On the reduction of metal oxides in non-equilibrium hydrogen plasmas

Abstract

Metal oxides are some of the most widely-used source materials for the primary production of metals. However, the technologies currently used for the reduction of metal oxides can produce deleterious byproducts. Hence, there exists a need to develop improved processes for reducing metal oxides. Atomic hydrogen, consisting of dissociated hydrogen radicals, has the potential to be such a reagent: due to its high chemical potential, atomic hydrogen is naïvely predicted to be able to reduce even the most stable metal oxides. Additionally, the primary byproduct of these reductions (water) is chemically benign. This research investigates the efficacy of atomic hydrogen (produced in a low-pressure microwave-frequency plasma) as a reducing agent for several metal oxide systems: copper oxide, molybdenum dioxide, titanium dioxide, and neodymium oxide. In situ thermogravimetric analysis is used to quantify the kinetics of the reduction reactions, while optical emission spectroscopy is employed to characterize the plasma. The experimental results indicate the existence of a finite depth-of-reaction in samples exposed to atomic hydrogen; i.e., only a thin surface region is affected by the hydrogen radicals. An analytical model is developed describing the evolution of this depth-of-reaction and its dependence on the plasma parameters. In order to avoid this depth-of-reaction constraint on the reduction, thin films of titanium and neodymium oxide were prepared on a variety of substrates by sol-gel and thermal oxidation techniques; ¬ex situ analysis of these thin-films was conducted by means of grazing-incidence X-Ray diffraction and X-Ray photoelectron spectroscopy. A greater degree of reduction is observed in samples exposed to atomic hydrogen than in those exposed only to molecular hydrogen, but complete reduction is found to be substrate-dependent; atomic hydrogen is unlikely to be useful for the large-scale reduction of stable metal oxides.