ausblenden:
Schlagwörter:
hydrogen, steam reforming, methanol, ammonia, zirconia oxynitride, catalysis, kinetics
Zusammenfassung:
This PhD thesis deals with the catalytic hydrogen production for mobile application, for example for the use in fuel cells for electric cars. Electric powered buses with fuel cells as driving system are well known, but the secure hydrogen storage in adequate amounts for long distance drive is still a topic of discussion. Methanol is an excellent hydrogen carrier. First of all it has a high H:C ratio and therefore a high energy density. Secondly the operating temperature of steam reforming of methanol is comparatively low (250 °C) and there is no risk of coking since methanol has no C-C bond. Thirdly methanol is a liquid, which means that the present gasoline infrastructure can be used.
For the further development of catalysts and for the construction of a reformer it is very important to characterize the catalysts very well. For the dimensioning and the control of an on-board production of hydrogen it is essential to draw accurately on the thermodynamic, chemical and kinetic data of the reaction. At the first part of this work the mesoporous Cu/ZrO2/CeO2-catalysts with various copper contents were characterized and their long-term stability and selectivity were investigated, and the kinetic data were determined.
Carbon monoxide is generated by reforming of carbon containing material. This process is undesired since CO poisons the Pt electrode of the fuel cell. The separation of hydrogen by metal membranes is technically feasible and a high purity of hydrogen can be obtained. However, due to their high densitiy this procedure is not favourable because of its energy loss. In this study a concept is presented, which enables an autothermal mode by application of ceramic membrane and simultaneously could help to deal with the CO problem.
The search for an absolutely selective catalyst is uncertain. The production of CO can be neither chemically nor thermodynamically excluded, if carbon is present in the hydrogen carrier. Since enrichment or separation are unfavourable for energetic and economic reasons, it is reasonable to investigate another reaction system, which is free of carbon. At the last part of this study the catalytic production of hydrogen from ammonia cracking was investigated. Ammonia is an interesting alternative: it has a high hydrogen densitiy, it is available and cheap. Since the Pt electrode is sensitive to reactive substances, it must be ensured, that for example no hydrazine is produced during the ammonia cracking .
A new type of ammonia cracking catalyst was investigated in this study, which unlike the conventional catalyst is not based on metal. Four different zirconium oxynitrides: ß’ ZrON, ß” ZrON, Zr2ON2 and Zr0.88Y0.12O1.72N0.15 (Y2O3 doped ZrON) were prepared by various methods and subsequently tested for their activity in ammonia cracking. A long-term study was carried out on the best catalyst and no hydrazine was detected. On the basis of the data from the accomplished investigations a reaction mechanism is proposed. The result provides a basis for the further improvement of the catalyst.
