Estudo teórico da interação metal−suporte em aglomerados de paládio depositados sobre as superfícies (110D) e (110C) da γ-alumina

Abstract

The metal−support interaction (MSI) existing in heterogeneous catalysts influences the structure−performance relationship of these materials. Concomitantly with the metal-metal interaction (MMI), the MSI hinders or favors the anchoring of adsorbate species on the catalyst and control the morphology and growth of the metal particle. γ-alumina (γ-Al2O3), an oxide widely used as support, presents a structural arrangement still not well-defined experimentally, being the main discussions regarding the crystal lattice and the distribution of octahedral (Alo) and tetrahedral (Alt) aluminum cations along with the bulk and surface. Quantum-mechanical calculations by Density Functional Theory (DFT) were performed to study the MSI effect in models proposed to represent γ-alumina (spinel-like)-supported palladium catalysts. Two approaches were adopted in this work: I – Analysis of the influence of different surfaces exposed by γ-alumina ((110D) and (110C)) on MSI in Pd4/γ-alumina models, facing the interaction with a fifth palladium atom and with one NO molecule and II – Study of the sequential growth (atom to atom) of a Pdn cluster (n = 1−7) on the (110D) surface of γ-alumina. The main results show that the magnitude of the MSI is twice as high when the (110C) surface is exposed. Distinct arrangements for Pd4 are produced on (110) terminations: boat (110D) and butterfly (110C). Natural Bond Orbital (NBO) calculations show that an electronic MSI (EMSI) is promoted at Pd/(110) interfaces, with Pd4→γ-alumina backdonation being the preferential direction. The EMSI effect modifies the intensity of the MMI, making the butterfly arrangement more cohesive than the boat. The MSI and the MMI act synergistically, decreasing the bonding energy of individual palladium atoms at most sites, mainly on the (110D) surface. Energy Decomposition Analysis (EDA) shows that the nature of the Pd−Al interaction depends on the coordination environment of the cation and the exposed surface of the γ-alumina, while Pd−O is mostly electrostatic. The most acidic sites on surfaces are Alo at (110D) and Alt at (110C). Palladium atoms bonded to oxygen anions also behave as acid centers, attracting adsorbates preferentially. The MSI/MMI effects make the nucleation of a fifth palladium atom more favored on Pd4/(110D), indicating lesser stability of this system face to Pd4/(110C). On the other hand, NO anchors preferentially in bridge − mode that most favors N−O bond break − on (110D) termination only. MSI and MMI effects are competitive along with the growth of Pdn on γ-alumina. From n=6, the MMI starts to command the process, suggesting an energetic favoring of sintering from this point on.