How molecular is the chemisorptive bond?

Abstract

Trends in adsorption energies as a function of transition metal differ for adsorbates that are attached atop a surface atom or are adsorbed onto a high coordination site. When adsorption onto early and late transition metals is compared variation in relative bond energies of adsorbates attached to different sites is large. A theoretical understanding is provided based on the analysis of the electronic structure of the respective chemical bonds. The electronic structure analysis is based on partial density of states (PDOS) and bond order overlap population densities from crystal orbital Hamiltonian population (COHP) calculations available from DFT electronic structure computations. This is complemented by calculations of Bader charge densities and electron density topology properties. Variation of the respective bond energies depends on the symmetry of the molecular orbitals that form the chemical bond. The key electronic structure parameters are the position of the Fermi level in the bonding or antibonding molecular orbital partial density of states region of the chemical bond and chemical bond polarity. These are very different for adsorbates adsorbed onto the same transition metal surface, but which have different coordination with surface metal atoms. The adsorption energies and the respective electronic structures of adatoms H, C and O and molecular fragments CHx (x = 1-3) are compared with those of the analogous molecules that contain a single transition metal atom. When adsorbed atop, trends in bond energies are remarkably similar to those of the corresponding molecules. The difference in bond energies of adsorbates and transition metal molecules, i.e. the embedding energy, is shown to consist of three contributions: quenching of the sometimes high molecular spin states, weakening of the adsorbate-surface interaction energy and weakening of the metal-metal atom bond energies next to the adsorbate. Conventional scaling rules of the interaction energies of adsorbed CHx (0 < x ≤ 3) fragments are satisfied only for adsorbates in high coordination sites. For the early transition metals a breaking of this rule is found for C and CH or N and NH when adsorbed atop a transition metal surface or when they are part of a transition metal molecule. The M-C bond energy is found to be only stronger than that of the M-CH bond as long as the Fermi level or the HOMO is located in the antibonding molecular orbital partial density of states of the chemical bond.

Chemical Causes of Metal Nobleness

“…The analysis thus relies on the linear relationship between M−O and M−O 2 energetics since very few experimental data are known for M−O 2 energies in gas phase. The validity of such a scaling is well established, and thus for simplicity we consider only the surface binding energy of the oxygen atom …”

“…One is the cycle of absolute redox potentials described notably by Trasatti, [68] and the other is the cycle relating adsorption energies to molecular binding as described e. g. by van Santen. [69] These cycles have been combined in Figure 3A, with full account of the free energy conversions. In this analysis the central metal atom in the solid state M(s) has several options to lose its metal character by reaction, and the tendency to do so reflects its reactivity, or inversely, nobleness.…”

Performance of Density Functional Theory for Transition Metal Oxygen Bonds

“…[39,40] Many of the 60 diatomic systems have been studied before at various levels of theory. [20,21,[46][47][48][49][50]22,23,30,34,35,40,44,45] However, this paper presents the first systematic benchmark of DFT for all 3d, 4d, and 5d MO and MO + systems as required to evaluate system dependencies and universality across the groups and periods of the d-block. To probe DFT broadly, we compared 30 different diverse functionals, including local, gradient-corrected, local and global hybrids, meta functionals and range-separated functionals, giving the most systematic and complete picture so far of the ability of DFT to describe MÀ O bonds.…”

Carboxylic acid formation by hydroxyl insertion into acyl moieties on late transition metals

“…For the bare hydroxyl fragment at a bridge site, we chose the second vector parallel to the surface. In the spirit of a previous analysis, 36 we show COOP spectra with orbital interactions decomposed into two contributions: (i) the σ-type components along the first axis (d-p σ ), and (ii) the sum the other the two π-type components (d-p ⊥ ) relative to this axis. As a second aspect of this analysis, we compare the COOP spectra of the IS and the corresponding TS; see Fig.…”

“…To ensure comparability of differently oriented atom pairs, we are treating two types of interactions: (i) the σ-component along the axis between these two atoms (d-p σ ), and (ii) the sum of the two π-contributions (d-p ⊥ ) relative to this axis. 36 All energies here and in the following discussion are relative to the Fermi energy ε F (Fig. 3).…”