Dopant Selection Guided By Sabatier's Principle: The Example of Doped Ceria for Promoting the Two-Step Water Dissociation

Use of ab-initio methodologies, based on Density Functional Theory, in describing surface science phenomenon has allowed for an increasing predictive ability of materials. In the case of H₂ synthesis, cerium based oxides have garnered significant interest given their oxygen buffering capability that facilitates the creation of active surface defect sites (vacancies). Tailoring the activity and selectivity of these materials towards low temperatures is paramount to improving reactivity. We started investigating the fundamentals governing the surface phases of cerium (IV) oxide under a range of environments. In the case of a pure ceria system in an O₂ environment, it was found that T > 1500 K or P_O2  < 10^-13 atm are required for any appreciable surface reduction. However, exposing the surface to a highly reducing environment such as CO or H₂ could help circumvent these harsh conditions. Given the aforementioned observations we sought to alter surface reducibility by doping with various alkali and transition metal elements. Doping ceria had an effect on the surface vacancy formation; low valence dopants favor defect formation, whilst high valence dopants suppress it. Armed with the insight dopants have on surface vacancy formation, their corresponding impact on the consecutive dissociation of water at these sites was studied. Using a carefully strategized multi-step screening approach abiding with the Sabatier's Principle, we were able to identify a subset of dopants that would promote surface reactivity towards H₂ synthesis. Dopants such as Sc, Au, Co, Pd, La, Y, Hg, Mn, Zr, Cr, and Fe possessed a critical balance in improving surface reducibility while sustaining the dissociation of water, hence are promising candidates in enhancing the two-step water dissociation process.

Figure 1: Measure of the Catalyst Performance Index for the class of dopants considered within ceria