Thermodesorption

Thermodesorption is a surface science technique in which adsorbed species are removed from a solid surface by controlled heating. It is used to study surface-adsorbate interactions, determine binding energies, and investigate desorption kinetics.

In a typical experiment, a sample is prepared under ultra-high vacuum with a known amount of a gas or vapor adsorbed onto its surface. The surface is then heated with a constant or programmable rate, while the flux of desorbed molecules is monitored by a mass spectrometer or other detectors. The resulting temperature-programmed desorption (TPD) spectrum, which plots desorbed quantity versus temperature, reveals peaks whose position, shape, and area provide information about binding energies, coverage, and surface heterogeneity.

Kinetics of thermodesorption are commonly described by the Polanyi-Wigner equation: rate = ν θ^n exp(-Ea/(RT)), where Ea is the activation energy for desorption, θ is surface coverage, n is the reaction order, and ν is the pre-exponential factor. Desorption order in TPD experiments often depends on coverage: sub-monolayer regimes frequently show first-order desorption, while multilayer desorption can approach zero-order kinetics. Peak temperatures shift with surface coverage and heating rate, reflecting changes in binding strength and coverage-dependent interactions.

Analysis methods include Redhead analysis for estimating Ea under first-order assumptions, and Kissinger or other approaches using multiple heating rates to extract kinetic parameters. More sophisticated fits may model the full TP spectrum with defects, adsorbate interactions, or heterogeneous sites.

Applications span catalyst and material characterization, adsorption energetics, sensor development, and fundamental studies of gas–surface interactions. Experimental accuracy depends on careful calibration, background correction, and consideration of surface cleanliness and coverage effects.