Metals zero-valent

None of the atoms involved have changed its valence the iron and oxygen are still in the divalent state, and the hydrogen is still univalent. One way to produce the ferrous ion needed in the above reaction is via the oxidation of metallic (zero valent) iron ... 

Mansour et al. (184) have observed that the area under the Pt Lit and LUi absorption edges for a Pt/Al203 catalyst decrease progressively to a value characteristic of small supported crystallites of platinum with increasing reduction temperature. The observed progressively decreasing area is indicative of reduction of platinum to a metallic (zero-valent) state. It is not immediately obvious that the relationship obtained for platinum compounds may be extended to other noble metal compounds. As pointed out by Mansour et al. (183), there is in general not a simple relationship between the area under the white line and the unoccupied d states. 

Ex situ remediation techniques require the excavation of polluted soil for subsequent treatment or disposal. Ex situ treatments can be broadly classified into extraction versus stabilization treatments that will render the polluted soil less harmful and suitable for deposition in a landfill or backfill. Soil washing is an example of an ex situ extraction technique in which the treated soil can either be returned to its original site (backfill) or be land filled, depending on the success of the cleanup stage. Asphalt incorporation, thermal treatment, and encapsulation are ex situ stabilization techniques in which the metal(loid)-contaminated soil is either incorporated (e.g., asphalt) or contained (encapsulation) by secondary materials that are subsequently land filled. Thermal treatments involve the incineration of the metal(loid)-polluted soil and the conversion of the pollutants into their metallic (zero-valent) states. In the following section we present an overview of the various technologies based on their mechanism of action. 

Dehalogenation of monochlorotoluenes can be readily effected with hydrogen and noble metal catalysts (34). Conversion of -chlorotoluene to Ncyanotoluene is accompHshed by reaction with tetraethyl ammonium cyanide and zero-valent Group (VIII) metal complexes, such as those of nickel or palladium (35). The reaction proceeds by initial oxidative addition of the aryl haHde to the zerovalent metal complex, followed by attack of cyanide ion on the metal and reductive elimination of the aryl cyanide. Methylstyrene is prepared from -chlorotoluene by a vinylation reaction using ethylene as the reagent and a catalyst derived from zinc, a triarylphosphine, and a nickel salt (36). 

A particularly elegant route to metallacarbor-anes is the direct oxidative insertion of a metal centre into a c/oso-carborane cluster the reaction uses zero-valent derivatives of Ni, Pd and Pt in a concerted process which involves a nett transfer of electrons from the nucleophilic metal centre to the cage ... 

Sulfmyl radicals have also been invoked in the synthetically useful reaction of alkane-and arenesulfinyl chlorides with activated zero-valent metals (Ag, Cu, Zn) to give the corresponding symmetrical thiosulfonates21. 

The general mechanism of coupling reactions of aryl-alkenyl halides with organometallic reagents and nucleophiles is shown in Fig. 9.4. It contains (a) oxidative addition of aryl-alkenyl halides to zero-valent transition metal catalysts such as Pd(0), (b) transmetallation of organometallic reagents to transition metal complexes, and (c) reductive elimination of coupled product with the regeneration of the zero-valent transition metal catalyst. 

Coordination-catalyzed ethylene oligomerization into n-a-olefins. The synthesis of homologous, even-numbered, linear a-olefins can also be performed by oligomerization of ethylene with the aid of homogeneous transition metal complex catalysts [26]. Such a soluble complex catalyst is formed by reaction of, say, a zero-valent nickel compound with a tertiary phosphine ligand. A typical Ni catalyst for the ethylene oligomerization is manufactured from cyclo-octadienyl nickel(O) and diphenylphosphinoacetic ester ... 

The radicals, such as methyl, trifluoromethyl, and trifluorosilyl, used in this work have been found to oxidize zero-valent metals to their highest oxidation-state upon cocondensation with these metals on a cold surface at - 196°C. 

This complex may be reduced either directly to the metal chalcogenide or to the zero-valent metal, which then reacts chemically with selenosulfate adsorbed at the cathode to form MSe. 

