Adsorption, comparison with chemisorption

There is a one-point modification of a chemisorption method, which is widely used for measurements of Ac. In this case, only one adsorption point of a chemisorption isotherm is measured, and is compared with only one point on a chemisorption isotherm on a reference material (usually, powder [black] or foil). The identity of the chemisorption properties of the active components in supported and pure form is postulated, but very often does not fulfill, making one-point modification an inaccurate procedure, which can hardly be used in scientific studies. For example, studies of supported Rh catalysts by 02 and CO chemosorption have shown that three different blacks of Rh yield three different results [88], The multipoint comparison of chemisorption isotherms shown that only one black had a chemisorption isotherm that had affinity to the isotherm on a supported metal. 

Generally, the bonding of adatoms other than hydrogen to a metal surface is highly coordination-dependent, whereas molecular adsorption tends to be much less discriminative. For the different metals the bond strength of an adatom also tends to vary much more than the chemisorption energy of a molecule. Atoms bind more strongly to surfaces than molecules do. Here we will discuss the quantum chemical basis of chemisorption to the transition metal surfaces. We will illustrate molecular chemisorption by an analysis of the chemisorption bond of CO [3] in comparison with the atomic chemisorption of a C atom. 

One effect of special interest to electrochemists is the potential-dependent chemisorption of ions and molecules on electrode surfaces. A particularly well-studied example is the adsorption of CO on platinum single-crystal electrodes. In collaboration with the Weaver group at Purdue, we have recently undertaken detailed DFT calculations of the potential-dependent chemisorption of CO on platinum-group (111) surfaces [47,54,55], modeled as clusters, for comparison with the extensive vibrational characterization of these systems as carried out by the Purdue group [56,57]. The electrode potential in these studies is modeled as a variable external electric held applied across the cluster, an approach many others have taken in the past. 

A comparison of this proposed selective oxidation mechanism (Scheme 11) with three other important mechanisms from the literature (Table VI) (27) shows that there is a considerable amount of discrepancy between the assignment of the role of the individual metallic components of the catalyst (72). Matsuura (25), based on low-temperature adsorption studies, attributes chemisorption and first hydrogen abstraction to Mo, while in Haber s... 

Some early proposals for the modes of adsorption of thiophenes on metal sulfides have been probed by comparisons with the structures of well-characterized metal complexes this has allowed the identification of the most reasonable alternatives and of new possibilities not previously considered. Tlieoretical studies on such complexes at increasing levels of sophistication have also contributed in an Important manner to provide a clear and consistent picture of the different possible bonding modes of thiophenes to metal centers. When these theoretical and experimental results from molecular chemistry are combined with the information available from surface techniques and heterogeneous catalysis, the chemisorption of this type of organosulfur compounds on metal sulfides arises as a very well understood phenomenon. This is no doubt one of the most important achievements of the organometallic modeling approach to HDS chemistry. 

The most extensive portion of the literature dealing with the application of absorption spectroscopy to catalysis is concerned with chemisorption. Changes in molecules produced by chemisorption are deduced from differences between the frequencies and intensities of the spectral bands of the adsorbate and those of the free molecule. The structure of the adsorbed species can be reliably ascertained only by direct comparison with spectra of its exact counterparts. When, as is frequently the case, spectra of the reactive intermediates are not available, some structural information can be obtained from changes in the spectrum of the adsorbate that are caused by various physical and chemical treatments e.g., classification of distinct species as strongly or weakly adsorbed can be made from the effects of temperature and pressure on the adsorption bands. 

The results confirm surface changes by the influence of water molecule. Partly during its equilibrium chemisorption the electron charge grows up both for Pt surface atoms and second component atoms in the sublayer. Besides, adsorption heat for water molecule on the binary nanocluster surface increases in comparison with pure platinum. In the result bond energy is weakened for platinum atoms with maximum values for Pt Co anA Thus, Pt oare the most hydrophilic... 

The influence of the Ni atoms becomes clear from a comparison of the actual reaction path, which consists of physical adsorption and subsequent dissociative chemisorption, with the theoretical alternative reaction path, consisting of dissociation of H2 followed by the formation of two Ni-H bonds. H2 is a very stable molecule and, as a consequence, the potential energy of the dissociated H-atoms is very high. In moving to the adsorbed state, Ni-... 

We have applied the FTIR method to investigate the surface properties of NiO/Ni(100) films by following the adsorption of CO. A comparison of FTIR spectra of CO on various surfaces is shown in Figure 3. Figure 3a is a typical CO chemisorption spectrum on a clean Ni(lOO) surface at 300 K. The CO coverage of this surface is about 0.5 ML with all CO molecules occupying the terminal sites on clean Ni(lOO). The vibrational feature observed at 2032 cm-i can be easily assigned to the... 

In comparison, too, direct calorimetric determination of heats of adsorption can be less accurate, although less tedious, than heat-of-immersion determinations. Lack of accuracy can occur with poorly conducting, non-metallic adsorbents, where long equilibrium times are required for vapor phase adsorption or where surface sites do not fill in strict accordance with the site energy distribution of the solid surface. Atoms or molecules can be expected to stick to the first part of the surface they strike when strong chemisorption occurs and then molecules are likely not to move freely over... 

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