A Low-Temperature Route Triggered by Water Vapor during the Ethanol-SCR of NO<i>x</i> over Ag/Al₂O₃

A negative temperature dependence was found for the selective catalytic reduction of NO<i>x</i> by ethanol (ethanol-SCR) over Ag/Al₂O₃ in the absence of water vapor. Activation energy measurements for this process confirmed that two reaction routes occurred in different temperature ranges. In situ DRIFTS experiments revealed that these temperature-dependent reactions were closely related to the process of the partial oxidation of ethanol. During the partial oxidation of ethanol at low temperatures below 400 °C, enolic species and acetates were produced, the former of which exhibited much higher activity for NO<i>x</i> reduction than the latter. Therefore, the formation of enolic species and their further transformation to produce N₂ governs the low-temperature route for ethanol-SCR. At temperatures above 400 °C, only acetate appeared during the partial oxidation of ethanol, and its further reaction with NO<i>x</i> accounts for the high-temperature route. More importantly, introduction of water vapor significantly enhanced the deNO<i>x</i> activity of Ag/Al₂O₃ for ethanol-SCR, especially in the low-temperature region. On pure Al₂O₃, however, the ethanol-SCR process was suppressed by the presence of water vapor, indicating that the promotion effect of water vapor is closely related to silver. Within the low-temperature region, water addition promoted the partial oxidation of ethanol to produce enolic species, the occurrence of which also enhanced the formation of NO₂ during the ethanol-SCR over Ag/Al₂O₃. The produced NO₂ in turn accelerated the formation of enolic species and also exhibited a higher reactivity toward enolic species compared with NO. Such synergistic effects of NO₂ and enolic species induced by water vapor addition thus triggered a cyclic reaction pathway for NO<i>x</i> reduction with high efficiency

Silver Valence State Determines the Water Tolerance of Ag/Al₂O₃ for the H₂–C₃H₆–SCR of NO<i>_x</i>

The influence of the silver valence state on Ag/Al₂O₃ on the water tolerance of H₂–C₃H₆–SCR of NO<i>_x</i> was investigated. The valence state of silver species on Ag/Al₂O₃, which was carefully characterized by XPS, UV–vis, and XANES measurements, was adjusted by varying the calcination temperature from 500 to 900 °C. Oxidized silver species were predominant on Ag/Al₂O₃ calcined at temperatures below 600 °C (LT-catalysts), while further increasing the calcination (temperatures above 600 °C, HT-catalysts) promoted the transformation of oxidized silver species into metallic silver clusters. The samples with higher amounts of oxidized silver species exhibited better water tolerance in the H₂–C₃H₆–SCR. Activation energy measurements confirmed that the mechanism of NO<i>_x</i> reduction on these catalysts was the same. In situ DRIFTS studies demonstrated that metallic silver species promoted the formation of active enolic species and the complete oxidation of formate, thus improving the low-temperature activity of HT-catalysts in the absence of water vapor. Water addition eliminated the formate, releasing the active Ag⁺ sites for enolic species formation, and thus promoted the low-temperature activity of LT-catalysts. From a comprehensive point of view, 60% oxidized silver species on Ag/Al₂O₃ catalysts is the optimal percentage for deNO<i>_x</i> performance and water tolerance

Discerning the Role of Ag–O–Al Entities on Ag/γ-Al₂O₃ Surface in NOx Selective Reduction by Ethanol

Alumina-supported silver catalysts (Ag/Al₂O₃) derived from AlOOH, Al­(OH)₃, and Al₂O₃ were investigated for the selective catalytic reduction of NOx by ethanol. In order to discern the role of support Al skeleton in anchoring silver species and reducing NOx, the series of alumina-supported silver catalysts calcined at different temperatures was characterized by means of <i>in situ</i> DRIFTS, XPS, UV–vis DRS, XRD, BET, and NMR. It was found that the NO_<i>x</i> reduction efficiency order as affected by alumina precursors could be generally described as AlOOH > Al₂O₃ ≫ Al­(OH)₃, with the optimum calcination temperature of 600 °C. XPS and UV–vis results indicated that silver ions predominated on the Ag/Al₂O₃ surface. Solid state NMR suggested that the silver ions might be anchored on Al tetrahedral and octahedral sites, forming Ag–O–Al_tetra and Ag–O–Al_octa entities. With the aid of NMR and DFT calculation, Al_octa was found to be the energetically favorable site to support silver ions. However, DFT calculation indicated that the Ag–O–Al_tetra entity can significantly adsorb and activate vital −NCO species rather than the Ag–O–Al_octa entity. A strongly positive correlation between the amount of Al_tetra structures and N₂ production rate confirms the crucial role of Al_tetra in NOx reduction by ethanol

Nature of Ag Species on Ag/γ-Al₂O₃: A Combined Experimental and Theoretical Study

The nature of silver species on Ag/Al₂O₃ catalysts with different silver loadings was studied by photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS) combined with theoretical calculation (DFT). On the basis of selective catalytic reduction of NO_<i>x</i> by ethanol experiments, it was found that the optimum silver content varies from 1 wt % to 2 wt %. The supported silver species are predominated by +1 oxidation state ions attached to surface oxygen atoms (Ag–O) under low silver loading of 2 wt %, which play a crucial role during the HC-SCR process. An Ag–Ag shell emerged clearly in analysis of EXAFS data when silver loading was increased to 2 wt %, which was beneficial for low-temperature activity. The theoretical models for Ag_n^δ+ species (1 ≤ <i>n</i> ≤ 4, both ions and oxidized silver clusters) on alumina were consistent with the coordination structure analysis by EXAFS. The predominant silver ions are most likely stabilized at isolated tetrahedral Al sites (Ag–O–Al_IVb) on the γ-Al₂O₃ (110) surface. However, the most reactive silver ion seems to be anchored on a tricoordinate Al_III site (Ag–O–Al_III). Density of states analysis revealed that the Ag–O–Al_III entity might be a very active silver species in terms of the hybridization of Ag, O, and Al orbitals to promote its catalytic activity

