(560es) Size Effect of Co3O4-Supported Pd Nanoparticle on the Catalytic Activities of CO Oxidation and Water-Gas Shift

It has been reported that nanoparticles with high surface-to-volume ratio dispersed on the support could efficiently enhance the catalytic activity. However, the fundamental understanding of this phenomenon is still not clear. In this study, we report an investigation of the size effect on CO oxidation and low temperature water-gas shift (LT-WGS) reaction over a series of Pd/Co₃O₄ catalysts. We controlled the size of Pd nanoparticle loaded on Co₃O₄ by changing the calcination temperature, and thus could tune the particle sizes from 2.5 nm to 10.6 nm. For both reactions, the catalysts with the smallest Pd particles showed the best activities.

In order to reveal the underlying reason for these results, we employed both experimental and theoretical approaches. Firstly, for CO oxidation, we experimentally found that smaller Pd particles have higher turnover frequency (TOF) compared to the larger particles. To elaborate this observation, DFT calculations were performed with taking Pd(211) and (111) facets as the representatives of small and large particles, respectively. The results suggested that CO adsorption is more favorable on Pd(111) than (211), indicating that CO poisoning effect is stronger for larger particles, thereby causing the lower activity. In contrast, Pd(211) has the lower reaction barrier of CO oxidation. Based on these results, it is thought that the active sites for CO oxidation are supposed to be on noble metal; since CO dominantly adsorbs the flat surfaces such as Pd(111) in the large particles, there are not much sites for O₂ to adsorb dissociatively to form CO₂. In contrast, the smaller particles provide both flat plates for CO adsorption and enough edges and/or vertexes for O₂ dissociative adsorption to result in the higher activity in CO oxidation.

On the other hand, for WGS reaction, we found that the TOF normalized by total area of interface was almost constant regardless of the particle size. This implies that the better WGS activity with smaller particles can be attributed to more metal-support interface area that is the active site in WGS. Therefore, we can conclude that for both reactions, Pd particle size effect showed the same activity trend, but their active sites are actually different. We believe that our results can provide a potential approach for developing highly efficient environmental catalysts.