Single-Atom Catalysis: Insights from Model Systems

Abstract

The field of single atom catalysis (SAC) has expanded greatly in recent years. While there has been much success developing new synthesis methods, a fundamental disconnect exists between most experiments and the theoretical computations used to model them. The real catalysts are based on powder supports, which inevitably contain a multitude of different facets, different surface sites, defects, hydroxyl groups, and other contaminants due to the environment. This makes it extremely difficult to determine the structure of the active SAC site using current techniques. To be tractable, computations aimed at modelling SAC utilize periodic boundary conditions and low index facets of an idealized support. Thus the reaction barriers and mechanisms determined computationally represent, at best, a plausibility argument, and there is a strong chance that some critical aspect is omitted. One way to better understand what is plausible is by experimental modelling, i.e. comparing the results of computations to experiments based on precisely defined single-crystalline supports prepared in an ultrahigh vacuum (UHV) environment. In this article, we review the status of the surface science literature as it pertains to SAC. We focus on experimental work on supports where the site of the metal atom are unambiguously determined from experiment, in particular the surfaces of rutile and anatase TiO2, the iron oxides Fe2O3 and Fe3O4, as well as CeO2 and MgO. Much of this work is based on scanning probe microscopy in conjunction with spectroscopy, and we highlight the remarkably few studies in which metal atoms are stable on low index surfaces of typical supports. In a perspective, we discuss the possibility for expanding such studies into other relevant supports such as N-doped carbon, graphene and metal carbides.

Does Cluster Encapsulation Inhibit Sintering? Stabilization of Size-Selected Pt Clusters on Fe3O4(001) by SMSI

“…A dissolution or diffusion of the clusters into the bulk, on the other hand, can be excluded by the only slightly decreasing Pt 4f signal in the more bulk-sensitive measurement at 921 eV photon energy, as well as by the STM measurements (discussed below), which is consistent with previous reports in the literature where neither Pt atoms nor clusters can be incorporated in bulk magnetite. 20,65,67 This experiment already gives insights into the morphology of the encapsulating layer. Although alloying cannot be excluded entirely, the reduction of the signal intensity indicates an additional layer on top of the clusters, which is also in line with the sputtering experiment described above.…”

“…Atomically resolving microscopy combined with highly sensitive reactivity measurements provides unmatched insights into encapsulation and sintering processes on the atomic scale, allowing us to discern whether the clusters remain size-selected or might even be completely integrated into the substrate. Such dynamic effects are increasingly discussed in the literature in the context of single-atom versus cluster catalysis and potential interchange between the two. − In our research, we study the cluster encapsulation and concomitant sintering behavior on the atomic scale with sound statistics and across a wide temperature range, with the aim to enable heterogeneous catalysis at stabilized subnanometer particles under harsh conditions.…”

A Surrogate Machine Learning Model for the Design of Single-Atom Catalyst on Carbon and Porphyrin Supports towards Electrochemistry

“…Single-atom catalysts (SACs) are extensively applied for various electrochemical reactions to produce value-added chemicals − due to their high atom utilization efficiency along with their unique properties. − According to their widespread applications, the rational design of SACs has received a lot of interest in improving the feasibility and efficiency of optimizing the desired products. , Density functional theory (DFT) calculations are mostly applied for the rational design of SACs with a focus on high activities and selectivities . However, DFT calculations are computationally expensive and time-consuming , due to the fact that the complexity of structure–activity relationships requires a huge number of nontrivial DFT calculations in a vast dimensional space, such as environmental coordination, SAC type, and reactants .…”

Photoinduced Hydration Boosts O2 Evolution on Co-Chelating Covalent Organic Framework

“…Photocatalysts with atomic metal sites exhibit unique structure and maximum atom efficiency 9 and have been attracted continuous interest. 10−15 In the photocatalytic water oxidation, the activity of metal sites strongly relies on the chemical microstructures composed of metal ions and the surrounding coordination environment.…”

“…Photocatalysts with atomic metal sites exhibit unique structure and maximum atom efficiency and have been attracted continuous interest. − In the photocatalytic water oxidation, the activity of metal sites strongly relies on the chemical microstructures composed of metal ions and the surrounding coordination environment. , The isolated metal site should be less active for dissociation of water and/or O–O bond formation, which is generally rate-limiting. , It is challenging to explore the semiconductor as supporter that can provide synergistic effects to mimic the synergy of multinuclear CaMn 4 O 5 for oxygen evolution in natural photosynthesis . In recent years, organic semiconductors such as C 3 N 4 , , conjugated microporous polymer, − and covalent organic frameworks − have been explored to support atomic metal sites for photocatalytic water oxidation, and they have demonstrated promising activities in O 2 evolution.…”