Size-Controlled Pd Nanoparticles Loaded on Co3O4 Nanoparticles by Calcination for Enhanced CO Oxidation

In accordance with Euro 6 emission standards, exhaust emissions are required to be substantially lowered especially via significant improvement in the efficiency of catalytic oxidation to reduce toxic carbon monoxide (CO). It has been reported that nanoparticles with high surface-to-volume ratio efficiently enhance the catalytic activity by providing additional active sites per unit area. However, the principle underlying this phenomenon is still not clear. To systematically elucidate the effect of metal nanoparticles on catalytic activity, we controlled the size of Pd nanoparticles loaded on Co3O4 by changing the calcination temperature. This approach was used to fine-tune the particle size from 2.5 to 10.6 nm. We found that Pd particle size is a dominant factor that affects the CO oxidation activity; smaller Pd particles yielded better catalytic activity. Three important reaction steps were identified through DFT calculations, based on which a series of temperature-programmed desorption and reduction measurements such as CO-TPD, CO chemisorption, O2-TPD, and CO-TPR were performed. The better abilities of CO desorption and O2 dissociation as well as the easier CO2 formation were found to be responsible for the higher activity of smaller Pd particles. We believe that our findings represent a potential strategy for the development of highly efficient catalysts.