The reaction mechanism for the SCR process on monomer V⁵⁺sites and the effect of modified Brønsted acidity

Abstract

The energetics, structures and activity of a monomeric VO3H/TiO2(001) catalyst are investigated for the selective catalytic reduction (SCR) reaction by the use of density functional theory (DFT). Furthermore we study the influences of a dopant substitute in the TiO2 support and its effects on the known properties of the SCR system such as Brønsted acidity and reducibility of vanadium. We find for the reduction part of the SCR mechanism that it involves two Ti-O-V oxygen sites. One is a hydroxyl possessing Brønsted acidity which contributes to the formation of NH4(+), while the other accepts a proton which charge stabilizes the reduced active site. In the reduction the proton is donated to the latter due to a reaction between NH3 and NO that forms a H2NNO molecule which decomposes into N2(g) and H2O(g). A dopant substitution of 10 different dopants: Si, Ge, Se, Zr, Sn, Te, Hf, V, Mo and W at each of the sites, which participate in the reaction, modifies the energetics and therefore the SCR activity. We find that Brønsted acidity is a descriptor for the SCR activity at low temperatures. Based on this descriptor we find that Zr, Hf and Sn have a positive effect as they decrease the activation energy for the SCR reaction.

NH3-SCR of NO with novel active, supported vanadium-containing Keggin-type heteropolyacid catalysts

“…In the generally accepted mechanisms for the SCR reaction, vanadium is surrounded by oxygen atoms whereas NO and NH 3 are in the second coordination sphere which is generally not detected by EPR spectroscopy. 44,54 This is in accordance with our results since all observed spectra correspond to V coordinated to oxygen atoms. 10 as the magnitude of the gap between the upper and lower plateaus.…”

In Situ DRIFTS Studies of NH3-SCR Mechanism over V2O5-CeO2/TiO2-ZrO2 Catalysts for Selective Catalytic Reduction of NOx

“…Topsoe et al [3] proposed a “Brønsted NH 4 + ” mechanism over a V 2 O 5 -based catalyst, which has gained the majority of support in the literature. Arnarson et al [4] observed the SCR reaction over the VO 3 H/TiO 2 catalyst and demonstrated that the Brønsted acid site served to capture the NH 3 and increased the NH 4 + stability (increased Brønsted acid strength), which impacted the catalytic rate in a negative direction. Marberger et al [5] had a similar conclusion for the V 2 O 5 -WO 3 /TiO 2 catalyst.…”

NH3-SCR over V–W/TiO2 Investigated by Operando X-ray Absorption and Emission Spectroscopy

“…On the other hand, there is no agreement on the nature of sites for adsorption of NH3, which can be V, [14][15][16][17] Ti, 20,24 or even sites formed by other dopants. 24 These uncertainties are a result of DRIFTS limited capability to distinguish between V and Ti adsorption sites, especially during catalyst working in high water vapor environments. Summing up, operando techniques allowing straightforward observation and quantification of redox dynamic of V sites as well as selective characterization of adsorbed species under realistic conditions (e.g.…”