Swetlana Schauermann

Nanoparticles for Heterogeneous Catalysis: New Mechanistic Insights

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 materials, we grew thin oxide films on metal single crystals under ultrahigh vacuum conditions and used these films as supports for metallic nanoparticles. We present four case studies on specifically designed materials with properties that expand our atomic-level understanding of surface chemistry. Specifically, we address (1) the effect of dopants in the oxide support on the growth of metal nanoclusters; (2) the effects of size and structural flexibility of metal clusters on the binding energy of gas-phase adsorbates and their catalytic activity; (3) the role of surface modifiers, such as carbon, on catalytic activity and selectivity; and (4) the structural and compositional changes of the active surface as a result of strong metal-support interaction. Using these examples, we demonstrate how studies of complex nanostructured materials can help revealing atomic processes at the solid-gas interface of heterogeneous catalysts. Among our findings is that doping of oxide materials opens promising routes to alter the morphology and electronic properties of supported metal particles and to induce the direct dissociation and reaction of molecules bound to the oxide surface. Also, the small size and atomic flexibility of metal clusters can have an important influence on gas adsorption and catalytic performance.

Catalytic activity and poisoning of specific sites on supported metal nanoparticles.

Typically, heterogeneous catalysts are based on nanometersized active particles, dispersed on an inert support material. In many cases it is assumed that the unique reactivities of such surfaces arise from the simultaneous presence of different active sites. On a molecular level, however, knowledge of the reaction kinetics of such systems is scarce (see e.g. refs. [1, 2] and references therein). Herein, we present first direct evidence for the different activity of coexisting sites on a well-defined supportednanoparticle system. As a model reaction we choose the decomposition of methanol on well-ordered Pd crystallites. For this reaction system two competing decomposition pathways exist (Figure 1): whereas dehydrogenation to CO

Size Dependence of the Adsorption Energy of CO on Metal Nanoparticles: A DFT Search for the Minimum Value

With a density functional theory method, we studied computationally the size dependence of adsorption properties of metal nanoparticles for CO as a probe on Pd(n) clusters with n = 13-116 atoms. For large particles, the values slowly decrease with cluster size from the asymptotic value for an (ideal) infinite surface. For clusters of 13-25 atoms, starting well above the asymptotic value, the adsorption energies drop quite steeply with increasing cluster size. These opposite trends meet in an intermediate size range, for clusters of 30-50 atoms, yielding the lowest adsorption energies. These computational results help to resolve a controversy on the size-dependent behavior of adsorption energies of metal nanoparticles.

Particle size dependent adsorption and reaction kinetics on reduced and partially oxidized Pd nanoparticles

Combining scanning tunneling microscopy (STM), IR reflection absorption spectroscopy (IRAS) and molecular beam (MB) techniques, we have investigated particle size effects on a Pd/Fe(3)O(4) model catalyst. We focus on the particle size dependence of (i) CO adsorption, (ii) oxygen adsorption and (iii) Pd nanoparticle oxidation/reduction. The model system, which is based on Pd nanoparticles supported on an ordered Fe(3)O(4) film on Pt(111), is characterized in detail with respect to particle morphology, nucleation, growth and coalescence behavior of the Pd particles. Morphological changes upon stabilization by thermal treatment in oxygen atmosphere are also considered. The size of the Pd particles can be varied roughly between 1 and 100 nm. The growth and morphology of the Pd particles on the Fe(3)O(4)/Pt(111) film were characterized by STM and IRAS of adsorbed CO as a probe molecule. It was found that very small Pd particles on Fe(3)O(4) show a strongly modified adsorption behavior, characterized by atypically weak CO adsorption and a characteristic CO stretching frequency around 2130 cm(-1). This modification is attributed to a strong interaction with the support. Additionally, the kinetics of CO adsorption was studied by sticking coefficient experiments as a function of particle size. For small particles it is shown that the CO adsorption rate is significantly enhanced by the capture zone effect. The absolute size of the capture zone was quantified on the basis of the STM and sticking coefficient data. Finally, oxygen adsorption was studied by means of MB CO titration experiments. Pure chemisorption of oxygen is observed at 400 K, whereas at 500 K partial oxidation of the particles occurs. The oxidation behavior reveals strong kinetic hindrances to oxidation for larger particles, whereas facile oxidation and reduction are observed for smaller particles. For the latter, estimates point to the formation of oxide layers which, on average, are thicker than the surface oxides on corresponding single crystal surfaces.

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

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 and in situ reflectivity 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) molecules cm(-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 nJ cm(-2)) corresponds to the detection limit for adsorption of less than 1.5 × 10(12) CO molecules cm(-2) or less than 0.1% of the monolayer coverage (with respect to the 1.5 × 10(15) surface Pt atoms cm(-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.

How Absorbed Hydrogen Affects the Catalytic Activity of Transition Metals

Heterogeneous catalysis is commonly governed by surface active sites. Yet, areas just below the surface can also influence catalytic activity, for instance, when fragmentation products of catalytic feeds penetrate into catalysts. In particular, H absorbed below the surface is required for certain hydrogenation reactions on metals. Herein, we show that a sufficient concentration of subsurface hydrogen, H(sub) , may either significantly increase or decrease the bond energy and the reactivity of the adsorbed hydrogen, H(ad) , depending on the metal. We predict a representative reaction, ethyl hydrogenation, to speed up on Pd and Pt, but to slow down on Ni and Rh in the presence of H(sub) , especially on metal nanoparticles. The identified effects of subsurface H on surface reactivity are indispensable for an atomistic understanding of hydrogenation processes on transition metals and interactions of hydrogen with metals in general.

