The extent of adsorption

The extent of surface coverage is normally expressed as the fractional coverage, θ:

θ=

The fractional coverage is often expressed in terms of the volume of adsorbate adsorbed by θ=V/V∞, where V∞ is the volume of adsorbate corresponding to complete mono layer coverage. The rate of adsorption, dθ/dt, is the rate of change of surface coverage, and can be determined by observing the change of fractional coverage with time. Among the principal techniques for measuring dθ/dt are flow methods, in which the sample itself acts as a pump because adsorption removes particles from the gas. One commonly used technique is therefore to monitor the rates of flow of gas into and out of the system: the difference is the rate of gas uptake by the sample. Integration of this rate then gives the fractional coverage at any stage. In flash desorption the sample is suddenly heated (electrically) and the resulting rise of pressure is interpreted in terms of the amount of adsorbate originally on the sample. The interpretation may be confused by the desorption of a compound (for example, WO₃ from oxygen on tungsten). Gravimetry, in which the sample is weighed on a microbalance during the experiment, can also be used. A common instrument for gravimetric measurements is the quartz crystal microbalance (QCM), in which the mass of a sample laid on the surface of a quartz crystal is related to changes in the latter’s mechanical properties. The key principle behind the operation of a QCM is the ability of a quartz crystal to vibrate at a characteristic frequency when an oscillating electric field is applied. The vibrational frequency decreases when material is spread over the surface of the crystal and the change in frequency is proportional to the mass of material. Masses as small as a few nanograms (1 ng = 10−9 g) can be measured reliably in this way.

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مواضيع ذات صلة

Units

Names of quantities

Working galvanic cells

Electrolysis

Experimental techniques

The rate laws

The electric potential at the interface

The structure of the interface

Catalytic activity at surfaces

Mechanisms of heterogeneous catalysis

Mobility on surfaces

The rate of desorption