Nanoparticles of late transition metals are used as catalysts and electrocatalysts for industrial chemical reactions that produce fuels, convert chemical energy to electricity, and clean up pollution associated with the generation and use of these fuels. For such applications, metals are usually bonded onto the surfaces of oxide or carbon support materials. To provide the energy needed for sustained economic development, we must develop new and improved solid catalysts and electrocatalysts for a variety of reactions that take better advantage of traditional and alternate energy sources (solar, wind, biomass, etc.) and avoid serious environmental problems. Additionally, since these metals are often expensive and in limited supply (e.g., Pt), we also need to develop catalysts that do not require precious metals. As a consequence, recent years have seen a dramatic increase in research aimed at providing the basic understanding needed to develop new and improved catalysts and electrocatalysts for a variety of reactions that involve nanoparticles of late transition metals supported on oxide or carbon materials. It is therefore timely to present a special issue that focuses on Catalytic Properties of Model Supported Nanoparticles. The collection highlights some of the most exciting new directions for research in this area, and provides in depth examples of some of the most cutting-edge methods, both experimental and theoretical, for studying this class of catalysts, which provide insights into structure, mechanisms, kinetics, and energetics with unprecedented accuracy and detail. As Guest Editors, we present below a few of the many excellent papers published in the collection.
Due to their complexity and nanoscale structural features, it is extremely challenging to experimentally characterize the structure of these catalyst materials. Ideally, one wants to know the size and spatial distribution of the nanoparticles and the surface composition, regarding both the metallic nanoparticle and support domains, and also at the nanoparticle-support interfacial perimeter. New synchrotron-based spectroscopies are offering powerful new approaches that greatly help. Liu et al.1 describe here a way to characterize these structures using x-ray absorption near-edge structure (XANES), where the spectra are quantitatively interpreted using a supervised machine learning algorithm that is trained on a set of theoretically generated spectra of simpler model structures that can be computationally simulated with sufficient accuracy. They demonstrate it for size-selected Cu oxide clusters on a flat support. Goodman et al.2 demonstrate the use of Quick EXAFS (QEXAFS) to characterize the structures and extent of oxidation of Pd nanoparticles with in situ measurements under methane combustion reaction conditions, demonstrating that the activity increases are associated with conversion to the Pd oxide phase and that particle size effects are related to this oxide conversion. Lee and Zaera3 show again the power of in situ vibrational spectroscopy of small probe adsorbates to reveal key structural information about the surface sites present on the working catalyst and how these depend on the structural parameters of the catalyst (particle size, support, etc.).
To simplify the challenge of structural characterization, many researchers have benefited tremendously by using model catalysts where the structural properties are built in by the way the catalyst is made. Many beautiful examples of this are presented here, where most have relied on planar single-crystal surfaces as the starting material, which allows better structural homogeneity and control. Sitja et al.4 made three exceptionally well-defined model catalysts consisting of a hexagonal array of Pd nanoparticles on an ultrathin alumina single crystalline film, where the Pd particles contain either 174, 360, or 768 atoms (within <8%) but have the same separations. They used this to demonstrate a dramatic effect of particle size (for ≤360 atoms) on the activation energy for the elementary Langmuir–Hinshelwood step where adsorbed CO reacts with an O adatom to make CO2. Ansari et al.5 made model catalysts consisting of Pt and Rh nanoparticles in the 2–4 nm range on graphene on Pt(111), and showed that methanol does not decompose as easily on these small Pt nanoparticles as on Pt(111), whereas the small Rh particles are just as active as Rh(111). Lykhach et al.6 characterized size-controlled Ir nanoparticles on CeO2(111) in unprecedented detail using synchrotron spectroscopies (RPES, NAP XPS) and STM, quantifying the extent of charge transfer and how it depends on metal loading, and correlating structural details with sintering rates and reactivity with hydrogen and oxygen. Chen et al.7 investigated CeO2 nanoparticles on the Pt(111) surface, a so-called “inverse model catalyst,” clarifying the changes in structural and local electronic properties that arise upon addition of K and Rh dopants, and elucidating the nature of charge transfers involved. Brandt et al.8 showed strong evidence that perimeter sites at the edges of Pt particles on TiO2 are most active for water-gas shift. Li et al.9 and Hamlyn et al.10 used Sn adatoms to modify the electronic structure and binding propensities of Pt7 clusters toward alkenes and promoters, and clarified the promoting effect of Cs on Cu-based catalysts in CO2 hydrogenation, respectively. Krause et al.11 elucidated the mechanism of acetylene and ethylene hydrogenation on size-controlled Pd clusters on MgO(100). Patel et al.12 used dilute Pd in Ag alloys and STM/STS to elucidate the effects of Ag neighbors and the size of the Pd ensemble on the reactive properties of the surface Pd atoms.
