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.     

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.     

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.     

Combination of Ag/Al2O3 and Fe-BEA for High-Activity Catalyst System for H2-Assisted NH3-SCR of NO x for Light-Duty Diesel Car Applications

Low-temperature active Ag/Al₂O₃ and high-temperature active Fe-BEA zeolite were combined and tested for H₂-assisted NH₃-selective catalytic reduction (SCR) of NO _x . The catalysts were either washcoated onto separate monoliths that were placed up- or downstream of each other (dual-brick layout) or washcoated on top of each other in a sandwiched layout (dual-layer). Our results showed that it is highly preferred to have Ag/Al₂O₃ as the upstream or outer layer catalyst. Fe-BEA showed a high NH₃ oxidation giving an NH₃ deficit over the Ag/Al₂O₃. Ag/Al₂O₃ formed NO₂ which enhanced the activity over Fe-BEA through the “fast”-SCR reaction when Fe-BEA was placed downstream or as inner layer. When no H₂, which is needed for the SCR reaction over Ag/Al₂O₃, was added, the dual-layer layout was preferred. The shorter diffusion distance between the layers is a probable explanation.     

Differences Between Al2O3 and Ag/Al2O3 for Lean Reduction of NO x with Dimethyl Ether

Lean reduction of NO _x with DME occurs with high selectivity to N₂ over Al₂O₃ between 300 °C and 550 °C with a maximum of 47% at 380 °C, and with lower selectivity over Ag/Al₂O₃ between 250 °C and 400 °C due to the catalysts’ sensitivity to gas phase radical reactions and activity for NO _x reduction with methanol.     

Influence of Ageing, Silver Loading and Type of Reducing Agent on the Lean NO x Reduction Over Ag–Al2O3 Catalysts

The influence of ageing temperature, silver loading and type of reducing agent on the lean NO _x reduction over silver–alumina catalysts was investigated with n-octane and bio-diesel (NExBTL) as reducing agent. The catalysts (2 and 6 wt% Ag–Al₂O₃) were prepared with a sol–gel method including freeze drying and the evaluation of NO _x reduction and aging were performed using a synthetic gas-flow reactor. The results indicate a relatively high NO _x reduction for both reducing agents. The hydrothermally treated 6 wt% Ag–Al₂O₃ sample displays a maximum NO _x reduction of 78 % at 350 °C for n-octane as reductant and the corresponding value for NExBTL is 60 %. Furthermore, the catalysts show high durability and an increase in activity for NO _x reduction after ageing at temperatures up to 650 °C, with n-octane as reducing agent.     

Organo-NOx formation–decomposition as the origin of the changes in the low temperature NO-TPD profile in the presence of n-decane on Ag/γ-Al2O3

The NO_x desorption profiles obtained in O₂–He and n-C₁₀–O₂–He were compared on γ-Al₂O₃ and Ag/γ-Al₂O₃. On Ag/γ-Al₂O₃, the low-temperature NO_x desorption profiles obtained in n-C₁₀–O₂–He were significantly different from those obtained in O₂–He. In particular, at 190–220 °C excess release of NO, instead of NO₂, was observed concomitantly with n-C₁₀ consumption. This is interpreted as the result of the formation–decomposition of organo-NO_x species issued from the interaction of NO₂ and hydrocarbons initially chemisorbed on Al₂O₃ and activated on Ag species, respectively. The occurrence of such a phenomenon at temperatures close to those at which the n-C₁₀-SCR reaction starts provides support for the involvement of the organo-NO_x species as intermediates.     

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.     

Effect of H2 admixture on the adsorption of NO,NO2 and propane at Ag/Al2O3 catalyst as examined by in situ FTIR

The interaction of DeNO_x components comprising NO, NO₂, propane, O₂ and H₂ with a selected Ag/Al₂O₃ catalyst was studied by in situ FTIR spectroscopy at a fixed temperature where a promoting effect of H₂ admixture on the catalytic NO_x reduction has been reported to occur. The significant enhancement of nitrato and acetate ad-species from NO and propane, respectively, could be confirmed. New findings are the relative inertness of these species for further reaction progress. Instead, nitrite, nitrito, and nitro species are reactive, and the H₂ co-fed favours the formation of those species. Nevertheless, the way H₂ interferes with the kind of species formed includes the promotion of oxidative reaction steps evidenced by different effect of H₂ on the interaction of the catalyst with NO/O₂ and NO₂, respectively. During NO₂/H₂ adsorption adsorbed NO_x species and Ag⁺ ions at the surface are reduced. This prevents abundant formation of stable nitrato species and favour the formation of largely unstable but reactive nitro and nitrito species. Otherwise, NO₂ is able to oxidize pre-reduced Ag/Al₂O₃. Furthermore, indications were found for minor propene formation from propane in the presence of hydrogen.     

