A spectroscopic and electrochemical investigation of the oxidation pathway of glycyl-d,l-methionine and its N-acetyl derivative induced by gold(III)

The ¹H nuclear magnetic resonance spectroscopy was applied to study the reaction of the dipeptide glycyl-d,l-methionine (H-Gly-d,l-Met-OH) and its N-acetylated derivative (Ac-Gly-d,l-Met-OH) with hydrogen tetrachloridoaurate(III) (H[AuCl₄]). The corresponding peptide and [AuCl₄]^– were reacted in 1:1, 2:1, and 3:1 molar ratios, and all reactions were performed at pD 2.45 in 0.01 M DCl as solvent and at 25°C. It was found that the first step of these reactions is coordination of Au(III) to the thioether sulfur atom with formation of the gold(III)-peptide complex [AuCl₃(R-Gly-Met-OH-S)] (R=H or Ac). This intermediate gold(III) complex further reacts with an additional methionine residue to generate the R-Gly-Met-OH chlorosulfonium cation as the second intermediate product, which readily undergoes hydrolysis to give the corresponding sulfoxide. The oxidation of the methionine residue in the reaction between H-Gly-d,l-Met-OH and [AuCl₄]^– was five times faster (k ₂ = 0.363 ± 0.074 M^-1s^-1) in comparison to the same process with N-acetylated derivative of this peptide (k ₂ = 0.074 ± 0.007 M^-1s^-1). The difference in the oxidation rates between these two peptides can be attributed to the free terminal amino group of H-Gly-d,l-Met-OH dipeptide. The mechanism of this redox process is discussed and, for its clarification, the reaction of the H-Gly-d,l-Met-OH dipeptide with [AuCl₄]^– was additionally investigated by UV-Vis and cyclic voltammetry techniques. From these measurements, it was shown that the [AuCl₂]^– complex under these experimental conditions has a strong tendency to disproportionate, forming [AuCl₄]^– and metallic gold. This study contributes to a better understanding of the mechanism of the Au(III)-induced oxidation of methionine and methionine-containing peptides in relation to the severe toxicity of anti-arthritic and anticancer gold-based drugs.     

Gold complexes with

The reactions of methane with the gold complexes [Au(OH)]⁻, [Au(OCH₃)₄]⁻, [Au(O(CO)₂O₂]⁻ and [Au(O₂CH)₂]⁺, [Au^I(acac)], [Au^III(acac)₂]⁺ (acac-acetylacetonato) were studied using the DFT/PBE method with the SBK basis set. High activation barriers were obtained for the electrophilic substitution in [Au(OH)]⁻, [Au(OCH₃)⁴]⁻, [Au(O(CO)₂O)₂]-and [Au^III(acac)₂]⁺ complexes, which excludes the possibility that these reactions might proceed under mild conditions. The reactions of the [Au(HCO₂)₂]⁺ and [AuI(acac)] complexes with methane have rather low energy barriers and proceed through the formation of an intermediate complex. The alternative mechanism of methane oxidation with a gold complex in the presence of oxygen is simulated.     

Di-(2-ethylhexyl) dithiophosphoric acid surface protected gold nanoparticles: micellar synthesis, stabilization, isolation, and properties

Gold nanoparticles (AuNPs) were synthesized in the organic solution by means of the reduction of HAuCl₄ by hydrazine in reverse micelles of oxyethylated surfactant Triton N-42, with decane as the dispersion medium. To isolate the powder of particles, the micelles were destroyed with chloroform in the presence of di-(2-ethylhexyl) dithiophosphoric acid as a surface protecting agent. According to the results of several experiments, the yield is within the limits of 90–98%, calculated for gold. The obtained preparations are dark blue hydrophobic powders containing aggregated but not agglomerated gold nanoparticles, as well as microcrystals (∼0.08–0.2 μm) of NaCl. The powders get re-dispersed in weakly polar organic solvents with the formation of colloidal solutions. The shape of the nanoparticles is spherical. Their nuclei are gold single crystals with a narrow size distribution; their diameter (d _Au) is about two times as large as the diameter of the aqueous nucleus (d _c) of initial micelles: d _Au = 7.7 ± 1.4 nm (d _c = 3.6 nm) and 8.8 ± 1.5 nm (4.6 nm). The preparations were studied by means of dynamic light scattering, atomic force microscopy, transmission electron microscopy, UV–vis spectroscopy, IR spectroscopy, X-ray powder diffraction, and thermogravimetric and elemental analyses. In the case of the particles with d _Au = 8.8 nm, the product is a mixture of AuNPs and the salt with the molar ratio Au/NaCl ≈ 1:4.54, while the gross composition of AuNPs per one gold atom is estimated as Au(C₁₆H₃₄O₂PS₂Na∙2N₂H₄)_0.16 with the number of gold atoms in one particle ∼21,000.     

