Figure 4. Effect of reactor length on the methanol and hydrogen mole fraction profiles along the length of the micro-structured heat-exchanger reactor for hydrogen production by steam methanol reforming.
The hydrogen and carbon dioxide mole fraction contour maps in the micro-structured heat-exchanger reactor are illustrated in Figure 5 for hydrogen production by steam methanol reforming. The steam reforming catalyst is involved in the production of hydrogen, typically hydrogen gas. Most often, the steam reforming catalyst is involved in the production of hydrogen from methanol and water. In some instances, the methanol steam reforming reaction can be represented by two reactions, namely, a first cracking reaction followed by a water gas shift reaction. When the steam reforming catalyst is coated on a monolith substrate, a suitable amount is provided to produce hydrogen. While the steam reforming catalyst may be in the form of a powder or pressed into pellets, in small-scale applications involving miniature devices and reactors with mesoscale and microscale features, including channels and other miniature device structures, pellets are not feasible due to their size. And both pellets and powders are not often mechanically stable in smaller configurations. In this connection, concerns over attrition, clogging of channels, sufficient adherence to the substrate, and stability during vibration are raised. Thus, while a steam reforming catalyst in the form of a powder or pellets is appropriate in larger devices, diminished performance may result when using the steam reforming catalyst in the form of a powder or pellets in miniature devices and reactors. The steam reforming catalyst is formed on or in a ceramic monolith substrate to provide for a miniaturized chemical reactor including a porous ceramic material having a catalyst immobilized within or upon the porous ceramic material. In miniature devices and reactors, it may be more desirable to have an immobilized support that retains the high porosity and surface area possible with bulk powders. When the steam reforming catalyst is formed on or in a ceramic monolith substrate, the immobilized catalyst is positioned in such a way as to allow reactants to intimately contact the immobilized catalyst, while not degrading the catalytic activity of the catalyst. When fabricating a monolithic steam reforming catalyst system using multilayer ceramic structures, the reactors that constitute the system which typically include a post fire deposition of a catalyst, do not provide for selective deposition of the catalyst material post firing. This is because, in part, it is difficult to deposit catalyst into small channels. In many instances, since the structure is fired prior to introduction of the steam reforming catalyst, the steam reforming catalyst is not able to be positioned where it is needed so as to provide optimum temperature profiles as desired. In order to make steam reforming catalyst system containing multilayer ceramic structures, a green multilayer ceramic structure is coated with the steam reforming catalyst described herein containing yttrium, palladium, a metal oxide and cerium, and optionally zinc, and then the coated green multilayer ceramic structure is fired to provide a steam reforming catalyst system. This steam reforming catalyst system may have a honeycomb structure, and may or may not contain spacers between individual reactors. The ceramic substrate may be porous ceramic substrate.