A reaction chamber enables a reaction between received elemental mercury gas and an oxidizing agent gas. The reaction chamber includes a porous (or permeable) medium through which to pass the elemental mercury gas and the oxidizing agent gas. Passing of the elemental mercury gas and the oxidizing agent gas through the porous medium supports a number of useful functions. For example, the porous medium enhances mixing of the elemental mercury gas with the oxidizing agent gas to enhance a reaction. Also, the porous medium increases an amount of surface area in a reaction chamber on which reactions (e.g., heterogeneous surface reactions) can take place between the elemental mercury gas and the oxidizing agent gas to form oxidized mercury gas. Accordingly, the reaction chamber configured to include a porous medium enhances a conversion of elemental mercury gas into oxidized mercury gas.