Sensing behavior of silica-coated Au nanoparticles towards nitrobenzene

In the present work, we report silica-stabilized gold nanoparticles (SiO₂/Au NPs) as a wide-range sensitive sensing material towards nitrobenzene (NB). Surface hydroxyl groups of silica selectively form Meisenheimer complex with electron-deficient aromatic ring of NB and facilitate its immobilization and subsequent catalytic reduction by Au cores. Silica-coated Au NPs were synthesized and characterized for their chemical, morphological, structural, and optical properties. SiO₂/Au NPs-modified electrodes were characterized with impedometric and cyclic voltammetric electrochemical techniques. SiO₂/Au NPs are found to have a higher optical detection window of range, 0.1 M to 1 μM and a lower electrochemical detection window of range, 10⁻⁴ to 2.5 × 10⁻² mM with a detection limit of 12.3 ppb. A significant enhancement in cathodic peak current, C ₁, and sensitivity (102 μA/mM) was observed with modified electrode relative to bare and silica-modified electrodes. The I _P was found to be linearly co-related to NB concentration (R ² = 0.985). The interference of cationic and anionic species on sensor sensitivity was also studied. Selectivity in the present sensing system may be further improved by modifying silica with specific functional moieties.     

Magnetic anisotropy of Ho–Fe–Co–Cr intermetallic compounds

Starting with a Ho₃(Fe_1−xCo_x)_29−yCr_y, (x,y) = (0.6,4.5) and (0.8,5.5) nominal stoichiometry, a disordered variant of the hexagonal 2:17 phase (Th₂Ni₁₇-type, S.G. P6₃/mmc) occurs, since both the monoclinic 3:29 and the transition-metal-rich disordered Th₂Ni₁₇-type hexagonal compounds have the same rare earth to transition metal ratio, 1:9.7. The magnetic properties and the magnetocrystalline anisotropy of these compounds have been investigated. The anisotropy constant, K's, and the anisotropy field, μ₀H_A, values have been deduced from the magnetization curves measured on powder samples magnetically aligned in a rotating magnetic field. The compound with (x,y) = (0.8,5.5) shows a compensation point at about 55 K. The magnetic anisotropy of both compounds is that of easy-plane from room temperature to low temperatures down to 5 K.     

Coordination polymers with cyanoaurate building blocks: Potential new industrial applications for gold

This report illustrates the concept that aurophilic interactions of gold-containing building blocks, particularly cyanoaurates, could be used as a tool to increase structural dimensionality in systems containingother metals in addition to gold(I). Such high-dimensionality systems may have useful optical, magnetic, conducting or porous materials properties. Recent successes from our group and others in using the neglected, luminescent [Au(CN)₂]⁻ building block to synthesize supramolecular coordination polymers with interesting and potentially commercially applicable physical properties will be surveyed. In most heterometallic [Au(CN)₂]-based polymers, aurophilic interactions increase the structural dimensionality of the system and can impart increased thermal stability. The gold(I) ion can mediate significant magnetic interactions between transition-metal centres or influence iron(II) spin-transition behaviour in the polymers. The Cu[Au(CN)₂]₂(solvent)_x polymer system is dynamically vapochromic, i.e., it shows large, reversible colour changes upon exposure to solvent vapours, thereby illustrating a sensor-type application. The related d⁸, square-planer [Au(CN)₄]⁻ building block, which has only recently been incorporated into coordination polymers, does not form any aurophilic interactions; weak Au-N(cyano) interactions control the intermolecular packing. Several structural examples of cyanoaurate-based coordination polymers are presented, including 2-D and 3-D arrays. The incorporation of cyanoaurates as components of advanced materials would provide a new utility and market for these key compounds of the gold mining and refining industry.     

