Transition metal cations , reduction

Table 20.4 Ease of Reduction ed (V) of Transition Metal Cations ...

As can be seen, the catalytic process over a zeolite-supported cation, or an oxide-supported cation, can be considered as a supported homogeneous catalysis, as far as adsorbed reactants and products behave like reactive ligands. The model developed for lean DcNO. catalysts over supported cations (function 3), as well as this supported homogeneous catalysis approach, is also suitable for stoichiometric mixture (TWC) comprising CO and H2 as reductants over supported transition metal cations [20-22],... 

The model can also be extended to three-way catalysis as far the active site is a cation of transition metal, the reductant being CO and the feed CO/NO/HC being very near stoichiometry [10,11],... 

Clay Minerals as Lewis Acids. Lewis acid sites in a clay mineral are exchangeable (2) or structural ( 0) transition metal cations in the higher valence state, such as Fe + and Cu +, and octahedrally coordinated aluminum exposed at the crystal edges (38). Reduction of both exchanged and structural (octahedral) transition metal cations in the upper oxidation state is a reversible process (12,... 

The reducibility of nickel and copper cations in Nb- and Al-containing MCM-41 It is known that transition metal cations, which occupy extra lattice positions in zeolites, are reduced while activated under vacuum or in an inert gas flow [20], The same occurs while applying mesoporous matrices for metal cations. The species formed during the auto reduction depend on the nature of a matrix, the conditions of cation modification, and activation They can be identified by H2-TPR, ESR, and NO/FTIR measurements. 

The results for Cr34 and the 3d5 cations Fe3+ and Mn2+ show that it is possible to derive values of the Racah B parameter for transition metal compounds from absorption bands in their crystal field spectra, enabling comparisons to be made with field-free ion values. In all cases, there is a decrease of the Racah B parameter for the bonded cations relative to the gaseous ions, which is indicative of diminished repulsion between 3d electrons in chemical compounds of the transition metals. This reduction is attributable to electron delocalization or covalent bonding in the compounds. Such decreases of Racah B parameters are expressed as the nephelauxetic Greek , cloud expanding) ratio, p, given by... 

Mossbauer spectroscopy may be important and useful when applied to electrodes which contain ferromagnetic components. It is basically an in situ tool which provides valuable information on possible orientation and oxidation states of ferromagnetic species in the electrodes as a function of the electrochemical process and the potential applied. For example, electrodes for oxygen reduction may be highly catalytic when containing macrocycles with transition metal cations such as Fez+, Niz+, Coz+ [89,90], A typical apparatus for this technique is described in Ref. 91. 

Efforts have also been made to improve the methodology used to prepare various mixed donor macrocycles especially applicable to binding transition metal cations <2006POL599>. Various approaches were tried including amide formation followed by reduction (see Figure 4). Activated leaving groups such as tosylate were employed and... 

The first transition metal cation which is unstable in water but which can be generated as a stable entity in HF was U3+ [30]. It was formed by oxidation of the metal by protons in a BF3-HF solution which is non-oxidising and relatively weakly acidic. The UV-vis spectrum of the lilac-colored solution was virtually identical with that observed for an acidified aqueous solution in which the uranium solution was under continuous electrolytic reduction to maintain U(III) as the aquo-cation. 

The electron-acceptor properties of oxidants are most readily evaluated by the reversible potentials red for the one-electron reduction, i.e. equation (6b). Values of red for many types of oxidiuits, particularly those based on transition metal cations, have been tabulated, and some of the more common ones in water are listed in Table 2. HowevCT thm ate a number of useful oxidants that undergo a multiple electron change, e.g. Tl + 2e" T1+, 02Cr + + 3er Ci , etc., and ied is known only for... 

Transition metal cations are known to be responsible for dimerization (8). If this Is true for ethylene-Isobutane Interaction. then deactivation of the active sites of dimerization occurs throuoh the reduction of the cations Into metal as follows ... 

Temperature-programmed reduction and desorption (TPR, TPD) have been applied to study the stability of Pt-Co bimetallic particles entrapped in NaY zeolite cages upon O2 oxidation and reaction with surface protons generated during the reduction of transition metal cations. Oxidation of Pt/NaY catalyst with O2 at 573 K causes shift of TPR peak to lower temperature due to formation of partially oxidized Pt particles. Similar treatment for Pt-Co/NaY bimetallic catalysts results in complete isolation of Pt and Co in Pt-Co particles, leaving Pt and Co in supercages and sodalite cages, respectively. 

Fig. 1.94. Catalytic cycles for the homogeneous reduction of nitrogen oxides by carbon monoxide mediated by various atomic transition metal cations M+. The ions Fe+, Os+, and lr+ were found to be most effectively performing the outhned reaction cycles out of 29 investigated transition metal elements. Adapted from [462]...

Catalytic activity in the homogeneous gas-phase conversion of nitrogen oxides and carbon monoxide to nitrogen and carbon dioxide was observed to be most effective for the atomic ions Fe+, Os+, and Ir+ out of the investigation of 29 different transition metal cations M+ [462]. The overall catalytic scheme that was established in this study consists of the three catalytic cycles shown in Fig. 1.94. The catalysis occurs in two steps in which NO is first reduced to N2O. An analogous three-step catalytic reduction of NO2, in which NO2 is first reduced to NO, was also discovered. The three cycles in Fig. 1.94 were characterized with laboratory measurements of reactions of each of the three nitrogen oxides NO2, NO, and N2O with the different transition metal ions in an inductively coupled plasma/selected-ion flow tube tandem mass spectrometer [462]. 

The ease of reduction increases, therefore, from Cr3+ to Ni3+ in the series of LaM03 perovskites. The same trend has been found for the simple oxides of Fe, Co, and Ni (115). On the other hand, the mixed oxides LaM03 (M = Fe, Co, Ni) were found to be more stable in a H2 atmosphere than the simple oxides NiO, Fe203, and Co304 (92) this shows the increased stability of transition-metal cations in a perovskite structure. The TPR experiments also show that the stability of perovskite oxides increases with increasing size of the A ion. 

The partial substitution of R by a divalent cation such as alkaline earth facilitates the reduction. This is due to an increase in anion vacancy concentration that facilitates oxygen diffusion and the existence of the transition metal cation in a higher, less stable oxidation state such as CoIV in I.ai SrxCo03 >,. 

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