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

Abstract

V-W/TiO2 based catalysts, which are used for the removal of NOx from the exhaust of diesel engines and stationary sources via Selective Catalytic Reduction with NH3 (NH3-SCR), were studied by operando X-ray absorption spectroscopy and emerging photon-in/photon-out techniques. In order to minimize the influence of highly X-ray absorbing tungsten and the fluorescence of titanium we used a high energy resolution fluorescence setup that is able to separate efficiently the V Kβ1,3 emission lines and additionally allows to record valence-to-core (vtc) X-ray emission lines. The High Energy Resolution Fluorescence Detected X-ray Absorption Spectroscopy (HERFD-XAS) and vtc X-ray Emission Spectroscopy (vtc-XES) proved to be the only way to perform operando V K edge X-ray spectroscopic study on industrially relevant V-W/TiO2 catalysts so far. The V-W/TiO2 and V/TiO2 samples were synthesized by incipient wetness impregnation and grafting and exhibit high activity towards NH3-SCR. Raman spectroscopy showed that they mainly contained highly dispersed, isolated and polymeric V-oxo species. HERFD-XAS and XES identified redox cycling of vanadium species between V 4+ and V 5+. However, no direct chemical bond between V and N was observed. As result of the obtained information we suggest that the primary role of V species in SCR is to provide redox sites, whereas, adsorption and reaction of NOx and NHx species may occur via the neighboring O atoms or on the acidic support.

Impact of gas phase reactions and catalyst poisons on the NH3-SCR activity of a V2O5-WO3/TiO2 catalyst at pre-turbine position

“…[17,30] Furthermore, propylene could slightly hinder the reoxidation of V-species, as shown in one of our previous studies. [32] According to our results (Fig. 4 and Fig.…”

“…Additionally, the presence of hydrocarbons might impede the reoxidation of V-species during the NH3-SCR reactions. [32] The deactivation became more pronounced with increasing temperature up to 300 °C. At this temperature the catalyst started to convert the majority of C12H26, thus leading to a competition for active sites between n-dodecane oxidation intermediates and SCR reactants.…”

Hydrocarbon and Soot Oxidation over Cerium and Iron Doped Vanadium SCR Catalysts

“…As mentioned above, this could be due to a switch in the reaction mechanism. At the same time, such a behavior might be caused by the inhibition of catalyst reoxidation due to the presence of NH 3 , CO and other carbonaceous species [2a] …”

In Situ XANES Characterization of V2O5/TiO2–SiO2–MoO3 Catalyst for Selective Catalytic Reduction of NO by NH3

“…XAFS is an effective method for determining the oxidation state and local geometry of vanadium-based catalysts. − However, there are only a few reported applications of XAFS for studying NH 3 -SCR catalysts under the reaction conditions. − Therefore, we investigated three kinds of industrial V 2 O 5 /TiO 2 -type catalysts for the NH 3 -SCR reaction, including V/TSM, using temperature-programmed reduction with H 2 (H 2 -TPR) to provide further insights into reducibility. Furthermore, we performed in situ X-ray absorption near edge structure (XANES) under the reaction conditions to clarify the local structure and oxidation state of the vanadium active center during NH 3 -SCR.…”