Iridium Stabilizes Ceramic Titanium Oxynitride Support for Oxygen Evolution Reaction.

Decreasing iridium loading in the electrocatalyst presents a crucial challenge in the implementation of proton exchange membrane (PEM) electrolyzers. In this respect, fine dispersion of Ir on electrically conductive ceramic supports is a promising strategy. However, the supporting material needs to meet the demanding requirements such as structural stability and electrical conductivity under harsh oxygen evolution reaction (OER) conditions. Herein, nanotubular titanium oxynitride (TiON) is studied as a support for iridium nanoparticles. Atomically resolved structural and compositional transformations of TiON during OER were followed using a task-specific advanced characterization platform. This combined the electrochemical treatment under floating electrode configuration and identical location transmission electron microscopy (IL-TEM) analysis of an in-house-prepared Ir-TiON TEM grid. Exhaustive characterization, supported by density functional theory (DFT) calculations, demonstrates and confirms that both the Ir nanoparticles and single atoms induce a stabilizing effect on the ceramic support via marked suppression of the oxidation tendency of TiON under OER conditions.

Improving the HER Activity and Stability of Pt Nanoparticles by Titanium Oxynitride Support.

Water electrolysis powered by renewables is regarded as the feasible route for the production of hydrogen, obtained at the cathode side through electrochemical hydrogen evolution reaction (HER). Herein, we present a rational strategy to improve the overall HER catalytic performance of Pt, which is known as the best monometallic catalyst for this reaction, by supporting it on a conductive titanium oxynitride (TiON x ) dispersed over reduced graphene oxide nanoribbons. Characterization of the Pt/TiON x composite revealed the presence of small Pt particles with diameters between 2 and 3 nm, which are well dispersed over the TiON x support. The Pt/TiON x nanocomposite exhibited improved HER activity and stability with respect to the Pt/C benchmark in an acid electrolyte, which was ascribed to the strong metal-support interaction (SMSI) triggered between the TiON x support and grafted Pt nanoparticles. SMSI between TiON x and Pt was evidenced by X-ray photoelectron spectroscopy (XPS) through a shift of the binding energies of the characteristic Pt 4f photoelectron lines with respect to Pt/C. Density functional theory (DFT) calculations confirmed the strong interaction between Pt nanoparticles and the TiON x support. This strong interaction improves the stability of Pt nanoparticles and weakens the binding of chemisorbed H atoms thereon. Both of these effects may result in enhanced HER activity.

Coordination of a Neutral Ligand to a Metal Center of Oxohalido Anions: Fact or Fiction?

Can a neutral ligand bond to a metal center of a square pyramidal oxohalido anion at the available sixth octahedral position? Crystal structures of some compounds indeed suggest that ligands, such as THF, pyridine, H2O, NH3, and CH3CN, can interact with the central metal atom, because they are oriented with their heteroatom toward the metal center with distances being within the bonding range. However, this assumption that is based on chemical intuition is wrong. In-depth analysis of interactions between ligands and oxohalido anions (e.g., VOX4-, NbOCl4-) reveals that the bonding of a neutral ligand is almost entirely due to electrostatic interactions between the H atoms of a ligand and halido atoms of an anion. Furthermore, ab initio calculations indicate that the ligand-VOF4- interactions represent only about one-quarter of the total binding of the ligand within the crystal structure, whereas the remaining binding is due to crystal packing effects. The current study therefore shows that relying solely on the structural aspects of solved crystal structures, such as ligand orientation and bond distances, can lead to the wrong interpretation of the chemical bonding.

Two-dimensional oxide quasicrystal approximants with tunable electronic and magnetic properties.

Recently, the discovery of the quasiperiodic order in ultra-thin perovskite films reinvigorated the field of 2-dimensional oxides on metals, and raised the question of the reasons behind the emergence of the quasiperiodic order in these systems. The effect of size-mismatch between the two separate systems has been widely reported as a key factor governing the formation of new oxide structures on metals. Herein, we show that electronic effects can play an important role as well. To this end, the structural, thermodynamic, electronic and magnetic properties of freestanding two-dimensional oxide quasicrystalline approximants and their characteristics when deposited over metallic substrates are systematically investigated to unveil the structure-property relationships within the series. Our thermodynamic approach suggests that the formation of these aperiodic systems is likely for a wide range of compositions. In addition, the magnetic properties and work functions of the thin films can be controlled by tuning their chemical composition. This work provides well-founded general insights into the driving forces behind the emergence of the quasiperiodic order in ternary oxides grown on elemental metals and offers guidelines for the discovery of new oxide quasicrystalline ultra-thin films with interesting physical properties.

