Heats of Atomic Chemisorption

In the subsequent BOC-MPAJBI-QEP relationships, the basic energetic parameter is the Morse constant Qo in equation 6.31, which corresponds to the maximum two-center M-A bond energy, Qoa, for atom A adsorbed on an on-top site. This value is not directly obtainable, but it can be readily determined from the experimental heat of adsorption value, Qa (the atomic binding energy), associated with the M -A bond energy, Q , namely 

For a single adatom A at low coverages, if the small changes in M-M interactions are neglected, equation 6.34 is 

Each two-center M-A interaction is described by the Morse potential (Equations 6.30 and 6.31). 

For a specified Mn-A configuration, n two-center M-A interactions are additive (Equations 6.32 and 6.33). 

Along any pathway the total M -X bond order is conserved and normalized to unity (Equations 6.34, 6.39, 6.40). 

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In the following BOC-MP interrelations, the basic energetic parameter is the Morse constant Q0 [cf. Eq. (2)], which corresponds to the maximum M-A two-center bond energy Q0A. The value of Q0A, although not directly observable, can be easily scaled from the experimental heat of atomic chemisorption QA (atomic binding energy) identified with the M -A bond energy Q(n), namely,... 

Now let us turn to the BOC condition of Eq. (8b). Although Eq. (8b) does not explicitly depend on n, it assumes the best possible coordination within the M -AB unit mesh, which reflects in the use of the experimental heats of atomic chemisorption, QA and QB, as the Morse constants in energy calculations. For the monocoordination rilpn in M -AB, the variational procedure now leads to an expression... 

Experimental Heats of Atomic Chemisorption QA on Some Metal Surfaces ... 

Table I lists the experimental values of QA for major adatoms such as H, O, N, and C on some close-packed metal surfaces (30-43). Typically, the heat of atomic chemisorption QA decreases while going from the left to right along a transition series and from the top to bottom of a column. This decrease AQA is the least pronounced for monovalent H when, within the series Pt-Ni-W, AQH does not exceed 7 kcal/mol [QH = 61, 63, and 68 kcal/mol for Pt(lll), Ni(lll), and W(110), respectively]. For divalent O and trivalent N, however, the changes in QA from Pt to Ni to W become very large, up to AQA = 40 kcal/mol i.e., Qq = 85-125 kcal/mol and (gN = 115-155 kcal/mol. For tetravalent C, the experimental measurements have been reported only for Ni(lll) and Ni(100), giving Qc = 171 kcal/ mol (43). So, for other metal surfaces we are to use extrapolated estimates of Qc. For C, we assume a somewhat larger spread in QA compared with O and N that is, A<2C = 50 kcal/mol, from Qc = 150 kcal/mol for Pt(l 11)...

Table 6.5. Experimental heats of atomic chemisorption, Qa on some metal surfaces ...

Strong chemisorption would be assumed to occur for surface species such as molecular radicals in which unpaired electrons retain most of their atomic character, and the adsorption pattern would resemble that for atoms, which includes a distinct preference for n-fold hollow sites. Examples would include radicals like CH, CH2, NH, OH and OCH3. In this case for monocoordination (Tri p,n,), such as M — AB, the Morse constants are better represented by the experimental heats of atomic chemisorption, Qa and Qb, and the use of equation 6.40 provides the following respective analogues for equations 6.41 and 6.52 ... 

Compared to the corresponding carbides the heats of oxygen chemisorption on metals are higher. For example, on metallic tungsten the heat of adsorption is 812 kJ/mole 02, while on metallic chromium it is 730 kJ/ mole 02n. These values are significantly higher that those of the carbides of the same metals (Table 16.2). Thus, carbon atoms, when implanted in the metal lattice, reduce the adsorption affinity of the metal atoms towards oxygen. 

Conceptually, the most important model conclusion is that for a diatomic AB, the heat of molecular chemisorption QAB relates to both the heats of chemisorption of coordinated atoms QA and QB and the A—B bond energy... 

