Adsorption of nitrogen and ammonia by polycrystalline iron surfaces in the temperature range 80–290 K studied by electron spectroscopy

Abstract

X-ray and uv induced photoelectron spectroscopy have provided information on the various molecular states of nitrogen formed on polycrystalline iron surfaces from dinitrogen and ammonia. At 85 K two distinct states are observed with N 2(g) which have N(1s) binding energy values of 405.3 and 400.2 eV. These are in equilibrium with N 2(g), are weakly held, and are desorbed on warming to 290 K leaving a nitrogen free surface. The two states are assigned to a molecularly adsorbed ▪ and linear ▪ species the former characterised by an N(1s) value of 400.2 eV and the latter by 405.3 eV. At 290 K nitrogen is adsorbed with a very low sticking probability (⩽10 −6) giving rise to an N(1s) value of 397.2 eV. This is undoubtedly the dissociatively chemisorbed ▪ species. At a nitrogen pressure of l Torr adsorption is “instantaneous” and the N(1s) value is 397 eV. No evidence for the unstable bridged and linear forms of nitrogen is obtained at 290 K although they may well be precursors to the formation of the strongly chemisorbed nitrogen species. Shifts in the N(1s) binding energy induced by subsequent oxygen adsorption are discussed briefly. At 85 K ammonia adsorbs largely in the molecular form with a broad N(1s) peak centred at about 400 eV but on warming to 290 K this splits to give two peaks one at 397 eV and the other at 400 eV. Interaction at 290 K leads to a dominant peak at 397.2 eV and a subidiary one at 400 eV. Helium (1) spectra support the assignment of the 397.2 eV peak to dissociated species (N, NH) and the 400 eV peak to molecular adsorption. The conclusions with N 2 and NH 3 are substantiated further by comparing the data with results for nitric oxide. The concentration of nitrogen adatom species formed from NO at 290 K and 10 −6 Torr is some ten times that formed from N 2 at 1 Torr and three times that from NH 3 at 10 −6 Torr and the same temperature.