Georgy M. Zhidomirov

Ion-exchanged binuclear Ca2OX clusters, X = 1–4, as active sites of selective oxidation over MOR and FAU zeolites

A new series of calcium oxide clusters Ca2OX (X = 1–4) at cationic positions of mordenite (MOR) and faujasite (FAU) is studied via the isolated cluster approach. Active oxide framework fragments are represented via 8‐membered window (8R) in MOR, and two 6R and 4R windows (6R+4R) possessing one common SiOSi moiety in FAU. Structural similarities between the Ca2OX(8R) and Ca2OX(6R+4R) moieties are considered up to X = 4. High oxidation possibilities of the Ca2O2(nR) and Ca2O3(nR) systems are demonstrated relative to CO, whose oxidation over the Ca‐exchanged zeolite forms is well studied experimentally. Relevance of the oxide cluster models with respect to trapping and desorption of singlet dioxygen is discussed.

First-Principle Study of Adsorption and Desorption of Chlorine on Cu(111) Surface: Does Chlorine or Copper Chloride Desorb?

First-principle density-functional calculations have been applied to study the interaction of molecular chlorine with the (111) plane of copper. Using transition-state search method, we considered the elementary processes (Cl2 dissociation, adsorption, diffusion, association, and desorption) on the chlorinated Cu(111) surface. A systematic study of possible desorption pathways has been carried out for different species (Cl, Cl2, CuCl, CuCl2, and Cu) at various chlorine coverage. As a result, we concluded that chlorine monolayer irrespective of the coverage desorbs in the form of CuCl molecules from step edges.

Computational Differentiation of Brønsted Acidity Induced by Alkaline Earth or Rare Earth Cations in Zeolites

For bi- and trivalent Me(q+) (Me = metal) cations of alkaline earth (AE) and rare earth (RE) metals, respectively, the formation of the nonacid MeOH((q-1)+) species and acid H-Ozeo group, where Ozeo is the framework atom, from water adsorbed at the multivalent Me(q+)(H2O) cation in cationic form zeolites was checked at both isolated cluster (8R or 6R + 4R) and periodic (the mordenite framework) levels. Both approaches demonstrate qualitative differences for the stability of the dissociated water between the two classes of industrial cationic forms if two Al atoms are closely located. The RE forms split water while the AE ones do not, that can be a basis of different proton transfer in the RE zeolites (thermodynamic control) than in the AE forms (kinetic control). The cluster models allow quantitatively explaining nearly equal intensities IHF ∼ ILF of the high frequency (HF) and low frequency (LF) OH vibrations in the RE forms and lowered IHF ≪ ILF in the AE forms, where HF bands are assigned to the Me-OH groups in the RE and AE forms, respectively, while LF bands are assigned to the Si-O(H)-Al groups. The role of electrostatic terms for water dissociation in the RE and AE forms is discussed.

Theoretical Aspects of Methanol Carbonylation on Copper-Containing Zeolites

The experimental data have been considered to match the theoretical mechanisms proposed previously to describe processes of oxidative carbonylation of methanol on copper-containing catalysts. The schemes examined cover methoxy intermediates, carbomethoxy intermediates, carbonates, and Cu(OCH3)2Cu binuclear clusters. The attack of the first methanol molecule on copper carbonate has been simulated in terms of the isolated cluster (8R) model with periodic boundary conditions (on CuMOR zeolite), and parameters of the individual steps involving description of the transition states have been evaluated.

STRUCTURE OF Bi- AND TRINUCLEAR CLUSTERS OF ALUMINUM IONS AT THE CATIONIC SITES OF MORDENITE

The structures and stability of bi- and trinuclear Al clusters at the cationic sites in an 8-membered ring of mordenite are calculated within the cluster approach using the DFT method. Adsorption heats of hydrogen and water, hydrogenation and hydrolysis reactions are compared with similar values for the Ga cluster at the cationic sites of a similar Ga-exchange form of mordenite whose existence is consistent with the experiment. The effect of a different arrangement of lattice aluminum atoms in the 8-membered ring of mordenite is analyzed.

