A kinetic model of the hydrogen assisted selective catalytic reduction of NO with ammonia over Ag/Al2O3

A global kinetic model which describes H₂‐assisted NH₃‐SCR over an Ag/Al₂O₃ monolith catalyst has been developed. The intention is that the model can be applied for dosing NH₃ and H₂ to an Ag/Al₂O₃ catalyst in a real automotive application as well as contribute to an increased understanding of the reaction mechanism for NH₃‐SCR. Therefore, the model needs to be simple and accurately predict the conversion of NO_x. The reduction of NO is described by a global reaction, with a molar stoichiometry between NO, NH₃ and H₂ of 1:1:2. Further reactions included in the model are the oxidation of NH₃ to N₂ and NO, oxidation of H₂, and the adsorption and desorption of NH₃. The model was fitted to the results of an NH₃‐TPD experiment, an NH₃ oxidation experiment, and a series of H₂‐assisted NH₃‐SCR steady‐state experiments. The model predicts the conversion of NO_x well even during transient experiments. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4325–4333, 2013     

Effect of the Presence of Ceria in the NSR Catalyst on the Hydrothermal Resistance and Global DeNOx Performance of Coupled LNT–SCR Systems

The global performance of coupled LNT–SCR systems, addressed to high NOx-to-N₂ conversion, minimal ammonia slip and null N₂O production, as well as the hydrothermal resistance of single NSR and SCR monolith catalysts and their coupling is discussed. Pt–Ba/Al₂O₃ and Pt–Ce–Ba/Al₂O₃ were washcoated on cordierite monoliths as NSR catalysts, and Cu/CHA was washcoated on similar monoliths as SCR catalysts. Both monoliths were coupled in two subsequent reactors to conform the LNT–SCR system. Previously to washcoating, the fresh powder catalysts and after severe hydrothermal aging were fully characterized by N₂ adsorption–desorption isotherms at 77 K, X-ray diffraction, NH₃ temperature-programmed desorption, and H₂ chemisorption to relate textural and chemical characteristics with the DeNO_x performance. The Cu/CHA catalyst shows an excellent hydrothermal resistance for the NH₃–SCR reaction. Incorporation of ceria to the model Pt–BaO/Al₂O₃is beneficial for the NO-to-NOx oxidation and NO₂ storage, improving NO conversion at low temperature and reducing the NH₃ slip. However, addition of ceria is detrimental for the hydrothermal resistance of the NSR catalyst. However, this detrimental effect is minimized when the NSR catalyst is coupled with the Cu/CHA monolith downstream of the NSR catalyst, achieving the coupled LNT–SCR device high NO conversion and minimal NH₃ slip with superior N₂ selectivity for an extended temperature windows, including as low as 220 °C, and maintaining performance even after severe hydrothermal aging.     

Experimental and Modelling Study of a Dual-Layer NH3 Slip Monolith Catalyst for Automotive SCR Aftertreatment Systems

The approach to the development of a chemically and physically consistent mathematical model of ASC dual-layer (SCR + PGM) washcoated monolith converters is herein presented. Steady-state and transient kinetic runs were performed over each one of the two ASC components (SCR and PGM) in the form of powders and also over the two mixed powdered catalysts, thus acquiring information on the interactions between the SCR and the PGM catalytic chemistries. Global kinetic models were fitted to the SCR and to the PGM catalyst data, and validated against experiments performed over both washcoated single-layered SCR and PGM monoliths and over a full dual-layer ASC honeycomb catalyst (SCR layer on top). It was found that the dual-layer (SCR + PGM) ASC architecture grants increased N₂ selectivities compared to a PGM-only washcoat.     

