Methane dehydrogenation on 3d 13-atom transition-metal clusters: A density functional theory investigation combined with Spearman rank correlation analysis

摘要

The rational use of non-renewable energy sources such as methane (CH4) has been considered a promising alternative to several high-value chemicals. Therefore, there is a considerable effort to identify the main physical-chemical features for modulating CH4 dehydrogenation, in particular, at the nanoscale regime, where new effects could promote the breaking of the C-H bond. Thus, at this work, we report a theoretical investigation of the dehydrogenation of CH4 based on a step-by-step process on 3d 13-atom transition-metal (TM) clusters (TM= Fe, Co, Ni, Cu) through ab initio density functional theory calculations combined with the Spearman rank correlation analysis and the unity bond index-quadratic exponential potential (UBI-QEP) model. Our results revealed that the adsorption/interaction of CH4 and its dehydrogenated species (CH3, CH2, CH, and C) is guided by three factors, namely, (i) charge transfer from the TM13 clusters to the CHn species, (ii) enhancement of the sp - d coupling between the electronic states of the CHn species (sp-) and TM13 (d-states) systems, (iii) number of unpaired electrons in the CHn species, in which (ii) and (iii) increases by decreasing n, which is consistent with the tendency of the C atom to restore its 4-fold coordination. The H co-adsorption promotes an increasing in the adsorption/interaction energy as it promotes an enhancement of sp - d hybridization, which has a maximum for, Fe-13 and Co-13 and minimum for Cu-13. From the Spearman correlation analysis, we identified that the effective coordination number, the Hirshfeld charge on the C atom, and the distance between the CHn and TM13 systems drive the magnitude of the adsorption energy, in particular, at the H co-adsorption regime. The thermodynamic and kinetic properties obtained via the UBI-QEP model indicates that the CH4 dehyrogenation is slightly favorable on Ni-13 cluster, while kinetically CH4 dehydrogenation process would take place much easier on Ni-13, Fe-13, and Co-13 rather than on the Cu-13 cluster.