Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal–support interaction, and metal–reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results o...

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles.

Advances in the development of novel cobalt Fischer-Tropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels.

Surface science studies of model fuel cell electrocatalysts

Ultrathin metal films and particles on oxide surfaces: structural, electronic and chemisorptive properties

Metal–organic frameworks (MOFs), established as a relatively new class of crystalline porous materials with high surface area, structural diversity, and tailorability, attract extensive interest and exhibit a variety of applications, especially in catalysis. Their permanent porosity enables their inherent superiority in confining guest species, particularly small metal nanoparticles (MNPs), for improved catalytic performance and/or the expansion of reaction scope. This is a rapidly developing interdisciplinary research field. In this review, we provide an overview of significant progress in the development of MNP/MOF composites, including various preparation strategies and characterization methods as well as catalytic applications. Special emphasis is placed on synergistic effects between the two components that result in an enhanced performance in heterogeneous catalysis. Finally, the prospects of MNP/MOF composites in catalysis and remaining issues in this field have been indicated.

Metal–organic frameworks meet metal nanoparticles: synergistic effect for enhanced catalysis

C–O activation is a crucial step in Fischer–Tropsch synthesis (FTS). Several pathways have been proposed to activate CO, namely, direct C–O dissociation, activation via hydrogenation, and activation by insertion into growing chains. Invariably, very high barriers are calculated for both direct C–O dissociation and for hydrogenation at the O atom in CO* and RCO*, while hydrogenation at the C atom leads to oxygenates. We demonstrate that surface hydroxyl groups open a new pathway for CO* and RCO* activation via proton transfer to the O atom. In combination with the CO insertion mechanism, the calculated rate for this new pathway is consistent with the selectivity in FTS, and is in agreement with the kinetic effect of water. Hydroxyl group formation from O* is sufficiently fast to be quasi-equilibrated, and is much faster than CO2 formation. The role of surface hydroxyl groups as hydrogenating species is likely general, and involved in several oxygenate transformation reactions.

Key Role of Surface Hydroxyl Groups in C–O Activation during Fischer–Tropsch Synthesis

The use of the metal organic framework MIL-101(Cr) as support for Pt nanoparticles is evaluated in three selective hydrogenation reactions. Homogenous Pt nanoparticles of ∼4 nm can be formed inside the porosity of MIL-101(Cr) in close contact with the Cr trimers in an egg-shell configuration by a wet impregnation procedure combined with sonication. The catalyst is applied in the selective hydrogenations of olefin mixtures, benzonitrile, and linoleic acid. In the case of an olefin mixture, 1-octene is selectively hydrogenated over 1-hexadecene, attributed to diffusion limitations that favor the hydrogenation of the smaller substrate. In the case of benzonitrile hydrogenation, benzylamine is selectively formed over dibenzylamine attributed to transition state selectivity. In the hydrogenation of linoleic acid, selectivities were similar to that for platinum on alumina.

Shape and Transition State Selective Hydrogenations Using Egg-Shell Pt-MIL-101(Cr) Catalyst

For several decades zeolites have found application as catalysts in the oil refining and petrochemical industry, because of their superior activity, stability and selectivity in major conversion and upgrading processes as compared with their amorphous equivalents. The continuing drive for higher quality transportation fuels and chemical products in addition to increasing environmental pressures provides opportunities for further enhancement of their role in industrial catalysis. The development of zeolite catalysis is increasingly supported by advances in preparation, characterisation and testing of catalysts for the determination of property/performance relationships. The explosive growth in computational chemistry and step changes in the range of hydrocarbons accessible to quantitative study have firmly established the role of zeolite modelling, and attractive alternatives to experimentation may be imminent. Development cycles will consequently shorten and future applications of zeolites can be envisaged in processing a wider range of feedstocks, in higher selectivity and stability catalysts and in non-acid catalysts.

Industrial applications of zeolite catalysis

Geometrical probability can be used as a basis for the quantification of XPS-signals stemming from random samples such as, for instance, supported heterogeneous catalysts. From a study of three ideal cases, namely extended layers of constant thickness, equally sized spheres and hemispheres on a randomly oriented support, it has been established that the effects of angular and layer thickness averaging are interconnected in such a way that a general model can be derived. It has been found that signal ratios as measured by XPS for convex particles are determined by the surface/volume ratios of the supported phases, nearly independent of particle shape.

The characterization of heterogeneous catalysts by XPS based on geometrical probability 1: Monometallic catalysts

Fourier-transform pulsed-field-gradient NMR measurements have been used to analyse the diffusion of some n-alkanes (methane, n-butane and n-pentane) in zeolite ZSM-5. The intention of the NMR study was to compare results obtained by molecular dynamics calculations and by uptake measurements, using the same systems. Methane clearly exhibits a bi-exponential spin-echo attenuation. This indicates two types of diffusion: intracrystalline diffusion and long-range diffusion, which is a combination of inter- and intra-crystalline diffusion. In the case of small crystals the diffusion of methane into the macropores between the crystals dominates the decay. However, as expected, for larger crystals, the contribution of intracrystalline (or micropore) diffusion increases significantly. From the curves, the coefficient of intracrystalline diffusion of methane in ZSM-5 has been determined (3.8 × 10–9 m2 s–1 at 25 °C). The NMR methane diffusion data are in good agreement with values obtained by molecular dynamics calculations. Subsequent NMR measurements of n-butane and n-pentane diffusion in ZSM-5 indicate that the diffusion decreases sharply with increasing chain length of the hydrocarbons (11 × 10–11 and 4.4 × 10–11 m2 s–1, respectively, at 25 °C and 20 kPa loading). To allow for a comparison with the diffusivities obtained independently by other techniques, the concentration dependence of the NMR self-diffusion coefficient of n-pentane in ZSM-5 was determined and was found to decrease with increasing sorbate concentration. In addition, from the temperature dependence of the diffusion rates the activation energies for the n-butane and n-pentane diffusion in ZSM-5 have been determined (8.4 and 12.6 kJ mol–1 at 20 kPa, respectively). The PFG NMR and MD results for the diffusion of light n-alkanes in ZSM-5 have also been compared with relevant diffusion data from the literature (obtained using other techniques, i.e. uptake methods, ZLC, MT). The microscopic self-diffusivity (from PFG NMR and MD) differs systematically by ca. two orders of magnitude from the much slower, macroscopic diffusion observed by uptake, ZLC and MT methods. On the other hand, there is satisfactory agreement between the self-diffusivity of n-butane obtained with PFG NMR and the transport diffusivity of the same system measured using the frequency response method.

H.P.C.E. Kuipers

Papers

Key Role of Surface Hydroxyl Groups in C–O Activation during Fischer–Tropsch Synthesis

Shape and Transition State Selective Hydrogenations Using Egg-Shell Pt-MIL-101(Cr) Catalyst

Industrial applications of zeolite catalysis

The characterization of heterogeneous catalysts by XPS based on geometrical probability 1: Monometallic catalysts

Fourier-transform pulsed-field-gradient 1H nuclear magnetic resonance investigation of the diffusion of light n-alkanes in zeolite ZSM-5