Influence of Al2O3 support on the activity of Ag/Al2O3 catalysts for SCR of NO with decane

The role of the Al₂O₃ support on the activity of supported Ag catalyst towards the selective catalytic reduction (SCR) of NO with decane is elucidated. A series of Ag/Al₂O₃ catalysts were prepared by impregnation method and characterized by N₂ pore size distribution, XRD, UV–Vis, in-situ FT-IR and acidity measurement by NH₃ and pyridine adsorption. The catalytic activity differences of Ag/Al₂O₃ are correlated with different properties of Al₂O₃ supports and the active Ag species formed. 4wt% Ag supported on sol-gel prepared Al₂O₃ (Ag/Al₂O₃ (SG), showed higher NO _x conversion (65% at 400 °C), compared with the respective catalysts made from commercial Al₂O₃ (Ag/Al₂O₃ (GB), Ag/Al₂O₃ (ALO), (∼26 and 7% at 400 °C). The higher surface area, acidity and pore size distribution in sol–gel prepared Al₂O₃ (SG) results in higher NO and hydrocarbon conversion. Based on the UV–vis characterization, the activity of NO reduction is correlated to the presence of Ag_n^δ+ clusters and acidity of Al₂O₃ support was found to be one of the important parameter in promoting the formation and stabilization of Ag_n^δ+ clusters. Furthermore from pyridine adsorption results, presence of more number of Bronsted acid sites in Ag/Al₂O₃ (SG) is confirmed, which could also contribute to low temperature hydrocarbon activation and improve NO conversion. In situ FT-IR measurements revealed the higher rate of –CN and –NCO intermediate species formation over 4wt% Ag/Al₂O₃ (SG). We conclude that the physico–chemical properties of Al₂O₃ play a crucial role in NO _x conversion over Ag/Al₂O₃ catalysts. Thus, the activity of the Ag/Al₂O₃ catalyst can be tailored by using a proper type of Al₂O₃ support.     

On the Detrimental Effect of Tungstates on the n-C10-SCR of NO x on Ag/γ-Al2O3

The influence of the addition of W to Al₂O₃, promoted or not by Ag, on the n-C₁₀ SCR of NO _x was investigated. It was shown that the addition of W was detrimental to the n-C₁₀ SCR reaction. Based on the NO _x -TPD, the XPS and the n-C₁₀ SCR measurements, it was concluded that the loss of activity observed at temperatures lower than 400 °C on the Ag/W(5)–Al₂O₃ catalyst compared with the Ag/Al₂O₃ sample is likely due to the preferential deposition of Ag on the tungstate phase, making it inactive for the n-C₁₀ SCR reaction which requires the active silver species to be in close contact with the Al₂O₃. At higher temperatures, the occupation, by the tungstates, of the Al₂O₃ sites responsible for the n-C₁₀-SCR reaction is proposed to be an additional drawback accounting for the detrimental effect of W on Al₂O₃-supported catalysts promoted or not by Ag.     

Effect of SO2 on NOx reduction by ethanol over Ag/Al2O3 catalyst

A new Ag/Al₂O₃ catalyst for removing NO_x in diesel engine exhaust gas was developed. The influence of SO₂ on the reduction of lean NO_x by ethanol over the Ag/Al₂O₃ catalyst was evaluated in simulated diesel exhaust and characterized using TPD, XRD, XPS, SEM and BET measurements. The Ag/Al₂O₃ catalyst was highly active for the reduction of NO_x with ethanol in the presence of SO₂ although the reduction of NO_x is suppressed at lower temperatures. The activity for NO_x reduction is high even on the Ag/Al₂O₃ catalyst exposed to a SO₂ (200 ppm)/O₂ (10%)/H₂O (10%) flow for 20 h at 723 K and comparable to that on the fresh Ag/Al₂O₃ catalyst. No crystallized Ag metal and Ag compounds were formed by the SO₂/O₂/H₂O exposure. On the other hand, crystallized Ag₂SO₄ was easily formed when the Ag/Al₂O₃ catalyst was exposed to a SO₂ (200 ppm)/O₂ (10%)/NO (800 ppm)/H₂O (10%) flow for 10 h at 723 K. XRD, SEM and XPS studies showed that the formation of crystallized Ag₂SO₄ results in growing of Ag particles in larger size and lowering the surface content of Ag particles. In addition, the specific surface area of the Ag/Al₂O₃ catalyst decreases from 221 to 193 m²/g. Although the dispersion of Ag particles was decreased by the formation of Ag₂SO₄, the activity for the reduction of lean NO_x was, remarkably, not affected. This suggests that the Ag–alumina sites created by the Ag₂SO₄ formation are still active for the lean catalytic reduction of NO_x. This revised version was published online in July 2006 with corrections to the Cover Date.     

