Single atom catalysis

Single atom catalysis, involving isolated metal atoms stabilized on appropriate carriers, is currently one of the most innovative and fastest growing research areas in the entire field of catalyst science. Several key factors have contributed to their rapid development. Continued advancements in techniques of characterization have increasingly facilitated the confirmation of the presence of atomically dispersed metal species, thereby enabling an improved understanding of their structure, stability, and catalytic properties. A key conceptual driver has been the maximization of atom efficiency through downsizing from the metal nanoparticles traditionally used in heterogeneous catalysis to single atoms and the potentially unique kinds of catalysis that the latter entities may display. Another attractive feature is the apparent simplicity that single atoms offer with respect to the identification of the catalytic loci and the establishment of novel reaction mechanisms. All this presents hitherto unequalled opportunities for theoreticians, as the tasks of coping with single atoms in a variety of perceived or actual (experimentally established) environments are both readily manageable and numerous.

The scope of single atom catalysis is broad. In addition to numerous distinct types of thermally activated reactions, they have demonstrated potential as electrocatalysts and, as outlined in one of the papers in this issue, may well figure in future programmes involving fuel cells (and the hydrogen economy). It has recently become known that photocatalytic conversions may also swiftly proceed at single atom active sites. For example, atomic palladium or platinum on graphitic carbon nitride are effective photocatalysts in converting carbon dioxide to formic acid or methane, respectively. Given the environmental and political pressure that demands diminution of global anthropogenic carbon dioxide emissions, this result is especially important, since a closed system – coupling the production of carbon dioxide to its subsequent conversion in hydrogen to methane by single atom photocatalysts – is feasible.

In this special issue, we have brought together leading researchers to provide a complementary selection of perspectives and research articles that provide a snapshot of current directions and challenges in the field.

Tang et al. (DOI: 10.1039/C7CY00723J) deliver a tutorial-like minireview, recapping the interesting facets of single atom catalysis before focusing on recent progress in addressing three key challenges in the development of these materials; i) the controllable and readily accomplished synthesis; ii) the characterization, and iii) the robust stabilization of single atoms on supports.

Taking stock of the developing understanding and discovery of new catalysts based on atomically dispersed metals, an essay by Gates et al. (DOI: 10.1039/C7CY00881C) sketches classes of these catalysts and suggests some key questions to guide future research.

The experimental work from Flytzani-Stephanopoulos and co-workers (DOI: 10.1039/C7CY00794A) explores the scope of silica supported and unsupported PdAu single atom alloys for the selective hydrogenation of 1-hexyne. By comparison with model catalyst analogues, the authors rationalize the enhanced hydrogenation activity observed upon introduction of small amounts of palladium into gold nanoparticles, which is attributed to the improved hydrogen activation.

Two further contributions represent the important standing of theory in the field. López et al. (DOI: 10.1039/C7CY01136A) present the first cross-comparison of platinum single atom catalysts based on different platforms (oxides, metals, and carbon nitrides). Perhaps unsurprisingly the findings highlight the vast potential impact of the scaffold on stability parameters, electronic structure, and activity towards H₂ activation. Li et al. (DOI: 10.1039/C7CY00704C) study the stability of various transition metals supported on zinc oxide for application in the water gas shift reaction, identifying the relative stability in the lattice and potential activity descriptors for catalyst screening.

Approaching the topic from a different angle, a minireview from Schwarz (DOI: 10.1039/C6CY02658C) updates us on how single atom catalysis can be achieved at a strictly molecular level by performing well-designed gas-phase experiments complemented by quantum mechanical calculations.

With a focus on electrochemical applications, Neyman and co-workers (DOI: 10.1039/C7CY00710H) constructively review the fabrication of single atom catalysts based on nanostructured ceria, offering interesting insights into the remarkable performance observed over these materials and guidelines for their rational development as anode catalysts in fuel cells.

We wish to thank all of the authors who have contributed to this special issue as well as members of the Editorial Office of Catalysis Science and Technology.

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