Adsorption of CO and

Even if it is assumed that the reaction is ionic, Occam s Razor would lead to the conclusion that the system is too complex and that the effort to keep it ionic is too great. It is difficult to undersand why step 8c is slow and why a simple uncharged complex would not be equally reasonable. We prefer a mechanism in which the carbon monoxide molecule is adsorbed parallel to the surface and in which the oxygen orbitals as well as the carbon orbitals of C=0 bond electrons interact with the metal. It seems reasonable that hydrogenolysis occurs exclusively only because the oxygen is held in some way while the two bonds are broken and it finally desorbs as water. The most attractive picture would be (a) adsorption of CO and H2 with both atoms on the surface... 

The volcano curve is bounded by the rate of dissociative adsorption of CO and hydrogenation of adsorbed carbon. This is illustrated in Figure 1.9. 

In this study, Pt/AliOj having high activity for CO oxidation and different affinities for fee adsorption of CO and Hi was selected as a catalyst/adsorbent In a conventional packed bed reactor (PBR), fee surface of fee catalyst is dominantly covered by COads with small amotmt of Oads fee CO conversion is therefore low. Several investigations on periodic operation have illustrated feat fee reaction front wife comparable amount of fee two adsorbed species leads to enhancement of fee CO conversion. Conceptually, this type of the reaction front should be generated by application of a CMBR, as well. Figure 1 illustrates an image of... 

As an introductory example we take one of the key reactions in cleaning automotive exhaust, the catalytic oxidation of CO on the surface of noble metals such as platinum, palladium and rhodium. To describe the process, we will assume that the metal surface consists of active sites, denoted as We define them properly later on. The catalytic reaction cycle begins with the adsorption of CO and O2 on the surface of platinum, whereby the O2 molecule dissociates into two O atoms (X indicates that the atom or molecule is adsorbed on the surface, i.e. bound to the site ) ... 

Therefore, a bifUnctional mechanistic scheme, including the participation of both the metal (via the adsorption of CO) and the support (via the formation of "oxygen vacancies" which are active sites for the H2O dissociative adsorption) seems quite relevant to explain the specific behavior, for the NO r uction in the presence of water, of samples containing Zr02 Such active sites would be located at the met -support interface and are linked to the redox properties of the support... 

The first IR studies detected the formation and adsorption of CO, and therefore CO was proposed as the poisoning intermediate [Beden et al., 1981 Nichols and Bewick, 1988 Corrigan and Weaver, 1988]. The formation of CO is stmcture-dependent and takes place at open circuit, and the maximum amount accumulated on the electrode... 

After adsorption of CO and solution exchange with pure base electrolyte, the oxidation of adsorbed CO during a triangular potential scan is observed (see Fig. 1.4a). In a second run after adsorption of CO the electrode is emersed and transferred to the UHV chamber in the same way as in the normal experimental procedure. The electrode is then transferred back to the cell and re-immersed in the base electrolyte. A potential scan is applied to oxidize the adsorbate. Fig. 1,4b shows... 

Combined with their kinetic measurements, the authors proposed CO from the gas phase could directly react with oxygen atoms in the surface oxides, accounting for relatively high reactivity of this phase for CO oxidation. This mechanism, termed as Mars-Van Krevelen mechanism, challenges the general concept that CO oxidation on Pt group metals is dominated by the Langmuir-Hinshelwood mechanism, which proceeds via (1) the adsorption of CO and the dissociative adsorption of 02 and (2) surface diffusion of COa(j and Oa(j atoms to ultimately form C02. 

The outcome of this research naturally raised the question whether the catalysts investigated differed also in chemisorptive and catalytic properties. We therefore examined how the adsorption of CO and CO2, and the reaction of benzene with deuterium proceed on these catalysts. This study forms the subject of the present article. 

On the basis of the nature of CO adsorption and of the nature of chain initiator intermediates, popular mechanistic proposals include the carbide mechanism,1-2 wherein CO adsorbs dissociatively and the carbide (C ) is the chain initiator intermediate, and the enolic mechanism,3 involving molecular adsorption of CO and the formation of an oxygen intermediate, the enol (HC OH). 

Figure 1.1 Schematic representation of a well known catalytic reaction, the oxidation of carbon monoxide on noble metal catalysts CO + Vi 02 —> C02. The catalytic cycle begins with the associative adsorption of CO and the dissociative adsorption of 02 on the surface. As adsorption is always exothermic, the potential energy decreases. Next CO and O combine to form an adsorbed C02 molecule, which represents the rate-determining step in the catalytic sequence. The adsorbed C02 molecule desorbs almost instantaneously, thereby liberating adsorption sites that are available for the following reaction cycle. This regeneration of sites distinguishes catalytic from stoichiometric reactions.

Adsorption kinetics. We can also study the adsorption kinetics of the nitrile component. This is illustrated by the IRRAS spectra shown in Figure 3, which demonstrate the influence of electrode potential on the competitive adsorption of CO and CjH CN. Curves a and b show the control experiments, in which spectra were recorded-at different potentials in saturated CO electrolyte with no nitrile added. A saturated CO layer is produced in both cases, but the frequency is different at the two potentials i.e., v(CO) 2085 cm at 0.55V, vs. v(C0) 2070 cm at 0.05 V. The magnitude of this shift is in agreement with the potential dependence of v(C0) discussed above. 

