A. N. Startsev

Sulphide catalysts on silica as a support. III: X-ray photoelectron spectroscopy study

Abstract Sulphide (Ni,W)/Si02 and (Ni,MO)/Si02 catalysts prepared by decomposition of surface organometallic complexes in H2S have been studied by X-ray photoelectron spectroscopy. The binding energy of electrons in the Ni 2p3/2level has been found to increase simultaneously with decrease of that on W 4f7/2 (Mo 3d5/2) level in bimetallic catalysts, compared with the corresponding values for monometallic sulphide catalysts. This effect is ascribed to the partial transfer of electron density from nickel atoms onto tungsten (molybdenum) atoms through the formation of mixed surface sulphides. A shift in the binding energies of Ni 2p3/2 and W 4f7/2 levels in supported catalysts in comparison with those in bulk sulphides is suggested to be evidence of chemical interaction between the supported sulphides and the silica surface.

Sulphide catalysts on silica as a support. i. effect of the preparation technique of (ni,w)/si02 and (ni,mo)/si02 catalysts on their activity in thiophen hydrogenolysis

Abstract Sulphide catalysts (Ni,W)/Si02 and (Ni,Mo)/Si02 have been prepared using surface allyl complexes of tungsten, molybdenum and nickel as precursors for sulphide particles. When Ni was added to sulphided Mo/Si02 and W/SiO2 catalysts 10-90 times increase of activity in thiophen hydrogenolysis was observe. The most active catalysts were prepared via anchoring of organometallic compounds of one element on dispersed sulphide particles of another element. Activity referred to the total amount of two metals has the maximum value at λ = Ni/[Ni+W(Mo)] = 0.2-0.4. Activity referred to one g atom of Ni increased with decreasing Ni content and reached the maximum value at a λ ≦0.2. Mixed catalysts prepared by conventional impregnation method are found to be some times less active than samples prepared from organometallic precursors. The data obtained are in favour of providing intimate mixing of two sulphides for preparation of active catalysts.

Sulphide catalysts on silica as a support. VI: On the structures of the active phase and active centers of hydrodesulphurization catalysts

Based on the experimental data of the study of highly dispersed sulphided (Ni-W)/SiO2 and (Ni-Mo)/SiO2 catalysts prepared from organometallic precursors, the structures of the active phase and active centers for CS bond hydrogenolysis are discussed. The active phase of hydrodesulphurization catalysts represents layered sulphide structures with the same MS and MM (MW or Mo) distances as in bulk disulphides. A single MS2 sheet is a sufficient structural element for the formation of active centers. It was suggested that the active centers include Ni atoms, on which thiophene adsorption occurs, and W(Mo) atoms of the side planes of the MS2 sheet. Sulphur atoms bound with one Ni atom are involved in the catalytic cycle.

Sulphide catalysts on silica as a support. ii. high resolution electron microscopy data

Abstract Bimetallic sulphided (Ni,W)/Si02 and (Ni,Mo)/Si02 catalysts prepared from W(Mo) and Ni organometallic precursors were studied by transmission high resolution electron microscopy. It was shown that addition of Ni to sulphided W(Mo)/Si02 catalysts with further sulphidization results in the formation of thread-like structures ∼ 25 A long. Mostly these structures are single; when they form assemblies the distance between neighbouring threads is, ∼ which corresponds to the distance between separate sheets in WS2 (MoS2) crystals. Comparison of morphology and catalytic activity of “organometallic” and “standard” catalyst samples gives evidence that the active centers of thiophen hydrogenolysis are located on the side planes (or edges) of dispersed WS2(MOS2) crystals.

