The Theoretical Study On Electronic And Catalytic Properties Of Transition Metal Monoxides

Transition metal monoxides (such as FeO, CoO and NiO) has attracted extensive attentions of researchers for several years due to their advanced advantages of single structure, easy to synthesis, nontoxic and low cost. They have been widely used as photoelectronic, magnetic, energy-storage and catalytic materials. Therefore, studying the structural and electronic properties of these materials could be benefit to understand the issue at mechanism level. However, transition metal monoxides fall in the realm of strongly-correlated systems. The interactions between Tm 3d electrons are quite strong. Conventional density function theory usually fail to predict the properties of these materials, especially for electronic properties. There is no unified method to investigate the properties of these systems yet, and the studies on comparison between different functional s are few.Based on the studying progress on transition metal monoxides, in current work, we choose FeO, CoO and NiO as the targets, then study the structural and electronic properties of three transition metal monoxides with GGA-PBE, PBE+U and hybrid screened functional HSE, the experimental data from previous works are listed. According to our results, these functional could give reasonable structural properties when compared to experiment. As for electronic properties, PBE obviously underestimate the band gaps of three monoxides, PBE+U could give a rational results when given a proper U, the U is often different for different properties. While HSE method could give rational prediction on lattice constant, magnetic moment and band gaps.According to above study, we then employ PBE+U method to study the surface energy, density of states and work function of different Miler index surfaces. The results show that, the surface energy of (100), (311) and (211) are lower than (111) and (110). As for polar surfaces, terminated by oxygen are more stable than which are terminated by transition metals. Then we calculate the Wulff construction, in FeO crystal, the major exposed surface is (110), while for CoO and NiO, the majority surface is (211). The percentage of exposed surfaces is not only determined by surface energy but also the surface orientation. The surface density of states illustrate that the band gaps of surfaces are smaller than bulk phase. We can also find that the work function of FeO surfaces is lower than it of CoO and NiO. And more, work function of metal terminated surfaces is lower than those terminated by oxygen.At last, we simulate the conversion of syngas (CO and H2) on the surface of Ni(111) and 4O/Ni(111) catalysts. It's interesting that on pure Ni(111), the CO and H2 were prefer to form CH4. While at the existence of O, the hydrogenation ability of Ni catalyst is obviously reduced, the selectivity of longer chain hydrocarbonates is improved.