First-Principle Study of Phosphine Adsorption on Si(001)-2 × 1–Cl

This paper presents a density functional theory study for phosphine adsorption on a Si(001)-2 × 1 surface covered by a chlorine monolayer, including adsorption on local defects, i.e., mono- and bivacancies in the adsorbate layer (Cl, Cl2), and combined vacancies with removed silicon atoms (SiCl, SiCl2). Activation barriers were found for the adsorbing PH3 to dissociate into PH2 + H and PH + H2 fragments; it was also established that phosphine dissociation on combined vacancies is possible at room temperature. If there is a silicon vacancy on the surface, phosphorus settles in the Si(001) lattice as PH (if the vacancy is SiCl) or as PH2 (if the vacancy is SiCl2). This paper suggests a method to plant a separate phosphorus atom into the silicon surface layer with atomic precision, using phosphine adsorption on defects specially created on a Si(001)-2 × 1–Cl surface with a scanning tunneling microscope tip.

Detailed Atomistic Modeling of Si(110) Passivation by Atomic Layer Deposition of Al2O3

Typical structural defects were studied theoretically in the course of O → Al → O atomic depositions on the basic Si(110) surface. The defects were determined by analyses of the band gap states and projected densities of the s- and p-states after the deposition aimed to form a Si(110)/SiOX/AlOY/γ-Al2O3 slab. The extent of Si(110) passivation after every deposition step was studied by scanning the band structure calculated using Density Functional Theory with periodic boundary conditions. The atomic structure of the optimized Si(110) surface was compared to the one of Si(100) for which more information is available. Our modeling reproduces most features of the use of trimethylaluminium or any other organic ligand as Al precursor along O2 plasma assisted atomic layer deposition (PA ALD) when the organic ligands are completely oxidized so that their participation can be neglected in the deposition as already shown experimentally. The final oxidation step corresponds to the junction of the slab deposited over Si(110) with a γ-Al2O3 fragment, whose super cell (SC) parameters have been selected to lead to the minimum mismatch. Different examples of either non-satisfactory or accurate junction of the oxidized Si(110) slab and γ-Al2O3 fragment (under two different forms) are discussed aiming to develop a route for understanding the dominant defect types at the interface. Such theoretical work should be the first step for the elaboration of computational tools for the passivation of silicon with amorphous oxides. The latter are mainly formed at the conditions of the PA ALD depositions. The list of formed typical defects at the Si(110)/SiOX/AlOY/γ-Al2O3 boundary is presented and characterized by the projected density of states and respective band structure around the band gap.

Molecular Models of the Stabilization of Bivalent Metal Cations in Zeolite Catalysts

A review of quantum chemical modeling of bivalent metal ion stabilization in zeolites is presented. Location of single metal ions in zeolite cationic positions and formation of polynuclear metal-oxo clusters are considered. Special attention is paid to the stabilization of single bivalent metal ions in the cationic positions with distant separation of the two lattice Al ions forming these exchange positions. It is shown that such a type of cation trapping generates increased number of the Lewis acid sites. Comparative stability and catalytic reactivity of different forms of cation species in zeolites are discussed on the example of Zn/HMFI. Dehydrogenation catalytic reaction of ethane molecule on the single Zn(II) and polynuclear zinc-oxo ions is considered. It is found that binuclear metal-oxo ions can be the effective traps of molecular oxygen and so they can direct the way of oxidation catalysis. This is demonstrated by the theoretical treatment of CO oxidation on alkaline earth zeolites. The CO2 molecule can be activated by the binuclear metal-oxo ions with possibility of further reaction functionalization. The last part of this review is devoted to discussion of the structure of single Fe(II) active sites in Fe/HZSM-5 zeolites. This system has attracted great attention as selective oxidation of hydrocarbons with N2O but up to now the understanding of the active structures remains challenging.