Chemisorption and Physisorption

The hydrophobic character exhibited by dehydroxylated silica is not shared by the metal oxides on which detailed adsorption studies have been made, in particular the oxides of Al, Cr, Fe, Mg, Ti and Zn. With these oxides, the progressive removal of chemisorbed water leads to an increase, rather than a decrease, in the affinity for water. In recent years much attention has been devoted, notably by use of spectroscopic and adsorption techniques, to the elucidation of the mechanism of the physisorption and chemisorption of water by those oxides the following brief account brings out some of the salient features. 

Physisorption and chemisorption of water on alumina, titania and ferric oxide selection of results (Morimoto ef a/. )... 

Theoretically, these intermolecular interactions could provide adhesion energy in the order of mJ/m. This should be sufficient to provide adhesion between the adhesive and the substrate. However, the energy of adhesion required in many applications is in the order of kJ/m. Therefore, the intermolecular forces across the interface are not enough to sustain a high stress under severe environmental conditions. It is generally accepted that chemisorption plays a significant role and thus, physisorption and chemisorption mechanisms of adhesion both account for bond strength. 

When an atom or molecule approaches the surface it feels the potential energy set up by the metal atoms in the solid. The interaction is usually divided into two regimes, namely physisorption and chemisorption, which we discuss separately. 

What is the essential difference between physisorption and chemisorption ... 

It is most convenient to explain catalysis using an example. We have chosen a hydrogenation catalysed by nickel in the metallic state. According to the schematic of Fig. 3.1 the first step in the actual catalysis is adsorption . It is useful to distinguish physisorption and chemisorption . In the former case weak, physical forces and in the latter case relatively strong, chemical forces play a role. When the molecules adsorb at an active site physisorption or chemisorption can occur. In catalysis often physisorption followed by chemisorption is the start of the catalytic cycle. This can be understood from Fig. 3.2, which illustrates the adsorption of hydrogen on a nickel surface. 

Two characterization techniques will be discussed in this chapter, viz. physisorption and chemisorption. Physisorption mainly yields information on catalyst texture and morphology, whereas chemisorption studies potentially give information regarding the active catalyst sites. 

The two extremes of physical adsorption and chemical adsorption are abbreviated to physisorption and chemisorption respectively. 

But why do we heat the mixture to effect reaction We will perform a thought experiment imagine immersing iron into nitrogen at different temperatures. We will see the effects of the relative differences between physisorption and chemisorption. 

Several authors reporting cluster ion reactions use the terms physisorption and chemisorption for reaction types 2 and 3, respectively (198,225). The reactions of the Nb clusters with several hydrocarbons have been studied but the reactions... 

Both of these methods are commonly used in the literature for analysis of irregular surface aggregates and porous materials, and both physisorption and chemisorption have been applied... 

Fig. 4.10 Current and targeted range (horizontal bar) of bond strengths for nanocarbons (red), shown against the ranges for physisorption and chemisorption, and compared with the bond strength for high-temperature MgH, and energetic molecules of and H O...

The process of formation of SAMs on the various substrates strongly depends on the nature of all three moieties of the surface active molecule. If a suitable mesogen is chosen, the affinity of the head group toward the substrate determines the kinetics of the physisorption and chemisorption as well as the stability of the resulting layer. 

Figure 10.1. Potential-energy diagram for physisorption and chemisorption of an A-A molecule.

By well defined we mean that all the steps of the catalyst synthesis have been followed stepwise (physisorption and chemisorption of the starting organometallic, chemical transformation on the surface, etc.) by an adequate variety of chemical and physical tools at the disposal of chemists such as the mass balance of any reaction occurring during adsorption, in situ IR, NMR, EXAFS, UV-visible, chemical analysis, and so on. 

There are two main classes of adsorption physisorption and chemisorption. 

The zeolites are also known as molecular sieves because of their capacity to discriminate between molecules they find numerous uses in separation and catalytic processes. Although they appear to be solid particles to the naked eye, they are highly porous, with a typical specific surface area of about 1000 m2/g. Catalysis is discussed in Chapter 9, but the scope of that chapter does not permit detailed discussions of the various types of catalysts and the role of physisorption and chemisorption in catalysis this vignette provides a glimpse of the rationale used in the molecular design of new materials of interest in surface chemistry and how the concepts introduced in Chapter 1 and Chapter 9 fit into the larger scheme. 

The difference between physisorption and chemisorption can be explained using a potential-energy diagram. The potential-energy diagram for physisorption and chemisorption of an A-A molecule (e.g., H2) is shown in Figure 10.1. Curve P in... 

Microporous carbon was also studied as a potential substrate for binding of AChE [24,35]. Discs cut from a commercial porous carbon rod were cleaned and then exposed to a solution of AChE in phosphate buffer for 20 h to allow for simple physisorption and chemisorption of the enzyme. Initial tests using electric eel AChE [35] gave linear detection of dichlorvos in the range 10 6-10 12M. The sensitivity of this method was increased still further by utilisation of the genetically engineered AChE mentioned earlier, with the detection limit of these systems being extended down to 10 17M [35]. 

Of course, 40 kj/mol is an arbitrary cutoff. For particular purposes, we might decide to modify this cutoff between physisorption and chemisorption. 

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. 

For Si(l 11)-(7 x 7), various possible physisorption and chemisorption geometries have been discussed in the framework of a Si25H24PH3 cluster model.54... 

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. 

For now, unimolecular electronic devices must be tested with inorganic electrodes, thinned out to atomic or oligoatomic sharp tips. Molecules either singly, or in parallel as a monolayer array (one molecule thick), have been shown to be either passive or active electronic components. There are two ways to connect a molecule to an inorganic metal electrode physisorption, and chemisorption. 

LEED has been used to determine the structure of a wide variety of surfaces, including clean and reconstructed surfaces of metals and semiconductors, and atomic and molecular physisorption and chemisorption on many different substrates (see part 5). As the theoretical and experimental tools of LEED have improved, the structure of systems with larger and more complex unit cells have been determined. Successful LEED structure determinations have been carried out for systems with several molecules adsorbed in unit cells up to 16 times larger than the substrate unit cell,/1 / and for reconstructed surfaces where the structural rearrangement involves several surface layers./2/... 

The term adsorption is the complex phenomenon of the interaction of an organic or inorganic molecule with a surface. Physisorption and chemisorption are generally distinguished according to the nature of forces involved. 

Adsorption fiom liquids can involve both physisorption and chemisorption however, one needs to consider the molecular interactions between the adsorbing species and the liquid, as well as those with the solid. The relative magnitudes of the liquid/adsorbate and solid/adsorbate interactions will determine the extent... 

The surface tension(i.e., surface free energy) decreases substantially with physisorption and chemisorption(2-3 orders of magnitude). Thus, even under reaction conditions, the pure metal surface tension dominates. 

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