Casey P. O'Brien

Trends in the binding strength of surface species on nanoparticles: how does the adsorption energy scale with the particle size?

How strongly does a molecule or an atom bind to a metal nanoparticle and how does this binding energy change with changing particle size? These questions are at the heart of many fundamental and practical problems, ranging from heterogeneous catalysis to important applied processes connected to materials science. In particular the interaction of oxygen with transition-metal nanoparticles is of pivotal importance for a variety of industrially and environmentally relevant processes such as CO oxidation in exhaust catalytic converters and methane combustion. Understanding the effect of a nanometer-scale confinement of matter on the binding strength of gaseous adsorbates is a current scientific challenge targeting the rational design of new catalytic and functional materials. Studies in this area provide a basis for the fundamental understanding of how the surface binds reactants and guides them through various elementary steps of a reaction to the products. The interaction of oxygen with palladium surfaces has been the subject of numerous studies, performed both on single-crystal surfaces and well-defined model systems consisting of Pd nanoclusters supported on thin oxide films. Presently, a very detailed microscopic-level understanding the interaction of oxygen with palladium is available, which proves to be a complex interplay between chemisorption, diffusion of oxygen into the subsurface region and bulk, 5, 10] formation of surface oxide layers, refaceting, particle reconstruction, and bulk oxide formation. The processes related to subsurface diffusion, refaceting, reconstruction, and oxidation are typically observed beyond a critical coverage of surface-adsorbed oxygen and temperatures above 300 K. Despite this comprehensive understanding and general agreement on the surface chemistry of the oxygen–palladium system, quantitative information on binding energies of oxygen on Pd nanoparticles is still missing, which is precisely because of the richness of the surface chemistry. When the binding strength is probed by a traditional desorption-based method, such as temperature-programed desorption (TPD), the O–Pd system must be heated to about 900–1000 K to desorb chemisorbed oxygen; this is often accompanied by subsurface O diffusion, surface oxide formation, and particle restructuring. These side processes together with the restrictions imposed by the kinetic modeling of the TPD spectra strongly limit the quantitative determination of binding energies of oxygen on Pd nanoparticles by traditional desorption-based methods, which results in a strong scatter of data available in literature. A strategy to overcome those shortcomings is a direct calorimetric measurement of adsorption enthalpies under isothermal conditions. At present, such fundamental information on the correlation between oxygen binding energies and the exact nature of the adsorption site as well as the size of the metal nanoparticles is not available. Herein we report on the first direct calorimetric measurement of oxygen binding energies on Pd nanoparticles investigated as a function of particle size and with the reference to a Pd(111) single crystal. The binding energies were obtained on well-defined Pd nanoparticles supported on thin oxide films prepared under ultra-high-vacuum (UHV) conditions. We apply a newly developed UHV single-crystal adsorption calorimeter (SCAC) based on molecular beam techniques in combination with infrared reflection adsorption spectroscopy (IRAS) to investigate the effect that the reduced dimensionality of metallic particles has on the interaction strength with oxygen. Complementary TPD experiments were performed to provide a link between the direct isothermal calorimetric studies and the traditional desorption-based approach. We show that there are two major structural factors determining the oxygen binding energy on Pd: the local configuration of the adsorption site, and the particle size. We provide direct experimental evidence that the change of the local adsorption environment from a multifold-bound position on the extended singlecrystal surface to an edge site of Pd nanoparticles results in a strong increase of the oxygen binding energy. On the other hand, if the local environment of the adsorbate is kept [*] Dipl.-Chem. M. Peter, Dr. J. M. Flores Camacho, Dr. S. Adamovski, Dipl.-Chem. K.-H. Dostert, Dr. C. P. O’Brien, Dr. S. Schauermann, Prof. Dr. H.-J. Freund Fritz-Haber-Institut der Max-Planck-Gesellschaft Faradayweg 4–6, 14195 Berlin (Germany) E-mail: schauermann@fhi-berlin.mpg.de

Water Interaction with Iron Oxides

We present a mechanistic study on the interaction of water with a well-defined model Fe3O4(111) surface that was investigated by a combination of direct calorimetric measurements of adsorption energies, infrared vibrational spectroscopy, and calculations bases on density functional theory (DFT). We show that the adsorption energy of water (101 kJ mol(-1)) is considerably higher than all previously reported values obtained by indirect desorption-based methods. By employing (18)O-labeled water molecules and an Fe3 O4 substrate, we proved that the generally accepted simple model of water dissociation to form two individual OH groups per water molecule is not correct. DFT calculations suggest formation of a dimer, which consists of one water molecule dissociated into two OH groups and another non-dissociated water molecule creating a thermodynamically very stable dimer-like complex.

