Weakly Bound Adsorbates

For weakly bound adsorbates, vibrational analysis may produce vibrational frequencies close to zero or even negative for bonds with small barriers to translation or rotation. Such calculated vibrational frequencies can produce erroneous results in the thermodynamic properties since Equation (8.20) diveiges as the frequency approaches zero. This situation can be avoided by assuming that the three smallest vibrational frequencies in weakly bound adsorbates are due to rotational and two-dimensional (2D) translational degrees of freedom. Then the state properties for weakly bound adsorbates are 

Assuming the partition function for 2D translation [9], one can easily derive the translational contributions to the state properties by applying the standard thermodynamic definitions as 

The rotational contribution to the state properties is in general one order of magnitude lower than the translational contributions at room temperature. A good approximation is that each rotational degree of freedom contributes to the thermodynamic properties as S =R/2 and Cp =R [15]. 

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Second, catalytic reactions do not necessarily proceed via the most stable adsorbates. In the ethylene case, hydrogenation of the weakly bound Jt-C2H4 proceeds much faster than that of the more stable di-cr bonded C2H4. In fact, on many metals, ethylene dehydrogenates to the highly stable ethylidyne species, =C-CH3, bound to three metal atoms. This species dominates at low coverages, but is not reactive in hydrogenation. It is therefore sometimes referred to as a spectator species. Hence, weakly bound adsorbates may dominate in catalytic reactions, and to observe them experimentally in situ spectroscopy is necessary. 

In this paper it has been shown that IR spectroscopy remains one of the most incisive tools for the study of both strong and weak bonding at surfaces. In addition to being able to study surface species structure in the chemisorbed layer, it is possible to obtain dynamical information about more weakly-bound adsorbates as they llbrate and rotate on the surface. These motions are controlled by local electrostatic forces due to polar surface groups on the surface. 

Cross sections for desorption of adsorbed species are very sensitive to the mode of bonding. In general, weakly bound adsorbates have a much higher cross section than adsorbates which are much more strongly bound. 

Stations and Inglesia [44] simulated for comparison the ordered surface of a crystal obtained by cutting the bulk material, the unrelaxed cut-off amorphous surface, as well as the latter relaxed. The last case was the random structure created by the Monte Carlo sphere packing method. They calculated the adsorption potential surface for some weakly bound adsorbates (N2, Ar, CH4) with the aim of judging the fidelity of the surface models by comparison with the available experimental data on the heats of adsorption and surface diffusivity. The adsorption energy profile in Fig. 5.12 gives an interesting look of the surface from the point of view of the problems discussed in this book the concrete data will be called for an analysis in later sections. 

As there is a difference between the heat of adsorption of this weakly bound adsorbate on the support and the more strongly bound irreversible ... 

As there is a difference between the heat of adsorption of this weakly bound adsorbate on the support and the more strongly bound irreversible adsorption on the metal, it is possible to determine monolayer capacity of the metal by noting the value of coverage at which the heat of adsorption abruptly decreases to give a constant value. Heats of adsorption may be obtained from isochore type measurements where variations in pressure are measured while varying the temperature after exposing the catalyst to different initial amounts of adsorbate. 

Structural information on the electrified interface, under real conditions, can be reliably obtained at the atomic level by AFM (as well as STM - see Chapter 3.1 of this volume). However, it should be noted that caution is often required with interpretation of the image, as the technique in standard use is relatively insensitive to chemical identification. Moreover, for very weakly bound adsorbates, the pressure exerted by the tip as it images the surface has been thought to, in some cases, lead to adsorbate detachment or surface deformation [9]. For the most powerful interpretation of interfacial structure, it is often useful to correlate AFM images with complementary techniques, such as voltammehy and in situ X-ray methodologies. 

