Ideal catalysts simultaneously lower operating temperatures, accelerate reaction rates, and preferentially select products without being consumed or altered. … The ideal photocatalytic metal should simultaneously exhibit good plasmonic and catalytic behaviors to increase the rates and selectivity of the reaction. Recently, the size- and shape-dependent plasmonic properties of rhodium (Rh) nanoparticles have been demonstrated at energies tunable throughout the ultraviolet and visible regions. Like Au and Pt, Rh is a transition metal without a native oxide coating, and direct bonding between adsorbates and the metal surface greatly facilitates the transfer of hot carriers for plasmonic photocatalysis. Supported Rh nanoparticles and molecular complexes are widely used as catalysts in automotive catalytic converters to reduce nitrogen oxides, as well as in industrial hydrogenation, hydroformylation, and ammonia oxidation reactions.

Here, we report the discovery that plasmonic Rh nanoparticles are photocatalytic, simultaneously lowering activation energies and exhibiting strong product photo-selectivity, as illustrated through the CO₂ hydrogenation reaction. CO₂ hydrogenation on transition metals at atmospheric pressure proceeds through two competing pathways: CO₂ methanation (CO₂+4H₂→CH₄+₂H₂O) and reverse water gas shift (RWGS, CO₂+H₂→CO+H₂O). We observe that mild illumination of the Rh nanoparticles not only reduces activation energies for CO₂ hydrogenation ∼35% below thermal activation energies, it also produces a strong selectivity towards CH₄ over CO.

Specifically, under illumination from low-intensity (∼W cm⁻²), continuous wave blue or ultraviolet light-emitting diodes (LEDs), the photocatalytic reactions on unheated Rh nanoparticles produce CH₄ with selectivity of >86% or >98%, respectively, with a reaction rate twice that of the thermocatalytic reaction rate at 623 K (350 °C). This high selectivity towards CH₄ disappears when the Rh nanoparticles are not illuminated, in stark contrast to plasmonic gold (Au) nanoparticles that only catalyse CO production whether illuminated or not. … Our discovery that plasmonic Rh nanoparticles exhibit a photocatalytic activity with strong product photo-selectivity opens an exciting new pathway in the long history of heterogeneous catalysis by offering a compelling advantage of light over heat.

—Zhang et al.

Having found a catalyst that can do this important chemistry using ultraviolet light, the team now hopes to develop a version that would run on natural sunlight, a potential boon to alternative energy.

Reaction mechanism on a rhodium nanocube. The thermocatalytic reaction activates both CO–Rh bonds and CH–O bonds to produce CO and CH4, respectively. Hot electrons generated in the photocatalytic reaction selectively activate the C–O bonds of the CHO intermediate and reduce the apparent activation energy to enhance the CH4 production rate. The black, red, and blue spheres are carbon, oxygen, and hydrogen atoms, respectively. The red corners of the cube show the intense electric field from the excitation of LSPRs. Zhang et al. Click to enlarge.

Over the past two decades, scientists have explored new and useful ways that light can be used to add energy to bits of metal shrunk down to the nanoscale, a field called plasmonics.

Effectively, plasmonic metal nanoparticles act like little antennas that absorb visible or ultraviolet light very efficiently and can do a number of things like generate strong electric fields. For the last few years there has been a recognition that this property might be applied to catalysis.

—Henry Everitt, co-author and adjunct professor of physics at Duke and senior research scientist at the Army’s Aviation and Missile RD&E Center at Redstone Arsenal

Xiao Zhang, a graduate student in Professor Jie Liu’s lab, synthesized rhodium nanocubes that were the optimal size for absorbing near-ultraviolet light. He then placed small amounts of the charcoal-colored nanoparticles into a reaction chamber and passed mixtures of carbon dioxide and hydrogen through the powdery material.

When Zhang heated the nanoparticles to 300 degrees Celsius, the reaction generated an equal mix of methane and carbon monoxide. When he turned off the heat and instead illuminated them with a high-powered ultraviolet LED lamp, Zhang was not only surprised to find that carbon dioxide and hydrogen reacted at room temperature, but that the reaction almost exclusively produced methane.

We discovered that when we shine light on rhodium nanostructures, we can force the chemical reaction to go in one direction more than another. So we get to choose how the reaction goes with light in a way that we can’t do with heat.

—Henry Everitt

This selectivity—the ability to control the chemical reaction so that it generates the desired product with little or no side-products—is an important factor in determining the cost and feasibility of industrial-scale reactions, Zhang says.

The team now plans to test whether their light-powered technique might drive other reactions that are currently catalyzed with heated rhodium metal. By tweaking the size of the rhodium nanoparticles, they also hope to develop a version of the catalyst that is powered by sunlight, creating a solar-powered reaction that could be integrated into renewable energy systems.

This research was supported by the National Science Foundation (CHE-1565657) and the Army Research Office (Award W911NF-15-1-0320). Additional support was provided by Duke University’s Katherine Goodman Stern Fellowship, the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program and the Center for the Computational Design of Functional Layered Materials, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award # DE-SC0012575.

Resources

• Xiao Zhang, Xueqian Li, Du Zhang, Neil Qiang Su, Weitao Yang, Henry O. Everitt and Jie Liu (2017) “Product selectivity in plasmonic photocatalysis for carbon dioxide hydrogenation,” Nature Communications doi: 10.1038/ncomms14542