Understanding the Role of Fe Doping in Tuning the Size and Dispersion of GaN Nanocrystallites for CO2-Assisted Oxidative Dehydrogenation of Propane

The Fe-doped silicalite-1 zeolite as a matrix was employed to implant GaN nanocrystallites via a combined wet impregnation with in situ nitridation method. Direct spectroscopic evidences under a vacuum unraveled that the incorporation of the isolated framework Fe atoms could cause the generation of more interior silanols and especially hydrogen-bonded groups of silanol nests, where the solvated Ga3+ ions and urea in an N, atmosphere at high temperatures were transformed into the highly dispersed GaN nanocrystallites that were embedded simultaneously in defects of the Fe-silicalite-1 matrix with the binding structure of Si-O-GaN. Moreover, the particle size distribution of GaN can be tuned from the nanoscale (14.6 +/- 3.5 nm) to sub-nanoscale (2.7 +/- 0.4 nm) by regulating the molar ratio of Si/Fe. Combining the results of the temperature-programmed surface reaction (TPSR) with the measurements of temperature-programmed desorption and mass spectra (TPD-MS) using the probe molecules of C3H8 and CO2, it was proposed that isolated framework Fe species were active sites for the catalytic conversion of CO2, while the nanostructured GaN was responsible for the adsorption and activation of C3H8. Catalytic results and kinetics show that the CO2-assisted oxidative dehydrogenation of propane (CO2-ODHP) over GaN/Fe-silicalite-1 proceeds by following a two-step coupling process (direct dehydrogenation of propane (DDP) + reverse water gas shift (RWGS)) and also is a structure-sensitive reaction characterized by the dependence of GaN particle size on the specific catalytic activity and apparent activation energy of converting C3H8 and CO2. Without the occurrence of GaN sintering, the slight loss of catalytic activity caused by coke deposition was confirmed by linearly correlating the normalized amount of coke with the deactivation rate.

Automated summary

In this study, Fe-doped silicalite-1 zeolite was used as a matrix to implant GaN nanocrystallites. The incorporation of isolated framework Fe atoms resulted in the generation of more interior silanols and silanol nests, leading to the formation of highly dispersed GaN nanocrystallites embedded in the Fe-silicalite-1 matrix. The particle size distribution of GaN could be adjusted by regulating the molar ratio of Si/Fe.