Controlled nanostructures for applications in catalysis

Catalysis has been a vital part of Physical Chemistry essentially since it emerged as a subdiscipline of Chemistry at the end of the 19th century. Many of the founding fathers of Physical Chemistry, such as Wilhelm Ostwald and Irving Langmuir, were active in catalysis research. Nanotechnology, on the other hand, is a much younger child of science, which came into being as such at the end of the 20th century, as foreseen by Feynman's lecture “There is Plenty of Room at the Bottom” which had been given in 1959. Catalysis researchers, at least those working in heterogeneous catalysis, however, addressed the “nanoscale” much earlier. Noble metal black powders and supported catalysts, which have been used since the early days of catalysis, were employed exactly because they provided metal particles with sizes in the sub-micrometre range. Nevertheless, the term “nanotechnology” implies a level of control that goes beyond the mere synthesis of metal blacks and supported catalysts by conventional means—as skilful as this may be. This level of control on the nanoscale has only been achieved over the last one or two decades, together with analytical techniques to assess the properties of such controlled nanostructures precisely. It is possible nowadays—at least for selected systems—to assemble supported catalysts from pre-fabricated components, localize functionality in specific regions of the catalytic material, create bi- or multi-metallic particles with desired distributions of the constituents in the particles, tune pore systems with pre-determined sizes and connectivities, and—finally—also to prove the success of the synthetic work using an array of advanced analytical methods. While the generation of catalysts is often still labelled “preparation”, a term in which the almost alchemical nature of some recipes shines through, the word “synthesis”, with its connotation of controlling the combination of the constituents, is fully adequate to describe many of the processes leading to modern catalysts.

The field of controlled nanostructures in catalysis has reached such a level of maturity that the compilation of a special issue appeared to be a worthwhile endeavour. Physical Chemistry is the discipline where many of the relevant threads come together: colloidal chemistry, porous solids, advanced instrumental analytics, the kinetics of catalytic reactions, and various others. Therefore, PCCP is the perfect journal for the publication of such a special issue, and many groups from laboratories all over the world have answered the invitation to submit their work to this issue.

The contributions that we received came from different fields of catalysis, and thus cover a wide range of topics. The most precise tuning of pore sizes is possible in crystalline porous solids, such as zeolites, and several papers in this issue deal with the influence of controlled pore sizes and topologies on the selectivity observed in heterogeneously catalyzed reactions. As the Physical Chemistry aspects are more important in the context of this special issue than specific catalytic reactions used in practice, it came as no surprise that CO oxidation is studied in many of the contributions as a convenient and sufficiently simple test reaction. Gold is the catalytically active component of choice in most of these papers, but other metals have also been studied. In the paper by Gabor Somorjai’s group (DOI: 10.1039/c0cp01858a), the influence of the composition of precisely controlled Rh–Pd bimetallic nanoparticle catalysts on the activity in CO oxidation is studied, and a model for the activity enhancement in the bimetallic system is proposed. Synergistic effects between the different components of a bicomponent catalyst are also described in the work by Oduro et al. (DOI: 10.1039/c0cp01832e) in which unsupported platinum nanoparticles were modified by other metals. It was found that cobalt-doping gave the highest selectivity in cinnamaldehyde hydrogenation, which is attributed to the preferential location of cobalt on low coordination sites of the platinum crystals. Such studies also show the importance of a multipronged analytical approach for the understanding of the catalytic behavior of controlled catalytic nanostructures: Correlations to catalytic performance can only be established if it is proven that the desired nanostructure really exists.

Other contributions highlight the effects of tailoring the support and/or nanoparticle structure to the demands of the catalytic reaction, or the tuning of the microenvironment around the catalytically active component, as described in the paper from Can Li's group (DOI: 10.1039/c0cp01828g). The communications and full papers of this issue are complemented by two perspective articles, one by Francesco Zaera on nanostructured solid catalysts (DOI: 10.1039/c0cp01688h) and one by Chunjiang Jia and Ferdi Schüth on pre-synthesized colloidal metal nanoparticles as component of designed catalysts (DOI: 10.1039/c0cp02680h).

We hope that this special issue gives an interesting survey of current trends in the field of nanostructured catalysts and provides food for thought for our readers.

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