Temperature Programmed
Desorption (TPD)TPD analysis permits to identify the strength, the number and the type of active sites that are available on the catalyst surface.
The technique consists in desorbing, by a linear temperature rate, a reactive gas previously chemisorbed on the surface. Usually, the catalyst is first degassed, then reduced by flowing hydrogen, degassed again and finally saturated with a suitable reactive gas. All the above operations are performed in flow. When the test gas saturates the active sites, a linear temperature increase is applied to the reactor.
At a certain temperature, the heat will overtake the activation energy of the gas/solid system, breaking therefore the bonding between the active site and the chemisorbed molecule. An inert carrier flow (typically argon, nitrogen or helium), before entering the reactor, passes through the reference filaments of a thermal conductivity detector. In the reactor, the inert carrier transports the gas molecule that are desorbed from the catalyst surface at different temperatures and finally enters the in the analytical detector channel.
During the desorption process under temperature increase, the detector bridge is unbalanced and the electrical signal is collected by the acquisition software. The volume of gas desorbed by the sample is calculated according the instrument calibration factor.
Finally, knowing the correct stoichiometry of the gas /solid reaction, it is possible to quantify the total number of active sites that are available on the catalyst surface. The desorption temperatures indicate the strength of the active sites in relation to the reactive gas.