Thermal,structural and textural studies on the double complex salt [Co(NH3)6][Fe(CN)6] and on its silica-supported catalysts

Thermal analyses were carried out for the simple complexes [Co(NH₃)₆]Cl₃ and K₃[Fe(CN)₆] together with the double complex [Co(NH₃)₆][Fe(CN)₆] using thermogravimetric analysis, differential thermal analysis (DTA) and derived DTA (DDTA). Two series of silica-supported catalyst samples were prepared with soaking periods ranging from 5 h to 7 days, and likewise investigated. Structural changes of the original samples and their thermally treated products obtained at temperatures < 500 °C were monitored using X-ray diffraction and infrared spectroscopy. Textural variations were also studied using the N₂ adsorption technique. The decomposition of the simple complex [Co(NH₃)₆]Cl₃ occurs in two main steps that give rise to two asymmetric endothermic peaks centred at 225 and 350 °C. The first step involves two decomposition processes in which primarily two chloride ions migrate to the inner coordination sphere, thereby releasing two coordinated NH₃ that are evolved, followed by simultaneous decomposition and reduction to CoCl₂ · 2NH₃. The second step involves the decomposition of this latter compound to CoCl₂ with some reduction to Co. K₃[Fe(CN)₆] decomposes in a more complex manner, yet two strong exotherms are observed at 345 and 400 °C — the former being accompanied by a loss in weight due to its decomposition, whereas the latter is accompanied by a small increase in weight where an oxidation process seems to take place with its further decomposition. The double complex [Co(NH₃)₆][Fe(CN)₆] exhibits five consecutive decomposition steps in the temperature range 210–305 °C. The first is reproduced as an endotherm at 180 °C, whereas the remaining four steps cannot be distinguished separately by DTA due to the strong exothermic effects and only two exotherms are observed at 255 and 280 °C. The steps are identified according to the simultaneous reaction of six molecules together, leaving a final solid product with empirical formula Fe(CN)₂Co. X-ray diffraction of the double complex retains the characteristics of the constituent simple complexes with the appearance of two new very strong bands with a d- spacing of 0.5483 and 0.3005 nm. The specific surface areas of the catalyst samples are found to depend on the ion moiety supported first during preparation as well as on the soaking period. The catalyst samples are predominantly microporous and the variations in surface characteristics are discussed. V_I-t plots reveal the presence of two groups of pore sizes in some cases.