SINGLE-SITE CATALYSIS

Catalytic sites are finally in sight

Controlled Pt loading on TiO2 nanoparticles enables single-site catalysts. With this, the coordination environment and catalytic activity can be obtained, allowing extraction of structure-function information.

Heterogeneous catalysis is responsible for the production of an estimated US$10 trillion per year of chemicals, fuels and foods. Moreover, due to their complexity the catalysts themselves have always been an area of great academic interest, with an intense focus on single-site catalysts in recent years1,2,3,4,5. Here, single metal atoms such as Pt, Pd and Au are deposited onto a support material and show great promise, both in terms of higher selectivity to desired products as well as enabling the efficient use of precious metals. There are several classes of single-site catalyst: single metal atoms on oxides, sometimes referred to as single-atom catalysts5; zeolites6; atomically dispersed metal alloys, called single-atom alloys2; and metal atoms on other supports such as graphite/graphene and boron nitride7. There have been many reports of highly dispersed metals on oxides, but there are open questions in the literature over the nature of the active site and whether nanoparticles, not single atoms, are the catalytically relevant species. This stems from the complexity of oxide support particles that can complicate interpretation. In addition to ideal surface terminations, oxides can possess other features such as surface reconstructions, a variety of exposed facets and defects such as steps, kinks and vacancies. Therefore, for the single-site catalyst field to develop beyond reports of very promising catalytic properties, the atomic-scale structure of active sites must be fully understood and correlated to chemical reactivity. Now, writing in Nature Materials, Phillip Christopher and co-workers8 have taken a big step along this path by using a holistic approach, encompassing different experimental and computational techniques for characterizing the different locations of single platinum atoms on titania particles at the atomic scale.

The team used an innovative approach where the platinum loading was so low that the vast majority of the titania particles contained at most one platinum atom. They convincingly demonstrated that Pt atoms predominantly occupied the same local coordination to the support, a case previously only seen for zeolite and single crystal supported systems. In this way they could conclusively assign the activity of single atoms. Crucially, Christopher and colleagues showed that platinum atoms occupy several different surface sites, ranging from Pt replacing Ti cations in the lattice to existing as PtOH species anchored to the TiO2 surface (Fig. 1). By varying pretreatment procedures they correlated each active site structure to its chemical reactivity, revealing that single metal atoms on an oxide support can occupy different sites, with each site having its own specific reactivity. This hints at why there is debate in the literature over the nature and reactivity of metals on oxides, and makes a clear point that single metal atoms on oxides can be catalytically relevant but sometimes need help from the support, which supplies oxygen for the reaction8,9.

Fig. 1: Single-site catalysts structures depend on surface pretreatment.
figure1

Depending on treatment, different structures form, such as a surface-bound PtO2 species after mild reduction (exposure to H2 at 250 °C), or Pt substituting for a sixfold coordinated Ti (Ti6c) after oxidation treatment (exposure to O2 at 300 °C). Pt in these structures is stable; it is only after formation of a PtOH species after harsh reduction (exposure to H2 at 450 °C) that Pt becomes somewhat mobile.

The authors also showed that the carefully prepared distinct local environments of the Pt atoms further evolved as they catalysed chemical reactions, demonstrating the need for in situ studies for developing structure-function relationships. This study highlights an important consideration in the characterization of supported single metal atoms. Probe molecule infrared spectroscopy (IR) is a ubiquitous technique used to assign structure in the single-site catalysis field, but using this technique in isolation has led to incorrect assignments. By combining state-of-the-art structural and theoretical techniques such as in situ IR, extended X-ray absorption fine structure spectra, scanning transmission electron microscopy and density functional theory, Christopher and colleagues can definitively assign IR signals to discreet and well-characterized platinum–titania active sites.

While this work does an excellent job at fully understanding structure-function relationships for platinum atoms on titania particles, there are many other promising single-site systems reported to date1,5. The challenge now is to flesh out characterization of these other catalysts with this same level of fundamental atomic-scale detail. This will take time; no one group has all these experimental tools in one lab and national facilities, in which every minute of allotted time counts, must be used. However, with the information obtained from these multi-technique studies, theoreticians should be able to build better models, and in the not too distant future it is hoped they will be able to survey catalytic materials and make predictions much faster than systems can be studied experimentally.

While Christopher and colleagues do their best at providing a surface-science-level picture of these systems, inherent heterogeneity and difficulty in imaging surface structures remains an issue in high-surface-area model systems. This is where using surface-sensitive techniques and scanning probes to characterize model single-crystal catalytic surfaces has begun to pay off9. For example, full structure-function correlations have been realized for metal atoms on magnetite4, and surface-science studies have led to the development of many new single-atom alloy catalysts2.

Despite these advances, the fundamental understanding of reaction mechanisms and catalyst tunability in the field of homogeneous catalysis remains significantly ahead of heterogeneous catalysis. This is because most heterogeneous catalysts are composed of nanoparticles of metals and alloys with a variety of different compositions, shapes, sizes, defect densities, surface reconstructions and interactions with the support that make understanding atomic-scale structure-function relationships very difficult, if not impossible. However, with the advent of single-site catalysis, this branch of heterogeneous catalysis is rapidly catching up with homogeneous catalysis in terms of full understanding of the active site and the individual reactions steps10. While homogeneous catalysts offer exquisite chemical selectivity and tunability, thermal stability limits overall catalytic activity, and the problem of separation of the catalyst itself from the products adds great inefficiencies to industrial processes. Single-site catalysts offer the opportunity to bridge this gap and offer great benefits in terms of thermal stability with no catalyst separation problems, but it is essential to carefully develop structure-function relationships in these well-defined systems to achieve these goals.

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Sykes, E.C.H. Catalytic sites are finally in sight. Nat. Mater. 18, 663–664 (2019). https://doi.org/10.1038/s41563-019-0400-x

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