The next set of examples show an entropy of adsorption roughly equal to the entropy change on losing the degree of translational freedom normal to the surface, i.e., in the adsorbed state the molecules are equivalent to a two-dimensional gas or vapor. The data for a variety of different adsorbates and adsorbents is given in Table V. The isotherms obtained by Armbruster (20) for the adsorption of CO and N2 on silver were not S-shaped, and they could be fitted to equations of the Langmuir type. The amount of adsorbate required to saturate the surface was given for each substance at both temperatures. Armbruster calculated the heats of adsorption by the method of Brunauer, Emmett and Teller (22) and there is some doubt about the validity of such heats. 

Some stability constants for ion pairs on Fe oxides are listed in Table 10.4. This model was applied by Davis and Leckie (1978, 1980) to adsorption of various cations and anions on ferrihydrite. The extended triple layer model of Sahai and Svenjensky (1997) incorporates recent advances in aqueous electrolyte chemistry which enable aqueous activity coefficients for electrolytes to be calculated over a wide range of ionic strengths. The model also considers the free energy of adsorption of an ion to be the sum of the contributions from an electrostatic term, a Born solvation term and a ion intrinsic term. This extended model has been applied to adsorption of Co and Cd on goethite. 

In Skirmer, H.G.W. Fitzpatrick, R.W. (eds.) Biomineralization processes of iron and manganese. Catena Verlag, Cremhngen-Destedt, Catena Suppl. 21 75—99 Ghoneimy, H.F. Morcos.T.N. Misak, N.Z. (1997) Adsorption of Co and Zn ions on hydrous Fe(lll), Sn(lV) and mixed Fe(lll)/ Sn(IV) oxides. Part 1. Characteristics of the hydrous oxides, apparent capacity and some equilibria measurements. Colloids Surfaces A. 122 13-26... 

NOg exceeded the standards. To reduce adsorption of CO and NO 2 on the dust of blasted material, it was recommended to drill... 

Sequence II O2—CO. Oxygen is first adsorbed on NiO(250) at 30°C. The sample is then evacuated at 30°C. (amount of irreversibly adsorbed oxygen, 1.9 cc. per gram), and carbon monoxide is adsorbed at the same temperature (Figure 3). The electrical conductivity of nickel oxide containing preadsorbed oxygen 1.8 X 10 5 (ohm cm.)"1 decreases during the adsorption of CO, and at the end of the adsorption, is identical to the conductivity of the pure oxide. Moreover, carbon dioxide is condensed in the cold trap. This shows that all ionized species are transformed into neutral species at the end of the interaction. 

Interactions 2 and 4 represent a Rideal mechanism and Interactions 2a and 4a a Langmuir-Hinshelwood mechanism. However, to form C03"(adS), by Interaction 1 in Mechanism I, CO must first be adsorbed since Interaction 1 is a fast process, the adsorption of CO would be the slow step of Mechanism I, and the kinetics of the reaction would depend on pco- However, it has been shown (8, 28) that the reaction is zero order with respect to CO, and therefore the adsorption of CO and its conversion to COa udsi are faster processes than Interaction 2 which is the rate-limiting step and hence may be written in the form of 2a (Langmuir-Hinshelwood mechanism). 

These surface modifications have little influence on the reactivity of the oxide towards the adsorption of CO and C02. Both gases must be adsorbed at room temperature on Ni2+ ions as has been shown through different experimental results (23, 34). However, participation of anions in the mechanism of adsorption of both gases is probable since oxygen from either CO or C02 is exchangeable with lattice anions at room temperature (2,3). 

It is probable that the conditions holding in the experiments of Dell and Stone are not to be compared too closely with those on the pure oxide. Teichner and Morrison (51) studied the adsorption of CO and 02 on finely divided NiO prepared by ignition of Ni(OH)2 in vacuo below 210° their material behaved as a normal p-type oxide should, in that CO was chemisorbed to a greater extent than 02 (Dell and Stone found the opposite), and they suggest the NiO films of Dell and Stone contained free metallic Ni. Teichner and Morrison found that the product of reaction of CO and 02 was stable on the surface at room temperature but corresponded in formula to something between C02 and CO 3 the reaction of C02 with the surface was not recorded, neither were any experiments carried out on the CO-oxidation. 

Fig. 6. Adsorption of CO and C2H4 on some Ni/SiOs catalysts in relation to each other.

As a whole, considerable experimental data have been accumulated in the literature concerning the influence of the gas phase on the surface state of adsorbents and catalysts. The surface state is used specially to preliminarily produce the required surface composition of a catalyst. At the same time, quantitative characteristics of these changes are mainly available only for binary alloys contacting with molecules of H2, CO, and 02 [41,42]. Investigations conducted in recent years reveal that the influence of adsorbed particles on the state of a solid surface is apparently more significant than is customarily considered at present. The structural transformations in the surface layers (an example is the rearrangement of the surface layer of platinum in the adsorption of CO and 02 molecules [43]) and the processes of formation of new phases in them, which are similar to three-dimensional topochemical processes [44], may be of a major significance. 

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