Preparation Of Iron Single Atom/Cluster Catalysts Via Ball Milling And Their Fenton-like Catalytic Performance

Single-atom/cluster catalysts have become one of the most promising alternatives to non-homogeneous Fenton catalysts due to their high atomic utilization and structural stability.However,current methods for the preparation of single-atom/cluster catalysts still suffer from complicated preparation processes,expensive and non-renewable precursors（e.g.,carbon nanotubes,formamide,polypyrrole,etc.）.In addition,in Fenton-like catalytic reactions dominated by the oxidation process of radicals generated by activated peroxymonosulfate（PMS）,there is problem of low catalytic reaction rates due to the short half-life of oxygen-containing radicals.To address the above issues,the soybean residue-derived nanocarbon anchored dual-site catalysts（Fe-NC）with Fe single atoms and clusters were prepared by ball milling method using natural soybean residue as raw materials,Iron（Ⅱ）phthalocyanine as the precursor,and potassium bicarbonate as activator.The crystal structure,micromorphology,chemical composition,valence,etc.of as-prepared catalyst were analyzed by a series of characterizations such as TEM,AC HAADF-STEM,XRD,Raman spectroscopy,BET,XPS,XAS,ICP and EA.Different catalysts,including iron nanocatalyst（Fe-NP-900）and iron single atom catalyst（Fe₁-NC-900）,were obtained by adjusting the pyrolysis temperature and acid treatment.Then,the Fenton-like catalytic performance of the obtained catalysts for the activation of PMS to degrade the Rhodamine B（Rh B）and tetracycline hydrochloride（TC）was compared.Moreover,the active species during the degradation of Rh B and TC were analyzed by radical trapping experiments,Raman spectroscopy,electrochemistry,and EPR measurements to reveal the catalytic mechanism of iron single-atom/cluster catalysts.The main contents and results of the study are as follows:（1）Iron single-atom/cluster catalysts（Fe-NC-750,Fe-NC-900,and Fe-NC-1000）,iron nanocatalysts（Fe-NP-900）,and iron single-atom catalysts（Fe₁-NC-900）were obtained by adjusting the pyrolysis temperature and acid treatment,and the dispersion state,valence state of metals and metal-metal interactions were revealed by characterization using TEM,AC HAADF-STEM,XRD,Raman spectroscopy,BET,XPS,XAS,ICP and EA.The results showed that the pyrrolic nitrogen content in the catalyst increased with the increase of pyrolysis temperature,in which the pyrrolic nitrogen content and iron loading of Fe-NC-900 were 1.48 at.%and 0.41 wt%,respectively.Furthermore,the Fe metal in Fe-NC-900 catalysts existed as Fe single atoms and ultra-small clusters,and the Fe ultra-small clusters could effectively modulate the electronic structure and geometry of single atom Fe N_xsites to expose more Fe N_xsites.In conclusion,ball milling can be used to successfully prepare dual-site catalysts with pyrrolic nitrogen and Fe N_xsites.（2）The performance of NC-900,Fe-NC-750,Fe-NC-900,Fe-NC-1000,Fe-NP-900,and Fe₁-NC-900 catalysts for activation of PMS to degrade Rh B was examined comparatively using catalytic degradation of Rh B as a probe reaction,and the results indicated that Fe-NC-900 had the best Fenton-like catalytic performance;Under the optimal experimental conditions(catalyst dosage of 0.8 g·L^-1,dosage of oxidant:0.6 g·L^-1,pH=3.0,reaction temperature:30℃,initial concentration:100 mg·L^-1),the removal rate of Rh B catalyzed by Fe-NC-900 could reach 99.5%within 10 min,and the mineralization rate could reach 68.2%within 210 min.After five cycles,the removal rate of Rh B was90.8%,and there was no significant change of the characteristic peaks in the XRD and XPS patterns of the catalyst before and after the reaction,which proved that the catalyst had good stability.Quenching experiments,radical active species trapping experiments,Raman spectroscopy,and electrochemical experiments determined that high-valent iron oxidation,electron transfer pathway and singlet oxygen（¹O₂）were involved in the reaction,but the ¹O₂was the predominant active species,which confirmed that pyrrolic nitrogen was the adsorption site for Rh B in Fe-NC-900 and the Fe N_xsites were the catalytic active site,resulting in greatly reducing the migration distance from the active radical to the target organic molecule and thus effectively improved the catalytic activity of the catalyst.In addition,the degradation products of Rh B were analyzed by liquid chromatography-mass spectrometry（LC-MS）,the degradation pathway of Rh B was investigated,and the catalytic mechanism of Fe-NC-900 on Rh B was proposed.（3）The catalytic degradation of TC was used as the probe reaction to explore the Fenton-like catalytic performance of Fe-NC-900,and the effects of catalyst dosage,PMS dosage,initial solution pH,initial solution concentration and solution temperature on the catalytic degradation of TC by Fe-NC-900activated PMS were investigated.The optimal experimental conditions were determined as follows:initial concentration of TC is 100 mg·L^-1,pH=5.0,30 ^oC of temperature,1.0 g·L^-1of catalyst dosage,0.8 g·L^-1of oxidizer dosage,30 min of reaction time,the removal rate of TC was 92.3%and the mineralization rate reached 56.8%within 300 min.After five cycles of Fe-NC-900,the removal rate of TC was 84.9%,and there were no significant changes in the XRD patterns and XPS patterns of the catalyst before and after the reaction,which confirmed the stability of the crystal structure of Fe-NC-900.During the quenching experiments and free radical active species trapping experiments,it was confirmed that O₂^·-radicals and ¹O₂were the main active substances were involved in the catalytic process,and the degradation products of TC were analyzed by LC-MS,the degradation pathway of TC was investigated,and the mechanism of degradation of TC by Fe-NC-900 was proposed.In summary,this study aims to reveal in detail the main reactive sites of single-atom/cluster catalysts and their efficient multiphase catalytic mechanisms,confirming their superior environmental remediation potential.