Experiments have been carried out to compare nanoscale catalysts composed of Fe-, Ni-, and Co-complexes of several porphyrins or cyanocobalamin (Dror et al. 2005). A cobalt-porphyrin complex and cyanocobalamin in the presence of Ti(III)citrate reduced the initial concentrations of tetrachloromethane and tetrachloroethene by —99.5%, and the porphyrin was equally effective with trichloroethene. The advantage of using heterogeneous catalysts was shown by experiments in repetitive cycling of tetrachloromethane. Zero-valent metals degrade vicinal dichlorides such as tetrachloroethene by a-elimination to produce dichloroacetylene and hnally acetylene (Roberts et al. 1996). 

Roberts AL, LA Totten, WA Arnold, DR Burris, TJ Campbell (1996) Reductive elimination of chlorinated ethylene by zero-valent metals. Environ Sci Technol 30 2654-2659. 

In summary, Dupont and coworkers have developed an organometalhc approach for the stabihzation of various zero-valent nanoparticles in the ionic liquid BMI PPe. Transition metal nanoparticles of 2.0-3.0 were obtained with... 

Both Ni and Pd reactions are proposed to proceed via the general catalytic pathway shown in Scheme 8.1. Following the oxidative addition of a carbon-halogen bond to a coordinatively unsaturated zero valent metal centre (invariably formed in situ), displacement of the halide ligand by alkoxide and subsequent P-hydride elimination affords a Ni(II)/Pd(ll) aryl-hydride complex, which reductively eliminates the dehalogenated product and regenerates M(0)(NHC). ... 

The synthesis of metal nanoparticles via the controlled decomposition of pre-prepared organometallic complexes or metal carbonyls where the metals are already in the zero valent or low-valent state has been known since 1970. The first examples were Pd- and Pt-dibenzylideneacetone complexes where the coordinated ligands detached using either hydrogen of carbon monoxide under mild conditions to give the respective metal nanoparticles [310]. 

Noble metal ions can be easily reduced to the corresponding zero-valent metal atoms. Therefore, bimetallic nanoparticles consisting of two different noble metals have been extensively investigated for purpose of novel catalysts and optical materials. A simultaneous reduction of two noble metal ions with alcohol is a simple and useful technique to prepare bimetallic nanoparticles. The alcohol reduction of metal ions M + is followed by Equation (1). 

Late transition metal or 3d-transition metal irons, such as cobalt, nickel, and copper, are important for catalysis, magnetism, and optics. Reduction of 3d-transition metal ions to zero-valent metals is quite difficult because of their lower redox potentials than those of noble metal ions. A production of bimetallic nanoparticles between 3d-transi-tion metal and noble metal, however, is not so difficult. In 1993, we successfully established a new preparation method of PVP-protected CuPd bimetallic nanoparticles [71-73]. In this method, bimetallic hydroxide colloid forms in the first step by adjusting the pH value with a sodium hydroxide solution before the reduction process, which is designed to overcome the problems caused by the difference in redox potentials. Then, the bimetallic species... 

The reduction of metal hydroxides or oxides powder by polyol was first reported by Figlarz and co-workers, which gave rise to fine powders of Cu, Ni, Co and some noble metals with micrometer sizes (polyol process) [32,33]. The polyol process was first modified for the preparation of PVP-protected bimetallic and monometallic nanoclusters such as Pt/Cu, Pd/Pd, Pt/Co, Pt, Pd, etc. [34-38]. The previous results definitely revealed that Pt, Pd, Cu and Co in these PVP-protected metal or alloy nanoclusters were in a zero-valent metallic state. 

The role of OSC materials in NO conversion is more complex. Most metals, specially Rh, are able to decompose NO, but this reaction is rapidly inhibited by O species resulting from this decomposition [60]. On Rh, for instance, Rh-0 species are replaced by Rh-NO+ ones in which NO is no longer dissociated [61]. O species may react with adsorbed species of the reducer (CO, HC) to form C02. The first role of the OSC materials could be to liberate metal sites by accepting O species. Indirect effects can also occur, with the Ce3+/Ce4+ redox system being able to regulate the metal state (zero-valent or ionic) during richness oscillations. 

The chemical state of the metal can play a decisive role on the reaction mechanism. In TWC, Rh is thought to remain in the zero-valent state, which favors NO dissociation [77,78], However, the role of the OSC materials is complex, and it is not inert with respect to NO activation. Ranga Rao et al. [79] showed that, when bulk oxygen vacancies are formed in a reduced Ce06Zr04O2 solid solution, NO was efficiently decomposed on the support to give N20 and N2. Further studies by the same group... 

---