The acid properties of A1-, Ga-, and Fe-substituted MCM-41-type mesoporous silicates have been probed using ammonia adsorption at 423 K [285]. Substitution led to the formation of Bronsted and Lewis acid sites of different type and strength. Initial heats of ammonia chemisorption decreased in the same order (185 (Al) > 162 (Ga) > 144 (Fe) kJ mol ) as the degree of isomorphous substitution of framework silicon by trivalent atoms. The ratio of strong to medium-strong Brdnsted sites fell in the same order. 

The relative ease with which hydrogen chemisorbs on the surface of a metal oxide surface mainly depends on the chemical nature of the oxide and on the O-vacancies. Thus, hydrogen adsorbs dissociatively on a perfect titanium oxide surface [10,11]. The energetically most favorable mode for the adsorption of atomic hydrogen is the adsorption on the outermost O atom, accompanied by the reduction of a Ti atom. In this mode, protons are formally adsorbed while an equivalent amount of Ti(IV) atoms are reduced to Ti(III). Theoretical calculations have demonstrated that H adsorption is less favorable on a defective surface than on a perfect surface. However, the best adsorption mode for the atomic chemisorption on a defective surface is heterolytic adsorption, which involves two different adsorption sites one H+/0= and one H on the surface. This adsorption mode is best on irreducible oxides such as MgO however, it is less favorable than adsorption on the perfect Ti02 surface [10]. The heat of atomic adsorption in all cases is very weak and dissociation onto the surface is unlikely. The molecular adsorption (physisorption), thus, remains the most stable system. 

Regardless of the exact extent (shorter or longer range) of the interaction of each alkali adatom on a metal surface, there is one important feature of Fig 2.6 which has not attracted attention in the past. This feature is depicted in Fig. 2.6c, obtained by crossploting the data in ref. 26 which shows that the activation energy of desorption, Ed, of the alkali atoms decreases linearly with decreasing work function . For non-activated adsorption this implies a linear decrease in the heat of chemisorption of the alkali atoms AHad (=Ed) with decreasing > ... 

Looking at the trends in dissociation probability across the transition metal series, dissociation is favored towards the left, and associative chemisorption towards the right. This is nicely illustrated for CO on the 4d transition metals in Fig. 6.36, which shows how, for Pd and Ag, molecular adsorption of CO is more stable than adsorption of the dissociation products. Rhodium is a borderline case and to the left of rhodium dissociation is favored. Note that the heat of adsorption of the C and O atoms changes much more steeply across the periodic table than that for the CO molecule. A similar situation occurs with NO, which, however, is more reactive than CO, and hence barriers for dissociation are considerably lower for NO. 

It has also to be remembered that the band model is a theory of the bulk properties of the metal (magnetism, electrical conductivity, specific heat, etc.), whereas chemisorption and catalysis depend upon the formation of bonds between surface metal atoms and the adsorbed species. Hence, modern theories of chemisorption have tended to concentrate on the formation of bonds with localized orbitals on surface metal atoms. Recently, the directional properties of the orbitals emerging at the surface, as discussed by Dowden (102) and Bond (103) on the basis of the Good-enough model, have been used to interpret the chemisorption behavior of different crystal faces (104, 105). A more elaborate theoretical treatment of the chemisorption process by Grimley (106) envisages the formation of a surface compound with localized metal orbitals, and in this case a weak interaction is allowed with the electrons in the metal. 

An important problem in surface chemistry concerns the nature of the bond formed when an atom or molecule is adsorbed onto the surface of a solid. The magnitude of the heat of adsorption provides a rough guide to the sort of interaction to be expected. K the heat is low, say 5 kcal. mole", we speak of physical adsorption and imply that the electronic structures of the solid and the adsorbate are not seriously modified when the two are in mutual interaction. If the heat is high, say, 50 kcal. mole", we speak of chemisorption and imply that a change in the electronic structures does occur. This change may be drastic, as with H2 and the transition metals where the gas is chemisorbed as atoms, or less obvious... 

De Boer (130) first drew attention to the contribution of the work-function effect to the heat of chemisorption of alkali atoms on a metal surface. With Cs on W, for example, the heat of adsorption is described by the equation,... 

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