Ammonium adsorption on Br?nsted acidic centers on low-index vanadium pentoxide surfaces

摘要

Vanadium-based catalysts are used in many technological processes, among which the removal of nitrogen oxides (NO_x) 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 N_2, and a possible reductant in this case is pre-adsorbed NH _3. In this paper, NH_3 adsorption on Br?nsted OH acid centers on low-index surfaces of V_2O_5 (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 NH_4 ~+ 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). [Figure not available: see fulltext.]