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Issue 4, 2007
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Comparison of the reactivity of different Pd–O species in CO oxidation

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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 PdOx 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 PdOx 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.

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

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Publication details

The article was received on 26 Jul 2006, accepted on 27 Nov 2006 and first published on 14 Dec 2006

Article type: Paper
DOI: 10.1039/B610719B
Citation: Phys. Chem. Chem. Phys., 2007,9, 533-540
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    Comparison of the reactivity of different Pd–O species in CO oxidation

    H. Gabasch, A. Knop-Gericke, R. Schlögl, M. Borasio, C. Weilach, G. Rupprechter, S. Penner, B. Jenewein, K. Hayek and B. Klötzer, Phys. Chem. Chem. Phys., 2007, 9, 533
    DOI: 10.1039/B610719B

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