ACB 202209 083022
Tailoring the coordination environment of cobalt in a single-atom catalyst through phosphorus doping for enhanced activation of peroxymonosulfate and thus efficient degradation of sulfadiazine
Keywords: CoN3-P-C; PMS; SDZ; 1O2; DFT; HPLC-QTOF-MS
磺胺类药物 (SAs); 磺胺嘧啶 (SDZ); AOP; Fenton; persulfate (PMS/PDS) (4). SAC; MN4 (3). MOF&SACs (greater stability and conductivity) (5). effectively control the coordination environment of metal atoms in SACs, we introduced phosphorus (P) into Co-based SACs through the host-guest technique to construct Co-N3P sites, thereby forming a novel catalyst: ZIF-CoN3P-C.
ZIF-C、ZIF-PC、ZIF -CoN4 -C and ZIF-CoN3P-C.
XRD, HR-TEM, HAADF-STEM, FT-IR, XPS, ICP-OES and ICP-MS.
Co K-edge XANES: Co(II); FT-EXAFS: Co-N (2.8, 1.856 Å), Co-P (0.9, 2.253 Å); WT: 5.43 Å-1 for ZIF-CoN4 -C and 4.85 Å-1 for ZIF-CoN3P-C, which are attributable to their different Co-N coordination.
SDZ: 10 mg·L-1, Catalyst: 0.05 g·L-1, PMS: 1 mM, 10 min, 96.9%, 1.074 × 105 M-1 min-1.
These quenching results indicate that 1O2, rather than ·OH, SO4·-, and high-valent Co-oxo species (Co(III) and Co(IV)), was the dominant ROS produced from PMS by ZIF-CoN3P-C.
Thus, based on the above analysis, the pathway of 1O2 generation may be proposed. Firstly, the dissociated PMS molecule (HSO5-) is chemically adsorption on the CoN3P site (≡CoN3P). Secondly, an electron transfer from ≡PN3Co(II) to the adsorption , generating HO2· and O2·-, and O2·- can be protonated (pk(HO2·/ O2·-) = 4.7). Thirdly, recombination of HO2· and O2·- occurs to produce 1O2.
The Co atom occupies the region with the highest electron density, while a lower ELF indicates a higher tendency of delocalization(离域) of electrons. After the substitution of one N atom with one P atom, the electron density and electron delocalization are further concentrated around the Co site instead of the introduced P site, implying that the Co active centre was not altered by the introduction of P atoms.
Electronegativity. Bader charge analyses. Fermi level. Hence, the introduction of the P heteroatom(杂原子) significantly enhanced the electron delocalisation of the Co atom in the catalyst.
The adsorption energy (Eads) of PMS on Co in CoN3P is -2.47 eV, whereas a smaller value of -2.81 eV is observed for CoN4. Bader charge. The reaction-free energy was calculated for the catalytic PMS activation process, including adsorption, action and regeneration (ZIF-CoN3P-C had a lower energy barrier(能垒) (1.14 eV) than ZIF-CoN4-C (1.48 eV)). Overall, the induction of P and construction of CoN3P enhanced the activity of ZIF-CoN3P-C for PMS activation.
Degradation pathway. Stability and universality (pH, anion, HA, DiffConta).
ACB 202204 083022
Engineering the low-coordinated single cobalt atom to boost persulfate activation for enhanced organic pollutant oxidation
Keywords: CoSA-N3-C; PDS; BPA; SO4·-; DFT; GC-MS;
·OH、SO4·-; persulfate; transition metal (poor stability and recyclability, sluggish kinetics and inhomogeneity of active centers); SACs; environmental catalysis (persulfate-based catalytic oxidation reaction).
Carbon-based materials (metal-free supports); Nitrogen (N) doping (exploited to enhance the interaction of metal (M) atoms with carbon substrates via the formation of strong M-N bonds); electronegativities (could regulate the local electronic structures or spin states of M-N4).
Coordination number x (M-Nx); lower coordination number; PDS; no reports.
Herein, cascade-protection strategy(级联保护策略); CoSA-N3-C and CoSA-N4-C; can increase the electron density (single Co atom in the active Co-N3 center to facilitate PDS adsorption and successive conversion)
XRD; XPS; SEM; TEM; HRTEM; SAED; HAADF-STEM; ICP-MS.
