DFT studies on catalytic properties of isolated and carbon nanotube supported Pd9cluster Part II. Hydro-isomerization of butene isomers

Abstract

The processes involved in the butene hydro-isomerization, occurring on a small palladium cluster in the presence of dissociated hydrogen, have been investigated by means of DFT and DFT/MM approaches. This study has been performed both on an isolated (unsupported) Pd₉ cluster and on the same cluster when it is supported on a portion of a single-walled armchair(6,6) carbon nanotube. The study follows another investigation which has already been published concerning the adsorption, fragmentation and diffusion of hydrogen on the same metal cluster. The main aspects involved in the parallel reaction steps of the whole hydro-isomerization mechanisms are not strongly affected by the presence of the support, which does, however, modify the energetics involved, likely due to the presence of strong metal surface interaction (SMSI) effects. Noticeably, a common step corresponding to the diffusion of one hydrogen atom is present. This diffusion step creates a characteristic semihydrogenated surface species along the occurrence of all the reaction pathways. Hence, the semihydrogenated species is a kind of molecular node able to connect the transformation pathways of the different surface species involved in the hydro-isomerization processes. Considering the energetics involved in the processes of both supported and unsupported systems and being aware of the simplification introduced in studying the same systems, it is still possible (i) to emphasize the basis importance of taking account of the support in modeling catalytic properties and (ii) to state that the models proposed here are able to capture the main characteristics of the title reaction.