Jörg Libuda

Support nanostructure boosts oxygen transfer to catalytically active platinum nanoparticles

Interactions of metal particles with oxide supports can radically enhance the performance of supported catalysts. At the microscopic level, the details of such metal-oxide interactions usually remain obscure. This study identifies two types of oxidative metal-oxide interaction on well-defined models of technologically important Pt-ceria catalysts: (1) electron transfer from the Pt nanoparticle to the support, and (2) oxygen transfer from ceria to Pt. The electron transfer is favourable on ceria supports, irrespective of their morphology. Remarkably, the oxygen transfer is shown to require the presence of nanostructured ceria in close contact with Pt and, thus, is inherently a nanoscale effect. Our findings enable us to detail the formation mechanism of the catalytically indispensable Pt-O species on ceria and to elucidate the extraordinary structure-activity dependence of ceria-based catalysts in general.

Structure and defects of an ordered alumina film on NiAl(110)

Abstract Via oxidation a well ordered Al 2 O 3 film may be grown on an ordered NiAl(110) surface. Its structure has been studied with SPA-LEED (spot-profile analysis) as well as with scanning tunneling microscopy (STM). The oxide overlayer grows strictly two-dimensional with a thickness of close to 5 A. Double diffraction spots have been observed but they are very weak, thus not excluding the existence of an interfacial layer between NiAl(110) and the oxide film. STM provides preliminary evidence for such a film and presents first clues to what the structure of the interface may be. The defect structure of the Al 2 O 3 film has been investigated. In addition to boundaries between two rotational domains constituting the Al 2 O 3 film, we also identify anti-phase domain boundaries through both the SPA-LEED as well as the STM measurements.

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.

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

Interaction of rhodium with hydroxylated alumina model substrates

In order to investigate how metal growth and metal-oxide interaction depend on the chemical properties of oxide surfaces, we describe a modification procedure which allows the introduction of surface hydroxyl groups on a well-ordered Al2O3 film on NiAl(110). The modification — based on deposition of metallic Al and subsequent water exposure — is characterized using LEED spot-profile analysis (SPA-LEED) and high-resolution photoelectron spectroscopy (PES). Upon Al deposition, small aggregates are formed, which are oxidized completely in the final preparation step as verified via PES. The presence of OH-groups is supported by the appearance of additional Al 2p and O 1s surface features. The origin of oxide core and valence level binding energy shifts induced by the modification procedure is discussed. Growth and metal-substrate interaction of Rh deposited onto the hydroxylated Al2O3 film is compared to Rh growth on the non-modified oxide surface. It is shown that at 300 K nucleation preferentially occurs on modified oxide areas (SPA-LEED). Photoelectron spectroscopy of both oxide and rhodium core levels points to a direct chemical interaction between the metal and surface hydroxyl groups.

Methane Activation by Platinum: Critical Role of Edge and Corner Sites of Metal Nanoparticles

Complete dehydrogenation of methane is studied on model Pt catalysts by means of state-of-the-art DFT methods and by a combination of supersonic molecular beams with high-resolution photoelectron spectroscopy. The DFT results predict that intermediate species like CH(3) and CH(2) are specially stabilized at sites located at particles edges and corners by an amount of 50-80 kJ mol(-1). This stabilization is caused by an enhanced activity of low-coordinated sites accompanied by their special flexibility to accommodate adsorbates. The kinetics of the complete dehydrogenation of methane is substantially modified according to the reaction energy profiles when switching from Pt(111) extended surfaces to Pt nanoparticles. The CH(3) and CH(2) formation steps are endothermic on Pt(111) but markedly exothermic on Pt(79). An important decrease of the reaction barriers is observed in the latter case with values of approximately 60 kJ mol(-1) for first C-H bond scission and 40 kJ mol(-1) for methyl decomposition. DFT predictions are experimentally confirmed by methane decomposition on Pt nanoparticles supported on an ordered CeO(2) film on Cu(111). It is shown that CH(3) generated on the Pt nanoparticles undergoes spontaneous dehydrogenation at 100 K. This is in sharp contrast to previous results on Pt single-crystal surfaces in which CH(3) was stable up to much higher temperatures. This result underlines the critical role of particle edge sites in methane activation and dehydrogenation.

