The effect of the alkaline environment on the oxidation of glucose into gluconic acid (gluconate ion) without the participation of a catalyst was studied by the density functional theory. It has been shown that the solvation environment destabilizes the intermediates of the reaction and, first of all, the glucose diolate R-CHO(OH)& oline;, the formation of which becomes unlikely. As a consequence, under real catalytic conditions, glucose diolate should be formed on the surface of the catalyst. Destabilization of intermediates, as well as an increase in the energy difference between the triplet and singlet states of the R-CHO(OH)& oline;... O2 complex in the solution enhances the activation barrier in an alkaline environment to 140 kJ/mol (101 kJ/mol in the gas phase). Of the two main factors, glucose diolate instability and high activation barrier, the first of them plays a decisive role in preventing the reaction from taking place in solution.

The authors summarize results from calculations using the density functional theory for atoms and small silver clusters on surfaces of nanostructured cerium(IV) oxide, along with the adsorption and transformations of O2 and CO molecules on these systems. Stoichiometric Ce21O42, which has {100} and {111} nanofacets with adsorption centers containing four and three oxygen atoms, is used to model surfaces of cerium oxide. It is shown the O4-center is a center of the selective adsorption of metal atoms. A silver atom on an O3‑center is less stable but it shows a greater ability to activate an O2 molecule. Results from calculations on the {100} and {111} faces of Ce21O42 nanoparticles are compared to data for infinite CeO2(100) and CeO2(111) surfaces. The efficiency of Ag/Ce21O42 atomic complexes is shown in the oxidation of carbon monoxide.

Density functional theory and the extended charge decomposition analysis (ECDA), the natural population analysis (NPA) as well as the quantum theory of atoms in molecules (QTAIM) were used to gain insight into the nature of metal-metal and metal-ligand interactions in binuclear manganese-platinum vinylidene complexes with phosphine ligands at the Pt atom: Cp(CO)(2)MnPt(mu-C=CHPh)(P-P) (1 P-P=(PPh3)(2), 2 P-P=dppm). The complexes 1 and 2 can be represented as pi-complexes of the Pt-phosphine moiety (M) with the metalla-allene ligand Cp(CO)(2)Mn=C=CHPh (L). The substitution of PPh3 groups at the Pt atom by dppm ligand leads to a growth of the L <- M back-interaction as the result of approaching the CO group at the Mn atom to the Pt atom induced by the decrease in steric disincentives caused by phosphine ligands. Topological analysis of the charge density points to the indirect metal - metal bonding mediated by a couple of the 2e-2c metal -carbon interactions and the semi-bridging coordinated carbonyl group. (C) 2022 Elsevier B.V. All rights reserved.

We report here the application of the low-temperature X-ray photoelectron spectroscopy (cryo-XPS) of fast-frozen dispersions as a quasi in situ technique for a case study of metal sulfides reacted in acidic aqueous solutions under non-oxidizing and moderate oxidizing conditions. The sulfide surfaces are known to tend to be depleted in metals, producing essentially sulfur-enriched surfaces and extended underlayers on Fe- and Cu-bearing sulfides, which have previously been examined using depth-sensitive HAXPES and cryo-XPS. The current study is focused on zinc and lead sulfides (natural sphalerite and galena), for whom both the experiment and theoretical DFT simulations suggest a low stability of sulfur-excessive structures. Cryo-XPS revealed the complicated behavior of the minerals under non-oxidative etching conditions, in particular, a notable concentration of polysulfide for PbS in dilute perchloric acid and a very minor one in hydrochloric acid. Oxidative etching with Fe3+ cations produced polysulfide anions and then elemental sulfur, which both volatized in the ultra-high vacuum at room temperature; the species can, nonetheless, be distinguished by considering the binding energies, electrostatic charging and evaporation rates. The cryo-XPS also detected interfacial products, e.g., ferrous chloride. DFT found that S-excessive centers are unstable in the undersurface regions of both materials, but are less unfavorable for ZnS surfaces, in agreement with the experimental data. It was concluded that cryo-XPS allows us to greatly reduce distortions of the interface composition in comparison with conventional techniques.

