Copper rate-limiting step

The hydrogenation of dioxomethylene, step (33) is, most likely, the rate-limiting step, although the hydrogenation of formate in (32) is a also candidate. By assuming that Eqs. (24), (23) and (29) are slow for the water-gas shift reaction and that (33) is slow for methanol synthesis, we arrive at the following set of equations, in which one site is assumed to consist of two copper atoms ... 

Fig. 25 Free energy profiles for stripping copper out of the organic phase. Note that the rate-limiting step, indicated by the double-headed arrow, has the reactant and the transition state in different cycles with respect to the vacant site.

The cuprous-cupric electron transfer reaction is believed to be the rate-limiting step in the process of stress corrosion cracking in some engineering environments [60], Experimental studies of the temperature dependence of this rate at a copper electrode were carried out at Argonne. Two remarkable conclusions arise from the study reviewed here [69] (1) Unlike our previous study of the ferrous-ferric reaction [44], we find the cuprous-cupric electron transfer reaction to be adiabatic, and (2) the free energy barrier to the cuprous cupric reaction is dominated in our interpretation by the energy required to approach the electrode and not, as in the ferrous-ferric case, by solvent rearrangement. 

The mechanism of the enantioselective 1,4-addition of Grignard reagents to a,j3-unsaturated carbonyl compounds (Scheme 5 R1 = alkyl R2 = alkyl, OR3), promoted by copper complexes of chiral ferrocenyl diphosphines (180), has been explored using kinetic, spectroscopic, and electrochemical analysis. The roles of the solvent, copper halide, and the Grignard reagent have been thoroughly examined. Kinetic studies support a reductive elimination as the rate-limiting step, in which the chiral catalyst,... 

The latter involves a copper(0)-catalyzed sequence(41) similar to that observed with iron (vide infra), and it is especially important with secondary and tertiary alkyl systems. The effects of structural variation are consistent with a rate-limiting step involving nucleophilic displacement of halide in Equation... 

The chemical component of CMP slurry creates porous unstable oxides or soluble surface complexes. The slurries are designed to have additives that initiate the above reactions. The mechanical component of the process removes the above-formed films by abrasion. In most planarization systems the mechanical component is the rate-limiting step. As soon as the formed porous film is removed, a new one is formed and planarization proceeds. Therefore, the removal rate is directly proportional to the applied pressure. To achieve practical copper removal rates, pressures greater than 3 psi are often required. These pressures should not create delamination, material deformation, or cracking on dense or relatively dense dielectrics used in silicon microfabrication on conventional dielectrics. However, the introduction of porous ultra-low-fc (low dielectric constant) materials will require a low downpressure (< 1 psi) polishing to maintain the structural integrity of the device [7-9]. It is expected that dielectrics with k value less than 2.4 will require a planarization process of 1 psi downpressure or less when they are introduced to production. It is expected that this process requirement will become even more important for the 45-nm technology node [10]. 

The theories proposed to explain the formation of passivation film are salt-film mechanism and acceptor mechanism [21]. In the salt-film mechanism, the assumption is that during the active dissolution regime, the concentration of metal ions (in this case, copper) in solution exceeds the solubility limit and this results in the precipitation of a salt film on the surface of copper. The formation of the salt film drives the reaction forward, where copper ions diffuse through the salt film into electrolyte solution and the removal rate is determined by the transport rate of ions away from the surface. As the salt-film thickness increases, the removal rate decreases. In the acceptor mechanism, it is assumed that the metal-ion products remain adsorbed onto the electrode surface until they are complexed by an acceptor species like water or anions. The rate-limiting step is therefore the mass transfer of the acceptor to the surface. Recent studies confirmed that water may act as an acceptor species for dissolving copper ions [22]. 

In the case of laccase and ascorbate oxidase, the observed ET rates for the reduction of the type-3 coppers (see Table VIII) are lower than the observed turnover number. This can be explained only by the possibility that the enzymes are in a resting form under the experimental conditions. A considerable reorganization energy seems to be necessary to get to the reduced state of the type-3 coppers (release of the bridging OH" and movement of the copper GU2 and GU3). From these data it cannot be decided what the rate-limiting step is in the catalytic cycle, either this intramolecular ET or the reaction of the dioxygen at the trinuclear copper site. 

The rate of dissolution, i.e., the rate that reaction (7.2) proceeds to the right, depends upon the concentration of Chi, not the total dissolved copper concentration. Complexing, which converts Cu to Cu(NH3)2, increases the dissolution rate by lowering the CIu ion concentration. As discussed in Section 7.3.3, reaction (7.2) appears to be the rate limiting step. 

The mechanism of the foregoing copper-catalyzed substitution at the sp carbon center is proposed as follows [Eq. (36) 88]. Formation of an intermediate alkylcopper RCu should play an important role, with the nucleophilic displacement of alkyl halides with this copper species being the rate-limiting step. 