Surprising Lateral Interactions between Negatively Charged Adatoms on Metal Surfaces.

Even something as conceptually simple as adsorption of electronegative adatoms on metal surfaces, where repulsive lateral interactions are expected for obvious reasons, can lead to unanticipated behavior. In this context, we explain the origin of surprising lateral interactions between electronegative adatoms observed on some metal surfaces by means of density functional theory calculations of four electronegative atoms (N, O, F, Cl) on 70 surfaces of 44 pristine metals. Four different scenarios for lateral interactions are identified, some of them being unexpected: (i) They are repulsive, which is the typical case and occurs on almost all transition metals. (ii, iii) They are atypical, being either attractive or negligible, which occurs on p-block metals and Mg. (iv) Surface restructuring stabilizes the low-coverage configuration, preventing atypical lateral interactions. The last case occurs predominantly on s-block metals.

Reactivity of VOF3 with N-Heterocyclic Carbene and Imidazolium Fluoride: Analysis of Ligand-VOF3 Bonding with Evidence of a Minute π Back-Donation of Fluoride.

Reaction of vanadium(V) oxide trifluoride (VOF3) and the new "naked" fluoride reagent [(LDipp)H][F] (LDipp = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2 H-imidazol-2-ylidene) leads to the isolation of [(LDipp)H][VOF4] (1) where the long sought discrete [VOF4]- anion was finally obtained. The neutral [(LDipp)VOF3] (2) complex was synthesized by a similar reaction between VOF3 and bulky N-heterocyclic carbene (NHC) ligand LDipp. In this context, we analyzed, by means of DFT calculations, intermolecular interactions between [(LDipp)VOF3] (2) complexes in the crystal structure and realized that these interactions have a significant effect on the V-Ftrans bond length. We further scrutinized ligand bonding within [(LDipp)VOF3] (2) and related complexes, because, in this kind of complexes, a rather short distance between CNHC and cis-halogen atoms has spurred some discussion about the type of interactions between them. We provide evidence of a minute π back-bonding into NHC ligands, which is larger for chloride [(NHC)VOCl3] than fluoride [(NHC)VOF3] complexes, although the fluoride ions are, counterintuitively and to a larger degree, involved in back-bonding than chloride ions. The influence of π back-bonding on V-Ftrans and V-Fcis bond lengths was also rationalized. Finally, the hydrolysis of [(LDipp)VOF3] (2) product was studied and [(LDipp)H][VO2F2] (3) salt was obtained and characterized as the most stable product in this system.

Decisive Role of Perimeter Sites in Silica-Supported Ag Nanoparticles in Selective Hydrogenation of CO2 to Methyl Formate in the Presence of Methanol.

Methyl formate synthesis by hydrogenation of carbon dioxide in the presence of methanol offers a promising path to valorize carbon dioxide. In this work, silica-supported silver nanoparticles are shown to be a significantly more active catalyst for the continuous methyl formate synthesis than the known gold and copper counterparts, and the origin of the unique reactivity of Ag is clarified. Transient in situ and operando vibrational spectroscopy and DFT calculations shed light on the reactive intermediates and reaction mechanisms: a key feature is the rapid formation of surface chemical species in equilibrium with adsorbed carbon dioxide. Such species is assigned to carbonic acid interacting with water/hydroxyls on silica and promoting the esterification of formic acid with adsorbed methanol at the perimeter sites of Ag on SiO2 to yield methyl formate. This study highlights the importance of employing combined methodologies to verify the location and nature of active sites and to uncover fundamental catalytic reaction steps taking place at metal-support interfaces.

Discrete GeF5- Anion Structurally Characterized with a Readily Synthesized Imidazolium Based Naked Fluoride Reagent.

The recently prepared novel naked fluoride reagent 1,3-bis(2,6-diisopropylphenyl)imidazolium fluoride ([(LDipp)H][F]), treated with an excess of MF4 (M = Si, Ge), results in isolation of [(LDipp)H][MF5] products with the elusive trigonal bipyramidal MF5- anions. Specific steric characteristics of the [(LDipp)H]+ cation readily support isolation of monomeric and discrete trigonal bipyramidal fluorido anions of silicon and germanium. Based on combination of experimental results and DFT calculations, we demonstrate that the role of bulky cation is not solely due to steric hindering but also due to electrostatic effects, which are important in the design of such uncommon species. The discrete GeF5- anion was characterized by X-ray single-crystal diffraction for the first time. We report the missing 19F NMR entries for the discrete GeF5- and GeF62- anions in acetonitrile. All the products were also characterized by Raman spectroscopy and elemental analysis and supported by quantum-mechanical calculations.

