Sodium Promoter Inducing a Phase Change in a Palladium Catalyst

Alkali metals act as promoters for the catalytic activity of transition metals, but it is unclear whether they only donate electrons or induce the formation of different sodium-containing phases. In the present work, calculations based on density functional theory and ab initio thermodynamics show that, under oxygen-rich conditions, addition of only 6 wt % of sodium to a palladium-based oxidation catalyst can induce a phase change in the catalyst. In presence of sodium, a cubic NaPd₃O₄ is more stable than tetragonal PdO, which is the stable phase in the absence of sodium. Moreover, the region of stability of the oxides is extended by ∼100 K with respect to the sodium-free case. Wulff construction predicts that (111), (100), and (110) facets should be present at the surface of NaPd₃O₄ particles at equilibrium. Carbon monoxide and methane adsorb strongest on NaPd₃O₄(100) and NaPd₃O₄(110), respectively, with adsorption energies of 3.75 and 3.03 eV, higher than on PdO. In all cases, there is a marked preference for adsorption on surface oxygen rather than on palladium, at variance from what happens on PdO. These results shed new light on the role of sodium as a promoter of the catalytic activity.