Metal Sulfide Species Research Articles

Synergy of Cu doping and SO42− adsorption in the self-optimized SO42−@Cu doped Co(OH)2 reconstructed from CuCo2S4 towards enhanced oxygen evolution reaction

Catalysts in-situ reconstructed from transition metal sulfides for oxygen evolution reaction (OER) have been regarded as potential alternatives for the noble metal catalysts. Intriguingly, it was found that the self-optimized SO42−@Cu doped Co(OH)2 catalyst was reconstructed from bimetallic sulfide of CuCo2S4. During the OER process, the copper was partially leached and meantime doped into the in-situ generated Co(OH)2 with SO42− adsorption, while only oxides were discovered in counterparts of mono-metal sulfides. As a result, the SO42−@Cu doped Co(OH)2 catalyst exhibited superior OER performances, with low overpotentials of 211/272 mV to achieve the current densities of 10/100 mA cm−2, exceeding the counterparts reconstructed from mono-metal sulfides. Experimental and theoretical results suggested that the in situ generated Cu doped Co(OH)2 possessed thin and loosely layered structure with expanded lattice spacing (lattice spacing expanded from 0.28 nm to 0.46 nm) due to the SO42− adsorption, making the intermediates more accessible to the active centers. In addition, the catalyst surface changed from hydrophobicity to super hydrophilicity, which greatly accelerated the mass transfer. This work indicated that the in-situ reconstruction process was greatly influenced by the interactions between metal sites and sulfide species, directly influencing the OER performances of the reconstructed catalysts.

Density functional theory study on the structural evolution of mercury sulfide adsorbed on zinc sulfide

The unclear structure evolution of mercury sulfides on metal sulfides (MSs) impedes the mechanistic understanding of the mercury immobilization processes on MSs-based sorbents. This work adopted the earth-abundant ZnS to investigate the transformation of the compositional unites of cinnabar comprised of repeated …S-Hg-S-Hg-S-Hg… sequences, on ZnS(110) surface. The results show that mercury mono-sulfide (HgS) was stable on ZnS(110) surface, indicating it would further grow into HgS2 or Hg2S unit. However, after exploring the stability of HgS2 and Hg2S, it was found that HgS2 was relatively stable, while Hg2S dissociated into HgS and an isolated Hg atom on ZnS(110) surface. This phenomenon suggests that no …S-Hg-S-Hg-S-Hg… chain was formed. The mechanistic understanding of the transformation of metal sulfide species on the ZnS surface is of great help to not only explain the evolution of mercury on metal sulfides, but also design efficient metal sulfides for mercury removal from anthropogenic sources.

Structure-property relationship of zeolite-Y/cubic ZnS nanohybrid material and its utilization for gaseous phase mercury removal

Zinc acetate dihydrate and thiourea based metal sulfide was synthesized and used to develop zeolite-ZnS (ZY-ZnS) nanohybrid material by using sonication process, solvothermal method and heat treatment process. The sulfur atom in thiourea is involved in formation of metal sulfide and ethylene glycol was used as solvents in the solvothermal method. The structure of the nanohybrid material was analyzed by Raman spectroscopy, FTIR, XRD and XPS. Raman spectra confirmed the presence of 4, 6- membered rings of zeolite and polar modes A1 and E1 and nonpolar mode E2 of ZnS nanoparticles. The morphology of the nanohybrid material was analyzed by TEM and SEM analysis. The uniform distribution of ZnS in the zeolite matrix was observed in TEM and SEM morphology. The spherical aggregates ZnS crystals size between 100 and 200 nm were observed. Cubical morphology was observed in pure zeolite samples in the range of 400 nm−500 nm. The X-ray photoelectron spectroscopy analysis was used to confirm the metal-sulfide species in the hybrid material. Mercury adsorption (elemental (Hg°)) in the vapor phase follows both pseudo-first-order and pseudo-second-order kinetic models. Kinetic study shows that the mercury adsorption on the zeolite-ZnS nanohybrid adsorbent was mostly regulated by the chemisorption. Vapor phase mercury adsorption capacity of zeolite-Y (ZY) and zeolite-ZnS (ZY-ZnS), is 2295 and 3461 µg/g at 180 °C, respectively. In ZY and ZY-ZnS the intraparticle diffusion rates were 86.1 and 141.2 μg/gmin-1/2, respectively. The higher intraparticle diffusion rate observed in ZY-ZnS confirm the uniformly decorated ZnS nano particles in ZY-ZnS nanohybrid material.

