apcatb.2017.12.059

About the author(s):

  • Bor-Rong Chen is a postdoctoral researcher at the SLAC National Accelerator Laboratory. As a member of the Center for Next Generation of Materials Design (CNGMD) Energy Frontier Research Center, she investigates the formation and evolution of oxides during water-based chemical reactions in real time by using X-ray analysis techniques. Her goal is to understand the nature of reaction pathways and to design more effective ways for synthesizing new materials. Chen has been studying materials science since 2005, and she graduated with a Ph.D. in materials science and engineering from Northwestern University in 2017.

More Information

Kim IS, Z Li, J Zheng, AE Platero-Prats, A Mavrandonakis, S Pellizzeri, M Ferrandon, A Vjunov, LC Gallington, TE Webber, NA Vermeulen, RL Penn, RB Getman, CJ Cramer, KW Chapman, DM Camaioni, JL Fulton, JA Lercher, OK Farha, JT Hupp, and ABF Martinson. 2018. “Sinter-Resistant Platinum Catalyst Supported by Metal-Organic Framework.” Angewandte Chemie International Edition 57(4):909. DOI: 10.1002/anie.201708092

Shan J, J Liua, M Li, S Lustig, S Lee, and M Flytzani-Stephanopoulos. 2018. “NiCu Single Atom Alloys Catalyze the C-H Bond Activation in the Selective Non-Oxidative Ethanol Dehydrogenation Reaction.” Applied Catalysis B: Environmental 226:534. DOI: 10.1016/j.apcatb.2017.12.059

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.