Comparison of the reactivity of different Pd–O species in CO oxidation

Abstract

The reactivity of several Pd–O species toward CO oxidation was compared experimentally, making use of chemically, structurally and morphologically different model systems such as single-crystalline Pd(111) covered by adsorbed oxygen or a Pd₅O₄ surface oxide layer, an oriented Pd(111) thin film on NiAl oxidized toward PdO_x suboxide and silica-supported uniform Pd nanoparticles oxidized to PdO. The oxygen reactivity decreased with increasing oxidation state: O(ad) on metallic Pd(111) exhibited the highest reactivity and could be reduced within a few minutes already at 223 K, using low CO beam fluxes around 0.02 ML s⁻¹. The Pd₅O₄ surface oxide on Pd(111) could be reacted by CO at a comparable rate above 330 K using the same low CO beam flux. The more deeply oxidized Pd(111) thin film supported on NiAl was already much less reactive, and reduction in 10⁻⁶ mbar CO at T > 500 K led only to partial reduction toward PdO_x suboxide, and the metallic state of Pd could not be re-established under these conditions. The fully oxidized PdO nanoparticles required even rougher reaction conditions such as 10 mbar CO for 15 min at 523 K in order to re-establish the metallic state. As a general explanation for the observed activity trends we propose kinetic long-range transport limitations for the formation of an extended, crystalline metal phase. These mass-transport limitations are not involved in the reduction of O(ad), and less demanding in case of the 2-D Pd₅O₄ surface oxide conversion back to metallic Pd(111). They presumably become rate-limiting in the complex separation process from an extended 3-D bulk oxide state toward a well ordered 3-D metallic phase.