Microwave-assisted synthesis of iron sulfide motifs for electrochemical applications

The syntheses of FeS2 and Fe3S4 nanomaterials were optimized using a novel facile, surfactant-free, and microwave-assisted, one-pot synthesis method, run under ambient and reasonably mild reaction conditions. Synthetic parameters, such as metal precursor salt identity, reaction time, reaction temperature, metal:sulfur molar ratios, and solvent combinations, were all systematically investigated and optimized. A series of FeS2 (pyrite) samples was initially fabricated using thioacetamide (TAA) as the sulfur precursor to generate a distinctive, uniform octahedra-based morphology. Switching the sulfur precursor from TAA to L-cysteine resulted in a corresponding transformation in not only chemical composition from FeS2 to an iron thiospinel structure, Fe3S4 (otherwise known as greigite), but also an associated morphological evolution from octahedra to nanosheet aggregates. The study of these materials has enabled crucial insights into the formation mechanisms of these materials under a relatively non-conventional microwave-assisted setting. Furthermore, in separate experiments, multi-walled carbon nanotubes (MWNTs) and graphene were added in with underlying metal sulfide species to create conductive Fe–S/MWNT composites and Fe–S/graphene composites, respectively. The method of addition of either MWNTs or graphene was also explored, wherein an ‘ex-situ’ synthetic procedure was found to be the least disruptive means of attachment and immobilization onto iron sulfide co-reagents as a means of preserving the latter’s inherent composition and morphology. The redox acidity for the parent material and associated composites demonstrates the utility of our as-developed synthetic methods for creating motifs relevant for electrochemical applications, such as energy storage.

Multiple Modifications of Li-B Alloy Anodes for Primary Batteries

Lithium-boron (Li-B) alloys have long been applied in thermal batteries and have recently been used in secondary batteries due to their stable three-dimensional (3D) framework. To extend the application of Li-B anodes to primary cells, especially primary cells with high rate performance, we developed a new surface treatment method to modify Li-B. Using silver trifluoromethanethiolate (AgSCF3) as the single reagent to react with Li to produce metal Ag, LiF, and sulfide species at the surface of Li-B. For the symmetric cell, the resulting multiple modified Li-B shows two orders of magnitude smaller charge transfer impedance than the pristine Li-B (1.10 ohms vs. 205.40 ohms) and improved reaction kinetics in the first cycle. The modified Li-B/MnO2 primary cells show improved rate performance in the current density range of 0.5 C − 5 C. Specifically, when discharging at 2 C, the full cell using modified Li-B anode show specific capacity of 197 mAh g−1 and specific energy of 485 Wh kg−1, 82% and 103% higher related to the cell using pristine Li-B anode, respectively. This study provides a new way to modify Li-B alloys that may have practical applications in high-power density primary batteries.

Positive phosphorous effect during co-processing of pyrolysis bio-oils and S-content model compounds over sulfide NiMo/P/HMS-Al catalysts

This article is focused on the problem of the inhibition of the hydrodeoxygenation (HDO) reaction by competitive adsorption of the S-containing compounds on the active sites. The phosphorous effect during co-processing of pyrolysis bio-oils and S-content model compounds over sulfide NiMo/P/HMS-Al catalysts was investigated. A series of NiMo catalysts supported on mesoporous P(x)/HMS-Al materials were prepared and characterized by several techniques. Guaiacol (GUA) was considered as a pyrolysis bio-oils model compound, while the dibenzothiophene (DBT) was used as a model molecule of S-content. The activity test revealed that support modification with 1.1 wt% of P (NM-P1.1) decreased the inhibition of HDO reaction by co-adsorption of DBT on the active sites. With the most active NM-P1.1 catalyst, the formation of a less susceptible to coke formation product (phenol) was favored. The main superiority of sulfided NM-P1.1 catalyst comes from the support decoration with P2O5 nanoparticles leading to its highest Brønsted acidity, the surface enrichment with metal sulfide species as well as with the diminishing of Al leaching from the framework of support during the HDO reaction. The activity tests demonstrated that support modification with phosphorous is an efficient method to solve the problem of the HDS inhibition by the HDO reaction.

