PNNL

The Science

Single atom catalysts (SACs), where single catalytically active atoms are isolated on a support structure, have attracted significant attention for their potential high activity/selectivity and use as model systems. However, the interactions of the catalyst atoms with their oxide support are complex and poorly understood. Research on the room temperature hydrogenation of ethylene using a rhodium catalyst on titanium dioxide (Rh₁/TiO₂) found that generated hydrogen adatoms “spillover” from the Rh onto the TiO₂ support. The hydrogen spillover adds electrons into the TiO₂, altering the electronic environment near the Rh and increasing its catalytic activity.

The Impact

SACs represent an exciting possibility for highly controlled, supported catalysts with tunable reactivity. Understanding how oxide support materials interact with the active sites is essential to developing effective catalysts. By systematically evaluating how the stability and activity of an oxide-supported single metal atom hydrogenation catalyst evolve, this work increases scientific understanding of the structure–function relationship of this class of catalysts. 

Summary

More precise than bulk materials and, theoretically, more stable than homogeneous catalysts, SACs are an active area of research. Reactions such as hydrogenation, essential for hydrogen storage and numerous industries, are potential applications. This work focused on room-temperature ethylene hydrogenation on Rh₁/TiO₂ SACs. Hydrogen (*H) spillover creates −OH/*H₂O on TiO₂ surfaces and injects electrons into its conduction band edge, enhancing the intrinsic activity of the Rh₁ 9-fold. The *H and ethylene adsorb competitively on the Rh₁/TiO₂. H₂ dissociation limits the overall reaction rate, while the reaction between *H and ethylene slows *H spillover. Without reduction at elevated temperature, <20% Rh₁ are exposed but catalyze >99% of the turnovers. H₂ reduction at ≥300°C sinters Rh₁, shifting the active site to Rh aggregates. As the reduction temperature further increases, the size and activity of the aggregates grow. This work demonstrates that most turnovers on SACs can be catalyzed by minority single atom sites, with reactivities that are drastically alterable through *H spillover at ambient conditions. It also provides a systematic understanding of the stability and evolution of single atoms under H₂, resulting in a description of the structure–function relationship in hydrogenation catalysis.

PNNL Contact

Janos Szanyi, Pacific Northwest National Laboratory, Janos.Szanyi@pnnl.gov 

Funding

This work was supported by the Department of Energy (DOE), Office of Science, Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, “Impact of catalytically active centers and their environment on rates and thermodynamic states along reaction paths,” FWP 47319. PNNL is a multiprogram national laboratory operated for DOE by Battelle. We acknowledge Dr. Mark E. Bowden, X. Shari Li, and Mark Engelhard at PNNL for XRD, BET, and XPS measurements, respectively. This research used resources of the Advanced Photon Source (APS), an Office of Science User Facility operated for the DOE Office of Science by the Argonne National Laboratory and was supported by the DOE under contract no. DE-AC02-06CH11357 and the Canadian Light Source and its funding partners.