M. Heemeier

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.

On the thermal stability of metal particles supported on a thin alumina film

The structure of metal (Co, Rh, Pd) particles deposited on a thin alumina film grown on a NiAl(1 1 0) substrate were studied as a function of annealing temperature. The results reveal at least three processes to occur upon heating: (a) rearrangement of metal atoms on the particle surface, (b) sintering of the particles, and (c) metal migration into the substrate. The latter process is probably mediated by the defect structure of the oxide film and dominates at higher temperatures. The thermal stability is found to be better for Co, which strongly interacts with the alumina support, as compared to Pd and Rh. Hydroxylation of the alumina film increases Pd and Rh dispersion and shifts processes (b) and (c) to higher temperatures, likely due to a stronger interaction of the metals with the hydroxylated film. 2002 Elsevier Science B.V. All rights reserved.

The influence of OH groups on the growth of rhodium on alumina: a model study

In order to investigate how the presence of surface hydroxyl groups on oxide surfaces affects the interaction with the supported metal, we have modified a well-ordered alumina film on NiAl(110) by Al deposition and subsequent exposure to water. This procedure yields a hydroxylated alumina surface as revealed by infrared and high-resolution electron energy loss spectroscopy. By means of scanning tunneling microscopy, we have studied the growth of rhodium on the modified film at 300 K. Clear differences in the particle distribution and density are observed in comparison to the clean substrate. While, in the latter case, decoration of domain boundaries as typical defects of the oxide film governs the growth mode, a more isotropic island distribution and a drastically increased particle density is found on the hydroxylated surface. From infrared data, it can be deduced that the growth is connected with the consumption of the hydroxyl groups due to the interaction between the metal deposit and the hydroxylated areas. This finding is in line with photoemission results published earlier.

The interaction of oxygen with alumina-supported palladium particles

Utilizing a combination of molecular beam techniques and scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions we have studied the interaction of oxygen with an alumina-supported Pd model catalyst as well as the influence of the oxygen pretreatment on the kinetics of the CO oxidation reaction. The Pd particles were deposited by metal evaporation in UHV onto a well-ordered alumina film prepared on a NiAl(110) single crystal. The particle density, morphology and structure are determined by STM both immediately after preparation and after oxygen adsorption and CO oxidation. The oxygen sticking coefficient and uptake in the temperature regime between 100 and 500 K and the kinetics of the CO oxidation reaction are quantitatively probed by molecular beam techniques. It is found that starting at temperatures below 300 K the Pd particles rapidly incorporate large amounts of oxygen, finally reaching stoichiometries of PdO>0.5. STM shows, that neither the overall particle shape nor the dispersion is affected by the oxygen and CO treatment. Only after saturation of the bulk oxygen reservoir are stable CO oxidation conditions obtained. In the low-temperature regime (<500 K), only the surface oxygen, but not the bulk and subsurface oxygen is susceptible to the CO oxidation. The activation energies for the Langmuir–Hinshelwood step of the CO oxidation reaction were determined both in the regime of high CO coverage and high surface oxygen coverage. A comparison shows that the values are consistent with previous Pd(111) single crystal results. Thus, we conclude that, at least for the particle size under consideration in this study (5.5 nm), the LH activation energies are neither affected by the reduced size nor by the oxygen pretreatment.

Nucleation and growth of transition metals on a thin alumina film

We have studied the growth of various metals (Pd, Rh, Co and Ir) on a thin well-ordered alumina film in order to elucidate the influence of film structure and deposition conditions on nucleation and growth behaviour. All metals exhibit a three-dimensional growth mode in agreement with thermodynamic considerations. The nucleation is, however, dominated by the defects of the substrate. At a deposition temperature of 90 K, point defects are the primary nucleation centres. At 300 K, the situation is different for some metals (such as Pd and Rh) since decoration of steps and film domain boundaries is favoured under these conditions. This temperature dependence points to a stronger interaction of the diffusing metal atoms with the line defects which, however, can only play a role if the thermal energy is sufficiently high to reach them. Metals which are expected to interact more strongly with the support (such as Ir and Co) do not show such a diversity with respect to their nucleation behaviour.

INFLUENCE OF CO ADSORPTION ON THE MAGNETISM OF SMALL CO PARTICLES DEPOSITED ON AL2O3

Abstract Ferromagnetic resonance under ultrahigh vacuum conditions was used to study the magnetic properties of small cobalt particles deposited on the reconstructed surface of a sapphire single crystal (( 31 × 31 )R±9° Al2O3(0001)). The intensity of the resonance line decreases after CO adsorption but the signal is not totally quenched. Therefore, only the atoms in the outer sphere of the cluster are influenced and a magnetic core still remains. STM measurements of Co particles on a thin alumina film Al2O3/NiAl(110) support this assumption of three-dimensional growth.

Metal particles on oxide surfaces: structure and adsorption behaviour

Although deposited metal particles play an important role as heterogeneous catalysts, there is only limited fundamental knowledge about the relationship between their structure, their adsorption behaviour and their catalytic activity. In this article, we describe a strategy giving access to suitable model systems which can be studied with most surface spectroscopic and microscopic techniques. These systems are based on thin oxide films as supports, onto which metal particles of controllable size are grown by vapour deposition. As concrete examples, the preparation of Ir, Pd and Rh aggregates as well as their interaction with CO and C 2 H 4 will be discussed.