Cu sites

Electrons from cytochrome c are transferred to Cu sites and then passed to the heme iron of cytochrome a. Cu is liganded by two cysteines and two histidines (Figure 21.18). The heme of cytochrome a is liganded by imidazole rings of histidine residues (Figure 21.18). The Cu and the Fe of cytochrome a are within 1.5 nm of each other. 

Figure 3. The probabililities for finding a charge between q and q+dq on a Cu atom for the five fee disordered alloys. The average charge on the Cu sites is indicated by the heavy line.

Figure 5. The solid line shows the probability for findin the charge on a Cu site between q and q+dq in a fee disordered alloy with any concentration. The dotted lines show the conditional probabilities corresponding to sites with a concentration of Cu atoms on the nearest-neighbor shell of 100%, 75%, 50%, 25%, and 0%.

PCu(ci,q) is clearly not a 5-function as has been suggested. Many more LSMS calculations would have to be done in order to determine the structure of Pcn(ci,q) for fee alloys in detail, but it is easier to see the structure in the conditional probability for bcc alloys. The probability Pcu(q) for finding a charge between q and q-t-dq on a Cu site in a bcc Cu-Zn alloy and three conditional probabilities Pcu(ci,q) are shown in Fig. 6. These functions were obtained, as for the fee case, by averaging the LSMS data for the bcc alloys with five concentrations. The probability function is not a uniform function of q, but the structure is not as clear-cut as for the fee case. The conditional probabilities Pcu(ci,q) are non-zero over a wider range than they are for the fee alloys, and it can be seen clearly that they have fine structure as well. Presumably, each Pcu(ci,q) can be expressed as a sum of probabilities with two conditions Pcu(ci,C2,q), but there is no reason to expect even those probabilities to be 5-functions. 

Figure 7. The Coulomb potential at a site plotted versus the net charge. The crosses are for the Cu sites and the plusses are for the Zn sites.

Figure 1 The local DOS for CU75PCI25 alloys. The solid line represents the result without lattice relaxation and the dashed with lattice relaxation, (a) at the Pd site (b) at the Cu site. Energies have been measured from the Fermi energy Ej...

Additional adsorption sites are provided on open metal sites, when available. [Cu3(BTC)2] is performant in the selective adsorption and separation of olefinic compounds. The highly relevant separations of propene from propane and of isobutene from isobutane have been accomplished with separation factors of 2.0 and 2.1, respectively [101, 102]. [Cu3(BTC)2] also selectively takes up pentene isomers from aliphatic solvent in liquid phase, and even discriminates between a series of cis- and trans-olefin isomer mixtures with varying chain length, always preferring a double bond in cis-position. This behavior is ascribed to tt -complexation with the open Cu sites [100]. 

Accessibility to Cu sites was determined by temperature programmed desorption of NO (NO TPD), using an experimental setup similar to that used for TPR, except the detector was a quadrupole mass spectrometer (Balzers QMS421) calibrated on standard mixtures. The samples were first activated in air at 673 K, cooled to room temperature in air, and saturated with NO (NO/He 1/99, vol/vol). They were then flushed with He until no NO could be detected in the effluent, and TPD was started up to 873 K at a heating rate of 10 K/min with an helium flow of 50 cm min. The amount of NO held on the surface was determined from the peak area of the TPD curves. 

For Ti02 and Z1O2, it is well known that sulfation induces a strong increase of acidity [17] and the participation of an add mechanism could then account for this promotion of activity. This mechamsm can be described as a bifunctional process oxidation of NO to NO on Cu sites, and nitration of a product of the oxidation of decane on the acid fiinction(8). The preparation of the catalyst must have a great influence on the activity. This has been shown by the comparison of three Cu/TiC catalysts prepared in different conditions one in which titania is first treated with sulfuric acid, then by Cu acetate (denominated Cu 04/Ti02, containing 0.S wt% Cu, 0.6 wt% S), one in which Cu is... 

