Cerium ions, reactions

When PAN block copolymers are synthesized with the help of such method the reaction is conducted during a period of time (20—30 min) shorter than the induction period of AN polymerization (45 min) in the presence of cerium ions. When the mechanism and the laws governing the reaction of AN copolymerization with PEO and PPO were studied, it was established that the initiation of the block copolymerization proceeds in accordance with the following scheme ... 

The induced reduction of chlorate can be inhibited by iodide, bromide and chloride ions. The effectiveness of these ions is about 400 10 1 in the given order. The order and the magnitude of the effect agree fairly well with the catalytic activity of these ions in the arsenic(III)-cerium(IV) reaction. This inhibition by halides is presumably connected with the opening of a new two-electron route for the arsenic(III)-cerium(IV) reaction. 

The oxidation of an anthracene suspension in sulfuric acid conducted in the presence of cerium salts can serve as an example of mediated oxidation. In the bulk solution the Ce" ions chemically oxidize anthracene to anthraquinone. The resulting Ce ions are then reoxided at the anode to Ce". Thus, the net result of the electrochemical reaction is the oxidation of anthracene, even though the electrochemical steps themselves involve only cerium ions, not anthracene. Since the cerium ions are regenerated continuously, a small amount will suffice to oxidize large amounts of anthracene. 

An example of the process of a passivating metal is the reaction of tetravalent cerium with iron (see Fig. 5.54D). Iron that has not been previously passivated dissolves in an acid solution containing tetravalent cerium ions, in an active state at a potential of Emix2. After previous passivation, the rate of corrosion is governed by the corrosion current ya and the potential assumes a value of Emixl. 

Results and Discussion. Of the 12 samples of starch graft copolymer synthesized, half were hydrolyzed to anionic polyelectrolytes. Synthesis data on these 6 samples are given in Table 2. These particular samples were chosen for hydrolysis because the samples can be intercompared to see the effect of synthesis variables on ultimate product properties. Samples 5, 8, and 11 have the same mole ratio of cerium ion to starch backbone, N, in their reaction mixture. Samples 7, 8, and 9 all have the same refctable mass per starch molecule,... 

This reaction appears to be first order because the cerium ions are in excess, so the concentration does not really change. We look at pseudo-order reactions on p. 387. 

Cerium phthalocyanine, 7 64 Cerium tetrafluoride, 20 63-67 lattice parameters, 20 64 Cerium trifluoroacetates, 17 5, 26 Cermets, nitrides in, 9 81-82 Cerous ion, reaction with hydroxyl ions, 3 187 Ceruloplasmin, 40 123, see also Blue copper oxidases... 

Observed stoichiometry of the cerium-alkoxyhydroperoxide reaction was a little more than 1 mole of peroxide per equivalent of Ce. This agrees well with Reaction 4s predominating over a bimolecular decomposition analogous to Reaction 3. Because the observed stoichiometry was so close to 1 1, chain reaction decomposition initiated by metal ions appears of little importance in this case. 

A typical chemical system is the oxidative decarboxylation of malonic acid catalyzed by cerium ions and bromine, the so-called Zhabotinsky reaction this reaction in a given domain leads to the evolution of sustained oscillations and chemical waves. Furthermore, these states have been observed in a number of enzyme systems. The simplest case is the reaction catalyzed by the enzyme peroxidase. The reaction kinetics display either steady states, bistability, or oscillations. A more complex system is the ubiquitous process of glycolysis catalyzed by a sequence of coordinated enzyme reactions. In a given domain the process readily exhibits continuous oscillations of chemical concentrations and fluxes, which can be recorded by spectroscopic and electrometric techniques. The source of the periodicity is the enzyme phosphofructokinase, which catalyzes the phosphorylation of fructose-6-phosphate by ATP, resulting in the formation of fructose-1,6 biphosphate and ADP. The overall activity of the octameric enzyme is described by an allosteric model with fructose-6-phosphate, ATP, and AMP as controlling ligands. 

The main processes occurring in this system are the following [219] bromate oxidizes trivalent cerium to tetravalent cerium Ce4+ oxidizes bromomalonic acid, and is reduced to Ce3+. The bromide ion, which inhibits the reaction, is isolated from the oxidation products of bromomalonic acid. During the reaction, the concentration of the Ce4+ ions (and Ce3+) oscillates several times, passing through a maximum and a minimum. The shape of the peaks of concentrations and the frequency depend on the reaction conditions. The autooscillation character of the kinetics of the cerium ions disappears if Ce4+ or Br are continuously introduced with a low rate into the reaction mixture. The autooscillation regime of the reaction takes place only in a certain interval of concentrations of the reactants [malonic... 

Several reports describe the mechanism of polymerisations using cerium compounds in combination with alcohols77,78, diols79,80 and polyols81. Only in a limited number of kinetic studies was the concentration of cerium ions measured during the polymerisation reaction. Mohanty et alm, Rout et al.m, Jayakrishnan et al.65 and Gupta and Behani.67 followed the reduction of Ce(IV) ions titrimetrically. 

