Nickel carbide, decomposition

Hofer et al. [29] used a magnetic method to measure the isothermal kinetics of the decompositions of cobalt and nickel carbides. The reaction of CojC was zero-order (0.20 < a< 0.75) with Z , = 226 kJ mol in the range 573 to 623 K, and became deceleratory thereafter. The behaviour of NijC differed in that there was a short induction period, but there was again a period of constant rate (0.3 < ar < 0.9) with , = 295 kJ mol between 593 and 628 K and the final period was deceleratory. There was no evidence for the intervention of a lower carbide. The mechanisms of these reactions were not discussed. 

Though salt dehydration was not accompanied [27] by particle disintegration, the anhydrous pseudomorph was shown by X-ray diffiaction measurements to be very poorly crystallized (a characteristic feature of many nickel carboxylates). Decomposition in air (554 to 631 K) proceeded at a constant rate (0.1 < nr < 0.8 and = 96 kJ mol" ), ascribed to the operation of an autocatalytic mechanism. The reaction in vacuum (562 to 610 K) gave a sigmoid ar-time curve which was fitted by the Prout-Tompkins equation. Because the activation energy was the same as that for reaction in air, it was concluded that the same mechanism operated. The reaction in air yielded residual nickel oxide, while reaction in vacuum gave the carbide with excess carbon and some oxide. In addition to carbon dioxide, the volatile products of decomposition included water and acetic acid. 

The decomposition of nickel llimarate (573 to 613 K, , = 208 8 kJ mol ) [120] gave sigmoid ar-time curves but with a short iiutial reaction a = 0.02). There was no evidence of reactant melting. It was concluded that anion breakdown was promoted by the nickel carbide product, in a nucleation and growth reaction. 

From the above comparisons it is evident that both structure and composition of the anion may influence the mechanism of decomposition of nickel carboxylates. The crystal structure of the reactant can probably be discounted as a rate controlling parameter because dehydration usually yields amorphous materials. Depending on temperature, carbon deposited on the surface of a germ metallic nucleus may effectively prevent or inhibit growth, it may be accommodated in the structure to yield carbide, or be deposited elsewhere (by carbide decomposition). These mechanistic interpretations are based on the relative reactivities of the nickel salt and of nickel carbide, for which the temperature of decomposition is known, 570 K [150]. 

In particular, the a and a states have been Identified as Individual carbon atoms, probably chemisorbed at two different sites (perhaps terrace and ledge or step sites (25). We made this Identification because (9 ) (1) these states are very reactive with hydrogen to produce methane, as we expect for C atoms (2) they are populated only up to about monolayer quantities In terms of CO adsorption capacity (3) they are not hydrocarbon fragments because they are populated by CO dissociation as well as hydrocarbon decomposition and (4) their reactivity Is only a little greater than the y carbon state, which we believe Is bulk nickel carbide because there are no nickel lines In the x-ray diffraction of G-56H catalyst with a carbon deposit composed of the y carbon state. Thus the a and y carbon states both have metal-carbon bonding rather than carbon-carbon bonding. 

Easily decomposed, volatile metal carbonyls have been used in metal deposition reactions where heating forms the metal and carbon monoxide. Other products such as metal carbides and carbon may also form, depending on the conditions. The commercially important Mond process depends on the thermal decomposition of Ni(CO)4 to form high purity nickel. In a typical vapor deposition process, a purified inert carrier gas is passed over a metal carbonyl containing the metal to be deposited. The carbonyl is volatilized, with or without heat, and carried over a heated substrate. The carbonyl is decomposed and the metal deposited on the substrate. A number of papers have appeared concerning vapor deposition techniques and uses (170—179). 

Partial melting was reported [27] to take place during decomposition of this salt in air. Kinetic behaviour included a decrease in rate at a about 0.3. In vaeuum the reaction was accompanied by sublimation. Kinetic behaviour showed some irreproducibility and the decrease in rate occurred at a about 0.1. The residue contained metal and oxide, but, unlike the nickel salt, not the metal carbide. 

A comparative study of the thermal reactions in oxygen of the above three reactants [118], together with nickel formate, showed that the shapes of the ar-time curves were comparable with those for vacuum decompositions. The magnitudes of the Anhenius parameters (with the exception of the decomposition of the formate which does not give carbide product) were diminished. It was concluded that the initiation of reaction was unaffected by the presence of oxygen, so that the geometry of each rate process was unchanged. In the presence of Oj, interfacial reactions on the nickel oxide product (-+ COj + HjO) proceeded appreciably more rapidly by a common mechanism and all values of E, were close to 150 kJ mol. ... 

Other reactions important to reforming are also considered in the reaction network in Figure 10, include the water-gas-shift reaction and its reverse, the reversible adsorption and decomposition of water, the desorption and adsorption of reforming products like CO, CO2, and H2, and the formation of hydrocarbons like CH. The formation of dissolved carbon, oxygen, and hydrogen in bulk nickel is also considered. Dissolved C, 0, or H may be important in the transport of those elements to or from interfaces with other solid phase (carbon, carbides, oxides, support). The possible formation of NiO from H2O is also shown. Finally, an important reaction to consider is the formation of a deactivating layer of carbons (6 or e carbon states). 

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