ABSTRACT
Hydrogen sulfide (H2 S) is an extremely hazardous chemical waste that is generated at large scale in many industries; its abatement has long been an energy-extensive and cost-ineffective liability due to the thermodynamic limitations of the selected approaches and low value of the final products, sulfur and water. Here we introduce an attractive method for H2 S abatement that yields value-added products via a thermodynamically favorable process. Specifically, sodium naphthalenide (Na-NAP) is used to capture H2 S to produce anhydrous Na2 S nanocrystals and 1,4-dihydronaphthalene, which are important materials for batteries and liquid fuels, respectively. This multipurpose process is driven by the acid/base neutralization reaction between hydrogen cations from H2 S and radical anions from naphthalenide. It is spontaneous and irreversible at ambient temperature and pressure, proceeding to completion very rapidly.
ABSTRACT
Pyrite (FeS(2)) thin films were synthesized using a H(2)S plasma to sulfurize hematite (Fe(2)O(3)) nanorods deposited by chemical bath deposition. The high S activity within the plasma enabled a direct solid-state transformation between the two materials, bypassing S-deficient contaminant phases (Fe(1-x)S). The application of plasma dramatically enhanced both the rate of conversion and the quality of the resulting material; stoichiometric FeS(2) was obtained at a moderate temperature of 400 °C using a chalcogen partial pressure <6 × 10(-5) atm. As the S:Fe atomic ratio increased from 0 to 2.0, the apparent optical band gap dropped from 2.2 (hematite) to ~1 eV (pyrite), with completely converted layers exhibiting absorption coefficients >10(5) cm(-1) in the visible range. Room-temperature conductivity of FeS(2) films was on the order of 10(-4) S cm(-1) and approximately doubled under calibrated solar illumination.
ABSTRACT
A kinetic model and mechanism were developed for the heterogeneous chelation reaction of thin CuO films with hexafluoroacetylacetone (hfacH) in supercritical CO2. This reaction has relevance for processing nanoscale structures and, more importantly, serves as a model system to tune the reaction behavior of solids using supercritical fluids. Precise control over reaction conditions enabled accurate etching rates to be measured as a function of both temperature [(53.5-88.4) +/- 0.5 degrees C] and hfacH concentration (0.3-10.9 mM), yielding an apparent activation energy of 70.2 +/- 4.1 kJ/mol and an order of approximately 0.6 with respect to hfacH. X-ray photoelectron spectroscopy and scanning electron microscopy were used to characterize the CuO surface, and a maximum etching rate of 24.5 +/- 3.1 A/min was obtained. Solvation forces between hfacH and the dense CO2 permitted material removal at temperatures more than 100 degrees C lower than that of the analogous gas-phase process. In the low concentration regime, the etching reaction was modeled with a three-step Langmuir-Hinshelwood mechanism. Small amounts of excess water nearly doubled the reaction rate through the proposed formation of a hydrogen-bonded hfacH complex in solution. Further increases in the hfacH concentration up to 27.5 mM caused a shift to first-order kinetics and an adsorption-limited or Rideal-Eley mechanism. These results demonstrate that relatively modest increases in concentration can prompt a heterogeneous reaction in supercritical CO2 to switch from a mechanism most commonly associated with a low-flux gas to one emblematic of a high-flux liquid.
