Fig. 1. (a-d) SEM images, (e) element mapping and (f-k) HRTEM images of the NiZn@N-G-900.
如Fig. 1所示，SEM、HRTEM和EDS-mapping結(jié)果表明所制備催化劑是由N摻雜石墨包覆Ni金屬和Ni3ZnC0.7合金納米顆粒組成的。
Fig. 2. XPS spectra of the Fresh, Used and Re-calcined NiZn@N-G-900: (a) the N 1s core level and (b) the Ni 2p core level.
XPS結(jié)果（Fig. 2）表明，N元素的改變可能對所致被催化劑的性能產(chǎn)生影響。而反應(yīng)前后并沒有檢測到Ni元素的價(jià)態(tài)變化。
Fig. 3. (a)CO2-TPD and (b) NH3-TPD profiles of the Fresh NiZn@N-G-900. (c) Tafel scans of the Fresh NiZn@N-G-900, Used NiZn@N-G-900, Re-calcined NiZn@N-G-900, Ni0 powder and the Zn@N-C-900.
如圖Fig. 3(a)所示，所制備NiZn@N-G-900催化劑表面同時(shí)存在酸性位點(diǎn)和堿性位點(diǎn)。通常，酸性位點(diǎn)容易與親核物質(zhì)（如NH3和HSO5-）結(jié)合，而堿性位點(diǎn)容易與親電物質(zhì)（如CO2）結(jié)合。因此，表面酸性位點(diǎn)有利于PMS的吸附與活化。Tafel曲線表明，所制備NiZn@N-G-900催化劑具有優(yōu)良的電荷傳輸性能(Fig. 3(b))。
Fig. 4. (a) A series of results for DFT calculations,including adsorption models (top view, side view), electrostatic potentialdistributions (ESP, Isosurface contour is 0.01 e/bohr3),adsorption energy (Eads), peroxide O-O bond length (lO-O)and electron gain and loss of the adsorbed PMS (Q). (b) The chargedifference distribution of different PMS adsorption models. (Isosurface contouris 0.002 e/bohr3. The light green and light yellow denote theelectron depletion and electron accumulation, respectively.). (c) The workfunction (Ф) of each slab model.
DFT計(jì)算結(jié)果表明(Fig. 4)，在納米金屬顆粒表面包覆石墨或N摻雜石墨，不僅可以降低催化劑表面的功函數(shù)(Ф)，增強(qiáng)催化劑與吸附質(zhì)之間的電子轉(zhuǎn)移，而且可以有效地控制表面電荷分布(ESP)，為PMS的吸附和活化創(chuàng)造更多的場所。同時(shí)，石墨或N摻雜石墨對納米金屬顆粒的包覆程度不同，也會導(dǎo)致PMS與催化劑之間吸附能的變化。通常，適當(dāng)?shù)奈侥苡欣?/span>PMS與催化劑之間的電荷轉(zhuǎn)移。PMS吸附模型都表明PMS具有獲得電子的趨勢。這種趨勢會使PMS中O-O過氧鍵鍵變長，進(jìn)而有利于PMS的活化。根據(jù)DFT計(jì)算結(jié)果，提出了PMS的活化機(jī)理：當(dāng)PMS吸附在活性位點(diǎn)上時(shí)，一些電子傾向于向PMS轉(zhuǎn)移，從而使O-O過氧鍵的長度變長。隨后，吸附在催化劑表面的、具有較長過氧鍵的PMS與相鄰PMS相互碰撞進(jìn)而轉(zhuǎn)化為ROS。
Fig. 5. (a) Degradation efficiency of BPA in the NiZn-N-G-900/PMS system under different quenching conditions. (b) Premixed experiments in the NiZn-N-G-900/PMS system. (c) The change of PMS residual in the presence of different scavengers. (d) Effect of β-carotene on BPA degradation in anhydrous methanol. (e) EPR spectra in the NiZn-N-G-900/PMS system. (f) Degradation of BPA in D2O. (Conditions: 20 mg·L?1 BPA, 200 mg·L?1 catalyst, 0.244 mM PMS, pH= 6.5 and temperature = 30 oC).
根據(jù)無水甲醇中β-胡蘿卜素淬滅實(shí)驗(yàn)、重水中BPA降解實(shí)驗(yàn)預(yù)混合實(shí)驗(yàn)和EPR結(jié)果得出NiZn@N-G-900/PMS體系屬于典型的1O2主導(dǎo)的非自由基體系。同時(shí)，體系中1O2大量產(chǎn)生的主要原因歸因于催化劑對PMS自分解反應(yīng)的催化作用。
