Theoretical Investigations On Different Crystals And Surfaces Of MnO₂

MnO2is a kind of very useful material, which has more than30different crystal structures and can act as electrode, supercapacitor, catalyst, ion sieve, adsorbent, etc. In the recent years, we have synthesized some kinds of MnO2such as a-MnO2(2×2tunnel), P-MnO2(1×1tunnel),δ-MnO2(1×∞), etc, and applied them to catalyze dimethyl ether (DME) and toluene combustion. Computational chemistry and molecular modelling tools are capable of simulating crystal and surface structures and advancing our understanding of adsorption and catalysis behaviors on these surfaces.To a deeper understanding of the adsorption and catalytic ability of α-, β-,δ-MnO2, DFT and periodic model have been applied to calculate their crystals and surfaces. All electron DFT calculations in the present thesis were performed using a DMol3package in Materials Studio (version5.5) at the same level of theory unless the different Monkhorst-Pack k points used for different supercells. The exchange-correlation interaction was treated within the generalized gradient approximation (GGA) with the functional parameterized by Perdew, Burke and Enzerhof (PBE). Atomic basis sets were applied numerically in terms of a double numerical plus polarization function and a global orbital cutoff of4.7A was employed. The geometry optimization convergence tolerances of the energy, gradient, and displacement were10-5Hartree,2×10-3Hartree A-1, and5×10-3A, respectively.Based on our calculations, the major conclusions are listed as following:(1) An idea α-MnO2supercell (2×1×4, Mn32O64) with a tetragonal structure was used to simulate the nonmagnetic (NM), ferromagnetic (FM), and four kinds of antiferromagnetic (AFM1to AFM4) states. By comparing the energies, the AFM1state was the ground state. It was3.10,18.72and18.94meV per formula unit lower than AFM2, AFM3and AFM4states, respectively. The lattice constants of AFM1such as a=b=9.8349A and c=2.8805A were in excellent agreement with experimental JCPDS44-0141values. Furthermore, metal cations should be important for its stability. The lattice constants became larger when the K+cations were put into the2×2tunnel, and these results fitted well with the natural mineral reported by Post et al..(2) The structure of α-MnO2(110) surface was simulated by the Mn12O24supercell and then the larger Mn48O96supercell. It was shown that the antiferromagnetic structure T1was the most stable one. The adsorption of OH-ion on this surface could fit with the experimental results from Yamamoto Post et al. To our best knowledge, it was the first systemic silulation of the most stable α-MnO2surface namely the (110) surface. The adsorption of oxygen molecule and OH-ion were also shown in this thesis.(3) Based on the calculations of the nonmagnetic (NM), ferromagnetic (FM), and antiferromagnetic (AFM) states, the AFM state was the basic state for the β-MnO2crystal. The lattice constants were a=4.4494A, b=4.4345A and c=2.8773A, which agreed with the experimental results from Bolzan et al. By comparing the experimental results and previous simulational results, the GGA method could provide credible results for the structures and energies.(4) The β-MnO2(110) surface was simulated by a five-layer-model with the bottom atoms fixed to their crystal structure. Our simulation results ageed with the previous calculation results from Oxford et al. and Tompsett et al.. Then OH-ion, water, OH radical and oxygen molecule were simulated on this surface. The adsorption abilities were OH-ion>water>OH radical>oxygen molecule.(5) From the calculation results of vacancy-free and defect δ-MnO2crystal, we found that the Mn4+defects could indeed facilitate photoconductivity by reducing the band-gap energy and separating electron and hole states. Based on our calculations, the interlayer species such as water and Zn2+could make the interlayer distance larger.(6) Based on the simulations of the crystal and surface of α-,β-,δ-MnO2, GGA method can give a sutable results for strutures and energies, whereas it can not give good results for the band structures. The PBE+U method may give better band structure results.