Fundamental Understanding of a Defect Density and Catalytic Activity for Methane Combustion Using Colloidally Engineered Pd Nanoparticles

Recently, defect engineering has been widely used in catalysis study to enhance catalytic activity. Defect sites in nanomaterials have a high energy state due to their unsaturated coordination or strain effects. Therefore, these sites have different absorption/desorption energy of reactant/intermediate molecules than those without defects, and the difference potentially affects the activity of catalysts. Among various types of defects, planar defects such as stacking fault, grain boundary, and twin boundary have been highly studied in the field of catalysis due to their stability during a reaction. There are various ways to introduce defects in catalysts, but those methods often inevitably introduce changes in the size or shape of nanoparticles which also have the potential to affect the catalytic activity. However, to address the direct correlation between the defect density and intrinsic activity of catalyst materials, it is required to control and exclude the change in other variables than defect densities, including the size and shape of nanoparticles. Therefore, the method to control defect density in nanoparticles while minimizing the change of other variables is highly required.<br/>Here we chose a Pd nanoparticle catalyst for methane combustion reaction as a model system. We vary the amount of phosphorous in Pd nanoparticles using wet chemical synthesis to modify defect density in nanoparticles without changing other factors such as shape and size. Firstly, we synthesize well-defined colloidal Pd nanoparticles and expose them to phosphorous sources to introduce a phosphorous into a Pd crystal structure. Next, nanoparticles are exposed to an oxygen environment to extract phosphorous. Structural rearrangement is expected to occur at this step due to the diffusion of phosphorous atoms, resulting in defects forming. Using this method, we successfully synthesize Pd nanoparticles with different defect densities and compare their intrinsic activity for the methane combustion reaction. Finally, we confirm that Pd nanoparticles with a high density of defects show higher intrinsic activity for this reaction. This study provides a way to introduce defects in nanoparticles and fundamental insights for the correlation between a defect density and activity.