Physical adsorption, definition

A large variety of problems related to the nature of the adsorption processes have been studied by infrared spectroscopy. The most extensive and productive application of this method has been in studies of chemisorption on supported-metal samples. Spectra of physically adsorbed molecules have provided important information on the interaction of these molecules with the surface of the adsorbent. Experimental developments have reached a state where it is evident that the infrared techniques are adaptable to practically all types of samples which are of interest to catalytic chemists. Not only are the infrared techniques applicable to studies of chemisorption and physical adsorption systems but they add depth and preciseness to the definitions of these terms. 

One might be inclined to think that only chemisorption would lead to an enhanced reactivity in this type of catalysis. Adsorption by physical forces only tends to lower the reactivity of the adsorbed molecules. It is, however, difficult to give such definitions of physical adsorption and chemisorption that the fields are clearly separated. We shall, therefore, discuss some of the differences and similarities between these two kinds of adsorption. 

Br0nsted theory, 23 Definition of Ka, 24 Lewis theory, 24 HSAB Theory, 12 Activation energy, 313-317 Chemisorption, 167 Physical adsorption, 167 Activity, 45-48, 51-53 Ionic strength, 45 Free metal-ions in solution, 45 Complex ionic species, 53 Activity coefficients, 45-48 Equations, 46 Ions in water, 21 Single-ions, 51... 

This definition is based on different physical adsorption phenomena of gases in pores of different size. Adsorption interactions of adsorbates are stronger in micropores and modify the bulk properties (density, surface tension) of the adsorbed fluids. The maximum size of ultramicropores corresponds to the bilayer thickness of nitrogen molecules adsorbed on a solid surface (2 x 0.354 nm). 

According to the definition given by lUPAC [46], adsorption is the enrichment [...] of one or more components in an interfacial layer and can be subdivided into chemical adsorption (chemisorption) and physical adsorption (physisorption). In chemisorption, the adsorbate becomes bound to the sohd surface by a chemical bond, forming a monolayer. In physisorption, which is a reversible process, adsorption takes place mainly by van der Waals and electrostatic forces between adsorbate molecules and the atoms composing the adsorbent surface. 

The amount of oxygen chemisorbed decreased with decreasing temperature except at —78°. This anomaly was not found when similar measurements were made with nickel and copper. A tentative explanation may be that at this low temperature, oxygen may be somewhat strongly physically adsorbed in addition to that required for oxide formation. This physical adsorption could accoimt for the larger amounts of oxygen taken up at this temperature on cobalt oxide. However, more experimental work is definitely needed before any decision can be made. 

Adsorption is an exothermic process and the magnitude of the heat of adsorption is used to distinguish chemisorption from physisorption. Heats of adsorption greater than 10 kcal/mol are definitely associated with chemisorbed species. Small heats of adsorption (2-5 kcal/mol) do not always indicate physisorption, however. Therefore, it is best to look at more than one property when trying to distinguish between chemisorption and physisorption. Physical adsorption is used to measure the area of high-area oxide catalysts and oxide-supported metal catalysts. Physisorption isotherms and their use are discussed in Chapter 7. The discussion that follows treats only chemisorption. 

All isotherms should reduce to the Henry s law form at extreme dilution. The Henry s constant is the most important factor for purification. From Eq. 3.1a, B is exponentially proportional to Q, the heat of adsorption. Here Q(Q = -AH) is positive by definition, because AH, the enthalpy change for physical adsorption, must be negative (Yang, 1987). For physical adsorption, Q is equal to the bond energy between the adsorbate molecule and the sorbent. Therefore, the bond energy is critical for purification. Strong bonds are needed for ultrapurification. 

The complete and actual terminology, symbols and definitions dealing with physical adsorption at various interfaces - among them those appropriate for adsorption at the solid/gas and solid/liquid interfaces - were prepared by the International Union of Pure and Applied Chemistry [15,62,66]. 

This task was pursued with such success that Schram" in 1967 wrote that, although no satisfactory theoretical basis has been developed for the equation first given by Dubinin and co-workers for physical adsorption on microporous substrates, the general applicability of this isotherm equation to represent almost all available experimental data has definitely introduced its use in the last few years. Not only is the surface area easily computed, but also isosteric heats have been calculated from this isotherm by several authors . 

The term physical adsorption or physisorption refers to the phenomenon of gas molecules adhering to a surface at a pressure less than the vapor pressure. The attractions between the molecules being adsorbed and the surface are relatively weak and definitely not covalent or ionic. In Table 1 definitions used in this book and in most of the hterature on physisorption are given [1]. 

Adsorbents such as some silica gels and types of carbons and zeolites have pores of the order of molecular dimensions, that is, from several up to 10-15 A in diameter. Adsorption in such pores is not readily treated as a capillary condensation phenomenon—in fact, there is typically no hysteresis loop. What happens physically is that as multilayer adsorption develops, the pore becomes filled by a meeting of the adsorbed films from opposing walls. Pores showing this type of adsorption behavior have come to be called micropores—a conventional definition is that micropore diameters are of width not exceeding 20 A (larger pores are called mesopores), see Ref. 221a. 

Hollabaugh and Chessick (301) concluded from adsorption studies with water, m-propanol, and w-butyl chloride that the surface of rutile is covered with hydroxyl groups. After evacuation at 450°, a definite chemisorption of water vapor was observed as well as of n-propanol. The adsorption of -butyl chloride was very little influenced by the outgassing temperature of the rutile sample (90 and 450°). A type I adsorption isotherm was observed after outgassing at 450°. Apparently surface esters had formed, forming a hydrocarbonlike surface. No further vapor was physically adsorbed up to high relative pressures. 

The physical processes by which natural gas liquids are recovered include phase separation, cooling, compression, absorption, adsorption, refrigeration, and any combination of these. Obviously the definition already stated excludes refinery light volatiles produced by the destructive decomposition of heavy petroleum fractions and it also excludes liquids that may be produced synthetically from natural gas. These distinctions are of economic importance in considering our basic energy reserves. Both the refinery volatiles and the synthetic liquids represent conversion products from other hydrocarbons and the conversion is usually attended by a considerable loss. Thus it has been stated that only about 47% (17) of the energy of natural gas is realized in the liquid hydrocarbon products of the Fischer-Tropsch type of synthesis. 

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