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 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.     

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.     

NO x uptake mechanism on Pt/BaO/Al2O3 catalysts

The NO _x adsorption mechanism on Pt/BaO/Al₂O₃ catalysts was investigated by performing NO _x storage/reduction cycles, NO₂ adsorption and NO + O₂ adsorption on 2%Pt/(x)BaO/Al₂O₃ (x = 2, 8, and 20 wt%) catalysts. NO _x uptake profiles on 2%\Pt/20%BaO/Al₂O₃ at 523 K show complete uptake behavior for almost 5 min, and then the NO _x level starts gradually increasing with time and it reaches 75% of the inlet NO _x concentration after 30 min time-on-stream. Although this catalyst shows fairly high NO _x conversion at 523 K, only ~2.4 wt% out of 20 wt% BaO is converted to Ba(NO₃)₂. Adsorption studies by using NO₂ and NO + O₂ suggest two different NO _x adsorption mechanisms. The NO₂ uptake profile on 2%Pt/20%BaO/Al₂O₃ shows the absence of a complete NO _x uptake period at the beginning of adsorption and the overall NO _x uptake is controlled by the gas–solid equilibrium between NO₂ and BaO/Ba(NO₃)₂ phase. When we use NO + O₂, complete initial NO _x uptake occurs and the time it takes to convert ~4% of BaO to Ba(NO₃)₂ is independent of the NO concentration. These NO _x uptake characteristics suggest that the NO + O₂ reaction on the surface of Pt particles produces NO₂ that is subsequently transferred to the neighboring BaO phase by spill over. At the beginning of the NO _x uptake, this spill-over process is very fast and so it is able to provide complete NO _x storage. However, the NO _x uptake by this mechanism slows down as BaO in the vicinity of Pt particles are converted to Ba(NO₃)₂. The formation of Ba(NO₃)₂ around the Pt particles results in the development of a diffusion barrier for NO₂, and increases the probability of NO₂ desorption and consequently, the beginning of NO _x slip. As NO _x uptake by NO₂ spill-over mechanism slows down due to the diffusion barrier formation, the rate and extent of NO₂ uptake are determined by the diffusion rate of nitrate ions into the BaO bulk, which, in turn, is determined by the gas phase NO₂ concentration.     

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     

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.     

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 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.     

CoAl2O4 spinel catalyst for soot combustion with NOx/O2

Mesoporous and nanosized cobalt aluminate spinel with high specific surface area was prepared using microwave assisted glycothermal method and used as soot combustion catalyst in a NO_x + O₂ stream. For comparison, zinc aluminate spinel and alumina supported platinum catalysts were prepared and tested. All samples were characterised using XRD, (HR)TEM, N₂ adsorption–desorption measurements. The CoAl₂O₄ spinel was able to oxidise soot as fast as the reference Pt/Al₂O₃ catalyst. Its catalytic activity can be attributed to a high NO_x chemisorption on the surface of this spinel, which leads to the fast oxidation of NO to NO₂.     

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.     

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.     

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.     

Effect of Low Concentration of SO x on NO Reduction with Propene Over Ag/Al2O3

Catalytic activity for NO reduction with propene was investigated at 0–80 ppm SO₂. NO was reduced more efficiently by propene on SO₂-treated than untreated catalyst. Simultaneously, combustion of reductant was observed to lower NO reduction efficiency. Thus, the role of surface-adsorbed SO _x species was regarded as depressing reductant combustion. NH₃ adsorption revealed that SO₂ treatment increased Bronsted acidity of the Ag/Al₂O₃ catalyst, which promoted propene activation. Reductant activation is a more important step, compared with NO activation to oxidative nitrate species. The NCO species, an index intermediate in NO _x reduction, was produced on SO₂-adsorbed Ag/Al₂O₃ at a lower temperature (473 K) than on the untreated catalyst. The reductive intermediates at low temperature are suggested to be alcohol, or aldehyde-adsorbed species, based on observed C=O band.     

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.     

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.     

Efficient Reduction of NO x by H2 Under Oxygen-Rich Conditions over Pd/TiO2 Catalysts: An in situ DRIFTS Study

The H₂/NO/O₂ reaction under lean-burn conditions has been studied by means of in situ DRIFTS, reactor measurements and temperature-programmed desorption with the aim of understanding the very different behavior of Pd/TiO₂ and Pd/Al₂O₃ catalysts. The former deliver very high NO _x conversions (70-80%) with good N₂ selectivity whereas the latter show very low activity. In addition, PdTiO₂ exhibits two distinct NO _x reduction pathways, thus greatly extending the useful temperature range. It is shown that the PdTiO₂ low-temperature channel involves adsorption and subsequent dissociation of NO on reduced (Pd⁰) metal sites. The low activity of PdAl₂O₃ is a consequence of palladium remaining in an oxidized state under reaction conditions. The high-temperature NO reduction channel found with PdTiO₂ is associated with the generation and subsequent reaction of NH _x species.     

Adsorption and temperature programmed desorption of NO,NO + O2, NO2 and NO + NO2 over CoH-ZSM-5

The NO₂, NO (O₂) adsorption and temperature programmed desorption (TPD) were studied systematically to probe into the selective catalytic reduction of NO by methane (CH₄–SCR) over CoH-ZSM-5 (SiO₂/Al₂O₃ = 25, Co/Al = 0.132–0.312). Adsorption conditions significantly affect the adsorption of NO, NO₂ and NO + O₂. Adsorbed NO species are unstable and desorbed below the reactive temperature 523 K. Increasing adsorption temperature results in the decrease of the adsorbed NO species amount. The amount of –NO_y species formed from NO₂ adsorption increases with the increase of NO₂ concentration in the adsorption process, while decreases significantly with the increase of adsorption temperature. Though NO species are adsorbed weakly on CoH-ZSM-5, competitive adsorption between NO and –NO_y species decreases the amount of adsorbed –NO_y species. Similar desorption profiles of NO₂ were obtained over CoH-ZSM-5 while they were contacted with NO₂ or NO + O₂ followed by TPD. If NO₂ was essential to form adsorbed –NO_y species, the amount of adsorbed –NO_y species for NO + O₂ adsorption should be the least among the adsorptions of NO₂, NO + O₂ and NO + NO₂ because of the lowest NO₂ concentration and highest NO concentration. In fact, the amount of adsorbed –NO_y species is between the other two adsorption processes. These indicate that the formation of adsorbed –NO_y species may not originate from NO₂.     

Preparation and characterization of Ag/MnOx/perovskite catalysts for CO oxidation

The spray decomposition method with lanthanum nitrate, manganese nitrate, silver nitrate and citric acid was used to synthesize Ag- and Mn-incorporated perovskites. The resulting samples were characterized by X-ray diffraction, BET adsorption measurement, X-ray photoelectron spectroscopy, and temperature-programmed oxygen desorption (O₂-TPD) measurement. The obtained composite Ag/MnO_x/perovskites catalysts exhibit higher activity by a few orders of magnitude at 338 K than that of LaMnO₃. From the O₂-TPD measurement, the high activity of the Ag/MnO_x/perovskites may result from the increase of weak oxygen adsorption below 373 K. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

The NO x reduction mechanism by H2 under near isothermal conditions over Pt–Ba/Al2O3 Lean NO x Trap systems

The study of the gas-phase NO reduction by H₂ and of the stability/reactivity of NO _x stored over Pt–Ba/Al₂O₃ Lean NO _x Trap systems allowed to propose the occurrence of a reduction process of the stored nitrates occurring via to a Pt-catalyzed surface reaction which does not involve, as a preliminary step, the thermal decomposition of the adsorbed NO _x species.