(49f) 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^-13atm 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