Abstract <p>Experimental means of surface analysis are used to investigate in situ the adsorption and reaction of nitrogen oxide (NO) molecules on the surfaces of a model metal–oxide system. The system is formed through the controlled deposition of nickel clusters under conditions of an ultrahigh vacuum on the surfaces of α-Al₂O₃(0001) aluminum oxide thin films grown on a Mo(110) substrate. X-ray photoelectron and Auger electron spectroscopy (XPS, AES), infrared Fourier spectroscopy (IFS), and temperature-programmed desorption (TPD) data reveal a conditional 2 nm size of Ni clusters that separates the character of the electronic state of NO molecules adsorbed on their surfaces and their reactive capability. It is shown that a distinctive feature of Ni clusters with typical diameters of less than 2 nm is the adsorption of NO molecules on their surfaces in the form of (NO)₂ dimers. In contrast, adsorption produces (NO) monomers on clusters of larger size. It is concluded that this difference is the reason for the different reaction behavior of the molecules. A key difference between clusters smaller and larger than 2 nm in size is that in the former, N₂O molecules form upon heating the system and are desorbed into the gas phase. This does not occur in the latter. The formation of N₂O is due to the mutual influence of NO molecules forming (NO)₂ dimers under the action of the metal–oxide interface. Results indicate the possibility of tuning the catalytic efficiency of the metal–oxide system by varying the size of the applied metal clusters.</p>