No abstract

Efficient tandem reactions on a single catalytic nanostructure would be beneficial to improving chemical transformation efficiency and reducing safety implications. It is imperative to identify the active sites for each single step reaction so that the entire reaction process can be optimized by designing and integrating the sites. Herein, hydrogen transfer reaction is taken as a proof-of-concept demonstration to show that the spatial integration of active sites is important to the catalytic efficiency of the entire process in tandem reactions. We identified specific active sites (i.e., various sites at faces versus corners and edges) for formic acid decomposition and alkene/nitrobenzene hydrogenation-the two steps in hydrogen transfer reactions, by employing three different shapes of Pd nanocrystals in tunable sizes. The investigation reveals that the decomposition of formic acid occurs preferentially at the edge sites of cubic nanocrystal and the plane sites of octahedral/tetrahedral nanocrystals, while the hydrogenation takes place mainly at the edge sites of both cubic and octahedral/ tetrahedral nanocrystals. The consistency of active edge sites during different step reactions enables cubic nanocrystals to exhibit a higher activity than octahedral nanocrystals in hydrogen transfer reactions, although octahedrons offer comparable activities to cubes in formic acid decomposition and hydrogenation reactions. Guided by these findings, we further improved the overall performance of tandem catalysis by specifically promoting the limiting step through nanocatalyst design. This work provides insights into the rational design of heterogeneous nanocatalysts in tandem reactions.

Metal-containing nanocrystals with well-designed surface structures represent a class of model systems for revealing the fundamental physical and chemical processes involved in heterogeneous catalysis. Herein it is shown how surface modification can be utilized as an efficient strategy for controlling the surface electronic state of catalysts and, thus, for tuning their catalytic activity. As model catalysts, the Pd-tetrahedron-TiO nanostructures, modified on the surface with different foreign atoms, showed a varied activity in the catalytic decomposition of formic acid towards H production. The catalytic activity increases with a reduction in the work function of modified atoms; this reduction can be well explained by a surface polarization mechanism. In this hybrid system, the difference in the work functions of Pd and modified atoms results in surface polarization on the Pd surface and, thus, in the tuning of its charge state. Together with the Schottky junction between TiO and metals, the tuned charge state enables the promotion of catalytic efficiency in the catalytic decomposition of formic acid to H and CO .