Coke Formation in Steam Reforming of Natural Gas over Rhodium/Alumina Catalysts: An Atomic Force Microscopy Study using the Oscillating Friction Mode

Abstract

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.