Langmuir chemisorption

Because of the relatively strong adsorption bond supposed to be present in chemisorption, the fundamental adsorption model has been that of Langmuir (as opposed to that of a two-dimensional nonideal gas). The Langmuir model is therefore basic to the present discussion, but for economy in presentation, the reader is referred to Section XVII-3 as prerequisite material. However, the Langmuir equation (Eq. XVlI-5) as such,... 

Since in chemisorption systems it is reasonable to suppose that the strong adsorbent-adsorbate interaction is associated with specific adsorption sites, a situation that may arise is that the adsorbate molecule occupies or blocks the occupancy of a second adjacent site. This means that each molecule effectively requires two adjacent sites. An analysis [106] suggests that in terms of the kinetic derivation of the Langmuir equation, the rate of adsorption should now be... 

Langmuir was first to model chemisorption and to relate the surface coverage, 0a, of an adsorbate A with the gaseous activity or partial pressure, pA, and temperature ... 

In addition to the preparation of Langmuir and Langmuir-Blodgett films, the use of self-assembly techniques also plays an important role in the formation of particle films. Both physisorption, as, for example, electrostatic adsorption of charged particles from colloidal solution, and chemisorption onto a substrate have been investigated. In Section V.A, electrostatic adsorption will be reviewed chemisorption is the subject of Section V.B. 

To understand the chemisorptive property of lr/ln/H-ZSM-5 in a comparison with that of ln/H-ZSM-5, the chemisorption data were analyzed in detail. The data shown in Fig. 3 are relatively well fitted to the Langmuir isotherm, as shown in Fig. 5. From these relations, the equilibrium constant (K) and the amount of NO2 adsorbed at saturation (Vo) were determined according to the following equations ... 

Henero E, Feliu JM, Blais S, Jerkiewicz G. 2000. Temperature dependence of CO chemisorption and its oxidative desorption on the Pt(lll) electrode. Langmuir 16 4779-4783. 

Kardash D, Korzeniewski C. 2000. Temperature effects on methanol dissociative chemisorption and water activation at polycrystalhne platinum electrodes. Langmuir 16 ... 

Park S, Tong YY, Wieckowski A, Weaver MJ. 2002a. Infrared spectral comparison of electrochemical carhon monoxide adlayers formed by direct chemisorption and methanol dissociation on carbon-supported platinum nanoparticles. Langmuir 18 3233-3240. 

The first STM evidence for the facile transport of metal atoms during chemisorption was for oxygen chemisorption at a Cu(110) surface at room temperature 10 the conventional Langmuir model is that the surface substrate atoms are immobile. The reconstruction involved the removal of copper atoms from steps [eqn (1)], resulting in an added row structure and the development of a (2 x 1)0 overlayer [eqn (2)]. The steps present at the Cu(llO) surface are... 

Chemisorption of oxygen at Pt(lll) has been studied in detail by Ertl s group25 and the STM evidence is for complex structural features present in the temperature range 54M60K (Figure 4.14). The limitations of the Langmuir model, frequently invoked for reactions at platinum surfaces, is obvious from... 

Plots of an amount of material adsorbed versus pressure at a fixed temperature are known as adsorption isotherms. They are generally classified in the five main categories described by Brunauer and his co-workers (4). In Figure 6.2 adsorbate partial pressures (P) are normalized by dividing by the saturation pressure at the temperature in question (P0). Type I is referred to as Langmuir-type adsorption and is characterized by a monotonic approach to a limiting amount of adsorption, which presumably corresponds to formation of a monolayer. This type of behavior is that expected for chemisorption. 

Michota A., Kudelski A., Bukowska J., Chemisorption of cysteamine on silver studied hy surface- enhanced Raman scattering, Langmuir 2000 16 10236-10242. 

Jaiswal A, Amaresh RR, Lakshmikantham MV, Honciuc A, Cava MP, Metzger RM (2003) Electrical rectification in a monolayer of zwitterions assembled by either physisorption or chemisorption. Langmuir 19 9043-9050... 

Improved chemisorption methods ( self-assembled monolayers ) and physi-sorption methods (Langmuir-Blodgett films). 

Since the early work of Langmuir (1), the chemisorption of carbon monoxide on platinum surfaces has been the subject of numerous investigations. Besides its scientific interest, an understanding of CO chemisorption on Pt is of considerable practical importance for example, the catalytic reaction of CO over noble metals (such as Pt) is an essential part of automobile emission control. 

Sorption of I O. A systematic investigation of the HjO sorption by analysing the wave propagation data has implied that I O sorption proceeds by two mechanisms, one of which follows a Langmuir isotherm (-AH = 33,5 kJ/mole) and the other one, probably chemisorption or capillary condensation in the micropores of the catalyst, has a capacitance which is saturated even at low partial pressures of t O (see (7)). 

The fitting of space relaxation data using Eq. (1) to this mechanistic scheme (space relaxation data are always isothermal, because transient temperature effects are not relevant for the amplitude change of a concentration disturbance this is just an advantage of wavefront analysis of reaction kinetics), reported in (3, 5, 12), supposing a Langmuir type chemisorption for (CO) and (I O) has confirmed that (see Figure 10, 11) ... 

Poisoning is caused by chemisorption of compounds in the process stream these compounds block or modify active sites on the catalyst. The poison may cause changes in the surface morphology of the catalyst, either by surface reconstruction or surface relaxation, or may modify the bond between the metal catalyst and the support. The toxicity of a poison (P) depends upon the enthalpy of adsorption for the poison, and the free energy for the adsorption process, which controls the equilibrium constant for chemisorption of the poison (KP). The fraction of sites blocked by a reversibly adsorbed poison (0P) can be calculated using a Langmuir isotherm (equation 8.4-23a) ... 

Data of the chemisorption of hydrogen on small copper particles were obtained by Ward (Proc Roy Soc London A133 506, 1031) in the units P, Torr, and V in cc/g STP. The plots show the Langmuir to be a better fit than the Freundlich. 

The Langmuir model first assumes the adsorption sites are energetically identical. Actually, this assumption is not borne out when adsorption occurs predominantly by physisorption. The spread of A/Tads values between the various sites can be as high as 2 kJmol-1, which is often a significant fraction of the overall enthalpy of adsorption when physisorption is the sole mode of adsorption. By contrast, energetic discrepancies between sites can be ignored when adsorption occurs by chemisorption. 

When chemisorption is involved, or when some additional surface chemical reaction occurs, the process is more complicated. The most common combinations of surface mechanisms have been expressed in the Langmuir-Hinshelwood relationships 36). Since the adsorption process results in the net transfer of molecules from the gas to the adsorbed phase, it is accompanied by a bulk flow of fluid which keeps the total pressure constant. The effect is small and usually neglected. As adsorption proceeds, diffusing molecules may be denied access to parts of the internal surface because the pore system becomes blocked at critical points with condensate. Complex as the situation may be in theory,... 

Recent reviews by Ertl and Engel have summarized most of the chemisorption and low pressure catalytic findings . In general, the reaction proceeds through a Langmuir-Hinshelwood mechanism involving adsorbed CO and O atoms. Under reaction conditions typical in most high pressure, supported catalyst studies, and most low pressure UHV studies, the surface is almost entirely covered by CO, and the reaction rate is determined by the rate of... 

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