DFT and TPD study of the role of steps in the adsorption of CO on copper: Cu(4 1 0) versus Cu(1 0 0).

Adsorption of carbon monoxide (CO) was studied on stepped Cu(4 1 0) by temperature programmed desorption (TPD) and density-functional-theory (DFT) calculations. For comparison, the adsorption of CO was characterized also on Cu(1 0 0) by DFT calculations. On Cu(4 1 0) TPD reveals two desorption peaks: a high temperature peak (∼210 K) is attributed to the desorption of CO from step-edge sites and low temperature peak (∼170 K) to desorption from terrace sites. According to DFT calculations, CO prefers to adsorb at step-edges of Cu(4 1 0), although the step-edge versus terrace site preference is rather small at low coverage of 1/16 ML, about 0.05 eV; the respective DFT predicted CO binding energies are -0.89 and -0.84 eV at the step-edge and terrace top sites, whereas the value calculated at top sites of Cu(1 0 0) is -0.86 eV. Although this small step-edge over terrace site preference of 0.05 eV cannot explain the temperature difference of 40 K between the two TPD peaks, when the lateral intermolecular interactions are neglected, it is sufficient that the CO adsorbs almost exclusively at step-edges at low coverage (at 200 K the 0.05 eV corresponds to 3 kT). The emergence of the two TPD peaks on Cu(4 1 0) is therefore attributed to a combination of step-edge preference and lateral repulsion between CO molecules, which increases with increasing coverages and diminishes the net desorption energy of CO. DFT calculations further reveal that the reason for the significant increase of saturation coverage on Cu(4 1 0) compared to Cu(1 0 0) is related to the geometry of the step-edge that allows the CO molecules adsorbed thereon to tilt away from the nearest neighboring CO molecules adsorbed at the terrace and therefore to effectively reduce the lateral repulsion.

Effect of Mercapto and Methyl Groups on the Efficiency of Imidazole and Benzimidazole-based Inhibitors of Iron Corrosion.

We report on the combined experimental and computational study of imidazole- and benzimidazole-based corrosion inhibitors containing methyl and/or mercapto groups. Electrochemical measurements and long-term immersion tests were performed on iron in NaCl solution, whilst computational study explicitly addresses the molecular level details of the bonding on iron surface by means of density functional theory calculations (DFT). Experimental data were the basis for the determination of inhibition efficiency and mechanism. Methyl group combined with mercapto group has a beneficial effect on corrosion inhibition at all inhibitor concentrations. The beneficial effect of mercapto group combined with benzene group is not so pronounced as when combined with methyl group. The latter is in stark contrast with the behaviour found previously on copper, where the effect of methyl group was detrimental and that of mercapto and benzene beneficial. Explicit DFT calculations reveal that methyl-group has a small effect on the inhibitor-surface interaction. In contrast, the presence of mercapto group involves the strong S-surface bonding and consequently the adsorption of inhibitors with mercapto group is found to be more exothermic.

A DFT study of adsorption of imidazole, triazole, and tetrazole on oxidized copper surfaces: Cu2O(111) and Cu2O(111)-w/o-CuCUS.

Azoles and their derivatives are known for their corrosion inhibition ability for copper. For this reason the bonding of imidazole, triazole, and tetrazole-used as archetypal models of azole corrosion inhibitors-to Cu2O(111) and Cu2O(111)-w/o-Cu(CUS) was characterized using density functional theory (DFT) calculations. The former surface contains coordinatively-saturated (CSA) and coordinatively-unsaturated (CUS) Cu sites, whereas the latter lacks the CUS sites. We find that the molecules preferentially bond with a single unsaturated N atom to a surface Cu ion and concomitantly form a hydrogen bond with the surface O ion. They adsorb rather strongly at CUS sites with an adsorption energy of about -1.6 eV (as calculated with the PBE functional), whereas the bonding at CSA sites is about three times weaker thus being similar as on metallic Cu(111). The impact of van der Waals dispersion interactions on molecular adsorption bonding is also addressed. Depending on specifics of the adsorption structure, they strengthen the adsorption bonding by about 0.2-0.5 eV. Due to this specific bonding enhancement, dispersion interactions alter the relative stability of adsorption modes for tetrazole. An atomistic thermodynamics approach was used to construct two-dimensional phase diagrams for all the three molecules. In the viable range of oxygen chemical potential only three phases appear in the phase-diagrams, two of which are the high coverage (1 × 1) molecular phases (one on Cu2O(111) and the other on Cu2O(111)-w/o-Cu(CUS)) and the third is clean Cu2O(111)-w/o-Cu(CUS). The current results indicate that molecular adsorption at CUS sites is strong enough to compensate the thermodynamic deficiency of stoichiometric Cu2O(111) thus making it more stable than Cu2O(111)-w/o-Cu(CUS), unless the conditions are too oxygen rich and/or for azole lean. This finding may tentatively suggest that the corrosion inhibition capability of azoles stems from their ability to passivate reactive surface sites.

