Mo Compounds Research Articles

Gas-phase conversion of U, Sr, and Mo compounds into water-soluble forms in a nitrating atmosphere

Gas-phase conversion of uranyl molybdates UO2MoO4, UMo1.7O7.2, and UMo0.4O3.4, of strontium molybdate SrMoO4, and of strontium uranate SrUO4 into water-soluble compounds in the NOx–H2O (vapor)–air or HNO3 (vapor)–air atmosphere (hereinafter, nitrating atmosphere) was studied. The conversion of UO2MoO4, UMo1.7O7.2, and UMo0.4O3.4 in nitrating atmosphere results in formation of water-soluble uranium compounds and of the water-insoluble phase MoO3·2H2O. Rapid separation of the solution from the precipitate in reaction of UO2MoO4, UMo1.7O7.2, and UMo0.4O3.4 conversion products with water allows efficient transfer of U into the solution, with Mo remaining in the form of an insoluble precipitate. The conversion of SrMoO4 depends on the composition of the nitrating atmosphere. In the NOx–H2O (vapor)–air atmosphere, there is no SrMoO4 conversion, whereas in the HNO3 (vapor)–air atmosphere it is possible to reach ~85% conversion of SrMoO4 into water-soluble Sr compounds. Complete conversion of SrUO4 into water-soluble U and Sr compounds is reached when performing the gas-phase conversion at 130°С in both nitrating atmospheres. In the case of the HNO3 (vapor)–air atmosphere, increasing the process temperature to 150°С leads to a decrease in the amount of water-soluble U(VI) phases.

Molybdenum isotope fractionation and speciation in a euxinic lake—Testing ways to discern isotope fractionation processes in a sulfidic setting

The molybdenum (Mo) isotope composition in euxinic shales has been used as a proxy for the global distribution of anoxic conditions in ancient oceans, and since more recently also as a proxy for sulfide concentrations in depositional environments. However, there is currently no way to distinguish isotope fractionation at low bottom water sulfide concentrations in ‘local’ basins from ‘global’ secular isotope variations associated with changing seawater composition. This uncertainty is challenging the use of Mo isotopes for paleoceanographic reconstructions.To explore this further, we present new data from sediments deposited over the past ~9800years in one of the best studied euxinic localities in the world: Lake Cadagno in Switzerland. The sample set allows us to test ways to discern isotope fractionation processes at play in a highly restricted euxinic basin.Most of our drill core samples (n=18) show high δ98Mo values similar to previously studied shallow sediments, indicative of quantitative Mo removal from the water column (Dahl et al. 2010a). However, a few samples (n=3) deposited between about 1200 and 3400years ago carry low δ98Mo values and have been isotopically fractionated in the lake. Sedimentological and geochemical characterizations show that these δ98Mo-fractionated sediments formed during times of frequent injection of O2- and sediment-rich river water into the deep sulfidic water column. A positive correlation between δ98Mo and sedimentary Mo contents suggests that isotope fractionation occurred during times of non-quantitative Mo removal, although Mn-oxide cycling at the chemocline might also contribute a subordinate proportion of (98Mo-depleted) molybdenum into the sulfidic zone. Sedimentary Mo/U enrichments relative to oxic lake water further supports the hypothesis that a particulate Mo shuttle was most efficient during times of quantitative Mo removal. Therefore, periods with inefficient Mo capture are ascribed to incomplete conversion of molybdate to particle reactive Mo species when bottom water H2S levels were low or less stable than today.Using XAFS spectroscopy, we found that the two distinct Mo compounds predominating in the sediments (MoIV-S and MoVI-OS) are not diagnostic for isotope fractionation that has occurred in Lake Cadagno. Instead, we infer that δ98Mo-fractionated products (forming via a low-sulfide Mo pathway) can be subsequently altered with little or no isotopic imprint during remobilization and re-precipitation (e.g., at higher sulfide levels in the sediments) as well as during post-depositional oxidation.Future work could investigate local δ98Mo-fractionation processes expressed in other euxinic settings and explore other sedimentary metrics to constrain the steps involved in the euxinic burial pathway(s). One tantalizing prospect of this is to distinguish between local bottomwater sulfide levels and variations in the fraction of global seafloor anoxia from the Mo isotope composition in ancient euxinic mudrocks.

