NITROGEN ADSORPTION AND NATIVE CONTAMINATION ON FE(211) SURFACES

Nitrogen adsorption has been studied on clean Fe(211) surfaces. Neglig ible adsorption occurred at low or room temperature. However, a surfac e with particular contamination showed appreciable adsorption. Heating a clean surface in nitrogen resulted in a small surface N concentrati on with a (2 x 5) structure, but since the same occurred after heating a nitrogen-rich surface region in a high vacuum, it indicates that in ternal N diffuses to the surface at heating temperatures as low as 580 K. Since there is no clear evidence that any appreciable adsorption o ccurs from 80 to 580 K, speculation about a special site existing for initial adsorption, such as the C7 site, is not supported. Internal ch lorine was detected on the surface after heavy exposures of nitrogen, hydrogen or oxygen, indicating that complex interactions occur with ga ses in the sub-surface region. Because used catalysts in ammonia catal ysis show the presence of C and O, the structures on Fe(211) formed wi th internal contaminants S, C and external O were studied. Various ord ered structures were found to form at fractional monolayer concentrati ons, and cannot be excluded from being involved in the catalysis proce ss. (C) 1997 Elsevier Science B.V.