Figure 5. Hydrogen and carbon dioxide mole fraction contour maps in the micro-structured heat-exchanger reactor for hydrogen production by steam methanol reforming.
The mesh and methanol mole fraction contour maps in the micro-structured heat-exchanger reactor are illustrated in Figure 6 for hydrogen production by steam methanol reforming. The alcohol steam reforming catalyst converts a lower alcohol such as methanol into hydrogen and carbon oxides. In the specific case where the lower alcohol is methanol, the alcohol steam reforming catalyst may be called a methanol steam reforming catalyst. The generation of carbon monoxide is undesired as carbon monoxide is a poison to the fuel cell electrode [49, 50]. Even generating small amounts of carbon monoxide can cause significant degradation [51, 52]. Consequently, any decrease in generating carbon monoxide is a positive advance in the steam reforming catalyst field. The alcohol steam reforming catalysts containing yttrium, palladium, a metal oxide and cerium, and optionally zinc generally display high carbon dioxide selectivity and thus have reduced or mitigated generation of carbon monoxide. It is also difficult to improve both good methanol activity and good carbon dioxide selectivity in alcohol steam reforming catalysts [53, 54]. Often, improvement in one of these properties results in a decrease in the other property [55, 56]. Consequently, merely combining two catalyst components, one that improves methanol activity and the other that improves carbon dioxide selectivity, does not typically result in a material that has both good methanol activity and good carbon dioxide selectivity. Even small increases in carbon monoxide generation are disfavored because of the large resultant poisoning effect [57, 58]. The fuel combustion and steam reforming processes can be stably and efficiently operated at lower temperatures, without the need for energy input to sustain or even to start the micro-combustion process. In some instances, the micro-combustor is started with hydrogen or vapors such as methanol. Heat losses can be effectively controlled and reduced. Another advantage is that the simplicity of the design and the materials used enable mass production at competitive costs. Another advantage is an extremely fast response time. When a suitable amount of yttrium is included in a steam methanol reforming catalyst based on palladium, a metal oxide and cerium, and optionally zinc, a steam reforming catalyst that departs from the undesired trade-off relationship results. Specifically, steam reforming catalysts containing yttrium, palladium, a metal oxide and cerium, and optionally zinc display both desirable properties of a high methanol conversion rate and high carbon dioxide selectivity. The steam reforming catalyst contains a suitable amount of at least one metal oxide and cerium to contribute to high methanol conversion properties. The cerium may or may not act as a support. In instances where the steam methanol reforming catalyst is subjected to high temperatures, such as during firing of a green ceramic substrate coated with the steam reforming catalyst, two or more metals of the steam reforming catalyst may form an alloy. If the steam methanol reforming catalyst does not contain cerium, then the steam reforming catalyst contains at least one metal oxide.