Hydrotalcite-like anionic clays of general formula [M(II)1–xM(III)x(OH)2]x+[CO32–x/2]x– · m H2O with M(III) = Al and M(II) = Mg, Ni, Zn have been prepared by precipitation of the hydroxycarbonates from the “homogeneous” solution after the thermally induced hydrolysis of urea. The effect of the temperature of precipitation, of the total metal cations concentration, of the molar fraction M(III)/M(III) + M(II) and of the molar fraction urea/M(II) + M(III) in solution on the composition and on the crystallinity degree of the samples has been investigated. The optimum conditions are reported to obtain micro-crystalline powders with a narrow distribution of particle size in a short time with a simple procedure. The compounds obtained have been characterised for chemical composition, thermal behaviour, particle-size distribution and BET-surface area. In addition, the crystal structure of Mg0.67Al0.33(OH)2 (CO3)0.165 · 0.4 H2O has been refined by X-ray diffraction powder methods. The carbonate form has been converted into the chloride form by letting gaseous HCl flow over the hydrotalcite-like compounds, heated at 150 °C. The exchange of Cl– anions with some alkoxide anions in the presence of the respective alkanols has been investigated. The exchange reaction was driven by the segregation of NaCl crystals poorly soluble in alkanols and led to the co-intercalation of the alkoxide ions together with the alkanol with the formation of a bi-layer of extended alkyl chain in the interlayer region of the Mg-Al hydrotalcite. The intercalation compound, washed with water, produces a hydrotalcite with Cl– and OH– as balancing anions.

New Synthetic Routes to Hydrotalcite-Like Compounds − Characterisation and Properties of the Obtained Materials

Ureases (urea amidohydrolases, EC 3.5.1.5) are a group of highly proficient enzymes, widely distributed in nature, whose catalytic function is to catalyze the hydrolysis of urea, its final products being carbonic acid and ammonia. The products and the resulting increase in pH of the reaction environment are consequential characteristics of the action of ureases. Apart from its natural significance, ureases-catalyzed hydrolysis of urea is important in that it has great potential for practical applications. In view of this importance, this article offers a review of the properties of the enzymes, where in addition to the established knowledge, the recent findings are presented. Special emphasis is put on the functional and practical properties of ureases that can be customized and exploited in a diversity of important applications, notably medical, analytical, environmental and engineering.

Ureases I. Functional, catalytic and kinetic properties: A review

Mono-dispersed sub-micron sized hydrotalcite-like compounds (HTlcs), [Mg1−xAlx(OH)2](CO3)y·2H2O, were synthesized by the urea hydrolysis method. The as-prepared compounds display platelet-like primary particles with a hexagonal shape. The breadth to thickness ratio can be varied by changing the synthesis conditions such as the precipitation rate or the medium nature. The factors influencing the secondary particle morphology (or aggregates) are also discussed. The results are compared to the constant pH coprecipitation method, commonly used for HTlcs.

Synthesis of Al-rich hydrotalcite-like compounds by using the urea hydrolysis reaction—control of size and morphology

Preparation and properties of monodispersed colloidal particles of lanthanide compounds: I. Gadolinium, europium, terbium, samarium, and cerium(III)

Gold on TiO2 prepared by cation adsorption (CA) and deposition–precipitation with urea (DP urea) and NaOH (DP NaOH) were characterized by various techniques during the preparation in order to determine the nature of the species deposited and the chemical phenomena occurring during these preparations. In the case of cationic adsorption of the [Au(en)2] 3+ complex, we showed that the preparation has to be performed at room temperature to avoid the decomposition of the complex and the reduction of gold. In such a way, small gold particles are obtained after calcination, but the gold loading is low. The methods of deposition–precipitation (DP NaOH and DP urea) involve both the deposition of a gold(III) species on the TiO2 surface, but the nature of these species is different. For the DP NaOH, we propose that [AuCl(OH)3] � , the main species present at the pH 8 of the preparation, reacts with hydroxyl groups of the TiO2 surface, and forms a grafted hydroxy-gold compound. This may explain the limited amount of gold deposited on TiO2 by this method. For the DP urea method, all the gold present in solution is deposited on the TiO2 surface as a gold(III) precipitate, which is not gold(III) hydroxide, but an amorphous compound containing nitrogen, oxygen and carbon. This compound arises from a reaction between the gold precursor and the products of decomposition of urea. The metallic gold particles obtained after calcination exhibit a decreasing size when the time of DP urea increases. We propose that the progressive increase of pH, due to urea decomposition at 80 8C, results in changes in the surface charge density of the gold precipitate particles, and leads to a fragmentation of the particles. # 2005 Elsevier B.V. All rights reserved.

John J. Bordeaux

Papers

The Decomposition of Urea in Aqueous Media

Inhibition of Phenol Alkylations by Ethers. Kinetic Evidence for Phenol-Ether Complexes