Carbon-coated mixed-metal sulfide hierarchical structure: MOF-derived synthesis and lithium-storage performances

With the aim to satisfy the demands for future-generation lithium-ion battery (LIB) with large reversible capacity, high energy density and long-period stability, more explorations focus on the metal sulfide electrode materials. A unique hollow hierarchical sphere of sulfur-doped carbon coated FeS2/ZnS (Fe-Zn-S@S-doped-C) is demonstrated to be an effective electrode candidate for Li-ion battery in this work. In favor of the robust hollow hierarchically porous spherical structure design, the doped-S in the carbon matrix along with the synergetic effect originated from the two metal sulfide species with different Li-storage potentials, the Fe-Zn-S@S-doped-C composite exhibits large reversible capacity and long-term stability when utilized as the Li-ion battery anode material. After 200 cycles, a large capacity (1321 mAh g−1) is retained, which is substantially higher than the theoretical value of FeS2 or ZnS. The Fe-Zn-S@S-doped-C composite also delivers good rate capabilities (679 and 317 mAh g−1 at 1 and 5 A g−1, respectively). Furthermore, the kinetic analysis based on cyclic voltammograms and Galvanostatic Intermittent Titration Technique reveal the fast lithium-storage kinetics of the Fe-Zn-S@S-doped-C electrode, which is largely originated from the dominated pseudocapacitive contribution.

In Situ Polysulfide Detection in Lithium Sulfur Cells.

Lithium sulfur batteries promise significant improvements in specific energy compared to Li-ion, but are limited by capacity fade upon cycling. Efforts to improve durability have focused on suppressing the solubility of intermediate polysulfides in the electrolyte. Here we describe an in situ electrochemical polysulfide detection method based on the cyclic volatmmetric response. The voltammetric peaks correlate with increased discharge, consistent with increased polysulfide species in the electrolyte as demonstrated by prior literature measurements using spectroscopic methods. We verified that adding metal sulfide species to the sulfur cathode and ceramic-coatings on the polyolefin separator result in reduced polysulfide concentration, consistent with improved cycle life reported earlier. Further, the use of highly concentrated electrolytes produces no detectable dissolved polysulfide species. Future advances in Li/S technology could utilize this method to determine the polysulfide contents in the electrolyte, and thus quantify the efficacy of the sulfur-sequestering strategies.

H2S removal from syngas using wastes pyrolysis chars

Pyrolysis chars from wastes were investigated as sorbents for H2S removal from syngas. The H2S removal tests were performed at ambient temperature in various dry gas matrices (N2, Air, Syngas) to study the effect of the gas composition on the adsorption efficiency. Two chars were produced by the pyrolysis of: used wood pallets (UWP), and a 50/50% mixture of food waste (FW) and coagulation-flocculation sludge (CFS). The chars were functionalized by low-cost processes without chemicals: gas phase oxygenation and steam activation. Activated chars were the most efficient materials due to their large specific surface area, alkaline pH, basic O-containing groups and structural defects in graphene-like sheets. Raman analysis evidenced that inherent mineral species (especially Ca and Fe) increased the H2S removal efficiency by promoting the formation of metal sulfide and metal sulphate species at the char surface. Mesopores lower than 70 Å were revealed to be important adsorption sites. Under dry Syngas matrix, the chars remained efficient and selective toward H2S removal despite the presence of CO2, while O2 in the Air matrix decreased their removal capacity due to the formation of sulfur acid species. The most efficient material was the steam activated char from FW/CFS, with a removal capacity of 65 mgH2S.g−1 under dry syngas. This char was proved to be completely regenerated with a thermal treatment under N2 at 750 °C. This study demonstrated that activated chars from food waste and sludge could be used as eco-friendly, affordable, and selective materials for syngas desulfurization even under dry atmosphere.

In Situ Transformation of MOFs into Layered Double Hydroxide Embedded Metal Sulfides for Improved Electrocatalytic and Supercapacitive Performance.

