Effect of titania oxidation state on the chemisorptive properties of titania-supported nickel

Temperature-programmed desorption (TPD) under ultrahigh vacuum conditions and transmission electron microscopy (TEM) were used to characterize the adsorption behavior of CO and H₂ on low-surface-area samples prepared by depositing nickel onto titania films. The effect of titania oxidation state on CO adsorption from Ni was probed by studying 0.2-nm Ni overlayers deposited in situ on titania surfaces vacuum-annealed at successively higher temperatures to vary the oxygen-to-titanium ratio from 2 to 1. The CO adsorption strength and saturation coverage on Ni decreased with increased extent of titania reduction. These effects could be attributed to electronic metal-support interaction between Ni and underlying titania in the absence of titania adspecies on the Ni surface. This type of interaction becomes unimportant for Ni overlayers or Ni crystallites thicker than about 3 atomic layers. The strength of CO adsorption was also found to be dependent on the initial nickel overlayer thickness for samples reduced at 650 K in hydrogen at 10^-3 Pa. The desorption peak temperature increased from 360 K for a 0.2-nm Ni overlayer to 420 K for a 2.5-nm Ni overlayer. It is proposed that the thinner nickel overlayers lead to smaller nickel particles during this treatment in hydrogen and that these smaller particles are more completely covered by titania adspecies from the support. Transmission electron micrographs suggested that the supported Ni crystallites develop a raft-like morphology upon reduction. The Ni particles were relatively resistant to sintering. Dissociative CO adsorption on the model nickel/titania surfaces was not observed under any conditions, suggesting that the Ni crystallites adopt flat morphologies and that sites active for CO dissociation under ultrahigh vacuum conditions are not created at nickel/titania interfaces.