Co K-edge XANES: is located between CoO and Co3O4 references, implying the Co atom is in an oxidation state between Co2+ and Co3+. FT-EXAFS: reveals a strong peak associated with the scattering(散射) of Co-N coordination, without the peak of Co-Co bonding; EXAFS fitting (in R and k spaces): Co-N (3.0, 1.94 Å); XPS: the appearance of the Co-N peak also attests to the coordination of Co with N atoms, compared to N-C, the pyridinic N peak shifts toward lower binding energy with the decoration of the Co atom in the that of CoSA-N3-C, suggesting that pyridinic N is inclined to immobilize the Co atom in the form of coordination bond; DFT calculations unraveled the formation energy for the CoSA-N3-C model was lower than that for the CoSA-N3-CV structure, manifesting the more favorable construction of CoSA-N3-C configuration without the formation of carbon defect.
BPA: 50 μM = 11.4 mg·L-1, Catalyst: 0.05 g·L-1, PMS: 2 mM, 4 min, 100%, 0.067 L min-1 m-2.
TOC; cycling experiments; ICP-OES; other recalcitrant(顽固) organics.
SO4·-; (since 1, 10-phenanthroline can effectively chelate with(螯合) Co atoms but hardly adsorb on carbon-based materials) validating that the single-atomic Co sites in CoSA-N3-C are the active centers for PDS accessibility and successive conversion. Co K-edge XANES: CoSA-N3-C after PDS activation (slight positive anergy shift) manifests the increase of oxidation state of the Co atom in the course of PDS conversion process. This means the occurrence of electron transfer from the single Co atom toward the PDS molecule leading to PDS reduction over the single-atomic Co sites for ·OH and SO4·- production. EIS: higher electron mobility(电子迁移率) and smaller acr radius(弧半径). DFT: PDOS of Co atoms in CoSA-N3-C near the Fermi lever is higher, implying that the CoSA-N3-C delivers a stronger interaction with PDS relative to CoSA-N4-C. Two-dimensional electron density distribution images: highest and greater electron density, which can facilitate PDS attachment onto the single- atomic Co site in CoSA-N3-C. The much large adsorption energy of PDS on CoSA-N3-C that on CoSA-N4-C corroborates the higher affinity(亲和力) of the single Co atom in CoSA-N3-C to the PDS molecule. The similar trend is also noted for the elongation(延长) of O-O bond length after PDS adsorption. The stretching of O-O bond is more significant when PDS was attached on the single Co atom in the CoSA-N3-C model, which implies the more efficient PDS conversion over the single atomic Co site of CoSA-N3-C.
Degradation pathway; Actual wastewater treatment application (anions, HA, actual dyeing wastewater).
EST 202205 083022
Origin of the Excellent Activity and Selectivity of a Single-Atom Copper Catalyst with Unsaturated Cu-N-2 Sites via Peroxydisulfate Activation: Cu(III) as a Dominant Oxidizing Species
Keywords: CuSA-N2C; PDS; 2,4-DCP; Cu (III); DFT; GC-MS; Column experiment.
·OH, and SO4·-; PMS and PDS; anion and HA (be consumed and hinder).
Nonradical oxidative pathways including mediated electron transfer, singlet oxygen, and high-valent metal; Cu(Ⅲ) in the heterogeneous catalytic system; SACs; PDS.
In this work, CuSA-N2C, this is the first study on the selective removal of pollutants by high-valent copper Cu(Ⅲ) on a single-atom catalyst with unsaturated Cu-N2 sites under PDS activation.
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XRD: exhibits two broadened peaks centered at 24.6 and 44.1°; HRTEM: curled two-dimensional nanosheets without any nanometal particles; BET: 758.7 m-2 g-1, 10-100 nm; HAADF-STEM: bright spots with a single-atom size; EDX-Mapping: uniformly distributed.
