Stabilization of chromium

Mena, M.L., A. Morales-Rubio, A.G. Cox, C.W. McLeod, and P. Quevauviller. 1995. Stability of chromium species immobilized on microcolumns of activated alumina. Quim. Anal. 14 164—168. 

The copper surface areas of fresh (S ) and used (S ) catalysts are demonstrated in Table l. The ratio of S1/S0 exhibits the extent of copper surface area reduced after reaction. The copper surface areas reduce after dehydrogenation reaction. This indicates that sintering occurs in reaction process for all of the catalysts. Chromium promoted catalysts have higher fresh copper surface areas than the unpromoted one as shown in Table 1. The previous results [5] indicated that the catalyst with Cr/Cu molar ratio of 1/10 had the highest stability for unsupported catalyst nevertheless, the catalyst with Cr to Cu molar ratio of 1/40 is the most stable one in Si02-supported case. The stability of chromium promoted catalyst decreases when the Cr/Cu molar ratio increases. 

Sintering occurs in reaction process for all of the catalysts, For Cr-promoted catalysts, the catalyst with Cr/Cu=i/4Q has the highest activity and stability. The activity and stability of chromium promoted catalyst decreases when the Cr/Cu molar ratio increases from 1/40 to 1/4. 

As mentioned before, subsequent phosphate treatment does not affect the stable sulfide, and TCLP results show excellent stabihzation of Cr in any oxidation state. Alternatively, a small amount of reductant in the waste will convert chromate into lower oxidation states. Such methods, however, are not preferred, because the reductant may also affect the solubility of other hazardous compounds. The exception is technetium-containing radioactive waste, in which chromate is also a contaminant. As we shall see in Chapter 17, a reductant is essential for stabihzation of technetium, and that will also help in stabilization of chromium. 

We recendy performed a didailed investigation of the stability of chromium-substituted molecular sieves [82]. As a model reaction we chose the allylic oxidation of a-pinene with TBHP (Reaction 19) which gives verbenone in high selectivity. 

Anand, V.D. and Ducharme, D.M. (1976). Stability of chromium ions at low concentrations in aqueous and biological matrices stored in glass, polyethylene, and polycarbonate containers. Natl. Bur. Standards Spec. Publ. 422. Accuracy in trace analysis Sampling, sample handling and analysis, pp. 611-619. 

The effect of a wide range of feed concentration of PCE from 30 to 10,000 ppm on the stability of chromium oxide supported on Ti02 and AI2O3 for the removal of chlorinated volatile organic compounds (CVOCs) has been investigated over a fixed bed flow reactor. Both chromium oxide catalysts exhibited stable PCE removal activity up to 100 h of reaction time without any catalyst deactivation when 30 ppm was introduced into the reactor. 

In the present study, the stability of chromium oxide catalysts has been systematically examined to elucidate the deactivation mechanism by chlorinated compounds. The operating conditions, including the feed concentration of perchloroethylene (PCE), were optimized to locate the optimal region of the reactor operating condition where the catalyst deactivation can be avoided. It may also resolve the hesitation in the use of chromium catalyst and suggest an optimal operating condition of the reactor system for the removal of CVOCs. 

The stability of chromium oxide catalysts supported on Ti02 and AI2O3 was examined at various feed concentrations of PCE from 30 to 10,000 ppm as shown in Fig. 1. The activity... 

Amide hydrolysis was described in the case of the chromium complex shown in Eq. 27.84 It is noteworthy that no alteration of the organometallic moiety occurs. In addition to the relative stability of chromium compoimds under sonication, the low-energy irradiation conditions selectively allowed the biphasic hydrolysis to occur, without promoting the more energy-demanding metal group sonolysis. 

The high stability and low solubility of chromium alloys and stainless steels are believed to be due to the formation of surface chromium oxide layers such as Cr203- H20 and Cr203-Fe0 (chromium spinel) (Ziemniak, Jones and Combs, 1998). Knowledge of the stability of chromium oxide and hydroxide phases is necessary to understand the solubility and stability of such surface layers on alloys and steels. 

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