(5bj) Factors Influencing Product Selectivity in Heterogeneous Catalysis: Examination of Monodisperse Platinum and Rhodium Nanoparticles

Identifying and understanding the key catalyst features that influence selectivity is a common goal in heterogeneous catalysis. The ability to control fundamental properties in order to design heterogeneous catalysts with high selectivity would greatly reduce energy consumption. For monometallic catalysts, features include surface structure, nature of the metal-oxide interface, and particle size. In the present work, monodisperse Pt and Rh nanoparticles were synthesized using colloid chemistry and influences of the catalysts' properties upon catalytic activity and selectivity were examined.

Ultrasmall (as small as ~ 1 nm) monometallic and bimetallic Pt and Rh nanoparticles were synthesized by using fourth generation PAMAM dendrimers as the template and capping agent. Nanoparticles between 2 and 10 nm were synthesized using polymer (PVP) capping. Shape controlled nanoparticles (5 nm and above) were also synthesized by various colloid chemistry routes. Nanoparticles were loaded onto mesoporous silica, multiwalled carbon nanotubes (MWCNTs), and commercial zirconia and alumina supports by sonication. Characterization data, by techniques such as TEM, DRIFTS, elemental analyses (ICP-MS), XPS, and chemisorption, are also discussed.

Using well studied reactions such as ethylene hydrogenation and CO oxidation, catalytic activities were screened and treatments prior to reaction studies were optimized. Then, influences of the catalysts' properties were examined for pyrrole hydrogenation, a model reaction for the study of hydrodenitrogenation (HDN). Hydrogenation of the ring was observed at lower temperatures, regardless of size, over Rh catalysts than Pt ones. However, the scission of C-N bonds occurred more readily over Pt catalysts. Moreover, C-N bond breaking was more rapid when the Pt particles were larger than 2 nm, and consequently n-butylamine and butane were more likely to form over larger nanoparticles. These results are believed to be caused by Rh and smaller Pt nanoparticles being poisoned by strongly bound amine groups. Additionally, in the study of (Pt,Rh) bimetallic catalysts, Pt-rich nanoparticles were able to break C-N bonds whereas Rh-rich and even mixtures were unable to. Further multipath reactions, such as CO hydrogenation and hydroisomerization, are being investigated.