H2-Induced NO x Adsorption/Desorption over Ag/Al2O3: Transient Experiments and TPD Study

NO adsorption/desorption over 1 wt% Ag/Al₂O₃ was studied by a combination of isothermal transient adsorption/desorption and NO _x temperature-programmed desorption (NO _x -TPD) methods. NO _x -TPD profiles obtained for Ag/Al₂O₃ were identified by comparison with decomposition profiles of “model” AgNO₃/Al₂O₃ and Al(NO₃)₃/Al₂O₃ prepared by impregnation of Al₂O₃ with individual AgNO₃ and Al(NO₃)₃ compounds. The data obtained indicate that H₂-induced NO adsorption leads to the formation of surface Ag and Al-nitrates. Their accumulation on the catalyst surface is accompanied by an intensive NO₂ evolution, which proceeds primarily via reaction of surface nitrates with NO. Thus, NO₂ formation appears to result from an intrinsic stage of the H₂-induced NO _x adsorption process, rather than from the direct oxidation of NO by gaseous oxygen catalyzed by Ag.     

Unusual Activity Enhancement of NO Conversion over Ag/Al2O3 by Using a Mixed NH3/H2 Reductant Under Lean Conditions

Addition of H₂ to a NO/NH₃/O₂/H₂O feed for selective catalytic reduction of nitrogen oxide over Ag/Al₂O₃ catalysts causes an unusual enhancement of activity, e.g., the marginal activity (<10%) of 1 wt% Ag impregnated on γ-Al₂O₃ or mesoporous Al₂O₃ modifications is boosted to nearly 100% over a broad temperature range from 200 to 550°C at a space velocity of 30,000cm³g^?1h^?1). Contrary, silver on SiO₂ or α-Al₂O₃ shows no improvement of activity in the presence of H₂. The effect is tentatively attributed to a higher percentage of intermediary nano-sized Ag clusters on high-surface area Al₂O₃ in the presence of hydrogen. This promotes oxygen activation and hence NO oxidation to reactive intermediate nitrite species. The required dispersion of Ag cannot be stabilized on SiO₂ or α-Al₂O₃.     

Lean NOx Reduction with H2 and CO in Dual-Layer LNT–SCR Monolithic Catalysts: Impact of Ceria Loading

A series of monolithic catalysts consisting of a layer of selective catalytic reduction (SCR) catalyst deposited on top of lean NO_x trap (LNT) catalyst were synthesized for lean reduction of NO_x (NO&NO₂) with H₂ and CO. The LNT catalyst exhibited a rather low NO_x conversion below 250 °C due to CO inhibition. The top SCR layer comprising Cu/ZSM5 significantly increased the NO_x conversion at low temperature by its reaction with NH₃ formed during the regeneration phase. The addition of CeO₂ to the LNT layer promoted the water gas shift reaction (CO + H₂O ? H₂ + CO₂). The WGS reaction mitigated the CO inhibition and the generated H₂ enhanced the low-temperature catalyst regeneration. The ceria addition decreased the performance at high temperatures due to increased oxidation of NH₃. The ceria loading was optimized by applying a non-uniform axial profile. A dual-layer catalyst with an increasing ceria loading axial profile improved the performance over a wide (low and high) temperature range.     

Steam effect on NO x reduction over lean NO x trap Pt–BaO/Al2O3 model catalyst

The effect of steam on NO _x reduction over lean NO _x trap (LNT) Pt–Ba/Al₂O₃ and Pt/Al₂O₃ model catalysts was investigated with reaction protocols of rich steady-state followed by lean–rich cyclic operations using CO and C₃H₈ as reductants, respectively. Compared to dry atmosphere, steam promoted NO _x reduction; however, under rich conditions the primary reduction product was NH₃. The results of NO _x reduction and NH₃ selectivity versus temperature, combined with temperature programmed reduction of stored NO _x over Pt–BaO/Al₂O₃ suggest that steam causes NH₃ formation over Pt sites via reduction of NO _x by hydrogen that is generated via water gas shift for CO/steam, or via steam reforming for C₃H₈/steam. During the rich mode of lean–rich cyclic operation with lean–rich duration ratio of 60 /20 s, not only the feed NO, but also the stored NO _x contributed to NH₃ formation. The NH₃ formed under these conditions could be effectively trapped by a downstream bed of Co²⁺ exchanged Beta zeolite. When the cyclic operation was switched into lean mode at T < 450 °C, the trapped ammonia in turn participated in additional NO _x reduction, leading to improved NO _x storage efficiency.     