According to Zhabotinskii, a simplified reaction scheme in the system consisting of malonic acid (MA) + bromate and cerium ions is of the form... 

In presence of 5% H20 and 0,05% S02 in the gas mixture, 15-30% loss of activity is observed at 573 K over Fe-MFI (101). The addition of cerium ions increases the stability towards the presence of H20+S02 (106). Above 573 K Yang et al. (104) reported that H20+S02 have little effect on the activity. According to these authors (104), water and sulfates increase the Bronstcd acidity of the catalysts and consequently the adsorption of ammonia (104). On overexchanged Fe-MFI prepared by sublimation, the presence of water (2.5%) in the gas mixture also promotes the NO conversion. S02 (0.02%) or H20+S02 inhibits the reaction only at temperatures below 650 K (103). On the other hand, upon addition of 8% H20 at 573 K, Ramachandran et al. (83) observe a drop of 40% in the NO conversion over a pelletized Fe-MOR containing an alumina binder. 

All reactions, save 10, contained O.SO g of lignin and O.IS ml of cerium ( + 4) ion. Reaction 10 contained 4.39 g of lignin and 1.28 ml of cerium ion. A = 2,2-dimethyl-3-imino-4-oxohex-5-ene-l-sulfonic acid. B = Hydroperoxide. Samples 1-7 Values arc weight of l,4-dioxa-2-hydropcroxycyclohexane in g. Samples 8-22 Values arc amount of aqueous solution of l,2-dioxy-3,3-dimethylbutane in ml. Equivalent/ml = 7.23 x 10". ... 

Copper(II) and cerium(IV) have been studied as oxidants in acetonitrile. The copper(II)-copper(I) couple has an estimated electrode potential of 0.68 V relative to the silver reference electrode. It has been studied as an oxidant for substances such as iodide, hydroquinone, thiourea, potassium ethyl xanthate, diphenylbenzidine, and ferrocene. Cerium(IV) reactions are catalyzed by acetate ion. Copper(I) is a suitable reductant for chromium(VI), vanadium(V), cerium(IV), and manganese(VII) in the presence of iron(III). For details on many studies of redox reactions in nonaqueous solvents, the reader is referred to the summary by Kratochvil. ... 

Oscillating reactions In these coupled autocatalytic reactions, repeated undamped oscillations occur in concentrations of reactants. These reactions have been reviewed by Field. For example, in the cerium-catalyzed reaction of bromate with malonic acid, changes in concentration of bromide ion and in the Ce(rV)-Ce(III) ratio over several orders of magnitude occur repeatedly. These systems as yet are not well understood. For the bromate-malonic acid reaction a 10-step process for the mechanism has been proposed. ... 

The molar concentrations of Ce(IlI), Ce(lV), and Fe(III) are easily computed at this point, but that for Fe(Il) is not. Therefore, Esystem computations based on the cerium half-reaction are more convenient. The concentrations of the two cerium ion species are... 

The main task of the cerium oxide washcoat component is oxygen storage, because the cerium ion is easily reduced and oxidized under typical catalyst operating conditions, formally according to the reaction... 

Speculations on the role if ceria have been already reported [5, 44]. It was found that cerium oxide has a positive effect on the nature of the active sites. Cerium ions may enhance the polarization of the Fe-0 bond and increase the basicity of this reaction center, and in addition they may hinder the reduction of Fc203. 

III C) 1967 Degn, H. Effect of Bromine Derivatives of Malonic Acid on the Oscillating Reaction of Malonic Acid, Cerium Ions and Bromate, Nature, vol. 213, 589-590... 

Molecular imprinted polymer recognition and on line electrogenerated chemiluminescence detection. Most CL results from a direct oxidation reaction or an oxidation reaction with energy transfer. Commonly used oxidants include hydroperoxide, oxygen, potassium permanganate, ferricyanide, tetravalent cerium ion, lead dioxide and oxygen free radical such as superoxide anion ( 02 ), hydroxy radical ( OH) and nitric oxide (NO). 

Now, we shall write equations for a reaction with diffusion assuming that at the front of a chemical wave z = 0 (z = [Ce4+]) which implies that the cerium ion is in the Ce3+ state and the first two reactions of the mechanism... 

The E° values listed in Table 12.1 are for the case where all species are at an activity of 1 M. However, the potential of a half-reaction may depend on the conditions of the solution. For example, the E° value for Ce + q Ce is 1.61 V. However, we can change this potential by changing the acid used to acidify the solution. (See Table C.5 in Appendix C.) This change in potential happens because the anion of the acid complexes with the cerium, and the concentration of free cerium ion is thereby reduced. 

As an example, let us examine the well-known Belousov-Zhabotinsky (BZ) reaction [15,16] of bromate with malonic acid catalyzed by cerium ions in acidic solution. 

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