Various COx species formed upon the adsorption and oxidation of CO on palladium and silver single atoms supported on a model ceria nanoparticle (NP) have been studied using density functional calculations. For both metals M, the ceria-supported MCOx moieties are found to be stabilised in the order MCO < MCO2 < MCO3, similar to the trend for COx species adsorbed on M-free ceria NP. Nevertheless, the characteristics of the palladium and silver intermediates are different. Very weak CO adsorption and the small exothermicity of the CO to CO2 transformation are found for O4Pd site of the Pd/Ce21O42 model featuring a square-planar coordination of the Pd2+ cation. The removal of one O atom and formation of the O3Pd site resulted in a notable strengthening of CO adsorption and increased the exothermicity of the CO to CO2 reaction. For the analogous ceria models with atomic Ag instead of atomic Pd, these two energies became twice as small in magnitude and basically independent of the presence of an O vacancy near the Ag atom. CO2-species are strongly bound in palladium carboxylate complexes, whereas the CO2 molecule easily desorbs from oxide-supported AgCO2 moieties. Opposite to metal-free ceria particle, the formation of neither PdCO3 nor AgCO3 carbonate intermediates before CO2 desorption is predicted. Overall, CO oxidation is concluded to be more favourable at Ag centres atomically dispersed on ceria nanostructures than at the corresponding Pd centres. Calculated vibrational fingerprints of surface COx moieties allow us to distinguish between CO adsorption on bare ceria NP (blue frequency shifts) and ceria-supported metal atoms (red frequency shifts). However, discrimination between the CO2 and CO32− species anchored to M-containing and bare ceria particles based solely on vibrational spectroscopy seems problematic. This computational modelling study provides guidance for the knowledge-driven design of more efficient ceria-based single-atom catalysts for the environmentally important CO oxidation reaction.

The density functional method was used to study vibrational frequenciesof the surface species involved in the reaction of the methanol dehydration via pathway CH₃OH→CH₃O→CH₂O→CHO→СОon a ideal Pt(111) surface and surface of nanoparticle Pt79. Connection of spectral features with stability and coordination of the adsorbed molecules was discussed. The determined vibrational features enable identification of themain reaction intermediates

Colloidal copper sulfides produced in reactions of aqueous copper and sulfide ions are important for many materials applications, environment and mineral processing. Here, CuxS nanoparticles formed and aged at varying copper sulfate to sodium sulfide ratios were studied using in situ UV-vis-NIR spectroscopy, dynamic light scattering, X-ray absorption spectroscopy, ex situ TEM, X-ray photoelectron spectroscopy and Raman scattering, and DFT + U calculations. It was established that the ratio of aqueous Cu2+ to S2- ions of 1:2 is critical for the reaction, which yields disordered covellite-like 4-6 nm Cu0.7S nanoparticles comprised polysulfide species at this and higher sulfide concentrations; upon aging, the particles release sulfur and transform to chalcocite-like structure (Cu2-xS, x < 1). Conversely, at the "excess" of copper ions, Cu2-xS-type particles grew into 12-14 nm "covellitic" nanoparticles. The optical absorbance at 1100-1200 nm commonly attributed to localized surface plasmon resonance increased with time and was lowest for Cu2+/S2- = 1:2. DFT + U calculations found that polysulfide stabilizes copper-deficit covellite, while Cu vacancies in chalcocite are more favorable and destabilized by S-S bonding; the Fermi level energy increases and the hole density decreases with polysulfide formation. We believe that CuS2 clusters form initially, and following conversion of disulfide to polysulfide and then monosulfide ions rather than release of Cu determines the character of CuxS nanoparticles.

The initial release of cations upon oxidation of metal sulfides commonly produces a metal-deficient surface and undersurface layers, which should greatly affect the properties of materials but are still poorly understood. We employed density functional theory + U simulation of chalcopyrite (012) and (110) surfaces with up to a half of surface iron removed together with X-ray photoelectron spectroscopy (XPS) of fast-frozen chalcopyrite oxidized in aqueous solutions. It was calculated that the centers comprising tri- or pentasulfide anions or tri- and disulfide complexes have the negative formation energy of 1.2-1.5 eV per one extracted Fe atom, while defects with disulfide anions are disadvantageous. The surfaces are typically "metallic" with comparable densities of S sp and Cu 3d states at the Fermi level. Upon performing cryo-XPS studies, it was found that sulfide surfaces depleted in iron but not in copper, and polysulfide anions S-n(2-) with n ( )>= 5 arose. As oxidation progresses, a deficit of Cu occurs, and S-S chains grow. Upon warming up to room temperature, polysulfide species partially volatilize, so S-s(2-) and S(3)(2- )anions appear to prevail, while the minor contribution of disulfide remains unchanged. The high stability of "polysulfide" centers is considered responsible for retarded oxidation and leaching ("passivation") of chalcopyrite; metallic DOS is important for the physical properties of the surfaces.

