Theoretical Insights into Heterogeneous Metallic Catalysts for Biomass Based Hydrogen Production

Abstract

The study of different metallic catalysts (Pt, Pd) for the 1,2-Propanediol (1,2-PDO) and formic acid (FA) to hydrogen conversion was addressed by means of computational methods. In order to shed light into the key factors governing the reactivity of these catalysts multiple models were used, ranging from Pt low Miller-index (111) and (100) like surfaces to supported sub-nanometric Pd clusters. Through a detailed study of the competition between C-H, O-H and C-C bond breakings, it was demonstrated that the 1,2-PDO Pt catalyzed decomposition is likely to occur via consecutive dehydrogenation steps, followed by C-C bond cleavages. These latter are more favoured once terminal CO moieties are formed. Hydroxyacetone resulted to be among the major by-products of the reaction. These findings are common both to (111) and (100) Pt surfaces, suggesting that the decomposition reaction may be structure insensitive. In parallel, the catalytic properties of a h-BN supported sub-nanometric Pd6 cluster toward FA dehydrogenation were investigated. A QM/MM approach was used for studying the nucleation mechanism of selected Pd clusters onto a boron nitride nanotube (BNNT) support and the information gained were exploited for the reactivity study. In this framework, the competi- tion between the formate (HCOO) and carboxyl (COOH) FA decomposition pathways was discussed. The Pd6 undercoordination was proposed to be at the basis of a major selectivity towards the formation of the HCOO intermediate. Conversely, the production of CO seems to be inhibited, and this could represent a major improvement for the CO-free production of hydrogen. A more realistic picture of the catalytic system was achieved by taking into account a variety of effects, such as support defectivity and metallic catalyst reconstruction. These were demonstrated to have a non-negligible impact on the FA decomposition reaction mechanism. In particular, the B vacancy-mediated dehydrogenation was found to be competitive with the Pd-mediated one, thus suggesting that the support could actively take part to the catalytic process.