Direct adoption of metal-organic frameworks (MOFs) as electrode materials shows impoverished electrochemical performance owing to low electrical conductivity and poor chemical stability. In this study, we demonstrate self-templated pseudomorphic transformation of MOF into surface chemistry rich hollow framework that delivers highly reactive, durable, and universal electrochemically active energy conversion and storage functionalities. In situ pseudomorphic transformation of MOF-derived hollow rhombic dodecahedron template and sulfurization of nickel cobalt layered double hydroxides (NiCo-LDHs) lead to the construction of interlayered metal sulfides (NiCo-LDH/Co9 S8 ) system. The embedment of metal sulfide species (Co9 S8 ) at the LDH intergalleries offers optimal interfacing of the hybrid constituent elements and materials stability. The hybrid NiCo-LDH/Co9 S8 system collectively presents an ideal porous structure, rich redox chemistry, and high electrical conductivity matrix. This leads to a significant enhancement in its complementary electrocatalytic hydrogen evolution and supercapacitive energy storage properties. This work establishes the potential of MOF derived scaffold for designing of novel class hybrid inorganic-organic functional materials for electrochemical applications and beyond.

Carbon Nanotubes Rooted in Porous Ternary Metal Sulfide@N/S‐Doped Carbon Dodecahedron: Bimetal‐Organic‐Frameworks Derivation and Electrochemical Application for High‐Capacity and Long‐Life Lithium‐Ion Batteries

Lithium ion battery is the predominant power source for portable electronic devices, electrical vehicles, and back‐up electricity storage units for clean and renewable energies. High‐capacity and long‐life electrode materials are essential for the next‐generation Li‐ion battery with high energy density. Here bimetal‐organic‐frameworks synthesis of Co0.4Zn0.19S@N and S codoped carbon dodecahedron is shown with rooted carbon nanotubes (Co‐Zn‐S@N‐S‐C‐CNT) for high‐performance Li‐ion battery application. Benefiting from the synergetic effect of two metal sulfide species for Li‐storage at different voltages, mesoporous dodecahedron structure, N and S codoped carbon overlayer and deep‐rooted CNTs network, the product exhibits a larger‐than‐theoretical reversible Li‐storage capacity of 941 mAh g−1 after 250 cycles at 100 mA g−1 and excellent high‐rate capability (734, 591, 505 mAh g−1 after 500 cycles at large current densities of 1, 2, and 5 A g−1 , respectively).

Combined effect of carbon dioxide and sulfur on vapor–liquid partitioning of metals in hydrothermal systems

Although CO2 is a ubiquitous volatile in geological fluids typically ranging from a few to more than 50wt%, its effect on metal vapor–liquid fractionation during fluid boiling and immiscibility phenomena in the Earth’s crust remains virtually unknown. Here we conducted first experiments to quantify the influence of CO2 on the partition of different metals in model water+salt+sulfur+CO2 systems at 350°C and CO2 pressures up to 100bar, which are typical conditions of formation of many hydrothermal ore deposits. In addition, we performed in situ Raman spectroscopy measurements on these two-phase systems, to determine sulfur and carbon speciation in the liquid and vapor phases. Results show that, in S-free systems and across a CO2 concentration range of 0–50wt% in the vapor phase, the absolute vapor–liquid partitioning coefficients of metals (Kvap/liq=Cvap/Cliq, where C is the mass concentration of the metal in the corresponding vapor and liquid phase) are in the range 10−6–10−5 for Mo; 10−4–10−3 for Na, K, Cu, Fe, Zn, Au; 10−3–10−2 for Si; and 10−4–10−1 for Pt. With increasing CO2 from 0 to 50wt%, Kvap/liq values decrease for Fe, Cu and Si by less than one order of magnitude, remain constant within errors (±0.2 log unit) for Na, K and Zn, and increase by 0.5 and 2 orders of magnitude, respectively for Au and Pt. The negative effect of CO2 on the partitioning of some metals is due to weakening of hydration of chloride complexes of some metals (Cu, Fe) in the vapor phase and/or salting-in effects in the liquid phase (Si), whereas both phenomena are negligible for complexes of other metals (Na, K, Zn, Mo). The only exception is Pt (and in a lesser extent Au), which partitions significantly more to the vapor of S-free systems in the presence of CO2, likely due to formation of volatile carbonyl (CO) complexes. In the S-bearing system, with H2S content of 0.1–1.0wt% in the vapor, Kvap/liq values of Cu, Fe, Mo, and Au are in the range 0.01–0.1, those of Pt 0.5–2.0, those of alkali metals are similar to the S-free system, and the partitioning of none of the studied metals is influenced by the presence of CO2 (up to 50wt% in the vapor). Our data thus confirm the large enhancement of volatility in the presence of reduced sulfur (H2S) due to formation of sulfide complexes for chalcophile metals such as Au, Pt, Mo and, to a lesser extent, Cu and Fe, as reported in previous studies of CO2-free water-salt systems. The negligible effect of CO2 on vapor–liquid partitioning of the studied metals in S-bearing systems is due to the lack of hydration of metal sulfide species making them little sensitive to changes in water activity and solvation power of CO2–H2O vapor. Our findings, combined with existing data over a wide range of temperature on vapor–liquid partitioning of metals in H2O-dominated systems, suggest that CO2 exerts mostly an indirect impact on metal fractionation, by extending vapor–liquid immiscibility to higher temperatures and pressures or depth compared to a CO2-free H2O-S-salt system. The deeper vapor–liquid separation, in particular in S-bearing systems, is expected to cause more significant partitioning of precious metals and molybdenum (Au, Pt, Mo) into the vapor phase while base metals (Fe, Zn, Cu) remain concentrated in the salt-rich (NaCl, KCl) liquid phase. In addition, irrespective of the presence of sulfur, an expansion of the immiscibility domain to higher temperature and pressure conditions in the presence of CO2 will also increase the depth of ore deposition and affect the vertical metal zonation in hydrothermal systems.

