Nanocatalysis

More than speed

The role of catalysts is greater than simply increasing the rate of a reaction. Modifying nanoparticles enhances two significant catalyst attributes: selectivity and thermal stability.

Most scientists understand that the activity of a catalyst represents its ability to accelerate the rate of a chemical reaction (A → B) without the catalyst itself being consumed in the process. Fewer scientists appreciate that an important problem is that of designing a catalyst that will select between two reaction pathways (A → B or A → C), perhaps to yield the product that is less energetically favoured. Moreover, it is the catalysis practitioner who truly values a catalyst with stability; long life under the most demanding chemical reaction conditions.

Two significant steps forward in catalyst design are reported on pages 126 and 132 of this issue by Somorjai et al.1 and Lee et al.2, respectively. The group lead by Gabor Somorjai use innovative methods of nanoparticle synthesis to prepare a core–shell Pt catalyst that is orders of magnitude more stable than bare Pt nanoparticles. Lee and co-workers (led by Francisco Zaera), meanwhile, exploit basic principles of surface chemistry to design and synthesize a Pt catalyst that selectively produces the less energetically favoured of two reaction products.

Many catalytic materials are nanometre-sized metal particles, usually supported on high-surface-area materials because small metal particles expose the greatest possible surface area per gram of metal. The activity and selectivity of these catalysts are determined by characteristics such as nanoparticle size and the atomic-level structure of the planes that they expose3,4. The concept that catalytic activity is linked to the structure of catalyst surfaces is commonly attributed to Taylor5. Much of the earliest work to elucidate the details of this relationship was achieved by preparing catalysts with narrow particle-size distributions such that the set of crystal planes exposed was well controlled6. It therefore should be appreciated that catalysis research was a well-defined 'nanoscience' many decades before the term was popularized.

Nevertheless, it must also be noted that the progress made by Somorjai and Zaera, and their colleagues, would not have been achievable without the significant advance in nanoparticle synthesis that has been made over the past decade as a direct result of the popular interest in nanoscience. It is now possible to synthesize all manner of metal nanoparticles by controlling the particle size and, more impressive still, the particle shape. Figure 1 shows examples of composite, metal and alloy nanoparticles that have been synthesized with control of size, shape and morphology, respectively7. From a catalysis science perspective, the virtue of shape control is the ability to synthesize particles that only expose one atomic plane. Whereas a small spherical nanoparticle exposes a variety of atomic environments for a catalytic reaction, a cubic nanoparticle (Fig. 1c) exposes surfaces with just one atomic structure. Homogeneity of surface structure translates directly into high catalytic selectivity and perhaps maximal activity8.

Figure 1: Chemically synthesized nanoparticles with well-defined size, composition and shape.
figure1

a, Size-controlled Ni nanorods coated with SiO2. b, Pt nanoparticles embedded in hexagonally shaped Fe nanoparticles. c, Self-assembled cubic FePt nanoparticles with their (100) planes parallel to the substrate.

Lee and colleagues2 take advantage of the ability to prepare tetrahedral-shaped Pt nanoparticles of uniform size that expose only (111) facets on which the atoms are arranged in a hexagonal lattice. This catalyst facilitates the interconversion of cis- and trans-2-butene with selectivity that favours the formation of cis-2-butene, despite the fact that, in the gas phase, it is energetically less stable than trans-2-butene. The team demonstrate that this subtle level of control over selectivity is possible because cis-2-butene is energetically more stable than trans-2-butene when adsorbed on the Pt(111) surface. What is most intellectually satisfying is that this achievement was made through a deep understanding of butene surface chemistry on Pt — temperature-programmed desorption experiments and density functional calculations indicated that Pt surface reconstruction is central to the adsorption of the isomers. The marriage of surface science methods and fundamental concepts of surface chemistry with nanoparticle synthesis has resulted in a catalyst that could not have been created a decade ago.

In their work, Somorjai and co-workers1 prevail over one of the phenomena that plague the practical application of many nanoparticulate metal catalysts: nanoparticle sintering. At high temperatures, and thus in the reactive environments of many catalytic processes, metal atoms are mobile to the point that metal nanoparticles change shape and large particles grow at the expense of smaller particles. This limitation is clearly illustrated, for instance, in the work of Lee et al., whose tetrahedral Pt nanoparticles do not survive heating to 575 K. Above this temperature the particles change shape, causing a reversal in the selectivity for cis-to-trans isomerization of 2-butene. To prevent catalyst nanoparticle agglomeration, Somorjai et al. use a creative synthesis that coats Pt metal nanoparticles with a mesoporous silica shell. The shell allows the particles to be heated to temperatures approaching 1,000 K without evidence of sintering. Although the nanoparticles are buried, the catalytic activity is not inhibited — the silica is sufficiently porous to allow transport of catalytic reactants and products to and from the Pt nanoparticle surface. This is likely to be the case for similar mesoporous shells surrounding nanoparticulate catalysts for reactions that are not limited by transport to and from the catalyst surface. The stability imparted on the Pt nanoparticles by the silica shell provides an additional benefit — thermal stability. Even after heating to almost 1,000 K, most of the Pt cores retain their surface morphology.

The field of catalysis science is often criticized as being ad hoc and empirical. The papers by Somorjai et al. and Lee et al. are testament to the fact that this need not be the case. Creative use of the modern methods of nanoparticle synthesis coupled with a deep understanding of fundamental surface chemistry has yielded significant progress in some of the most important and challenging problems in the field.

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Gellman, A., Shukla, N. More than speed. Nature Mater 8, 87–88 (2009). https://doi.org/10.1038/nmat2363

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