Donor-type chemisorption

It is evident from examples like these that the investigation of electron transfer in catalysis is dependent on the availability of test reactions of well-known acceptor or donor type. Lately, it has become clear that sometimes the same reaction can exert both functions, depending on the conditions. Thus, the carbon monoxide oxidation is a donor reaction on most p-conducting catalysts, like nickel oxide 13) when the chemisorption of carbon monoxide governs the reaction rate. However, on zinc oxide, the chemisorption of the acceptor oxygen is rate-determining. 

Fig. 1.16. The qualitative type of dependencies ait) (curve 1) and flit) (curve 2) in case of linear kinetics of chemisorption of donors.

It has been proven by experiment that there are donor acceptor atoms and molecules of absorbate and their classification as belonging to one or another type is controlled not only by their chemical nature but by the nature of adsorbent as well (see, for instance [18, 21, 203-205]). From the standpoint of the electron theory of chemisorption it became possible to explain the effect of electron adsorption [206] as well as phenomenon of luminescence of radical recombination during chemisorption [207]. The experimental proof was given to the capability of changing of one form of chemisorption into another during change in the value of the Fermi level in adsorbent [208]. 

Let us dwell on existing key models describing chemisorption induced response of electric conductivity in semiconductor adsorbent. Let us consider both the stationary values of electric conductivity attained during equilibrium in the adsorbate-adsorbent system and the kinetics of the change of electric conductivity when the content of ambient atmosphere changes. Let us consider the cases of adsorption of acceptor and donor particles separately. In all cases we will pay a special attention to the issue of dependence of the value and character of signal on the structure type of adsorbent, namely on characteristics of the dominant type of contacts in microcrystals. 

One of the characteristics of chemisorption is that it permits the formation of different types of bonds between a given adsorbed species and the same adsorbent. Thus, an atom can be attached to an ionic crystal by a weak covalent bond, a strong covalent bond, or an ionic bond. The first is characterized by a localized electron and an induced dipole moment that may be larger by several orders of magnitude than the moment due to physical adsorption. When bonding is augmented by a free electron from the crystal lattice, the adsorbed atom (in the case of monovalent electropositive atom) is held by a strong covalent bond. On the other hand, localization of a hole near a weakly adsorbed atom leads to the formation of an ionic bond. Thus the same atom can represent an acceptor or a donor at the same time. 

The toxicity of an element such as sulfur is dependent on the presence, in the valency shell of the toxic element, of free electron pairs which are evidently necessary for the formation of the link with the catalyst. The toxicity—i.e., the power of forming a relatively strong chemisorptive bond—disappears if the structure of the molecule is of a shielded type in which this element is already associated with a completely shared electron octet. Thus, it appears (Maxted, 8) that the chemical bond by means of which the poison is linked to the metallic surface resembles the ordinary dative bond in which the poison is the donor. In the case of methyl sulhde adsorbed on palladium, indications have been obtained (Dilke, Eley, and Maxted, 9) by means of magnetic susceptibility measurements that electrons from the methyl sulfide enter the d-band of the adsorbing metal to give a coordinate link, the process being probably accompanied (Maxted, 10) by a filling up of the fractional deficiencies or holes in the d-band of the metal due to d- -band overlap which seem to be responmble for the catalytic activity of the transition metals (11). 

As expected the />-type semiconductor NiO is the better catalyst. An increased /7-type conduction due to Li20 and a decrease due to Cr203 is understandable. It is assmned that a donor step is rate-determining, namely, die chemisorption of CO. 

The influence of donors on ZnO at first appears remarkable. Here one would expect that increasing the -type conductivity by adding trivalent donors would lower the reaction rate. However, this does not happen. This leads to the conclusion that in this case an acceptor reaction is the rate-determining step. Presumably it is the chemisorption of oxygen, since considerable dependence of the reaction rate on the oxygen partial pressure was observed. The example shows how the reaction mechanism can change from catalyst to catalyst. 

It should be noted that minerals, and oxide minerals in particular, have different types of OH groups, depending on the coordination of the O atoms, as revealed by spectroscopic studies. Goethite (a-FeOOH) has four types of surface hydroxyls whose reactivities are a function of the coordination environment of the O in the FeOH group (Sposito 1984 Sparks 2002). The FeOH groups are A-, B-, or C-type sites, depending on whether the O is coordinated with 1,3, or 2 adjacent Fe(III) ions. The fourth type of site is a Lewis acid-type site, which results from chemisorption of a water molecule on a bare Fe(III) ion. Only A-type sites are basic (can bind H+), and, on the contrary, A-type and Lewis acid sites can release a proton. The B- and C-type sites are considered unreactive. Thus, A-type sites can be either a proton acceptor or a proton donor (i.e., they are amphoteric). Other spectroscopic studies have shown that boehmite (y-AlOOH) and lepidocrocite (y-FeOOH) have two types of OH, presumably associated with different crystal faces (Lewis and Farmer 1986). 

The donor and acceptor type interaction of a gas and solid are often described by a charge transfer model, which assumes that adsorbed particles induce extrinsic two-dimensional surface states. As mentioned earlier, a distinction is to be made between localized chemisorption and delocalized chemosorption also called iono-sorption. Localized chemisorption is due to a charge transfer between an adsorbent (e.g., surface site = surface atom or group of atoms) and an adsorbate. It is important to state that the changes induced by localized chemisorption processes are not easy to read out because they are not influencing the resistance (conductance) of the sensing layer. 

Chemisorption [114] on an oxide surface differs significantly from that on metals. One of the main reasons for this difference is the ionic character of the solid, which favors acid-base or donor-acceptor reactions. Lewis sites are localized on the cations and basic sites on the anions. An example of this type of interaction is given by CO2, which reacts with basic to give a surface carbonate COj . Similarly, a donor molecule such as H2O or NH3 can be molecularly adsorbed via its lone-pair electrons, which react with an acidic (cation) site. An alternative to the molecular adsorption is that resulting from the heterolytic dissociation of the molecule. It may occur by abstraction of H atom transferred to a basic site, producing a hydroxyl group. 

At the point when adsorption ceases, the difference between the bulk chemical potential of the solid and the surface is balanced by a potential difference between the bulk and the surface. The surface is effectively at the chemical potential of the adsorbate. The amount of adsorption depends Intimately on the electronic properties of the solid. For example the term "depletive" chemisorption is used to describe the adsorption oxygen (an electron acceptor) on "n" type zinc oxide (an electron donor). Equilibrium is reached when no further electrons are available at the surface and the electrical conductivity has dropped. The similarity to contact charging is obvious. 

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