Calculating the adsorption energy of a charged adsorbent in a periodic metallic system – the case of BH4− hydrolysis on the Ag(111) surface

Abstract

The hydrolysis of borohydride on the Ag(111) surface is explored theoretically to obtain the in-depth reaction mechanism. Many heterogeneously catalyzed reactions like this involve the adsorption of charged species on metals. DFT calculations of charged systems, with periodic boundaries, face serious problems, concerning convergence and reliability of the results. To study the heterogeneously catalyzed reactions, a simple method to calculate the adsorption energy of charged systems in metallic periodic cells is proposed. In this method, a counter ion is placed at a non-interactive distance, in an aqueous medium, so that the calculated system is neutral. Bader analysis is used to validate that the calculated couple is charged correctly. Adsorption energies of F⁻, Cl⁻, Br⁻, OH⁻, BH₄⁻, ClO₄⁻ and H⁻ ions on the Ag(111) surface in an aqueous medium were determined using Na⁺ and K⁺ as counter ions, to evaluate the performance of this method. The adsorption of the divalent ions S²⁻, Se²⁻ and SO₄²⁻ on different surfaces was studied as well. Then this method was used to explore the hydrolysis of BH₄⁻ ions, which have a high theoretical hydrogen storage capacity, on the Ag(111) surface. The results point out that during the catalytic hydrolysis only one hydrogen atom from borohydride is transferred to the surface. In the first step one hydrogen atom from BH₄⁻ is transferred to the silver surface; this H atom reacts with a hydrogen atom that is released from an adsorbed water molecule; in addition, a hydrogen molecule is released in the second step (one atom from *BH₄⁻ and one from *H₂O). Thus, the mechanisms of the catalyzed reductions by BH₄⁻ and the hydrogen evolution reactions must be reconsidered.