Adsorption metastable

Systems involving an interface are often metastable, that is, essentially in equilibrium in some aspects although in principle evolving slowly to a final state of global equilibrium. The solid-vapor interface is a good example of this. We can have adsorption equilibrium and calculate various thermodynamic quantities for the adsorption process yet the particles of a solid are unstable toward a drift to the final equilibrium condition of a single, perfect crystal. Much of Chapters IX and XVII are thus thermodynamic in content. 

In this picture, the kinetic barriers hindering the exchange between the two adlayers are related to the presence of metastable, but rather strongly bound, adsorbed species (Hupd and OHad), which cannot be removed easily, and which block the surface for adsorption of the respective other species. The nonequilibrium situation is also reflected in the shape of the corresponding peaks A and A, where the anodic one (A) is less sharp and extends over a larger potential range. 

Adsorbed layers, thin films of oxides, or other compounds present on the metal surface aggravate the pattern of deactivation of metastable atoms. The adsorption changes the surface energy structure. Besides, dense layers of adsorbate may hamper the approach of metastable atom sufficiently close to the metal to suppress thus the process of resonance ionization. An example can be work [130], in which a transition from a two- to one-electron mechanism during deactivation of He atoms is exemplified by the Co - Pd system (111). The experimental material on the interaction of metastable atoms with an adsorption-coated surface of... 

The results of work [ 135] are of specific interest. The work surveyed the influence of the nature and structure of adsorbed layers upon the mechanism of deactivation of He(2 S) atoms. It has been shown that on a surface of pure Ni(lll) coated with absorbed bridge-positioned molecules of CO or NO, the deactivation of metastable atoms proceeds by the mechanism of resonance ionization with subsequent Auger-neutralization. With large adsorbent coverages, when the adsorbed molecules are in a position normal to the surface, deactivation proceeds by the one-electron Auger-mechanism. The adsorbed layers of C2H4 and H2O on Ni(lll) de-excite atoms of He(2 S) by the two-electron mechanism solely. In case of NH3 adsorption, both mechanisms of deactivation are simultaneously realized. Based on the given data, the authors infer that the nature of metastable atoms deactivation on an adsorbate coated metal surface is determined by the distance the electron density of adsorbate valance electrons is removed from the metal lattice. 

Fig. 4.8 The Frumkin adsorption isotherm. The value of the interaction coefficient a is indicated at each curve. The transition in the metastable region is indicated by a dashed...

The phenomena of surface precipitation and isomorphic substitutions described above and in Chapters 3.5, 6.5 and 6.6 are hampered because equilibrium is seldom established. The initial surface reaction, e.g., the surface complex formation on the surface of an oxide or carbonate fulfills many criteria of a reversible equilibrium. If we form on the outer layer of the solid phase a coprecipitate (isomorphic substitutions) we may still ideally have a metastable equilibrium. The extent of incipient adsorption, e.g., of HPOjj on FeOOH(s) or of Cd2+ on caicite is certainly dependent on the surface charge of the sorbing solid, and thus on pH of the solution etc. even the kinetics of the reaction will be influenced by the surface charge but the final solid solution, if it were in equilibrium, would not depend on the surface charge and the solution variables which influence the adsorption process i.e., the extent of isomorphic substitution for the ideal solid solution is given by the equilibrium that describes the formation of the solid solution (and not by the rates by which these compositions are formed). Many surface phenomena that are encountered in laboratory studies and in field observations are characterized by partial, or metastable equilibrium or by non-equilibrium relations. Reversibility of the apparent equilibrium or congruence in dissolution or precipitation can often not be assumed. 

Precipitation can occur if a water is supersaturated with respect to a solid phase however, if the growth of a thermodynamically stable phase is slow, a metastable phase may form. Disordered, amorphous phases such as ferric hydroxide, aluminum hydroxide, and allophane are thermodynamically unstable with respect to crystalline phases nonetheless, these disordered phases are frequently found in nature. The rates of crystallization of these phases are strongly controlled by the presence of adsorbed ions on the surfaces of precipitates (99). Zawacki et al. (Chapter 32) present evidence that adsorption of alkaline earth ions greatly influences the formation and growth of calcium phosphates. While hydroxyapatite was the thermodynamically stable phase under the conditions studied by these authors, it is shown that several different metastable phases may form, depending upon the degree of supersaturation and the initiating surface phase. 

Figure 1 shows the crystallization kinetics of ZSM-48. A good agreement is shown between the crystallinity evaluated by X-ray and adsorption of n-hexane. These kinetic curves confirm the metastability of ZSM-48 zeolite. Indeed the conversion of ZSM-48 into cristobalite, a dense and stable phase, occurs for long reaction times. The difference between the two curves at start reaction times is due to the presence of hydrated silica (Aerosil) that also adsorbs n-hexane. 

Fig. 36. Cross section versus Electron Energy. The cross section for excitation of metastable molecules is from ref. and for adsorption from ref. . The data was taken under very similar conditions in almost identical tubes...

