Superbasic Surface Centres with Ionic Character

Superbasic Surface Centres with Ionic Character. - As mentioned earlier reactions between alkali metal atoms and surface acceptor centres such as hydroxyl groups or holes near cationic vacancies lead to the creation of centres of higher basicity. In the first of these examples the reason is the replacement of a hydrogen atom by a more electropositive element, such as an alkali metal atom in the second example it is the result of introduction of an electron from the alkali metal to the hole trapped on the O anion, the vacancy being filled by a univalent cation. It should be noted that both of the surface configurations so formed can cause strong one-electron or two-electron donor activity. Closer physico-chemical examination has shown that these centres tend to be electron pair rather than one-electron donating in character. They mostly occur on surfaces on which alkali-metal vapours have reacted with oxides heated at the lower temperatures, e.g., MgO calcined 

It is possible that alkali-metal atoms retained on the MgO surface after reaction with surface hydroxyl groups or VJ defects interact with more than one 0 anion, thereby increasing their basic strength. After evaporation of alkali metal on the oxide surface the quantity of new basic sites is greater than the quantity of alkaU-metal atoms introduced. (When sodium has been evaporated on MgO surface calcined at 823 K, the increase in the concentration of basic centres is 0.03 mmol m but the quantity of metal retained is 0.006 mmol m ).  

Consideration of the MgO[100] surface and that 0 either belongs to the crystalline lattice or comes from adsorbed water molecules or OH groups indicate that the ratio of 0 anions to the quantity of sodium atoms retained on 100 of surface is approximately 4 (in the case of the most basic preparations). It is also possible that sodium bound to the surface has a coordination number of 4. 

The quantity of alkali metal retained on the MgO surface and the concentration of the newly created ionic superbasic centres depends on the position of metal in the Periodic Table. The greater the electropositivity in the sequence sodium, potassium, caesium, the greater is the reactivity with surface acceptor centres of MgO surfaces. It is possible that metals having lower ionization energy, such as potassium or caesium (Table 1), react with these surface centres of MgO, which are not affected by sodium atoms. In consequence an oxide surface that has been heated to a particular temperature is able to bind more caesium than sodium atoms. The increase of the quantity of metal retained on MgO surfaces is not followed by a simultaneous increase in the number of newly created ionic superbasic centres. The largest quantity of such centres is formed on MgO surfaces doped with potassium. It is interesting to note that in the case of MgO-K and MgO-Cs systems two types of superbasic centres occur, one with a basic strength of 33 H 35, the second one with H 35 (Table 1).  

The introduction of two alkali metals by the evaporation method on an MgO surface leads to systems that have the greatest concentration of extremely strong basic sites (Table 2)P The number of basic centres depends not only on the metal deposited, but also on how it is introduced. It can be assumed that, in the case of simultaneous deposition by evaporation of two metals during the reaction with the surface, a eutectic mixture of metals takes part, since this has greater volatility than the volatility of each component. When two metals have been successively introduced by evaporation (the first being 

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