Cluster embedding in an elastic polarizable environment: Density functional study of Pd atoms adsorbed at oxygen vacancies of MgO(001)

Abstract

Adsorption complexes of palladium atoms on F s , F s ϩ , F s 2ϩ , and O 2Ϫ centers of MgO͑001͒ surface have been investigated with a gradient-corrected ͑Becke-Perdew͒ density functional method applied to embedded cluster models. This study presents the first application of a self-consistent hybrid quantum mechanical/molecular mechanical embedding approach where the defect-induced distortions are treated variationally and the environment is allowed to react on perturbations of a reference configuration describing the regular surface. The cluster models are embedded in an elastic polarizable environment which is described at the atomistic level using a shell model treatment of ionic polarizabilities. The frontier region that separates the quantum mechanical cluster and the classical environment is represented by pseudopotential centers without basis functions. Accounting in this way for the relaxation of the electronic structure of the adsorption complex results in energy corrections of 1.9 and 5.3 eV for electron affinities of the charged defects F s ϩ and F s 2ϩ , respectively, as compared to models with a bulk-terminated geometry. The relaxation increases the stability of the adsorption complex Pd/F s by 0.4 eV and decreases the stability of the complex Pd/F s 2ϩ by 1.0 eV, but it only weakly affects the binding energy of Pd/F s ϩ . The calculations provide no indication that the metal species is oxidized, not even for the most electron deficient complex Pd/F s 2ϩ . The binding energy of the complex Pd/O 2Ϫ is calculated at Ϫ1.4 eV, that of the complex Pd/F s 2ϩ at Ϫ1.3 eV. The complexes Pd/F s and Pd/F s ϩ exhibit notably higher binding energies, Ϫ2.5 and Ϫ4.0 eV, respectively; in these complexes, a covalent polar adsorption bond is formed, accompanied by donation of electronic density to the Pd 5s orbital.

Density Functional Embedded Cluster Study of Cu4, Ag4 and Au4 Species Interacting with Oxygen Vacancies on the MgO(001) Surface

“…For this purpose, we performed density functional calculations of cluster models, applying our recently developed approach of cluster embedding in an elastic polarizable environment. [10] All tetramers exhibit a distorted rhombus structure, favoring the same adsorption mode on both vacancy sites, where the adsorbed metal particles are oriented upright, perpendicular to the (001) surface, with one of their atoms bound to the defect and another one attached to a nearby surface oxygen anion. Similarly to dimer and trimer complexes of the coinage metals, Au 4 exhibits the strongest adsorption interaction on MgOA C H T U N G T R E N N U N G (001) among all tetramers under scrutiny, followed by Cu 4 and Ag 4 .…”

“…Some coalescence processes on This work rounds off a series of our systematic theoretical studies of small MgO-supported transition metal particles, in particular, those formed by coinage metals. [10,[12][13][14][15] A variety of data, obtained with the same accurate computational strategy, enabled us to quantify structural and binding parameters of adsorbed coinage metal particles and to draw some general conclusions which are expected to be applicable also to similar systems of other metals. For instance, we have shown that agglomeration (sintering) processes of the smallest metal species depend in a complicated way on the electronic state, nuclearity, and the type of adsorption site of the "reactant" and "product" complexes.…”

“…[10] A specially designed QM/molecular mechanical (MM) interface ensures a consistent description of the mutual influence of electronic and geometric changes that the QM cluster region and its environment undergo relative to the regular unperturbed surface. [10] We employed a Mg 9 O 8 A C H T U N G T R E N N U N G (Mg pp* ) 12 cluster as QM part of the system, modeling F s and F s + defect sites on the MgOA C H T U N G T R E N N U N G (001) surface. Here, Mg pp* labels pseudopotential centers Mg 2 + , entirely without electrons, which saturate the coordination spheres of O anions at the cluster boundaries.…”

“…[10,11] These methods allow one to describe accurately adsorption on metal-oxide surfaces of varying degrees of covalence while taking relaxation effects into account. We employed EPE embedding for ionic oxides to elucidate the adsorption of isolated Pd atoms on regular sites and oxygen vacancies of MgOA C H T U N G T R E N N U N G (001).…”

“…We employed EPE embedding for ionic oxides to elucidate the adsorption of isolated Pd atoms on regular sites and oxygen vacancies of MgOA C H T U N G T R E N N U N G (001). [10] We went on to apply the EPE method in a consistent fashion to investigate systematically the adsorption of 17 single d-metal atoms across the periodic table with the regular O 2À sites on MgOA C H T U N G T R E N N U N G (001) terraces [12] as well as with F s and F s + oxygen vacancies. [13] These studies showed that, at variance with general belief, some d-metal atoms do not form stronger bound adsorption complexes with F-type surface defects than with regular sites and that metal atoms in M 1 /F s complexes accumulate considerable amount of electron density, which governs the trend of the adsorption energies.…”