ADSORPTION OF CARBON MONOXIDE AU/PD(100) ALLOYS IN ULTRAHIGH VACUUM: IDENTIFICATION OF ADSORPTION SITES

ZHENJUN LI; FENG GAO; OCTAVIO JAVIER FURLONG; WILFRED T. TYSOE

SURFACE SCIENCE

ELSEVIER SCIENCE BV

Año: 2010 vol. 602 p. 136 - 143

0039-6028

The adsorption of carbon monoxide is studied on Au/Pd(1 0 0) alloys by means of reflection absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD). The alloy was formed by adsorbing a four-monolayer thick gold film on a Pd(1 0 0) substrate and by heating to various temperatures to form alloys with a range of palladium coverages. The alloy was characterized using X-ray photoelectron spectroscopy and the composition of the outermost layer measured using low-energy ion scattering spectroscopy. CO adsorbs on palladium bridge sites only for palladium coverages greater than 0.5 monolayers (ML) suggesting that next-nearest neighbor sites are preferentially populated by palladium atoms. CO adsorbs on atop palladium sites and desorbs at ~350 K corresponding to a desorption activation energy of ~117 kJ/mol. However, at lower palladium coverages, these sites are not occupied and CO desorption states are detected 170 and 112 K corresponding to desorption activation energies of ~53 kJ/mol and ~35 kJ/mol, respectively, for these states. It is suggested that these states are due to a restructuring of the surface to form low-coordination gold sites that obscure the atop palladium site.