Photooxidation of NOx using scheelite-type ABO4 (A = Ca, Pb; B = W, Mo) phases as catalyts

The scheelite-type compounds have been investigated due to their interesting properties of photoluminescence and recently, they have been applied as photocatalysts in air and water purification trough the removal of different organic and inorganic pollutants. In this work, four scheelite-type ABO4 (A=Ca, Pb; B=W, Mo) compounds were prepared by co-precipitation method and its capacity to act as photocatalyst was evaluated in the removal of NOx gases using the oxidation of nitric oxide (NO) as model reaction. The photocatalytic activity of the samples decreased in the following order: CaMoO4>PbWO4>PbMoO4>CaWO4, which was related with the electronic properties associated with each sample. When the photocatalyst with the highest photocatalytic activity (CaMoO4) was activated under UV radiation, the selectivity for the formation of NO3− ions was 35%, revealing the ability of the photocatalyst to carry out a deep oxidation of NO until innocuous products. Based in the modification of the experimental conditions, the mechanism proposed for the photooxidation of NO to NO3− ions was mainly by the action of the ion superoxide (O2−) formed from the O2 adsorbed on CaMoO4 surface.

Hydrocracking of Vacuum Gasoil over NiMo/ААС-Al2O3 Catalysts Prepared with Citric Acid: The Influence of Temperature of Thermal Treatment of the Catalyst

Bimetal Ni-Mo catalysts were prepared by impregnating the support consisting of amorphous aluminosilicate (AAS) and alumina with the solution of Ni, Mo compounds and citric acid; temperature of the thermal treatment was varied from 120 to 550 °C. XPS, TEM and HCNS techniques were used for studying physicochemical properties of the prepared catalysts. The catalysts were tested for hydrocracking of vacuum gasoil. Particles of the active sulfide component (NiMoS phase) were established to reside predominantly on the alumina surface and only partially on the AAS surface. As the temperature of the catalyst calcinations rose, the average number of layers in the NiMoS particles increased due to removal of citric acid that indicated a stronger interaction between the active sulfide component and alumina. The temperature elevation also was accompanied by an increase in the proportion of bulky Ni-Mo particles. The morphology of the active sulfide component was shown to influence the catalyst activity to hydrodesulfurization and hydrodenitrogenation but not to hydrocracking. The optimal temperature of thermal treatment of NiMo/AAS-Al 2 O 3 catalysts prepared with citric acid was 120 °C. Recommendations for the thermal treatment of the catalysts under industrial conditions were provided.

Fission of Highly Excited $^{88}$Mo Compound Nucleus

Fission of Highly Excited $^{88}$Mo Compound Nucleus

Molybdenum Metallopharmaceuticals Candidate Compounds - The "Renaissance" of Molybdenum Metallodrugs?

Metal-based drugs, also called "metallopharmaceuticals" or "metallodrugs", are examples of sophisticated compounds that have been used in inorganic medicinal chemistry as therapeutic agents for a long time. Few of them have shown substantially promising results and many of them have been used in different phases of clinical trials. The Mo-based metallodrugs were successfully applied in the past for treating conditions such as anemia or Wilson's disease. Moreover, Mo complexes are supposed to exert their effect by intercalation/ cleavage of DNA/RNA, arrest of the cell cycle, and alteration of cell membrane functions. However, in the current literature, there are no reliable and in-depth reviews about the hypothetical therapeutic applications of all of the known molybdenum complexes as metallopharmaceuticals/ metallodrugs. The main emphasis was on the in-depth review of the potential applications of Mo-based complexes in medicinal chemistry as metallopharmaceuticals in treating diseases such as cancer and tumors, Wilson's disease, diabetes mellitus, Huntington's disease, atherosclerosis, and anemia. It must be emphasized that today the development of innovative and new Mo-based metalo-pharmaceuticals is not rapid, and hence the aim of this paper was also to inspire colleagues working in the field of Mo compounds who are trying to find "signpost" for research. The authors hope that this article will increase interest and initiate the Renaissance of Mo-compounds among medicinal inorganic chemists. This paper is the first review article in the literature that refers to and emphasizes many different and complex aspects of possible applications and capabilities of Mo-based metallodrugs.

