Heats of Molecular Chemisorption

For chemisorption of a molecule AB, from equation 6.34 bond-order conservation gives 

Qn/Qo = 2 — 1/n (equation 6.35 or 6.38) Possible high coordinations on rough surfaces. 

The difference between the last two equations is that different Morse constants are used to describe the interaction of an atom A in a molecule AB with a metal surface, i.e., the two-center M-A energy Qoa for equation 6.39 and the polycenter M - AB energy Qa for equation 6.40. Consequently, a choice must be made about which molecules are best described by each representation, and this essentially depends on whether chemisorption is weak or strong . 

If a molecule adsorbs in a dicoordinated mode (nri ) via two atoms, i.e., a bridge site (1x2), then for this (11 1x2) configuration, where A and B can be either atoms or groups treated as quasi-atoms  

For a bridge-bonded homonuclear molecule A2, equations 6.43 and 6.44 reduce to  

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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... 

Heats of Molecular Chemisorption QAB Strongly Bound Radicals ... 

Heats of Adsorption and Activation Barriers on Metal Surfaces Table 6.6. Heats of molecular chemisorption Qab diatomic molecules ... 

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 ... 

Sequences such as the above allow the formulation of rate laws but do not reveal molecular details such as the nature of the transition states involved. Molecular orbital analyses can help, as in Ref. 270 it is expected, for example, that increased strength of the metal—CO bond means decreased C=0 bond strength, which should facilitate process XVIII-55. The complexity of the situation is indicated in Fig. XVIII-24, however, which shows catalytic activity to go through a maximum with increasing heat of chemisorption of CO. Temperature-programmed reaction studies show the presence of more than one kind of site [99,1(K),283], and ESDIAD data show both the location and the orientation of adsorbed CO (on Pt) to vary with coverage [284]. 

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. 

HjO, heat of chemisorption, 37 120-121 HOMO, see Highest occupied molecular orbital... 

Quantum-chemical cluster models, 34 131-202 computer programs, 34 134 methods, 34 135-138 for chemisorption, 34 135 the local approach, 34 132 molecular orbital methods, 34 135 for surface structures, 34 135 valence bond method, 34 135 Quantum chemistry, heat of chemisorption determination, 37 151-154 Quantum conversion, in chloroplasts, 14 1 Quantum mechanical simulations bond activation, 42 2, 84—107 Quasi-elastic neutron scattering benzene... 

Relevant to the synthesis of ammonia over iron catalysts is the observation of Ertl et al. (54) that potassium preadsorbed by an iron catalyst ((7 = 0.1) increased the rate of synthesis at 430 K by a factor of about 300. This effect the authors attributed to an enhancement of the heat of adsorption of molecular nitrogen due to transfer of electronic charge from potassium to the surface of the iron catalyst. This would be entirely in keeping with the precursor model proposed for nitrogen chemisorption (55). 

The heat of chemisorption is rather high, which involves a low level for the minimum D. Molecular hydrogen can penetrate (in atomic form) into iron, provided that the kinetic energy is high enough to reach level E,from which it can move into the metal (level F). The exact place of the surface cannot be pictured in such a potential-curve scheme it is somewhere in... 

When a contamination is present which produces a dipole layer, as sulfur does, the dissociative chemisorption of molecular hydrogen is given by (Fig. 40) ABC D ) there is an activation energy (difference between levels C and A) the heat of adsorption is severely reduced it is even negative (endothermic chemisorption). The dissolution of molecular hydrogen proceeds less easily than in the case of a pure-iron surface. The kinetic energy has to be sufficient to overcome the difference between C ... 

One can add that the dissociation barrier AE%B% from a chemisorbed state will be larger than AEABg (one- or multidimensional) just by the amount of the molecular heat of chemisorption QAB,... 

The general conclusion from Eqs. (9)—(14) is that the molecular heat of chemisorption ( AB rapidly decreases as the gas-phase dissociation (total bond) energy DAB increases. The values of QAB are smaller than QA(QB), typically by a factor of 5-10 but sometimes even 15-20. For this reason, the periodic changes in QAB for molecules such as CO, NH3, NO, H20, C2H4, and C2H2 are expected to be small and potentially irregular, unlike the large and systematic variations in QA observed for the relevant multiply bonded adatoms A. 

It is worth repeating that the high accuracy of Eq. (24) for atomic-molecular recombination A-BC stems from the strong inequality Qk > QBC leading to A BC < QBC. Because QBC < QB, Qc (cf. Section III,A), the recombination barrier A -bc will be confined to a small range of a few kilocalories/mole and show a very weak (periodic) dependence on the heats of chemisorption of the atomic constituents Qk, QB, and Qc. 

The BOC-MP method provides reasonably accurate estimates of the heats of chemisorption Q and the dissociation and recombination barriers AE for various molecules and molecular fragments. Combined with the knowledge of the molecular total bond (gas-phase dissociation) energies, this allows one to construct potential energy profiles of surface reactions. 

In the zero-coverage extreme, the BOC-MP interrelations are exact for atomic adsorbates and well defined for molecular adsorbates, the same analytic formalism being used to treat both diatomic and polyatomic molecules. Moreover, these interrelations are expressed in terms of observables only (the heats of chemisorption and various constants), which makes comparison with experiment direct and unambiguous. With rare exceptions, the agreement with experiment is remarkably good, qualitatively and quantitatively. 

IN THIS SURVEY of current concepts in adsorption and chemisorption, it is pointed out that entropy relations, both thermodynamic and kinetic, have made a relatively late appearance on the scene of adsorption research. Exaggerated preoccupation with heats of adsorption and energies of activation has led to a frozen formalism which appears to have outlived much of its usefulness. This situation is now being corrected by more attention to molecular structure of adsorbed layers and its relation to entropies of adsorption. 

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