Single atom catalysis: a decade of stunning progress and the promise for a bright future

Abstract

Controlling the hybridization of single atoms in suitable host materials opens unique opportunities for catalyst design, but equally faces many challenges. Here, we highlight emerging directions from the last, highly productive, decade in single-atom catalysis and identify frontiers for future research. Origins and evolution of single-atom catalysis The isolation of elements as atoms in chemically-distinct substances played fundamental catalytic roles, for example, in metalloenzymes, organometallic complexes, and open framework structures, long before this concept was extended to widely applied heterogeneous catalysts based on supported metals. In pioneering early work, Flytzani-Stephanopoulos et al. provided convincing evidence that ionic gold or platinum species strongly associated with the surface of ceria, and not the metal nanoparticles, were responsible for the activity observed in the water-gas shift reaction (Fig. 1, L1) 1. Subsequently, Bashyam and Zelenay proposed that oxidized cobalt and iron species coordinated to nitrogen and oxygen in functionalized carbons deliver high performance in the electrochemical oxygen reduction reaction (Fig. 1, L2) 2. After years of speculation about the catalytic role of single atoms of this group of elements, advances in experimental techniques made it possible to confirm the exclusive presence of isolated centers. In their landmark paper, Zhang et al. confirmed the high efficiency of platinum atoms supported on iron oxide for CO oxidation, introducing a new paradigm in heterogeneous catalysis (Fig. 1, L3) 3. The first catalytic applications of single-atom alloys based on metal hosts followed shortly after (Fig. 1, L5) 4. In just a decade, the topic of single-atom catalysis has become a highly transversal field of contemporary chemical research. Early researchers quickly recognized the potential cost benefits of using atomically-dispersed species for precious metals, and improving the utilization became a central driver in the topic. As the area developed, the motivation also evolved. Comparative studies revealed that the strategy of using SACs does not apply to all applications and depends on the reaction requirements 5. Another strength is the higher uniformity of potential active sites in heterogeneous catalysts based on single atoms compared to nanoparticles. The well-defined nature is attractive for both defining structure-function relationships and computational modeling. Furthermore, the close structural resemblance to molecular complexes provides a bridge between homogeneous and heterogeneous catalysis. On the 10-year journey since the consolidation of the topic, single-atom catalysis has crossed the periodic table. The diversity of host materials has also expanded. While metal oxides were initially most widely studied, tailored carbons have superseded all other types in the last years.

Recent advances in understanding oxygen evolution reaction mechanisms over iridium oxide

“…67 This type of study would cultivate a better understanding of OER catalysis from a comprehensive perspective, and furthermore would eventually bridge the gap between heterogeneous and homogeneous catalysis. [83][84][85] As described above, recent in situ and operando characterization studies captured the oxidation state of the active site and surface intermediate during the OER. The experimental evidence in Fig.…”

Single Atom Catalysts: A Review of Characterization Methods

“…There are several earlier reviews that have explored recent developments in studies of SAC activity, such as capturing reaction intermediates and monitoring the dynamics of both the geometric structure and electronic environment of atomically dispersed sites, [7] and optimizing their reactivity by tailoring the local environment of active sites and speciations of the metal ions they embed. [8] Similar advances in the use of SACs for electrocatalysis have also been reviewed. [9] In general terms, progress in the SAC field has gone hand in hand with an increasing recognition of the various forms of complexity that are exhibited by these materialsinitially unexpected due to the assumed "homogeneity" of their atomically dispersed sites.…”

“…While “single atom catalysts” is the ubiquitous term used to describe these systems, the phrase “atomically dispersed catalysts” may better convey that the individual metal atoms are located at the surface of another material. There are several earlier reviews that have explored recent developments in studies of SAC activity, such as capturing reaction intermediates and monitoring the dynamics of both the geometric structure and electronic environment of atomically dispersed sites, [7] and optimizing their reactivity by tailoring the local environment of active sites and speciations of the metal ions they embed [8] . Similar advances in the use of SACs for electrocatalysis have also been reviewed [9] .…”

Single‐Atom Catalysts: A Sustainable Pathway for the Advanced Catalytic Applications

“…[ 219 ] The H 2 production via HER has been researched as an alternative clean fuel for various uses. [ 220 ] In this regard, the H 2 O electrolysis in alkaline media is of primary importance because it is the only largely employed technology worldwide and may become a game‐changer to achieve a sustainable future. [ 221 ] Recently, Ni 5 P 4 –Ru active sites were found highly active and long term stability for alkaline H 2 production with the low onset potential of 17 mV and an overpotential of 54 mV at a current density of 10 mA cm −2 with a Tafel slop of 53 mV decade −1 .…”