Self-consistent (DFT + U) study of electronic, structural and magnetic properties in A2NiMoO6 (A = Ba, Sr) compounds

This work aims to study the double perovskites A2NiMoO6 (A = Ba, Sr) by using the first principle calculation, within the framework of the self-consistent Hubbard correction. The value of this correction, for Ni and Mo depends strongly on the type of the studied compounds. Such values are determinate as 7.35 eV for Ni and 5.42 eV for Mo in the compound Ba2NiMoO6, whereas in the case of Sr2NiMoO6 the calculated values are 8.71 eV and 5.48 eV for Ni and Mo compound respectively. Based on the densities of state calculation we show that both the compounds are semiconductors with gap energies of 2.20 eV and 2.10 eV for Ba2NiMoO6 for Sr2NiMoO6, respectively. The total energies comparison shows clearly that the ground state is G-type anti-ferromagnetic order in agreement with experimental results, the magnetic interactions are due to the super-exchange mechanism acting in long range via hybridization throw Ni-O-Mo-O-Ni path.

Ground state of the U2Mo compound: Physical properties of the Ω-phase

Using ab initio calculations, unexpected structural instability was recently found in the ground state of the U2 Mo compound. Instead of the unstable I4/mmm and the Pmmn structures, in this work the P6/mmm (#191) space group, usually called Ω-phase, is proposed as the fundamental state. Total energy calculations using Wien2k code slightly favoured the last structure. Electronic and elastic properties are studied in this work in order to characterize the physical properties of this new phase. The stability of the Ω-phase is studied by means of its elastic constants calculation and phonon dispersion spectrum. Analysis of isotropic indices shows that the new phase is a ductile material with a minimal degree of anisotropy, suggesting that U2 Mo in the P6/mmm structure is an elastic isotropic material. Analysis of charge density, density of electronic states (DOS) and the character of the bands revealed a high level of hybridization between d-molybdenum electronic states and d- and f-uranium ones.

Development and research of nanostructured multilayer composite coatings for tungsten-free carbides with extended area of technological applications

This paper discusses aspects of the development of nanostructured multilayer composite coatings (NMCCs) formed using the processes of filtered cathodic vacuum arc deposition (FCVAD) for application to the tungsten-free carbides (cermets) based on TiC-(Ni,Mo) and TiCN-(Ni,Mo) compounds in order to improve cutting properties of tools and to expand the area of their technological application. NMCCs were used not only to improve the physical and mechanical properties of the working surfaces of tools but also to ensure the control over contact processes during cutting. The study has shown that despite their high hardness, thermal stability and resistance to scaling, low tendency to diffusion with the material being machined, and tungsten-free carbides are characterized by relatively low fracture toughness and bending strength, low thermal conductivity. With regard to the above properties, tungsten-free carbides are inferior not only to tungsten WC-Co carbides but also to WC-TiC-Co carbides with binder content of less than 8 % (by weight). Therefore, cutting tools made of tungsten-free carbides have a limited range of technological application in interrupted cutting, machining of hard-to-cut alloys and steels. With respect to this, the paper considers the possibility of directional control over contact processes during cutting with the use of NMCCs to create more balanced properties of tungsten-free carbides with regard to hardness and toughness. This work has developed architecture of three-component nanostructured multilayer composite coatings, the methods for selecting functions and rational component parameters of architecture for tools made of tungsten-free carbides. The developed compositions of NMCCs have improved cutting properties of tungsten-free carbides and expanded the area of their technological application in cutting of heat-treated steels of increased hardness and machining of heat-resistant alloys.

Influence of Temperature, Roughness and Washer Shape on Crevice Corrosion for 254 SMO Alloy

Effect of temperature, roughness and washer shape on 254 SMO alloy was applied to study the susceptibility of alloy’s crevice corrosion. XPS measurements have been carried out to detect and define the products which formed on the surface of 254 SMO in 22% NaCl at 30oC at applied potential 600 mVSCE. The formation of Fe, Cr and Mo compounds which play a great role to protect the alloy was found.

Charged particle decay of hot and rotatingMo88nuclei in fusion-evaporation reactions

A study of fusion-evaporation and (partly) fusion-fission channels for the $^{88}$Mo compound nucleus, produced at different excitation energies in the reaction $^{48}$Ti + $^{40}$Ca at 300, 450 and 600 MeV beam energies, is presented. Fusion-evaporation and fusion-fission cross sections have been extracted and compared with the existing systematics. Experimental data concerning light charged particles have been compared with the prediction of the statistical model in its implementation in the Gemini++ code, well suited even for high spin systems, in order to tune the main model parameters in a mass region not abundantly covered by exclusive experimental data. Multiplicities for light charged particles emitted in fusion evaporation events are also presented. Some discrepancies with respect to the prediction of the statistical model have been found for forward emitted $\alpha$-particles; they may be due both to pre-equilibrium emission and to reaction channels (such as Deep Inelastic Collisions, QuasiFission/QuasiFusion) different from the compound nucleus formation.