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

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

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 Fe(3)O(4)/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 Fe(3)O(4) 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.

Role of low-coordinated surface sites in olefin hydrogenation: a molecular beam study on Pd nanoparticles and Pd(111).

hydrogenation of the olefinic double bond was shown to crucially depend on the presence of hydrogen species absorbed in the subsurface region of a metal catalyst. [2–4] Particularly for Pd nanoparticles, slow replenishment of these species required for hydrogenation was identified as a rate-determining process in a broad range of reaction conditions. [2–6] In view of these results, the permeability of the metal surface for hydrogen can play a crucial role in its activity in hydrogenation catalysis. Since subsurface hydrogen diffusion is known to be a strongly structure-sensitive process on Pd surfaces, [7] the degree of coordination of the surface Pd atoms can be very important in determining the formation rate of subsurface hydrogen species, and therefore may be decisive for the hydrogenation activity. In line with this suggestion, it was recently observed experimentally that modification of the low-coordinated surface sites of Pd nanoparticles, such as edges and corners, with carbon significantly affects the hydrogenation activity and results in a sustained hydrogenation rate that is not possible on the C-free surface. [2, 5] This observation was proposed to arise from faster subsurface diffusion of hydrogen through the edge sites modified with carbonaceous deposits. Recent theoretical calculations supported the hypothesis that carbon adsorbed in the vicinity of particle edges strongly reduces or nearly eliminates the activation barrier for subsurface hydrogen diffusion on Pd particles. [8] This effect was ascribed mainly to C-induced expansion of the surface openings for penetration of H into the subsurface region. In contrast, no notable reduction of the activation barrier was found with carbon on the intrinsically rigid regular Pd(111) surface. [8] The conceptual importance of atomic flexibility of sites near particles edges in subsurface hydrogen diffusion, demonstrated by the theoretical calculations, suggests that the low-coordinated surface sites can play a crucial role in the hydrogenation process. In order to obtain more insight into the role of low-coordinated surface sites in hydrogenation, we studied the reaction of cis-2-butene with deuterium on well-defined model Pd nanoparticles supported on a thin Fe3O4/Pt(111) film. Two complementary strategies were applied herein to address the role of low-coordinated surface sites experimentally: first, we compared the hydrogenation activity of Pd particles with an extended Pd(111) single crystal surface. Secondly, we selectively modified the low-coordinated surface sites by deposition of carbon, which has previously been determined to predominantly occupy edges and corners of Pd nanoclusters, [5] and follow the reactivity changes upon such modification. Herein, we demonstrate that modification of the low-coordinated surface sites of Pd particles with carbon promotes sustained hydrogenation activity in a pronounced manner, while carbonaceous deposits adsorbed on the extended Pd(111) surface do not noticeably affect the reactivity. We ascribe this phenomenon to facilitation of hydrogen subsurface diffusion through carbon-modified low-coordinated sites of Pd nanoparticles, which are not available on Pd(111). Generally, alkene conversions promoted by transition-metal catalysts are accounted for by the Horiuti–Polanyi mechanism, [9] which proceeds through a series of successive hydrogenation-dehydrogenation steps:

Adsorption, decomposition and oxidation of methanol on alumina supported palladium particles

We have investigated the adsorption, decomposition and oxidation of methanol on a well-defined supported Pd model catalyst, utilizing a combination of molecular beam methods, reflection absorption IR spectroscopy (RAIRS) and temperature-programmed desorption (TPD). The Pd model catalyst is prepared under ultrahigh-vacuum (UHV) conditions on a well-ordered Al2O3 film grown on NiAl(110). In previous studies, this model system has been characterized in detail with respect to its geometric and electronic structure. On the alumina support, two molecular adsorption states of methanol are distinguished by RAIRS and TPD. Moreover, we can differentiate between adsorption on the Pd particles and on the alumina support, enabling us to follow surface diffusion from the alumina film to the Pd particles during the adsorption process. Upon heating, methanol partially desorbs from the Pd particles and partially undergoes decomposition, with a reaction probability that is sensitively dependent on the initial methanol coverage. At 100 K, preadsorbed CO suppresses methanol adsorption on the Pd particles, whereas preadsorbed oxygen reduces the reaction probability. As a first intermediate, methoxy species are formed, which are stable up to temperatures of 200 K. Isotope exchange experiments indicate that a fast equilibrium is established between molecular methanol and methoxy species and that both species are rapidly exchanged with the gas phase. Further decomposition of methanol proceeds via two competing reaction pathways. The dominant pathway is dehydrogenation to CO, followed by CO2 formation in the presence of oxygen. Adsorbed oxygen has a pronounced inhibiting effect on the rate of decomposition. As a second pathway, we observe slow breakage of the carbon–oxygen bond, leading to formation of carbon and hydrocarbon species.