Field emission study of ammonia adsorption and catalytic decomposition on individual molybdenum planes

A probe-hole field emission microscope was used to investigate the crystallographic specificity of ammonia adsorption at 200 and 300 K on (110), (100), (211) and (111) molybdenum crystal planes. Chemisorbed NH₃ causes a large work function decrease, especially at 200 K in agreement with an associative adsorption model which can also explain that this decrease is more important on the crystal planes of highest work function (At 200 K, Δφ = ?2.25 eV on Mo(110) compared to Δφ = ?1.55 eV on Mo (111). The decomposition of NH₃ was followed by measuring the work function changes for stepwise heating of the Mo tip covered with NH₃ at 200 K. On the four studied planes NH₃ decomposition and H₂ desorption are completed at about 400 K. Δφ changes above 400 K depend on the crystal plane and have been related to two different nitrogen surface states. No inactive plane towards NH₃ adsorption and decomposition has been found but the noted crystallographic anisotropy in this low pressure study is relevant to the structure sensitive character of the NH₃ decomposition and synthesis reactions.