Showing papers on "Molybdenum published in 1984"

Partial oxidation of methane by nitrous oxide over molybdenum on silica

Abstract: Kinetic data are presented for the partial oxidation of methane to methanol and formaldehyde by nitrous oxide catalysed by molybdenum supported on silica when steam is present in the system. The data are used as evidence to show how the selective oxidation mechanism fits into an overall reaction scheme. Moreover, in support of this selective mechanism, spectroscopic evidence is given for the formation of methyl radicals and methoxide ions on the surface. 12 references, 7 figures, 1 table.

Molybdenum atom ligand substitution reactions of molybdenum-iron-sulfur (MoFe3S4) cubane-type clusters: synthesis and structures of clusters containing molybdenum-bound pseudosubstrates of nitrogenase

Abstract: Resolution des structures cristallines des trois complexes (Et 4 N) 4 [Mo 2 Fe 6 S 8 (S-p-C 6 H 4 Cl) 6 (cat) 2 ], (Et 4 N) 3 [MoFe 3 S 4 (SEt) 3 (Cl 4 cat)(CN)] et (Et 4 N) 3 [MoFe 3 S 4 (S-p-C 6 H 4 Cl) 3 (Cl 4 cat)N 3 ]•MeCN. Spectres RMN

EXAFS studies of amorphous molybdenum and tungsten trisulfides and triselenides

Abstract: lower freezing point of N 2 relative to argon, and hence a slightly slower rate of freezing during the deposition process and slightly greater mobility of the reactant species. It is also interesting to note that a recent study2 of the S 0 2 / H 2 0 system found the large majority of the sulfur either in the form of SO2 or as S2052-, while very little of the intermediate SO3,was noted. The disulfite S205,anion might form by one of several routes under these conditions. One is the simultaneous, three-body collision of two SO2 molecules and T120, but this is quite unlikely at the concentrations employed. More likely, formation of S2052involves initial formation of the SO3,anion, followed by addition of a second SO2 unit to form the final product anion. The fact that the ‘*O label appears in the bridging position as well as in the terminal oxygen positions is consistent with this view as well. If this is the mechanism by which S20,2is formed, then the addition of SO, to SO3,must be very facile, with effectively no activation energy, since in many cases nearly all of the SO3,is converted to S2052during the condensation process. A final possible mechanism, the reaction of T120 with sulfur dioxide dimer, (SO,),, cannot be ruled out but does not seem likely in that this dimer is weakly bound and should not be abundant a t the concentrations employed. Conclusions

Biosynthesis of iron-molybdenum cofactor in the absence of nitrogenase.

Abstract: Klebsiella pneumoniae accumulates molybdenum during nitrogenase derepression. The molybdenum is primarily in nitrogenase component I in the form of iron-molybdenum cofactor (FeMo-co). Mutations in any of three genes (nifB, nifN, and nifE) involved in the biosynthesis of FeMo-co resulted in very low molybdenum accumulation and in a molybdenum-free nitrogenase component I. A mutant lacking both subunits of nitrogenase component I accumulated 60% of the amount of molybdenum present in the wild type. The molybdenum was in protein-bound form and behaved differently than that in the wild type with respect to electrophoretic mobility, size, and extractability by organic solvents. Two forms of molybdenum could be extracted from the protein fraction of the mutant; one of them was not detected in the wild type, and the other behaved like FeMo-co in nonaqueous gel filtration chromatography. Crude extracts of this mutant were able to complement in vitro K. pneumoniae or Azotobacter vinelandii mutants unable to produce FeMo-co. These data show that biosynthesis of FeMo-co does not require the presence of nitrogenase component I. In its absence, FeMo-co is accumulated on a different protein, presumably an intermediate in the normal FeMo-co biosynthetic pathway.

Surface coatings and subcoats

Abstract: Coatings are disclosed (a) which are useful by themselves to substantially improve the surface properties of substrates to which they are applied and (b) which are useful as subcoats to substantially improve the bonding of coatings of nitrides, carbides and borides of titanium, hafnium and zirconium to such substrates. The coatings which are disclosed are novel materials characterized by a microcrystalline, single-phase, solid solution structure comprising: (a) about 20 to 70% by weight of at least one element from the group consisting of cobalt and nickel; (b) about 8 to 35% by weight of chromium; (c) about 3 to 20% by weight of at least one element from the group consisting of molybdenum and tungsten; and (d) about 0.5 to 10% by weight of at least one element from the group consisting of carbon, nitrogen and boron.

