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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.