Effect of Silver Loading on the Lean NOx Reduction with Methanol Over Ag–Al2O3

The influence of silver loading on the lean NO_x reduction activity using methanol as reductant has been studied for alumina supported silver catalysts. In general, increasing the silver loading (0–3 wt%), in Ag–Al₂O₃, shifts or extends the activity window, for lean NO_x reduction towards lower temperatures. In particular Ag–Al₂O₃ with 3 wt% silver is active for NO_x reduction under methanol-SCR conditions in a broad temperature interval (200–500 °C), with high activity in the low temperature range (maximum around 300 °C) typical for exhaust gases from diesel and other lean burn engines. Furthermore, increasing the C/N molar ratio enhances the reduction of NO_x. However, too high C/N ratios results in poor selectivity to N₂.     

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.     

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.     

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.     

Performance Improvement of NO x -Storage BaO/Al2O3 by Using Barium Peroxide as the Precursor of BaO

Comparison of barium peroxide, Ba(OH)₂ and Ba(NO₃)₂ as the precursor of BaO for the preparation of NO _x -storage BaO/Al₂O₃ material was carried out. The as prepared materials were calcined at 550 and 800 °C and characterized by N₂ physisorption, XRD, Raman and FT-IR spectroscopy. Measurements of the NO _x storage performances of these BaO/Al₂O₃ materials by NO₂ adsorption and NO _x -TPD experiments showed that the use of barium peroxide as the precursor of BaO inhibited the formation of BaAl₂O₄ and led to remarkable improvements in the thermal stability as well as NO _x storage capacity of the final BaO/Al₂O₃ material calcined at 800 °C.     

Influence of low concentration of SO2 for selective reduction of NO by C3H8 in lean-exhaust conditions on the activity of Ag/Al2O3 catalyst

The effect of SO₂ for the selective reduction of NO by C₃H₈ on Ag/Al₂O₃ was investigated in the presence of excess oxygen and water vapor. The NO_x conversion decreased permanently even in the presence of a low concentration of SO₂ (0.5–10 ppm) at <773 K. The increase in SO₂ concentration resulted in a large decrease in NO_x conversion at 773 K. However, when the reaction temperature was more than 823 K, the activity of Ag/Al₂O₃ remained constant even in the presence of 10 ppm of SO₂. The sulfate species formed on the used Ag/Al₂O₃ were characterized by a temperature programmed desorption method. The sulfated species formed on silver should mainly decrease the deNO_x activity on the Ag/Al₂O₃. The sulfated Ag/Al₂O₃ was appreciably regenerated by thermal treatment in the deNO_x feed at 873 K. The moderate activity remains at 773 K in the presence of 1 ppm SO₂ for long time by the heat treatment at every 20 h intervals.     

On the Characterisation of Silver Species for SCR of NO x with Ethanol

Reducing of nitrogen oxides (NO _x ) in a lean exhaust gases has become one of the most important environmental concerns. Among the different active phases studied for NO _x reduction reaction, silver-based catalysts supported over alumina show good performances using, as reducing agents, either hydrocarbons or oxygenated compounds. Nevertheless, a good understanding of the mechanism reaction has not been reached yet. This comprehension requires a better characterisation of the silver-based catalysts system. In our study, Ag/Al₂O₃ catalysts showed high efficiency in NO _x reduction using ethanol as reducing agent. The conversion plots, in steady state conditions for the different samples Ag/Al₂O₃ (0.8–3.5% Ag wt), show a great dependance of the activity with the metal loading. The optimal silver loading has been established around 2 wt.% Increasing the silver loading, the temperature of maximal NO _x conversion shifted toward the lower temperatures. According to the literature, a reduced and an oxide phase of silver have been observed by UV–Vis spectroscopy. The ratio between the two phases is changing with the silver loading. However, temperature programmed reduction (TPR) measurements reveal the presence of two types of oxide phases. TPR reveal the coexistence of a silver oxide phase (Ag₂O), according to a production of water in the course of the reaction, and a non-oxygenated phase attributed to isolated Ag⁺ cation. Thus, an original way using TPR measurements has been developed to differentiate the various oxidized phases. The aim of this characterisation is to correlate the catalyst’s activity with the observed silver phases, in order to understand the nature of phase active for NO _x reduction at low temperatures.     

