Shamil K. Shaikhutdinov

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.

Role of ceria in oxidative dehydrogenation on supported vanadia catalysts

The effect of the suppport on oxidative dehydrogenation activity for vanadia/ceria systems is examined for the oxidation of methanol to formaldehyde by use of well-defined VO(x)/CeO(2)(111) model catalysts. Temperature-programmed desorption at low vanadia loadings revealed reactivity at much lower temperature (370 K) as compared to pure ceria and vanadia on inert supports such as silica. Density functional theory is applied and the energies of hydrogenation and oxygen vacancy formation also predict an enhanced reactivity of the vanadia/ceria system. At the origin of this support effect is the ability of ceria to stabilize reduced states by accommodating electrons in localized f-states.

Size and support effects for CO adsorption on gold model catalysts

CO adsorption on gold particles deposited on well-ordered alumina and iron oxide films was studied with temperature-programmed desorption. Scanning tunneling microscopy was used to provide correlative structural characterization. The results show that the adsorption of CO on gold exhibits a size effect in that small particles adsorb CO more strongly. For a given particle size (∼3 nm), CO desorption temperature (at ∼170 K) is essentially independent of the supports studied. Therefore, support effects seen in CO oxidation on real catalytic systems must arise from the interaction of oxygen rather than CO with these catalysts.

Gold supported on thin oxide films: From single atoms to nanoparticles

[Figure: see text]. Historically, people have prized gold for its beauty and the durability that resulted from its chemical inertness. However, even the ancient Romans had noted that finely dispersed gold can give rise to particular optical phenomena. A decade ago, researchers found that highly dispersed gold supported on oxides exhibits high chemical activity in a number of reactions. These chemical and optical properties have recently prompted considerable interest in applications of nanodispersed gold. Despite their broad use, a microscopic understanding of these gold-metal oxide systems lags behind their application. Numerous studies are currently underway to understand why supported nanometer-sized gold particles show catalytic activity and to explore possible applications of their optical properties in photonics and biology. This Account focuses on a microscopic understanding of the gold-substrate interaction and its impact on the properties of the adsorbed gold. Our strategy uses model systems in which gold atoms and clusters are supported on well-ordered thin oxide films grown on metal single crystals. As a result, we can investigate the systems with the rigor of modern surface science techniques while incorporating some of the complexity found in technological applications. We use a variety of different experimental methods, namely, scanning probe techniques (scanning tunneling microscopy and spectroscopy, STM and STS), as well as infrared (IR), temperature-programmed desorption (TPD), and electron paramagnetic resonance (EPR) spectroscopy, to evaluate these interactions and combine these results with theoretical calculations. We examined the properties of supported gold with increasing complexity starting from single gold atoms to one- and two-dimensional clusters and three-dimensional particles. These investigations show that the binding of gold on oxide surfaces depends on the properties of the oxide, which leads to different electronic properties of the Au deposits. Changes in the electronic structure, namely, the charge state of Au atoms and clusters, can be induced by surface defects such as color centers. Interestingly, the film thickness can also serve as a parameter to alter the properties of Au. Thin MgO films (two to three monolayer thickness) stabilize negatively charged Au atoms and two-dimensional Au particles. In three dimensions, the properties of Au particles bigger than 2-3 nm in diameter are largely independent of the support. Smaller three-dimensional particles, however, showed differences based on the supporting oxide. Presumably, the oxide support stabilizes particular atomic configurations, charge states, or electronic properties of the ultrasmall Au aggregates, which are in turn responsible for this distinct chemical behavior.

Preparation and characterization of model catalysts: from ultrahigh vacuum to in situ conditions at the atomic dimension

Abstract In situ characterization and reaction studies of working catalytic systems are an issue of current interest. We present studies on well characterized model systems, i.e., deposited metal nanoparticles, applying a variety of experimental techniques in an attempt to bridge gaps between surface science and catalysis. In particular, we investigate methanol dehydrogenation and ethene hydrogenation under UHV as well as ambient conditions and apply nonlinear optical techniques. We use electron spin resonance to study intermediately formed radicals in Ziegler–Natta polymerization of ethene. It is concluded that there is a chance to transfer results from studies on model systems toward an understanding of catalysis.

The Atomic Structure of a Metal-Supported Vitreous Thin Silica Film**

Clear as glass: The atomic structure of a metal-supported vitreous thin silica film was resolved using low-temperature scanning tunneling microscopy (STM). Based on the STM image, a model was constructed and the atomic arrangement of the thin silica glass determined (see picture). The total pair correlation function of the structural model shows good agreement with diffraction experiments performed on vitreous silica.

Resolving the atomic structure of vanadia monolayer catalysts: monomers, trimers, and oligomers on ceria

