Molecular adsorption, 5.21

It is possible to extend the formulism discussed in Section 6.2.2 to calculate the extent to which a gas will chemisorb to a given surface. In equilibrium, the rate of desorption of molecules from the surface will be equal to their rate of adsorption. The ratio of the rate of adsorption. Tads, to the rate of desorption, rdes, is approximately 

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The interaction of an electrolyte with an adsorbent may take one of several forms. Several of these are discussed, albeit briefly, in what follows. The electrolyte may be adsorbed in toto, in which case the situation is similar to that for molecular adsorption. It is more often true, however, that ions of one sign are held more strongly, with those of the opposite sign forming a diffuse or secondary layer. The surface may be polar, with a potential l/, so that primary adsorption can be treated in terms of the Stem model (Section V-3), or the adsorption of interest may involve exchange of ions in the diffuse layer. 

The first step consists of the molecular adsorption of CO. The second step is the dissociation of O2 to yield two adsorbed oxygen atoms. The third step is the reaction of an adsorbed CO molecule with an adsorbed oxygen atom to fonn a CO2 molecule that, at room temperature and higher, desorbs upon fomiation. To simplify matters, this desorption step is not included. This sequence of steps depicts a Langmuir-Hinshelwood mechanism, whereby reaction occurs between two adsorbed species (as opposed to an Eley-Rideal mechanism, whereby reaction occurs between one adsorbed species and one gas phase species). The role of surface science studies in fomuilating the CO oxidation mechanism was prominent. 

In the last decade two-dimensional (2D) layers at surfaces have become an interesting field of research [13-27]. Many experimental studies of molecular adsorption have been done on metals [28-40], graphite [41-46], and other substrates [47-58]. The adsorbate particles experience intermolecular forces as well as forces due to the surface. The structure of the adsorbate is determined by the interplay of these forces as well as by the coverage (density of the adsorbate) and the temperature and pressure of the system. In consequence a variety of superstructures on the surfaces have been found experimentally [47-58], a typical example being the a/3 x a/3- structure of adsorbates on a graphite structure (see Fig. 1). 

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 most widely used model of adsorption is Langmuir s equation for reversible molecular adsorption [163]. However, this is inappropriate for charged latex particles, because... 

The description of molecular adsorption is very similar to that of atoms, provided we account for the molecules internal degrees of freedom. Hence we need to consider how these degrees change in going from the gas phase to the transition state of adsorption. The most general form for the rate constant of adsorption in the transition state theory is... 

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. 

Derive the Langmuir adsorption isotherm for the molecular adsorption of CO on a metal with equivalent adsorption sites. Do the same for the dissociative adsorption of H2, and, finally, for the case when CO and H2 adsorb together on the same surface. 

Molecular Adsorption and Decomposition on Clean and Sulfiir-Mod ed Metal Surfaces... 

Coadsorption and Decomposition Reactions. The final area we discuss are cases where there are coadsorbed species present, which may react, and cases where molecular adsorption converts to dissociative (or associative) products. 

In situ infrared spectroscopy allows one to obtain stracture-specific information at the electrode-solution interface. It is particularly useful in the study of electrocat-alytic reactions, molecular adsorption, and the adsorption of ions at metal surfaces. 

Flux Decline Plugging, Fouling, Polarization Membranes operated in NFF mode tend to show a steady flux decline while those operated in TFF mode tend to show a more stable flux after a short initial decline. Irreversible flux decline can occur by membrane compression or retentate channel spacers blinding off the membrane. Flux decline by fouling mechanisms (molecular adsorption, precipitation on the membrane surface, entrapment within the membrane structure) are amenable to chemical cleaning between batches. Flux decline amenable to mechanical disturbance (such as TFF operation) includes the formation of a secondary structure on the membrane surface such as a static cake or a fluid region of high component concentration called a polarization layer. 

Zygmunt, S. A., Mueller, R. M., Curtiss, L. A., Iton, L. E., 1998, An Assessment of Density Functional Methods for Studying Molecular Adsorption in Cluster Models of Zeolites , J. Mol. Struct. (Theochem), 430, 9. 

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