Electroanalytical methods in characterization of sulfur species in aqueous environment

Electroanalytical (voltammetric, polarographic, chronoamperometric) methods on an Hg electrode were applied for studying of different sulfur compounds in model and natural water systems (anoxic lakes, waste water, rain precipitation, sea-aerosols). In all investigated samples typical HgS reduction voltammetric peak, characteristic for many different reduced sulfur species (RSS: sulfide, elemental sulfur, polysulfide, labile metal sulfide and organosulfur species) was recorded at about -0.6 V vs. Ag/AgCl reference electrode. In addition, in anoxic waters which are enriched with sulfide and iron species, voltammetric peaks characteristic for the presence of free Fe(II) and FeS nanoparticles (NPs) were recorded at -1.4 V and around -0.45 V, respectively. Depending on the used electroanalytical method and experimental conditions (varying deposition potential, varying time of oxidative and/or reductive accumulation, sample pretreatment i.e. acidification followed by purging) it is possible to distinguish between different sulfur species. This work clearly shows a large potential of the electrochemistry as a powerful analytical technique for screening water quality regarding presence of different reduced sulfur species and their speciation between dissolved and colloidal/nanoparticle phases.

Quantum Chemical Study of the Reactions between Pd+/Pt+and H2O/H2S

The study of the reactions of water and hydrogen sulfide with palladium and platinum cations has been completed in this work, in both low- and high-spin states. Our calculations predict that only the formation of platinum sulfide is exothermic (in both spin states), whereas for the remaining species the oxides and sulfides are found to be more reactive than their corresponding bare metal cations. An in-depth analysis of the reaction paths leading to metal oxide and sulfide species is given, including various minima, and several important transition states. All results have been compared with existing experimental and theoretical data, and earlier works covering the reaction of nickel cation with water and hydrogen sulfide to observe the trends for the group 10 transition metals.

Zinc sulfide surface formation on Hg electrode during cyclic voltammetric scan: an implication for previous and future research studies on metal sulfide systems

Cyclic voltammetry on the Hg electrode was used to investigate the electrochemical behavior of NaCl/NaHCO3 electrolyte solutions supersaturated with respect to Zn sulfide phases. The voltammetric results clearly show how an Hg electrode, due to exchange between Hg2+ from an HgSadlayer and Zn2+ from solution, becomes the site for surface ZnSadlayer formation in the potential range −0.45 to −0.6 V. The exchange reaction is reversible, and the surface-formed ZnSadlayer persists at the Hg electrode surface until −1.3 V during cathodic scans. Near −1.3 V, it is reduced. In the same solution, evidence for reduction of bulk Zn sulfide species including nanoparticles was not obtained. The approach emphasized here can be readily extended to any other system consisting of metal electrode and chalcogenide anions, pointing to the importance of choosing experimental conditions (i.e., deposition potential, stirring, and accumulation times) to avoid artifacts and wrong interpretation of data due to surface formation of metal sulfide species.

