CO oxidation, although seemingly a simple chemical reaction, provides us with a panacea that reveals the richness and beauty of heterogeneous catalysis. The Fritz Haber Institute is a place where a multidisciplinary approach to study the course of such a heterogeneous reaction can be generated in house. Research at the institute is primarily curiosity driven, which is reflected in the five sections comprising this Review. We use an approach based on microscopic concepts to study the interaction of simple molecules with well-defined materials, such as clusters in the gas phase or solid surfaces. This approach often asks for the development of new methods, tools, and materials to prove them, and it is exactly this aspect, both, with respect to experiment and theory, that is a trade mark of our institute.

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CO oxidation as a prototypical reaction for heterogeneous processes

Seed-mediated growth is a powerful and versatile approach for the synthesis of colloidal metal nanocrystals. The vast allure of this approach mainly stems from the staggering degree of control one can achieve over the size, shape, composition, and structure of nanocrystals. These parameters not only control the properties of nanocrystals but also determine their relevance to, and performance in, various applications. The ingenuity and artistry inherent to seed-mediated growth offer extensive promise, enhancing a number of existing applications and opening the door to new developments. This Review demonstrates how the diversity of metal nanocrystals can be expanded with endless opportunities by using seeds with well-defined and controllable internal structures in conjunction with a proper combination of capping agent and reduction kinetics. New capabilities and future directions are also highlighted.

Seed-Mediated Growth of Colloidal Metal Nanocrystals.

A benchmark database for adsorption bond energies to transition metal surfaces and comparison to selected DFT functionals

Metallic nanoparticles finely dispersed over oxide supports have found use as heterogeneous catalysts in many industries including chemical manufacturing, energy-related applications and environmental remediation. The compositional and structural complexity of such nanosized systems offers many degrees of freedom for tuning their catalytic properties. However, fully rational design of heterogeneous catalysts based on an atomic-level understanding of surface processes remains an unattained goal in catalysis research.Researchers have used surface science methods and metal single crystals to explore elementary processes in heterogeneous catalysis. In this Account, we use more realistic materials that capture part of the complexity inherent to industrial catalysts. We assess the impacts on the overall catalytic performance of characteristics such as finite particle size, particle structure, particle chemical composition, flexibility of atoms in clusters, and metal–support interactions.To prepare these materia...

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Nanoparticles for heterogeneous catalysis: new mechanistic insights.

Nanocatalysis: size- and shape-dependent chemisorption and catalytic reactivity

We present calorimetric measurements of the effect of cluster size on the adsorption enthalpy of carbon monoxide on Pd nanoclusters sized from 120 to 4900 Pd atoms per particle, which were grown in situ on a well-ordered ${\text{Fe}}_{3}{\text{O}}_{4}/\text{Pt}(111)$ film. A substantial decrease in the initial heat of adsorption amounting to about $20--40\text{ }\text{kJ}\text{ }{\text{mol}}^{\ensuremath{-}1}$ was observed on the smallest Pd nanoparticles as compared to the larger Pd clusters and the extended Pd(111) single-crystal surface. We discuss this effect in terms of the size-dependent properties of the Pd nanoparticles.

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Particle-size dependent heats of adsorption of CO on supported Pd nanoparticles as measured with a single-crystal microcalorimeter

A new ultrahigh vacuum microcalorimeter for measuring heats of adsorption and adsorption-induced surface reactions on complex single crystal-based model surfaces is described. It has been specifically designed to study the interaction of gaseous molecules with well-defined model catalysts consisting of metal nanoparticles supported on single crystal surfaces or epitaxial thin oxide films grown on single crystals. The detection principle is based on the previously described measurement of the temperature rise upon adsorption of gaseous molecules by use of a pyroelectric polymer ribbon, which is brought into mechanical/thermal contact with the back side of the thin single crystal. The instrument includes (i) a preparation chamber providing the required equipment to prepare supported model catalysts involving well-defined nanoparticles on clean single crystal surfaces and to characterize them using surface analysis techniques andin situreflectivity measurements and (ii) the adsorption/reaction chamber containing a molecular beam, a pyroelectric heat detector, and calibration tools for determining the absolute reactant fluxes and adsorption heats. The molecular beam is produced by a differentially pumped source based on a multichannel array capable of providing variable fluxes of both high and low vapor pressure gaseous molecules in the range of 0.005‐1.5 × 10 15 moleculescm −2 s −1 and is modulated by means of the computer-controlled chopper with the shortest pulse length of 150 ms. The calorimetric measurements of adsorption and reaction heats can be performed in a broad temperature range from 100 to 300 K. A novel vibrational isolation method for the pyroelectric detector is introduced for the reduction of acoustic noise. The detector shows a pulse-to-pulse standard deviation ≤15 nJ when heat pulses in the range of 190‐3600 nJ are applied to the sample surface with a chopped laser. Particularly for CO adsorption on Pt(111), the energy input of 15 nJ (or 120 nJcm −2 ) corresponds to the detection limit for adsorption of less than 1.5 × 10 12 CO moleculescm −2 or less than 0.1% of the monolayer coverage (with respect to the 1.5 × 10 15 surface Pt atomscm −2 ). The absolute accuracy in energy is within ∼7%‐9%. As a test of the new calorimeter, the adsorption heats of CO on Pt(111) at different temperatures were measured and compared to previously obtained calorimetric data at 300 K. © 2011 American Institute of Physics.

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An improved single crystal adsorption calorimeter for determining gas adsorption and reaction energies on complex model catalysts

The heat of adsorption and sticking probability of CO on well-defined Pd nanoparticles were measured as a function of particle size using single crystal adsorption microcalorimetry. Pd particles of different average sizes ranging from 120 to 4900 atoms per particle (or from 1.8 to 8 nm) and Pd(111) were used that were supported on a model in situ grown Fe3O4/Pt(111) oxide film. To precisely quantify the adsorption energies, the reflectivities of the investigated model surfaces were measured as a function of the thickness of the Fe3O4 oxide layer and the amount of deposited Pd. A substantial decrease of the binding energy of CO was found with decreasing particle size. Initial heat of adsorption obtained on the virtually adsorbate-free surface was observed to be reduced by about 20–40 kJ mol−1 on the smallest 1.8 nm sized Pd particles as compared to the larger Pd clusters and the extended Pd(111) single crystal surface. This effect is discussed in terms of the size-dependent properties of the Pd nanoparticles. The CO adsorption kinetics indicates a strong enhancement of the adsorbate flux onto the metal particles due to a capture zone effect, which involves trapping of adsorbates on the support and diffusion to metal clusters. The CO adsorption rate was found to be enhanced by a factor of ∼8 for the smallest 1.8 nm sized particles and by ∼1.4 for the particles of 7–8 nm size.

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Jan-Henrik Fischer-Wolfarth

Papers

Particle-size dependent heats of adsorption of CO on supported Pd nanoparticles as measured with a single-crystal microcalorimeter

An improved single crystal adsorption calorimeter for determining gas adsorption and reaction energies on complex model catalysts

Adsorption energetics of CO on supported Pd nanoparticles as a function of particle size by single crystal microcalorimetry.