Structural and Magnetic Studies of Novel 1-D Cyanido-bridged [CuII(Me2en)][CuII(Me2en)2][MoIV(CN)8]6H2O Chain

Background: The design, synthesis and physicochemical characterization of new switchable magnetic materials is among the most important tasks in the search of multifunctional compounds. Along these lines, intense efforts are focused on the investigation of cyanido–bridged coordination polymers: Prussian Blue Analogs and bimetallic coordination networks constructed of octacyanidometallates. Within this family, the CuII–[Mo(CN)8]4- assemblies have been extensively studied for their photomagnetic properties. The main aim of this work was the design, synthesis, as well as structural, spectroscopic and magnetic characterization of novel molecular material based on copper(II) complex with aliphatic bidentate polyamine and octacyanidometallate( IV) which shows a high number of cyanido bridges. The consequent objective was the study of potential photomagnetic properties of this new material. Methods: A newly synthesized compound [CuII(Me2en)][CuII(Me2en)2][MoIV(CN)8]·6H2O (1), Me2en = N,N– dimethylethylenediamine, was investigated structurally using single crystal X-ray diffraction, spectroscopically by studying IR and UV/Vis spectra, and photo- and magnetically characterized by studying its magnetic response in the dark and under irradiation in a variety of magnetic fields and temperatures. Results: Incorporation of N,N–dimethylethylenediamine (Me2en) into the CuII–[MoIV(CN)8]4- system leads to the construction of a novel coordination polymer [CuII(Me2en)][CuII(Me2en)2][MoIV(CN)8]·6H2O (1). The assembly reveals the unique structure consisting of 1–D chains constructed of {[Cu(Me2en)][Mo(CN)8]}n 2n- ladders with {[Cu(Me2en)2]2n+ pendant arms connected through cyanido–bridges to the Mo(IV) centers. Structural parameters of 1 were compared with other Cu(II)–Mo(IV) compounds with bidentate aliphatic polyamine ligands. Magnetic studies of 1 indicate paramagnetic behavior with weak antiferromagnetic interactions at low temperature. Photomagnetic studies with the application of 405, 473, 532 and 650 nm light sources and white light show no detectable photomagnetic effect. Conclusion: Novel 1–D ladder with pendant arms [CuII(Me2en)][CuII(Me2en)2][MoIV(CN)8]·6H2O (1) coordination polymer reveals unprecedented topological pattern and shows one of the highest complexities among 1–D octacyanidometallate– based chains. The local environments of metal centers were compared with other Cu(II)–Mo(IV) assemblies containing bidentate chelating polyamine ligands. In accordance with this analysis, two types of copper(II) moieties were distinguished among this kind of assemblies. Moreover, the analysis of [Mo(CN)8]4- for systems with various dimensionalities shows the same geometry and structural parameters of the moieties, differing in the number of bridging cyanides. Despite similar structural, spectroscopic and magnetic properties of 1 in comparison with other copper(II) complexes with aliphatic polyamines and octacyanidomolybdate(IV) revealing photomagnetic effect, no light induced increase of magnetization was observed. This leads to the observation that a large number of Cu(II)–NC–Mo(IV) linkages does not guarantee the presence of photomagnetic behavior.

Effect of Laser Surface Treatment on the Corrosion Behavior of FeCrAl-Coated TZM Alloy

The current study involves the coating of Titanium-Zirconium-Molybdenum (TZM) alloy with FeCrAl through plasma thermal spraying which proved effective in improving the oxidation resistance of the substrate. A post-laser surface melting treatment further enhanced the surface protection of the TZM alloy. Oxidation tests conducted at 1100 °C in air indicated that some Mo oxides were formed at the surface but a relatively small amount of weight reduction was observed for FeCrAl-coated TZM alloys up to 60 min of treatment. The post-laser surface treatment following the plasma thermal spray process apparently delayed the severe oxidation process and surface spalling of the alloy. It was suggested that the slow reduction in weight in the post-laser-treated specimen was related to fewer defects in the coating layer. It was also found that a surface reaction layer formed through the diffusion of Fe into the Mo alloy substrate at high temperature. The layer mainly consisted of Fe-saturated Mo and FeMo intermetallic compounds. In order to observe the corrosion behavior of the laser-treated alloy in 3.5% NaCl solution, electrochemical characteristics were also investigated. A proposed equivalent circuit model for the specimen indicated localized corrosion of coated alloy with some permeable defects in the coating layer.