Supported vanadium oxide catalysts have received considerable attention owing to their high activity for selective oxidation reactions. The reactivity has been shown to depend strongly on the oxide support, with reducible oxides (e.g., ceria, titania, and zirconia) exhibiting much higher turnover frequencies for oxidative dehydrogenation (ODH) reactions than irreducible oxides (e.g., silica and alumina). Structural characterization of the catalysts has been performed primarily using Raman and UV/Vis spectroscopy (see Ref. [4, 6,7] and references therein), as well as X-ray absorption spectroscopy. These results have been used to postulate that vanadia catalysts consist of isolated and polymer structures that wet the supporting oxide (so-called “monolayer catalysts”). To elucidate the surface chemistry of vanadia, different model systems, such as vanadia single crystals and thin films as well as vanadia clusters supported on planar metal oxide substrates, have been studied experimentally by surface-science techniques and computational means. To rationalize structure–reactivity relationships, welldefined systems are required. Of the reducible metal oxide supports that are known to be highly active in ODH reactions, ceria is particularly suited, because well-ordered thin films can be grown with a known surface termination. Previously, the structure and reactivity of vanadia supported on CeO2(111) has been studied using photoelectron spectroscopy (PES) and temperature-programmed desorption (TPD). 15] However, the atomic structure of ceria-supported vanadia monolayer catalysts has not been resolved. Herein, using a combination of high-resolution scanning tunneling microscopy (STM), infrared reflection absorption spectroscopy (IRAS), and PES with synchrotron radiation, we unambiguously demonstrate the formation of monomeric O=VO3 species on the CeO2(111) surface at low vanadia loadings. For the first time, we show a direct relationship between the nuclearity of vanadia species (monomeric vs. polymeric) as observed by STM and their vibrational properties. We show that ceria stabilizes the vanadium + 5 oxidation state, leading to partially reduced ceria upon vanadium deposition. These experimental results are fully supported by density functional theory (DFT) calculations. The results indicate that ceria surfaces stabilize small vanadia species, such as monomers and trimers, that sinter into two-dimensional, monolayer islands. Such stabilization probably plays a crucial role in the enhanced activity observed for ceriasupported vanadia in ODH reactions. Indeed, low-nuclearity species revealed reactivities at much lower temperatures than those with higher nuclearity, which in turn show strong similarities to the reactivity of vanadia clusters supported on alumina and silica. 13] Figure 1 presents compelling evidence for the presence of vanadia monomers on ceria at low coverage (ca. 0.3 V atoms nm ). The STM image in Figure 1 a shows that highly dispersed and randomly distributed species are formed upon deposition of vanadium in an ambient oxygen atmosphere onto a CeO2(111) thin film (see the Experimental Section). The absence of preferential nucleation sites indicates a strong interaction between vanadia species and the underlying ceria support. In the atomically resolved image (inset of Figure 1a), the two protruding spots (ca. 3 in diameter and 1.2 in height) appear to be monomers positioned atop protrusions in the ceria substrate. The apparent height of these vanadia species depends on the tunneling bias and monotonically decreases from approximately 1.8 at 2.2 V to approximately 0.9 at 3 V. At certain voltages, a dark “halo” is visible around a few of the monomeric species (see Figure 1a), which may be related to defect structures of the ceria film. The IR spectrum corresponding to the sample shown in Figure 1a is depicted in Figure 1d with an absorption feature at 1006 cm . This peak is assigned to the vanadyl (V=O) stretching vibration on the basis of comparison with other vanadia systems and reference compounds in which the V=O [*] M. Baron, Dr. H. Abbott, Dr. O. Bondarchuk, Dr. D. Stacchiola, Dr. A. Uhl, Dr. S. Shaikhutdinov, Prof. Dr. H.-J. Freund Fritz Haber Institute of the Max Planck Society Chemical Physics Department Faradayweg 4–6, 14195 Berlin (Germany) Fax: (+ 49)30-8413-4105 E-mail: shamil@fhi-berlin.mpg.de

Ultrathin Oxide Films on Metal Supports: Structure-Reactivity Relations

Well-ordered, thin oxide films have drawn some attention in recent years as suitable oxide supports for modeling highly dispersed metal catalysts at the atomic scale. It turned out, however, that ultrathin oxide films may exhibit interesting catalytic properties in their own right. In this review, we discuss phenomena specifically connected to ultrathin oxide films to explain and understand the physicochemical basis of their reactivity in oxidation reactions. Two sets of systems are discussed, i.e., transition metal oxide films grown on metal substrates and native oxide films formed upon oxidation of metal surfaces.

A case of strong metal-support interactions: combining advanced microscopy and model systems to elucidate the atomic structure of interfaces.

A symbiosis of advanced scanning probe and electron microscopy and a well-defined model system may provide a detailed picture of interfaces on nanostructured catalytic systems. This was demonstrated for Pt nanoparticles supported on iron oxide thin films which undergo encapsulation by supporting oxide as a result of strong metal-support interactions.

Thin silica films on Ru(0001): monolayer, bilayer and three-dimensional networks of [SiO4] tetrahedra

The atomic structure of thin silica films grown over a Ru(0001) substrate was studied by X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, low energy electron diffraction, helium ion scattering spectroscopy, CO temperature programmed desorption, and scanning tunneling microscopy in combination with density functional theory calculations. The films were prepared by Si vapor deposition and subsequent oxidation at high temperatures. The silica film first grows as a monolayer of corner-sharing [SiO(4)] tetrahedra strongly bonded to the Ru(0001) surface through the Si-O-Ru linkages. At increasing amounts of Si, the film forms a bilayer of corner-sharing [SiO(4)] tetrahedra which is weakly bonded to Ru(0001). The bilayer film can be grown in either the crystalline or vitreous state, or both coexisting. Further increasing the film thickness leads to the formation of vitreous silica exhibiting a three-dimensional network of [SiO(4)]. The principal structure of the films can be monitored by infrared spectroscopy, as each structure shows a characteristic vibrational band, i.e., ∼1135 cm(-1) for a monolayer film, ∼1300 cm(-1) for the bilayer structures, and ∼1250 cm(-1) for the bulk-like vitreous silica.