0383500 - ÚACH 2013 RIV US eng M - Část monografie knihy
Šubrt, Jan
Homogeneous precipitation with urea – a versatile way to metal oxide nanoparticles.
.UREA SYNTHESIS PROPERTIES USES. New York: Nova Science Publishers, 2012, s. 109-148. ISBN 978-1-62257-061-4
Výzkumný záměr: CEZ:AV0Z40320502
Klíčová slova: urea * homogeneous precipitation * metal oxide nanoparticles * hydrolysis.
Kód oboru RIV: CA - Anorganická chemie

Homogeneous precipitation of aqueous metal salt solutions with urea is used for synthesis of inorganic materials based on metal oxide nanoparticles. Contrary to the heterogeneous precipitation method, when the acidic metal salt solution is mixed together with the basic neutralizing agent, much milder concentration gradients occur during the homogeneous precipitation, resulting in significantly different properties of the precipitation product. Whereas the heterogeneous precipitation method leads usually to voluminous mostly amorphous metal hydroxides, the homogeneous method regularly gives rise to (nano) crystalline material with significantly lower content of impurities from the mother liquor. Regularly, these nanocrystals are subjected to self assembly processes resulting in formation of rather big spherical aggregates or their layers on surfaces of solid admixtures. Urea is the most widely used neutralizing agent for the homogeneous precipitations because its hydrolysis in water is sufficiently slow and occurs at acceptable temperatures (mostly 60-100 °C). The hydrolysis of urea takes place in two steps. The first one, the reversible formation of ammonium cyanate, is followed by the irreversible hydrolysis of cyanate ions. The concentration of HCO3− increases linearly with time and pseudo-zero order kinetics results. During the urea decomposition, NH4+ ions are slowly produced in the whole volume of the solution resulting in absence of H+ gradients in the solution during metal ions hydrolysis. Among the most important advantages of the urea precipitation method in comparison with heterogeneous precipitation processes are that the reaction products are both pure and uniform with respect to their shape and the waste solution contains mostly ammonium sulfate which can be utilized as fertilizer.
Trvalý link: http://hdl.handle.net/11104/0216639