Chemisorption and decomposition of pyrrole on Pd{111}

XPS, TPD and HREEL results indicate that molecular pyrrole is a fragile adsorbate on clean Pd{111}, At 200 K and for low coverages, the molecule remains intact and adopts an almost flat-lying geometry. With increasing coverage, pyrrole molecules tilt away from the surface and undergo N-H bond cleavage to form strongly tilted pyrrolyl (C₄H₄N) species. In addition, a weakly bound, strongly tilted form of molecular pyrrole is observed at coverages approaching saturation. Heating pyrrole monolayers results in desorption of ∼15% of the overlayer as molecular pyrrole and H_a+C₄H₄N_a recombination with formation of flat-lying pyrrole molecules. This strongly bound species undergoes decomposition to adsorbed CN, CH_x and H, leading ultimately to desorption of HCN and H₂. The implications of these results for the production of pyrrole by a heterogeneously catalysed route are discussed.