J. Phys. Chem. C 2007, 111, 7957-7962
7957
Methane Oxidation on Pd(111): In Situ XPS Identification of Active Phase Harald Gabasch,†,‡ Konrad Hayek,† Bernhard Klo1 tzer,† Werner Unterberger,† Evgueni Kleimenov,‡ Detre Teschner,‡ Spiros Zafeiratos,‡ Michael Ha1 vecker,‡ Axel Knop-Gericke,‡ Robert Schlo1 gl,‡ Balazs Aszalos-Kiss,§ and Dmitry Zemlyanov*,§,⊥ Institut fu¨r Physikalische Chemie, UniVersita¨t Innsbruck, A-6020, Innsbruck, Austria, Abteilung Anorganische Chemie, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany, Materials and Surface Science Institute, UniVersity of Limerick, Limerick, Ireland, and Birck Nanotechnology Center, Purdue UniVersity, West Lafayette, Indiana 47907-2057 ReceiVed: December 6, 2006; In Final Form: February 22, 2007
The reaction between CH4 and O2 (1:5) was studied by in situ XPS during heating and cooling in a 0.33 mbar reaction mixture. During heating, the reaction rate exhibited an activity maximum at 650 K, whereas no activity maximum was found during the subsequent cooling ramp. This kinetic hysteresis was assigned to the spectroscopically observed difference in the surface oxidation state. During heating, the reaction rate approached the 650 K maximum in the stability range of bulk PdO seeds among the otherwise Pd5O4 2D oxide covered surface. On the other hand, no PdO seeds were formed during cooling, most likely due to kinetic limitations of PdO nucleation on a passivating surface oxide layer containing less oxygen than Pd5O4.
Introduction Palladium is well known for its catalytic activity in a number of oxidation reactions such as the complete oxidation of hydrocarbons in automotive exhausts and the combustion of methane in gas-powered turbines. In comparison to other metals, palladium shows the highest rate per unit metal surface for methane oxidation.1,2 Catalytic combustion is carried out under conditions varying from low temperatures, where PdO is the thermodynamically stable phase, to high temperatures, where Pd metal is stable. Although previous work3,4 showed fully oxidized Pd particles to be nearly inactive, PdO is generally believed to be more active than the metal in methane combustion.5-9 A number of research groups10-15 observed different methane combustion rates when the catalyst was either cooled or heated in the reaction mixture. This unusual kinetic behavior, referred to as activity hysteresis, was assigned to the decomposition of PdO to Pd and its re-formation.10,14 A few hypotheses can be found in the literature. First, it was suggested that strongly bound chemisorbed oxygen forms on the palladium surface during cooling and that this oxygen species passivates the surface and inhibits further oxidation.14,16 Second, Salomonsson et al.13 explained the hysteresis in terms of an equilibrium in a threephase system: gas-phase O2 and two solid phases, Pd and PdOx. Third, a more complex four-phase scheme proposed by Wolf et al.15 includes Pd metal, PdO bulk, surface PdO, and chemisorbed oxygen. However, the general conclusion is straightforward: the catalytic activity of palladium in hydrocarbon oxidation reactions depends on the oxygen/palladium chemistry. The interaction of palladium with oxygen has been studied extensively under high-vacuum conditions (
