Initial Steps in the Selective Catalytic Reduction of NO with NH₃ by TiO₂-Supported Vanadium Oxides

Abstract

Electronic structure calculations at the density functional theory/ B3LYP level (selectively benchmarked by CCSD(T)) were performed on neutral and protonated monomer and dimer clusters of vanadium oxide (V x O y ) on a cluster model of a TiO 2 support to predict the first steps in the mechanism of the selective catalytic reduction (SCR) of nitric oxide by ammonia. The vanadium cluster structures are based on experimental NMR measurements. The first step is Lewis acid−base addition of NH 3 to a vanadium site for the neutral and formation of an "NH 4+ " site on the protonated surface. Different proton transfer pathways, which depend on the initial neutral or protonated sites coupled with addition of NO lead to the formation of NH 2 NO surface species. The mechanisms can be complicated involving many different pathways for the proton transfers, especially for the initial protonated surface. The addition of the doublet NO leading to the formation of NH 2 NO leads to a reduction of a vanadium and transfer of the spin to this site. NH 2 NO desorbs and then undergoes gas-phase rearrangements to form the final products N 2 + H 2 O. The barrier heights for the gas-phase rearrangement process leading to final product formation are comparable in a number of cases to the barrier heights on the catalyst and may represent the rate-determining step. The reaction on the cluster model with a reduced vanadium(+4) proceeds by different paths with addition of NO leading to a second reduced vanadium site. The predicted pathways are consistent with the available experimental data and show that the complete SCR mechanism is very complicated as additional H 2 O molecules will be removed from the surface by the addition of O 2 to fully regenerate the catalyst and oxidize V back to the formal +5 state.

Analogous Mechanistic Features of NH3-SCR over Vanadium Oxide and Copper Zeolite Catalysts

“…First, V­(V) or Cu­(II) species is reduced to V­(IV) or Cu­(I) species under NH 3 + NO; this is accompanied by the formation of N 2 and H 2 O, presumably via NH 2 NO ,,− or NH 4 NO 2 − intermediate. For both V − and Cu catalysts, − there has been a long debate on the behavior of the reactive NH 3 adspecies in this step (the so-called reduction half-cycle); it is unclear whether the NH 3 adsorbed on Brønsted acid sites (B-NH 3 ) or Lewis acid sites (L-NH 3 ) acts as an intermediate in NH 3 -SCR. This is generally examined via in situ infrared (IR) spectroscopy during NH 3 -SCR or after the adsorption of NH 3 on V- and Cu-based catalysts.…”

“…Operando spectroscopic results have assisted in clarifying that L-NH 3 , or in other words, the NH 3 adsorbed on an oxidized metal cation, V­(V) or Cu­(II), is directly involved in this step. ,,− Computational investigations of the reduction half-cycle of V , and Cu ,, catalysts have shown a common transition state in which a gas-phase (or weakly adsorbed) NO molecule directly reacts with L-NH 3 and the adjacent oxidized metal cation species, V­(V) or Cu­(II), to yield N 2 , H 2 O, a reduced cation [V­(IV) or Cu­(I)], and a proton on the catalyst. The formation of protons (H + ) through the reduction half-cycle has been evidenced by an increase in the number of Brønsted acid sites, , which further interact with NH 3 to form NH 4 + on the V , and Cu − catalyst surfaces. There is an ongoing debate regarding the oxidation half-cycle; one of the widely accepted pathways involves the oxidation of the reduced cation [V­(IV) or Cu­(I)] by O 2 to produce oxidized cations [V­(V) or Cu­(II)] and H 2 O. ,,, …”

Theoretical design of transition metal-doped TiO2 for the selective catalytic reduction of NO with NH3 by DFT calculations

“…† It can be seen that the adsorption and desorption processes showed significant differences regarding the electronic energies and Gibbs free energies due to the change of entropy. 38 However, the reaction energies and barriers showed little difference between the electronic energies and free energies (150 °C and 250 °C), where the results for the key steps and trends for the activity and N 2 selectivity did not change. Therefore, the DFT calculation results in the study are reliable.…”

“…It can be seen that NH 3 adsorption would not change the spin state of these metal-doped TiO 2 catalysts, while NO can bring an unpaired spin, resulting in the occurrence of redox processes. 38…”

“…33-35. There are also some theoretical studies on TiO 2 -based catalysts for understanding the NH 3 -SCR mechanism by the DFT method. 12,[36][37][38] The primary proposed NH 3 -SCR mechanism is summarized as: [39][40][41] adsorbed NH 3 on the surface Lewis acid sites is activated by the lattice oxygen to form NH…”