Adsorption and C-C bond cleavage of benzene on hematite α-Fe2O3 surfaces: a DFT mechanistic study

Reforming tar molecules into smaller gaseous molecules has been a critical challenge for biomass energy utilization. Hematite (alpha-Fe2O3) has been demonstrated as an effective catalyst for the catalytic reforming of tar, nevertheless, the detailed mechanism of alpha-Fe2O3 catalyzed tar reforming remains unclear. In this work, we apply the density functional theory method to investigate this problem. Specifically, we study both (0001) and (0112) surface structures of alpha-Fe2O3 and then use the structures to investigate the adsorption and C-C bond cleavage of benzene on these surfaces. Our results show that the dominant interactions between benzene and a single Fe-terminated (0001) surface are van der Waals forces, yet benzene could be chemisorbed on the Fe and O co-exposed (0112) surface via strong C-O interactions. As a result, the (0001) surface is not active towards benzene cleavage, whereas the (0112) surface can promote the aromatic C-C bond breaking. Furthermore, our calculations indicate that chain-like alkene species and carbonyl species are the two types of potential products that form after the C-C bond cleavage of benzene on the alpha-Fe2O3 (0112) surface, with the activation energy of 1.78 eV and 2.62 eV, respectively. In summary, we reveal the importance of co-adsorption on both Fe and O centers and oxidative addition on C-C bond cleavage of aromatic compounds on the alpha-Fe2O3 surface, which provides novel insights into the mechanisms of tar cracking on oxide catalysts.