Open experimental teaching design and practice based on cultivating cognition of mass transfer and diffusion processes

Mass transfer and diffusion are core concepts in environmental engineering and pollution control. However, undergraduate students are often exposed to these concepts mainly through mathematical equations or simplified verification experiments, making it difficult for them to relate transport phenomena to complex catalytic reaction systems. In porous catalysts used for advanced oxidation processes, catalytic performance is governed by their active-site chemistry, pore architecture, channel connectivity, reactant accessibility, and diffusion pathways. To address this educational gap, an open experimental teaching module was developed using Fe-, N-, and S-codoped carbon (FeNSC) catalysts with different pore structures as the instructional platform. The module was designed to help students understand how pore architecture affects peroxymonosulfate (PMS) activation and pollutant degradation while developing their ability to interpret catalytic behavior through a structure–diffusion–performance correlation.

Three FeNSC catalysts were selected as experimental materials: FeNSC/SBA-15, FeNSC/KIT-6, and FeNSC/SiO₂-NPs, representing a two-dimensional unidirectional channel structure, a three-dimensional ordered interconnected pore network, and a three-dimensional disordered multidirectional porous structure, respectively. The experiment integrated catalyst characterization, PMS activation, pollutant degradation, and comparative data analysis. The FeNSC catalysts were pre-synthesized by the instructor using different SiO₂ templates and supplied to students for characterization and performance evaluation. Transmission electron microscopy and nitrogen adsorption–desorption analysis were used to characterize pore morphology, channel architecture, and pore-size distribution, while X-ray diffraction and X-ray photoelectron spectroscopy were employed to determine crystal structure, elemental composition, and surface chemical states. These characterization results provided the structural basis for analyzing the internal migration behavior of PMS, p-nitrophenol (p-NP), and reaction intermediates within different pore systems. PMS decomposition tests were first conducted to evaluate the PMS activation efficiency of each FeNSC catalyst. Residual PMS concentrations were determined using the KI/NaHCO₃ colorimetric method using ultraviolet–visible (UV–vis) spectrophotometry at 352 nm. Pollutant degradation experiments were then performed using p-NP as the model pollutant. Adsorption tests were conducted before PMS addition to distinguish adsorption from catalytic oxidation. After PMS was introduced, samples were collected at predetermined time intervals, filtered, and analyzed via UV–vis spectrophotometry to generate degradation kinetic curves. Students compared PMS consumption, pollutant removal efficiency, initial reaction rates, residual concentrations, and late-stage reaction behavior under different PMS concentrations and pollutant loadings.

The experiment enabled students to distinguish the three pore architectures and relate their structural characteristics to diffusion behavior. FeNSC/SBA-15 showed ordered unidirectional channels, providing an effective model for demonstrating one-dimensional diffusion and local mass-transfer limitations resulting from weak lateral connectivity. FeNSC/KIT-6 possessed an ordered three-dimensional interconnected pore network, making it suitable for illustrating multipath diffusion, continuous reactant transport, and improved accessibility to internal active sites. By contrast, FeNSC/SiO₂-NPs exhibited a more open but disordered porous structure, helping students understand multidirectional diffusion through irregular pore channels. By correlating structural characterization results with PMS activation and p-NP degradation profiles, students recognized that catalytic performance cannot be explained only by the number of active sites or the apparent reaction rate. Under low reactant loadings, differences in catalytic performance among the three catalysts were relatively small. However, at high PMS concentrations or pollutant loadings, pore connectivity and diffusion pathways became increasingly important. The resulting kinetic curves reflected the combined effects of reactant transport, active-site accessibility, reactive-species utilization, and sustained catalytic activity. Furthermore, correlating performance curves with schematic illustrations of pore structures transformed otherwise invisible diffusion processes into observable and interpretable experimental phenomena.

An open experimental teaching module based on FeNSC catalysts with different pore architectures was established for undergraduate environmental engineering education. By integrating material characterization, PMS activation, pollutant degradation, and mechanism-oriented analysis, the module provides a distinct connection between microscopic pore structures and macroscopic reaction behavior. Concepts such as diffusion pathways, mass-transfer resistance, active-site accessibility, and reaction sustainability are transformed into experimentally observable phenomena supported by structural characterization and quantitative data. This module improves students’ experimental skills, data analysis, graphical interpretation, and mechanism-based reasoning while providing a practical model for developing comprehensive open laboratory experiments in environmental engineering.