In some cases, small biological redox partner proteins such as heme-containing cytochromes, ferredoxins comprising an iron-sulfur cluster, or azurin with a mononuclear Cu site have been used as natural mediators to facilitate fast electron exchange with enzymes. A specific surface site on the redox protein often complements a region on the enzyme surface, and enables selective docking with a short electron tunneling... 

Figure 17.5 The protein environment around the Cu centers (gold spheres) of laccase from Melanocarpus albomyces (PDB file IGWO) showing a substrate O2 molecule bound in the trinuciear Cu site [Hakulinen et al., 2002], The protein is depicted in stick representation with atoms in their conventional coloring. (Courtesy of Armand W. J. W. Tepper.) (See color insert.)...

Figure 18.6 Energetics of the ORR at the heme/Cu site of CcO the enzyme couples oxidation of ferroc3ftochrome c (standard potential about —250 mV all potentials are listed with respect to a normal hydrogen electrode) to reduction of O2 (standard potential at pH 7 800 mV). Of the 550 mV difference, only 100 mV is dissipated to drive the reaction 220 mV is expanded to translocate four protons from the basic matrix compartment to the acidic IMS (inter-membrane space). In addition 200 mV is converted into transmembrane electrostatic potential as ferroc3ftochrome is oxidized in the IMS, but the charge-compensating protons are taken from the matrix. The potentials are approximate.

Section 18.2). The latest generation of such catalysts (1 in Fig. 18.17) reproduces the key features of the site (i) the proximal imidazole ligation of the heme (ii) the trisi-midazole ligation of distal Cu (iii) the Fe-Cu separation and (iv) the distal phenol covalently attached to one of the imidazoles. As a result, binding of O2 to compound 1 in its reduced (Fe Cu ) state appears to result in rapid reduction of O2 to the level of oxides (—2 oxidation state) without the need for outer-sphere electron transfer steps [Collman et ah, 2007b]. This reactivity is analogous to that of the heme/Cu site of cytochrome c oxidase (see Section 18.2). 

Collman JP, Boulatov R. 2002. Electtocatal3dic O2 reduction by S3mthetic analogs of the heme/ Cu site of cytochrome oxidase incorporated in a Upid film. Angew Chem Int Ed 41 3487. 

It is well known that Rh(I) complexes can catalyze the carbonylation of methanol. A heterogenized catalyst was prepared by ion exchange of zeolite X or Y with Rh cations.126 The same catalytic cycle takes place in zeolites and in solution because the activation energy is nearly the same. The specific activity in zeolites, however, is less by an order of magnitude, suggesting that the Rh sites in the zeolite are not uniformly accessible. The oxidation of camphene was performed over zeolites exchanged with different metals (Mn, Co, Cu, Ni, and Zn).127 Cu-loaded zeolites have attracted considerable attention because of their unique properties applied in catalytic redox reactions.128-130 Four different Cu sites with defined coordinations have been found.131 It was found that the zeolitic media affects strongly the catalytic activity of the Cd2+ ion sites in Cd zeolites used to catalyze the hydration of acetylene.132... 

The CO-TPD technique together with DFT calculations were previously successfully used to characterize monovalent copper positions in Cu-ZSM-5 and Cu-Na-FER catalysts[4, 5]. Recently it was observed that the CO molecule can also form adsorption complexes, where the CO molecule is bonded between two extra-framework cations [6]. It is likely that the formation of similar species between the Cu+ and K+ ions can also occur. The presence of adsorption complexes on such heterogeneous dual cation site was evidenced by the FTIR experiments [7]. The formation of CO complexes on dual cation sites was not considered in our previous TPD models where three types of Cu+ sites were taken into account. 

The model based on formal kinetics was used to model the TPD curves of adsorbed CO molecules, based on the model previously reported [4], The desorption is strongly affected by the fast readsorption of CO on unoccupied Cu+ ions, thus, a quasiequilibrium state is a suitable approximation for the description of adsorption. A Langmuir type of adsorption isotherm was assumed for the CO adsorption on the Cu+ sites in zeolite, without considering lateral interactions among adsorbed molecules. 

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