XAS has traditionally been rather limited in gleaning the effective surface chemistry involved in catalysis (i.e., weakly-bound adsorbate interactions) due to its bulk-averaging nature. This limitation has been alleviated by the Ap XANES analysis technique pioneered by Koningsberger and Ramaker, and has turned XAS into a truly surface sensitive technique (see Fig. 2a). Only recently, the Ap XANES technique has been applied to the adsorption of H, O, and OH on Pt and Pt-M (M=Cr, Fe, Co, and Ni) cathodes in an electrochemical cell, as well as CO, O and OH on alloyed Pt-Ru electrodes. Results recently obtained have been successful in providing detailed binding site information for adsorbed CO simultaneously with OH, and even coverage levels... 

Electron attachment to the adsorbate is a resonant process, because the electron attaches to the LUMO of the adsorbate. Even for weakly bound adsorbates the resonance is broadened by adsorbate-substrate interaction. However, as illustrated in Figure 27.11, only electrons belonging to a given energy window (marked with dashed lines in the figure) will be effective for attachment. 

Green, I. andYates Jr, J. (2010). Vibrational spectroscopic observation of weakly bound adsorbed molecular oxygen on powdered titanium dioxide, J. Phys. Chem. C, 114, pp. 11924-11930. 

Hereby the concentration of the weakly-bound adsorbed oxygen species which is believed to be mainly responsible for the non-selective surface reactions is minimized, resulting in an enhancement of C2+ selectivity. 

Molecular adsorbates usually cover a substrate with a single layer, after which the surface becomes passive with respect to fiirther adsorption. The actual saturation coverage varies from system to system, and is often detenumed by the strength of the repulsive interactions between neighbouring adsorbates. Some molecules will remain intact upon adsorption, while others will adsorb dissociatively. This is often a frinction of the surface temperature and composition. There are also often multiple adsorption states, in which the stronger, more tightly bound states fill first, and the more weakly bound states fill last. The factors that control adsorbate behaviour depend on the complex interactions between adsorbates and the substrate, and between the adsorbates themselves. 

Owing to their strong bond on Ru(OOOl), mixed COa 0.55 V, the shift of the equilibrium between water and adsorbed OHad/Oad towards the latter increases the density of the respective species in the intermixed adlayer, which increases the repulsions between the adsorbed species and hence leads to more weakly bound OHad/Oad and COad species. These latter species are less stable against COOHad or CO2 formation, because of the reduced reaction barrier ( Brpnsted-Polanyi-Evans relation [Bronstedt, 1928]), and can support a reaction via (14.9) or (14.12), respectively, at low rates. (Note that the total density of the adlayer does not need to remain constant, although also this is possible.)... 

The results of these experiments enable one to draw an unambiguous conclusion that there is no trivial desorption of weakly bound silver atoms because both in the first and the second experiments there were silver atoms adsorbed on the surface, yet, the emission was observed... 

In Sections III(l) and 111(2) the lability principle has been illustrated for processes involving the transfer of weakly bound electrons, including the reactions of solvated and trapped electrons and F-centers and processes of electrochemical generation of solvated electrons. In Sections IV and V, it will be illustrated also by atom transfer reactions and, in particular, by reactions involving adsorbed atoms. 

If the spectrum of adsorbed propylene is observed in the presence of gaseous propylene, additional bands to those shown in Figs. 15 and 16 are observed. These additional bands are due to a more weakly bound form of propylene which is readily removed by a brief evacuation. The salient... 

A simple example of the redox behaviour of surface-bound species can be seen in Figure 2.17, which shows the behaviour of a bare platinum electrode in N2-saturated aqueous sulphuric acid when a saw tooth potential is applied. There are two clearly resolved redox processes between 0.0 V and 0.4 V, and these are known to correspond to the formation and removal of weakly and strongly bound hydride, respectively (see section on the platinum CV in chapter 3). The peak currents of the cathodic and anodic reactions for these processes occur at the same potential indicating that the processes are not kinetically limited and are behaving in essentially an ideal Nernstian fashion. The weakly bound hydride is thought to be simply H atoms adsorbed on top of the surface Pt atoms, such that they are still exposed to the... 

However, Bewick s results require the reassessment of the nature of the weakly bound form of adsorbed hydrogen. It has been since suggested that this form of hydrogen is multiply bonded, sitting between the surface atoms in a multi-coordination site. 

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