Factors affecting luminescence and aurophilicity on digold(I) complexes and their potential as cation probes

In the last few years we have analysed the factors that affect the structures and luminescence properties of Au(I) compounds, specifically in relation with the presence of aurophilic contacts and their application as cation probes. EXAFS studies have allowed us to obtain for the first time direct structural data of dissolved Au(I) compounds. An overview of the work reported to date is presented here. The optical properties of complex [Au₂Cl₂(μ-dpephos)] (dpephos =bis(2-diphenylphosphino) phenylether, 1) have been revisited and new results are now included. New aspects on the use of the complexes as Ag(I) probes are also discussed.     

Influence of Co substitution on magnetoelastic properties of Er2Fe14−xCoxB (x = 1, 3 and 5) intermetallic compounds

The magnetostriction and thermal expansion of Er₂Fe_14−xCo_xB (x = 1, 3 and 5) intermetallic compounds were measured, using the strain gauge method in the temperature range 75–450 K under applied magnetic fields up to 1.5 T. For all samples the longitudinal magnetostriction (λ_l) undergoes an anomaly around the spin reorientation temperature (T_SR). It is also observed that λ_l decreases with increasing the Co content. All compounds show saturation type behaviour in their anisotropic magnetostriction curves at different temperatures and applied fields. The saturation behaviour of the compound with x = 3 occurs at higher temperatures than with x = 1 and 5. The volume magnetostriction strongly increases below μ₀H = 0.3 T, then monotonically rises with applied field up to the spin reorientation temperature. An invar type behaviour is observed above 200 K in the compound with x = 1. The results are discussed based on the temperature dependence of magnetocrystalline anisotropy of compounds below and above their T_SR.     

Magnetic and structural properties of the chromium-based Mn1−xCdxCr2S4 thiospinel

The (Mn_1−xCd_x)Cr₂S₄ phases (0 ≤ x ≤ 0.6) have been synthesized from the corresponding elements at 1123 K. These samples were characterized by powder X-ray diffraction (XRD) and magnetic susceptibility. The (Mn_1−xCd_x)Cr₂S₄ compounds crystallize in the space group Fd-3m with cell parameters a = 10.101(6) Å, 10.139(3) Å, 10.165(2) Å, and 10.192(1) Å for x = 0, 0.2, 0.4 and 0.6, respectively. An overall ferrimagnetic behavior is observed for all samples. The ferromagnetic component increases rapidly when manganese is substituted by non-magnetic cadmium, as shown by ZFC/FC measurements. At the same time, the value of the magnetization M₅₀ at 50 kOe, deduced from M(H) loops, also increases with increasing cadmium content because the antiferromagnetic alignment between chromium and manganese spins is progressively lost, leading toward well aligned moments pointing into the same direction. These results are explained by a rearrangement of the chromium spins when Mn located at the tetrahedral sites, is substituted by Cd.     

Infrared brazing of Fe3Al intermetallic compound using gold-based braze alloy

Infrared-brazing Fe₃Al with Au–44Cu as filler metal has been investigated. The brazed joint consists mainly of a β-phase, Au_8 − x Cu_4 + x Al₄, caused by the dissolution of Al from Fe₃Al substrate into the braze alloy. The depletion of Al from Fe₃Al substrate results in the formation of a layer of β-phase particles dispersed in the Fe-rich phase. The highest shear strength for AuCu filler is 327 MPa for specimens infrared brazed at 880°C for 180 s. The brazed joint is mainly fractured along the central β-phase in which the fractograph exhibits quasi-cleavage with dimples. Increasing the brazing time or temperature will deteriorate the bonding strength of the joint, and the fracture mode is prone to cleavage of brittle fracture. Au–44Cu filler demonstrates a great potential for bonding Fe₃Al intermetallic compound.     

Support effects in the selective gas phase hydrogenation ofp-chloronitrobenzene over gold

The catalytic continuous gas phase hydrogenation of p-chloronitrobenzene (P=1 atm;T=423 K) has been investigated over a series of oxide (Al₂O₃, TiO₂, Fe₂O₃ and CeO₂) supported Au (1 mol %) catalysts. The application of two catalyst synthesis routes,i.e. impregnation (IMP) and deposition-precipitation (DP), has been considered where the DP route generated smaller mean Au particle sizes (1.5-2.8 nm) compared with the IMP preparation (3.5-9.0 nm). The catalysts have been characterised in terms H2 chemisorption and BET area measurements where the formation of metallic Au post-activation has been verified by diffuse reflectance UV-Vis, XRD and HRTEM analyses.p-Chloroaniline was generated as the sole reaction product over all the Au catalysts with no evidence of C-Cl and/or C-NO₂ bond scission and/or aromatic ring reduction. The specific hydrogenation rate increased with decreasing Au particle size (from 9 to 3 nm), regardless of the nature of the support. This response extends to a reference Au/TiO₂ catalyst provided by the World Gold Council. A decrease in specific rate is in evidence for smaller particles (< 2 nm) and can be attributed to a quantum size effect. The results presented establish the basis for the design and development of a versatile catalytic system for the clean continuous production of high value amino compounds under mild reaction conditions.     

