Phase transition metal carbides

The application of ly transition metal carbides as effective substitutes for the more expensive noble metals in a variety of reactions has hem demonstrated in several studies [ 1 -2]. Conventional pr aration route via high temperature (>1200K) oxide carburization using methane is, however, poorly understood. This study deals with the synthesis of supported tungsten carbide nanoparticles via the relatively low-tempoatine propane carburization of the precursor metal sulphide, hi order to optimize the carbide catalyst propertira at the molecular level, we have undertaken a detailed examination of hotii solid-state carburization conditions and gas phase kinetics so as to understand the connectivity between plmse kinetic parametera and catalytically-important intrinsic attributes of the nanoparticle catalyst system. 

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

The present study made use of diffusion couples where metals were reacted with carbon or nitrogen with the purpose of determining both phase equilibria and nonmetal diffusivities for Group 4 and 5 transition metal carbide and nitride systems. 

Group 5 transition metal-carbon systems. The diffusion bands of the phases of the transition metal carbides are generally rather narrow so that an unambiguous proof of their presence or absence is difficult to obtain. Therefore wedge-type or thin plane-sheet samples were... 

A new method of interpreting Auger electron spectroscopy (AES) sputter profiles of transition metal carbides and nitrides is proposed. It is shown that the chemical information hidden in the shape of the peaks, and usually neglected in depth profiles, can be successfully extracted by factor analysis (FA). The various carbide and nitride phases of model samples were separated by application of FA to the spectra recorded during AES depth profiles. The different chemical states of carbon, nitrogen and metal were clearly identified. 

Many binary transition metal carbides, especially the 5-phases of group 4 elements, exist over a broad range of composition with an upper limit of the carbon to metal ratio near 1. Practically all solid-state properties show a gradual change with the [C]/[T] ratio (T = transition metal). Only some carbides such as tungsten monocarbide, WC, and the chromium carbides have a very narrow homogeneity region. 

The stmeture of transition metal carbides are closely related to those of the transition metal nitrides. However, transition metal carbides feature generally simpler stmeture elements as compared to the nitrides. In carbides, the metal atoms are arranged in such a way that they form close-packed arrangements of metal layers with a hexagonal (h) or cubic (c) stacking sequence or with a mixtme of these (see Nitrides Transition Metal Solid-state Chemistry). The carbon atoms in these phases occupy the octahedral interstitial sites. A crystallochemical rule claims that the phases of pure h type can have a maximum carbon content of [C]/[T] = 1/2 and the c type phases a maximum carbon content of [C]/[T] = 1 hence in stractures with layer sequences comprising h and c stractme elements the maximum nonmetal content follows suit. 

The phenomenon of vacancy ordering is often observed in the transition metal carbides. " While in the group 4 carbides the tendency towards carbon ordering is weak and can be observed only by extended annealing of 5-TCi samples at comparably low temperatures (<800 °C), the tendency becomes more pronounced in the group 5 and 6 carbides. By the ordering of vacancies at low temperatures, even the metal atom positions are affected. In some cases, however, such ordered structures probably cannot be regarded as equilibrium phases nor can an impurity influence on the stabihty of the structures be ruled out. 

The chlorides of the transition metals can also be employed to obtain transition metal carbides (equation 4). This reaction proceeds in the gas phase at temperatures above approximately 600 °C. Because transition metal halogenides are rather volatile, this reaction is usually used for chemical vapor deposition of carbide layers on solid substrates or to produce carbide powders with very fine (submicron) grain sizes ( nanopowders ). In the latter case, the nucleation of the... 

In the preparation of standards, diffusional techniqnes are of special importance. Using snch techniqnes diffusion layers are formed. With a special modification by use of wedge-type samples, these layers are broadened and are thus better accessible for the microprobe.The samples consist of several diffusion bands because of the various phases in the respective carbide system. Usually, however, the samples should be single phase and homogeneous in order to make chemical gross compositional analysis applicable. Only with a comparison of different techniques a real standardization is possible. Such transition metal carbide standards have been made from the phases VC, NbC, TaC and Cr3C2. ... 

Because of the evident structmal similarities between transition metal carbides and transition metal nitrides the carbon atoms in group 4 and 5 carbides can be replaced completely by nitrogen without changing the structme of the binary phases. So far only one distinct ternary phase Cr2 (C,N)2 has been reported. Intersolubihty between the binary nitrides and carbides in the group 6 carbonitride systems Cr-C-N and Mo-C-N is not complete because of the differences in the crystal structmes of the carbide and nitride phases. 

The transition metal carbides are some of the highest melting known materials. 5-TaCo.89 melts congruently at 3985 °C and 5-HfCo.94 congruently at 3950 °C. The melting temperatures can be read from the phase diagrams presented in Section 5.2. 

Thermal expansion is an important property of transition metal carbides. " They are practically never used in pure form but mostly in composite materials with matrices of other materials (metals). Upon thermal load, the difference in the thermal expansion coefficients of the carbide phase and the matrix may cause degradation of the composite. Generally, the thermal expansion of transition metal carbides is higher than that of the pure metal component. Table 1 gives average thermal expansion coefficients of various carbides. For WC, the thermal expansion has even been measured at various pressures. ... 

The XRD pattern (Figure 7.18(a)) indicates that the obtained sample is cubic phase TiC, with cell constant a= 4.34A. TEM imaging (Figure 7.18(b)) reveals that it consists of nanoparticles of size 10-20 nm [67]. The co-reduction carburization method may be used to prepare other transition metal carbides. For example, as shown in Reaction (20), nanocrystalline ZrC, size 10-25 nm was synthesized on heating at 550 °C for 12 h [68]. 

Table 2 also shows that the group V transition metal carbides, NbCx and TaC, are less active than the carbides of the group VI metals. At 1223 K, both NbCx and TaCx deactivated rapidly, even at elevated pressure, while catalyst stability and high reactant conversions were achieved at 1373 K with NbCx, where autothermal gas-phase reactions are likely to play a significant role in the reaction and carbon deposition was observed, as is demonstrated by the high H2/CO ratio. The reason for the low activity of these materials is the relative ease of their conversion back to the oxide, i.e. the rate of carbidation over these materials is slower than the rate of oxidation. 

H.W. Hugosson, U. Jansson, B. Johansson, O. Eriksson, Restricting dislocation movement in transition metal carbides by phase stability tuning, Science, 293, 2434-2437 (2001). 

In the absence of experimental thermochemical evidence about the strength of the metal-carbon bonds in metal carbonyl carbide systems, we can turn to the binary compounds formed between transition metals and carbon for information about the last point, the strength of metal-carbon bonds to core carbon atoms. Transition metal carbides are important. They include, in substances such as tungsten carbide, WC, some of the hardest substances known, and the capacity of added carbon to toughen metals has been known since the earliest days of steel-making. Information about them is, however, patchy. They are difficult to prepare in stoichiometric compositions of established structure and thermochemistry the metals we are most interested in here (osmium, rhenium, and rhodium) are not known to form thermodynamically stable binary phases MC and the carbides of some other metals adopt very complicated structures. Enough is, however, known about the simple structures of the carbides of the early transition metals to provide some useful pointers. 

Gas-Phase Reactions Gas-phase reactions are typically used for the preparation of thin films or nano-sized transition metal carbides and nitrides. High volatility of metal chloride precursors is useful and nitrides can be obtained by reaction with ammonia (M is the corresponding transition metal) ... 

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