Mechanistic Of Photocatalysis-assisted PMS Activation With Variable Valence Metals (Co,Cu) Modified ZIF-8-derived ZnO

The photocatalysis-assisted peroxymonosulfate（PMS）activation mediated by ZIF-8-derived ZnO has been recognized for its potential in remediating water pollution.However,ZnO suffers from limitations in its narrow visible light absorption range,low utilization efficiency of photogenerated charge carriers,and weak direct activation capability towards PMS,which restrict its catalytic degradation ability.The modification of ZnO with variable-valent metals possessing PMS activation capability is considered a direct and effective approach in current research.Variable-valent metals such as Co and Cu,which possess similar atomic properties and coordination environments to Zn,can be easily introduced into ZIF-8 and its derived ZnO.By constructing Z-scheme heterojunctions or incorporating them as heteroatoms,the introduced Co and Cunot only provide stronger activation sites,but also hold the potential to regulate and optimize the band structure and charge behavior of ZnO,thus enabling more efficient utilization of the introduced variable-valent metals.However,the methods to achieve different modification strategies and the reaction mechanism of the variable-valent metals remain unclear.Based on the above,variable valence metals（Co,Co）were introduced by two strategies of Z-scheme heterostructure construction and heteroatom doping to enhance the catalytic activation performance of ZIF-8-derived ZnO in photocatalysis-assisted PMS activation reaction system.The purpose of this study is to explore and reveal the synergistic mechanism of variable valence metals introduced by different strategies for reaction performance,and to realize the efficient utilization of variable valence metals.The main research results are listed as follows:（1）Z-scheme heterojunctions consisting of Co₃O₄and ZnO,as well as CuO and Cu-doped ZnO,were separately constructed by direct energy level matching and energy level modulation using in-situ synthesized ZIF-67/ZIF-8 and Cu-ZIF-8synthesized in Cu²⁺-rich environment as templates.The introduction of variable-valent metals in the Z-scheme heterojunction enhanced the utilization efficiency of visible light and photogenerated charge carriers in the catalyst,while maintaining a high-energy energy band.Additionally,the construction of Z-scheme heterojunction enhanced the electron-donating ability of the variable-valent metals（Co,Cu）,which primarily served as reduction-activating sites in the reaction,and improved the oxidative properties of Zn sites and their adsorption capacity for PMS.The metal sites on the surface of the catalyst and the high-energy conduction and valence band of the Z-scheme heterojunction synergistically enhanced the electron gain/loss activation reaction of PMS,resulting in the mixed active species mechanism of radical/non-radical in the catalytic degradation reaction.The Z-scheme heterojunction of Co-Zn and Cu-Zn oxides exhibited removal rates for MB in the photocatalysis-assisted PMS activation system that were 42.0 and 7.5 times higher than that of pristine ZnO,respectively.Furthermore,the construction of the Z-scheme heterojunction ensured the stability of the variable-valent metals in the catalyst.The catalyst/visible light/PMS composite reaction system also demonstrated environmental friendliness and excellent degradation adaptability.（2）Co-doped ZnO,Cu-doped ZnO,and Cu/Co co-doped ZnO catalysts were prepared by introducing low doses of Co and Cuinto ZIF-8 as precursors.The doping of Co and Cuenhanced visible light response and improved the efficiency of photogenerated charge transfer and separation in ZnO.Simultaneously,the doping of variable-valent metals reduced the bandgap and elevated the valence band of ZnO,thereby enhancing its oxidative properties under illumination.The variable-valent metals Co and Cuin the doped phase mainly acted as oxidative activation sites in the photocatalysis-assisted PMS activation reaction,cooperating with the high-energy valence band,strengthening the PMS electron-loss activation path,leading to abundant production of singlet oxygen（¹O₂）.Co or Cudoping could increase the degradation efficiency of ZnO in the photocatalytic assisted PMS activation reaction by 27.2 and 2.7 times,respectively,and displayed good stability and anti-interference capability.The more significant enhancement effect of Co doping was attributed to the stronger oxidation ability of Co（III）.Cu/Co co-doping provided the catalyst with a more abundant population of photogenerated charge carriers and oxidative Co sites,enhancing the catalyst’s oxidation of PMS.Simultaneously,co-doping increased the tendency for O-H bond cleavage in PMS adsorbed on the ZnO surface,thereby increasing the possibility of PMS electron loss decomposition and ultimately forming an active species system dominated by non-radical ¹O₂.Cu/Co co-doped ZnO demonstrated excellent catalytic activation performance,with a degradation efficiency60.9 times higher than that of ZnO.（3）Compared with Co₃O₄/ZnO possessing the best catalytic degradation performance in the Z-scheme heterostructure construction strategy,the variable-valent metals in Cu/Co co-doped ZnO contributed more to catalytic activation degradation under the premise of lower relative content.Based on the result,it is considered that Cu/Co co-doped ZnO might be the best catalyst for the reaction system.The photocatalytic fuel cell was constructed using Cu/Co co-doped ZnO immobilized as the photoanode.The cell system exhibited higher catalytic degradation performance than the pristine photoanode,and could achieve simultaneous oxidation-reduction removal of MB and Cr（VI）（100%and 69.9%removal within 30 min,respectively）.The fill factor of the constructed photocatalytic fuel cell was calculated as 0.32.Compared with the approximate research,it is considered that the system possessed favorable power generation performance and photoelectric conversion efficiency.In this paper,the directional construction of different forms of variable valence metal modified ZIF-8 derived ZnO catalysts was realized.Based on the catalytic reaction mechanism of each catalyst in the reaction,the synergistic mechanism and efficient utilization strategy of the introduction of variable valence metals are clarified,which provides new insights for the rational utilization and purposeful introduction of variable valence metals.