Properties of the Active Metals

The other alkali metals all form oxides or peroxides (containing the peroxide ion, 0 )  

Potassium, rubidium, and cesium also form superoxides (containing the superoxide ion, OJ)  

The type of oxide that forms when an alkali metal reacts with oxygen has to do with the stability of the various oxides. Because these oxides are all ionic compounds, their stability depends on how strongly the cations and anions attract one another. Lithium tends to form predominantly the oxide because lithium oxide is more stable than lithium peroxide. 

The reactions of alkaline earth metals with water vary considerably. Beryllium does not react with water magnesium reacts slowly with steam and calcium, strontium, and barium react vigorously with cold water. 

The reactivity of the alkaline earth metals toward oxygen also increases ifom Be to Ba. Beryllium and magnesium form oxides (BeO and MgO) only at elevated temperatures, whereas CaO, SrO, and BaO form at room temperature. 

Student Annotation Because they have less metallic character than the other Group 2A elements, beryllium and magnesium form some molecular compounds such as BeH2 and MgH2. 

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To optimize the use of the amorphous sodium titanate powders as catalyst substrates, it is important to fully characterize the ion-exchange properties of the material. Further, the solution properties of the active metal to be loaded onto the support will be an important parameter in the control of the adsorption process. For example, exposure of sodium titanate to a nickel salt solution does not guarantee that nickel will be loaded onto the sodium titanate, or that the nickel, if loaded, will be dispersed on an atomic level. Sodium titanate only behaves as a cation exchange material under certain pH conditions. The solution pH also influences the hydrolysis and speciation of dissolved nickel ions (3), which can form large polymeric clusters or colloidal particles which are not adsorbed by the sodium titanate via a simple ion-exchange process. 

Catalysis. Ion implantation and sputtering in general are useful methods for preparing catalysts on metal and insulator substrates. This has been demonstrated for reactions at gas—soHd and Hquid—soHd interfaces. Ion implantation should be considered in cases where good adhesion of the active metal to the substrate is needed or production of novel materials with catalytic properties different from either the substrate or the pure active metal is wanted (129—131). Ion beam mixing of deposited films also promises interesting prospects for the preparation of catalysts (132). 

Unfortunately, at present the information characterizing the properties of the active bond in polymerization catalysts is very scant. The analogy between the features of the active bonds in the propagation centers and those of the transition metal-carbon bond in individual organometallic compounds is sure to exist, but as in the initial form the latter do not show catalytic activity in olefin polymerization this analogy is restricted to its limits. 

An advantage of nickel catalysts over other metal systems is that the properties of the active species can easily be tuned by the addition of suitable ligands. For example, the presence of PPhj was shown to have a direct influence on the regiochem-istry of hydroalumination of 1,1-dimethylindene la [33]. While the reaction of BU2AIH with la gave a 4 1 mixture of regioisomeric products 13a/13b after deuterolytic workup, the same reaction carried out in the presence of PPh, yielded 13a and 13b in a ratio of >99 1 (Scheme 2-14). 

Gold forms a continuous series of solid solutions with palladium, and there is no evidence for the existence of a miscibility gap. Also, the catalytic properties of the component metals are very different, and for these reasons the Pd-Au alloys have been popular in studies of the electronic factor in catalysis. The well-known paper by Couper and Eley (127) remains the most clearly defined example of a correlation between catalytic activity and the filling of d-band vacancies. The apparent activation energy for the ortho-parahydrogen conversion over Pd-Au wires wras constant on Pd and the Pd-rich alloys, but increased abruptly at 60% Au, at which composition d-band vacancies were considered to be just filled. Subsequently, Eley, with various collaborators, has studied a number of other reactions over the same alloy wires, e.g., formic acid decomposition 128), CO oxidation 129), and N20 decomposition ISO). These results, and the extent to which they support the d-band theory, have been reviewed by Eley (1). We shall confine our attention here to the chemisorption of oxygen and the decomposition of formic acid, winch have been studied on Pd-Au alloy films. 

There is a one-point modification of a chemisorption method, which is widely used for measurements of Ac. In this case, only one adsorption point of a chemisorption isotherm is measured, and is compared with only one point on a chemisorption isotherm on a reference material (usually, powder [black] or foil). The identity of the chemisorption properties of the active components in supported and pure form is postulated, but very often does not fulfill, making one-point modification an inaccurate procedure, which can hardly be used in scientific studies. For example, studies of supported Rh catalysts by 02 and CO chemosorption have shown that three different blacks of Rh yield three different results [88], The multipoint comparison of chemisorption isotherms shown that only one black had a chemisorption isotherm that had affinity to the isotherm on a supported metal. 

In addition to modification of surfaces by non-metals, the catalytic properties of metals can also be altered greatly by the addition of a second transition metaP ". Interest in bimetallic catalysts has arisen steadily over the years because of the commercial success of these systems. This success results from an enhanced ability to control the catalytic activity and selectivity by tailoring the catalyst composition . A long-standing question regarding such bimetallic systems is the nature of the properties of the mixed-metal system which give rise to its enhanced catalytic performance relative to either of its individual metal components. These enhanced properties (improved stability, selectivity and/or activity) can be accounted for by one or more of several possibilities. First, the addition of one metal to a second may lead to an electronic modification of either or both of the metal constituents. This... 

A guide to the manner in which structural theory may be applied to a detailed consideration of the mechanism of a surface-catalyzed reaction is found in papers by Cossee (113), Arlman (114), and Arlman and Cossee (115) concerning the mechanism of the stereoregular heterogeneous catalyzed polymerization of propylene. Particular crystallographic sites are shown to be the active centers at which the reactants combine and ligand field theory is used to demonstrate a plausible relationship between the activation energy for the conversion of adsorbed reactants to the product and the properties of the transition metal complex which constitutes the reaction center. 

A highly interesting class of catalysts is represented by bimetallic systems, which in many important catalytic processes show improved activity or selectivity compared with catalysts involving only one metal. Understanding their better performance is still a challenge. One metal can tune and/or modify the catalytic properties of the other metal as the result of both electronic or/and structural effects. Several mechanisms for synergism can be proposed, but it is difficult to assess their relative importance. It is clear that each metal can play a very important role in proper circumstances [41]. 

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