Supramolecular bisporphyrin cages: Design and ways of self-assembly of supramolecular bisporphyrin structures for molecular nanomotors and nanosensors

Self-assembly of supramolecular ensembles of coordination bisporphyrins was studied in reactions of meso-tetrakis(3-hydroxyphenyl)porphyrinatozinc with LaCl₃, MnCl₂, CrCl₃, and SnCl₂ and exodentate ligands (1,4-diazabicyclo[2.2.2]octane and 4,4’-bipyridyl). A design and ways of self-assembly of bisporphyrins structures were developed. Original Russian Text ? Yu.P. Yashchuk, V.S. Tyurin, I.P. Beletskaya, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 6, pp. 608–614.     

Phase Equilibria,Crystal Chemistry and Physical Properties of Au-Ba-Ge Clathrates

In the Au-Ba-Ge system the clathrate type I solid solution, Ba₈Au _x Ge_46−x−y □ _y , extends at 800 °C from binary Ba₈Ge₄₃□₃ (□ is a vacancy) to Ba₈Au₆Ge₄₀. For the clathrate phase (1 ≤ x ≤ 6) cubic primitive symmetry (space group Pm[`3]n Pm{\bar{{3}}}n ) was confirmed by x-ray powder diffraction assisted by x-ray single crystal analyses of Ba₈Au_4.6Ge_40.3□_1.1. The lattice parameters of the solid solution show an almost linear increase with increasing gold content. Site preference from x-ray refinement shows that gold atoms preferably occupy the 6d site in random mixture with Ge and vacancies, which vanish at the solubility limit. Clathrate type ΙX (Ba₆Ge₂₅ type) has a maximum solubility of 2.7 at.% gold at 800 °C. Phase equlilibria at 800 °C are characterized by four ternary phases in the investigated region up to 33.3 at.% barium. The homogeneity range of Ba(Au_1−x Ge _x )₂ (AlB₂-type) and BaAu_1+x Ge_3−x has been established: Ba(Au_1−x Ge _x )₂ extends from BaAu_0.5Ge_1.5 to BaAu_0.9Ge_1.1 and BaAu_1+x Ge_3−x from BaAu_1.1Ge_2.9 (BaNiSn₃-type) to BaAu_2.7Ge_1.3 (Ce(Ni,Sb)₄-type). The crystal structures of two phases in the gold-rich part have been determined from single crystal x-ray data and were found to form new structure types: BaAu₃Ge with BaAu₃Ge-type (space group P4/nmm, a = 0.6459(2), c = 0.5487(2) nm) and BaAu_5+x Ge_2−x (x = 0, BaAu₅Si₂-type, space group Pnma, a = 0.8981(2), b = 0.7106(2) and c = 1.0363(2) nm), the latter revealing with increasing gold content a closely related derivative structure type (BaAu_5.3Ge_1.7, a = a_{\textBaAu5 \textSi2} ,  b = b_{\textBaAu5 \textSi2} ,  c = 2c_{\textBaAu5 \textSi2} a = a_{{{\text{BaAu}}_{5} {\text{Si}}_{2} }} ,\;b = b_{{{\text{BaAu}}_{5} {\text{Si}}_{2} }} ,\;c = 2c_{{{\text{BaAu}}_{5} {\text{Si}}{}_{2}}} ). Transport properties and particularly the thermoelectric behavior were studied for Ba₈Au₆Ge₄₀.     