The effect of the nature of paramagnetic adsorption centers of the dehydroxylated silica surface - nonbridging oxygens (NBO) or silicon atoms with dangling bond (E') - on the oxidation of adsorbed subnanometer-sized silver clusters Ag-n (n = 3, 4, 7) has been clarified by density functional theory using embedded cluster models. The interaction with NBO centers results in the formation of positively charged Ag moieties, while at E' centers metal species remain rather neutral. At both surface defects, the electronic and structural properties of Ag species resemble the features of free Ag-n(+) clusters. As a result, O-2 molecular adsorption on the supported Ag clusters follows the trends established for cationic species. O-2 is weakly adsorbed in a terminal mode on the Ag trimers and heptamers and is strongly bound to the Ag tetramers in a bridge mode. The stability of O-2 molecular and dissociative adsorbed forms at the supported Ag-3 and Ag-7 species is similar, whereas on Ag-4 cluster O-2 dissociation is preferred. The heights of the O-2 dissociation barriers are determined by the initial activation of molecularly adsorbed O-2 and the deformation of supported metal clusters along the reaction pathway. The high activation energies make O-2 dissociation unlikely and manifest that small Ag clusters trapped by silica paramagnetic defects highly resistant to the oxidation.

Silver single atoms and small clusters supported on a model Ce21O42 nanoparticle have been studied computationally at a DFT+U level. It was found that silver atoms, trimers and tetramers are oxidized upon interaction with the {100} and {111} nanofacets of the ceria particle. The most stable surface complexes are formed via interaction of the silver species with two-coordinated O-2c atoms of the {100} nanofacet. The binding energies of the silver atom, trimer, and tetramer are calculated to be similar to 2.2, similar to 2.9 and 3.1 eV, respectively. Binding of these silver species located on the {111} facet is less severe, by 1.0 eV for the single atom and by 0.4-1.1 eV for Ag-3 and Ag-4 clusters. Stability of the lowest-energy supported silver species on the two types of ceria nanofacets decreases in the order of Ag/Co21O42{100} > Ag-4/Ce21O42{100} > Ag-3/Ce21O42{100} > Ag-3/Ce21O42{111} > Ag-4/Ce21O42{111} > Ag/Ce21O42{111}, pointing out on the supremacy of the denucleation trend.

The review is dedicated to the elaboration and application of hybrid quantum mechani-cal/molecular mechanical methods for heterogeneous catalytic systems, including single atoms and clusters of transition metals immobilized on covalent oxide supports. The following issues are considered: (1) elaboration of the hybrid covEPE method for modeling of covalent sys-tems of the zeolite and silicate types, (2) computations of the properties of atoms and small titanium, rhodium, iridium, and gold clusters localized in cavities or embedded in the zeolite framework, and (3) computations of small silver and tantalum clusters anchored at the dehydr-oxylated and hydroxylated silica surfaces. The calculations were performed by the density functional theory (DFT) with the Becke—Perdew (BP) exchange-correlation potential.

The depletion of oxidized metal sulfide surfaces in metals due to the preferential release of cations is a common, but as yet poorly understood phenomenon. Herein, X-ray photoelectron spectroscopy using excitation energies from 1.25 keV to 6 keV, and Fe K- and S K-edge X-ray absorption near-edge spectra in total electron and partial fluorescence yield modes was employed to study natural chalcopyrite oxidized in air and etched in an acidic ferric sulfate solution. The metal-depleted undersurface formed was found to consist of a thin, 1-4 nm, outer layer containing polysulfide species, a layer with a pronounced deficiency of metals, mainly iron, and an abundant disulfide content but negligible polysulfide content (about 20 nm thick after the chemical etching), and a defective underlayer which extended down to about a hundred nm. DFT+U was used to simulate chalcopyrite with increasing numbers of removed Fe atoms. It was found that the structure with disulfide anion near double Fe vacancies, and the 'defective' structure comprising Cu in the position of Fe and Cu vacancy are most energetically favorable, especially when using a higher Hubbard-type parameter U, and have a large density of states at the Fermi level, whereas polysulfide anions are stable only near the surface. We propose a mechanism explaining the formation of the layered undersurface and 'passivation' of metal sulfides by (i) arrested decomposition of a nearly stoichiometric sulfide surface, and (ii) faster interfacial transfer and solid diffusion of cations towards the surface; (iii) stability limits for specific defect structures, promoting their expansion in depth rather than through compositional changes, excluding surface layers; (iv) decay of surface polysulfide layer yielding elemental sulfur.

