Zero valent state

Qiemical shift from zero-valent state... 

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... 

Some of these coupling reactions can be made catalytic if hydrogen is eliminated and combines with the anion, thus leaving the nickel complex in the zero-valent state. Allylation of alkynes or of strained olefins with allylic acetates and nickel complexes with phosphites has been achieved (example 38, Table III). 

If the reaction in which the metallic fraction serves as a catalyst produces water as a by-product, it may well be that the catalyst converts back to an oxide. One should always be aware that in fundamental catalytic studies, where reactions are usually carried out under differential conditions (i.e. low conversions) the catalyst may be more reduced than is the case under industrial conditions. An example is the behavior of iron in the Fischer-Tropsch reaction, where the industrial iron catalyst at work contains substantial fractions of Fe304, while fundamental studies report that iron is entirely carbidic and in the zero-valent state when the reaction is run at low conversions [6],... 

The last explanation for methanol formation, which was proposed by Ponec et al., 26), seems to be well supported by experimental and theoretical results. They established a correlation between the gfiethanol activity and the concentration of Pd , most probably Pd. Furthermore, Anikin et al. (27) performed ab initio calculations and found that a positive charge on the palladium effectively stabilizes formyl species. Metals in a non-zero valent state were also proposed by Klier et al. (28) on Cu/ZnO/Al O, by Apai (29) on Cu/Cr O and by Somorjai for rhodium catalyts (30). Recently results were obtained with different rhodium based catalysts which showed the metal was oxidized by an interaction with the support (Rh-0) (on Rh/Al 0 ) by EXAFS ( -32) and by FT-IR ( ) and on Rh/MgO by EXAFS ( ). The oxidation of the rhodium was promoted by the chemisorption of carbon monoxide (, ). ... 

Because the oxidation process is reversible, a measure of its reversibility is the yield of antimony in the zero-valent state ... 

Temperature Programmed Reduction. Temperature-programmed reduction (TPR), one of the indirect analysis methods, yielded data that suggested that Sn was not reduced to zero-valent state (10,16). Burch (15) has reviewed early work on the characterization of this type of catalyst. Lieske and Volter (21) reported, based on the results obtained from TPR studies, that a minor part of the tin is reduced to the metal, and this Sn(O) combined with Pt to form "alloy clusters" but the major portion of the tin is reduced to only the... 

In addition to performing acid/base catalysis, zeolite structures can serve as hosts for small metal particles. Transition metal ions, e.g., platinum, rhodium, can be ion exchanged into zeolites and then reduced to their zero valent state to yield zeolite encapsulated metal particles. Inside the zeolite structure, these particles can perform shape selective catalysis. Joh et al. (16) reported the shape selective hydrogenation of olefins by rhodium encapsulated in zeolite Y (specifically, cyclohexene and cyclododecene). Although both molecules can be hydrogenated by rhodium supported on nonmicroporous carbon, only cyclohexene can be hydrogenated by rhodium encapsulated in zeolite Y since cyclododecene is too large to adsorb into the pores of zeolite Y. 

As a standard procedure the catalysts are reduced with hydrogen in water at 363 K for half an hour. In this way all the platinum is in the zero-valent state and any changes induced by temperature effects during reaction can be ruled out. 

Only metals in carbonyl complexes are in the zero-valent state and can be polynuclear. 

This potassium salt, K4Ni2(CN)6, may be further reduced by potassium in liquid ammonia to yield a yellow substance, K4Ni(CN)4. This has nickel in the zero-valent state and is thus comparable to the metal carbonyls, Fe(CO)5 and Ni(CO)4 (p. 157), to cobalt nitrosyl carbonyl Co(CO)4NO, and to the metal ammoniates Ca(NH3)6 and Pt(NH3)2. However, K4Ni(CN)4, and the closely related acetylene derivative, K4Ni(C=CH)4, are especially unusual, for in them, the zero-valent metal has been incorporated into an anion, whereas in the carbonyls and metal ammoniates, the zerovalent metals are present as uncharged species. 

Consequently, the elements to the left of the noble metals show strongest (ft)-character in their zero-valent oxidation state. Thus iron(O), cobalt(O) and nickel(O) are typically (b), forming inter alia strong carbonyl complexes, while the higher oxidation states of these elements have no marked ( )-character at all. Elements in zero-valent state in fact display (b) -character as far left in the periodic system as chromium, or even vanadium, which in higher oxidation states behave as very typical (a)-acceptors. To the right of the noble metals, on the other hand, the metals in their zero-valent states do not show any marked (6)-character they do not form e.g. carbonyl or olefin complexes. 

Reduction to the zero-valent state or formation of other metal solid phases like oxides, causes the element to deposit onto the semiconductor surface. The efficiency of the photocatalytic reaction depends on different factors. One of the most critical aspects is the high probability of electron-hole recombination, which competes with the separation of the photogenerated charges. On the other hand, as there is no physical separation between the anodic reaction site (oxidation by holes) and the cathodic one (reduction by electrons), back reactions can be of importance. The low efficiency is one of the most severe limitations of heterogeneous photo catalysis. 

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. 

As a result, the reduction of cobalt from the divalent to the zero-valent state changes the chemistry of the system, since Co° readily forms complexes with hydrocarbons. This was confirmed by subsequent adsorption of propene, which produced the EPR spectrum shown in Figure 1.23D, with spin Hamiltonian parameters ofg, = 2.096, gy = 1.924, = 2.297, A = 12, Ay = 52, = 99G. This... 

In contrast, multivalent ions are reduced down to the zero-valent state according to a multi-step reaction mechanism involving intermediate valencies by disproportionation. These redox reactions, which are often diffusion-controlled, have extensively been studied by pulse radiolysis in the case of several free or complexed metal ions. 

Our site-time yields were measured on catalyst granules smaller than 0.2 mm in diameter in order to ensure that CO hydrogenation rates were unaffected by diffusion-limited CO and H2 arrival at catalytic sites. Product removal is still slow on such small pellets, but it affects only the selectivity and not the rate of CO hydrogenation reactions. Our site-time yields were measured on catalysts with more than 95% of the Co atoms in a zero-valent state, in order to avoid complicating factors associated with partially reduced Co surfaces. 

CARBON MONOXIDE co-ordinates much more strongly with surface Ni than with Ni i adsorbed CO can be pumped off Nii Y at 25°C but from Ni requires at least 200°C. Since this desorption requires temperatures at which surface Nii normally disproportionates it appears that Nil is stabilized by the carbonyl ligand. This strong bonding with CO was expected since CO is a very weak electron donor that favours co-ordination with atoms in the zero-valent state or in negatively charged complexes. 

The chemistry of palladium is dominated by two stable oxidation states the zero-valent state [Pd(0), d ] and the +2 state [(Pd(II), d ]. Each oxidation state has its own characteristic reaction pattern. Thus, Pd(0) complexes are electron-rich nucleophilic species, and are prone to oxidation, ligand dissociation, insertion, and oxidative-coupling reactions. Pd(II) complexes are electrophilic and undergo ligand association and reductive-coupling reactions. A large amount of literature deals with these reactions. However, a few fundamental principles, such as oxidative addition, transmetalation, and reductive elimination, provide a basis for applying the chemistry of palladium in research. 

Reactions of the second type are carried out with palladium compounds or complexes of either bivalent or zero-valent states. Since these reactions proceed catalyti-cally without using reoxidants they are more useful than the stoichiometric processes. Telomerization of conjugated dienes, reactions of allylic and alkenyl esters and ethers, and various organic halides belong to this type. 

A wide variety of metals are used in the reduced (zero-valent) state as catalysts. Most often the metal is... 

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. 

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