Transformation of ethylene to higher hydrocarbons on silica-supported Ir catalysts: the nature of carbonaceous deposits

The first stage of ethylene decomposition followed by second stage of temperature-programmed surface reduction (H2-TPSR) to produce higher hydrocarbons at different temperatures over silica-supported iridium catalysts has been investigated. The catalysts for the two stepwise reactions are characterized by X-ray diffraction, Raman and Fourier transformed infrared spectroscopies, temperature-programmed reduction, and mass spectroscopy. These studies reveal that ethylene decomposition at low temperatures (≤673 K) in the first stage produces mainly C1 hydrocarbon moieties on the Ir surface via dissociative adsorption, the sequential hydrogenation in the second stage will give arise to CH4. The surface polymerization of C1 to higher hydrocarbon species and metathesis reactions under these temperatures are also clearly evident. When ethylene is decomposed at 773–973 K, stable graphitic carbon deposits with poor propensity for hydrogenation are obtained. Interestingly, water formed from surface dehydroxylation on silica can produce a significant quantity of CO/H2 with these carbons during the H2-TPSR at elevated temperature.