Chemisorption enthalpies

First H2 is physically adsorbed on the surface. The forces are weak and the adsorption enthalpy is only slightly negative. Subsequently, chemical bonding between Ni atoms and the hydrogen atoms of H2 molecule takes place while simultaneous dissociation of the H2 molecule takes place. The chemisorption enthalpy is strongly negative (exothermic reaction). 

Figiue 5-15 describes the adsorption of hydrogen. The chemisorption enthalpy increases from group 4 to 6, then decreases. In groups 8-10 it remains almost constant [T20]. 

The chemisorption characteristic is the strong interaction between molecules and surfaces. The chemisorption enthalpy (—AHads) is of the order of 10-100 kcal/mol, thus, of the same order of the reaction involved in chemical bindings. 

The chemisorptive bond A-M is a chemical bond, thus chemisorption is reactant- and catalyst-specific. The enthalpy, AH, of chemisorption is typically of the order of -1 to -5 eV/atom (-23 to -115 kcal/mol, leV/molecule=23.06 kcal/mol). 

In view of the potential-work function equivalence of solid state electrochemistry (Eq. 4.30 or 5.18) and of the fact that for non-activated adsorption, AEd>Pt=0=A AHo,pt, where AHo.pt is the enthalpy of chemisorption of O on Pt, these equations can also be written as ... 

If the reverse of equation (2) above is activated, then v2 = k29x e2iX6- However, in the event that chemisorption of X2 is not activated, all the change in enthalpy of activation will appear in the activation energy (i.e. c, = 1) and... 

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) ... 

There is a wide range of adsorption enthalpies AH(adsi, ranging from effectively zero to as much a 600 kJ per mole of adsorbate. The adsorptive interaction cannot truly be said to be a bond if the enthalpy is small the interaction will probably be more akin to van der Waals forces, or maybe hydrogen bonds if the substrate bears a surface layer of oxide. We call this type of adsorption physical adsorption, which is often abbreviated to physisorption. At the other extreme are adsorption processes for which A//(ads) is so large that real chemical bond(s) form between the substrate and adsorbate. We call this type of adsorption chemical adsorption, although we might abbreviate this to chemisorption. 

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. 

C complexes, 32 185-186 CFjHCFjH, 39 340 chemisorption complexes, 32 170-172 CjH, enthalpies, 37 141, 143 "C-labeling studies, 25 166-172 commercial, 6 197 complex molecules, 30 58-72 medium-sized rings, 30 68-72 polymethylcycloalkanes, 30 59-65 substituted aromatics, 30 65-68 cyclic-acyclic product ratio, 30 8-9 cycloalkanes, 30 68-69 function, hydrogen pressure, 30 12, 15-16 hydrocarbon reaction models, 32 202-205 hydrogenolysis and, 23 93, 103 interconversion, 30 81-82 isopentane, 30 17 label scrambling, 30 7, 12-13 mechanism, 30 5-16 bifunctional, 30 4 catalyst particle size and, 30 72-85 concerted, 30 20... 

Hg/OH on which the standard electrochemical free enthalpy of anion adsorption is positive AGli > 0), only the ph3 ical adsorption of hydrated anions occurs at the OHP whereas, the chemisorption of anions takes place at the IHP on those electrodes whose standard electrochemical free enthalpy of anion adsorption is negative < 0). 

Chemisorption denotes the situation in which an actual chemical bond is formed between the molecules and the surface atoms. A molecule undergoing chemisorption may lose its identity as the atoms are rearranged, forming new compounds that better satisfy the valences of the surface atoms. The enthalpy of chemisorption is much greater than that of physical adsorption. The basis of much catalytic activity at surfaces is that chemisorption may organize molecules into forms that can readily undergo reactions. It often is difficult to distinguish between chemisorption and physical sorption, because a chemisorbed layer may have a physically sorbed layer deposited above it. 

Adsorption desorption. Gas can stick to surfaces either by physisorption or chemisorption. Generally, in physisorption, there is a weak Van der Waals interaction between the surface and the adsorbed species. The enthalpy of adsorption is about the same as the enthalpy of condensation, e.g. the maximum observed values of the enthalpy of physisorption of H2, N2 and H20 are -84, -21 and 57kJmol-1, respectively. (In chemisorption, the adsorbing species sticks to the surface as a result of chemical bond formation. The energies involved are much greater than in physisorption.)... 

Moreover, for coverage close to 1, a sudden decrease of the adsorption enthalpy (Fig. 1) can be explained by adsorption of species such as HS or undissociated H2S. A study of the nickel-sulfur interactions shows that the adsorbed state is energetically more stable than the bulky Ni3S2 sulfide (14). The same result was found for Ir catalysts (15). This shows that the contact of a metal with H2S will lead to a widely covered surface without any sulfur dissolution in the metal. The chemisorption energies of sulfur were also defined on Pt (16), Ir (15), Ru (17), and Fe and Co (18). For example, in the case of Pt, which is known as more resistant than Ni to sulfur poisoning, sulfur is weakly chemisorbed (16). 

It was suggested by Alstrup et al. (19) that the heat of adsorption varies linearly with coverage according to the Temkin law, but Wise et al. (20) proposed a modified Temkin equation taking into account a linear variation of the enthalpy and entropy of sulfur chemisorption. 

Enthalpy changes on adsorption and desorption of probe molecules on catalyst surfaces may also be followed by differential thermal analysis (DTA) (67) although this method has been used only sporadically in the past. The experimental techniques have been described by Landau and Molyneux (67) very recently. As an example, Bremer and Steinberg (68) observed three endothermic peaks during the desorption of pyridine from a MgO-Si02 catalyst these peaks were assigned as three different chemisorption states of pyridine. 

Chromatographic methods are widely used for the study of both physisorption and chemisorption. In its simplest form the technique consists of passing a pulse of the adsorbate through a column of the adsorbent and measuring the retention time and registering the elution curve. Measurement of the variation in the retention time as a function of temperature permits the evaluation of the enthalpy of adsorption, and analysis of the shape of the elution curve provides information about the adsorption isotherm. 

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