A Model for the Active-Site Formation Process in DMSO Reductase Family Molybdenum Enzymes Involving Oxido-Alcoholato and Oxido-Thiolato Molybdenum(VI) Core Structures.

New bis(ene-1,2-dithiolato)-oxido-alcoholato molybdenum(VI) and -oxido-thiolato molybdenum(VI) anionic complexes, denoted as [Mo(VI)O(ER)L2](-) (E = O, S; L = dimethoxycarboxylate-1,2-ethylenedithiolate), were obtained from the reaction of the corresponding dioxido-molybdenum(VI) precursor complex with either an alcohol or a thiol in the presence of an organic acid (e.g., 10-camphorsulfonic acid) at low temperature. The [Mo(VI)O(ER)L2](-) complexes were isolated and characterized, and the structure of [Mo(VI)O(OEt)L2](-) was determined by X-ray crystallography. The Mo(VI) center in [Mo(VI)O(OEt)L2](-) exhibits a distorted octahedral geometry with the two ene-1,2-dithiolate ligands being symmetry inequivalent. The computed structure of [Mo(VI)O(SR)L2](-) is essentially identical to that of [Mo(VI)O(OR)L2](-). The electronic structures of the resulting molybdenum(VI) complexes were evaluated using electronic absorption spectroscopy and bonding calculations. The nature of the distorted O(h) geometry in these [Mo(VI)O(EEt)L2](-) complexes results in a lowest unoccupied molecular orbital wave function that possesses strong π* interactions between the Mo(d(xy)) orbital and the cis S(p(z)) orbital localized on one sulfur donor from a single ene-1,2-dithiolate ligand. The presence of a covalent Mo-S(dithiolene) bonding interaction in these monooxido Mo(VI) compounds contributes to their low-energy ligand-to-metal charge transfer transitions. A second important d-p π bonding interaction derives from the ∼180° O(oxo)-Mo-E-C dihedral angle involving the alcoholate and thiolate donors, and this contributes to ancillary ligand contributions to the electronic structure of these species. The formation of [Mo(VI)O(OEt)L2](-) and [Mo(VI)O(SEt)L2](-) from the dioxidomolybdenum(VI) precursor may be regarded as a model for the active-site formation process that occurs in the dimethyl sulfoxide reductase family of pyranopterin molybdenum enzymes.

The metallurgical characteristics of non-precious alloys using Nd:YAG laser welding.

BackgroundThis study aimed to determine the effect of hardness change according to penetration depth in the laser fusing zone and observed the correlation of the microstructure as an Nd:YAG laser was irradiated to Ni-Cr alloy for dental use by setting the spot diameter size with respect to defocusing distances. In all groups, the hardness depth profiles in the laser fusing zone and heat-affecteded zone (HAZ) had larger values than those of the base metal. In addition, the hardness values in places beyond the fusing zone and the HAZ were measured as being quantitatively lower.MethodsThe alloys used in this study were Verabond 2 V, Noritake Super, and Bellabond Plus, which are commercially used non-precious dental alloys. The specimens were cut to have a plate shape with a size of 0.5 × 3.0 × 2.5 mm. This was followed by setting the Nd:YAG laser output, pulse duration, and frequency to 60 W, 10 ms, and 5 Hz, respectively. The laser was then irradiated as the spot diameter condition varied between 0.5 mm and 1.4 mm in accordance with defocusing distance from 0.0 mm to 2.0 mm. After the laser irradiation, a cross-section of the fusing zone in the specimens was observed in terms of laser melted depth, hardness depth profile, and the microstructure of each alloy.ResultsThe observation result of the diffusion of the constituent elements and microstructure using field emission scanning electron microscopy, energy dispersive spectroscopy (EDS), and electron probe micro-analyzer showed that the fusing zone revealed a much finer dendritic form than the base metal due to the self-quenching effect after laser melting, while no change in constituent elements was found although some evaporation of the main elements was observed.ConclusionsThese results suggest that each Mo and Si combined inter-metallic compounds were formed on the interdendritic area. Through this study, the laser fusing zone had better hardenability due to the inter-metallic compound and grain refinement effect.

