9459

Temperature-Programmed Desorption

Catalytic mechanisms and desorption from solids are studied using temperature-programmed desorption. In such a measurement, a molecule is adsorbed on a solid surface, the temperature is ramped linearly with time, and the rate of desorption is measured, usually with a mass spectrometer. This is essentially a small batch reactor whose temperature increases linearly with time. This Demonstration shows temperature-programmed desorption curves for three heating rates (1, 3, and 10 K/s). Changing heating rate is often used to determine heats of desorption. You can change the order of reaction, activation energy, and coverage. The rate of desorption is plotted either as a function of time or as a function of temperature.

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As the heating rate increases, the temperature at which the desorption rate is a maximum (peak temperature) increases, and the maximum rate of desorption increases. When plotted on a time scale, as the heating rate increases, the maximum rate of desorption occurs at shorter times. As the activation energy for desorption increases, the peak temperature increases. This is one of the most widely-used techniques for characterizing catalysts and porous materials and for studying catalytic mechanisms.
Mole balance:
= concentration of A
= saturation coverage
= activation energy
= pre-exponential factor
= ideal gas constant
= temperature
= order of reaction
= time
Temperature-time relation:
= heating rate
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