Assessing the Importance of V(IV) During NH₃−SCR Using Operando EPR Spectroscopy

Abstract

Suggested mechanisms for NH 3 À SCR on V 2 O 5 /TiO 2 based catalysts contain both oxidized and reduced vanadium species. The degree of reduction and the importance of the redox properties of the catalyst are however still widely discussed. Here we show the presence of significant amounts of oxidovanadium(IV), VO 2 + using electron paramagnetic resonance (EPR) spectroscopy of V 2 O 5 /TiO 2 and V 2 O 5 /WO 3 /TiO 2 under operando selective catalytic reduction (SCR) of NO with NH 3 in a fixed bed plug flow reactor operating at high space velocity: 420,000 h À 1. The V(IV) content at the completely oxidized and reduced states is also analyzed. It is found that a significant part of the vanadium is reduced to the EPR active species, VO 2 + at operando SCR conditions and even more when O 2 is removed from the SCR gas.

Role of V and W Sites in V2O5−WO3/TiO2 Catalysts and Effect of Formaldehyde during NH3−SCR of NOx

“…Previously, the signal with the lower Δ /Δ value has been assigned to the vicinity of isolated vanadate and tungstate sites, though without being connected by a direct oxygen bridge, while signal II with the higher value was attributed to species with direct V−O−W connections [17] . VO 2+ species with very similar spin Hamiltonian parameters were also reported by others in V 2 O 5 /WO 3 /TiO 2 catalysts [35–36] . It was proposed that a mixed polymeric V−W monolayer is formed which stabilizes the V sites on the surface and prevents too strong interaction with the TiO 2 matrix [35] …”

“…[17] VO 2 + species with very similar spin Hamiltonian parameters were also reported by others in V 2 O 5 /WO 3 /TiO 2 catalysts. [35][36] It was proposed that a mixed polymeric VÀ W monolayer is formed which stabilizes the V sites on the surface and prevents too strong interaction with the TiO 2 matrix. [35] Exposure of the preoxidized VÀ Ti and VÀ WÀ Ti catalysts to a flow of 0.2 vol % NH 3 /He led to a slight increase of the total VO 2 + EPR intensity, due to reduction of VO 3 + surface sites (Figure S7).…”

Cu-Speciation in Dehydrated CHA Zeolites Studied by H2-TPR and In Situ EPR

“…The reactor was placed in a variable temperature dewar inside an ER 4102ST cavity. The sample was heated by a heated air flow (see ref ). The measurements were typically performed with a frequency of 9.46 GHz and a power of 6.7 mW.…”

Dependence of the Al Distribution in CHA Zeolite on the Presence of Na+ during the Synthesis. An EPR Investigation of Cu Species in CuCHA

“…We have previously observed a correlation between the size of the “redox window” defined in Eq. 2 and the activity of the catalyst in the SCR reaction [44,45] . $$$\vcenter{\openup.5em\halign{$\displaystyle{#}$\cr \Delta {\rm E}{\rm P}{\rm R}{\rm \ }{\rm a}{\rm c}{\rm t}{\rm i}{\rm v}{\rm e}{\rm \ }{\rm C}{\rm u},{\rm \ }{\rm o}{\rm x}-{\rm r}{\rm e}{\rm d}={{{I}_{ox}-\ {I}_{red}}\over{{I}_{fresh}}}\ \cdot{}\hfill\cr {\rm E}{\rm P}{\rm R}{\rm \ }{\rm a}{\rm c}{\rm t}{\rm i}{\rm v}{\rm e}{\rm \ }{\rm C}{\rm u},{\rm \ }{\rm f}{\rm r}{\rm e}{\rm s}{\rm h}\hfill\cr}}$$$ …”