| Rosasite and aurichalcite are the main active phases in the precursors ofCu/ZnO catalyst. These two phases can transform into CuO-ZnO solid solutionafter calcination, resulting into strong interections between Cu and Zn afterreduction, and therefore promoting the catalytic activity and stabilty.McGuinessite is an interesting and favorable replacement for rosasite, which canpromote the nanostructuring and increase the Cu dispersion. For(Cu1-xMgx)2(OH)2CO3system, it is difficult to confirm the cationic arrangementby experimental characterization methods, and the single crystal synthesis islimited, so it is hard to explore and clarify the microstructures and electronicproperties of (Cu1-xMgx)2(OH)2CO3from the perspective of experiment. Inaddition, the active additives (such as Al3+, Zr4+) in the Cu/ZnO catalyst canobtain higher effective Cu specific surface area, well Cu dispersion and improvethe reduction performance of CuO. But the preparation of single Al/Zr doped(Zn1-xCux)5(OH)6(CO3)2phase is difficult. It is also difficult to accurately judgethe composition and distribution of Al3+/Zr4+by characterization means and theelectronic properties of Al/Zr doped aurichalcite is ambiguous.In this study, we employ the first-principles calculation method tosystematically study on the energetic, structural and electronic properties ofMg/Zn doped Cu2(OH)2CO3and Al/Zr doped (Zn1-xCux)5(OH)6(CO3)2systems.The main conclusions, which can provide clear theoretical clues to design andmodify Cu/ZnO catalyst, are presented as follows: (1) Based on theoretical calculation of formation energies for(Cu1-xMgx)2(OH)2CO3system (x=1/8,2/8,1/8,2/8and5/8), it can be found thatformation energies of Mg doped Cu2(OH)2CO3are negative (-1.647~-1.606eV), and with the increase of the Mg doping ratio, the formation energies showfluctuation change. Among them,(Cu6/8Mg2/8)2(OH)2CO3obtains the lowestformation energy and the density of states value at Fermi level is the lowest,implying that the most stable crystal structure and is relatively easy to form. Bycomparing Mg/Zn doped Cu2(OH)2CO3(x=2/8,4/8) system, it can be foundthat under the condition of same doping proportion, the formation energies ofMg doped malachite are negative, while the formation energies of zincmalachite are all positive. In addition, for Mg/Zn doped Cu2(OH)2CO3system,all Zn and Mg atoms prefer to substitute the Cu2sites with less Jahn-Tellerdistortion.(2) The Mg/Zn doped Cu2(OH)2CO3crystal structures do not showsignificant distortion, keeping for monoclinic system. The equatorial averagebond lengths of Zn–O and Mg–O are longer than that of Cu–O bond. For(Cu6/8Zn1/8Mg1/8)(OH)2CO3system, the apical average bond lengths of Mg–Oare the shortest, showing the weakest Jahn-Teller effect in this compound. WhenMg/Zn doping ratio increases, the Jahn-Teller distortion of the systemstrengthening, reducing the stability of crystal structure. Besides, it can be seenthat Jahn-Teller effect not only exists in the Cu2+with3d9orbitals, may alsoexist in Mg2+with2p6orbitals, and unsaturated d and p orbitals affect metal ionsJahn-Teller distortion degree together. In addition, atomic Mulliken charges,overlap population analysis and density of states analysis results show thecovalent bonds strength are as the following order, Zn–O> Cu–O, while Mg–Obond is a typical ionic bond.(3) After geometry optimization of Zn5(OH)6(CO3)2crystal, the cellparameters are consistent with experimental values. Further calculated resultsshow that the formation energy of Al/Zr doped (Zn1-xCux)5(OH)6(CO3)2system are negative, indicating the stability in thermodynamics.When x=0.3, Al prefersto interstitial doping in aurichalcite, forming AlO6octahedron with the nearest Oatoms; While Zr is favorable to substitute Zn3(7) site, and forming a triangularbipyramid (3+2) coordination configuration with the surrounding O atoms.Whenx=0.4, Al/Zr prefers to occupy octahedral coordination Zn1(1) and Zn2(5). Inconclusion, when Cu/Zn ratio is not the same time, the way of Al/Zr doped(Zn1-xCux)5(OH)6(CO3)2and priority position of doping are totally different, andthe formation of Zr doped aurichalcite is easier.(4) After geometric optimization, the crystal structure of Al/Zr doped(Zn1-xCux)5(OH)6(CO3)2system do not show significant deformation, and stillremain similar to the structure of Zn5(OH)6(CO3)2structure. When x=0.3, afterAl/Zr doping the bond lengths of Zn–O and Cu–O increase. Namely the ZnO6and CuO6octahedrons increase. At the same time, the bond lengths of Al–O andZr–O are less than that of Zn–O and Cu–O. When x=0.4, Al/Zr doping makesthe CuO6octahedra slightly increases. Al doping prompts the increase of ZnO6octahedron at the same time, while Zr doping makes the decrease of ZnO6octahedra. In addition, atomic Mulliken charges, overlap population analysis anddensity of states analysis results show the covalent bonds strength of Al–O andZr–O are greater than that of Al–O and Zr–O bonds. |
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