Nitric oxide adsorption on the Pt(111) surface

Abstract

The adsorption of nitric oxide has been studied on the Pt(111) surface using electron energy loss spectroscopy (EELS), thermal desorption spectroscopy (TDS), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) over the temperature range 100 K to 1100 K. The chemisorption of nitric oxide is predominantly molecular on the Pt(111) surface, however a small amount of dissociation occurs with heating. The low temperature vibrational spectrum for a saturation coverage of adsorbed NO is dominated by two nitrogen-oxygen stretching bands at 1490 and 1710 cm −1. These bands can be rationalized using either a site model or a dimerization model. The low frequency mode could be assigned either to adsorption in a multiply bonded bridge site or to an adsorbed nitric oxide monomer. The high frequency mode could be assigned either to singly bound nitric oxide adsorbed in a terminal configuration or to nitric oxide dimers. The authors feel that data available favors the adsorption site model. The thermal desorption spectrum resulting from NO adsorption is complex. At least three distinct desorption processes are observed as well as some dissociation to form molecular nitrogen and oxygen. The desorption peaks below 350 K apparently correspond to desorption from nitric oxide adsorbed in a terminal configuration. As desorption proceeds with increasing temperature the adsorbed nitric oxide changes from an adsorbed configuration containing primarily terminally bonded nitric oxide at saturation coverage to bridge bonded nitric oxide at lower coverages. Some of the nitric oxide which desorbs in the high temperature shoulders above, 350 K is adsorbed molecularly in a distinct configuration as evidenced by the observation of new EELS bands at 1600 and 1820 cm −1. The thermal desorption results suggest some of this adsorbed nitric oxide is also dissociatively adsorbed as evidenced by the observation that some of this nitric oxide exchanges oxygen with preadsorbed oxygen eighteen. The nitric oxide which adsorbs above 350 K is responsible for most of the nitrogen formation which occurs with heating.