The structure and the first step of hydrolysis of hydrated Pu(IV) ion in aqueous medium were studied using relativistic density functional theory and the polarizable continuum model (PCM). According to calculations, the Pu(IV) ion was mainly coordinated with eight water molecules. The calculated distance Pu-O of 238-241 pm was in good agreement with the experimental value ((239 +/- 2) pm). It was demonstrated that for the correct replication of constants log K-1(0) of the first hydrolysis step, it was important to use scaling multipliers adequate to the charge of the hydrated complex when constructing the density that included the solvated complex in PCM model. The calculated values of log K-1(0) of -1.1...-0.2 were close to the experimentally defined range (-0.6-0.6). The correct replication of hydrolysis constants allowed considering the calculated range of the hydratation free energies of the Pu(IV) ion of 6070-6157 kJ/mol as a reasonable prediction of experimental values.

Vladimir A. Nasluzova, Konstantin M. Neymanb, c, Aleksey M. Shora, Svetlana S. Laletinaa and Elena A. Ivanova-Shor *a aInstitute of Chemistry and Chemical Technology SB RAS FRC “Krasnoyarsk Science Center SB RAS” 50/24, Akademgorodok, Krasnoyarsk, 660036, Russia bDepartament de Ciència de Materials i Química Física Universitat de Barcelona 1 C/Martí i Franquès, 08028, Barcelona, Spain cICREA (Institució Catalana de Recerca i Estudis Avançats) 23 Pg. Lluís Companys, 08010, Barcelona, Spain

Structures and relative energies of binuclear iron-manganese complexes with the phosphine ligand L, which exist in vinylidene Cp(CO)(L)MnFe(μ-C=CHPh)(CO)4 (2) and benzylidene ketene η4-{C[Mn(CO)(L)Cp]∙ ∙(CO)CHPh}Fe(CO)3 (3) forms are calculated by the B3LYP density functional method. Four isomers with different positions of ligand L relative to the phenyl ring (conformers a and b) and the substituent Ph relative to the С=С bond (conformers E and Z) are considered for each form and their relative stability is determined. It is shown that all isomers of 2 have approximately the same energy (within 4 kcal/mol) whereas the energies of isomers of 3 differ within 21 kcal/mol. Isomer 3Ea in which the PPh3 ligand contacts with the phenyl substituent of the vinylidene group is most energetically favorable. It is found that with an increase in the L ligand size in the order PH3 < PH2Ph < PHPh2 < PPh3 the Mn–P bond length increases to 2.37 Å in the most stable isomer of form 3 and to 2.43 Å in the isomers of 2 and three conformers of 3. A more substantial increase in the Mn–P bond length in complexes 2 and 3 correlates with their lower stability as compared to isomer Ea of 3, which is consistent with experimental data on the presence of only one conformer 3Ea in solution.

To identify the coordination modes of bare and hydrogenated trinuclear tantalum species on hydroxylated silica, we computationally examined models of Ta3H n (n = 0, 3, 5–9) species grafted to a β-cris-tobalite surface. Ta3H n clusters are bound to the surface by substitution of hydrogen atoms of vicinal (…O-)3SiOH and geminal (…O-)2Si(OH)2 groups via three and six, respectively, Ta–O bonds of ~193 pm on average, in both types of models. The maximum Ta–O coordination number of non-hydrogenated Ta3 species to a silica surface is seven for the second type model surface; the additional Ta–O bond is due to an oxygen atom located in a bridging position to Ta–Ta bond. In the latter case, the mean Ta–O bond distance to one of =Si(O–)2 group is increased by 15 pm. For the complexes bound via vicinal silanol groups, each additional unit of hydrogen loading on the metal elongated the average Ta–Ta distance by ~2 pm, covering a range of 258–277 pm. For the most stable hydrogenated trimers, Ta3H9, the desorption energies of hydrogen atoms are relatively high, above 70 kJ/mol. The average Ta–Ta distances increase by ~19 pm on going from the complex (=SiO–)3Ta3H9 to complex (=SiO–)3Ta3 and by ~5 pm when the hydrogen loading is increased by one unit for (=Si(O–)2)3Ta3H n complexes, reaching the maximum value 319 pm when n = 9. The desorption energies of hydrogen atoms for the most stable tantalum trimer species grafted to the surface by geminal silanol groups, (=Si(O–)2)3Ta3H7, are rather low, less than 40 kJ/mol.