Aspects of the Role of Hydrogen in H2-Assisted HC–SCR Over Ag–Al2O3

The role of hydrogen in H₂-assisted HC–SCR of NO _x over Ag–Al₂O₃ is investigated by XPS and in situ DRIFT spectroscopy. Hydrogen does not reduce the surface silver species to metallic silver, however direct reduction of surface nitrates by hydrogen is observed. It is proposed that one important role of hydrogen is the removal of nitrates from the Ag–Al₂O₃ surface.     

Hydrogen promotes the selective catalytic reduction of NOx by ethanol over Ag/Al2O3

To understand the effect of H₂ on the selective catalytic reduction of NO_x with C₂H₅OH over Ag/Al₂O₃, surface intermediates were examined using in situ DRIFTS spectra, and by-products were identified using GC–MS. Results showed that H₂ addition promoted the partial oxidation of C₂H₅OH to form enolic species, and enhanced the reaction of NCO with NO + O₂ at low temperature. We propose that the enhancement of the enolic species was the main contributor in accelerating NO_x reduction under the presence of H₂ over Ag/Al₂O₃ at low temperatures.     

Cooperation of Ag/Al2O3 and Sn/Al2O3 Catalysts for the Selective Reduction of NO by Propene

Compared to the Ag/Al₂O₃ catalyst, a two‐stage catalyst composed of an Ag/Al₂O₃ layer followed by a Sn/Al₂O₃ layer shows higher low‐temperature activity and a wider temperature window. Its activity below 350 °C is enhanced in the presence of SO₂. Even in the presence of H₂O and SO₂, the performance of the same catalytic system is still satisfactory.     

Support modification to improve the sulphur tolerance of Ag/Al2O3 for SCR of NOx with propene under lean-burn conditions

Ag/Al₂O₃ catalysts with 1 wt% SiO₂ or TiO₂ doping in alumina support have been prepared by wet impregnation method and tested for sulphur tolerance during the selective catalytic reduction (SCR) of NO_x using propene under lean conditions. Ag/Al₂O₃ showed 44% NO_x conversion at 623 K, which was drastically reduced to 21% when exposed to 20 ppm SO₂. When Al₂O₃ support in Ag/Al₂O₃ was doped with 1 wt% SiO₂ or TiO₂ the NO_x conversion remained constant in presence of SO₂ showing the improved sulphur tolerance of these catalysts. Subsequent water addition does not induce significant deactivation. On the contrary, a slight promotional effect on the activity of NO conversion to nitrogen is observed after Si and Ti incorporation. FTIR study showed the sulphation of silver and aluminum sites of Ag/Al₂O₃ catalysts resulting in the decrease in the formation of reactive intermediate species such as –NCO, which in turn decreases NO_x conversion to N₂. In the case of Ag/Al₂O₃ doped with SiO₂ or TiO₂, formation of silver sulphate and aluminum sulphate was drastically reduced, which was evident in FTIR resulting in remarkable improvement in the sulphur tolerance of Ag/Al₂O₃ catalyst. These catalysts before and after the reaction have been characterized with various techniques (XRD, BET surface area, transmittance FTIR and pyridine adsorption) for physico-chemical properties.     

Promoting Effect of Triglyme on Lean NO<Subscript>x</Subscript> Reduction Over Ag/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

Abstract  

The highly oxygenated hydrocarbon triethylene glycol dimethyl ether or triglyme (CH₃O–(C₂H₄O–)₃CH₃) was found to efficiently reduce NO_x under lean conditions over Ag/Al₂O₃, but gave a low NO_x conversion over Cu-ZSM-5. Furthermore, triglyme showed an extraordinary promoting effect when added together with propene as reducing agent for NO_x over Ag/Al₂O₃ at low temperature. This is most likely due to that triglyme promotes the activation of propene.     

