Sequential precipitation systems

Ternary systems have been prepared in this way as well. The sequential precipitation of aluminum hydroxide, lanthanum hydroxide and, finally, nickel hydroxide gave, after calcination and reduction, a lanthamun activated Ni/Al203 catalyst which had smaller metal crystallites and was somewhat more active than a catalyst prepared by the simultaneous coprecipitation of the three... 

Colloidal Pt/RuO c- (C5 0.4nm) stabilized by a surfactant was prepared by co-hydrolysis of PtCU and RuCls under basic conditions. The Pt Ru ratio in the colloids can be between 1 4 and 4 1 by variation of the stoichiometry of the transition metal salts. The corresponding zerovalent metal colloids are obtained by the subsequent application of H2 to the colloidal Pt/Ru oxides (optionally in the immobilized form). Additional metals have been included in the metal oxide concept [Eq. (10)] in order to prepare binary and ternary mixed metal oxides in the colloidal form. Pt/Ru/WO c is regarded as a good precatalyst especially for the application in DMECs. Main-group elements such as A1 have been included in multimetallic alloy systems in order to improve the durability of fuel-cell catalysts. PtsAlCo.s alloyed with Cr, Mo, or W particles of 4—7-nm size has been prepared by sequential precipitation on conductant carbon supports such as highly disperse Vulcan XC72 [70]. Alternatively, colloidal precursors composed of Pt/Ru/Al allow... 

Thus, there is in essence a critical supersaturation concentration below which nucleation is very slow and above which nucleation is very fast. The range between this critical supersaturation concentration and the solubility concentration is the domain of deposition-precipitation discussed elsewhere (Chapter 6), while impregnation (Chapter 4) is performed in the concentration range below the solubility limit. Especially for multicomponent systems, it is important to operate at high supersaturation levels so that the solubility product of all products is simultaneously exceeded, otherwise a possibly undesired sequential precipitation may occur. 

Fig. 2 Crystallization pathways under thermodynamic and kinetic control Whether a system follows a one-step route to the final mineral phase (pathway A) or proceeds by sequential precipitation (pathway B), depends on the free energy of activation associated with nucleation (N), growth (g), and phase transformation (T). Amorphous phases are common under kinetic conditions. Reproduced from [40] with permission of Wiley...

It has been demonstrated that sequential precipitation in a moderately acid pH range for the binary metal systems Cu/Al, Cu/Cr, Cu/Fe and Nl/Fe in the presence of oxalic acid with aluminum or ferric hydroxide as the first stage and adsorption/precipitation of copper or nickel as the second stage provides metal oxides (after 250-350° C air calcine) with considerable enhancement in dispersion and in catalytic activity, notable for Cu/Al, for the room temperature decomposition of hydrogen peroxide and benzaldehyde oxidation by hydrogen peroxide. 

The holistic thermodynamic approach based on material (charge, concentration and electron) balances is a firm and valuable tool for a choice of the best a priori conditions of chemical analyses performed in electrolytic systems. Such an approach has been already presented in a series of papers issued in recent years, see [1-4] and references cited therein. In this communication, the approach will be exemplified with electrolytic systems, with special emphasis put on the complex systems where all particular types (acid-base, redox, complexation and precipitation) of chemical equilibria occur in parallel and/or sequentially. All attainable physicochemical knowledge can be involved in calculations and none simplifying assumptions are needed. All analytical prescriptions can be followed. The approach enables all possible (from thermodynamic viewpoint) reactions to be included and all effects resulting from activation barrier(s) and incomplete set of equilibrium data presumed can be tested. The problems involved are presented on some examples of analytical systems considered lately, concerning potentiometric titrations in complex titrand + titrant systems. All calculations were done with use of iterative computer programs MATLAB and DELPHI. 

A three-necked, round-bottomed flask equipped with a thermometer, reflux conductor, N, inlet and an overhead mechanical stirrer was charged sequentially with H O (5 mL), 50% NaOH (2 mL. 39 mmol), and adenine (2 g, 15 mmol). After the adenine had dissolved, a solution of benzylating reagent (16 mmol) and phase-transfer catalyst (5 mol%) in organic solvent (30-40 mL) was added. The system was stirred under reflux and then cooled to rt. The precipitated solids were filtered, washed with lIjO (2x10 mL), and dried in vacuo to give the crude bcnzylated adenine mixtures. For separation see ref 82. 

In this chapter we demonstrate the efficiency of P on Pb immobilization and discuss various mechanisms involved in the processes. Mechanisms include sorption, cation substitution, and chemical precipitation, which may occur simultaneously and/or sequentially. The complexity of the kinetics in various systems was discussed, and the data shown encourage further research to identify the reactions... 

A Unal example of direct bioanalysis was recently published by Dethy et al. and involves the appUcation of infusion nanoelectrospray (nano-ESI) from a silicon chip [110]. In this example, supernatant obtained from protein precipitation was directly infused with an automated pipette-tip delivery system. Individual, conductive pipette tips that contain sample were sequentially introduced to the backplane of a silicon chip for analysis. The front plane of the chip that consisted of 100 individual nano-ESI nozzles, was positioned near the API orifice of a TQMS for direct serial analysis. Quantitation of verapamil and its metaboUte norverapamil occurred in human plasma over a range of 5-1000 ng/mL. It is possible to achieve analysis times of less than 1 minute per sample with this technology. An important advantage, demonstrated by this work, is the unique abiUty to avoid system carryover with this device [110]. 

Drug-drug iuteractions Calcium salts A previous warning from the FDA that ceftriaxone and intravenous calcium products should not be co-administered to any patient to prevent precipitation leading to end-organ damage [SEDA-33, 494] was retracted in April 2009. In a search of the FDA Adverse Event Reporting System for reports of ceftriaxone-calcium interactions that resulted in serious adverse events, with ceftazidime-calcium as a comparator, 104 and 99 events respectively were identified [23 ]. For ceftriaxone-calcium-related adverse events, 7.7% and 20% were classified as probable and possible respectively for embolism. Ceftazidime-calcium resulted in fewer probable embolic events (4%) but more possible embolic events (30%). The authors claimed that their analysis supported the revised FDA recommendation that patients over 28 days old may receive ceftriaxone and calcium sequentially. This has been supported by the results of a matched cohort study in 142 patients who were exposed to the combination of ceftriaxone and intravenous calcium and in whom mortality and other adverse outcomes were not increased... 

The term upconversion describes an effect [1] related to the emission of anti-Stokes fluorescence in the visible spectral range following excitation of certain (doped) luminophores in the near infrared (NIR). It mainly occurs with rare-earth doped solids, but also with doped transition-metal systems and combinations of both [2, 3], and relies on the sequential absorption of two or more NIR photons by the dopants. Following its discovery [1] it has been extensively studied for bulk materials both theoretically and in context with uses in solid-state lasers, infrared quantum counters, lighting or displays, and physical sensors, for example [4, 5]. Substantial efforts also have been made to prepare nanoscale materials that show more efficient upconversion emission. Meanwhile, numerous protocols are available for making nanoparticles, nanorods, nanoplates, and nanotubes. These include thermal decomposition, co-precipitation, solvothermal synthesis, combustion, and sol-gel processes [6], synthesis in liquid-solid-solutions [7, 8], and ionothermal synthesis [9]. Nanocrystal materials include oxides of zirconium and titanium, the fluorides, oxides, phosphates, oxysulfates, and oxyfluoiides of the trivalent lanthanides (Ln ), and similar compounds that may additionally contain alkaline earth ions. Wang and Liu [6] have recently reviewed the theory of upconversion and the common materials and methods used. 

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