Generalized schematic of a simple system containing a plasmonic antenna coupled through localized near-field enhancements to a catalytic reactor metal nanoparticle. Swearer et al. Click to enlarge.

… catalytic processes utilizing transition metal nanoparticles are often energy-intensive, relying on high temperatures and pressures to maximize catalytic activity. A transition from extreme, high-temperature conditions to low-temperature activation of catalytically active transition metal nanoparticles could have widespread impact, substantially reducing the current energy demands of heterogeneous catalysis.

Light-driven chemical transformations offer an attractive and ultimately sustainable alternative to traditional high-temperature catalytic reactions. Metallic plasmonic nanostructures are a new paradigm in photoactive heterogeneous catalysts. … Plasmonic metal nanoparticles have been shown to induce chemical transformations directly on their surfaces … Although these “good” plasmonic metals show initial promise for plasmon-induced photocatalytic chemistry, in general they are not universally good catalytic materials despite finding niche applications in a few industrial processes.

In comparison, noncoinage transition metals have historical precedence as excellent catalysts, yet are generally considered poor plasmonic metals, because they suffer from large nonradiative damping, which results in broad spectral features and weak absorption across the visible region of the spectrum. … Here we show that the optical antenna effects of plasmonic metal nanoparticles can be used to directly enhance light absorption and modify the catalytic activity of directly adjacent reactive metal nanoparticle surfaces.

—Swearer

Antenna-reactor plasmonic catalysts combine a light-harvesting nanomaterials with highly efficient metal catalysts. In this image, “islands” of reactive palladium dot the aluminum oxide surface of an underlying aluminum crystal, which serves as a photonic antenna to capture light and activate the catalytic islands. (Image courtesy of D. Swearer/Rice University) Click to enlarge.

The new catalyst is the latest innovation from LANP, a multidisciplinary, multi-investigator research group headed by photonics pioneer Naomi Halas. Halas, who also directs Rice’s Smalley-Curl Institute, said a number of studies in recent years have shown that light-activated “plasmonic” nanoparticles can be used to increase the amount of light absorbed by adjacent dark nanoparticles. Plasmons are waves of electrons that slosh like a fluid across the surface of tiny metallic nanoparticles. Depending upon the frequency, these plasmonic waves can interact with and harvest the energy from passing light.

In summer 2015, Halas and study co-author Peter Nordlander designed an experiment to test whether a plasmonic antenna could be attached to a catalytic reactor particle. Graduate student Dayne Swearer worked with them, Rice materials scientist Emilie Ringe and others at Rice and Princeton University to produce, test and analyze the performance of the “antenna-reactor” design.

Swearer began by synthesizing 100-nanometer-diameter aluminum crystals that, once exposed to air, develop a thin 2- to 4-nanometer-thick coating of aluminum oxide. The oxidized particles were then treated with a palladium salt to initiate a reaction that resulted in small islands of palladium metal forming on the surface of the oxidized particles. The unoxidized aluminum core serves as the plasmonic antenna and the palladium islands as the catalytic reactors.

Swearer said the chemical industry already uses aluminum oxide materials that are dotted with palladium islands to catalyze reactions, but the palladium in those materials must be heated to high temperatures to become an efficient catalyst.

You need to add energy to improve the catalytic efficiency. Our catalysts also need energy, but they draw it directly from light and require no additional heating.

—Dayne Swearer

One example of a process where the new antenna-reactor catalysts could be used is for reacting acetylene with hydrogen to produce ethylene, Swearer said.

Ethylene is the chemical feedstock for making polyethylene, the world’s most common plastic, which is used in thousands of everyday products. Acetylene, a hydrocarbon that’s often found in the gas feedstocks that are used at polyethylene plants, damages the catalysts that producers use to convert ethylene to polyethylene. For this reason, acetylene is considered a “catalyst poison” and must be removed from the ethylene feedstock—often with another catalyst—before it can cause damage.

One way producers remove acetylene is to add hydrogen gas in the presence of a palladium catalyst to convert the poisonous acetylene into ethylene—the primary component needed to make polyethylene resin. But this catalytic process also produces another gas, ethane, in addition to ethylene. Chemical producers try to tailor the process to produce as much ethylene and as little ethane possible, but selectivity remains a challenge, Swearer said.

As a proof-of-concept for the new antenna-reactor catalysts, Swearer, Halas and colleagues conducted acetylene conversion tests at LANP and found that the light-driven antenna-reactor catalysts produced a 40-to-1 ratio of ethylene to ethane, a significant improvement in selectivity over thermal catalysis.

Swearer said the potential energy savings and improved efficiency of the new catalysts are likely to capture the attention of chemical producers, even though their plants are not currently designed to use solar-powered catalysts.

The antenna-reactor design is modular, which means we can mix and match the materials for both the antenna and the reactor to create a tailored catalyst for a specific reaction. Because of this flexibility, there are many, many applications where we believe this technology could outperform existing catalysts.

—Naomi Halas

Additional co-authors include Hangqi Zhao, Linan Zhou, Chao Zhang, Hossein Robatjazi, Sadegh Yazdi and Michael McClain, all of Rice, and Emily Carter, John Mark Martirez and Caroline Krauter, all of Princeton. The research was supported by the Air Force Office of Science and Research, the Welch Foundation, the National Science Foundation and the German Academic Exchange Service.

Resources

• Dayne F. Swearer, Hangqi Zhao, Linan Zhou, Chao Zhang, Hossein Robatjazi, John Mark P. Martirez, Caroline M. Krauter, Sadegh Yazdi, Michael J. McClain, Emilie Ringe, Emily A. Carter, Peter Nordlander, and Naomi J. Halas (2016) “Heterometallic antenna−reactor complexes for photocatalysis” PNAS doi: 10.1073/pnas.1609769113