Ab initio modeling of the bonding of benzotriazole corrosion inhibitor to reduced and oxidized copper surfaces.

The bonding of benzotriazole-an outstanding corrosion inhibitor for copper-on reduced and oxidized copper surfaces is discussed on the basis of density functional theory (DFT) calculations. Calculations reveal that benzotriazole is able to bond with oxide-free and oxidized copper surfaces and on both of them it bonds significantly stronger to coordinatively unsaturated Cu sites. This suggests that benzotriazole is able to passivate the reactive under-coordinated surface sites that are plausible microscopic sites for corrosion attack. Benzotriazole can adsorb in a variety of different forms, yet it forms a strong molecule-surface bond only in deprotonated form. The bonding is even stronger when the deprotonated form is incorporated into organometallic adcomplexes. This is consistent with existing experimental evidence that benzotriazole inhibits corrosion by forming protective organometallic complexes. It is further shown that adsorption of benzotriazole considerably reduces the metal work function, which is a consequence of a large permanent molecular dipole and a properly oriented adsorption structure. It is argued that such a pronounced effect on the work function might be relevant for corrosion inhibition, because it should diminish the anodic corrosion reaction, which is consistent with existing experimental evidence that benzotriazole, although a mixed type inhibitor, predominantly affects the anodic reaction.

Molecular modeling of organic corrosion inhibitors: why bare metal cations are not appropriate models of oxidized metal surfaces and solvated metal cations.

The applicability of various models of oxidized metal surfaces - bare metal cations, clusters of various size, and extended (periodic) slabs - that are used in the field of quantum-chemical modeling of corrosion inhibitors is examined and discussed. As representative model systems imidazole inhibitor, MgO surface, and solvated Mg(2+) ion are considered by means of density-functional-theory calculations. Although the results of cluster models are prone to cluster size and shape effects, the clusters of moderate size seem useful at least for qualitative purposes. In contrast, the bare metal cations are useless not only as models of oxidized surfaces but also as models of solvated cations, because they bind molecules several times stronger than the more appropriate models. In particular, bare Mg(2+) binds imidazole by 5.9 eV, while the slab model of MgO(001) by only 0.35 eV. Such binding is even stronger for 3+ cations, e.g., bare Al(3+) binds imidazole by 17.9 eV. The reasons for these fantastically strong binding energies are discussed and it is shown that the strong bonding is predominantly due to electron charge transfer from molecule to metal cation, which stems from differences between molecular and metal ionization potentials.

Triazole, benzotriazole, and naphthotriazole as copper corrosion inhibitors: I. Molecular electronic and adsorption properties.

The gas-phase adsorption of 1,2,3-triazole, benzotriazole, and naphthotriazole-considered as corrosion inhibitors-on copper surfaces was studied and characterized using density functional theory (DFT) calculations. We find that the molecule-surface bond strength increases with increasing molecular size, thus following the sequence: triazole

DFT study of gas-phase adsorption of benzotriazole on Cu(111), Cu(100), Cu(110), and low coordinated defects thereon.

