Hydrogen Adsorption and Dissociation Reactions Using Single Metal Atom Doped Calix [4] Pyrrole as a Catalyst: A DFT Study

Abstract

The increasing reliance on fossil fuel combustion and the corresponding surge in CO₂ emissions have raised a global energy crisis and intensified climate change. It is essential to shift to sustainable energy sources due to limited nature of fossil fuels reserves and their environmental consequences. Among the various alternatives, hydrogen is predominantly regarded as the optimal alternative to fossil fuels and a prominent energy source, owing to its clean and environmentally friendly nature. Hydrogen offers numerous advantages, including its high energy density, no emission of CO2, and diverse applications in various fields. In industries, the hydrogen dissociation step holds significant importance in hydrogenation reactions. Therefore, an effective and low-cost catalyst is desirable for this step. The preparation of single-atom catalysts (SAC) is an emerging approach in the field of catalysis. In single-atom catalysis (SAC), an isolated metal atom is uniformly dispersed onto the support material and minimizes the use of metal and consequently maximizes catalytic activity while reducing costs. In this study, DFT simulations are utilized to investigate the adsorption and splitting of molecular hydrogen on 1st-row transition metal atoms doped into calix[4]pyrrole. Each TM@C4P complex is examined to distinguish the most stable spin state, as transition metals exhibit variable spin states. Interaction energy is calculated to analyze the stability of all transition metal doped calix[4]pyrrole complexes, with the highest interaction energy observed for Mn@C4P (-2.0 eV). Furthermore, NBO, FMO, IRI, and QTAIM analyses reveal the transfer of charge from C4P to the transition metal and confirm the non- covalent interactions among the TM-doped complexes. The adsorption of molecular hydrogen on the TM@C4P complexes exhibits negative adsorption energy, confirming the exothermic nature of H₂ adsorption, except for Ni@C4P. Notably, the homolytic dissociation of H₂ on the Ti@C4P complex displayed the lowest activation barrier (0.04 eV), highlighting its potential as an effective catalyst for hydrogen dissociation reaction(HDR). NBO and EDD analyses are performed to examine the transfer of charge from the metal d-orbitals to the hydrogen antibonding orbital(σ*), which weakens the H-H bond and facilitates the adsorption of hydrogen atoms on the catalyst. Our investigation provides insights into the factors that influence the electronic properties and catalytic performance of TM-doped C4P complexes in hydrogen dissociation reactions, paving the way for the advancement of enhanced hydrogen energy technologies