Irreversible chemisorption

The acid monolayers adsorb via physical forces [30] however, the interactions between the head group and the surface are very strong [29]. While chemisorption controls the SAMs created from alkylthiols or silanes, it is often preceded by a physical adsorption step [42]. This has been shown quantitatively by FTIR for siloxane polymers chemisorbing to alumina illustrated in Fig. XI-2. The fact that irreversible chemisorption is preceded by physical adsorption explains the utility of equilibrium adsorption models for these processes. 

The described adsorption phenomena are characteristic for electrodes of sp metals. Transition metal electrodes are usually connected with irreversible chemisorption phenomena, discussed in Section 5.7. 

These and other data (10) show that hydrogen chemisorption is operationally of two types Type I chemisorption which is removed by evacuation for 15 min at room temperature, and type II chemisorption which is not removed by evacuation at room temperature even after several hours. The type I chemisorption appears to be independent of the amount of type II chemisorption (compare runs 3 and 5). Figure 2 show s an isotherm for type I adsorption, as defined. This is a typical curve for chemisorption and suggests that type I chemisorption occurs on sites corresponding to roughly 5% of the BET Vm value. (The designation type I and type II chemisorption was chosen in preference to fast and slow because not all of the type II chemisorption is slow. For example, the amount of adsorption in curve 1 of Fig. 1 is 0.154 cm3/gm after 2 min. We would estimate at least one-third of this adsorption is type II. Thus, some type II irreversible chemisorption is quite rapid.)... 

Values for the irreversible chemisorption of CO on Pt/Si02 and PtSn-BM are included in Table 6.3. The tendency is similar to that of hydrogen isotherms. The CO/Pt value changes from 0.56 to 0.25 when the monometallic catalyst is modified by hn addition at an atomic ratio of Sn/Pt = 0.71. 

Heats of adsorption are due to three fundamental processes, physical adsorption, reversible chemisorption, and irreversible chemisorption. Physical adsorption can be attributed to van der Waals forces (dispersion forces). Reversible and irreversible chemisorption are due to bond formation between the adsorbent and the adsorbate. Generally heats of adsorption less than 10-15 kcal/mole are considered due to physical adsorption alone, although some physical adsorption processes may exceed these values. Any chemisorption process generally involves all three processes so that the heat of adsorption value reflects the sum of the average contribution of each. The interaction of transition metals with unsaturated compounds is attributed by most authors, at least in part, to chemisorption. 

The irreversible chemisorption of Garner is thus shown to be, in fact, reversible at higher pressures w ithout appreciable reduction of the surface. Nevertheless, on pumping out at say 250° after an exchange reaction some CO was held tenanciously by the surface and could be removed rapidly at this temperature only by the addition of 02 the total unsaturation towards 02 amounted to <5% of the surface. During the exchange reactions of CO on ZnO at 2-cm. pressure, a small amount of C02 was... 

Degassing of the silica results in a complete desorption of the hexamethyldisiloxane. Irreversible chemisorption of alkoxysiloxanes starts at reaction temperatures above... 

Quantitative and qualitative changes in chemisorption of the reactants in methanol synthesis occur as a consequence of the chemical and physical interactions of the components of the copper-zinc oxide binary catalysts. Parris and Klier (43) have found that irreversible chemisorption of carbon monoxide is induced in the copper-zinc oxide catalysts, while pure copper chemisorbs CO only reversibly and pure zinc oxide does not chemisorb this gas at all at ambient temperature. The CO chemisorption isotherms are shown in Fig. 12, and the variations of total CO adsorption at saturation and its irreversible portion with the Cu/ZnO ratio are displayed in Fig. 13. The irreversible portion was defined as one which could not be removed by 10 min pumping at 10"6 Torr at room temperature. The weakly adsorbed CO, given by the difference between the total and irreversible CO adsorption, correlated linearly with the amount of irreversibly chemisorbed oxygen, as demonstrated in Fig. 14. The most straightforward interpretation of this correlation is that both irreversible oxygen and reversible CO adsorb on the copper metal surface. The stoichiometry is approximately C0 0 = 1 2, a ratio obtained for pure copper, over the whole compositional range of the... 

Several observations have already indicated that the amorphous copper solute induces irreversible chemisorption of carbon monoxide, and it therefore behooves us to correlate the methanol yields with the surface concentration of irreversibly chemisorbed CO. The best understanding of the synthesis pattern is provided with the help of the representation in Fig. 17. The turnover rate, defined as the number of CO molecules converted to methanol in 1 sec on one site titratable by irreversibly bound carbon monoxide, depends smoothly on the concentration of these sites for each zinc... 

FiC5. 14. NMR spectra of Ru/SiOi. The initial number of silanol protons has been reduced by exchange with deuterium. Both traces are difference spectra with respect to the state after initial evacuation. The continuous line represents a sample under 20 Torr of H, gas, and the dashed line represents a sample after pumping away the reversible hydrogen. There is both reversible and irreversible spillover to the support (signal at. 3 ppm), and ther e is rever sible and irreversible chemisorption on the metal (sigiral at 65 ppm). [Reproduced with permission from Uner et al. (47). ... 

