Leonidas Petrakis

Effects of impregnation pH on the surface structure and hydrodesulfurization activity of Mo/Al2O3 catalysts

A series of Mo/Al/sub 2/O/sub 3/ (molybdenum/alumina) catalysts was prepared by incipient wetness impregnation at various pHs (4.0, 5.4, 7.1, and 11.0) using a fixed amount of Mo (8% Mo). The effect of pH on the state and dispersion of Mo species in the dried, calcined, and reduced catalysts was investigated by the combined use of two surface sensitive techniques: X-ray photoelectron spectroscopy (XPS, ESCA) and ion scattering spectroscopy (ISS). The surface properties were correlated to thiophene hydrodesulfurization activities for the various catalysts. A tentative interpretation of the influence of pH of the Mo impregnating solution on the surface structure of Mo/Al/sub 2/O/sub 3/ catalysts at various stages of their preparation is proposed. 8 figures, 1 table.

Surface characterization of thorium-nickel intermetallic catalysts

Electron spectroscopy for chemical analysis (ESCA), Auger electron spectroscopy (AES), and ion scattering spectroscopy (ISS) have been used to study a series thorium-nickel alloys (Th7Ni3, ThNi, ThNi2, ThNi5), both untreated and treated with different gases (O2, air, COH2). ESCA results indicate that the surfaces of untreated alloys are Ni-poor with respect to the bulk. Treatment of these alloys with synthesis gas (COH2), O2, or air results in decomposition of the intermetallic into NiThO2 or NiOThO2. In all cases Ni was observed to segregate to the surface after reaction. The degree of Ni surface segregation is not affected by different gas treatments but varies for different alloys. The relationship between the relative Ni signal intensity and the bulk Ni content was examined and compared by ESCA, AES, and ISS which have different sampling depths. Catalysts with large surface concentrations of Ni generally show high activity for the methanation reaction. Comparison of the Ni surface concentration with catalytic activity of Th7Ni3 and ThNi5 has shown that they do not correlate mathematically. This incompatibility can be explained by the presence of different active sites, a spongy model, or both.

The spectroscopic characterization of intermetallic synthesis gas conversion catalysts and the correlation of their activity with surface structure

Abstract Two series of Ni x Si y (NiSi 2 , Ni 3 Si 2 , Ni 2 Si and Ni 5 Si 2 ) and Ni x Th y (Ni 3 Th 7 , Ni 2 Th, NiTh and Ni 5 Th) intermetallics have been prepared by melting the component metals in an induction furnace. Surface and bulk characterization were carried out by the combined use of several techniques: Ion Scattering Spectroscopy (ISS), X-ray Photoelectron Spectroscopy (XPS, ESCA) and X-ray diffraction (XRD). The results show that the surfaces of the untreated Ni x Si y and Ni x Th y alloys are all nickel-poor compared to the bulk. Oxidation treatment further decreases the surface Ni content of Ni x Si y alloys. Surface enrichment in nickel is, however observed in the case of Ni x Th y intermetallics upon oxidation. Oxidized Ni x Si y alloys which are reportedly active in methanation (Ni 5 Si 2 , Ni 2 Si) show a relative surface enrichment in nickel and evidences of partial decomposition whereas silicon-rich alloys are essentially stable. Oxidized Ni 5 Th, which exhibits the greater methanation activity within the Ni x Th y system, shows equally the highest surface concentration in nickel. The correlation between Ni surface content and CO conversion data will be discussed in terms of the relative configuration of the Ni phase with respect to ThO 2 .

Surface Characterization Of Supported Nickel-Thorium Catalysts

X-ray photoelectron spectroscopy (XPS, ESCA) and x-ray diffraction analysis (XRD) have been used to investigate the influence of thorium on the state and dispersion of nickel oxide supported on silica and γ-alumina. The results indicate that the dispersion of the nickel is much greater on the γ-alumina than on the silica support. On the γ-alumina support, the surface concentration of the nickel decreases as the thorium content increases. On the silica support, the surface concentration of the nickel increases as the thorium content increases.

Association of Ni-mesoporphyrin IX dimethyl esteran NMR study of the mesoproton chemical shifts

Abstract Nickel-mesoporphyrin IX dimethyl ester (Ni-MPDME) has been studied by NMR as a function of solvent, concentration and temperature. The very low concentration- and temperature-dependent data reported indicate that environmental effects within the porphyrin molecule as well as solvent-solute interaction play a significant role in determining the chemical shifts of the mesoprotons. Intermolecular association contributes significantly to the complexity of the resonances at higher concentrations. The energy of dissociation of the porphyrin dimers is of the order of 3 kcal mole −1 . This value is significantly lower than the energy reported earlier for the vanadyl porphyrin analogue.

Surface characterization and CO hydrogenation activity of silane-treated nickel/aluminas

High-surface-area supported nickel-silicon alloys were prepared by treating Ni/Al2O3 with silane (SiH4) at 300–400°C in flowing hydrogen. Microbalance studies indicated that the supported nickel-silicon alloys were easily decomposed, compared with alloys prepared by bulk melting. However, the decomposed alloys were difficult to reduce, indicating formation of an interaction species. ESCA and ISS studies indicate that silane treatment brings about considerable shielding of the nickel surface which coincides with a drastic decrease in CO hydrogenation activity.