Influence of Pt location on BaCO3 or Al2O3 during NOx storage reduction

Catalysts for NO_x storage–reduction (NSR) were made selectively with Pt on either the Al- or the Ba-components without altering significantly the Al₂O₃ or BaCO₃ crystal sizes, Al/Ba weight ratio, specific surface area, porosity, and Pt dispersion using a two-nozzle flame spray pyrolysis (FSP) unit. The NO_x storage performance at 300 °C was best for Pt located near Al₂O₃ as it facilitates the oxidation of NO to NO₂ during the fuel lean period but the reduction rate during the subsequent short fuel rich period was much slower resulting in incomplete regeneration. This contributed to a gradual decrease of the NO_x conversion at increasing cycling. In contrast, Pt on BaCO₃ resulted in an initially lower NO_x storage rate but during ten storage–reduction cycles a stable NO_x conversion of about 50% was reached. When using NO₂ instead of NO or higher NO_x oxidation-reduction temperatures (e.g. 350 °C) the Pt location did not affect the NSR performance of the Pt/Ba/Al₂O₃ catalysts.     

Lean NOx catalysis over alumina‐supported catalysts

aluminasupported catalysts show promise as lean NO_x catalysts. The role of alumina in influencing the structural and chemical properties of the active phase supported on it is discussed using some effective aluminabased lean NO_x catalysts. These include Ag/Al₂O₃, CoO_x/Al₂O₃ and SnO₂/Al₂O₃. Alumina plays an important role in stabilizing Ag in the oxidic phase and cobalt in the 2+ oxidation state. For SnO₂/Al₂O₃, alumina increases the SnO₂ surface area. On both Ag/Al₂O₃ and SnO₂/Al₂O₃, alumina also participates actively in the NO_x reduction reaction. An active organic intermediate is formed on Ag or Sn oxide which reacts with NO_x subsequently on alumina to form N₂.     

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.     

Synthesis and Dispersion of Dendrimer-Encapsulated Pt Nanoparticles on γ-Al2O3 for the Reduction of NO x by Methane

Dendrimer encapsulated Pt nanoparticles were prepared by using hydroxyl terminated generation four (G4OH) PAMAM dendrimers (DEN) as the templating agents. The encapsulated Pt nanoparticles were dispersed on γ-Al₂O₃ at room temperature by impregnation. Pt/Al₂O₃ (DEN) catalysts were then subjected to thermal treatments in oxidizing and reducing atmospheres at different temperatures. These catalysts were characterized by Transmission Electron microscopy (TEM) and In situ Fourier-Transform Infrared (FTIR) spectroscopy. The TEM analysis of the as synthesized catalysts revealed that the Pt nanoparticles were found to be 2–4 nm in size. It is observed that the Pt particle size in 0.5% Pt/Al₂O₃ (DEN) catalyst increased upon thermal decomposition of the dendrimer. The in situ FTIR results suggested that the presence of oxygen and the Pt nanoparticles in the Pt-dendrimer nanocomposite accelerate the dendrimer decomposition at low temperatures. All the catalysts were tested for the reduction of NO _x with CH₄ in the temperature range of 250–500 °C. NO _x reduction efficiency of Pt/Al₂O₃ (DEN) catalysts were compared with the Pt/Al₂O₃ (CON; conventional) catalyst. The conversion of NO _x was started from the low temperatures over Pt/Al₂O₃ (DEN) catalysts. The high selectivity of NO _x to N₂ of 74% was obtained over 0.5% Pt/Al₂O₃ (DEN) catalyst at low temperatures around 350 °C.