Ammonium adsorption on Brønsted acidic centers on low-index vanadium pentoxide surfaces

Vanadium-based catalysts are used in many technological processes, among which the removal of nitrogen oxides (NOx) from waste gases is one of the most important. The chemical reaction responsible for this selective catalytic reaction (SCR) is based on the reduction of NOx molecules to N2, and a possible reductant in this case is pre-adsorbed NH3. In this paper, NH3 adsorption on Brønsted OH acid centers on low-index surfaces of V2O5 (010, 100, 001) is studied using a theoretical DFT method with a gradient-corrected functional (RPBE) in the embedded cluster approximation model. The results of the calculations show that ammonia molecules are spontaneously stabilized on all low-index surfaces of the investigated catalyst, with adsorption energies ranging from -0.34 to -2 eV. Two different mechanisms of ammonia adsorption occur: the predominant mechanism involves the transfer of a proton from a surface OH group and the stabilization of ammonia as an NH4 (+) cation bonded to surface O atom(s), while an alternative mechanism involves the hydrogen bonding of NH3 to a surface OH moiety. The latter binding mode is present only in cases of stabilization over a doubly coordinated O(2) center at a (100) surface. The results of the calculations indicate that a nondirectional local electrostatic interaction with ammonia approaching a surface predetermines the mode of stabilization, whereas hydrogen-bonding interactions are the main force stabilizing the adsorbed ammonia. Utilizing the geometric features of the hydrogen bonds, the overall strength of these interactions was quantified and qualitatively correlated (R = 0.93) with the magnitude of the stabilization effect (i.e., the adsorption energy).