Reactivity of Surface Nitrates in H2-Assisted SCR of NO x Over Ag/Al2O3 Catalyst

The role of nitrate ad-species in H₂-assisted SCR over Ag/Al₂O₃ was compared in NH₃-SCR and n-C₆H₁₄-SCR processes. It was found that nitrates could be reduced by NH₃ or n-C₆H₁₄ at similar rates with H₂ co-feeding which indicates a common rate-limiting step. However, contributions of surface nitrate reduction to the overall NH₃-SCR or n-C₆H₁₄-SCR are different as revealed by comparing the rates of nitrate reduction with the rates of steady-state processes. The rate of the steady-state n-C₆H₁₄-SCR is virtually identical to the rate of surface nitrate reduction suggesting a significant contribution of the surface nitrates reduction to the overall n-C₆H₁₄-SCR process. On the other hand, the steady-state rate of NH₃-SCR is by ~15 times higher, which indicates that the reduction of surface nitrates plays a marginal role in the overall NH₃-SCR.     

The role of silver species on Ag/Al2O3 catalysts for the selective catalytic oxidation of ammonia to nitrogen

The role of Ag species on Ag/Al₂O₃ catalyst for the selective catalytic oxidation (SCO) of NH₃ to N₂ was studied using 10 wt% Ag/Al₂O₃ catalysts prepared with impregnation, incipient wetness impregnation and sol–gel methods. The catalyst characterization was preformed using N₂ adsorption–desorption, UV/Vis, TEM and XRD. O₂-chemisorption and H₂–O₂ titration were measured to confirm the metal dispersion on the catalyst. The Ag species state and Ag particle size have significant influence on the Ag/Al₂O₃ activity and N₂ selectivity of the SCO of NH₃ at low temperature. Ag⁰ is proposed to be an active species on the H₂ pretreated catalyst at low temperature (<140 °C). It is evident that well-dispersed and small particle Ag⁰ enhances catalytic activity at low temperature, whereas large particle Ag⁰ is relate to a high N₂ selectivity. In contrast, Ag⁺ could also be the active species at temperatures above 140 °C.     

Transient reaction analysis and steady-state kinetic study of selective catalytic reduction of NO and NO + NO2 by NH3 over Fe/ZSM-5

The reaction kinetics of selective catalytic reduction (SCR) by NH₃ on NO (standard SCR) and on NO + NO₂ (fast SCR) over Fe/ZSM-5 were investigated using transient and steady-state analyses. In the standard SCR, the N₂ production rate was transiently promoted in the absence of gaseous NH₃; this enhancement can be attributed to the negative reaction order of NH₃ (between −0.21 and −0.11). The steady-state data for the standard SCR could be fit to a Langmuir–Hinshelwood-type reaction between NO_ad and O_ad to form NO₂. In the fast SCR, however, the promotion behavior in the absence of gaseous NH₃ was not observed and the apparent NH₃ order changed from positive to negative with NH₃ concentration. The steady-state rate analysis combined with elementary reaction modeling suggested that competitive adsorption between NO₂ and NH₃ was occurring due to strong NO₂ adsorption; this must be the main reason for the absence of the promotion effect.     

Performance of V2O5/NPTiO2–Al2O3-nanoparticle- and V2O5/NTiO2–Al2O3-nanotube model catalysts in the SCR–NO with NH3

In this study, the NO reduction by NH₃ over V₂O₅/NPTiO₂–Al₂O₃ (nanoparticles) and V₂O₅/NTiO₂–Al₂O₃ (nanotubes) catalysts synthesized by the sol–gel method with 10 and 5 wt.% of Al₂O₃ and V₂O₅, respectively, is reported. The V₂O₅/NPTiO₂–Al₂O₃ and V₂O₅/NTiO₂–Al₂O₃ catalysts showed remarkable conversion, high acidity, structure stability, N₂ selectivity and a high resistance to deactivation in the presence of 10 vol.% of H₂O within the 200 to 500 °C temperature interval. The nanostructured catalysts developed in this work are an excellent alternative to improve the SCR–NH₃ process, both expanding its operation window and preventing deactivation by H₂O at high temperatures.     

MnOx-WO3/TiO2NH3选择性还原NOx的催化性能与动力学

研究了Mn-W/TiO2用于NH3选择性催化还原NOx体系的催化反应性能,在很宽的温度范围和各种气体条件下,该催化剂显示了较高的催化活性.在GHSV 18900 h-1、100～350℃条件下,NOx转化率高达80.3%～99.6%,Nz选择性达98.7%～100%;当反应气体中有0.01%SO2(分压比,下同)和6%...