Heterogeneous catalysts play a vital role in many industries such as commodity chemical production, pollution control, fine chemical synthesis, and pharmaceutical manufacturing. In these venues, chemical reactions take place on surfaces that typically consist of supported metal and metal oxide clusters or nanoparticles. Understanding the functionality of these catalysts requires correlating their local electronic, chemical, and morphological structure to catalytic activity and selectivity. It is also well known that dramatic changes in catalytic activity can occur due to finite size effects (i.e., unsaturated atoms at surfaces, edges, kinks, and corners), preferred morphologies, or modification of the nanoparticle’s electronic structure by the support. Unfortunately, many traditional catalyst preparation methods (e.g. precipitation and impregnation) can result in unknown particle size and shape distributions that make it difficult to establish connections between reactivity and morphology.
In this work, we utilize colloidal synthesis to realize Ag on Pt nanoparticle catalysts with well-defined morphology to critically evaluate the influence of shape (e.g. cubes, cuboctahedra, and octahedra) and surface composition (Pt vs. Ag) on C2 hydrogenation activity and selectivity. Ag on Pt nanoparticles were synthesized using polyvinylpyrrolidone (PVP) and AgNO3 as growth promoters; nanoparticles were subsequently supported on colloidal silica, redox cycled to remove PVP, chemically etched to remove Ag, and evaluated for hydrogenation activity. IR, Raman, and NMR were used throughout the synthesis process to understand how different adsorbates (CO, C2H2, C2H4, and PVP) associate with specific particle facets as a function of Ag content. This data was used in conjunction with continuous flow reactor experiments to rationalize how shape and surface composition affect catalyst behavior. In particular, it was found that (1) surface-segregated Ag on Pt dramatically decreases hydrogenation activity while increasing selectivity for C2H4; (2) Ag removal produces a highly-active, yet non-selective hydrogenation catalyst; and (3) cuboctahedra, as compared to cubic and octahedral particles, tend to favor coke formation. The catalytic behavior of Ag on Pt nanoparticles was also compared with a reverse catalyst of Pt on Ag (nanocubes) synthesized by galvanic exchange. Nanoparticle synthesis, in-situ spectroscopy, and catalytic testing results will be highlighted in the talk.