The adsorption of benzotriazole--an outstanding corrosion inhibitor for copper--on Cu(111), Cu(100), Cu(110), and low coordinated defects thereon has been studied and characterized using density functional theory (DFT) calculations. We find that benzotriazole can either chemisorb in an upright geometry or physisorb with the molecular plane being nearly parallel to the surface. While the magnitude of chemisorption energy increases as passing from densely packed Cu(111) to more open surfaces and low coordinated defects, the physisorption energy is instead rather similar on all three low Miller index surfaces. It is pointed out that due to a large dipole moment of benzotriazole the dipole-dipole interactions are rather important. For perpendicular chemisorption modes the lateral repulsion is very long ranged, extending up to the nearest-neighbor distance of about 60 bohrs, whereas for parallel adsorption modes the lateral interactions are far less pronounced and the molecules experience a weak attraction at distances ≲25 bohrs. The chemisorption energies were therefore extrapolated to zero coverage by a recently developed scheme and the resulting values are -0.60, -0.73, and -0.92 eV for Cu(111), Cu(100), and Cu(110), respectively, whereas the zero-coverage physisorption energy is about -0.7 eV irrespective of the surface plane. While the more densely packed surfaces are not reactive enough to interact with the molecular π-system, the reactivity of Cu(110) appears to be at the onset of such interaction, resulting in a very stable parallel adsorption structure with an adsorption energy of -1.3 eV that is ascribed as an apparent chemisorption+physisorption mode.

What determines the inhibition effectiveness of ATA, BTAH, and BTAOH corrosion inhibitors on copper?

Three corrosion inhibitors for copper-3-amino-1,2,4-triazole (ATA), benzotriazole (BTAH), and 1-hydroxybenzotriazole (BTAOH)-were investigated by corrosion experiments and atomistic computer simulations. The trend of corrosion inhibition effectiveness of the three inhibitors on copper in near-neutral chloride solution is determined experimentally as BTAH ≳ ATA ≫ BTAOH. A careful analysis of the results of computer simulations based on density functional theory allowed to pinpoint the superior inhibiting action of BTAH and ATA as a result of their ability to form strong N-Cu chemical bonds in deprotonated form. While these bonds are not as strong as the Cl-Cu bonds, the presence of solvent favors the adsorption of inhibitor molecules onto the surface due to stronger solvation of the Cl(-) anions. Moreover, benzotriazole displays the largest affinity among the three inhibitors to form intermolecular aggregates, such as [BTA-Cu](n) polymeric complex. This is another factor contributing to the stability of the protective inhibitor film on the surface, thus making benzotriazole an outstanding corrosion inhibitor for copper. These findings cannot be anticipated on the basis of inhibitors' molecular electronic properties alone, thus emphasizing the importance of a rigorous modeling of the interactions between the components of the corrosion system in corrosion inhibition studies.

Density functional theory study of ATA, BTAH, and BTAOH as copper corrosion inhibitors: adsorption onto Cu(111) from gas phase.

A low-coverage gas-phase adsorption of three corrosion inhibitors-3-amino-1,2,4-triazole (ATA), benzotriazole (BTAH), and 1-hydroxybenzotriazole (BTAOH)-on perfect Cu(111) surface has been studied and characterized using density functional theory calculations. We find that the molecules in neutral form chemisorb weakly to the perfect surface in an upright geometry. The strength of the chemisorption increases in the order BTAH < BTAOH < ATA with adsorption energies of -0.40, -0.53, and -0.60 eV, respectively. The molecules bond to the surface with triazole nitrogen atoms and also through X-H···Metal hydrogen bonds (X = N or O). In addition to chemisorption, BTAH and BTAOH can also physisorb with the molecular plane being nearly parallel to the surface and the energies of the physisorption are -0.72 and -0.97 eV, respectively, hence being more exothermic than the corresponding chemisorption energies. On the other hand, the molecules in dehydrogenated form chemisorb strongly to the surface and the strength of the chemisorption increases in the order BTAO· < ATA· < BTA· with the adsorption energies of -1.65, -2.22, and -2.78 eV, respectively. This order is compatible with the trend of experimentally observed corrosion inhibition effectiveness on copper in near-neutral chloride solutions. Although the calculations are performed at the metal/vacuum interface, they provide enough insight to rationalize why in some experiments the BTAH was observed to be adsorbed with an upright geometry and in the others with parallel geometry.

Ethene stabilization on Cu(111) by surface roughness.

The molecular vibrations of ethene adsorbed on roughened Cu(111) surfaces have been investigated with high resolution electron energy loss spectroscopy and density-functional-theory calculations. The roughness was introduced by sputtering or evaporation of copper, respectively, on the cooled surface. We found stabilization of the ethene layer compared to ethene adsorbed on pristine Cu(111). Furthermore, two new vibrational features observed on the rough surface can be assigned to frustrated translations and rotations of the ethene molecule on surface defects and are indicative of a different binding on the rough surface.