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| Adsorption of Gases on Solids |
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| Adsorption of gases on solids depends on the following factors: |
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| i) Nature of the gas (adsorbate) |
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| ii) Nature of the solid (adsorbent) |
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| iii) Specific area of the solid |
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| iv) Pressure of the gas |
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| v) Temperature |
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| vi) Activation of adsorbent. |
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| How these factors affect adsorption are discussed in the following sections. |
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| i) Nature of the adsorbate |
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| Since physical adsorption is non-specific, every gas gets adsorbed on the surface of any solid. How much will get adsorbed depends on the nature of the gas. Under a given condition of temperature and pressure, the easily liquefiable gases such as NH3, HCl and SO2 are adsorbed more than the gases like H2, N2 and CO. The ease with which a gas can be liquefied is determined by its critical temperature, Tc. Tc is the minimum temperature above which a gas cannot be liquefied, however high the applied pressure may be. This implies that gases with high critical temperature values can be easily liquefied as compared to gases with low critical temperature values. The below table illustrates the correlation between the extent of adsorption of gases on charcoal with their ease of liquefaction. |
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| Volume of gases at N.T.P adsorbed by 1g charcoal at 288 K |
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| Since chemisorption is specific in nature, a gas is chemisorbed only when it forms chemical bonds with the adsorbent. A gas which is physisorbed at a certain temperature can be chemisorbed when the temperature is increased dramatically. For e.g., at 83 K nitrogen (N2) is physisorbed on iron surface as N2 molecules. At room temperature there is no physisorption of N2 on iron surface. However, at 773K nitrogen is chemisorbed on iron surface as nitrogen atoms. |
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| ii) Nature of the adsorbent |
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| The nature of the adsorbent has profound effect on the process of adsorption. Solids with porous structure are potentially good adsorbents. Among them, activated carbon is the most common adsorbent for gases which are easily liquefied. Activated carbon is used in gas masks to adsorb poisonous gases such as methane (CH4). |
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| Activated carbon is prepared from a variety of raw materials including wood, lignite, coal, bone, nut shells and petroleum residues. The new material is activated in an atmosphere of CO2, CO, O2 water vapor air or other selected gases at a temperature between 573 K and 1273 K. This is often followed by quenching in air or water. Adsorption on activated carbon is associated with the presence of small and uniform pores. The surface area of activated carbon is approximately 1000 m2/g. |
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| Apart from activated carbon, silica gel, aluminium oxide and clay are used as adsorbents. It is common knowledge that small satchets containing silica gel are used in electronic equipments such as camera to keep the lenses dry. As mentioned earlier, silica gel is used to adsorb water vapor, which is always present in the humid weather conditions. |
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| iii) Specific area of the adsorbent. |
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| Specific area of an adsorbent is the surface area available for adsorption per gram of the adsorbent. As discussed before, specific area of the adsorbent increases tremendously when finely divided forms of the solid adsorbent are used. Porous solids are very good adsorbents too, however, the pores should be large enough to allow the gas molecules to enter them. |
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| iv) Pressure of the gas - Adsorption Isotherms |
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| The extent of adsorption of a gas on a solid generally increases with pressure. The extent of adsorption is given by x/m, the ratio of the mass (x) of the adsorbate and the mass (m) of the adsorbent when dynamic equilibrium has been attained. The plot of extent of adsorption versus pressure of the gas at a given temperature is called the adsorption isotherm. Adsorption isotherm of different shapes have been observed for various experiments, that is different adsorption isotherms are usually described by different empirical equations. Here, the two most common adsorption isotherms namely, Freundlich isotherm and Langmuir isotherm will be discussed. |
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