Metal Sulfide Species Research Articles

Metal-sulfide species in oxic waters

Several analytical techniques were employed to characterize oxidation-resistant metal-sulfide species that have recently been suggested to be present in oxic waters. Sulfide in synthetic Cd-, Zn-, Pb-, Cu-sulfide solutions was found to persist in oxic waters for a prolonged period of time (2 to more than 10 weeks) whereas it was oxidized rapidly in Fe-, Mn- and Ni-sulfide solutions. Multiple lines of evidence did not support the presence of soluble molecular metal-sulfide clusters in these solutions as suggested previously. Instead, the metal-sulfide species resistant to oxidation in oxic waters were more likely a mixture of truly dissolved metal-sulfide complexes and dynamic metal-sulfide colloids. The morphology and size of the colloids, as monitored by transmission electron microscopy (TEM), changed significantly with time. Nanomolar to sub-micromolar levels of acid-volatile sulfide (AVS) and chromous-volatile sulfide (CVS) were measured in oxic surface waters of six lakes located on the Canadian Shield and in the nearby areas. Thermodynamic calculations indicated that at the AVS levels measured, the dissolved metal-sulfide complexes play a minor role in the speciation of Class B metal ions such as Pb, Cd, Cu, Hg. The relative importance of metal-sulfide colloids in these natural oxic waters remains unknown.

Catalytic activity of Co, Mo and CoMo supported on NaY zeolite: hydrodesulfurization of gas oil at high pressure

Three series of Co/NaY, Mo/NaY and CoMo/NaY zeolite catalysts with variable metal content, prepared by a conventional impregnation method, were characterized by XRD, IR spectroscopy (oxide state) and acidity measurements (sulfide state), and tested in hydrodesulfurization (HDS) of gas oil at high pressure in the temperature range 275–350°C. The combined results of surface area, XRD and IR showed that in the catalysts with high metal loading a small loss in crystallinity and a partial blockage of the zeolite supercages were produced by Mo oxide species. The number of acid sites, which was lower for the Co/NaY than for the Mo/NaY catalysts, increased with increasing Co or Mo loading, but the strength of the acid sites was stronger for the Co/NaY series. HDS specific activities of the Co/NaY and Mo/NaY monometallic catalysts reached a maximum at very low loadings of Co (∼ 0.10 at. nm−2) or Mo (∼ 0.16 at. nm−2) by the double action of the metal sulfide species and the strong acid sites generated on the zeolite by the Co or Mo incorporation. In the binary CoMo/NaY catalysts, the synergy between Co and Mo species was significant for high Mo contents only.

A review of carbon-supported hydrodesulfurization catalysts

The purpose of this paper is to provide a comprehensive review of carbon-supported hydrodesulfurization (HDS) catalysts and to serve as a guide to investigators exploring the potential of these catalysts. The preparative techniques and physical properties of the commonly used carbon supports are discussed, as is the influence of the surface functional groups on the carbon surface charge and metal species adsorption. The various techniques which have been applied to the characterization of the active metal sulfide species in HDS catalysts are also described. Where necessary, the properties and performance of conventional γ-Al 2O 3-supported hydrodesulfurization catalysts are provided and compared to those of the carbon-based counterparts. The results of several investigations indicate that carbon-supported catalysts have a potential advantage over HDS catalysts in current use, both with respect to high desulfurization activity and low coking propensity.

The influence of phosphorus on the structure and hydrodesulfurization activity of sulfided Fe and Fe-Mo catalysts supported on carbon and alumina

Phosphorus-containing Fe and Fe−Mo sulfide catalysts supported on γ-Al2O3 and activated carbon were evaluated for their thiophene HDS activities at atmospheric pressure. The thiophene HDS activity of the carbon-supported catalysts decreased considerably in the presence of phosphorus, while the activity of the alumina-supported catalysts was not affected. It was shown by Mossbauer spectroscopy that in both the oxidic carbon- and alumina-supported catalyst precursors the presence of phosphorus resulted in an improved dispersion of the Fe phase. In the sulfided carbon-supported Fe and Fe−Mo catalysts, the presence of phosphorus resulted in the formation of an “Fe(II)-phosphate” phase, which was held responsible for the decrease in HDS activity. With regard to the sulfided alumina-supported catalysts, it was found that phosphorus did not influence the structure of the metal sulfide species.

Bacteria as nucleation sites for authigenic minerals in a metal-contaminated lake sediment

Sediment samples contaminated with metals arising from mine tailings drainage were obtained from Lower Moose Lake in the Onaping region near Sudbury, Ontario, Canada. The samples were examined by electron microscopy, selected-area electron diffraction and energy-dispersive X-ray spectroscopy. Individual bacterial cells and their remains were prominent as nucleation sites for both metal sulfides and a complex polymorphic (Fe,Al)-silicate. The principal metal sulfide species associated with the bacteria were amorphous mackinawite (FeS 1 - x ) and microcrystalline millerite (NiS). Trace amounts of Cu and Zu were also detected in some of the sulfide precipitates. At least two structural forms of the (Fe,Al)-silicate were present, and energy-dispersive X-ray spectroscopy point analyses revealed corresponding differences in chemical composition. Poorly ordered limonitic clay-type phases had a granular morphology and contained less Fe than well-developed crystalline material which generated hexagonal diffraction patterns with reflections ( d = 4.60 and 2.55 Ȧ) characteristic of an interstratified chamositic clay.

The reactivity and stability of mixed-sulfide structures in unsupported MoS 2-based hydrodesulfurization catalysts promoted by group VIII metals

Unsupported CoMo, NiMo and FeMo catalysts have been prepared by the homogeneous sulfide precipitation method and the coprecipitation method. The presence of a group VIII metal (Me) causes a decrease in the reactivity with O 2 and an increase in the reducibility in H 2. These effects are correlated with the increase in the catalytic activity. The stability of cobalt and iron sulfide species in CoMo and FeMo catalysts was studied by Mössbauer spectroscopy. A significant amount of MeMoS structure remains stable after strong reducing treatment. Catalysts contain also sulfur-rich group VIII metal sulfide species which appear stabilized by MoS 2. The role of these species in promoting HDS activity is evaluated.