PNNL

The Science

Stable, but relatively inactive catalysts, can be transiently activated in the presence of surface intermediates. Researchers employed a surface science approach to prepare single rhodium (Rh) atom sites with different coordinations. They found that metastable Rh adatoms (Rh_ad) become 5-fold oxygen-coordinated Rh sites (Rh_oct) after high temperature annealing. While reacting with formic acid, a fraction of the stable Rh_oct sites transiently convert to the highly active Rh_ad sites. When the reaction completes, the Rh_ad transition back to Rh_oct and maintain the overall catalyst stability. The experimental findings are corroborated by density functional theory calculations.

The Impact

Catalysis often requires tradeoffs between the activity and stability of a catalyst. This work highlights how a catalyst can be activated by the presence of reaction intermediates. Transiently active catalysts can effectively maintain high activity with high stability. These results also highlight how small amounts of highly active species can dominate the overall reactivity of a catalyst.

Summary

Researchers prepared single Rh atoms on a single crystalline iron oxide [Fe₃O₄(001)] surface. Metastable Rh_ad are coordinated to two surface oxygens and convert to 5-fold oxygen-coordinated Rh sites that are incorporated in the Fe₃O₄(001) surface upon annealing to 500–700 K. These sites are very stable and, based on prior studies, dominate high surface area catalysts. 

Catalytic activity studies with formic acid, which readily deprotonates to formate and hydroxyls, show that Rh_ad are highly active toward dehydrogenation of formate to carbon dioxide. Spectroscopic studies show that during the reaction, a small fraction of Rh_oct transiently converts to the active species Rh_ad. Removal of the surface reaction intermediates from the catalyst surface leads to back conversion of Rh_ad to Rh_oct. Additional studies of Rh_oct/Fe₃O₄(001) catalysts show that hydroxyls are the primary species responsible for the Rh_oct activation. The Rh_oct conversion is initiated by water formation that utilizes lattice oxygen and reduces Rh_oct coordination, leading to destabilization. Since lattice oxygen participation via the Mars-van Krevelen mechanism is common in many acid-base and redox catalytic reactions, this single-atom catalyst activation mechanism can broadly apply to many systems.

Contact

Zdenek Dohnalek, Pacific Northwest National Laboratory, Zdenek.Dohnalek@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, Catalysis Science Program, FWP 47319. PNNL is a multiprogram national laboratory operated for DOE by Battelle under Contract DE-AC05-76RL01830. Computational resources were provided by a user proposal at the National Energy Research Scientific Computing Center located at Lawrence Berkley National Laboratory.