Nanometer-sized Pt particles were prepared on an atomically flat surface of rutile TiO2. Trimethyl acetate (TMA) adsorbed on the Pt-modified surface was photochemically decomposed under ultraviolet light irradiation in a vacuum. Residing TMA anions were imaged by a scanning tunneling microscope to deduce the local rate of decomposition. Increasing the number density of Pt particles led to an enhancement of the initial reaction rate. The degree of this enhancement did not depend on the distance from the Pt particles.