Physisorbed state

Frequently, adsorption proceeds via a mobile precursor, in which the adsorbate diffuses over the surface in a physisorbed state before finding a free site. In such cases the rate of adsorption and the sticking coefficient are constant until a relatively high coverage is reached, after which the sticking probability declines rapidly. If the precursor resides only on empty surface sites it is called an intrinsic precursor, while if it exits on already occupied sites it is called extrinsic. Here we simply note such effects, without further discussion. 

CO occupies the atop site. The authors argue that adsorption, even at 4 K, is in the chemisorbed state with the molecular axis oriented perpendicular to the surface. In a physisorbed state, variations in the orientation, including where the C-0 axis is parallel to the surface, would be expected to maximise the van der Waals interaction. The oxidation of CO at Cu(110) is discussed elsewhere (Chapter 5). 

A combination of matrix isolation IR and DFT studies have shown that condensation of monomeric NiX2 (X = C1, Br) in solid N2 matrices initially gives species NiX2- -(N2) with a physisorbed type of interaction. These can be converted by photolysis into pseudotetrahedral NiX2(r/l-N2)2 (115) moieties with end-on bound N2 in a chemisorbed type of interaction below 10 K. On warming to ca. 20 K the chemisorbed species revert to the physisorbed state.457... 

Figure 5.22 Lennard-Jones potential of hydrogen approaching a metallic surface. Far from the metal surface the potential of a hydrogen molecule and oftwo hydrogen atoms is separated by the dissociation energy. The first attractive interaction ofthe hydrogen molecule is the van der Waals force leading to the physisorbed state. Closer to the surface the...

Ex situ measurements in the presence of dissolved oxygen have proved that the mixed monolayer was stable in the solution free of 6TG and guanine. Madueno etal. [Ill] have also studied adsorption and phase formation of 6TG on mercury electrode. At high potentials, the molecules were chemisorbed and were able to form a self-assembled monolayer. When the potential was scanned to more negative values, reductive desorption of the monolayer was observed. Cathodic voltam-metric peaks, which are typical of a 2D condensed phase transition, divided the potential window into two regions one, in which self-assembled monolayer was stable, and the second, in which a physisorbed state existed. 

The adsorption of saturated hydrocarbons on metallic substrates is typically considered as an example of a weak physical interaction, which is dominated by van der Waals forces. The classification of this type of interaction, denoted physisorption where no direct chemical bonds are formed between the adsorbate and substrate, has been based on the heat of adsorption. A physisorbed state is considered to be one in which the heat of adsorption is comparable to the heat of vaporization or... 

Another PES topology for molecular dissociation occurs when an intermediate molecularly chemisorbed state lies parallel to the surface between the physisorption well and the dissociated species as shown in Figure 3.2(b). This molecular state is usually described in terms of a diabatic correlation to a state formed by some charge transfer from the surface to the molecule [16]. In this case, there can be two activation barriers, V] for entry into the molecular chemisorption state of depth Wx and barrier V2 for dissociation of the molecularly chemisorbed state. This PES topology is relevant to the dissociation of some it bonded molecules such as 02 on metals, although this is often an oversimplification since distinct molecularly adsorbed states may exist at different sites on the surface [17]. In some cases, V < 0 so that no separate physisorbed state exists [18]. If multiple molecular chemisorption... 

Spectroscopic measurements of 02 adsorption at very low Ts (and low En) demonstrate that only the physisorbed state is populated under this condition, and this converts thermally to chemisorbed 02 at Ts = 35 [30,325]. It was therefore proposed that the dissociation of 02 at low E is due to sequential precursors the physisorbed state acts as a precursor to molecular chemisorption and the molecularly chemisorbed state is a precursor to dissociation [30]. This can be written as the following kinetic equation ... 

Blaszkowski et al. (221) demonstrated that the methanol molecule is capable of adsorbing in a physisorbed state in two different modes, the end-on mode, shown in the first part of Fig. 12, and a side-on mode, shown in Fig. 13a. In this side-on mode, a C-H bond of the methanol CH3 group is directed toward the zeolitic basic oxygen site, while the acidic zeolite proton retains its strong hydrogen bond with the methanol oxygen. The authors used TST (4) to determine the equilibrium constants for the two modes of adsorption from the computed adsorption energies. The equilibrium constant for the side-on mode is a factor of 106 smaller than that for the end-on mode at 300 K. Thus, nearly all methanol molecules adsorb in an end-on manner, but the dehydration reaction necessitates conversion to the side-on form. 

