In order to investigate any influence of steps and possible positive effects of making surface alloys the ammonia synthesis has been investigated over Au modified Fe(111) and Ag and Cu modified Fe(100) single crystals in the temperature range 603-773 K, using a system combining ultra-high vacuum and a high-pressure cell. Ammonia was synthesized from a stoichiometric (N-2:3H(2)) gas mixture at a pressure of 2 bar. By deposition of small amounts of An, the ammonia production activity of the Fe(1 1 1) surface can be enhanced. More important, for the gold modified surface, the reaction order in ammonia pressure is less negative. The activity goes through a maximum as the An coverage increases, and then to zero as the coverage approaches 1 ML. At 713 K and 400 ppm NH3, a maximum enhancement of 50% is observed at 0.04 ML An. Based on features in the TPD spectra, the enhancement is suggested to be due to lifting of the nitrogen induced reconstruction of the Fe(I 11). On the Fe(l 00) surface strong effects were expected from modification with Cu and Ag since the in general more reactive atomic steps always present on such surfaces would be blocked. However, only minor and modest negative effects were found. The Cu and Ag were found to be inefficient step blockers as they were forced away from the surface by the formation of the much more stable surface nitrogen.
Surface Science, 2003, Vol 543, Issue 1-3, p. 207-218