Blurring the Boundary between Homogenous and Heterogeneous Catalysis using Palladium Nanoclusters with Dynamic Surfaces

Using a magnetron sputtering approach that allows size-controlled formation of nanoclusters, we have created palladium nanoclusters that combine the features of both heterogeneous and homogeneous catalysts. Here we report the atomic structures and electronic environments of a series of metal nanoclusters in ionic liquids at different stages of formation, leading to the discovery of Pd nanoclusters with a core of ca. 2 nm surrounded by a diffuse dynamic shell of atoms in [C4C1Im][NTf2]. Comparison of the catalytic activity of Pd nanoclusters in alkene cyclopropanation reveals that the atomically dynamic surface is critically important, increasing the activity by a factor of ca. 2 when compared to compact nanoclusters of similar size. Catalyst poisoning tests using mercury and dibenzo[a,e]cyclooctene show that dynamic Pd nanoclusters maintain their catalytic activity, which demonstrate their combined features of homogeneous and heterogeneous catalysts within the same material. Additionally, kinetic studies of cyclopropanation of alkenes mediated by the dynamic Pd nanoclusters reveal an observed catalyst order of 1, underpinning the pseudo-homogeneous character of the dynamic Pd nanoclusters. Introduction Catalysts are used in nearly 80 % of industrial processes, 1 demanding the maximum catalytic efficiency of rare elements, such as Au, Pt and Pd. 2-4 Increasing the metal active surface area by utilising catalysts in the form of nanoclusters is one of the most powerful approaches. 5-9 When the size of metal shrinks down to the nanoscale, the structure and dynamics and thus properties of the nanocluster cannot be interpreted as a linear function of its size, which makes it challenging to correlate them with their catalytic performance. 7,10-12 In order to establish a structure-property relationship for nanoclusters and the link with their catalytic performance, we must explore the series of related nanoclusters at their boundaries between discrete atoms and crystalline nanoclusters, with atomic precision, and using a set of complementary analytical methods, thus shedding light on their nature. 13 In this context, magnetron sputtering offers exciting new opportunities for the manufacture of novel catalytic materials with controlled sizes and shapes of metal nanoclusters on a variety of supports. 14-18 Crucially, this approach generates highly active clean surfaces, since no chemica