Natural gas steam reforming (SR) over technically used rhodium/alumina (Rh/Al 2 O 3 ) honeycomb catalysts is studied experimentally at temperatures between 923 and 1073 K and steam-to-carbon ratios (S/C) of unity, with regard to coke deposition caused by the decomposition of the product species ethylene (C 2 H 4 ) and carbon monoxide (CO). Furthermore, the process is modeled using detailed reaction mechanisms, and numerical simulations are carried out to describe the coke formation on Rh/Al 2 O 3 catalysts quantitatively. The amount of deposited carbon was detected and analyzed for varying feed mixtures of the products CO and C 2 H 4 diluted in N 2 . During the decomposition of CO, the saturation of the amount of coke is monitored by feeding CO in high concentrations. No saturation occurs for the same amounts of coke resulting from the decomposition of C 2 H 4 . The coking rate caused by the decomposition of C 2 H 4 is found to be ∼25 times higher than the coking rate caused by the decomposition of CO. The differences in coking behavior caused by C 2 H 4 and CO, respectively, are described by coking models.

The initial stage of coke formation in steam reforming of natural gas over rhodium/alumina catalysts was studied microscopically. A well-defined model catalyst prepared by an aerosol technique was placed in a flow reactor to very mildly coke the catalyst sample. Therefore, a natural gas–steam mixture at steam-to-carbon ratios of unity was fed to the reactor operated for thirty minutes at atmospheric pressure and moderate temperatures of 650 ºC. Fresh and used catalyst samples were characterized by SEM-EDX and a recently developed AFM technique, the Oscillating Friction Microscopy (OFM), to analyze the friction characteristics of the sample. OFM combined with SEM-EDX allowed to distinguish between coke depositions, alumina support (Al2O3), and Rh particles and to locate the initial carbon deposition in the process. It was found that coke formation starts on the catalyst particle. The carbonaceous overlayer can be removed from the catalyst and the closely surrounding support by multiples scans with the AFM tip.

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