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1.
Ag/Al 2O 3 catalysts with 1 wt% SiO 2 or TiO 2 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/Al 2O 3 showed 44% NO x conversion at 623 K, which was drastically reduced to 21% when exposed to 20 ppm SO 2. When Al 2O 3 support in Ag/Al 2O 3 was doped with 1 wt% SiO 2 or TiO 2 the NO x conversion remained constant in presence of SO 2 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/Al 2O 3 catalysts resulting in the decrease in the formation of reactive intermediate species such as –NCO, which in turn decreases NO x conversion to N 2. In the case of Ag/Al 2O 3 doped with SiO 2 or TiO 2, 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/Al 2O 3 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. 相似文献
2.
The role of the Al 2O 3 support on the activity of supported Ag catalyst towards the selective catalytic reduction (SCR) of NO with decane is elucidated.
A series of Ag/Al 2O 3 catalysts were prepared by impregnation method and characterized by N 2 pore size distribution, XRD, UV–Vis, in-situ FT-IR and acidity measurement by NH 3 and pyridine adsorption. The catalytic activity differences of Ag/Al 2O 3 are correlated with different properties of Al 2O 3 supports and the active Ag species formed. 4wt% Ag supported on sol-gel prepared Al 2O 3 (Ag/Al 2O 3 (SG), showed higher NO
x
conversion (65% at 400 °C), compared with the respective catalysts made from commercial Al 2O 3 (Ag/Al 2O 3 (GB), Ag/Al 2O 3 (ALO), (∼26 and 7% at 400 °C). The higher surface area, acidity and pore size distribution in sol–gel prepared Al 2O 3 (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 nδ+ clusters and acidity of Al 2O 3 support was found to be one of the important parameter in promoting the formation and stabilization of Ag nδ+ clusters. Furthermore from pyridine adsorption results, presence of more number of Bronsted acid sites in Ag/Al 2O 3 (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/Al 2O 3 (SG). We conclude that the physico–chemical properties of Al 2O 3 play a crucial role in NO
x
conversion over Ag/Al 2O 3 catalysts. Thus, the activity of the Ag/Al 2O 3 catalyst can be tailored by using a proper type of Al 2O 3 support. 相似文献
3.
Reactivity of surface isocyanate (NCO(a)) species with NO, O 2 and NO+O 2 in selective reduction of NOχ over Ag/Al 2O 3 and Al 2O 3 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/Al 2O 3 reacted with O 2 or NO+O 2 mixture gas to produce N 2 effectively above 200°C, while the reaction of NCO(a) with NO hardly produced N 2 even at 350°C. In the case of Al 2O 3 alone, less N 2 was detected in the reaction of NCO(a) with NO+O 2, indicating that silver plays an important role in the N 2 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/Al 2O 3 and Al 2O 3 catalysts is discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
Ag/Al 2O 3 catalysts (2 wt% Ag) have been prepared and calcined at different temperatures to render catalysts with different silver
particle size or silver configuration. The differences in activity and selectivity of these catalysts are related to the activity
for oxidation of NO and hydrocarbons, the NO x storage properties, and to the oxidation state of silver. 相似文献
5.
The NO
x
storage performance at low temperature (100–200 °C) has been studied for model NO
x
storage catalysts. The catalysts were prepared by sequentially depositing support, metal oxide and platinum on ceramic monoliths. The support material consisted of acidic aluminium silicate, alumina or basic aluminium magnesium oxide, and the added metal oxide was either ceria or barium oxide. The NO
x
conversion was evaluated under net-oxidising conditions with transients between lean and rich gas composition and the NO
x
storage performance was studied by isothermal adsorption of NO 2 followed by temperature programmed desorption of adsorbed species. The maximum in NO
x
storage capacity was observed at 100 °C for all samples studied. The Pt/BaO/Al 2O 3 catalyst stored about twice the amount of NO
x
compared with the Pt/Al 2O 3 and Pt/CeO 2/Al 2O 3 samples. The storage capacity increased with increasing basicity of the support material, i.e. Pt/Al 2O 3·SiO 2 < Pt/Al 2O 3 < Pt/Al 2O 3 · MgO. Water did not significantly affect the NO
x
storage performance for Pt/Al 2O 3 or Pt/BaO/Al 2O 3. 相似文献
6.
The effect of various parameters on the NO
x
conversion over NO
x
storage and reduction catalysts supported on alumina was investigated. The Pt/BaO/Al 2O 3 catalyst exhibited a higher NO
x
reduction activity than the Pt/Al 2O 3 catalyst under the static and cycling conditions. The activity of Pt/BaO/Al 2O 3 catalyst was improved in the cycled feedstream. The Pt/SrO/Al 2O 3 was found to have as high activity as Pt/BaO/Al 2O 3 for NO
x
reduction. In order to achieve effective reduction of NO
x
, NO
x
storage in the form of Me(NO 3) 2 (Me = Ba or Sr) is more favorable than other nitrates and the rich condition should be chosen in such a way that the sorption capacity can be fully regenerated at a fast rate and the inhibition effect by strongly adsorbed molecules derived from C 3H 6 and CO can be minimized. 相似文献
7.
A series of Cu catalysts were studied as a function of support (Al 2O 3, TiO 2 and SiO 2) and Cu precursor (ex-SO 4 and ex-NO 3) for activity in the SCR-NH 3 reaction. The catalysts were characterized using NO x TPD and SEM/EDAX analysis and the effects of residual sulphur interpreted in terms of site-blocking and NH 3 activation mechanisms. 相似文献
8.
