Energy of Supported Metal Catalysts: From Single Atoms to Large Metal Nanoparticles

Many catalysts consist of late transition metal nanoparticles dispersed across oxide supports. The chemical potential of the metal atoms in these particles correlate with their catalytic activity and long-term thermal stability. This chemical potential versus particle size across the full size range between the single isolated atom and bulklike limits is reported here for the first time for any metal on any oxide. The chemical potential of Cu atoms on CeO2( 111) surfaces, determined by single crystal adsorption calorimetry of gaseous Cu atoms onto slightly reduced CeO2(111) at 100 and 300 K is shown to decrease dramatically with increasing Cu cluster size. The Cu chemical potential is similar to 110 kJ/mol higher for isolated Cu adatoms on stoichometric terrace sites than for Cu in nanoparticles exceeding 2.5 nm diameter, where it reaches the bulk Cu(solid) limit. In Cu dimers, Cu's chemical potential is similar to 57 kJ/mol lower at step edges than on stoichiometric terrace sites. Since Cu avoids oxygen vacancies, these monomer and dimer results are not strongly influenced by the 2.5% oxygen vacancies present on this CeO2 surface and are thus considered representative of stoichiometric CeO2(111) surfaces.