The activation energies for the decomposition of ammonia on the nitrides of the transition elements from titanium to nickel have been determined. With the exception of nickel and cobalt nitrides, where the temperature range of investigation was limited, all these nitrides show more than one activation energy, the change from one to the other being at a sharp temperature. In these cases, except for titanium nitride, the lower activation energies occurred at the lower temperatures, iron nitride showing three energies of activation.As nitrides are believed to play an intermediate role in the catalytic synthesis of ammonia on metals, and as, with the exception of iron nitride, no quantitative data on the behaviour of nitrides as catalysts for the decomposition of ammonia are available, the activation energies for ammonia decomposition on the nitrides of the transition elements from titanium to nickel have been determined. Scandium nitride was omitted as the preparation, requiring a temperature of 1800" C, was beyond our resources.
EXPERIMENTAL
MATERIALS
CATALYST PREPARATIONThe catalysts prepared were interstitial nitrides of the metals titanium, vanadium, chromium, manganese, iron, cobalt and nickel. At least two separate samples of each nitride were prepared and examined for catalytic activity.Titanium nitride was prepared by heating pure titanium dioxide (Ti02) in pure dry ammonia at 1500" C, following Ruff.1 The golden-yellow nitride is face-centred cubic 2~4 and analysed 22-5 and 22-3 % N(TiNl.00 requires 228 % N).Vanadium nitride was prepared by heating ammonium vanadate in ammonia at 1100" C following Epelbaum and Brager.3 The grey-brown nitride, face-centred cubic,3.4,5, analysed 21.2 and 21.4 % N (VN1.00 requires 21.6 % N).The remainder of the nitrides were prepared by the action of pure ammonia on spectrographically pure metals, obtained from Johnson, Matthey & Co.Chromium nitride, prepared at 850" C, following Henderson and Galletly,6 analysed 84.7 and 85-0 % Cr (Cr3N2 requires 84.8 % Cr). It is hexagonal close-packed with nitrogen incorporated in a non-uniform manner. It is possible that some CrN, a true compound, not interstitial,' may be present .Manganese nitride, prepared from electrolytic manganese at 750" C,6, * analysed 13.8, 13.9, 13.9 % N (Mn3N2 requires 14.6 % N). This probably consisted of mixed crystals of two face-centred tetragonal structures.8.9Iron nitride, from iron sponge,S analysed 10.8 and 11.0 % N (6 FezN requires 11.1 % N).The nitrides of cobalt and nickel required a special experimental procedure as they exist only in a narrow range of temperature and required in their preparation a very rapid