Three different crystallographic modifications of the anhydrous salt can be prepared. The decompositions of these reactants (430 to 506 K) exhibit sigmoid a-time curves which have been attributed [11] to a chain-type process through the formation of the volatile and unstable copper(I) salt as intermediate. This was indicated by the formation of a copper mirror on the containing vessel. Schuffenecker et al. [11] identified the rate limiting step as electron transfer. Erofeev et al. [10,12] showed that the kinetic behaviours of two forms of the salt were different. This was ascribed to variations in the dispositions of copper ions in the crystal structures of the reactants. 

The initial or rate limiting step for anion breakdown in metal oxalate decompositions has been identified as either the rupture of the C - C bond [4], or electron transfer at a M - O bond [5], This may be an oversimplification, because different controls may operate for different constituent cations. The decomposition of nickel oxalate is probably promoted by the metallic product [68] (the activity of which may be decreased by deposited carbon, compare with nickel malonate mentioned above [65]). No catalytically-active metal product is formed on breakdown of oxalates of the more electropositive elements. Instead, they yield oxide or carbonate and reactions may include secondary processes [27]. There is, however, evidence that the decompositions of transition metal oxalates may be accompanied by electron transfers. The decomposition of copper(II) oxalate [69] (Cu - Cu - Cu°) was not catalytically promoted by the metal and the acceleratory behaviour was ascribed to progressive melting. Similarly, iron(III) oxalate decomposition [61,70] was accompanied by cation reduction (Fe " - Fe ). In contrast, evidence was obtained that the reaction of MnC204 was accompanied by the intervention of Mn believed to be active in anion breakdown [71]. These observations confirm the participation of electron transfer steps in breakdown of the oxalate ion, but other controls influence the overall behaviour. Dollimore has discussed [72] the literature concerned with oxalate pyrolyses, including possible bond rupture steps involved in the decomposition mechanisms... 

The maximum value of the permeation copper flux is observed at the minimum thicknesses of the stagnant layer of adjacent to the feed phase and the stripping phase side of the respective interfaces with the hollow-fiber SLM. The residence time of the complex of the metal in the membrane has reached optimum value when the maximum copper permeation flux is observed [126, 127]. The rate-limiting step of copper permeation through the hollow-fiber SLM was the transport of the Cu-carrier complex through the hoUow-fiber SLM [126, 127]. 

As a mimic of the well-studied galactose oxidase [37], a copper(II) thiophenol complex catalyzes the oxidation of primary alcohols to aldehydes in the presence of (Scheme 12) [38]. The latter also promotes the oxidation of secondary alcohols to diols (Scheme 12). The catalytic cycle starts with the oxidation of copper by O, leading to a biradical species. The intermediate 39 is produced from 38 by coordination of two alkoxide substrates. The rate-limiting step is the formation of 40 from 39 by a hydrogen atom transfer from the secondary alcoholate to the oxygen-centered radicals of the aminophenols ligands. The cycle is then closed by radical dimerization which leads the formation of the diol [39]. 

AG° = 31.9 kJmol-1 for reaction 32. This value should be compared to the thermodynamic redox potentials for the process with 1 M 02 as the standard state. The relevant redox potentials are 0.158 and —0.16 V for the reduction of Cu(II) and 02 (Sawyer and Valentine, 1981), respectively. From these thermodynamic data we calculate AG° = — nFE° = 30.1 kJmol-1. The close agreement indicates that the redox kinetics of copper in natural waters is, indeed, governed by reaction 32 as the rate-limiting step. 

The inhibiting effect of nitrobenzene on the reaction [1] is in agreement with the involvement of radicals in the rate limiting step [18]. However Ais effect is not significant enough to exclude the possibility of a mechanism in which free radicals would not exist as free species such as a mechanism involving aryl copper complexes [5, 13, 14] which could also be considered [1]. 

Kinetic isotope effects using CH3OH and CH3OD show that the O-H bond is at least partially involved in the rate-limiting step. TPD experiments with pure Cu°, pure ZnO, and the catalyst Cu/ZnO showed that methanol can be activated by both ZnO and copper. On the ZnO surface, methanol can form intermediates, which in the presence of copper might react and desorb more easily probably via a reverse spillover process. The isotopic product distribution of H2, HD, D2,H20, HDO and D2O in the... 

The highest rate constant observed for intramolecular ET in AO is 1100 s , which still is considerably smaller than the turnover number of about 14,000 s . Thus, interaction between dioxygen and the trinuclear site is not sufficient to ensure maximal enzymatic activity. Under optimal conditions, the concentration of reducing substrate (e.g., ascorbate) is sufficiently high to maintain a steady state of fully reduced copper sites. Thus, an antithetical approach was very recently taken by Tollin and co-workers studying the reoxidation of fully reduced AO by a laser-generated triplet state of 5-deazariboflavin 41). Subsequent to the assumed one-electron oxidation of the reduced trinuclear cluster, a rapid, biphasic intramolecular ET occurs from Tl[Cu(I)] (and presumably) to the oxidized trinuclear center. The faster of the two observed rate constants (9500 and 1400 s, respectively) is comparable to the turnover number determined for AO under steady-state conditions and renders it likely that this is the rate-limiting step in catalysis. 

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