The Application Of Microstructure Analysis Of XRD On Fused Iron Catalyst For Ammonia Synthesis And Nanomaterials

The ammonia synthesis catalysts and their catalytic process are important in theoretical research and industrial field. Although the traditional fused-iron catalysts, namly Fe₃O₄-based catalyst, have been studied systematically and thoroughly, the investigation of Fe_1-xO-based ammonia synthesis catalysts, invented by Profesor Liu Huazhang and his coworkers, are still preliminary. Profesor Liu et al. have investigated the relationship between the catalytic activities and their microstructures and found that the catalytic activities of two tpyes of catalysts differ greatly from each other despite the sameα-Fe activated phase produced after reduction. However, up to now, there is no work on the factors responsible for such a difference by analyzing the microstructure of both precursors and active phases, especially the microstructural evolution by in-situ XRD.In this thesis, X-ray Diffraction (XRD) method, combining with some other characterizartion methods, such as Scanning Electron Micscopy (SEM), Energy Diverse Spectrum (EDS) and Transmission Electron Micscopy (TEM) was employed to systemically analyse the microstructures of catalyst's precursors and active phase, the distribution feature of the promoters, and their impact on the reduction process. Two traditional Fe₃O₄-based catalysts, A110 and ICI74-1, and two Fe_1-xO-based catalysts, A301 and ZA-5, were chosen and investigated by in situ XRD measurements under almost the same conditions that ammonia synthesis reduction. The Rietveld full-spectroscopy-fit structure analysis, Warren-Averbach microscale and microstrain analysis and Scherrer's equation were employed to systematically analyze fine XRD patterns at each stage during the reduction process of catalysts, and the variations of crystallize size, microstrain, cell parameters and crystal shape of the activated catalyst phases were obtained. The microstructure evolution model of the activated phase,α-Fe, of each catalyst and the dynamic evolution model of crystal growth of each face were proposed. The relationship between intrinsic factors (microstructure difference and development difference of activated sites) and catalytic activities of corresponding catalysts were revealed. The high active mechanism of Fe_1-xO-based ammonia synthesis catalysts was also explored.The results show that the reduction speed of Fe_1-xO-based catalyst is faster, and the reduction temperature is lower than that of the traditional Fe₃O₄-based catalyst, Fe_1-xO-based catalyst is a new type of facile reduction catalyst. Much more pore channels were provided to the Fe_1-xO-based catalyst precursors than to those of the Fe₃O₄-based precursors by the addition of cataltic promoter. Therefore, the distribution of Fe_1-xO-based catalysts on the faces and interfaces of particles as amorphous ultrafine "films" is homogeneous. Much more developped pores were obtained from the Fe_1-xO-based catalyst precursors than those of Fe₃O₄-based precursors. These pore structures are not only good to the reduction of the catalysts, but also can form fine particles of activated phases and intergranular developped pores after reduction and therefore increase the specific surface area. The crystal size evolution of activated phases of catalysts are all obeyed a degenerate law, namely the crystal size of catalysts becomes larger when activated phase,α-Fe, begins to appear, especially for that of the (110) crystal face. The average crystal size of the catalysts initially decreases and then increases a little bit with the increses of the temperature. The average crystal size of the catalysts holds stable within the main functional field of the catalysts. Therefore, the increase of the content of activated phases only related to the increase of the amount of grain. The crystal size distribution of activated phases becomes broad gradually with the increase of temperature and the minimum of the size distribution leans to the smaller size direction. In additional, the ring or core/shell crystals existed inside the interiors can often be seen. Therefore, the crystal growth of activated phase forms gradually from the shell of the precursors to their interiors with the course of alteration-breakage-re-alteration.For four catalysts, the order of development degree of high active crystal faces of (211) is ZA-5＞A301＞A110＞ICI74-1, the value of microstrain follows the order of A301＞ZA-5＞ICI74-1＞A110, and the order of the value of influence of microstrain on crystallize size is ZA-5＞A301＞A110＞ICI74-1. The consistence among the three orders demonstrates that the high activity of Fe_1-xO-based ammonia synthesis catalysts is mainly attributed to the development of active sites of activated phase and interior crystal defects.The simulation of three-dimensional growth morphology of the activated phase has revealed that the activated phase of Fe₃O₄-based ammonia catalysts is mainly cubic, while that of Fe_1-xO-based ammonia catalysts is mainly spherical. The latter has more exposure of high active crystal faces of (111) and more developed active sites than the former. In addition, the activated phases of Fe_1-xO-based ammonia catalysts are evolved from the octahedrons and match well with the morphology of octahedron precursors. This is also one of the intrinsic reasons for facile reduction of the catalysts.XRD was also used to determine the particle size of nanotitania powers. The results were compared with those obtained by other techniques such as SEM and TEM. It has been found that that the mean diameter of the particles estimated from the XRD data is consentient with that obtained by TEM images with the latter being 1.6 % larger than the former. However, the average size obtained by SEM images is 53.4 % larger than that estimated from the XRD data.Our results have also shown that the dispersity and the morphology of Mg(OH)₂ particles can be adjusted by changing the temperature of the hydrothermal approach. The lattice of Mg(OH)₂ crystallite becomes well and well, the ratio of lattice twisted and the cell parameter value of Mg(OH)₂ crystallite become smaller and smaller, as the temperature of hydrothermal approach rising. This can also produce higher angle regular displacement of XRD diffraction peaks of (001) and (101) crystal planes than expected.In addition, we have also employed XRD,SEM,TEM (HRTEM) and EDS to study the morphology, microstructure and chemical composition of the M_xH_yTi₃O₇ nanotubes prepared by ion-change approach, and found that the physical and chemical properties of these nanotubes can be controlled by changing the preparation conditions. Besides the M_xH_yTi₃O₇ nanotubes, nano WS₂ particles with inorganic fullerene-like structure; prepared form the precursor of WS₃ during reduction at high temperatures were also studied by XRD,TEM (HRTEM) and EDS, and the results have revealed that polyglycol dispersant play a important in the formation such a inorganic fullerene-like structure.