Interactions between oxygen and carbon monoxide on a Pd(111) surface

Adsorption, mutual interactions and transient product formation between oxygen and carbon monoxide on a Pd(111) surface were studied by means of LEED, UPS and isothermal as well as temperature-programmed desorption spectrometry at temperatures between 200 and 550 K. CO adsorption at 200 K was found to cause the formation of additional periodic overlayer structures above θ = 0.5, apart from the already known surface structures at θ < 0.5. Saturation is reached with a hexagonal close-packed layer at θ = 0.66 (≜1 × 1015moleculescm2). Depending on the sequence of gas exposure and on the partial coverages of COad andOad, different stages of coadsorption (island formation, “compression” of the 2 × 2-Oad layer into a √3 × √3 R 30° structure, growth of a mixed phase with 2 × 1 periodicity etc.) may be distinguished, which influence the electronic properties and the reactivity of the adsorbates. At high surface concentrations CO2 formation proceeds with an appreciable rate already at 200 K, whereas at low coverages temperatures above 400 K are needed. The results demonstrate that no appropriate description of the kinetics of catalytic CO2 formation over the whole range of surface concentrations may be achieved in terms of simple rate laws involving coverage-independent rate constants (and activation energies) and mere surface concentrations of the reactants. There is on the other hand no indication for a reaction path involving the collision of a gaseous CO molecule with an adsorbed oxygen atom (Eley-Rideal mechanism), i.e. the reaction proceeds always with chemisorbed CO via Oad + COad → CO2 (Langmuir-Hinshelwood mechanism).

[1]  H. Bonzel,et al.  Mechanisms of the catalytic carbon monoxide oxidation on Pt (110) , 1972 .

[2]  G. Ertl,et al.  Semiempirical model calculations for chemisorption on transition metals: III. CO, N2 and NO on Ni, Cu, Pd and Ag , 1977 .

[3]  J. S. Close,et al.  On the oxidation of carbon monoxide catalyzed by palladium , 1975 .

[4]  G. Ertl,et al.  Adsorption of CO on Pd single crystal surfaces , 1974 .

[5]  V. P. Ivanov,et al.  The CO oxidation reaction on the Ir(111) surface , 1977 .

[6]  W. H. Weinberg,et al.  The chemisorption of carbon monoxide on the iridium (III) surface , 1976 .

[7]  T. Matsushima.  Kinetics of the reaction of oxygen with carbon monoxide adsorbed on palladium*1 , 1975 .

[8]  G. Ertl,et al.  Theory of carbon monoxide chemisorption on transition metals , 1974 .

[9]  A. Bradshaw,et al.  The chemisorption of carbon monoxide on palladium single crystal surfaces: IR spectroscopic evidence for localised site adsorption , 1978 .

[10]  J. White,et al.  On the mechanism and kinetics of the CO-oxidation reaction on polycrystalline palladium: I. The reaction paths , 1975 .

[11]  J. S. Close,et al.  On the mechanism and kinetics of the CO-oxidation reaction on polycrystalline palladium: II. The kinetics☆ , 1975 .

[12]  Gerhard Ertl,et al.  Adsorption of hydrogen on palladium single crystal surfaces , 1974 .

[13]  A. Plagge,et al.  Interaction of CO and O2 with Ir(111) surfaces , 1976 .

[14]  J. White,et al.  The adsorption of carbon monoxide on palladium during the catalyzed reaction CO + 12O2 → CO2☆ , 1976 .

[15]  G. Wedler.  Adsorption—Desorption Phenomena , 1974 .

[16]  G. Ertl,et al.  Chemisorption und katalytische Reaktion von Sauerstoff und Kohlenmonoxid an einer Palladium (110)-Oberfläche , 1969 .

[17]  G. Ertl,et al.  Adsorption of CO on clean and oxygen covered Ni(111) surfaces , 1976 .

[18]  G. Ertl,et al.  Chemisorption of CO on the Pt(111) surface , 1977 .

[19]  G. Ertl,et al.  Interaction of NO and O2 with Pd(111) surfaces. II , 1977 .

[20]  G. Ertl,et al.  Coadsorption of hydrogen and carbon monoxide on a Pd (110) surface , 1974 .