Adsorption and Interaction of CO and NO Molecules on Pure and Oxidized Surfaces of Al-Mo(110) Alloy

The coadsorption of carbon-monoxide- and nitric-oxide molecules and their interactions on surfaces of Al-Mo(110) alloy and the system resulting from its oxidation (Al-Mo-O) are investigated using an array of spectroscopic techniques (X-ray photoelectron, Auger electron, infrared, and thermal-desorption spectroscopies), low-energy electron diffraction, and work-function measurements in ultra-high vacuum. Al-Mo(110) alloy is fabricated by the thermal annealing (at 800 K) of an aluminum film several layers thick deposited onto the Mo(110) surface. Aluminum atoms diffuse into the substrate to yield a surface alloy of hexagonal structure characteristic of stoichiometric Al2Mo alloy. Unlike dissociative adsorption on both Mo(110) and Al(111) surfaces, CO and NO adsorb molecularly at the surface of Al-Mo(110) alloy. At 200 K, the adsorption of CO molecules on the Al-Mo(110) surface containing pre-adsorbed NO molecules has a dramatic effect on the latter by displacing them to higher-coordinated adsorption sites and at the same time causing their molecular axis to tilt toward the adsorbent surface plane. Heating this system to 320 K results in the reduction of nitric oxide by carbon monoxide to yield CO2 and surface nitrides. This can be a consequence of surface reconstruction that leads to the formation of additional adsorption/reaction sites at the Al/Mo interface and to changes in the substrate's d-band filling as a result of alloying. The oxidation of CO by NO proceeds with a markedly higher efficiency at the surface of the Al-Mo-O system; this system results from oxidation of the Al-Mo(110) alloy by oxygen at 700 K and exposure up to 1500 L.