Nanocomposites Based on Metal and Metal Sulfide Clusters Embedded in Polystyrene

Transition-metal alkane-thiolates (i.e., organic salts with formula Me(SR)x, where R is a linear aliphatic hydrocarbon group, –CnH2n+1) undergo a thermolysis reaction at moderately low temperatures (close to 200 °C), which produces metal atoms or metal sulfide species and an organic by-product, disulfide (RSSR) or thioether (RSR) molecules, respectively. Alkane-thiolates are non-polar chemical compounds that dissolve in most techno-polymers and the resulting solid solutions can be annealed to generate polymer-embedded metal or metal sulfide clusters. Here, the preparation of silver and gold clusters embedded into amorphous polystyrene by thermolysis of a dodecyl-thiolate precursor is described in detail. However, this chemical approach is quite universal and a large variety of polymer-embedded metals or metal sulfides could be similarly prepared.

Role of Au Nanoparticles and NiTiO<SUB>3</SUB> Matrix in H<SUB>2</SUB>S Sensing and Its Catalytic Oxidation to SO<SUB><I>x</I></SUB>

The role of Au nanoparticles inside TiO2-NiO sol-gel thin films for hydrogen sulfide optical detection has been assessed by combining optical spectroscopy, X-ray diffraction (XRD), reaction products analysis and X-ray photoemission spectroscopy (XPS) data. Sulfur oxides (i.e., SO2 and SO3, hereafter SOx) are clearly distinguished as reaction products and SOx species are detected on the surface of the samples by XPS. After exposure to H2S, no metal sulfide species can be detected, neither by XRD nor XPS, so we propose that the reaction mechanism does not involve the formation of Ni, Ti or Au sulfides, but a direct catalytic oxidation of H2S to SOx. The role of Au is to provide an optical probe sensitive to the H2S presence, while the oxide matrix is responsible for the conversion of H2Sto SOx.

Growth kinetics and long-term stability of CdS nanoparticles in aqueous solution under ambient conditions

The ubiquity of naturally occurring nanoparticles in the aquatic environment is now widely accepted, but a better understanding of the conditions that promote their formation and persistence is needed. Using cadmium sulfide (CdS) as a model metal sulfide species, thiolate-capped CdS nanoparticles were prepared in the laboratory to evaluate how aquatic conditions influence metal sulfide nanoparticle growth and stability. This work examines CdS nanoparticle growth directly in aqueous solution at room temperature by utilizing the size-dependent spectroscopic properties of semiconductors detectable by UV/vis. CdS nanoparticle growth was governed by oriented attachment, a non-classical mechanism of crystallization in which small precursor nanoparticles coalesce to form larger nanoparticle products. Nanoparticle growth was slowed with increasing capping agent and decreasing ionic strength. In addition to examining the short-term (hours) growth of the nanoparticles, a long-term study was conducted in which cysteine-capped CdS nanoparticles were monitored over 3 weeks in solutions of various ionic strengths. The long-term study revealed an apparent shift from small nanoparticles to nanoparticles twice their original size, suggesting nanoparticle growth may continue through oriented attachment over longer time scales. High-ionic strength solutions resulted in salt-induced aggregation and eventual settling of nanoparticles within days, whereas low-ionic strength solutions were stable against settling over the course of the experiment. Sulfide recovery from cysteine-capped CdS nanoparticles as acid volatile sulfide was nearly quantitative after 2 weeks in fully oxygenated water, demonstrating significantly slowed oxidation of sulfide when complexed to Cd(II) within CdS nanoparticles. The nanoparticles were also shown to be resistant to oxidation by Fe(III) (hydr)oxide. This study illustrates that aggregation, rather than chemical oxidation, is likely more important to the lifetime of many metal sulfide nanoparticles in the aquatic environment.

The impact of Co and Ni speciation on methanogenesis in sulfidic media—Biouptake versus metal dissolution

The speciation of the trace nutrients Co(II) and Ni(II) in sulfide containing media can control the methanogenic activity of Methanosarcina sp., which is of importance for the optimisation of anaerobic treatment of wastewater containing methanol. To obtain more insight in the mechanistic backgrounds, dissolved metal concentrations and methane production rates were measured for a highly enriched culture, obtained from anaerobic sludge, at various total metal and sulfide concentrations. The methanogenic activity increased with increasing total metal and decreased with increasing total sulfide. Dissolution kinetics and the influence of soluble metal sulfide species explain the trends in the concentrations of the dissolved metal species. The results suggest that the stimulation of the methanogenic activity is related to the free metal ion concentration. Flux calculations and measurements of uptake by a pure culture of Methanosarcina barkeri show that limitation by slow dissolution of the solid metal sulfides is unlikely. Growth therefore seems to be primarily controlled by the biouptake characteristics of the microorganism.

