The Investigation Of Fundamental And Applications Of Chromium Hydroxide

Chromium hydroxide is of great importance for its application in inorganic pigments, chromium product intermediates, trivalent chromium electroplating source and chemical materials. In the chemical production, a quick dissolution of chromium hydroxide can save cost and time, and will improve production efficiency as well. It has been a key subject for the researchers to develop a low cost method for preparation of quick soluble chromium hydroxide with simple process. Crystalline chromium hydroxide with excellent dissolve performance was prepared by one step precipitation through controlling the temperature and precipitation agent. We examined the factors that influence the chromium hydroxide crystallization, and analyzed its thermal dehydration behavior. The catalytic role of crystalline chromium hydroxide on glucose and ammonium perchlorate, and the application of chromium hydroxide as a source of trivalent chromium in electroplating were investigated in detail. The following results have been obtained.?1? The chromium hydroxide prepared by ammonia precipitating agent was crystalline state at room temperature, meanwhile crystallization at low temperature was slightly better than that under the room temperature. This is because that Cr3+ and NH4+ ions will form stable complexes Cr?NH3?63+, and hydroxyl replaces ammonium ions slowly,meanwhile, Cr3+ ion also slows down the precipitation speed, all the phenomena aforementioned are beneficial to the growth of the crystallization. When preparing chromium hydroxide by sodium hydroxide, precipitation temperature has a great influence on chromium hydroxide crystallization. The solution under the low temperature condition can quickly achieve the supersaturated state, high degree of supersaturation can be beneficial to the nucleation of tiny crystal nucleus. As regard for strong base, good chromium hydroxide crystallization can be obtained by controlling the temperature below 15 °C. Crystalline chromium hydroxide is very stable under the low temperature, its structure will not change. At the same time, it can be directly distinguished between crystalline chromium hydroxide and amorphous chromium hydroxide through the significant difference in IR absorption at low wave number.?2? The dissolve performance of crystalline chromium hydroxide is obviously better than that of the amorphous chromium hydroxide in acid, which is due to that the crystalline chromium hydroxide has the unique structure Cr?H?OH···O?H?Cr. The hydrogen bond is easier to fracture in acid, which makes hydroxyl react with hydrogen ions faster and more fully; At the same time, crystalline chromium hydroxide has regular morphology and the reunion phenomenon is not obvious, which leads to more room of contact between chromium hydroxide and acid.?3? The comparison of water molecules loss between theoretical calculation and the experimental results showed that amorphous chromium hydroxide more easily absorbed water than nanocrystalline chromium hydroxide at the room temperature observed from the first dehydration endothermic peak and the intermediate for amorphous chromium hydroxide during heating was complicated. XRD characterization and the theoretical calculation of water molecules indicated that when the quality "platform" appeared at 300 °C to 400 °C, the amorphous chromium hydroxide dehydrated products contain crystalline intermediate Cr OOH; Meanwhile, crystalline chromium hydroxide dehydrated products have amorphous Cr OOH. In situ XRD at several temperature test points showed that the chromium compounds evolved without phase transition. No structure change happened no matter at low temperature or high temperature. The dehydration of nanocrystalline chromium hydroxide occurred at 105 °C, 289 °C, and 409 °C, with the number of water molecules lost being 3, 5, and 1, respectively. Meanwhile, the dehydration of amorphous chromium hydroxide occurred at 70 °C, 289 °C, 406 °C, and 443 °C, with the number of water molecules lost being 2.1, 6, 0.5, and 0.5, respectively.?4? The chrome oxide obtained by calcination of crystalline chromium hydroxide behaved excellent crystallization, which had uniform particles with grain size of about 45 nm. The powder had bright green colour and lustre beautiful appearance; The crystallization of the Cr₂O₃ obtained by amorphous chromium hydroxide was poor, which turned up as blue green fine particles with grain size of around 60 nm. Both of the two forms of chromium hydroxide precursors can prepare nano-Cr₂O₃ powder. Looking from the apparent colour and lustre, crystalline chromium hydroxide as paint raw materials was better than amorphous chromium hydroxide. On the other hand, during the the short-term aging of crystalline chromium, no big change occured in the dehydration temperature; However when the aging time exceed more than a month, the dehydration temperature shifted to a high temperature, which was due to the large increase in polymer, resulting that the Cr-O-Cr bond in the polymer ruptured more difficult, therefore the decomposition temperature rised.?5? The Ni?OH?2/Cr?OH?3 hybrid prepared by NH3·H2O was in nanocrystalline. The octahedral nanocrystalline structure of Cr?OH?3 could provide a high surface area for easy and fast ion/electron transfer and then induce high sensitivity to glucose detection. The performance of the proposed sensor was assessed using the effect of glucose concentration on current response, and nanocrystalline Ni?OH?2/Cr?OH?3 electrode after water washing reached a better sensitivity of 697.4 ?A/?mmol/L cm-2? and detection limit of 0.30 ?mol/L. The electrode responded rapidly and reached a steady state within 5¹0 s when glucose was injected, indicating a very rapid electron transfer rate on the modified electrode. Furthermore, the sensor exhibited a excellent selectivity in the presence of common interfering species such as AA and UA at their physiological concentrations.?6? The addition amount of 20% nanocrystalline chromium hydroxide received the best catalytic performance on the decomposition of ammonium perchlorate?AP?. Nanocrystalline chromium hydroxide reduced the decomposition temperature from 450 °C, 313 °C to 318 °C, 295 °C, respectively. Meanwhile, at different amount addition of nanocrystalline chromium hydroxide, part of AP was found to decompose before crystal structure transformation and the decomposition temperature was 214 °C. This is the first time to use nanocrystalline chromium hydroxide as a catalyst to make the decomposition of AP happen at 214 °C.?7? In the sulfate trivalent chromium plating system, based on the glycine as the primary ligand, the influence of the second ligand such as succinic acid, 1,6-hexanediol, urea and ethylenediamine on trivalent chromium plating coating was investigated respectively. The results have shown that all these four kinds of the second ligands were able to enhance cathodic polarization, among them urea as the second ligand solution system produced bright coating with the best corrosion resistance. In the system of urea as second ligand, the optimum conditions were obtain by investigating the coating brightness and thickness: Solution temperature 40 °C; Current density of 7.5 A/dm2; The electroplating time 15 min; Solution p H3.0.