Cluster-Surface Interaction and Catalytic Activity

This thesis is devoted to the study of the catalytic activity of small supported metal clusters. The TiO2(110)-(1×1) supported platinum cluster activity towards CO oxidation, its correlation with the cluster morphology, support properties, and the cluster stability are studied. For this purpose a new ultra-high vacuum (UHV) compatible reactor has been developed, which, in combination with a low temperature scanning tunneling microscope, a size selected cluster source and standard techniques for sample preparation, constitutes a unique experimental setup that allows for in situ investigations of the morphology and catalytic properties of size-selected cluster based model catalysts. First, large platinum clusters (100 to 200 atoms per cluster), grown from deposited atoms, were investigated. This system initially shows a low catalytic activity, due to the formation of a TiOx (0 < x < 2) encapsulation layer on the clusters upon annealing, known as strong metal-support interaction (SMSI) state. We observed that a soft sputtering and a second annealing to 1100 K give rise to a highly active catalyst for CO oxidation. We attribute the activation of the catalytic activity to the destruction of the encapsulation layer by the soft sputtering and to the absence of its reformation during the second annealing. This procedure could in principle be used to activate other initially passive SMSI systems. Second, the effect of the reduction state of the TiO2 support on the catalytic activity of the supported Pt clusters has been analyzed. It is demonstrated that small platinum clusters (in this case Pt7) are two orders of magnitude more active toward CO oxidation when they are supported on slightly reduced TiO2 crystals than when they are supported on strongly reduced ones. This is attributed to the consumption of the oxygen absorbed on the surface by the crystal interstitial titanium atoms, which are present in higher concentration in strongly reduced crystals than in weakly reduced ones. The effect could take place also for other catalytic processes involving oxygen on TiO2 supported metal clusters and allow for the tuning of oxidation and de-oxidation reactions. Platinum cluster size effects on the catalytic CO oxidation have been investigated depositing Pt3, Pt7 and Pt10 clusters on TiO2(110)-(1×1). From Pt3 clusters a CO2 production, per platinum atom, two times bigger than the one from Pt7 and Pt10 has been observed. The cluster morphology was strongly modified by the exposure to the reaction environment (temperatures up to 600 K and CO and O2 doses). This highlights the necessity to study the stability of small platinum clusters as a function of the temperature and gas exposure. Finally, as suggested by the preceding measurements, the stability of Pt7 clusters on TiO2 has been investigated as a function of the temperature, from 300 K to 600 K, and of the exposure to gases. Pt7 clusters are proved to be stable in UHV up to 430 K on the TiO2(110)-(1×1) surface. If the annealing is performed while exposing the sample to gases (CO, O2 and a mixture of the two) important cluster sintering takes place already at 430 K. This effect can be attributed to a modification of the cluster-support interaction induced by the adsorption of gases and to the additional energy injected in the system by the CO combustion taking place on the clusters when both CO and O2 are dosed.

Transition metal clusters on h-BN/Rh(111)

