desorptionsmaxima

Desorptionsmaxima, literally desorption maxima, denotes the peaks that appear in the rate of desorption from a surface during desorption experiments. They are most commonly observed in temperature-programmed desorption (TPD), where a surface bearing adsorbed species is heated at a constant rate. The desorption rate, typically measured as the amount desorbed per unit time, increases with temperature to a maximum and then declines as coverage diminishes, producing a characteristic peak when plotted against temperature or time. These maxima provide information about the energetics and distribution of adsorption sites on the surface.

Origin and interpretation: The shape and position of desorptionsmaxima are governed by the kinetics of desorption, often described by the Polanyi–Wigner rate law r = ν N^n exp(-Ea/RT). Depending on the order n and the activation energy Ea, the peak temperature Tp shifts with the heating rate and surface coverage. On heterogeneous surfaces, multiple maxima can appear, corresponding to different binding sites or adsorption states (physisorption vs chemisorption, monolayer vs multilayer desorption). The analysis of Tp can yield estimates of binding energies and site distributions when combined with appropriate kinetic models.

Measurement, analysis, and limitations: In practice, Tp is used to extract adsorption energies via methods such as Redhead or complete kinetic analysis, but results depend on the assumed mechanism and pre-exponential factor. Desorptionsmaxima thus reflect both intrinsic surface energetics and experimental conditions (heating rate, initial coverage). While informative for surface science and catalysis, they require careful interpretation within a chosen kinetic framework and with awareness of potential site heterogeneity or complex desorption pathways.

Applications: Desorptionsmaxima aid in characterizing catalyst surfaces, adsorbent materials, sensor substrates, and any system where surface-adsorbate interactions govern performance. They provide a qualitative and, with proper modeling, quantitative handle on adsorption energy distributions and surface heterogeneity.