Kinetic modeling of ammonia temperature-programmed desorption on titanium silicalite-1

Abstract

In this work, we present a comprehensive kinetic analysis of ammonia desorption from a series of titanium silicalite-1 (TS-1) samples, including both commercial (ZD-07031) and laboratory-synthesized materials with varying titanium content and textural properties. The classical Polanyi–Wigner model, despite yielding high correlation coefficients (R2 ≥ 0.98), fails to provide convergent kinetic parameters across different samples, with pre-exponential factors varying by orders of magnitude. To overcome this limitation, we employed the Schroeder–Eyring equation, which explicitly accounts for the temperature dependence of the pre-exponential factor through the thermodynamic parameters of activation – enthalpy (ΔrH0≠) and entropy (ΔrS0≠). This modification not only improves the consistency of the fitted parameters but also reduces the number of kinetic curves required to describe the experimental data. Kinetics analysis reveals that ammonia desorption from TS-1 can be adequately described by a two-site model with characteristic thermodynamic parameters: for type I sites, ΔrS0≠ = 5.03 ± 0.19 J·(K·mol)−1 and ΔrH0≠ = 100.7 ± 0.7 kJ·mol−1; for type II sites, ΔrS0≠ = 59 ± 3 J·(K·mol)−1 and ΔrH0≠ = 124.5 ± 0.8 kJ·mol−1. These values are consistent across all samples, despite variations in titanium content and specific surface area (438–518 m2/g). The observed third-order kinetics is rationalized by a collision-mediated desorption mechanism: adsorbed ammonia molecules diffuse along the surface in a fluid-like state within the micropores; triple molecular collisions can concentrate sufficient energy to overcome the desorption barrier. This interpretation suggests that for weakly acidic zeolites such as TS-1, ammonia TPD primarily probes the volume-filling properties of the porous structure rather than surface acidity per se. The proposed kinetic modeling approach provides a physically meaningful framework for interpreting TPD-NH3 data, enabling the extraction of consistent thermodynamic parameters that are independent of experimental conditions. This work opens the possibility of standardizing TPD methodology for comparative analysis of zeolitic materials.