Synthesis and Reactions of Group 4 Imido Complexes Supported by Cyclooctatetraene Ligands

The reactions of the pseudo-two-coordinate titanium imido complexes [Ti(NtBu)(COT)] (1) (COT = η8-C8H8), [Ti(NtBu)(COT‘ ‘)] (2) (COT‘ ‘ = η8-1,4-C8H6(SiMe3)2), and [Ti(NAr)(COT)] (3) (Ar = 2,6-iPr2C6H3) with a variety of organic substrates are reported. Reaction of 1 with CO2, tBuNCO, or ArNCO and reaction of 3 with CO2 or tBuNCO afforded the organic products tBuNCO, tBuNCNtBu, tBuNCNAr, ArNCO, and ArNCNtBu, respectively, and a titanium oxo species. These reactions proceeded via an initial [2 + 2] cycloaddition to form an N,O-bound intermediate of the type [Ti{N(R)C(O)R‘}(COT)], which subsequently underwent a retrocycloaddition to give an organic product and the titanium oxo species. In contrast, reaction of 3 with ArNCO led to the formation of the N,N-bound [2 + 2] cycloaddition product [Ti{N(Ar)C(O)N(Ar)}(COT)] (7). In general, the reactions of 1 and 3 with CS2 and isothiocyanates also resulted in an initial [2 + 2] cycloaddition to form an N,S-bound intermediate of the type [Ti{N(R)C(S)R‘}(COT)], which also subsequently underwent a retrocycloaddition to give an organic product and a metal sulfide species. However, the N,S-bound compound [Ti{N(Ar)C(S)S}(COT)] (10) was stable to retrocycloaddition and was isolated. Proton transfer reactions occurred between pinacol and compounds 1−3 to form the bis(alkoxide) species [Ti{OC(Me)2C(Me)2O}(COT)] (11) (from 1 or 3) or [Ti{OC(Me)2C(Me)2O}(COT‘ ‘)] (12) (from 2) and the corresponding free amine. The reactions between 1−3 and 2 equiv of the thiols tBuSH and HS-2,4,6-iPr3C6H2 all resulted in the oxidation of the thiol to the disulfides tBuS−StBu and (2,4,6-iPr3C6H2)S−S(2,4,6-iPr3C6H2). Treatment of 1 with tBuNC in the presence of 1,3,5,7-cyclooctatetraene led to formal nitrene group transfer and the formation of the Ti(II) species [Ti(COT)(η4-C8H8)] (13) and tBuNCNtBu. The analogous reactions between 2 and 3 and tBuNC resulted in the formation of the adducts [Ti(NtBu)(COT‘ ‘)(CNtBu)] (15) and [Ti(NAr)(COT)(CNtBu)] (17), and similarly the reaction between 1 and pyridine led to the isolation of the adduct [Ti(NtBu)(COT)(py)] (19) (py = pyridine). Complex 19 was crystallographically characterized. DFT studies indicated that the interaction between pyridine and the Ti center in 19 and tBuNC and the Ti center in 17 was electrostatic in nature. Complexes of the type [Ti(NR)(COT)(AlMe3-xClx)] (R = tBu, x = 0 (20); R = Ar, x = 0 (21); R = tBu, x = 1 (22); R = Ar, x = 1 (23)) were formed through the reactions of 1 and 3 with AlMe3 and AlMe2Cl, and DFT studies indicated that they contained four-membered metallacyclic rings. Attempts to prepare monomeric zirconium imido cyclooctatetraene complexes through the reactions of [Zr2(μ-NR)2Cl4(THF)x] (R = tBu, x = 3; R = 2,6-Me2C6H3(Ar‘), x = 4) with K2COT, Li2COT‘ ‘·1.8(THF), or Li2[COT*] (COT* = 1,4-C8H6(SiMe2tBu)2) were unsuccessful. Only the crystallographically characterized dimeric species [Zr2(μ-NAr‘)2(COT‘ ‘)2] (24) was isolated.