Dissociation Dissociative adsorption

Hydrogen adsorption on MgO can, in principle, be either molecular or dissociative. Dissociative adsorption of hydrogen on high-surface-area MgO has already been reported, and both homolytic and heterolytic pathways have been proposed 12). Homolytic splitting is supposed to operate under UV-irradiation only (117-119) and is not discussed further here. Heterolytic splitting takes place in the dark and at 300 K on coordinatively unsaturated (cus) Mg O surface pairs following the schematic mechanism illustrated in Scheme 2. 

Figure A3.9.7. A representation of the Leimard-Jones model for dissociative adsorption of H2. Curves (a) interaction of intact molecule with surface (b) interaction of two separately chemisorbed atoms with surface.

Darling G R and Holloway S 1994 The role of parallel momentum in the dissociative adsorption of H2 at highly corrugated surfaces Surf. Sc/. 304 L461... 

Figure A3.10.17 Potential energy diagram for the dissociative adsorption of N2 [46].

Beebe T P, Goodman D W, Kay B D and Yates J T Jr 1987 Kinetics of the activated dissociation adsorption of methane on low index planes of nickel single crystal surfaces J. Chem. Phys. 87 2305... 

The industrial catalysts for ammonia synthesis consist of far more than the catalyticaHy active iron (74). There are textural promoters, alumina and calcium oxide, that minimise sintering of the iron and a chemical promoter, potassium (about 1 wt % of the catalyst), and possibly present as K2O the potassium is beheved to be present on the iron surface and to donate electrons to the iron, increasing its activity for the dissociative adsorption of N2. The primary iron particles are about 30 nm in size, and the surface area is about 15 m /g. These catalysts last for years. 

The reaction shown above for the steam reforming of methatie led to die formation of a mixture of CO and H2, die so-called synthesis gas. The mixture was given this name since it can be used for the preparation of a large number of organic species with the use of an appropriate catalyst. The simplest example of this is the coupling reaction in which medrane is converted to ethane. The process occurs by the dissociative adsorption of methane on the catalyst, followed by the coupling of two methyl radicals to form ethane, which is then desorbed into the gas phase. 

For dissociative adsorption, i.e., for systems in which the gas phase is predominantly molecules which dissociate into fragments A and B on the surface (not necessarily atoms), the desorption rate is given by... 

Alternatively, an intermediate formation of an adsorbed methylene on the catalyst surface through the dissociative adsorption of carbon monoxide has been considered ... 

A classical example of promotion is the use of alkalis (K) on Fe for the ammonia synthesis reaction. Coadsorbed potassium (in the form of K20) significantly enhances the dissociative adsorption of N2 on the Fe surface, which is the crucial and rate limiting step for the ammonia synthesis5 (Fig. 2.1). 

For alkali modified noble and sp-metals (e.g. Cu, Al, Ag and Au), where the CO adsorption bond is rather weak, due to negligible backdonation of electronic density from the metal, the presence of an alkali metal has a weaker effect on CO adsorption. A promotional effect in CO adsorption (increase in the initial sticking coefficient and strengthening of the chemisorptive CO bond) has been observed for K- or Cs-modified Cu surfaces as well as for the CO-K(or Na)/Al(100) system.6,43 In the latter system dissociative adsorption of CO is induced in the presence of alkali species.43... 

The alkali promotion of CO dissociation is substrate-specific, in the sense that it has been observed only for a restricted number of substrates where CO does not dissociate on the clean surface, specifically on Na, K, Cs/Ni( 100),38,47,48 Na/Rh49 and K, Na/Al(100).43 This implies that the reactivity of the clean metal surface for CO dissociation plays a dominant role. The alkali induced increase in the heat of CO adsorption (not higher than 60 kJ/mol)50 and the decrease in the activation energy for dissociation of the molecular state (on the order of 30 kJ/mol)51 are usually not sufficient to induce dissociative adsorption of CO on surfaces which strongly favor molecular adsorption (e. g. Pd or Pt). 

