Understanding the Nanoparticle Size Effect in Catalysis

Catalysts with high activity and selectivity could dramatically reduce the energy required for many energy intensive catalytic processes. Many factors could affect the catalytic activity and selectivity of metal nanoparticles, such as particle size, shape, capping agent, and oxide support. With the help of dendrimers, monodis-perse nanoparticles 1 nm in diameter can be synthesized repeatedly, which extends the study of size-induced activity and selectivity changes of metal nanoparticles to the sub-nm regime. 

11 (a) Pyrrole hydrogenation reaction network, (b) Pyrrole hydrogenation selectivity (color coordinated to (a)) as a function of Pt NP size (7 =413 K and 11 2 % conversion). Feed was 4 Torr of pyrrole and 400 Torr of H2 with the balance He. Reprinted with permission from ref. [34], Copyright 2008 American Chemical Society 

Using Pd DENs of 1.3-1.9 nm, Crooks et al. also conducted an interesting study about the size effect on the activity for catalytic hydrogenation of allyl alcohol [97], A clear correlation was observed between the size of the DENs and their catalytic activity in allyl alcohol hydrogenation, as shown in Fig. 4.12a. Nanoparticles 1.3, 1.4, and 1.5 nm in size show similar activity for the reaction. When nanoparticles are bigger than 1.5 nm (1.7 and 1.9 nm), activity increases with increasing particle size. 

Electronic and/or geometric effects could be used to explain the size-induced activity changes for metal catalysts [99], To understand which effect dominates, the authors plotted the size-dependent normalized turnover frequency (TOE) calculated based on the number of Pd atoms located at the facets, at the defect sites, the total surface atoms, and the number of Pd particles of different size (blue triangles), as shown in Fig. 4.12b. If the geometric effect dominates the observed size-induced activity changes, the TOE should remain constant for particles of different sizes. The TOFs calculated based on the total surface atoms, defect atoms, and number of particles increased monotonically, while the TOFs ealculated based on face atoms remained constant for particle sizes larger than 1.5 nm. These results indicate that for Pd nanoparticles 1.5-1.9 run in size, the geometric effect dominates the 

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