Physical Processing Controls the Structure, Mesoporosity, and Suspension-Phase Functionality of Nanostructured MnOx Materials
Physical processing can modify the structural, textural, and dispersion characteristics of nanostructured oxide materials, thereby altering their functional state under suspension conditions. Here, poorly crystalline, sol–gel-derived porous MnOx materials were used to establish how post-synthetic physical processing affects the relationship between nanoscale structure, accessible mesoporosity, powder-to-suspension transfer, and chromogenic response in 3,3′,5,5′-tetramethylbenzidine (TMB) oxidation. Two compositionally distinct processing series were examined: an ultrasonic processing/recovery route for Sr- and Fe-containing MnOx and vibratory milling followed by identical ultrasonic dispersion for Sr-free Fe-containing MnOx. The recovered SrFeMn-US-S solid showed higher N2-accessible surface area and pore volume, stronger hydration signatures, and a larger low-temperature H2 temperature-programmed reduction (H2-TPR) contribution than SrFeMn-S. In contrast, vibratory milling of FeMn-S preserved the bulk Fe/Mn ratio but decreased SBET from 305.9 to 127.1 m2 g−1, Vtot from 0.533 to 0.215 cm3 g−1, total H2 uptake from 0.38 to 0.34 mmol g−1, and the Mn concentration in the operationally defined stable suspension fraction from 50.5 to 17.3 mg L−1. At an identical assay concentration of 500 ng Mn mL−1, milled FeMn-S5 also exhibited a lower time-summed ΣA652 response than FeMn-S. Thus, milling affected both the efficiency of powder-to-suspension transfer and the Mn-normalized functional response of the dispersed material. The contrasting outcomes show that the functional state of nanostructured, powder-derived MnOx is route-dependent and cannot be predicted from a single solid-state descriptor.
