Figure 5

Left: Sketch of the potential energy surface for the reduction of MoO₃ to MoO₂ to depict the possible reaction pathways. The z-axis is the energy coordinate, while x- and y-axes represent the not further defined reaction coordinates. MoO₂ is most stable with respect to MoO₃ and hydrogen, however, hydrogen bronze H_xMoO₃ is a metastable state. The corresponding enthalpies of formations serve as quantitative estimates for the position on the energy potential surface²⁶. The activation energy of the reactions depends on the path over the potential energy surface. The corresponding experimental activation energies E_A for the high temperature reduction are indicated¹⁵. Hydrogen diffusion is relatively fast in MoO₃, which results from the small activation energy for diffusion ²⁹. The latter is related to the electronic structure on one hand, but the reaction path is also affected by the external conditions temperature, hydrogen and water partial pressure on the other. These conditions depend on the experimental setup as sketched in the right figures: catalysts are usually made of MoO₃ particles, which are basically inert at room temperature, i.e., they do not dissociate hydrogen nor is desorption of water facilitated (bottom left). At higher temperature, an oxygen deficient, dissociatively active surface is formed with high oxygen mobility (bottom right). To overcome the high dissociation at room temperature, one can coat a MoO₃ thin film on an inert substrate with Pd (typical sensor setup, top left). In our membrane approach, the MoO₃ thin film is hydrogenated via the Pd substrate, i.e., potentially formed water can leave the film.