The Knudsen cell has been used in determining the kinetics of heterogeneous processes since its introduction by Golden, Spoke and Benton. A Knudsen cell is a high vacuum flow reactor and operates at very low gas pressures such that the mean free path of the gas is greater than the dimensions of the reactor and at least 10 times larger than the size of the exit aperture. Under these conditions the gas molecules undergo gas-wall collisions, instead of collisions with each other.

The Knudsen cell consists of:

  • Gas handling system (GDS) (1)
  • Gas inlet (2)
  • Reaction chamber (3)
  • Detection chamber (4) 

Fig. 1: theoretical scheme of a Knudsen cell

(1) Gas handling system (GDS):

The Gas under study is introduced into the Knudsen reactor by the gas handling system. The gas handling system represents a glass tube with outlets for the pump, for reactor and three stop-cocks. The stop-cocks are used for the connection with the glass flaks containing the reactive gases. The pressure inside is regulated by a membrane pump and measured by an absolute pressure gauge.

(2) Gas inlet:

Reactive gas can enter the Knudsen reactor by two ways determining different types of measurements: (a) Steady state measurements: .the gas is introduced constantly through a capillary. (b) Pulse measurements: the gas is introduced through an electromagnetic valve. The gas leaves the reaction compartment through a small orifice with an adjustable diameter. This orifice determines the residence time and the concentration of the gas molecules in the chamber.

(3) Reaction chamber:

The reaction chamber consists of a stainless steel cross, whose inner walls are coated with a fluoropolymer in order to eliminate adsorption or reaction of the gas species at the wall surfaces. The sample is placed on the bottom of the reaction chamber and can be separated by a movable plunger. The sample compartment represents only 2% to 4% of the total volume of the reaction chamber.

(4) Detection chamber:

After escaping the reaction chamber through the orifice, the reactive gas species enter the differently pumped vacuum chamber housing the quadrupole mass spectrometer (QMS), in which the gas beam is modulated by a chopper operating at 70 to 80 Hz and finally detected by a QMS. The pressure inside the vacuum chamber is regulated by two turbo molecular pumps and measured by two compact full range gauges

Fig. 2: Experimental Setup Knudsen cell