SO₂ Photoaging Enhances the Surface Conversion of NO₂‑to-HONO on Elemental Carbon

Chemical interactions between soot and NO2 are believed to play a significant role in the formation of HONO in the atmosphere. Despite extensive studies, the present understanding of how soot chemistry influences HONO formation remains contentious due to the rapid deactivation of surface reactive sites. In this study, we reveal the novel mechanism that the photoaging of SO2 can notably accelerate the reduction of functionalized elemental carbon (EC) in soot by rapidly removing surface hydroxyl functional groups. The reduced EC can further drive continuous HONO formation due to the rejuvenation of the surface reduction reactivity. We verify that the increase in surface vacancy defects created by the removal of OH groups is the key contributing factor and the reactive centers driving NO2 adsorption and reduction. This finding challenges the existing notion that fresh soot is rapidly deactivated due to the decline in reductive capacity. Our work suggests that aged graphene-like EC on soot may have a significant effect on the chemical conversion of NO2-to-HONO in polluted air, contributing to a better understanding of air pollution chemistry

Spatial Distribution of Brønsted Acid Sites Determines the Mobility of Reactive Cu Ions in the Cu-SSZ-13 Catalyst during the Selective Catalytic Reduction of NO_<i>x</i> with NH₃

The formation of dimer-Cu species, which serve as the active sites of the low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR), relies on the mobility of CuI species in the channels of the Cu-SSZ-13 catalysts. Herein, the key role of framework Brønsted acid sites in the mobility of reactive Cu ions was elucidated via a combination of density functional theory calculations, in situ impedance spectroscopy, and in situ diffuse reflectance ultraviolet–visible spectroscopy. When the number of framework Al sites decreases, the Brønsted acid sites decrease, leading to a systematic increase in the diffusion barrier for [Cu­(NH3)2]+ and less formation of highly reactive dimer-Cu species, which inhibits the low-temperature NH3-SCR reactivity and vice versa. When the spatial distribution of Al sites is uneven, the [Cu­(NH3)2]+ complexes tend to migrate from an Al-poor cage to an Al-rich cage (e.g., cage with paired Al sites), which effectively accelerates the formation of dimer-Cu species and hence promotes the SCR reaction. These findings unveil the mechanism by which framework Brønsted acid sites influence the intercage diffusion and reactivity of [Cu­(NH3)2]+ complexes in Cu-SSZ-13 catalysts and provide new insights for the development of zeolite-based catalysts with excellent SCR activity by regulating the microscopic spatial distribution of framework Brønsted acid sites

Spatial Distribution of Brønsted Acid Sites Determines the Mobility of Reactive Cu Ions in the Cu-SSZ-13 Catalyst during the Selective Catalytic Reduction of NO_<i>x</i> with NH₃

The formation of dimer-Cu species, which serve as the active sites of the low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR), relies on the mobility of CuI species in the channels of the Cu-SSZ-13 catalysts. Herein, the key role of framework Brønsted acid sites in the mobility of reactive Cu ions was elucidated via a combination of density functional theory calculations, in situ impedance spectroscopy, and in situ diffuse reflectance ultraviolet–visible spectroscopy. When the number of framework Al sites decreases, the Brønsted acid sites decrease, leading to a systematic increase in the diffusion barrier for [Cu­(NH3)2]+ and less formation of highly reactive dimer-Cu species, which inhibits the low-temperature NH3-SCR reactivity and vice versa. When the spatial distribution of Al sites is uneven, the [Cu­(NH3)2]+ complexes tend to migrate from an Al-poor cage to an Al-rich cage (e.g., cage with paired Al sites), which effectively accelerates the formation of dimer-Cu species and hence promotes the SCR reaction. These findings unveil the mechanism by which framework Brønsted acid sites influence the intercage diffusion and reactivity of [Cu­(NH3)2]+ complexes in Cu-SSZ-13 catalysts and provide new insights for the development of zeolite-based catalysts with excellent SCR activity by regulating the microscopic spatial distribution of framework Brønsted acid sites

Spatial Distribution of Brønsted Acid Sites Determines the Mobility of Reactive Cu Ions in the Cu-SSZ-13 Catalyst during the Selective Catalytic Reduction of NO_<i>x</i> with NH₃

The formation of dimer-Cu species, which serve as the active sites of the low-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR), relies on the mobility of CuI species in the channels of the Cu-SSZ-13 catalysts. Herein, the key role of framework Brønsted acid sites in the mobility of reactive Cu ions was elucidated via a combination of density functional theory calculations, in situ impedance spectroscopy, and in situ diffuse reflectance ultraviolet–visible spectroscopy. When the number of framework Al sites decreases, the Brønsted acid sites decrease, leading to a systematic increase in the diffusion barrier for [Cu­(NH3)2]+ and less formation of highly reactive dimer-Cu species, which inhibits the low-temperature NH3-SCR reactivity and vice versa. When the spatial distribution of Al sites is uneven, the [Cu­(NH3)2]+ complexes tend to migrate from an Al-poor cage to an Al-rich cage (e.g., cage with paired Al sites), which effectively accelerates the formation of dimer-Cu species and hence promotes the SCR reaction. These findings unveil the mechanism by which framework Brønsted acid sites influence the intercage diffusion and reactivity of [Cu­(NH3)2]+ complexes in Cu-SSZ-13 catalysts and provide new insights for the development of zeolite-based catalysts with excellent SCR activity by regulating the microscopic spatial distribution of framework Brønsted acid sites