(609g) An Analysis of the Carbothermic Reduction of Tungsten Oxide for Solar Fuels Production

The production of fuels, like hydrogen or syngas, through solar-driven thermochemical cycles offers an atractive path that minimizes the damage to the environment. Nowadays, various thermochemical cycles have been studied, however two-step ones based on oxide redox pair systems are of particular interest. In this approach, there is a transition between the oxidized and reduced forms of a metal oxide. In the first step, the metal oxide is reduced at high temperature in an endothermic reaction. Subsequently, in a second step, the reduced metal oxide is oxidized-back by reacting with water and/or CO₂ to generate H₂ and/or CO. The regenerated metal oxide can be reduced again establishing a cycling process. Recently, the W/WO₃ redox pair has been proposed as an alternative option for solar fuels production presenting interesting chemical properties. However, purely thermal reduction of WO₃ to W requires temperatures above 3000 °C. This elevated temperature implies several challenges in the reactor, especially in the materials, therefore reducing this temperature is an imperative issue.

In this work the use of carbonaceous materials, like carbon and methane, as reducing agents for lowering the temperature of the WO₃/W redox pair reduction step is investigated. Thermodynamic and experimental investigations were performed in order to quantify and compare the chemical performance of this reaction with different forms of carbon (graphite, activated charcoal, carbon black and methane). Thermodynamics predicts that, when using carbon as reducing agent, the main products are W and CO at temperatures above 1000 °C. Concerning carbothermic reduction with methane, thermodynamic calculations indicate that the stables species are W, CO and H₂at temperatures above 1000 °C. The reactivity of the WO₃ with carbon (graphite, activated charcoal and carbon black) and methane was then investigated by thermogravimetry, tube furnace and solar reactor experiments. For carbothermic reduction with carbon, chemical conversions above 80% were achieved with reaction rates depending on type of carbon and stoichiometry of the sample. When using methane as reducing agent, chemical conversions above 60% were accomplished with reaction rates depending on temperature. Finally, it was found that the form of carbon highly impacts the final composition.