Metal carbonyls thermal decomposition

Nagy et al. [127] used thermal decomposition of [Fe(CO)s] sorbed in HY zeolite to form highly dispersed iron partides, which were observed by transmission electron microscopy to be 10-50 A in size. Photodiemical decomposition gave smaller partides (too small to be observed with transmission electron micros-copy), presumaUy because of the strong interaction between the carbonyl dusters and the surface resulting from the eledronic exdtation of the metal carbonyl during decomposition. 

If the metal in the precursor is zerovalent, such as in carbonyls, thermal decomposition initially leads to formation of the metal, but two-step procedures can be used to produce oxide nanoparticles as well [30]. In a related work, the synthesis of highly crystalline and monodisperse y-Fe Oj nanocrystallites is reported. High-temperature (300°C) aging of iron-oleic acid metal complex, which was prepared by the thermal decomposition of iron pentacarbonyl in the presence of oleic acid at 100°C, was found to generate monodisperse iron nanoparticles [31]. 

Miscellaneous. Electron beams can be used to decompose a gas such as silver chloride and simultaneously deposit silver metal. An older technique is the thermal decomposition of volatile and extremely toxic gases such as nickel carbonyl [13463-39-3] Ni(CO)4, to form dense deposits or dendritic coatings by modification of coating parameters. 

The heavy metal salts, ia contrast to the alkah metal salts, have lower melting points and are more soluble ia organic solvents, eg, methylene chloride, chloroform, tetrahydrofiiran, and benzene. They are slightly soluble ia water, alcohol, ahphatic hydrocarbons, and ethyl ether (18). Their thermal decompositions have been extensively studied by dta and tga (thermal gravimetric analysis) methods. They decompose to the metal sulfides and gaseous products, which are primarily carbonyl sulfide and carbon disulfide ia varying ratios. In some cases, the dialkyl xanthate forms. Solvent extraction studies of a large number of elements as their xanthate salts have been reported (19). 

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). 

Recent work on [CpFe(CO)2]2 was intended to test whether once again a complex molecule could be found to have a high yield and also to test a possible preferential formation of metal carbonyls over metal sandwich compounds. In this compound, thermal decomposition of the starting compound gives rise predominantly to ferrocene (28, 68). The data (50) given in Table VIII show that indeed the carbonyl is preferentially formed... 

The most intensive development of the nanoparticle area concerns the synthesis of metal particles for applications in physics or in micro/nano-electronics generally. Besides the use of physical techniques such as atom evaporation, synthetic techniques based on salt reduction or compound precipitation (oxides, sulfides, selenides, etc.) have been developed, and associated, in general, to a kinetic control of the reaction using high temperatures, slow addition of reactants, or use of micelles as nanoreactors [15-20]. Organometallic compounds have also previously been used as material precursors in high temperature decomposition processes, for example in chemical vapor deposition [21]. Metal carbonyls have been widely used as precursors of metals either in the gas phase (OMCVD for the deposition of films or nanoparticles) or in solution for the synthesis after thermal treatment [22], UV irradiation or sonolysis [23,24] of fine powders or metal nanoparticles. 

Metallic powders are made several different ways. They can be prepared by reducing salts in a stream of a reducing gas, such as hydrogen chlorides of metals are commonly used but oxides are used too. Thermal decomposition in a vacuum of metal carbonyls or metal salts of organic acids, such as formates, produces metal powders. Surface areas of such powders are around 1.5 m2/g. Powders can also be made from electrolytic reduction of salts in organic solvents and by atomization of the metal. 

Metal carbonyl compounds are other suitable precursors for the synthesis of NPs by thermal decomposition. The main advantage is the formation of CO that is expelled from the IL phase due to its poor solubility. However, high temperatures are commonly used to decompose such precursors. Metal NPs of Cr(0), Mo(0), and W(0) were prepared by thermal or photolytic decomposition of their respective monometallic carbonyl compounds [M(CO)6] dispersed in ILs [52]. Similarly, the precursors [Fe2(CO)9], [Ru3(CO)i2], and [Os3(CO)12] were employed in order to obtain stable metal NPs (1.5-2.5 nm) in BMI.BF4 [53]. The same procedure was extended to the preparation of lr(0), Rh(0), and Co(0) NPs in ILs [54]. 

J. A. Connor, H. A. Skinner, Y. Virmaai. Microcalorimetric Studies. Thermal Decomposition and Iodination of Metal Carbonyls. J. Chem. Soc., Faraday Trans. 11972, 68, 1754-1763. 

Early studies of the interaction of lr4(CO)i2 with a silica surface indicate that physisorption of the cluster takes place. Although the cluster can sublime during thermal treatments after impregnation [198], the loss of metal carbonyl can be avoided by mild thermal treatments that produce a redispersion of the physisorbed lr4(CO)i2 onto the silica surface [199]. An XPS and FTIR study of the evolution of physisorbed lr4(CO)i2 under different conditions pointed to the formation of metallic particles by mild thermal decomposition under Ar or H2, with the particle size increasing with increasing temperature [200]. 

It was reported early on that thermal decomposition above 110°C of Ir4(CO)i2 adsorbed on partially hydroxylated alumina gives carbonyl decomposition and renders metal parhcles of iridium below 1 nm [201]. 

Unlike the related Na3[M (CO)5], where M = V, Nb, and Ta, which undergoes thermolysis below O0C (vide infra), these materials possess remarkable thermal stabilities for metal carbonyls and briefly survive without melting at temperatures as high as 300°C. By comparison, K2[Fe(CO)4], another metal carbonyl salt of high thermal stability, has been reported to melt at 270-273°C with decomposition (20). The related K[Co(CO)4] melts at about 203°C with decomposition (21). 

The fragmentation patterns of a number of phosphine-metal carbonyl complexes have been reported 164). Complexes of bis(diphenylphos-phino)ethane (diphos) appear to lose ethylene to give ions of the type (Ph2P)2Mo+. In contrast to thermal decomposition by loss of phosphine,... 

Among monomeric metal carbonyls, Mo(CO)6, Cr(CO)6, Fe(CO)5, and Ni(CO)4 have been most studied. Their stepwise decomposition on the support may lead to the formation of grafted species. For example, Mo(CO)6 is first physically adsorbed onto hydroxylated alumina at room temperature [2, 62, 63, 68]. Thermal decomposition leads to the formation of adsorbed subcarbonyl species such as Mo(CO)s and then Mo(CO)3 at 373 K. Complete dccarbonylation at 573 K is observed upon oxidative addition of the metal on surface hydroxy groups ... 

Various active nickel catalysts obtained not via reduction of nickel oxide with hydrogen have been described in the literature. Among these are the catalysts obtained by the decomposition of nickel carbonyl 10 by thermal decomposition of nickel formate or oxalate 11 by treating Ni-Si alloy or, more commonly, Ni-Al alloy with caustic alkali (or with heated water or steam) (Raney Ni) 12 by reducing nickel salts with a more electropositive metal,13 particularly by zinc dust followed by activation with an alkali or acid (Urushibara Ni) 14-16 and by reducing nickel salts with sodium boro-hydride (Ni boride catalyst)17-19 or other reducing agents.20-24... 

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