| Cu/ZnO is an important industrial catalyst, which is not only applicable to methanol production by CO hydrogenation, but also used for ethanol and dimethyl ether synthesis, low-temperature water gas shift reaction, and hydrogen production via methanol cracking process, etc. Rosasite and aurichalcite are the main active phases in the precursors of Cu/ZnO catalyst. These two phases transform into CuO-ZnO solid solution after calcination. Cu embeds in ZnO lattice after reduction, resulting into strong interections between Cu and Zn, and therefore promoting the catalytic activity and stabilty. Thus, it is crucial to increase the contents of (Cu,Zn)2(CO3)(OH)2 and (Cu,Zn)5(CO3)2(OH)6 in the precursors in order to prepare highly active and long-life Cu/ZnO catalyst. During the co-precipitation process, it is difficult to control the direction of substitution reaction, because Cu2+, Zn2+, CO32-, OH- co-exist in the system and competition takes place in the precipitation of Cu2+ and Zn2+. When the ratio of Cu (or Zn) is low, single pure phase in the catalyst can be gained. However, if the content of Cu (or Zn) is relatively high, the as-prepared precusors are composed of mixed phases with probably carbonates, subnitrates, etc. Therfore, via the co-precipitation method, it is hard to obtain the pure phase with high substitution ratio, neither to explain which phase possesses a better activity, nor to analyze the rules of crystal phase transition during the formation of (Cu,Zn)2(C03)(OH)2 and (Cu,Zn)5(CO3)2(OH)6.In this dissertation, the phase-pure (Cu1-x,Znx)2(CO3)(OH)2 and (Cux,Zn1-x)5(CO3)2(OH)6 with different Cu/Zn ratios were prepared by oriented isomorphous substitution. The micro structures of the precursors were characterized with XRD, AAS, TG-DTG, FTIR, UV-vis, SEM and TEM. First-principles based on DFT were employed to analyze the energy, structure and electronic properties of Cu2(CO3)(OH)2/Zns(CO3)2(OH)6 system with different Zn/Cu contents. The theories of the highest Zn and Cu substitution content and the preferential substitution sites of Zn(or Cu) were studied for the preparation of (Cu1-xZnx)2(CO3)(OH)2 and (Cux,Zn1-x)5(CO3)2(OH)6 via isomorphous substitution. At the same time, the structure properties of Cu/ZnO catalyst and its catalytic performance for syngas-to-methanol in slurry bed were investigated for the two pure phases after calcination. Finally, microwave irradiation-instantaneous cooling technique was utilized in the aging step, and the effects of microwave irradiation on the crystal phase transition and the microstructure of precursors were studied. The main conclusions are as follows:(1) The phase-pure (Cu1-x,Znx)2(CO3)(OH)2 and (Cux,Zn1-x)5(CO3)2(OH)6 with different Cu/Zn ratio can be obtained by oriented isomorphous substitution. Among them, the maximum Zn substitution amount in (Cu1-x,Znx)2(CO3)(OH)2 is x=0.5, much higher than that of x=0.27 by co-precipitation. The maximum Cu substitution content in (Cux,Zn1-x)5(CO3)2(OH)6 is x=0.41, which is similar with that of x=0.4 by co-precipitation. The two phases obtained by isomorphous substitution are similar to the stuctures of malachite and hydrozincite, but are different from the mineral phase of rosasite and aurichalcite.(2) The isomorphous substitution of Cu2+ by Zn2+ in Cu2(CO3)(OH)2 is easy to proceed even without the aging step. The crystal shape of the samples transformed from snowflake-shaped crystals and platelets into uniform-size and ordered radial needle-like meso-structure. However, the rate of isomorphous substitution of Zn2+ with Cu2+ in Zn5(CO3)2(OH)6 is slow, and thus a certain period for aging is necessary. The sample shape transformed from needle-like structure into flake and spherical crystals. With increasing substitution amount of Zn or Cu, the crystal sizes of the two pure phases decrease, the thermal decompositon tempereature rises, DTG peak broadens, and the structure stability is enhanced. At the same time, the increase of Zn2+ with a relatively stronger electronic negativity changes the electronic environment of nearby OH- and CO32-, leading to that the O-H vibrational frequency and C-O antisymmetric stretching vibration shift to higher wavelength; the increase of Cu2+ with a relatively weaker electronic negativity weakens the O-H bond energy, resulting that the O-H stretching vibration shift to lower wavelength.