Mesoporous tantalum oxide catalysts for nitrogen fixation.

摘要

A variety of mesoporous tantalum oxide supported catalytic systems were synthesized and investigated for their activities in nitrogen activation, including the Schrauzer-type photocatalytic process and the Haber-Bosch type thermocatalytic process. Mesoporous Ta oxide possesses a high thermal stability, high surface area, tunable wall composition and, most interestingly, variable oxidation states of the transitional metal sites. For this reason it represents a unique support for heterogeneous catalysis. Modification of the surface properties by doping active metal agents and various thermal treatments can further improve the activity. Results obtained in this work showed that for the photocatalytic process pure Ta oxide, with suitable band gap near-UV, exhibited relatively low activity; however, 1 wt% Fe3+ doping increased the activity by a factor of 3. For the Haber process, the Ba-Ru-Ta material was the most active system. Ru3(CO)12 proved to be the best precursor for the active Ru metal component, and Ba(NO 3)2 was the best precursor for the BaO promoter. Remarkably, this system shows a very low activation energy of 9.3 kJ/mol as well as a clear involvement of Ta specie(s) during the catalytic reaction. This suggests a different mechanism than that proposed for standard Ru-based Haber synthesis, which uses alumina, silica and magnesia supports, might be functioning. The results in this thesis clearly show the enormous potential of mesoporous transition metal oxides in catalysis, the first porous support materials offering variable oxidation states. All materials in this work were characterized by a combination of techniques including XRD, TEM, nitrogen adsorption, XPS, EDS, and H 2-TDA.