Interaction of oxygen with palladium deposited on a thin alumina film

The interaction of oxygen with Pd particles, vapor deposited onto a thin alumina film grown on a NiAl(1 1 0) substrate, was studied by STM, AES, LEED, XPS, TPD and molecular beam techniques. The results show that O2 exposure at 400–500 K strongly influences the oxide support. We suggest that the oxygen atoms formed by dissociation on the Pd surface can diffuse through the alumina film and react with the NiAl substrate underneath the Pd particles, thus increasing the thickness of the oxide film. The surface oxygen inhibits hydrogen adsorption, and readily reacts with CO at 300–500 K. For large and crystalline Pd particles, the system exhibits adsorption–desorption properties which are very similar to those of the Pd(1 1 1) single crystal surface. The molecular beam and TPD experiments reveal that, at low coverage, CO adsorbs slightly stronger on the smaller Pd particles, with an adsorption energy difference of � 5–7 kJ mol � 1 for 1 and 3–5 nm Pd particles studied. 2002 Elsevier Science B.V. All rights reserved.

Bridging the pressure and materials gaps between catalysis and surface science: clean and modified oxide surfaces.

The preparation of model systems based on thin epitaxial oxide films and oxide single crystals is discussed. A variety of surface sensitive techniques has been applied to study the geometric and electronic properties of these systems. The findings are correlated with adsorption and reaction of probe molecules on the surfaces. Metal vapor deposition under controlled conditions leads to the formation of metal aggregates with narrow size distributions. Their properties have been characterized, establishing that we can begin to bridge the materials gap between catalysis and surface science. While mainly performed under UHV conditions, adsorption measurements can be pushed to ambient conditions using non-linear optical techniques such as sum frequency generation. Results for systems with deposited metal aggregates will be discussed.

Hydroxy1 driven reconstruction of the polar NiO(111) surface

Abstract We have studied the reconstruction of the polar NiO(111) surface predicted recently by D. Wolf with low energy electron diffraction. Thin NiO films (10–20 A) are used as substrates. As prepared, the films with a p(1 × 1) NiO(111) structure are covered with hydroxyl groups, which may be removed through a simple heat treatment. As the hydroxyl groups are desorbed, the surface reconstructs, exhibiting a diffuse p(2 × 2) structure. Readsorption of water onto the reconstructed surface lifts the reconstruction and again leads to the formation of the p(1 × 1) hydroxyl covered surface.

Toward Ionic-Liquid-Based Model Catalysis: Growth, Orientation, Conformation, and Interaction Mechanism of the [Tf2N]− Anion in [BMIM][Tf2N] Thin Films on a Well-Ordered Alumina Surface

Aiming at a better understanding of the interaction of ionic liquid (IL) thin films with oxide supports, we have performed a model study under ultrahigh vacuum (UHV) conditions. We apply infrared reflection absorption spectroscopy (IRAS) in combination with density functional theory (DFT). Thin films of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][Tf(2)N] are grown on an atomically flat, well-ordered alumina film on NiAl(110) using a novel UHV-compatible evaporator. Time-resolved IRAS measured during the growth and subsequent thermal desorption points toward reversible molecular adsorption and desorption. There was no indication of decomposition. The vibrational bands are assigned with the help of DFT calculations. Strong relative intensity changes in individual [Tf(2)N](-) bands are observed in the monolayer region. This indicates pronounced orientation effects for the anion. The adsorption geometry of [Tf(2)N](-) is determined on the basis of a detailed comparison with DFT. The results suggest that [Tf(2)N](-) anions adopt a cis conformation in the submonolayer region. They adsorb in a slightly tilted orientation with respect to the surface, mainly interacting with the support via the sulfonyl groups.