Equilibrium concentrations of Mo, V, and Ni compounds in the combustion products of the carbon-containing solid residues of tar hydroconversion

The equilibrium composition of the combustion products of the carbon-containing solid residue of hydroconversion was calculated by chemical thermodynamics methods over a wide range of temperatures taking into account V and Ni constituents and MoS2 used as a nanosized catalyst.

Influence of Temperature, Roughness and Washer Shape on Crevice Corrosion for 254 SMO Alloy

Effect of temperature, roughness and washer shape on 254 SMO alloy was applied to study the susceptibility of alloy's crevice corrosion. XPS measurements have been carried out to detect and define the products which formed on the surface of 254 SMO in 22% NaCl at 30oC at applied potential 600 mVSCE. The formation of Fe, Cr and Mo compounds which play a great role to protect the alloy was found.

Single crystal growth and characterization of the first reduced lanthanum molybdenum oxychloride, La20Mo12O63Cl4, with an unusual trigonal prismatic MoO6 unit

The use of a KCl/CsCl alkali eutectic flux under reducing conditions facilitated the crystal growth of the mixed valent Mo(V)/Mo(VI) compound La20Mo12O63Cl4. La20Mo12O63Cl4 crystallizes in the noncentrosymmetric hexagonal space group P-62m with a = 10.2241(2) Å and c = 15.0768(4) Å, which represents a new structure type exhibiting a three-dimensional crystal structure containing three different molybdenum coordination environments, MoO5 square pyramids, MoO6 trigonal prisms, and MoO5 trigonal bipyramids. The temperature dependence of the magnetic susceptibility was measured and La20Mo12O63Cl4 was found to be paramagnetic across the entire 2–300 K temperature range measured. Although noncentrosymmetric, La20Mo12O63Cl4 was not found to be second harmonic generation active. La20Mo12O63Cl4 was further characterized by UV/Vis spectroscopy.

SmO thin films: A flexible route to correlated flat bands with nontrivial topology

Using density functional theory based calculations, we show that the correlated mixed-valent compound SmO is a 3D strongly topological semi-metal as a result of a 4$f$-5$d$ band inversion at the X point. The [001] surface Bloch spectral density reveals two weakly interacting Dirac cones that are quasi-degenerate at the M_bar-point and another single Dirac cone at the Gamma_bar-point. We also show that the topological non-triviality in SmO is very robust and prevails for a wide range of lattice parameters, making it an ideal candidate to investigate topological nontrivial correlated flat bands in thin-film form. Moreover, the electron filling is tunable by strain. In addition, we find conditions for which the inversion is of the 4f-6s type, making SmO to be a rather unique system. The similarities of the crystal symmetry and the lattice constant of SmO to the well studied ferromagnetic semiconductor EuO, makes SmO/EuO thin film interfaces an excellent contender towards realizing the quantum anomalous Hall effect in a strongly correlated electron system.

Experimental dissolution of molybdenum‐sulphides at low oxygen concentrations: A first‐order approximation of late Archean atmospheric conditions

AbstractThe abundance of atmospheric oxygen and its evolution through Earth's history is a highly debated topic. The earliest change of the Mo concentration and isotope composition of marine sediments are interpreted to be linked to the onset of the accumulation of free O2 in Earth's atmosphere. The O2 concentration needed to dissolve significant amounts of Mo in water is not yet quantified, however. We present laboratory experiments on pulverized and surface‐cleaned molybdenite (MoS2) and a hydrothermal breccia enriched in Mo‐bearing sulphides using a glove box setup. Duration of an experiment was 14 days, and first signs of oxidation and subsequent dissolution of Mo compounds start to occur above an atmospheric oxygen concentration of 72 ± 20 ppmv (i.e., 2.6 to 4.6 × 10−4 present atmospheric level (PAL)). This experimentally determined value coincides with published model calculations supporting atmospheric O2 concentrations between 1 × 10−5 to 3 × 10−4 PAL prior to the Great Oxidation Event and sets an upper limit to the molecular oxygen needed to trigger Mo accumulation and Mo isotope variations recorded in sediments. In combination with the published Mo isotope composition of the rock record, this result implies an atmospheric oxygen concentration prior to 2.76 Ga of below 72 ± 20 ppmv.