Reactivity of metal nanoclusters: nitric oxide adsorption and CO+NO reaction on Pd/MgO model catalysts

Abstract Palladium clusters epitaxially grown on MgO(100) have been used as model catalysts to study the oxidation of CO by NO. The size distribution and the shape of the Pd particles are determined by transmission electron microscopy (TEM). Three samples corresponding to particle mean sizes of 3, 7 and 16 nm have been studied. The adsorption of NO and its reaction with CO have been studied, at low pressure, using a molecular beam of NO and an isotropic pressure of CO. NO dissociates on all the particles between 170°C and 450°C. During the reaction, CO 2 and N 2 are produced with a little amount of N 2 O below 300°C. The curve of the steady state reaction rate as a function of the temperature has a “volcano” shape. The maximum of activity is shifted toward higher temperature when the CO pressure increases. The turnover rate for CO 2 production increases when particle size decreases. However, as the reaction is limited at high temperature by NO and CO adsorption, it is necessary to correct the reaction rate by the fact that a large part of the reactant molecules adsorbed on the Pd clusters comes from the capture of molecules physisorbed on the support. After this correction, which depends on particle size, the medium-sized particles appear to be the more active. A possible explanation based on the particle shape is discussed.