Role of surface defects in the activation of supported metals: A quantum-chemical study of acetylene cyclotrimerization on Pd-1/MgO

We report results of first principle density functional calculations on the catalytic activity of isolated Pd atoms deposited on different MgO surface sites. The reaction of interest is the cyclization of acetylene to benzene . 3C(2)H(2) --> C6H6 Experimentally, it has been observed that the reaction is catalyzed by size-selected Pd clusters and that even a single Pd atom d eposited on MgO is enough for the reaction to occur. In this theoretical st udy we have analyzed in detail the role of the support. It is found that a gas-phase Pd atom is not active in promoting the reaction as it has not eno ugh electron density to bind and activate three acetylene molecules. The re action, however, occurs when Pd is bonded to low-coordinated oxygen anions of the surface, located at sites such as steps and corners. Oxygen anions o n (001) terraces, in fact, are not a sufficiently good electron donor to in crease the electron density on the metal. Another group of surface defects which play a role in the activation of the Pd atom are F centers, oxygen Va cancies with two trapped electrons. These defect centers, independently of their location, terrace, edge, or corner, are very strong "basic" sites: th ey efficiently transfer electronic charge to the adsorbed metal atom and th us improve its catalytic propel-ties. This study demonstrates the importanc e of morphological defects and anion vacancies at the metal-oxide interface and the noninnocent role of the substrate in catalysis by supported metal particles.