Bo et al.13 beautifully demonstrated that elegant structural control in such systems is not limited to planar model catalysts, but can also be accomplished on more practical high-surface-area powders. Since the performance of such catalysts often depends strongly on particle size and the nature of the support material below, there is great interest in learning to separately control particle size and support material. For precious metals like Pt, small (<1 nm) particles are usually most desirable, to ensure that nearly 100% of the Pt atoms are at the surface (i.e., <1 nm in size). Bo et al. demonstrates here a powerful new way to produce uniform Pt nanoparticles below 1 nm in size attached to alumina support material. This was achieved by a very creative combination of synthesis methods involving atomic layer deposition of a silica coating on the alumina support, selectively blocked in tiny places by an adsorbed molecular template, followed by strong electrostatic adsorption of Pt at a pH between the isoelectric point of silica and alumina so that the Pt only binds to the alumina patches which had previously been blocked by the template molecules.
Computational studies form a very important component of this field. Rice and Hu14 present a brief, insightful perspective-type review of the progress that has been made in understanding the catalytic properties of supported metal nanoparticles using density functional theory (DFT). Computational studies of these complex materials are very challenging and require unusual approaches to achieve desired accuracy. Several computational studies are presented that demonstrate creative ideas and powerful new ways to achieve this. Given the multi-element and two-phase nature of these materials, and the large size of the structures needed to model them, finding the minimum-energy structure is one major challenge. The papers by Sun et al.15 (for Pt on alumina) and by Huang et al.16 [for Au clusters on CeO2(111)] demonstrate powerful new ways to approach this that involve a grand canonical genetic algorithm or stochastic surface walking global optimization based on a global neural network potential (established based on a DFT global dataset), respectively. Kauppinen et al.17 computationally explore Pt and Pd clusters on ZrO2, and show that the perimeter sites at the cluster edges offer the important possibility of breaking the so-called “tyranny of the scaling relations” for water dissociation on these metals. These scaling relations, which offer many important insights into structure–function relations in surface chemistry, also often impose serious limitations in the catalytic performance that can be achieved with pure metal sites alone, no matter what the metal or its local surface atomic arrangement. This paper thus points to potential ways around these limitations using the complex chemical nature of mixed sites at the support/metal interface. Mammen and Narasimhan18 uncover scaling relations associated with the calculated binding and migration barriers of small (1–4 atom) Pt clusters on MgO(100) that provide important new insights into sintering kinetics. Streibel and co-workers19,20 extend and enhance their important new “alloy stability model” that predicts the stability of metal atoms in bimetallic nanoparticles with site-by-site resolution, presenting accelerated approaches of parameterizing it and an approach for including strain effects. Schmidt et al.21 applied the generalized coordination number approach with kinetic Monte Carlo simulations to predict the oxygen reduction activity of supported PtNi particles. Perhaps the most important potential catalyst is one that could convert methane directly into methanol. Zhang et al.22 presented DFT calculations of this reaction on catalysts consisting of single metal atoms on Mo6S8 clusters, predicting low barrier pathways, especially for Fe. Ellaby et al.23 and Iyemperumal and co-workers24 explore TiO2-supported clusters of Pt and Cu, respectively, showing the importance of allowing the oxide surface atoms to relax in interacting with the metal cluster to understand their interactions with small molecules (O2 and CO), and clarifying the energetics and pathways of those interactions. Zhu et al.25 addressed the effects of particle size on electrochemical dissolution of Pt.
The examples above give only a sampling of the many excellent papers in this special issue. The topic is one of intense interest in catalysis for energy and environmental technologies. We are confident that this collection will help bring new ideas to the field, and thereby help accelerate its already rapid advance. We wish to sincerely thank all the authors for their insightful contributions!
C.T.C. acknowledges the Department of Energy, Office of Basic Energy Sciences, Chemical Sciences Division Grant No. DE-FG02-96ER14630, for support of this work. S.V. acknowledges support from the European Union’s Horizon 2020 Program, H2020-EU.4.c (No. FP7/2018-6/2023) under Grant Agreement No. 810310, which corresponds to the J. Heyrovský Chair project (“ERA Chair at J. Heyrovský Institute of Physical Chemistry AS CR – The institutional approach toward ERA”). The funders had no role in the preparation of the article.
