Transition Metal Catalysts, X-ray Absorption And Diffraction Spectroscopy Study

In this thesis, XAFS and XRD are the main tools used to study the local and interface structures of NiB nano-amorphous alloys, Pd-Pt/Al₂O₃ hydrogen catalyst and CoxO/SiO₂ catalyst of CO oxidation. The relationships among preparing conditions, structures and catalysis properties have been established. The annealed behavior, atom structures and electronic structures, studied by XRD, EXAFS and XANES, revealed that two different amorphous structures can be formed under different preparation conditions. The first nearest coordination structures of Pd-Pt/Al₂O₃ and its fluorin promoted state are studied by EXAFS and XANES. The hole number of Pt 5d orbits described the Pt-Pd bi-metallic electronic structure. XRD, Raman, XPS and XAFS are used to study the characters of local structure of CoxO/SiO₂ catalysts in different calcination temperatures and establish the relationship between surface species and catalyst ability.1. NiB nano-amorphous alloyThe series NiB nano-amorphous alloys were synthesized by chemical reduction method. Their catalytic properties are characterized by the reaction of benzene hydrogenation with 10 MPa H₂ at 313K. The results of ICP show the same atomic composition of NiB samples prepared at different temperature, pH and concentration of precursor salt but with different catalyst activity. The NiB(313K) has the highest benzene hydrogenation conversion of 33.6% among all the samples prepared at different temperatures (273 K, 293 K, 313 K, and 333 K), which is one times higher than others. The NiB with the 6.1 pH value of Ni(CH₃COO)₂ solutions also has the 33.5% conversion, higher than the low pH value ones.The SEM results revealed that all the NiB samples have the similar particle sizes with 20-30nm. Upon annealing at 573K, the XRD results indicated that the NiB(313K) together with NiB(333K) have Ni₃B intermediate product but the NiB(313K) and NiB(333K) samples have not. The NiB sample with 6.1 pH value of Ni(CH₃COO)₂ solution also has the Ni₃B intermediate product. We considered that there may be two kinds of amorphous structures of NiB nano-amorphous alloy.The in-situ XAFS shows that the crystallization of the NiB(313K) is completed in two steps: the generation of intermediate product Ni₃B starting from 548 K and finishing at about 573 K; the unique formation of crystalline Ni from 598 K till 623 K. However, the NiB(273K) crystallizes only in one step with the formation of fcc-Ni in a wide crystallization temperature range beginning from ca. 498 K and ending at 598 K. These indicate that the NiB(273K) and NiB(313K) samples have different initial amorphous structure with different thermal stability. The NiB(313K) is more symmetrical than the NiB(273K) which has broad crystallization temperature range.The XANES spectra of metal Ni considering Debye-Waller factor and 1-8 shells are calculated based on FEFF8.0. While the metal Ni with only one or two shells nearest atoms and higherσ², the Ni K edge XANES spectra is like the NiB amorphous. The hump at 40 eV in the XANES of NiB is attributed to the boron numbers in the first nearest shell of Ni atoms, the higher with the more. The fitting results of EXAFS shown that the Ni-B coordinate number of first nearest shell with NiB(313K) is 3.0, and theσ_s of Ni-Ni shell is about 2.20 A. But the Ni-B coordinate number is only 0.7 with NiB(273K) and theσ_s of Ni-Ni is about 0.24A. As the atomic composition is Ni₇₅B₂₅ It conclude that the NiB(313K) has the Ni₃B like symmetrical atom structure and The NiB(273K) has amorphous Ni like atom structure with poor symmetry accompanied with Ni and B enrichment respectively.In summary, the low preparing temperature of 273K and low pH value (pH<4) is favorable to form the local structure of amorphous Ni with Ni-rich and B-rich regions which have low thermal stability and poor benzene hydrogenation ability. When preparing the NiB nano-amorphous alloy at 313K and normal pH value (pH=6), the NiB catalyst would be good to benzene hydrogenation.2. Pd-PtAy-Al₂O₃ catalystA series of bi-metallic Pt and Pd catalysts supported on both pristine and fluorine-promotedγ-alumina supports were investigated with x-ray absorption spectroscopy. A combination of the XAFS analysis at both Pt and Pd-edges leads to a model which explains the metal-support interaction as a change in the valence states of the Pt atoms: increase of Pt ionization potential with increasing acidity. This model is based upon an electrostatic interaction induced by changes in charge density of the support oxygen ions. Both structure and electronic properties of bimetallic Pt-Pd catalysts were modified by the change in metal-support interaction caused by the fluorine pretreatment of alumina support. The EXAFS analysis indicated that the formation of a new bimetallic phase favored by the addition of fluorine might be an important factor for this increase in sulfur resistance. The high sulfur tolerance of noble metals supported on acidic supports should be attributed to the electron-deficient characteristic of noble metals resulting from the interaction with acidic support (electron acceptor).3. CoOx/SiO₂ catalystCoOx/SiO₂ catalysts with 6 wt% loading were prepared by conventional incipient wetness impregnation method on the silica support and subsequent calcination at elevated temperatures. The catalysts were characterized with X-ray diffraction, laser Raman spectroscopy, Brunauer-Emmett-Teller method, XPS, XAFS and their catalytic activities towards CO oxidation were also tested. The catalysts behave a significant activity for CO combustion reaction, and it was found that the activity decreases with increasing calcination temperature. The CO oxidation achieves an 100% conversion at 200℃over the CoOx/SiO₂ catalyst calcined at 200℃(CoOx(200)/SiO₂). XPS and XAFS results clearly demonstrate the dominance of Co(II) interface with SiO₂ in CoOx(200)/SiO₂. With increasing calcination temperature, Co(II) is oxidized to Co₃O₄, accompanied by the agglomeration, resulting in the decrease of the catalytic activity. This can be explained by the interaction between the cobalt oxide and the silica, together with the dispersion and sintering of the cobalt active species varying with the calcination temperature.