Desulfurization, deoxygenation and denitrogenation of heterocycles by a palladium surface: A mechanistic study of thiophene, furan and pyrrole on Pd(111) using laser-induced thermal desorption with Fourier-transform mass spectrometry

The following contains a review of recent work, plus new results, from our laboratory using laser-induced thermal desorption with Fourier transform mass spectrometry (LITD/FTMS) to elucidate the decomposition mechanism of thiophene, furan and pyrrole on Pd(111). The results show that all three heterocycles react differently on Pd, despite their similarity in structure. Thiophene decomposes at 300 K via a C4H4 intermediate species, which subsequently hydrogenates and desorbs as 1,3-butadiene. The cleavage between the C-S bands of thiophene results in the deposition of sulfur, which remains on the Pd(111) surface. In direct contrast to thiophene, furan decomposition on Pd(111) is shown to proceed at 300 K Dia elimination of alpha-H and CO, leaving a C3H3 species on the surface. Heating to 350 K causes dimerization of the C-3 species, forming benzene. In this case, oxygen is removed efficiently from the Pd(111) surface. Preliminary results of pyrrole on Pd(111) indicate that decomposition occurs at around 230 K, a lower temperature than either thiophene or furan. The only reaction product observed is HCN. The data cannot be readily explained by a pathway involving either a C-4 or a C-3 species, however, our results indicate the presence of some hydrocarbon species that decomposes above 325 K yielding hydrogen and above 500 K to give HCN. Removal of nitrogen from Pd(111) is similar to that of furan, in that the formation and subsequent desorption of HCN removes a significant fraction of nitrogen from the surface. (C) 1997 Elsevier Science Ltd.