This thesis presents the results obtained from experiments performed in an Ultra High Vacuum (UHV) apparatus, named Parallel Screening, consisting of a preparation chamber combined to a High Pressure Cell (HPC) for testing catalytic activity at 1 bar. Two catalytic reactions have been carried out on different transition metals and metal alloys supported on sputtered Highly Ordered Pyrolytic Graphite (HOPG). The first is the H-D exchange reaction, used as probe reaction for H2 dissociation, which is relevant for Proton Exchange Membrane (PEM) fuel cells. Experiments have been performed on Pt, Ru, and Rh nanoparticles and on Pt/Ru alloys in the pure H2/D2 mixture and also in the presence of 10 ppm CO, since this is a poison for the anode of the PEM fuel cells, where the H2 dissociation takes place. It is found that the activity for Ru and Rh is strongly dependent on the particle size, with the larger particles being more active, whereas the activity for Pt does not show any dependence on size. The apparent energy of desorption shows a dramatic increase with decreasing particle diameter for Ru and Rh, whereas it is only weakly dependent on particle diameter for Pt. It is suggested that the strong variation for Ru and Rh is due to the formation of compressed hydrogen adlayers on the terraces of the larger particles. In the case of the Pt/Ru alloys, it is found that the activity exceeds the values measured on the individual metals and have a maximum on alloys with surface composition ratio of Pt:Ru equal to 1:1. In the presence of CO the exchange rate decreases significantly, for all the metals. Alloying Pt with Ru improves signicantly the resistance towards CO poisoning with respect to pure Pt and the resistance is found to increase with increasing amount of Ru in the alloys. The results from the exchange reaction on the nanoparticles and on alloys have been summarized in two scientific articles which have been recently published [1, 2]. The exchange reaction is carried out on supported catalysts. The effect of the interaction between metals and support on the catalytic activity has been investigated in the case of Pt and Ru deposited on HOPG, which were annealed to high temperatures in the UHV chamber. It is found that upon annealing the amount of carbon present in the films increases up to 95%, as derived by surface analysis, indicating the formation of a carbon layer on the metal films. The exchange rate decreases dramatically with increasing carbon content in the films for both metals, indicating a decrease in the ability of the films to adsorb hydrogen. It is also shown that performing He+ sputtering enables the removal of the carbon layer and regenerates the Pt catalysts. The results from this investigation have been summarized in a scientific article which has been submitted to the Journal of Physical Chemistry. The second reaction investigated in this thesis is the NH3 decomposition reaction, relevant for hydrogen storage and production in a COx free environment, which has been performed on Ru, Ir, Rh and Pt thin films, on Ru nanoparticles and on Ir/Ru alloys. Preliminary results indicated that Ru is the most active among the selected metals, but no definitive conclusions can be drawn on the effect of the particle diameter on the decomposition rate. In the case of the Ru/Ir alloys, it is found that the activity exceeds the values measured on the individual metals and have a maximum on alloys containing roughly 10% Ir on the surface.