“Soft” oxidative coupling of methane to ethylene: Mechanistic insights from combined experiment and theory

The oxidative coupling of methane to ethylene using gaseous disulfur (2CH4 + S2 → C2H4 + 2H2S) as an oxidant (SOCM) proceeds with promising selectivity. Here, we report detailed experimental and theoretical studies that examine the mechanism for the conversion of CH4 to C2H4 over an Fe3O4-derived FeS2 catalyst achieving a promising ethylene selectivity of 33%. We compare and contrast these results with those for the highly exothermic oxidative coupling of methane (OCM) using O2 (2CH4 + O2 → C2H4 + 2H2O). SOCM kinetic/mechanistic analysis, along with density functional theory results, indicate that ethylene is produced as a primary product of methane activation, proceeding predominantly via CH2 coupling over dimeric S–S moieties that bridge Fe surface sites, and to a lesser degree, on heavily sulfided mononuclear sites. In contrast to and unlike OCM, the overoxidized CS2 by-product forms predominantly via CH4 oxidation, rather than from C2 products, through a series of C–H activation and S-addition steps at adsorbed sulfur sites on the FeS2 surface. The experimental rates for methane conversion are first order in both CH4 and S2, consistent with the involvement of two S sites in the rate-determining methane C–H activation step, with a CD4/CH4 kinetic isotope effect of 1.78. The experimental apparent activation energy for methane conversion is 66 ± 8 kJ/mol, significantly lower than for CH4 oxidative coupling with O2. The computed methane activation barrier, rate orders, and kinetic isotope values are consistent with experiment. All evidence indicates that SOCM proceeds via a very different pathway than that of OCM.

Composite elastomeric materials filled with modified mineral fillers

In the paper, the influence of modified Angren kaolin on the properties of composite elastomeric materials based on butadiene-styrene rubber has been established that the addition of modified Angren kaolin into rubber mixtures instead of gas pedals, activators and mineral fillers causes intensive connection of sulfur with macromolecules. Adsorbed sulfur on the surface of particles of modified Angren kaolin and its uniform distribution in the composition and formation of the structure with equal force were determined. This effect shows that the modified Angren kaolin in the curing processes accelerates and activates and affects the formation of the curing grid with equal strength and helps distribution throughout the composite. It has been established that introduction of MAK into the composition of elastomers more than 50 wt. % per 100 wt. % of rubber changes technological and technical properties of composite elastomeric materials. The application of modified Angren kaolin instead of the gas pedal, activator of vulcanization and mineral filler in composite elastomeric materials allows making rubber goods of various purposes.

Identification of an AgS2 Complex on Ag(110)

Adsorbed sulfur has been investigated on the Ag(110) surface at two different coverages, 0.02 and 0.25 monolayers. At the lower coverage, only sulfur adatoms are present. At the higher coverage, there are additional bright features which we identify as linear, independent AgS2 complexes. This identification is based upon density functional theory (DFT) and its comparison with experimental observations including bias dependence and separation between complexes. DFT also predicts the absence of AgS2 complexes at low coverage, and the development of AgS2 complexes around a coverage of 0.25 monolayers of sulfur, as is experimentally observed. To our knowledge, this is the first example of an isolated linear sulfur-metal-sulfur complex.

Numerical Investigation on Surface Coverage of Weakly-Adsorbed Molecular SO2 Contaminant in a PEM Fuel Cell Cathode

This paper describes attempt to numerically predict surface coverage of SO2 contaminant in a PEMFC cathode, as a step towards assessing its impact towards cell performance. Three-dimensional macro-homogeneous conservation equations of two-phase fluid flow is coupled with micro-scale cathode ORR kinetics to solve for surface coverage distribution of O-ad and SO2-ad at the surface of the catalyst layer for bulk SO2 concentrations of 2.5 and 5.0 ppm. At 2.5 ppm, SO2-ad is predicted to block ca. 20% of the active sites at cell current density of 0.2 A/cm2. The effect of SO2-ad blockage is then correlated with loss in cell performance. The numerical results are compared with experimental data from literature, which confirms that though the model successfully predicted higher potential loss with higher bulk SO2 concentration in the reactant feed, inclusion of only weakly-adsorbed SO2 will under-predict the exact potential loss experienced by the cell. This means strongly adsorbed sulfur containing species must be adopted into the model in order to better predict the severity of degradation of the cell due to SO2 contamination.