CO Induces Clustering Of Metal Atoms

To get the most out of particulate precious-metal catalysts, catalyst manufacturers strive to disperse the metal as finely as possible. The aim is to maximize the fraction of exposed atoms because atoms buried in a particle’s interior are inaccessible to reactants and therefore uninvolved in catalysis. Some reaction processes thwart that aim, causing catalyst particles to coalesce, which deactivates them. Gareth S. Parkinson, Ulrike Diebold, and coworkers at Vienna University of Technology have uncovered the mechanism of one such process—and found a way to stop it. By using scanning tunneling microscopy to track individual palladium atoms on Fe₃O₄, a model catalyst system, the team finds that the presence of just a small amount of CO, which is common to many catalytic processes, causes otherwise stationary Pd atoms to form highly mobile Pd–CO species. As the Pd–CO species grow in number, they nucleate and form small Pd clusters that diffuse, coalesce, and grow into nanoparticles. The team also finds that surface OH groups impede this detrimental process (Nat. Mater. 2013, DOI: 10.1038/nmat3667). A surface hydroxyl coating could significantly improve catalyst stability, they suggest.