The Significance of Lewis Acid Sites for the Selective Catalytic Reduction of Nitric Oxide on Vanadium‐Based Catalysts

Abstract

The long debated reaction mechanisms of the selective catalytic reduction (SCR) of nitric oxide with ammonia (NH3 ) on vanadium-based catalysts rely on the involvement of Brønsted or Lewis acid sites. This issue has been clearly elucidated using a combination of transient perturbations of the catalyst environment with operando time-resolved spectroscopy to obtain unique molecular level insights. Nitric oxide reacts predominantly with NH3 coordinated to Lewis sites on vanadia on tungsta-titania (V2 O5 -WO3 -TiO2 ), while Brønsted sites are not involved in the catalytic cycle. The Lewis site is a mono-oxo vanadyl group that reduces only in the presence of both nitric oxide and NH3 . We were also able to verify the formation of the nitrosamide (NH2 NO) intermediate, which forms in tandem with vanadium reduction, and thus the entire mechanism of SCR. Our experimental approach, demonstrated in the specific case of SCR, promises to progress the understanding of chemical reactions of technological relevance.

What News in the Surface Chemistry of Bulk and Supported Vanadia Based SCR‐Catalysts: Improvements in their Resistance to Poisoning and Thermal Sintering

“…But there is no clear consensus because several investigations suggested that Lewis acid centers could be also involved. Indeed, Marberger et al . through time‐resolved spectroscopic measurements pointed out the involvement of Lewis acid sites on vanadate species which seems consistent with higher Turn‐Over‐Frequency values compared to those estimated on Brønsted acid sites …”

“…Despite, adsorbed NH 4 + species (ads‐NH 4 + ) dominates, adsorbed NH 3 intermediates on Lewis sites were found exhibiting higher TOF values. Marberger et al . also found from time resolved‐Diffuse Reflectance Infrared spectroscopy (DRIFT) and Diffuse Reflectance Visible spectroscopy (DR‐Vis) that NO reacts preferentially with adsorbed ammonia species bonded to Lewis sites on V 2 O 5 −WO 3 /TiO 2 instead of Brønsted acid sites.…”

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

“…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. That is, the Brønsted acid sites hardly contributed to the SCR activity and mainly served as an NH 3 pool to replenish the Lewis sites.…”

Insights into Redox Dynamics of Vanadium Species Impregnated in Layered Siliceous Zeolitic Structures during Methanol Oxidation Reactions

“…Vanadium‐based materials play an important role in heterogeneous catalysis. Redox properties present in vanadium oxides are very useful to promote significant chemical processes, like oxidations, ammoxidations and reductions . An important parameter that induces catalytic properties in vanadium‐based materials is the support's surface coverage.…”