Kinetics of oxygen atom transfer reactions involving oxomolybdenum complexes. General treatment for reactions with intermediate oxo-bridged molybdenum(V) dimer formation

Abstract: Cinetique des reactions Mo VI O 2 L 2 +X⇄Mo IV OL 2 +XO L=R 2 NCS 2 − , X=PPh 3 , dans le dichloro-1,2 ethane

Hydride formation rates of titanium-based b.c.c. solid solution alloys☆

Abstract: Titanium has an h.c.p. structure at room temperature and reacts with hydrogen at an appreciable rate only at elevated temperatures. However, the metal undergoes an allotropic transformation to a b.c.c. structure at 882.5 °C. The b.c.c. phase can be stabilized by suitable alloying with niobium, molybdenum, vanadium etc. so that the b.c.c. phase is retained at room temperature. Rapid reactions at room temperature of these b.c.c. solid solution phases with hydrogen have been observed when small amounts of a third element are added. Suitable additions are those elements which have radii 5% or more smaller than that of titanium.

Mono- and binuclear tri- and tetracarbonyl complexes of chromium(0), molybdenum(0), and tungsten(0) with the 2,2'-bipyrimidine radical anion

Abstract: Etude par RPE a haute resolution des complexes [((CO) 4 M) x bpym] − (x=1,2; M=Cr,Mo,W; bpym=bipyrimidyle-2,2'). Interpretation des resultats par une localisation electronique

Molybdenum(VI)-dioxo complexes with sterically bulky ligands

Abstract: Synthese de nouveaux complexes MoO 2 LH 2 avec LH 4 =N,N'-bis(hydroxy-2 t-butyl-3 benzyl) ethylenediamine. Etude de la reduction coulometrique. Etude RX, RPE

Quantitative transfer of the molybdenum cofactor from xanthine oxidase and from sulphite oxidase to the deficient enzyme of the nit-1 mutant of Neurospora crassa to yield active nitrate reductase.

Abstract: An assay method is described for measurement of absolute concentrations of the molybdenum cofactor, based on complementation of the defective nitrate reductase ('apo nitrate reductase') in extracts of the nit-1 mutant of Neurospora crassa. A number of alternative methods are described for preparing, anaerobically, molybdenum-cofactor-containing solutions from sulphite oxidase, xanthine oxidase and desulpho xanthine oxidase. For assay, these were mixed with an excess of extract of the nit-1 mutant, incubated for 24 h at 3.5 degrees C then assayed for NADPH:nitrate reductase activity. In all cases, the specific activity of the molybdenum cofactor, expressed as mumol of NO2-formed/min per ng-atom of Mo added from the denatured molybdoenzyme , was 25 +/- 4, a value that agrees with the known catalytic activity of the nitrate reductase of wild-type Neurospora crassa. This indicates that, under our conditions, there was quantitative transfer of the molybdenum cofactor from denatured molybdoenzyme to yield fully active nitrate reductase. Comparable cofactor assay methods of previous workers, apparently indicating transfer efficiencies of at best a few per cent, have never excluded satisfactorily the possibility that cofactor activity arose, not from stoichiometric constituents of the molybdoenzymes , but from contaminants. The following factors were investigated separately in developing the assay:the efficiency of extraction of the cofactor from the original enzyme, the efficiency of the complementation reaction between cofactor and apo nitrate reductase, and the assay of the resultant nitrate reductase, which must be carried out under non-inhibitory conditions. Though the cofactor is unstable in air (t1/2 about 15 min at 3.5 degrees C), it is stable when kept anaerobic in the presence of sodium dithionite, in aqueous solution or in dimethyl sulphoxide (activity lost at the rate of about 3%/24 h at 20-25 degrees C). Studies of stabilities, and investigations of the effect of added molybdate on the assay, permit conclusions to be drawn about the ligation of molybdenum to the cofactor and about steps in incorporation of the cofactor into the apoenzyme. Though the development of nitrate reductase activity is slow at 3.5 degrees C (t1/2 1.5-3 h) the complementation reaction may be carried out in high yield, aerobically. This is ascribed to rapid formation of an air-stable but catalytically inactive complex of the cofactor, as a precursor of the active nitrate reductase.(ABSTRACT TRUNCATED AT 400 WORDS)

Superconductivity in MoN films with NaCl structure

Abstract: Stoichiometric molybdenum nitride with sodium chloride-type structure has been prepared by reactive sputtering and by ion implantation of nitrogen into molybdenum. The lattice parameter of this phase is 0.4212 nm. The superconducting transition temperature is only around 3K in contrast to expectations raised by theoretical and empirical predictions.