Catalysts gaseous hydrocarbons reforming

While natural gas reforming is the primary process for the industrial production of H2, the reforming of other gaseous hydrocarbons such as ethane, propane, and n-butane have been explored for the production of H2 for fuel cells.52,97 The reforming of propane and n-butane received particular attention in recent years, because they are the primary constituents of liquefied petroleum gas (LPG), which is available commercially and can be easily transported and stored on-site. LPG could be an attractive fuel for solid oxide fuel cells (SOFCs) and PEMFCs for mobile applications.98 01 The chemistry, thermodynamics, catalysts, kinetics, and reaction mechanism involved in the reforming of C2-C4 hydrocarbons are briefly discussed in this section. 

The design of catalysts for steam reforming of liquid hydrocarbons is dictated mainly from the need to avoid carbon formation. As illustrated in the simplified mechanism (1) in Table 2, the hydrocarbons are adsorbed on the nickel surface, and the C -species, formed by successive a-cleavage of the carbon/carbon bonds, are dehydrogenated stepwise into adsorbed carbon atoms which may dissolve in the nickel crystal. When the concentration of carbon is above saturation, a carbon whisker will nucleate. These reactions compete with the reaction of the Ci-species with adsorbed oxygen atoms to gaseous products. The concentration of adsorbed oxygen depends on the steam adsorption (and CO2 -adsorption) on the catalyst. 

The platforming catalyst was the first example of a reforming catalyst having two functions.43 44 93 100-103 The functions of this bifunctional catalyst consist of platinum-catalyzed reactions (dehydrogenation of cycloalkanes to aromatics, hydrogenation of olefins, and dehydrocyclization) and acid-catalyzed reactions (isomerization of alkanes and cycloalkanes). Hyrocracking is usually an undesirable reaction since it produces gaseous products. However, it may contribute to octane enhancement. n-Decane, for example, can hydrocrack to C3 and C7 hydrocarbons the latter is further transformed to aromatics. 

The selective oxidation or preferential oxidation of CO in hydrogen-rich stream is another important object for ceria based catalysts. The gas mixture from steam reforming/partial oxidation of alcohols or hydrocarbons, followed by the WGS reaction contains mainly FI2, CO2 and a small portion of CO, H2O, and N2. When such gaseous stream would be taken as input for hydrogen fuel cells, the CO has to be removed to avoid poisoning of the anode electrocatalysts. Ceria based nanomaterials, such as ceria/gold, ceria/copper oxide catalysts exhibit suitable catalytic activities and selectivities for CO PROX process. 

Carbon Formation. Steam reforming involves the risk of carbon formation by the decomposition of methane and other hydrocarbons or by the Boudouard reaction (reactions (7) -(10)). Reactions (7) - (8) are catalyzed by nickel (Rostrup-Nielsen, 1984a). The carbon grows as a fibre (whisker) with a nickel crystal at the tip. The methane or carbon monoxide is adsorbed dissociatively on the nickel surface (Alstrup, 1988). Carbon atoms not reacting to gaseous molecules are dissolved in the nickel crystal, and solid carbon nucleates at the non-exposed side of the nickel crystal, preferably from Ae dense (111) surface planes. Reaction (10) results in pyrolytic carbon encapsulating the catalyst. 

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