Hamed Achour

The main aim of this dissertation is to study the catalytic aspects of very small transition metal clusters supported on h-BN/Rh(111). The catalytic reactions of interest were ammonia synthesis and CO oxidation on mass selected and soft landed iron and platinum clusters, respectively. A special focus was devoted to the investigation of the stability of these very small clusters on h-BN before and after the reaction. It was found that Pt7 clusters soft landed at room temperature with an energy of 1.2 eV per atom and annealed to 700 K exhibit Smoluchowski ripening. Above this temperature, these clusters undergo partial intercalation between the h-BN monolayer and the Rh(111). The intercalation initiates at 900 K and becomes more pronounced when the clusters are annealed under gas reaction conditions. A high catalytic activity was observed when the Pt catalyst remains supported on h-BN. In this case, the reaction starts at 480 K and follows Langmuir-Hinshelwood mechanism. However, the catalytic activity strongly reduces when the Pt clusters undergo partial intercalation. Nevertheless, in this case the reaction starts at only 380 K, revealing a reduction by 100 K in the Pt poisoning as a result of substrate effect and charge redistribution. In a second trail, the cluster-h-BN interaction was studied through h-BN irradiation with Pt clusters, at room temperature, within an energy window of 30-416 eV/atom. The results show that even irradiation at 30 eV/atom can lead to entire non thermal intercalation of the clusters. The energetic Pt clusters were found to be site selective as they settle only under the h-BN wires, in contrast to soft landed Pt7 clusters that were found to settle at the side edge of the h-BN depressions. After being exposed to irradiated Pt clusters, at an energy above 100 eV/atom, the h-BN layer starts to display visual cavity-type defects. These defects, induced by collision, disappeared after annealing to 600 K under gas reaction leaving behind an h-BN with no visual defects. CO temperature desorption spectroscopy (TDS) indicates that the intercalated Pt is inactive towards CO oxidation due to h-BN screening. Subsequently, we were able to produce a catalyst system made of soft-landed Pt/h-BN/intercalated Pt/Rh(111) using a combination between soft and energetic Pt deposition with which a reduction by 100 K in CO poisoning was obtained. Finally, ammonia synthesis was conducted on soft landed Fe clusters, supported on h-BN/Rh(111) and deposited at 100 K under Ultra High Vacuum (UHV). Scanning tunneling microscopy (STM) displays two imaging states of the as deposited clusters; a ring state surrounding h-BN depression which disappears above 300 K and a dot like structure. The Fe clusters are found to grow by Ostwald ripening after annealing to 600 K and above this temperature, they endure partial intercalation. It was found that, in the presence of iron, the nitrogen reaction gas experienced an exchange with the nitrogen species forming the h-BN layer. TPR (Temperature Programmed Reduction) of N

_2

with hydrogen revealed that nitrogen reduction occurs at 620 K under high vacuum following the Haber-Bosch method. The detection of reaction intermediates NH and NH

_2

together with desorbed atomic nitrogen, during ammonia synthesis, confirms that NH

_3

formation involves the stepwise hydrogenation of adsorbed nitrogen.

EPFL2017

Submonolayer growth of cobalt on metallic and insulating surfaces studied by scanning tunneling microscopy and kinetic Monte-Carlo simulations

Philipp Buluschek

Submonolayer epitaxial growth is obtained by the deposition of less than a complete layer of atoms on a single crystal surface. It is of fundamental interest for industrial applications (e.g. in the semiconductor industry) as well as from the point of view of basic research. For example, it is known that nanometer-sized atomic structures (nanostructures) exhibit remarkable physical and chemical properties which can differ greatly from those of bulk matter. In order to investigate these properties, it is often necessary to create large quantities of well defined and possibly spatially ordered nanostructures. One way to achieve this result is self-organized growth where one deposits atoms on a clean crystalline surface and lets the growth process evolve freely. Here, nanostructures result from the atomic diffusion and aggregation processes taking place at the surface. Understanding the exact nature of these processes is of ongoing interest in the field of nanostructure growth. In this thesis we report about submonolayer growth experiments of the ferromagnetic transition metal cobalt. Cobalt was chosen because it exhibits remarkable magnetic properties. These experiments were performed in an ultra-high vacuum chamber and molecular beam epitaxy was used to grow the nanostructures. Observations of the surface were made using a variable-temperature scanning tunneling microscope (STM). In order to get a better understanding of the atomic processes happening at the surface, we developed two adapted computational simulation methods. Kinetic Monte-Carlo (KMC) simulations were used to get an atomistic picture of the surface while mean field rate equations were integrated numerically to yield cluster densities. We study the growth of cobalt on three different surfaces. By depositing Co on a clean Pt(111) crystal surface, we observe that the platinum surface reconstructs by forming star-shaped partial dislocations for sample temperatures above 180 K (-93°C). We also observe that island densities deviate from predictions of all known models towards higher values for these same temperatures. By simulation we are able to show that insertion of Co into the topmost platinum layer creates a repulsive network of dislocations. We show that these dislocations act as diffusion inhibiting barriers and thus influence the island density by constraining the free movement of atoms at the surface. We also show that the Co dimer and trimer diffuse on Pt(111) before dissociating and are able to extract corresponding activation barriers. We also study the deposition of Co on a Ru(0001) surface which shows a more conventional temperature dependence. By comparing simulation and experiment we extract all relevant diffusion activation barriers and show that at high temperatures the Co dimer and trimer dissociate. Finally, we investigate the growth of Co on the hexagonal boron nitride superlattice which forms on a Rh(111) surface. On this surface Co clusters form three-dimensional hemispherical dots. We show how the substrate corrugation influences diffusion of Co atoms and that clusters up to the size of the pentamer can diffuse. By simulation, we also extract the relevant migrations and desorption barriers.

