No abstract

X-Ray diffraction, F.t.i.r., scanning electron microscopy (SEM) and ionscattering spectroscopy (ISS) have been applied to characterize a series of Moo,-TiO, (anatase) samples prepared by the grafting and impregnation methods. The sample prepared with the grafting method exhibits good dispersion of MOO,, whereas the impregnation technique leads to a poorer dispersion of molybdena on the surface of titania. The poorer dispersion of MOO, is due to the clustering of molybdena when the MOO, loading does not exceed the theoretical monolayer coverage (6.3 wt % MOO,). At higher MOO, loading, crystals of MOO, of morphology different from pure MOO, crystals, appear. A possible explanation of this phenomenon is discussed.

No abstract

ChemInform Abstract A series of MoO3-anatase catalysts are prepared by grafting and impregnation methods and characterized by XRD, FTIR, SEM, XPS, and ISS in order to determine the nature of MoO3 when the monolayer coverage is exceeded. It is established that well dispersed MoO3 can be obtained as a monolayer with the grafting method. The impregnation method leads to worse dispersion of MoO3 on the anatase surface due to clustering of molybdena. At MoO3 loadings higher than the theoretical monolayer coverage (6.3 wt.% MoO3) epitaxially grown MoO3 crystals appear and their epitaxial growth produces a specific morphology of the MoO3 crystals.

Two chromium-doped Raney nickel catalysts have been prepared from Al-Ni-Cr alloys where Cr was introduced in such a way a# to substitute for nickel in the Ni A13 intermetallic phaae. an3 metallic surface areas remain little affected but more aluminium is retained after etching. X.P.S. show# that aluminium and nickel are in the metallic state whereas chromium is completely oxidized. Very large amounts of chromium are found on the aurface, which auggests migration of chromium to the aurface during loaching. Analytical electron microscopy indicates that the local chromium concentration is not perfectly uniform. INTRODUCTIONThe catalytic propertiea of a metal can be modified by bi-or multi-metallic asaociations. In the case of Raney nickel catalysts, the metal moat often combined with nickel u e Fe, Cr, Co, Mo, Cu or W 11-101. Chromium has retained particular attention as a dope for enhancing the activity and/or changing the selectivity of certain reactions 14, 6-10].Such a change of selectivity waa reported recently in a competitive hydrogenation of cyclohexene and acetophenone i the ratio of the reaction rates VcldVclc is ten times higher with a Raney nickel catalyst doped by 2 at. % Cr than with an undoped catalyst 1101. The cauaes of the changes of the catalytic properties induced by doping are uaually ascribed to a change of the electronic denaity of the active mitea and/ or to a restructuring of the metal atom ensamblea at the aurface ly, Faaman et a1 have also invoked the preaence of oxidea in doped Raney nickel catalysts obtained by doping the precursor Ni-A1 alloy 121. Careful characterization of doped catalyats is thus essential. Surprisingly, such characterizations are acarce in the literature. In a previous work based on XPS analysis of Raney nickel catalyst. obtainod from pure NiAlj and Ni2A13 intermetallic compounds 1121, we have ahown that

No abstract