Structure Sensitivity of NO Adsorption-Dissociation on Pdn (n=8, 13, 19, 25) Clusters

NO adsorption and dissociation on subnanometer Pd-n (n = 8, 13, 19, 25) clusters were first studied with GGA-DFT. The transition structures of the NO dissociating on the potential-energy surfaces were derived using the climbing image nudged-elastic-band (CI-NEB) method. The preferred NO adsorption positions are bridge sites on Pd-n (n = 8, 19) and hollow sites on Pd-n (n = 13, 25). The NO adsorption energy values on hollow sites of icosahedron-based Pdn (n = 13, 25) are relatively higher than that on the bridge site of octahedral Pd-19. However, the NO dissociation barrier on octahedral Pd-19 is lower than that on icosahedron-based Pd-n (n = 13, 25) clusters. These results suggest on a Pd-n (n = 8, 13, 19, 25) scale the NO activity may no longer rely on the cluster size but rather on the geometric structure of Pd-n clusters. The coordination number of the NO adsorption site is found to be the key factor to determine the structure sensitivity of NO adsorption and dissociation. The charge difference and Hirshfeld charges reveal that the charge transfer is from the Pd-n clusters to NO and increases upon NO dissociation. PDOS reveals that the 3 sigma, 4 sigma, 5 sigma, 1 pi, and 2 pi peaks of NO are sensitive not to Pd-n cluster size but to NO adsorption sites. Our calculations may provide an insight into structure-sensitive Pd-based catalysts for NO removal on a subnanometer scale.