XPS: Cu-NX (399.3 eV), suggesting that Cu atoms in CuSA-N2C probably coordinate with N atoms; ICP-OES: 1.03%; k3-weight FT-EXAFS spectrum: 1.47 Å, which can be assigned to the backscattering of Cu-N coordination; WT-EXAFS: 5.2 Å-1, which is very close to Co-N in CuPc; EXAFS fitting curve: Cu-N (2.0, 1.93 Å).
The above results indicate that the single-atom Cu on the N-doped carbon support is the main active site.
2,4-DCP: 100 μM, Catalyst: 0.04 g·L-1, PMS: 0.5 mM, 30 min, 100%, 0.067 L min-1 g-1.
pH = 9, 60%. pHPZC = 3.8, and an increase in pH can lead to a more negative charge on the surface of CuSA-N2C, which results in stronger electrostatic repulsion(静电排斥力) with both PDS and 2,4-DCP. anion: exhibits excellent activity performance, which suggest that free radicals (·OH, SO4·-) may not be the active oxidants. The addition of HA slightly inhibits the removal of 2,4-DCP probably because of the easily degraded electron-rich groups in HA. Different Pollutants: the degradation preference of CuSA-N2C for electron-rich phenolic pollutants further indicates the existence of non-free radical oxidants.
As shown in Fig 4b, no signals belonging to DMPO-·OH, DMPO- SO4·-, and DMPO- O2·- on 2,4-DCP degradation, whereas a dominant signal of DMPOX appears in the CuSA-N2C system, which is attributed to direct oxidation rather than the free radical attack of DMPO.
FFA distinctly inhibits 2,4-DCP degradation, but the characteristic signal of 1O2-TEMP (aN = 17 G) adducts is not observed in the TEMP-trapping EPR experiment. There is no predominant difference in the 2,4-DCP degradation rates between the D2O and H2O systems, excluding the presence of 1O2. FFA would affect the surface properties of the catalyst (passivation钝化) and ultimately lead to the inhibition of pollutant degradation rather than 1O2.
Typical electron transfer mechanism is no applicable to the CuSA-N2C system. The concentration of PDS significantly decreases at approximately the same rate regardless of the presence of 2,4-DCP. PDS is activated by CuSA-N2C and transformed to SO42- instead of being adsorbed. The addition of PDS results in an obvious negative current owing to the transformation of Cu(Ⅱ) to Cu(Ⅲ) with PDS activation.
Raman spectra: the new peak around 616 cm-1 in CuSA-N2C + PDS system, confirming the formation of Cu(Ⅲ), because it is the typical Cu-O stretching vibration peak of a metastable(亚稳态) Cu(Ⅲ). BA and NB with electron-poor groups can hardly be degraded in the CuSA-N2C system, which is attributed to the selective oxidation of Cu(Ⅲ) reported in previous studies. The main intermediates in the GC-MS result are also consistent with the phenomenon in a study on high-valent iron species Fe(Ⅴ).
DFT: The adsorption energies of PDS on saturated Cu-N4 (-1.18 eV) and unsaturated Cu-N2 (-2.64 eV). The energy barrier of PDS dissociation on Cu-N2 was calculated to be 0.50 eV, which is much small than that on Cu-N4 (0.90 eV), indicating that the PDS activation to SO42- on the unsaturated Cu-N2 is energy-favorable than that on the saturated Cu-N4.
PDOS: the Cu 3d states hybridize(杂化) the N 2p states strongly, indicating the strong interaction between Cu and N atoms in CuSA-N2C due to the formation of Cu-N bonds. The Cu 3d states (CuSA-N2C) also exhibit significant orbital overlaps with the O 2p states (PDS) and C 2p orbital (2,4-DCP), indicating that electrons can easily transfer between single-atom Cu and PDS/2,4-DCP.
The PDS molecule prefers to adsorb on the Cu atom of CuSA-N2C, and is activated/ dissociates to from two SO42- (temporarily attached to the Cu site), the pollutant 2,4-DCP molecule adsorbs and is oxidized to form the intermediate. We proposed that the Cu in the unsaturated Cu-N2 is the main active site of CuSA-N2C, and the high-valent copper species Cu(Ⅱ) generated by activating PDS can directly oxidize pollutants selectively.
Anion; different aquatic systems; dyeing wastewater; column experiment.