Jobbágy Matías, Blesa Miguel A, Regazzoni Alberto E
INQUIMAE, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Pabellón II, Ciudad Universitaria, C1428EHA Buenos Aires, Argentina.
J Colloid Interface Sci. 2007 May 1;309(1):72-7. doi: 10.1016/j.jcis.2007.01.010. Epub 2007 Feb 21.
An adequate account of the hydrolytic properties of Cr3+ and Ni2+ allows setting the conditions for homogeneous nucleation of layered Ni(II)-Cr(III) double hydroxides; water exchange and hydrolysis rate constants indicate that, at very high rates of base dosing, formation of heteronuclear Cr(III)-Ni(II) hydroxo species should prevail over precipitation of active Cr(OH)3. This is realized by the urea method under microwave-assisted hydrothermal conditions. This approach yields crystalline Ni1-xCrx(OH)2(CO3)x/2nH2O (x approximately 0.32-0.36) in less than 5 min at 453 K; higher degrees of crystallinity are obtained at higher temperatures and/or longer aging times. Formation of Ni(II)-Cr(III) LDHs upon microwave-assisted hydrothermal aging of freshly coprecipitated Ni(OH)2+Cr(OH)3 mixtures takes longer, due to a different operating mechanism. The implications of the advanced rationale for the design of synthesis procedures are stressed. It is proposed that homogeneous nucleation of Ni(II)-Cr(III) LDHs involves the edge-on condensation of planar heteronuclear Cr(III)-Ni(II) hydroxo trimers. Ordered aggregation of primary particles leads to the final platelet crystals, a process that involves the exchange of CO2-(3) ions dangling at the crystallites' edges by bridging OH-.
对Cr3+和Ni2+水解特性的充分了解有助于设定层状Ni(II)-Cr(III)双氢氧化物均匀成核的条件；水交换和水解速率常数表明，在极高的碱剂量速率下，异核Cr(III)-Ni(II)羟基物种的形成应优先于活性Cr(OH)3的沉淀。这可通过微波辅助水热条件下的尿素法实现。该方法在453 K下不到5分钟就能得到结晶的Ni1-xCrx(OH)2(CO3)x/2nH2O（x约为0.32 - 0.36）；在更高温度和/或更长老化时间下可获得更高的结晶度。新鲜共沉淀的Ni(OH)2 + Cr(OH)3混合物在微波辅助水热老化过程中形成Ni(II)-Cr(III)层状双氢氧化物所需的时间更长，这是由于操作机制不同。强调了先进原理对合成程序设计的影响。有人提出，Ni(II)-Cr(III)层状双氢氧化物的均匀成核涉及平面异核Cr(III)-Ni(II)羟基三聚体的边缘凝聚。初级颗粒的有序聚集导致最终的片状晶体，这一过程涉及通过桥连OH-交换在微晶边缘悬挂的CO2-(3)离子。
