Dissociative chemisorption experimental values

The easiest test concerns Eq. (22a) describing the LJ-type dissociation. The equation establishes the linear correlation between the dissociation barrier A for a homonuclear admolecule A2 and the atomic heat of chemisorption Qh with the slope of k = 3/2. As seen from Table V, for H2, 02, and N2 dissociated on surfaces of metals as varied as Fe, Ni, Cu, W, and Pt, the experimental values of k lie within the range k = 1.4-1.7, that is, within 10-15% of the theoretical LJ value of k = 1.5. It should be stressed that, unlike similar linear relations between the activation barriers and the heats of reactions (Brpnsted, Polanyi, Frumkin-Temkin-Semye-nov, etc.), Eq. (22a) is not a postulate but a corollary of the general principle (BOC-MP) applied to the one-dimensional dissociation ABS As + Bs. 

As far as phenomenological modeling is concerned, an excellent review of earlier thermodynamic approaches to chemisorption and surface reactivity was given by Benziger (156), who also developed some general thermodynamic criteria for dissociative versus nondissociative adsorption of diatomic and polyatomic molecules on transition metal surfaces (137, 156). In particular, for quantitative estimates of QA, A = C, N, or O, Benziger (156) used the heats of formation of bulk metal carbides, nitrides, and oxides. The BOC-MP approach is different, however, not only analytically but also in making direct use of experimental values of QA. 

The dissociative adsorption of alkanes on Ir(l 1 0) surface was investigated in a series of studies performed by the Madix group [13, 32, 33]. A study performed by Hamza et al. [32] probed the dynamics of the dissociative chemisorption of methane, ethane, propane, and n-butane on the Ir(l 1 0)-(l x 2) surface. These investigations were complemented by a later study of propane dissociation on the Ir(l 1 0)-(l x 2) surface by Soulen and Madix [13]. Shown in Figs 6 and 7 are plots of S0 vs. E obtained for propane at various surface temperatures [32] and a plot of experimental and theoretical values of S0 for propane (at an incident translational energy of 50kcal/mol) on Ir(l 1 0)-(l x 2) as a function of surface temperature [13]. 

Our first example is the widely discussed problem of CO chemisorption on transition metal surfaces. Ray and Anderson/106/ investigated CO chemisorption on a Pt (111) surface using the ASED-MO theory. In their work, Pt4 and Pt10 clusters were used to model the Pt (111) surface. The bulk Pt-Pt bond distance was used and other theoretical parameters are reproduced in Table II. The calculated Pt-Pt and C-O bond distances and harmonic force constants are within 8% of experimental values for isolated Pt2 and CO. The results for dissociation energies are less satisfactory 94.08 and 116 kcal/mole in comparison with experimental values of 84.5 and 256 kcal/mole, respectively. 

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