Oxygen-free precursor for chemical vapor deposition of gold films: thermal properties and decomposition mechanism

Thermal properties of oxygen-, phosphorus-, and halogen-free dimethylgold(III) diethyldithiocarbamate complex (CH₃)₂AuS₂CN(C₂H₅)₂ (gold, dimethyl(diethylcarbamodithioato -S,S′)-) having excellent storage stability and the mechanism of its decomposition to elemental gold were studied. Saturated vapor pressure was found to be ~10⁻³–10⁻¹ Torr at 50–90°C. Decomposition of the vapor on the surface starts at T = 210°C. The temperature dependence of gas phase composition was studied using the original mass spectrometric technique, it was established that the decomposition of the compound on the surface in vacuum follows three main pathways. Two of them result in the formation of elemental gold, saturated C2–C4 alkanes and (1) protonated ligand or (2) methylated ligand. The third one results in elemental gold and gaseous products: C2–C3 alkylmercaptanes and CH₃SCN(C₂H₅)₂. The formation of gold as a sole solid product within the temperature range 210–240°C was confirmed by X-ray photoelectron spectroscopy analysis. It was shown that the compound exhibits the best combination of volatility, thermal, and storage stability among volatile organogold complexes and thus it may be a promising precursor for obtaining gold films by chemical vapor deposition.     

Study on the boundary structures in a series of lanthanide hexacyanoferrate(III) n-hydrates

The properties of a series of lanthanide hexacyanoferrate(III) n-hydrates were studied by means of thermal analysis, IR spectroscopy, Raman spectroscopy and X-ray crystallography. Thermal analyses showed that there were two kinds of complexes in this series, Ln[Fe(CN)₆]·5H₂O (Ln=La–Nd) and Ln′[Fe(CN)₆]·4H₂O (Ln′=Sm–Lu). The boundary complex between them was Nd[Fe(CN)₆]·5H₂O. The IR spectra of the two kinds of complexes were obviously different. For the pentahydrates, there were two sharp CN stretching bands at 2050 and 2140 cm⁻¹, and one band at 1600 cm⁻¹ assigned to the HOH bending. On the other hand, for the tetrahydrates besides the two CN stretching bands at 2050 and 2140 cm⁻¹, a new band was observed at 1940 cm⁻¹, and the HOH bending band split into three bands around 1600 cm⁻¹. From the X-ray crystal analysis, the structure of the boundary complex Nd[Fe(CN)₆]·5H₂O was determined. It belonged to hexagonal, P6₃/m, with a=7.467(2) Å, c=13.793(3) Å and Z=2 (R=0.082, R_w=0.126). Neodymium was nine-coordinated in the form of the NdN₆(H₂O)₃ group. The three coordinated water molecules of the 5H₂O complex with Nd have a large value for the equivalent isotropic thermal parameter. One of the three water molecules was dissociated easily and the 5H₂O complex changed into the stable 4H₂O complex with Nd. The crystal of the 4H₂O complex is orthorhombic, and belongs to the space group Cmcm as well as the other Ln[Fe(CN)₆]·4H₂O (Ln=Sm–Lu). Therefore, the structure of Nd[Fe(CN)₆]·5H₂O is regarded as the boundary structure.     

Thermal Expansion and Phase Stability Investigations on Cs-Substituted Nanocrystalline Calcium Hydroxyapatites

The high-temperature phase stability of Ca_10−x Cs _x (PO₄)₆(OH)₂, (x = 0–3) compositions synthesized by various wet chemical methods was investigated. The thermal expansion property of Ca₁₀(PO₄)₆(OH)₂ (abbreviated as CaHAp) and Cs-substituted CaHAp was measured by high-temperature XRD and dilatometry. The average crystallite size of the powders synthesized by wet chemical methods was found to be 10–50 nm range as shown by XRD and TEM. Up to 30 mol% Cs loading was observed to show only the apatite phase by XRD when the apatite powder was nanocrystalline in nature. However, high-temperature stability of the Cs-substituted system is limited to ≤5 mol%. Cs₃(PO₄) is observed to be separated out on heating the material above 773 K for compositions substituted with more than 5 mol% of Cs in the Ca-sublattice. The coefficient of thermal expansion measured by HTXRD is α_a = 12.42 × 10⁻⁶ K⁻¹, α_c = 14.98 × 10⁻⁶ K⁻¹; and α_a = 12.62 × 10⁻⁶ K⁻¹, α_c = 12.57 × 10⁻⁶ K⁻¹ for CaHAp and Ca_9.78Cs_0.2(PO₄)₆(OH)_1.96, respectively, in the temperature range of 298-1083 K. Bulk thermal expansion measurements are seen to be in agreement with the lattice expansion results.     

