Interaction of Ag, Rh, and Pd atoms with MgO thin films studied by the CO probe molecule

The interaction of carbon monoxide with Ag, Rh, and Pd atoms deposited on MgO thin films has been studied by thermal desorption and Fourier transform infrared spectroscopies. To obtain an atomistic view of the surface complexes, we performed density functional calculations on cluster models of the MgO(100) surface. The combined experimental-theoretical information leads to the following picture. Ag atoms interact weakly with the surface and do not form stable complexes with CO. Pd atoms become trapped at oxygen vacancies (F centers) already at low temperature; at these sites Pd(CO)(2) complexes form after exposure to CO. The CO molecules desorb from these sites at T = 230 K. At higher temperature, 370 K, we observe formation of small Pd aggregates with bridge-bonded CO molecules, indicating diffusion of Pd atoms or Pd(CO) units. Rh atoms bind quite strongly to the oxide anions at steps where they form relatively stable Rh(CO)(2) or even Rh(CO)(3) complexes; a minority of the Rh atoms populates the F centers. A first CO desorption occurs at 180 K and leaves on the surface rather mobile, very stable Rh(CO) units which diffuse until they become trapped at F centers; at 390 K a second CO desorption occurs leaving on the surface isolated Rh atoms, with no indication of formation of small Rh aggregates.