XPS and SIMS studies of carbon deposits on Pt/Al2O3 and Pd/SiO2 catalysts applied in the synthesis of hydrogen cyanide and selective hydrogenation of acetylene

The morphology, chemical composition and graphiticity of carbons deposited or generated on catalyst surfaces during operation under technical conditions were analyzed and compared with respect to the catalytic activity. The growth of carbon filaments was observed on the surfaces of high-temperature Pt/Al2O3 catalysts used for the synthesis of hydrogen cyanide as well as, surprisingly, low-temperature Pd/SiO2 catalysts used in the vinyl chloride process if enhanced concentrations of Fe were present. At low impurity levels the deposition of glossy pyrocarbon and microcrystalline carbon was observed at T approximate to 1200 degrees C and polymeric and microcrystalline carbon at T<200 degrees C. Selected features of the C1s XPS signals turned out to be suitable to characterize the graphiticity of the products of coke formation at a high temperature. SIMS fragment ion ratios were utilized as rough qualitative probes for the microstructural properties of the coke in comparison with well-defined grades of carbon. (C) 1999 Elsevier Science B.V. All rights reserved.