EPFL2008

Propriétés magnétiques de nanostructures de cobalt adsorbées

Nicolas Weiss

This thesis reports results on magnetic properties of supported cobalt nanostructures. The nanostructures were grown on single crystal metal surface by Molecular Beam Epitaxy in an Ultra High Vacuum chamber. Our experimental setup is based on two in situ measurement techniques. The first is the Scanning Tunneling Microscope (STM) allowing to investigate the nanostructure morphology. The second, called Surface Magneto-Optical Kerr Effect (SMOKE), probes the magnetism of the nanostructures. By combining the data obtained with both techniques we were able to investigate the magnetism of 2D nanoparticles down to the atomic level. The first aim of the thesis consisted in disclosing the role played by the differently coordinated atoms in determining the Magnetic Energy Anisotropy in 2D nanostructures. To this purpose, we performed experiments on the Co/Pt(111) system, chosen as a model system since the Pt surface is well known to increase the out-of-plane uniaxial anisotropy as well as the magnetic moment of the Co islands. A preliminary work has been devoted to the growth of the Co islands as a function of the deposition and annealing temperatures. As a result, we learned to grow three different island's shapes : ramified, compact and double-layer islands. We then concentrated on the magnetism of these islands. Due to the nonlinear relationship between perimeter length and surface area, we were able to distinguish the different contributions to the anisotropy energy of surface and perimeter atoms. We found that the magnetic anisotropy in 2D Co nanostructures is predominantly due to the low-coordinated edge atoms, for which the anisotropy energy can be as large as 20 times the bulk value. This finding opens new possibilities to separately tune the anisotropy and moment of nanostructures. To exemplify this, and to illustrate once more the role of perimeter atoms, we produced Co island with a non-magnetic Pt core. As expected, these bimetallic islands have identical anisotropy and lower magnetic moment to their equally shaped pure Co counterpart. The second aim we focused on was the investigation of the magnetic properties of ultra-high density arrays of nanostructures created by self-assembly. We chose Co/Au(788) as a model system since the nanoscopic Co dots were found to have a very narrow size distribution and to be placed onto a lattice which is phase coherent on a macroscopic length scale. Moreover, the array has a density of 26 Tdot/in2 which allows the investigation of inter-particle interaction in ultra high density arrays. We showed that the narrow size distribution of the dots corresponds to an unprecedented narrow Magnetic Anisotropy Energy distribution with 35% width at half of the maximum. Moreover, we demonstrated the weak magnetic coupling between the Co dots despite their high density. In parallel to these fundamental researches, we developed a new experimental setup that will replace the current one. We designed a UHV chamber equipped with a homemade variable temperature STM combined with a SMOKE experiment. A quadrupole, focused on the sample, generates a 0.35 T magnetic field. This field is one order of magnitude larger than the present one and can be uniformly rotated in the plane of the quadrupole. Furthermore, the design of the new chamber allows STM imaging in this magnetic field. In this report, we present the adopted solutions as well as the first tests of the components (STM, SMOKE, cryostat).

EPFL2004