Figure 71 shows the 13C spectra of pyridine at different loading levels and after a pretreatment with HC1. It is seen that the resonance of the y carbon of pyridine (middle peak) is very sensitive to its chemical state, and Maciel et al. developed a model of pyridine exchange between Bronsted, Lewis, and hydrogen-bonding acid sites as well as the physisorbed state. The same authors used the 15N signal of a fixed quantity of adsorbed pyridine as a kind of urface indicator. Pyridine is displaced by n-butylamine according to the sequence... 

The methane might be supposed to react either from the gas phase or from a physisorbed state. 

Several other studies have appeared for T1 deposition on polycrystalline Ag which are pertinent to this discussion. The work of Furtak et al. [148] shows the sensitivity to the deposition of thallium on mechanically polished silver surfaces. They attribute a decrease in observed intensity prior to T1 deposition to a physisorbed state of T1 on the silver surface. In another study, Robinson and Richmond [54] performed time-resolved measurements on the deposition of T1 on electrochemically polished polycrystalline silver. Sharper UPD peaks were observed in this study compared to the previous work. The interesting aspect of this work was the sequential nature by... 

Broad Lines. - The width of an n.m.r. line, At>1/2, is defined as the width in Hz at half signal height. Narrow lines, i.e., Ap1/2 < 10 Hz, are desirable in order to make use of chemical shift information and to follow chemical change. N.m.r. line widths in the liquid and the physisorbed state tend to be very narrow, with Ar>1/2 of the order of 10-1Hz. This fortuitous state arises because the molecular motion is sufficiently rapid and random in a liquid to average out the line broadening features present in solids, namely dipolar interactions, chemical shift anisotropy, quadrupolar interactions, and paramagnetic interactions which render the spectrum unusable under conventional or liquid-state experimental conditions. The mechanisms of each of these features will be described. The treatment will perforce be cursory, but an indication will be given to where a full theoretical treatment can be found. 

Figure 1 One-dimension potential energy diagram for precursor-mediated chemisorption of molecule R-H. The dotted line barrier above the vacuum zero is for an activated system, whereas the solid line barrier below the vacuum zero is for a facile system. Ed is the activation energy of the physisorbed state and Er is the activation barrier to the chemisorbed state.

Both physisorption and chemisorption contribute to S0. For Ag(l 00) the physisorbed part is dominant, while for the stepped surfaces substantial chemisorption occurs already at the lowest translational energies as demonstrated by the independence of S0 on crystal temperature up to 140 K, as shown in Fig. 16. The lifetime of the physisorbed state diminishes indeed rapidly when T is increased from 105 to 140 K, while the initial sticking probability does not. 

Figure 9 Adsorption process of NO on Pd particles supported on MgO(l 00). (a) Global adsorption probability as a function of surface temperature and for various particle sizes (from Ref. [89]). (b) Schematic representation of die elementary processes in die molecular adsorption of NO on supported Pd particles (1) quasi-elastic redection on die bare support, (2) physisorption-diffusion-desorption from the bare support, (3) direct chemisorption on die Pd particles, (4) NO chemisorption on the Pd particles via a precursor physisorbed state on die bare support. Xs is die mean diffusion length of die NO molecules on the support and p is die width of die collection zone around die Pd particles.

Similar observations were reported by Heegemann et al. (85). They observed the value of 9 at saturation coverage to be 0.5 and 0.38 for the (100) and (111) planes, respectively, when S2 was adsorbed on these planes. Saturated layers of Pt(lll) and (100) adsorbed more sulfur at room temperature, giving values of 9 of 1.12 and 1.08, respectively. Evacuation at room temperature reduced the value of 9 to 0.92 for both surfaces, indicating some fraction of S2 to be in a physisorbed state. When both these surfaces were heated to temperatures above 575 K, values of 9 = 0.5 and 0.38 were obtained for the (100) and (111) planes, respectively further continued heating to 723 K resulted in no further reduction in the value of 9. 

Other extensions on the ZGB model include addition of an Eley-Rideal step in which A from the gas phase reacts with adsorbed B fragments [58] and addition of physisorbed states [45,59]. The Eley-Rideal step has the same effect as inclusion of associative B desorption the second-order phase transition is removed. In other words, the value of Yj shifts to Yi = 0. Inclusion of precursor states for A makes the surface more reactive and lowers the transition to A poisoning (Ti) to lower values. 

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