The studies of Garner and his co-workers in the years 1928-1939, which had established the existence of two types of carbon monoxide and hydrogen chemisorption on oxides and which identified irreversible chemisorption with incipient reduction, were followed in the immediate postwar period by an intensive study of the properties of copper oxide (12-15). The work was later extended to nickel oxide (16) and cobalt oxide (17,18). With each of these oxides it was established that carbon monoxide was capable of reacting not only with lattice oxygen, but also with adsorbed oxygen. The concept of irreversible chemisorption involving a carbonate ion and ulti-... 

Elemental analysis, solid-state, and solution EUR measurements demonstrate the removal of three molecules of hexyne per organometallic group deposited, along with the formation of Snfc . -0-Sn-C bonds. The presence of a maximum chain density from a certain precursor concentration dismisses the possibility of any continuous polycondensation of 1 at the oxide surface, which is consistent with the irreversible chemisorption of 1 as a monolayer. Moreover, the modified Sn02 materials show remarkable chemical stability both in organic and aqueous media in the pH range 3-10. 

Chemical adsorption (chemisorption) occurs when the attraction between the adsorbent and the adsorbate can form a covalent bond, or when a chemical reaction occurs between the adsorbent and adsorbate. Usually chemical adsorption will only allow a single layer of molecules (monolayer) to accumulate on the surface of the adsorbent. Chemical adsorption is usually irreversible. Chemisorption is typically more specific, kineticaUy slower and has a larger heat of adsorption (>3AHvap). 

Similar set of results for oxygen chemisorption ( first and second cycles, Fig.4) reveal more interesting aspects. OTj (25) increases with Sn content, due to irreversible chemisorption of oxygen by Sn while OT2 (25) follows the reverse trend. Thus OT2 (25) represents chemisorption due to unalloyed Pt only. Both OT, and OTj at 150°C increase with increasing Sn, due to the fact that at 150°C, both alloyed and unalloyed parts of Pt as well as alloyed Sn chemisorb oxygen. 

Structure of Active Sites. - Klier and others have claimed that the active phase is a Cu" species dissolved in ZnO. Estimating the amount of dissolved Cu" reflected irreversible chemisorption of CO in proportion to the dissolved Cu" ". The existence of Cu in the active state is verified by means of Auger electron spectroscopy (AES), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), but it is also pointed out that the Cu" concentration depends upon the total content of Cu in the catalyst. 

Using i.r. evidence, a mechanism may be postulated whereby SO2 is adsorbed on hydroxyl sites and COS on aluminium sites on the oxide. The latter sites become blocked by the irreversible chemisorption of COg. The reaction of S02 with carbon at elevated temperatures has been shown to involve the following reactions ... 

A typical kinetic response of conductance as a function of an introduction of a concentration step is shown in Fig. 8.2. After the reducing species is introduced at time ti, the sensor conductance G, increases to Gf, in the time needed to reach the new thermodynamic equilibrium of the surface reactions. If the metal oxide is not stable, or if there is an irreversible chemisorption, a steady state may not be reached. 

This section summarizes the investigations dealing with purely physical adsorption of PH3 on adsorbents like activated carbon or silica gel, dissociative adsorption on various adsorbents which can already occur at low temperatures, and irreversible chemisorption involving the decomposition of PH3 on certain surfaces at higher temperatures. The products formed by PH3 reacting with the surface atoms were generally not studied. This section also includes studies dealing with the coadsorption of PH3 with Hg, Dg, O2, HgO, DgO, or CO on various surfaces. 

The metal areas of the unactivated metal-carbons (226A and 249A) measured by Hj chemisorption appeared to be zero ie the reversible and irreversible chemisorption isotherms were co-incident. Therefore, although a small amount of reversible chemisorption did take place, this was shown to be due to chemisorption on the carbon as chemisorption isotherms for the carbon containing no metal and sample 249A (Rh/C) were virtually superimposable. 

Industrial adsorption processes normally are cyclic processes in which adsorption and desorption steps of the sorbent material alterate periodically. Often the desorption or regeneration step is cmcial and essentially determine the period and the energetic efficiency of the cycle [1.2, 1.14-1.16]. An important quantity to characterize the desorption process is the (molar) enthalpy (AH ) needed to desorb the leading component either of product or waste - of a gas mixture from the sorbent. In Table 1.2 some examples of desorption processes and their industrial applications together with typical values of the molar desorption enthalpy are given. Summarizing it can be stated that in reversible physidesorption processes molar enthalpies of about (10-50) kJ/mol are needed whereas in irreversible chemisorption processes (70-200) kJ/mol are necessary for desorption. ... 

This is the result of an essentially irreversible chemisorption of some impurity in the feed stream. 

The irreversibility of adsorption of phenols has been established since the late 1960s. The dominant features of irreversibility (chemisorption of phenol) were summarized by Magne and Walker (1986) and include (a) irreversibility increases with time spent by the phenol on the carbon surface, (b) irreversibility increases with increasing temperature of the system, (c) irreversibility is inhibited by the presence of surface oxygen functionality on the carbon surface, (d) the sites of formation of chemisorbed phenol are oxygen-free sites at the edges of graphene layers and (e) physisorption occurs over the entire surface of the carbon. 

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