(3) When Zn/(Cu+Zn)≥30%, Zn is enough to disperse nearby Cu to prevent aggregation and keep its small size. In addition, CuO-ZnO solid solution is formed to generate strong interaction between Cu and Zn, showing better performance of methanol synthesis. In addtion, Cu content in (Cu1-x,Znx)2(CO3)(OH)2is obviously higher than that in (Cux,Zn1-x)5(CO3)2(OH)6, thus there is higher effective Cu surface area and CuO/ZnO contact interface in the former catalyst when Zn content is ≥30%. The methanol yield of the two catalysts after calcination is:(Cu1-x,Znx)2(CO3)(OH)2>(Cux,Zn1-x)5(CO3)2(OH)6. In summary, the Cu/(Cu+Zn) ratio needs to be kept between 30%～70% for CuO/ZnO catalyst prepared by isomophous substitution to ensure its high activity.(4) Based on theoretical calculation of formation energies for the (Cu1-xAZnx)2(CO3)(OH)2 system, it is found that when x≤0.5, all Zn atoms prefer to substitute the Cu2 sites with less Jahn-Teller distortion. When x> 0.5, Zn starts to occupy the Cu1 sites with stronger Jahn-Teller effect, resulting in the obvious increase of formation energy, larger than the driving force of isomorphous substitution, which explains that the maximum Zn substitution ratio is 0.5 in the preparation of (Cu1-xZnx)2(CO3)(OH)2. From the calculated formation energy of the (Cux,Zn1-x)5(CO3)2(OH)6 system, it is found that the system obtains the lowest formation energy and the most stable crystal structure when x=0.4, which is consistent with the experimental result.For x=0.4, Cu atoms occupy simultaneously Zn1、Zn2 and Zn3 sites at the ratio of 1:1:2 in the lattice. This verifies the experimental phenomenon that Cu occupies not just one type of Zn positions in (Cux,Zn1-x)5(CO3)2(OH)6-. Interestingly, the calculated results show that (Cuo.75Zno.25)2(C03)(OH)2 system has the lowest formation energy, which is relatively easy to form for Zn-doped Cu2(CO3)(OH)2 system. This is probably the main reason that the maximum Zn substitution ratio in (Cu1-xZnx)2(CO3)(OH)2 is x≈0.27 by co-precipitation.(5) According to the calculation results, the crystal structure of (Cu1-xZnx)2(CO3)(OH)2(x=1/8,2/8,3/8 and 4/8) and (Cux,Zn1-x)5(CO3)2(OH)6(x =0,0.1,0.2,0.3,0.4 and 0.5) do not show significant distortion and remain similar to the structure of malachite and hydrozincite, respectively. The theoretical calculation results are in line with the experimental results of isomorphous substition. The covalent bonds strength are as the following order, (Cu,Zn)2(CO3)(OH)2:Zn-O> Cu2-O> Cu1-O; (Cu,Zn)5(CO3)2(OH)6:M3-O> M2-O> M1-O; the bond energy Zn-O> Cu-O. The covalent features gradually enhance in Zn-doped Cu2(CO3) (OH)2 system and the contribution of Zn states gradually increases for the total density of states with the increasing incorporation amount of Zn dopant. The bond populations of M-O gradually decreases with the Cu concentrations increasing and the covalent features gradually weaken in (CuxZn1-x)5(CO3)2(OH)6 system, following the transformation of covalent crystals to ionic crystals.(6) Microwave irradation was found to increase the system entropy by using the microwave irradiation-instantaneous cooling technique to eliminate the thermal effects. Microwave facilitates the growth of (Cu,Zn)5(CO3)2(OH)6 grains, while suppresses (Cu,Zn)2(CO3)(OH)2 grains growth, enabling the crystal structure with higher stability. |
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