Abstract
The utilization of renewable feedstocks and the development of cleaner and more efficient processes are the two main challenges facing us in order to satisfy the increasing energy demand and eliminate climate problems caused by CO2 emission. In this PhD study, the catalytic conversion of glycerol and gold catalyzed CO oxidation have been studied, and the aim is to show how a better understanding of complicated catalytic systems could be obtained via the combination of knowledge in material science and the usage of novel characterization methods, especially through the design of materials—either as the catalyst or the support, in a controlled manner on a nanometer scale.
The combination of glycerol and iron oxide (cobalt oxide) has been explored from both the material point of view and the catalytic point of view, and remarkable findings have been obtained from both sides:
Firstly, glycerol was found to be able to pseudomorphically reduce mesostructured Co3O4 and Fe2O3 to CoO and Fe3O4. The finding itself is exciting, while even more exciting is that, this probably reveals a general method for the synthesis of metal oxides in a lower oxidation state with delicate morphologies. Actually it has been proved that several other alcohols besides glycerol are also active.
Secondly, an iron oxide catalyst has been developed to transfer glycerol to allyl alcohol, and a dehydration and consecutive hydrogen transfer mechanism has been proposed. Although the detailed mechanism is still unknown and commercial application based on this process seems still far away, it is still very remarkable that one single catalyst (iron oxide) can catalyze such a complicated reaction and end up with a fairly high yield of allyl alcohol through an unusual pathway.
Supported gold catalysts have been prepared via the colloidal deposition method and their catalytic activities in glycerol oxidation in the liquid phase have been evaluated. The almost identical particle size distributions of the gold nanoparticles allowed the investigation on the influence of the support in this reaction. A glycerol/NaOH molar ratio of 1:2 was found necessary to reach the total conversion of glycerol, and the activities of these catalysts can be ranked as follows: Au/AC ~ Au/MC > Au/TiO2 > Au/ZnO > Au/Mg(OH)2 > Au/Al2O3.
CaF2 materials were synthesized through different routes including nanocasting, direct thermolysis of Ca(CF3COO)2 and the hydrothermal route, and used as support for gold nanoparticles via colloidal deposition. Catalytic activities of these catalysts in CO oxidation were then evaluated, and several conclusions concerning gold catalysis could be drawn. First, the fact that rather active catalysts can be obtained using a non-oxide support, CaF2, suggests that framework oxygen is most probably not necessary for high activity in gold catalyzed CO oxidation. Second, the preparation method was found to strongly affect the activity of the supported gold catalyst. For CaF2 synthesized using Ca(CF3COO)2 as precursor, carbonaceous species which originate from the decomposition of the precursor can remain on the surface and significantly inhibit the catalytic activity. By calcining the sample in oxygen, the carbonaceous residues can be effectively removed.
A highly active iron oxide supported gold catalyst has been prepared through the colloidal deposition method, and STEM-HAADF analysis confirmed that for the current catalyst, gold nanoclusters have diameters larger than 1.5 nm. Further investigation combining the 2-D image and the intensity profiles allowed the determination of the number of gold atoms along the electron beam. One representative gold particle with a diameter around 1.8 nm was thoroughly studied and it was confirmed that the gold particle has up to 4-5 layers packed along the beam. Although the current catalyst is prepared via colloidal deposition and the catalyst in the literature were prepared via co-precipitation and the difference in the preparation method may lead to different conclusions, our work is at least a clear proof that the bilayered structure and/or diameter ~0.5 nanometer are not mandatory to achieve the high activity for a gold catalyst.
