Time-Resolved Formation and Operation Maps of Pd Catalysts Suggest a Key Role of Single Atom Centers in Cross-Coupling

Abstract

An approach to the spatially localized characterization of supported catalysts over a reaction course is proposed. It consists of a combination of scanning, transmission, and high-resolution scanning transmission electron microscopy to determine metal particles from arrays of surface nanoparticles to individual nanoparticles and individual atoms. The study of the evolution of specific metal catalyst particles at different scale levels over time, particularly before and after the cross-coupling catalytic reaction, made it possible to approach the concept of 4D catalysis–tracking the positions of catalytic centers in space (3D) over time (+1D). The dynamic behavior of individual palladium atoms and nanoparticles in cross-coupling reactions was recorded with nanometer accuracy via the precise localization of catalytic centers. Single atoms of palladium leach out into solution from the support under the action of the catalytic system, where they exhibit extremely high catalytic activity compared to surface metal nanoparticles. Monoatomic centers, which make up only approximately 1% of palladium in the Pd/C system, provide more than 99% of the catalytic activity. The remaining palladium nanoparticles changed their shape and could move over the surface of the support, which was recorded by processing images of the array of nanoparticles with a neural network and aligning them using automatically detected keypoints. The study reveals a novel opportunity for single-atom catalysiseasier detachment (capture) from (on) the carbon support surface is the origin of superior catalytic activity, rather than the operation of single atomic catalytic centers on the surface of the support, as is typically assumed.

4D Catalysis Concept Enabled by Multilevel Data Collection and Machine Learning Analysis

“…Recently, our research group conducted a study of a specific site of the catalyst at the level of nanoparticles and individual atoms before and after the reaction in cross-coupling. 18,56 The results made it possible to reveal the previously unknown key role of single palladium atoms in C−C cross-coupling reactions; in addition, the study surprisingly suggested that metal nanoparticles can act as catalyst poisons rather than active centers.…”

“…Such multicomponent catalytic systems are difficult to direct experimental study using in situ or IL EM, but in recent years developments in this area have also been observed. For example, catalytic C–C bond formation reactions represented by Suzuki–Miyaura and Mizoroki–Heck on Pd/C catalyst reactions have recently been investigated using IL-SEM, IL-TEM, and IL-HAADF-STEM . In addition to palladium nanoparticles (approximately 3 nm), individual atoms and clusters were also present on the catalyst surface.…”

“…For example, catalytic C−C bond formation reactions represented by Suzuki−Miyaura and Mizoroki−Heck on Pd/ C catalyst reactions have recently been investigated using IL-SEM, IL-TEM, and IL-HAADF-STEM. 18 In addition to palladium nanoparticles (approximately 3 nm), individual atoms and clusters were also present on the catalyst surface. Moreover, individual atoms were the most active species in these cross-coupling reactions.…”

“…In this context, we highlight the concept of 4D catalysis, which has been described recently. 18 Within the framework of this approach, studies are carried out using electron microscopy to track specific microand nanoassembles of the catalyst (3D), which can be used to record morphological changes in catalytically active particles and/or regions over time (+1D) (Figure 1C). The setup, closer to reality, would be to study the reaction outside the electron microscope in conventional (i.e., macrosized) reactors with measurable yields/selectivities of the catalytic reaction, where a heterogeneous catalyst could be periodically placed in an electron microscope chamber to monitor the same micro-and nanoscale changes as the reaction proceeds.…”

“…This is justified from the point of view of studying complex, multicomponent, and multiphase systems, such as, for example, cross-coupling reactions, where the four-dimensional approach to catalysis made it possible to reveal the key role of catalysis of this reaction by single atomic palladium centers. 18…”