Showing papers on "Ammonium tetrathiomolybdate published in 1998"

In Situ Investigation of the Thermal Decomposition of Ammonium Tetrathiomolybdate Using Combined Time-Resolved X-ray Absorption Spectroscopy and X-ray Diffraction

Abstract: The thermal decomposition of ammonium tetrathiomolybdate, a process widely used for the preparation of molybdenum sulfides, has been studied in situ for the first time, using combined Mo K edge XAFS and X-ray powder diffraction. Isothermal decomposition at 120 °C produced pure amorphous molybdenum trisulfide (a-MoS3), after 135 min, with no further reaction observed on continued heating. XAFS spectra and diffraction patterns collected during the heating showed the disappearance of crystalline (NH4)2MoS4 and the formation of a-MoS3 without the formation of an intermediate. Thermal decomposition of (NH4)2MoS4 by heating from 100 to 400 °C produced ultimately poorly crystalline MoS2. It is shown that the Mo−Mo nonbonded correlation of ∼3.2 A found by some previous workers in a-MoS3 is most likely due to the presence of small quantities of poorly crystalline MoS2. This allows certain structural models for a-MoS3 to be ruled out. A detailed comparison between XAFS data collected from crystalline 2H−MoS2 at a v...

Thermogravimetric-differential thermal analysis of the solid-state decomposition of ammonium tetrathiomolybdate during heating in argon

Abstract: The formation of MoS2 by thermal decomposition of ammonium tetrathiomolybdate (ATT) solids under an argon atmosphere has been studied by simultaneous thermogravimetric and differential thermal analysis. The sequential products for the decomposition upon heating to 700 °C is ATT (hydrated)→(NH4)2MoS4→(NH4)HMoS4→H2MoS4→MoS3→ Mo2S5→MoS2. MoS2 forms between 230 and 260 °C and remains stable up to about 360 °C when it tends to be oxidized by residual oxygen, if present in the atmosphere. These findings suggest that the synthesis of MoS2 from (NH4)2MoS4 via formation of MoS3 is not a direct process, as previously reported, but rather a complex process involving a number of intermediate products (NH4)HMoS4, H2MoS4 and Mo2S5 which have not been reported before. That these products are only specific to the very narrow temperature regimes as revealed suggests that they are very unstable and short lived, that their presence is transient in nature and thus that ex-situ characterization of them is normally difficult. The presence of these intermediate products, as justified experimentally, is further interpreted in terms of their mutual structural similarities which improve understanding as to why MoS2 can usually be prepared from ATT by thermal decomposition, as in this case, or by other techniques, such as anodizing. Laminar morphology of MoS2 is revealed by transmission electron microscopy and its crystal structure examined by selected-area diffraction. Further ex-situ examination by X-ray photoelectron spectroscopy of this end product supports the feasibility of preparing MoS2 from aqueous solutions by anodizing. © 1998 Kluwer Academic Publishers

Studies of molybdenum sulfide catalyst ex ammonium tetrathiomolybdate: effect of pretreatment on hydrodesulfurization of dibenzothiophene

Abstract: Alumina-supported molybdenum sulfide catalyst was prepared by the decomposition of ammonium tetrathiomolybdate (ATTM) and tested in the hydrodesulfurization (HDS) of dibenzothiophene (DBT). The HDS activity of unpromoted and Co-promoted molybdenum sulfide catalysts was found to depend strongly on the activation procedure of the catalyst. In all cases, the activity of the Co-promoted catalyst was superior to that of the ATTM counterpart. Surface characterization of the catalyst subjected to different pretreatments was carried out by X-ray photoelectron spectroscopy and FTIR spectroscopy of NO probe. The pretreatments were effective in obtaining a highly dispersed heterometallic sulfide phase and a good synergy between Co and molybdenum sulfide.