Flat monolayer clusters of platinum on graphite show one order of magnitude higher catalytic activity than taller Pt clusters for the H2-D2 exchange reaction. Significantly lower CO adsorption energy (73 kJ/mol) has also been found on the monolayer Pt clusters deposited on the terrace of the graphite surface. Based on the measurements of scanning tunneling microscopy, scanning tunneling spectroscopy and inelastic electron tunneling spectroscopy, the origin of such specific characteristics of the monolayer Pt cluster has been ascribed to the Pt-C hybridization, which leads to a shift of the d-band center of platinum away from the Fermi level.