The adsorption of C02 on metal surfaces is rather weak, with the exception of Fe, and no molecular or dissociative adsorption takes place at room temperature on clean metal surfaces. At low temperatures, lower than 180 to 300 K, a chemisorbed COf" species has been observed by UPS6 on Fe(lll) and Ni(110) surfaces, which acts as a precursor for further dissociation to CO and adsorbed atomic oxygen. A further step of CO dissociation takes place on Fe(l 11) above 300 to 390 K. 

Similar to the case of CO, the dissociation propensity of NO depends largely on the substrate, following the same general trends. Alkali introduction on metal substrates promotes the dissociative adsorption of NO, both by weakening the N-O intramolecular bond and by stabilizing the molecular state which acts as a precursor for dissociation. 

Alkali promoted NO dissociation is clearly illustrated in the case of NO adsorption on K/Pt(lll), as NO is not adsorbed dissociatively on the alkali-clean surface. The dissociative adsorption of NO on K/Pt(l 11) takes place at temperatures higher than 300 K and the number of dissociated NO molecules... 

A general conclusion regarding H2 adsorption on alkali modified metal surfaces is that alkali addition results in a pronounced decrease of the dissociation adsorption rate of hydrogen as well as of the saturation coverage. 

On K modified Ni(100) and Ni(lll)62,63 and Pt(lll)64 the dissociative adsorption of hydrogen is almost completely inhibited for potassium coverages above 0.1. This would imply that H behaves as an electron donor. On the other hand the peaks of the hydrogen TPD spectra shift to higher temperatures with increasing alkali coverage, as shown in Fig. 2.22a for K/Ni(lll), which would imply an electron acceptor behaviour for the chemisorbed H. Furthermore, as deduced from analysis of the TPD spectra, both the pre-exponential factor and the activation energy for desorption... 

Figure 2.23. Changes in the initial sticking coefficient for N2 dissociative adsorption on K-covered Fe(Ul) as a function of K coverage. Ta=430 K.72 Reprinted with permission from Elsevier Science.

The same trends regarding the effect of sulfur have been reported for NO adsorption on Pt(lOO)90 and Rh(100).6 In the case of Pt(100) dissociative adsorption is completely inhibited upon formation of a p(2x2) overlayer at a sulfur coverage equal to 0.25, while the binding strength of molecularly adsorbed NO is lowered by more than 50 kJ/mol, as calculated by analysis of NO TPD data. Due to this complete inhibition of dissociative adsorption, the CO+NO reaction is completely deactivated, although it proceeds easily on sulfur free Pt(100). In the case of Rh(100) a sulfur coverage of only 0.08 suffices to completely inhibit NO dissociation at 300 K. 

Figure 2.34. The effect of Cl coverage on the rate of oxygen dissociative adsorption on Ag(l 10).96 Reprinted with permission from Elsevier Science.

Despite the poisoning action of Cl for oxygen dissociative adsorption on Ag, it is used as moderator in the ethylene epoxidation reaction in order to attain high selectivity to ethylene oxide. The presence of Cl adatoms in this... 

The influence of electronegative additives on the CO hydrogenation reaction corresponds mainly to a reduction in the overall catalyst activity.131 This is shown for example in Fig. 2.42 which compares the steady-state methanation activities of Ni, Co, Fe and Ru catalysts relative to their fresh, unpoisoned activities as a function of gas phase H2S concentration. The distribution of the reaction products is also affected, leading to an increase in the relative amount of higher unsaturated hydrocarbons at the expense of methane formation.6 Model kinetic studies of the effect of sulfur on the methanation reaction on Ni(lOO)132,135 and Ru(OOl)133,134 at near atmospheric pressure attribute this behavior to the inhibition effect of sulfur to the dissociative adsorption rate of hydrogen but also to the drastic decrease in the... 

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