Sodium Promoter Inducing a Phase Change in a Palladium Catalyst
journal contributionposted on 2012-11-01, 00:00 authored by Nicola Seriani
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.