硫化鉛の水素還元について

抄録

Reduction rate of PbS with H₂ was measured thermogravimetrically in the temperature range 380°to 750°C by using thin disks prepared from synthetic and natural PbS powder.
The reduction rate of synthetic PbS was about twice that of galena below 450°C, whereas above 650°C no difference was detected in their rates. The rate decreased when a sample was covered with the formed metallic Pb at 550°C, but the tendency was not observed at 710°C. The measured rates surpassed the vaporization rates of PbS significantly and the Arrhenius plot yielded two straight lines. The activation energies were determined to be 15 kcal/mol below 450°C and 33 kcal/mol above 650°C respectively.
It can be presumed from these results that the reduction in the lower temperature range is governed by a mechanism different from that in the higher temperature. At temperatures below 450°C, the experimental results suggest that the rate is chemically-controlled by the heterogeneous reaction between H₂ and solid PbS. Absolute reaction rate theory was used to develop an analytical expression for this heterogeneous reaction. The rate constants calculated with the equation assuming that the surface of PbS is sparsely covered by adsorbed hydrogen agreed favorably with the experimental data.
On the other hand, above 650°C, it can be suggested that homogeneous reaction between gaseous PbS and H₂ is dominant and the reaction proceeds not at the surface or in the bulk stream of H₂, but in the boundary gas layer and enhances vaporization of PbS. The model can be supported by the following observations:(1) The surface of the reduced PbS at high temperature shows a similar structure to the surface which is allowed to evaporate under reduced pressure.(2) Reduction rate is independent on the size of the grains which comprise of a sample disk and also on the much adhesion of metallic Pb on the surface.(3) Serious difference in the activation energies in the reduction and vaporization.