Interaction of Isophorone with Pd(111): A Combination of Infrared Reflection−Absorption Spectroscopy, Near-Edge X‑ray Absorption Fine Structure, and Density Functional Theory Studies

Atomistic level understanding of interaction of α,β-unsaturated carbonyls with late transition metals is a key prerequisite for rational design of new catalytic materials with the desired selectivity toward C=C or C=O bond hydrogenation. The interaction of this class of compounds with transition metals was investigated on α,β-unsaturated ketone isophorone on Pd(111) as a prototypical system. In this study, infrared reflection–absorption spectroscopy (IRAS), near-edge X-ray absorption fine structure (NEXAFS) experiments, and density functional theory calculations including van der Waals interactions (DFT+vdW) were combined to obtain detailed information on the binding of isophorone to palladium at different coverages and on the effect of preadsorbed hydrogen on the binding and adsorption geometry. According to these experimental observations and the results of theoretical calculations, isophorone adsorbs on Pd(111) in a flat-lying geometry at low coverages. With increasing coverage, both C=C and C=O bonds of isophorone tilt with respect to the surface plane. The tilting is considerably more pronounced for the C=C bond on the pristine Pd(111) surface, indicating a prominent perturbation and structural distortion of the conjugated π system upon interaction with Pd. Preadsorbed hydrogen leads to higher tilting angles of both π bonds, which points to much weaker interaction of isophorone with hydrogen-precovered Pd and suggests the conservation of the in-plane geometry of the conjugated π system. The results of the DFT+vdW calculations provide further insights into the perturbation of the molecular structure of isophorone on Pd(111).

Selective Hydrogenation of Acrolein Over Pd Model Catalysts: Temperature and Particle-Size Effects

The selectivity in the hydrogenation of acrolein over Fe3 O4 -supported Pd nanoparticles has been investigated as a function of nanoparticle size in the 220-270 K temperature range. While Pd(111) shows nearly 100 % selectivity towards the desired hydrogenation of the C=O bond to produce propenol, Pd nanoparticles were found to be much less selective towards this product. In situ detection of surface species by using IR-reflection absorption spectroscopy shows that the selectivity towards propenol critically depends on the formation of an oxopropyl spectator species. While an overlayer of oxopropyl species is effectively formed on Pd(111) turning the surface highly selective for propenol formation, this process is strongly hindered on Pd nanoparticles by acrolein decomposition resulting in CO formation. We show that the extent of acrolein decomposition can be tuned by varying the particle size and the reaction temperature. As a result, significant production of propenol is observed over 12 nm Pd nanoparticles at 250 K, while smaller (4 and 7 nm) nanoparticles did not produce propenol at any of the temperatures investigated. The possible origin of particle-size dependence of propenol formation is discussed. This work demonstrates that the selectivity in the hydrogenation of acrolein is controlled by the relative rates of acrolein partial hydrogenation to oxopropyl surface species and of acrolein decomposition, which has significant implications for rational catalyst design.

Catalysis beyond frontier molecular orbitals: Selectivity in partial hydrogenation of multi-unsaturated hydrocarbons on metal catalysts

Broadening of inner molecular orbitals upon adsorption can predict chemoselectivity of metal catalysts. The mechanistic understanding and control over transformations of multi-unsaturated hydrocarbons on transition metal surfaces remains one of the major challenges of hydrogenation catalysis. To reveal the microscopic origins of hydrogenation chemoselectivity, we performed a comprehensive theoretical investigation on the reactivity of two α,β-unsaturated carbonyls—isophorone and acrolein—on seven (111) metal surfaces: Pd, Pt, Rh, Ir, Cu, Ag, and Au. In doing so, we uncover a general mechanism that goes beyond the celebrated frontier molecular orbital theory, rationalizing the C═C bond activation in isophorone and acrolein as a result of significant surface-induced broadening of high-energy inner molecular orbitals. By extending our calculations to hydrogen-precovered surface and higher adsorbate surface coverage, we further confirm the validity of the “inner orbital broadening mechanism” under realistic catalytic conditions. The proposed mechanism is fully supported by our experimental reaction studies for isophorone and acrolein over Pd nanoparticles terminated with (111) facets. Although the position of the frontier molecular orbitals in these molecules, which are commonly considered to be responsible for chemical interactions, suggests preferential hydrogenation of the C═O double bond, experiments show that hydrogenation occurs at the C═C bond on Pd catalysts. The extent of broadening of inner molecular orbitals might be used as a guiding principle to predict the chemoselectivity for a wide class of catalytic reactions at metal surfaces.