The lowest energy atomic site for H chemisorbed on Cu(l 11) is the fee hollow site with W = 2.3 eV. Smaller and different W exists at the other high symmetry sites. Thus, the PES is very laterally corrugated, both in energy and geometry. In addition, there are metastable subsurface sites inside the surface plane, e.g., one site exists below the fee hollow with W 0.9 eV above that of the most stable surface adsorption site [140]. This is made metastable by abarrier of 0.4 eV relative to the bottom of the subsurface well. Bulk octahedral absorption sites have essentially the same stability as the subsurface sites, with presumably similar barriers to migration into the bulk. Thus, populating the subsurface site represents the initial step in bulk absorption of H. 

The interaction of N2 with transition metals is quite complex. The dissociation is generally very exothermic, with many molecular adsorption wells, both oriented normal and parallel to the surface and at different sites on the surface existing prior to dissociation. Most of these, however, are only metastable. Both vertically adsorbed (y+) and parallel adsorption states (y) have been observed in vibrational spectroscopy for N2 adsorbed on W(100), and the parallel states are the ones known to ultimately dissociate [335]. The dissociation of N2 on W(100) has been well studied by molecular beam techniques [336-339] and these studies exemplify the complexity of the interaction. S(Et. 0n Ts) for this system [339] in Figure 3.36 (a) is interpreted as evidence for two distinct dissociation mechanisms a precursor-mediated one at low E and Ts and a direct activated process at higher These results are similar to those of Figure 3.35 for 02/ Pt(lll), except that there is no Ts... 

In Figure 4 for Pocahontas coal the methane isotherms at —195°, —78°, 0°, 30°, and 50°C., determined in the sequence indicated, are shown as solid curves, and the isotherms at 0°, —78° and —195°C. after the initial sequence are shown as dashed curves. For the Pittsburgh coal, only the isotherms in a rising series of temperatures were determined (Figure 5). Figures 4 and 5 show a plot of methane isotherms at —195°C. on a relative pressure basis (pressure of methane/vapor pressure) because the vapor pressure is only about 10 torr. Isotherms determined at —195°C. represent metastable equilibrium and those at 30°, 50°, and possibly 0°C. equilibrium. Adsorption was... 

To calculate the pore size distributions we have constructed two kernels of theoretical isotherms in cylindrical channels corresponding to the metastable adsorption and equilibrium desorption branches. These kernels were employed for calculating pore size distributions from experimental isotherms following the deconvolution procedure described elsewhere [21, 24] In Figs 6-7 we present the pore size distributions of the enlarged MCM-41 samples [2-4] calculated from the experimental desorption branches by means of the desorption kernel and the pore size distributions calculated from the experimental adsorption branches by means of the adsorption kernel The pore size distributions obtained from the desorption and adsorption branches practically coincide, which confirms that the NLDFT quantitatively describes both branches on the adsorption-desorption isotherm. 

The adsorption of macromolecules is rarely an equilibrium process. Just as the properties of synthetic polymers are often dependent on non-equilibrium processes and relaxation phenomena30), so do the properties of adsorbed proteins depend on time, metastable states, and hysteresis processes. 

Adsorption on the (100) plane is complicated through the reconstruction observed for the Pt and Ir surfaces. On the Rh(100) surface which is unreconstructed, Castner et al. (89) have observed that room temperature adsorption leads to a 2 x 2 pattern at low coverages and a c2 x 2 pattern at higher coverages that presumably correspond to 0 = 0.25 and 9 = 0.5, respectively. Adsorption of 02 on the reconstructed clean Pt(100) surface has not been seen (69, 98, 154), whereas adsorption on the metastable 1 x 1 surface occurs rapidly with the formation of 5 x 1 and 2 x 1 patterns (69,152). The 5 x 1 pattern is formed directly upon exposure to oxygen whereas the 2 x 1 pattern is formed from the 5 x 1 structure after partial reduction with H2 (152). 

The reconstruction of the Ir and Pt surfaces also complicates the adsorption behavior on these metals. Exposure of the reconstructed 1 x 2 Ir(110) surface to oxygen results in a 2 x 2 pattern (54. 124) which on the basis of thermal desorption spectra has been assigned to a coverage of 0 = 0.25 (124), whereas adsorption on the reconstructed 1 x 2 Pt surface leads to a 1 x 2 structure with streaks in the (100) direction (134). Adsorption of 02 on the metastable 1 x 1 Ir(l 10) surface, which is stabilized by the random... 

In the second mechanism the topology of the pore network plays a role [394], During the desorption process, vaporization can occur only from pores that have access to the vapor phase, and not from pores that are surrounded by other liquid-filled pores. There is a pore blocking effect in which a metastable liquid phase is preserved below the condensation pressure until vaporization occurs in a neighboring pore. Therefore, the relative pressure at which vaporization occurs depends on the size of the pore, the connectivity of the network, and the state of neighboring pores. For a single ink bottle pore this is illustrated in Fig. 9.15. The adsorption process is dominated by the radius of the large inner cavity while the desorption process is limited by the smaller neck. 

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