Structure and magnetic properties of Gd3(Fe1−xTix)29 (x=0.011–0.034)

Single-phase compounds Gd₃(Fe_1−xTi_x)₂₉ (x=0.0110.034) have been synthesized. Gd₃(Fe_1−xTi_x)₂₉ crystallises in a monoclinic lattice with space group P2₁/c, and the crystal structure is refined by the Rietveld technique based on X-ray powder diffraction data. Thermomagnetic analysis indicates that the Curie temperature of the compounds ranges from 517 K to 538 K. The saturation magnetizations of the Gd₃(Fe_1−xTi_x)₂₉ (x=0.011, 0.022, 0.034) at 1.5 K are 103.6, 102.0 and 94.3 Am²/kg, and the anisotropy fields at 1.5 K are 6.0, 6.2 and 6.4T, respectively.     

Investigation of the dependency of the upper critical magnetic field on the content x in Y1−xYbx/2Gdx/2Ba2Cu3O7−y superconducting structures

The Y_1−xYb_x/2Gd_x/2Ba₂Cu₃O_7−y superconducting samples for x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 were prepared by using the solid-state reaction technique. Resistivity measurements of the samples were performed in QD–PPMS system under different magnetic fields up to 5 T in zero fields cooling regime. Using the resistivity data, the upper critical magnetic field H_c2(0) at T = 0 K for 50% of R_n was calculated by the extrapolation H_c2(T) to the temperature T = 0 K. The coherence length in T = 0 K were calculated from H_c2(0) and the effects of x in the composition on both the coherence length and the upper critical magnetic field were examined. The results showed that H_c2(0) varied from 84.05 to 122.26 T with the content x. The upper critical magnetic field in the temperature T = 0 K slightly decreased with increasing the content x. Using the content x, the upper critical magnetic field can be controlled and this can be used in the superconductivity applications.     

Photodecomposition of H<Subscript>2</Subscript>S to H<Subscript>2</Subscript> over Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Zn<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>S composite photocatalysts

A series of Cd _x Zn_1−x S (x = 0.1–0.9) photocatalysts were prepared by coprecipitation. They could form solid solution semiconductors with hexagonal phase in agreement with pure CdS by characterization of XRD. The photophysical properties of Cd _x Zn_1−x S photocatalysts were measured by UV-Vis diffuse reflectance spectrum and surface photovoltage spectroscopy (SPS). The band gap energy gradually reduced with the increasing of x value in CdxZn_1−x S, and when x = 0.7, the Cd_0.7Zn_0.3S photocatalyst had the strongest surface photovoltage. Cd _x Zn_1−x S photocatalysts were used in the photodecomposition of H₂S to H₂. The evolution rate of H₂ over the Cd_0.7Zn_0.3S photocatalyst was also the highest among Cd _x Zn_1−x S photocatalysts. And the effect of calcination temperature on the evolution rate of H₂ was investigated and the optimum temperature was 650°C.     

The Effect of Water Vapor on the Initial Stages of Oxidation of the FeCrAl Alloy Kanthal AF at 900 °C

The effect of water vapor on the initial stages of oxidation of the FeCrAl alloy Kanthal AF is reported. Polished samples were exposed isothermally at 900 °C for 1, 24, 72 and 168 h in a well-controlled environment consisting of dry O₂ or O₂ + 40% H₂O. The samples were investigated using a combination of gravimetry and several surface-analytical techniques, including XRD, SEM, EDX, FIB, AES and TEM. The presence of water vapor significantly accelerates oxidation during the first 72 h. A two-layered oxide forms in both the dry and wet environments. The bottom layer consists of inward-growing α-Al₂O₃ while the outer layer initially consists of outward-growing γ-Al₂O₃. A straight and narrow Cr-enriched band is present at the top of the lower (α-Al₂O₃) oxide, corresponding to the original sample surface. In dry O₂, the top (γ-Al₂O₃) layer is converted into a mixture of γ-Al_2−x (Mg,Fe) _x O_3−(x/2), MgAl₂O₄ and α-Al₂O₃. This transformation does not occur in O₂ + H₂O. The initial acceleration of oxidation by H₂O is attributed to the stabilization of the outward-growing γ-alumina layer by the hydroxylation of the γ-Al₂O₃ surface. A schematic mechanism of the early stages of oxidation of FeCrAl alloys is presented, emphasizing the influence of water vapor.