Hydrogen adsorption on metal‐functionalized benzene and B‐substituted benzene

Abstract

Hydrogen storage properties of metal doped benzene complexes vary with positions of boron atom substitution in a benzene ring at MP2/6-311++g(d,p) level. Two carbon atoms of a benzene (C6H6) ring positioned at 1-2, 1-3 and 1-4 are replaced by two boron atoms and the structures named as BB1-2, BB1-3 and BB1-4, respectively. Further, pristine as well as boron substituted benzene complexes are decorated with Li, Be,Ti and Ca atoms. C6H6Li, BB1-2Li, BB1-3Li and BB1-4Li complexes can interact with three, three, two and three H2 molecules, respectively, with 6.64, 6.82, 4.65 and 6.82 wt% H2 uptake capacities. Only one H2 molecule gets adsorbed on each C6H6Be, BB1-2Be, BB1-3Be and BB1-4Be complexes with respective H2 uptake capacity of 2.26, 2.32, 2.32 and 2.32 wt%. One additional hydrogen molecule gets adsorbed on Ti(Ca) decorated boron substituted benzene than C6H6Ti(C6H6Ca) with respective H2 uptake capacity 7.54(7.98) and 6.02(9.46) wt%. Electron density of metal atom gets altered significantly after adsorption of H2 molecules. The averaged H2 adsorption energies with Gibbs free energy correction ΔEG are found to be negative for all Li- and Ca-doped complexes indicates thermodynamically unfavorable H2 adsorption where ΔEG values are positive for all Ti- and Be-doped complexes at room temperature indicating thermodynamically favorable H2 adsorption. Interaction between H2 molecules and Be- as well as Ti-doped complexes is found to be stronger than Li- and Ca-doped complexes. Be-doped complexes considered do not satisfy the target set by DOE regarding H2 uptake capacity. Boron substitution improves molecular hydrogen uptake of Benzene-Ti complex by 1.52 wt% and satisfy the target set by DOE. C6H6Li, BB1-2Li, BB1-3Li and BB1-4Li complexes cannot be used for hydrogen storage since hydrogen adsorption on these complexes is endothermic. Ti-doped boron substituted complexes are found to more promising H2 storage material because of their higher H2 uptake capacity as well as their strong interaction with adsorbed hydrogen molecules than Li-, Be- and Ca-doped complexes.