Periodic DFT+D molecular modeling of the Zn−MOF−5(100)/(110)TiO2Zn-MOF-5(100)/(110)TiO_{2} interface : electronic structure, chemical bonding, adhesion and strain

Electronic structure, bonding characteristics, adhesion, and stress energy of the Zn-MOF-5(100)/(110) rutile interface were modeled by using periodic DFT+D calculations, corroborated by simulation of high resolution transmission electron microscopy (HR-TEM) images. Adjustment of the flexible metal–organic framework (MOF) moiety to the rigid rutile substrate was achieved within a supercell comprised of (1 × 1) Zn-MOF-5 and (4 × 9) TiO2 units. It was shown that binding of the Zn-MOF-5 layer takes place via bidentate 1,4-benzenedicarboxylate (BDC)–titania bridges. A coherent interface can be formed with the minimal periodicity along the [11̅0] direction defined by nine Ti5c adsorption sites (9 × 2.96 Å = 26.64 Å) and two consecutive linkers of the Zn-MOF-5 chain (2 × 12.94 Å = 25.88 Å). The MOF part is tuned to the oxide substrate by tilting the BDC linkers by 10° and twisting around their long axis by 34°. The resultant lattice strain of the Zn-MOF-5 layer was equal to ε[001] = 0.31% and ε[11̅0] = 2.86%, and the associated stress energy to σtotal = 4.8 eV. Pronounced adhesion energy of the Zn-MOF-5 layer deposited on the rutile surface (−0.33 eV/nm2) stems from the sizable dispersion (−0.39 eV/nm2) contribution, counterbalancing the unfavorable lattice strain and bonds distortion components. The calculated density of states structure of the Zn-MOF-5(100)/(110)TiO2 interface revealed that it can be described as an electronically coupled, staggered (Type II) charge injection system, where a photoinduced electron may be directly transferred from the Zn-MOF-5 moiety to the conduction band of the titania substrate

Mono- and diatomic reactive oxygen species produced upon O2O_2 interaction with the (111) facet of cobalt spinel at various conditions : molecular DFT and atomistic thermodynamic investigations

Periodic spin unrestricted DFT + U calculations joined with atomistic thermodynamics were used to study the location, structure, and stability along with the electronic and magnetic properties of various surface oxygen species and oxygen vacancies, produced under different thermodynamic conditions on the (111) surface of the cobalt spinel nano-octahedra. The density of state alignment diagrams between dioxygen and cobalt centers were used to rationalize speciation of the surface oxygen varieties into diatomic superoxo (μ-CoO3c–O2–CoT3c, η2-O2–CoO3c) or intrafacial peroxo ([O–Osurf.]2–) and monoatomic metal–oxo (CoT3c–O, CoO3c–O) entities. It was shown that the surface cobalt cations work in tandem constituting dual CoO3c–CoT3c sites for O2 adsorption, where the CoO3c dxz(β) and dyz(β) states act as spin-polarized electron donor centers, producing the most stable bridging μ-CoO3c–O2–CoT3c adducts (ΔEa = −1.86 eV). The single site mono- and bidentate binding modes η1-O2–CoO3c (ΔEa = −1.66 eV) and η2-O2–CoO3c (ΔEa = −1.12 eV) are less stable. These results imply that the most probable pathway of dioxygen activation involves an η1-O2–CoO3c → η2-O2–CoO3c → μ-CoO3c–O2–CoT3c sequence. Subsequent dissociation of the O–O bond in the bridging peroxide moiety leads to the formation of ferromagnetically coupled [↑CoO3cIVO↑]3+ and [↑↑↑CoT3cIIO↑]+ species, characterized by a large difference of their stability (ΔEa = −1.24 and −0.43 eV respectively). First principles thermodynamic modeling revealed that in typical catalytic pressures of dioxygen (p O2/p° ∼ 0.01 ÷ 1), the most stable bridging μ-CoO3c–O2–CoT3c species persist on the surface below 375 °C, whereas above this temperature the surface is covered with monoatomic species (CoO3c–O stable up to 475 °C). At T > 475 °C, bare and then oxygen vacancy bearing (T > 575 °C) surfaces are thermodynamically preferred. The cobalt–oxo species may diffuse across the surface with the involvement of the thermodynamically metastable O2O,1T–O and O3O–O peroxy species. The obtained results provide a convenient conceptual background for the interpretation of the redox catalytic and electrocatalytic processes over cobalt spinel with dioxygen participation

Copper ionic pairs as possible active sites in N2ON_{2}O decomposition on CuOx/CeO2CuO_{x}/CeO_{2} catalysts

Synthesized by impregnation and coprecipitation methods, ceria-supported copper samples were tested in catalytic decomposition of N 2 O (deN 2 O) under dry and wet conditions. Basing on the structural (XRD, SEM), spectroscopic (EPR, RS) and catalytic characterization, supported by DFT calculations, the role of mono- and dimeric copper as active centers in the deN 2 O reaction was evaluated. Particular attention was paid to elucidation of the structure and localization of the Cu 2+ Cu 2+ pairs within the CeO 2 matrix, their stability and chemical conditions of their formation

Cobalt Spinel at Various Redox Conditions: DFT+U Investigations into the Structure and Surface Thermodynamics of the (100) Facet

Periodic spin unrestricted DFT-PW91+U calculations together with ab initio thermodynamic modeling were used to study the structure, defects, and stability of different terminations of the (100) surface of cobalt spinel under various redox conditions imposed by different oxygen partial pressure and temperature. Three terminations containing under-stoichiometric (100)-<b>O</b>, stoichiometric (100)-<b>S</b>, and overstoichiometric (100)-<b>R</b> amount of cobalt ions were analyzed, and their atomic and defect structure, reconstruction, and stability were elucidated. For the most stable (100)-<b>S</b> and (100)-<b>O</b> facets, formation of cationic and anionic vacancies was examined, and a surface redox state diagram of possible spinel (100) terminations in the stoichiometry range from Co_2.75O₄ to Co₃O_3.75 was constructed and discussed in detail. The results revealed that the bare (100)-<b>S</b> surface is the most stable at temperatures and pressures of typical catalytic processes (<i>T</i> ∼ 200 °C to ∼500 °C, <i>p</i>_O2/<i>p</i>° ∼ 0.001 to ∼1). In more reducing conditions (<i>T</i> > 600 °C and <i>p</i>_O2/<i>p</i>° < 0.0001), the (100)-<b>S</b> facet is readily reduced by formation of oxygen vacancies, whereas in the oxidizing conditions (<i>T</i> < 200 °C and <i>p</i>_O2/<i>p</i>° > 10), coexistence of (100)-<b>S</b> and (100)-<b>O</b> terminations was revealed. Formation of the oxygen vacancies involves reduction of the octahedral trivalent cobalt and is accompanied by migration of the divalent tetrahedral cobalt into empty, interstitial octahedral positions. It was also found that the constituent octahedral Co cation proximal to the interstitial cobalt adopts a low spin configuration in contrast to the distal one that preserves its surface high spin state. In the case of the Co depleted surfaces, the octahedral vacancies are thermodynamically disfavored with respect to the tetrahedral ones in the whole range of the examined <i>T</i> and <i>p</i>_O2 values. The obtained theoretical results, supported by TPD-O₂ and TG experiments, show that the octahedral cobalt ions are directly involved in the redox processes of Co₃O₄