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.     

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.     

Kinetics of Selective Oxidation of Dimethyl Ether to Formaldehyde Over Al2O3-Supported VO x and MoO x Catalysts

The object of this work is to study the effect of hydrothermal treatment on the TS-1/SiO₂ catalyst in propylene epoxidation in fixed bed. The TS-1/SiO₂ catalysts before and after hydrothermal treatment were characterized by means of XRD, XRF, BET, UV–Vis and EPR techniques. It was found by EPR characterization that two types of Ti(IV)-superoxide radicals, A (g _z = 2.0271; g _y = 2.0074; g _x = 2.0010) and B (g _z = 2.0247; g _y = 2.0074; g _x = 2.0010), were observed for the TS-1/SiO₂ catalyst. The superoxo species A was converted to B after the TS-1/SiO₂ catalyst was hydrothermally treated. The results show that hydrothermal treatment temperature and time have marked effects on the activity and the PO selectivity. The optimal hydrothermal treatment temperature and time are 170 and 4 h, respectively. In the long-term propylene epoxidation reaction, about 95% H₂O₂ conversion and above 94% PO selectivity are obtained over TS-1/SiO₂ catalyst hydrothermally treated at 170° C for 4 h.     

Reactivity of surface isocyanate species with NO, O2 and NO+O2 in selective reduction of NOχ over Ag/Al2O3 and Al2O3 catalysts

Reactivity of surface isocyanate (NCO(a)) species with NO, O₂ and NO+O₂ in selective reduction of NOχ over Ag/Al₂O₃ and Al₂O₃ catalysts was studied by a pulse reaction technique and an in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. The NCO(a) species on Ag/Al₂O₃ reacted with O₂ or NO+O₂ mixture gas to produce N₂ effectively above 200°C, while the reaction of NCO(a) with NO hardly produced N₂ even at 350°C. In the case of Al₂O₃ alone, less N₂ was detected in the reaction of NCO(a) with NO+O₂, indicating that silver plays an important role in the N₂ formation from NCO(a). These behaviors of the reactivity of NCO(a) species with reactant gases were in good agreement with the changes in NCO(a) bands shown by in situ DRIFT measurements. Based on these findings, the role of NCO(a) species in the selective reduction of NOχ on Ag/Al₂O₃ and Al₂O₃ catalysts is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

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 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.     

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.     

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.     

Selective catalytic reduction of NO x over Ag/Al2O3 using various bio-diesels as reducing agents

The effect of bio-diesel compounds (vegetable methyl and ethyl laurate and hexadecane) as reducing agents on the selective catalytic reduction of NO _x over a 2 wt.% Ag/Al₂O₃ was investigated. These components were found to have a two-fold effect on the SCR over Ag/Al₂O₃. First, the reduction activity below 400 °C was higher with bio-diesel than with n-octane, which is a representative compound for fossil fuels. This effect is attributed to the presence of the ester group in these molecules. However, the conversion above 400 °C decreased sharply and was considerable lower than with n-octane. The most interesting observation was found when the reduction efficiency of bio-diesel components was tested in the presence of hydrogen. The well known low temperature boosting effect of hydrogen was visible not only at lower temperatures, but also above 400 °C. Mechanistically the observation is extremely interesting and indicates that hydrogen effect cannot directly be connected to reduction of surface nitrates, which can be operative only at low temperature domain.     

Catalytic properties of Ag/Al2O3 catalysts for lean NO x reduction processes and characterisation of active silver species

Topics in Catalysis - A study of the lean NO x reduction activity employing different reductants over Ag/Al2O3 samples prepared from reverse microemulsions or impregnation with EDTA-complexes is...     

Effect of Pt and Ba content on NO x Storage and Reduction Over Pt/Ba/Al2O3

Effects of Pt and Ba content of Pt/Ba/Al₂O₃ on NO _x storage reduction (NSR) catalyst were investigated by means of kinetic analysis of surface nitrates using in situ FT/IR. Turnover frequencies of both storage and reduction of nitrate were significantly enhanced at higher Ba content, while those were higher at lower Pt content below 0.2 wt%.     

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.     

Sulfur deactivation and regeneration of Pt/BaO/Al2O3 and Pt/SrO/Al2O3 NO x storage catalysts

Transient experiments were performed to study sulfur deactivation and regeneration of Pt/BaO/Al₂O₃ and Pt/SrO/Al₂O₃ NO _x storage catalysts. It was found that the strontium-based catalysts are more easily regenerated than the barium-based catalysts and that a higher fraction of the NO _x storage sites are regenerated when H₂ is used in combination with CO₂ compared to H₂ only.     

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 NO x Reduction with Various Bio-Diesels as Reducing Agents

Commercial Cu-ZSM-5- and Ag/Al₂O₃-based lean NO _x catalysts were evaluated in a synthetic exhaust gas bench with the fuels RME, B30, B15, Agrodiesel 15, GTL, NExBTL, and MK1 as reducing agents. The influence of reductant was larger for Ag/Al₂O₃, albeit moderate, whereas the Cu-zeolite showed the highest NO _x conversion at lower temperature for all alternative fuels tested.     

Exceptional activity for NO x reduction at low temperatures using combinations of hydrogen and higher hydrocarbons on Ag/Al2O3 catalysts

Topics in Catalysis - The effect of the addition of hydrogen on the SCR of NO x with a hydrocarbon reaction was investigated. It was found that hydrogen had a remarkable effect on the temperature...     

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.