CO oxidation on Pd(100): A study of the coadsorption of oxygen and carbon monoxide

Abstract

Oxygen adsorption, and coadsorption and reaction with CO were studied with temperature programmed reaction spectroscopy (TPRS), low energy electron diffraction (LEED), and high resolution electron energy loss spectroscopy (EELS). Oxygen adsorption at 300 K was studied for coverages up to 0.5, while coadsorption with CO was studied in the temperature range of 80 to 450 K, and O and CO coverages of 0 to 0.25 and 0 to 0.8, respectively. Oxygen adsorbed into p(2 × 2) islands for coverages in excess of 0.05 and yielded a fully developed p(2 × 2) structure for 1 4 monolayer coverage at 300 K. The p(2 × 2)O pattern was gradually replaced by a c(2 × 2)O structure as the oxygen coverage was increased to 0.5. Oxygen desorption occurred in three temperature programmed desorption states at 840, 730, and 695 K. The highest temperature state was populated at all coverages, whereas the 730 and 695 K states appeared as the oxygen coverage exceeded 0.25 and 0.36, respectively. The 695 K state was unusually narrow suggesting attractive interactions in the oxygen adlayer for coverages of 0.36 to 0.5. For the surface precovered with 0.25 monolayer of oxygen, CO adsorption at 80 K caused a disordering of the p(2 × 2)O structure. At lower oxygen coverages CO initially adsorbed at sites apart from the oxygen domains, but also adsorbed within oxygen islands upon filling of the exterior sites. CO adsorption in the interior of the islands produced a directly interacting COPdO complex characterized by a CO stretching frequency of 2125 cm-1. Reaction-limited CO2 evolution occurred in three states; viz. reaction of CO with (1) isolated oxygen atoms or p(2 × 2)O islands at 420 K, (2) disordered oxygen islands at 360 K, and (3) by a low temperature reaction between 100 and 310 K due to the COPdO complexes. No evidence of molecularly adsorbed CO2 or carbonate (CO3) reaction intermediates was observed by EELS at 80 K. © 1984.