The Chemisorption of Carbon Monoxide, Oxygen and Nitric Oxide on the Ruthenium (001) Surface

The vibrational energies of small molecules adsorbed on the Ru(001) surface were studied by means of inelastic electron scattering. The instrument used to carry out this work is an electron spectrometer of the Kuyatt-Simpson type. The design and operation of this instrument, which is capable of carrying out a number of electron spectroscopies important in chemisorption research, is discussed in detail. The vibrations of carbon monoxide, oxygen and nitric oxide chemisorbed on the basal plane of Ru provide insight into their chemical structures. Carbon monoxide exhibits the carbon-oxygen stretching frequency (1980 cm-1 - 2080 cm-1) characteristic of a linear CO, with a single carbon-metal bond. This is true at all coverages, although at saturation coverage the low-energy electron diffraction pattern shows that the adsorbed CO overlayer is out of registry with the substrate. An interadsorbate interaction is proposed to explain the observed increase in the carbon-oxygen stretching frequency of 100 cm-1 with coverage. Oxygen is adsorbed dissociatively on Ru(001) and exhibits a single coverage-dependent stretching frequency of approximately 570 cm-1. The low frequency indicates that atomic oxygen is bonded in a bridged or threefold site. A study of chemisorbed oxygen over the temperature range, 100 K to 1000 K, shows the existence of energy gain peaks in the electron energy distribution. Nitric oxide adsorbs molecularly at 130 K but dissociates at approximately 400 K, depending on coverage. Comparison with the stretching frequencies of metal nitrosyls suggests that NO is bound in a threefold or bridged site at low coverage and as a linear NO with a single metal-nitrogen bond at high coverage. The vibrational spectrum of adsorbed NO changes irreversibly if the surface is heated to a temperature below the onset of desorption (depending on coverage). A mechanism is proposed for this in which the bridged NO dissociates at room temperature and the atomically bound nitrogen and oxygen atoms block further dissociation.