Energies of activation for decomposition of ammonia catalysed by the nitrides of the 4th series transition elements

Abstract

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

Lithium Imide Synergy with 3d Transition‐Metal Nitrides Leading to Unprecedented Catalytic Activities for Ammonia Decomposition

“…The difficulty in dehydrogenation of NH 3 on Fe (100) surface, as simulated recently, increases with the increase in the surface N content owing to the insufficient Fe coordination to the NH x (x = 0, 1, and 2) intermediates, [14] in agreement with the low catalytic activity of neat Fe nitrides observed previously [10] and in this study. Li 2 NH, repeatedly consumed and regenerated by R1 and R2, is beyond the role of electronic promoter.…”

“…The general catalytic features are as follows: 1) Universal catalytic activity. All Li 2 NH-TM(N) tested here exhibit catalytic activities towards NH 3 decomposition at temperatures above 623 K, marking the drastic changes of the inert or low active neat TM(N) especially for the early 3d TM(N) and Cu [10,17] (Figure 3 b; Supporting Information, Figure S7). 2) Unprecedentedly high activity.…”

“…[10] Such a situation changes drastically when Li 2 NH takes part in. It should be put forth here that, to focus on the neat interaction between 3d transition metal and Li 2 NH and to facilitate the related characterizations, a high content of Li 2 NH was employed in this study.…”

Experimental investigation of the enthalpy, entropy, and free energy of formation of GaN

“…No lower bound to this range could be determined because up to 1300 K no NH 3 generated from the decomposition of GaN in H 2 gas was found in the mass spectra, in contrast to the report of Logan and Thurmond. Thus we conclude that the dominant nitrogen species desorbed from the surface is N 2 gas, as is observed in NH 3 dissociation over other metals [18][19][20].…”

Adsorption and dehydrogenation of ammonia on vanadium and niobium nitride cluster cations

“…Among them, vanadium nitride, which is an interstitial compound, has been shown to be an active catalyst for synthesis and decomposition of ammonia [2][3][4][5]. For instance, Choi et al studied vanadium nitride particles with the diameters of ∼100 nm and revealed that the smaller particle has the lower activation energy in the ammonia decomposition [5].…”