An experimental setup combining a highly sensitive detector for reaction products with a mass-selected cluster source and a low-temperature STM for advanced nanocatalysis measurements

We report on a home-built detector for catalytic reaction measurements offering good gas isolation from the surrounding ultrahigh vacuum components, high sensitivity for reaction products and a fast response time of 10 ms enabling dynamic studies correlated to reactant gas pulses. The device is mounted in ultrahigh vacuum and combined with a low-temperature scanning tunneling microscope and a source for the deposition of mass-selected clusters. This combination allows for a direct correlation between surface morphology and catalytic properties of model catalysts. The performances of the new detector are illustrated by measurements on two model systems. Thermal desorption spectroscopy of CO carried out on morphologically well characterized Pt on TiO2(110)-(1×1) reveals several desorption features, which can be attributed to different surface sites. Catalytic CO oxidation performed by alternatingly pulsing isotopic CO and O2 on a Pt film on yttria stabilized zirconia reveals the CO or O rich temperature regimes. The CO2 production rate correlated with either one of the reactants can perfectly be reproduced by a kinetic reaction model giving access to the respective adsorption energies.

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

Cluster-Surface Interaction and Catalytic Activity

Simon Bonanni

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.

EPFL2012

Reaction-Induced Cluster Ripening and Initial Size-Dependent Reaction Rates for CO Oxidation on Pt-n/TiO2(110)-(1x1)

Simon Bonanni, Harald Brune, Wolfgang Harbich

We determined the CO oxidation rates for size-selected Pt-n (n is an element of {3,7,10}) clusters deposited onto TiO2(110). In addition, we investigated the cluster morphologies and their mean sizes before and after the reaction. While the clusters are fairly stable upon annealing in ultrahigh vacuum up to 600 K, increasing the temperature while adsorbing either one of the two reactants leads to ripening already from 430 K on. This coarsening is even more pronounced when both reactants are dosed simultaneously, i.e., running the CO oxidation reaction. Since the ripening depends on the size initially deposited, there is nevertheless a size effect; the catalytic activity decreases monotonically with increasing initial cluster size.

American Chemical Society2014