Considerations for Employing Atomic Hydrogen as a Reductant for Metal Oxides, A Case Study: Conversion of Cupric Oxide to Copper
摘要
Reduction of particulate cupric oxide spheres to copper by both atomic hydrogen (produced in a low-pressure microwave-frequency plasma) and molecular hydrogen was investigated at temperatures between 100 °C and 450 °C. A custom-built reactor system with thermogravimetric capability was employed in the investigation. A coupled thermal and gas diffusion in a porous medium mathematical model was developed to provide for a quasi-quantitative portrayal of the sigmoidal character of the extent-of-reaction vs time curves observed in the experiments. It is revealed that for a sphere of cupric oxide for which the surface region constitutes less than 1 pct of the bulk phase, there is no significant difference in the conversion time with atomic hydrogen relative to that achieved with molecular hydrogen. Thus, it is apparent that as the (porous) copper layer develops during the early stage of conversion, the competing process of recombination of atomic hydrogen results in only molecular hydrogen being available at the conversion–reaction interfacial region beyond a relatively small length-scale depth layer dc. It is noted that the energy released due to recombination is confined to this outer layer of size dc and contributes somewhat differently to the thermal phenomenology occurring within the pellet compared to when molecular hydrogen is the reductant. Nonetheless, the broad-detail conceptualization of the limitations imposed when atomic hydrogen is employed as a reductant remains valid.