Sulphide catalysts on silica as a support: VIII. Peculiarities of thiophene hydrogenolysis and probable nature of “synergetic effect”

Abstract The main kinetic features of thiophene hydrogenolysis on mono- and bimetallic sulphide catalysts were analyzed. Sulphide molybdenum and tungsten catalysts were shown to differ from nickel catalysts in that they exhibited higher hydrogenation activity. According to the data from the temperature-programmed reduction (TPR) the amount of sulphur removed at TPR was not functionally dependent on the activity of bimetallic sulphide catalysts in thiophene hydrogenolysis. A new reaction mechanism of thiophene hydrogenolysis is proposed with consideration given to the structure of the active component of bimetallic catalysts. The adsorption and activation of a thiophene molecule is supposed to occur on nickel atoms stabilized on the side planes of MoS2 (WS2) single slabs, while hydrogen activation is achieved on the Mo(W) atoms. A high rate of chemical reaction seems to be achieved by synchronous interaction of thiophene and hydrogen molecules in the coordination sphere of a bimetallic sulphide species in the rate-determining reaction step.

Composition and catalytic activity of supported sulphided metal catalysts prepared via anchored complexes I. Tungsten catalysts prepared via anchoring of W(C4H7)4 ON SiO2

Anchored organometallic complexes of tungsten were prepared by supporting tetramethallyltungsten on SiO2. The composition of the surface compounds obtained by treatment of the anchored complexes with H2S was studied. The activities of sulphided catalysts containing different surface species of tungsten in the hydrogenolysis of thiophen were measured. An increase in turn-over number (per surface atom of tungsten) was observed for catalysts containing small sulphide aggregates. The electron-deficient state of W and S in these aggregates by comparison with WS2 was shown by the x.p.s. technique.

Nature of active centers of molybdenum catalysts for propylene disproportionation, obtained in the reaction of tetrakis(π-allyl)molybdenum with silica

The activities of surface compounds of Mo(VI), Mo(IV) and Mo(II) obtained via interaction of Mo(π-C3H5)4 with silica have been compared using the disproportionation of propylene. The surface compounds of tetravalent molybdenum were shown to be the active centers for disproportionation.AbstractСопоставлялась активность поверхностных соединении MoVI, MoIV и MoII, полученных взаимодействием Mo(π-C3H5)4 с снликагелем, в реакции диспропорционирования пропилена. Показано, что поверхностные соединения четырехвалентного молибдена являются активными центрами в реакции диспропорционирования.

TEM and XPS studies of Ni/WS2 catalysts for thiophene hydrogenolysis

Differences in the structure of WS2 samples with SBET=7.6 and 75 m2/g are manifested in decreasing lengths of basal faces of some small crystallites in the more dispersed WS2. This effect increases the relative fraction of lateral faces on the catalyst surface. Ni ions in the Ni/WS2 catalysts are in an electrondeficient state compared to NiS.AbstractРазличие струры образоцв WS2 с поверхностью 7,6 м2/qg и 75 М2/г проявляется в уменьшении длины базальных плоскостей у отдельных мелких кристаллов и в увеличении выходов на их поверхность боковых граней для более дисперсного WS2. Ионы Ni в катализаторах Ni/WS2 нахочятся в электронодефицитном (по сравнению с NiS) состоянии.

Sulphide catalysts on silica as a support: VII. Isotope exchange of hydrogen sulphide with sulphide catalysts

Abstract Isotope exchange of H235S with the surface of sulphide catalysts Mo/SiO2 and (Ni-M)/SiO2 (M = Mo, W), prepared both by synthesis of surface metal complexes and by traditional impregnation, was studied. The rate of heteroexchange over the temperature range 150 to 300°C was shown to be almost the same on mono- and bimetallic sulphide catalysts obtained by the above procedure. The rate of thiophene hydrogenolysis increased with decreasing reactivity of surface sulphide groups towards the H235S heteroexchange.

Sulphide catalysts on silica as a support. : V. EXAFS study of the nearest environment of tungsten and nickel in w/SiO2 and Ni,W/SiO2 catalysts.

Abstract Analysis of the extended X-ray absorption fine structure (EXAFS) of W LIII edge showed that in sulphided W/SiO2 and Ni,W/SiO2 catalysts, th