Three model catalysts (Pt/Al 2O 3, Pt/TiO 2, Pt/V 2O 5/TiO 2) were examined in regard to their NO 2 formation ability under a changing lean gas composition. The results show that the NO to NO 2 oxidation function as well as the NO
x
reduction under lean gas conditions is affected by a change in the lean gas atmosphere. The NO oxidation activity also decreased with time, for Pt/Al 2O 3 and Pt/TiO 2, and a possible explanation may be platinum oxide formation. This deactivation was not observed for Pt/V 2O 5/TiO 2. 相似文献
9.
The reduction of cobalt catalysts supported on Al 2O 3, SiO 2, and TiO 2 was investigated using a closed system filled with hydrogen gas. Effects of support and metal loading on the rate of reduction were also discussed. The activation energy of reduction increased in the following order: Co/TiO 22O32. For different metal loadings, it was found that the catalyst with the higher loading was more readily reducible than that with the lower metal loading. This was confirmed using the results from measurements of particle size, amount of CO adsorbed and activity. 相似文献
10.
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–Al 2O 3, shifts or extends the activity window, for lean NO x reduction towards lower temperatures. In particular Ag–Al 2O 3 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 N 2. 相似文献
11.
The oxidation of perchloroethylene (PCE) was investigated over chromium oxide catalysts supported on TiO 2, Al 2O 3, SiO 2, SiO 2–Al 2O 3 and activated carbon. The phase of chromium oxide on the catalyst surface is critical for the oxidation of PCE. The catalytic
activity of PCE removal enhances as the formation of Cr(VI) species on the catalyst surface increases. The surface area and
the type of the catalyst supports were also essential for high performance in the PCE oxidation. In addition, the structure
of Cr(VI) on the catalyst surface also plays an important role for the decomposition of PCE. The polymerized Cr(VI) mainly
formed by the interaction of metals with the support is the active reaction site for the present reaction system. CrO x/TiO 2 reveals the strongest PCE removal activity among the catalysts examined in the present study.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
12.
The present study explores the possibilities of catalysts of Ag/Al 2O 3, in which silver has been deposited using reverse microemulsions with the aim of getting maximum dispersion and homogeneity in the active superficial species, for the selective catalytic reduction of NO x in excess of oxygen, using both propene and ethanol as reductants and in the scope of the control of the emissions produced by vehicles that operate in conditions of lean mixture like the diesel engine or those of gasoline direct injection. The promotional effect of the hydrogen presence in the reactive mixture has also been analyzed. For both reductants, when in presence of hydrogen, an important enhancement in NO x conversion is produced, in particular for a catalyst with 3 wt.% silver. The production of acetaldehyde during the reaction employing ethanol is also analyzed and its role on the NO x reduction process has been examined. The interpretation of catalytic properties has been complemented by means of in-situ DRIFTS. 相似文献
13.
The aim of this study was to select, to test and to optimise supported Pt catalysts for nitrogen oxide sensors application in order to improve their selectivity. A series of catalysts based on platinum dispersed onto different supports: SiO 2, γ-Al 2O 3, Al 2O 3–SiO 2, ZrO 2, TiO 2, CeO 2 and YSZ (formulae: (ZrO 2) 0.92(Y 2O 3) 0.08) were prepared and tested for NO oxidation. The effect of the nature of the support, and of the Pt dispersion and loading on the catalytic activity has been examined under operating conditions similar to those encountered by NO x sensors, i.e. traces of NO in air. Catalytic activity and TPD measurements have clearly shown the prevailing role of the support. Silica is the most adequate support because it weakly adsorbed NO and did not store NO 2 as nitrates, then favouring the reaction between NO molecules coming from the support and oxygen adsorbed on Pt. The effect of the Pt particles sizes was also important and our results have shown that larger particles exhibit the highest activity. Finally, the most effective catalyst was found to be Pt/SiO 2 containing 2 wt.% of platinum and presenting a low Pt dispersion. This kind of catalyst enables to fix a determined value of the NO/NO 2 ratio near the sensing layer of the sensor by reaching the thermodynamic equilibrium over the whole working temperature range. 相似文献
14.
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/Al 2O 3 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/Al 2O 3 (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 (Ag 2O), 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. 相似文献
15.
The NO
x
storage performance at low temperature (100–200 °C) has been studied for model NO
x
storage catalysts. The catalysts were prepared by sequentially depositing support, metal oxide and platinum on ceramic monoliths. The support material consisted of acidic aluminium silicate, alumina or basic aluminium magnesium oxide, and the added metal oxide was either ceria or barium oxide. The NO
x
conversion was evaluated under net-oxidising conditions with transients between lean and rich gas composition and the NO
x
storage performance was studied by isothermal adsorption of NO2 followed by temperature programmed desorption of adsorbed species. The maximum in NO
x
storage capacity was observed at 100 °C for all samples studied. The Pt/BaO/Al2O3 catalyst stored about twice the amount of NO
x
compared with the Pt/Al2O3 and Pt/CeO2/Al2O3 samples. The storage capacity increased with increasing basicity of the support material, i.e. Pt/Al2O3 · SiO2 < Pt/Al2O3 < Pt/Al2O3 · MgO. Water did not significantly affect the NO
x
storage performance for Pt/Al2O3 or Pt/BaO/Al2O3. 相似文献
16.
This paper describes the selective oxidation of ammonia into nitrogen over copper, silver and gold catalysts between room temperature and 400 °C using different NH 3/O 2 ratios. The effect of addition of CeO x and Li 2O on the activity and selectivity is also discussed. The results show that copper and silver are very active and selective toward N 2. However the multicomponent catalysts: M/Li 2O/CeO x/Al 2O 3 (M: Au, Ag, Cu) perform the best. On all three metal containing catalysts the activity and selectivity is influenced by the particle size and the interaction between metal particles and support. 相似文献
17.
Pt–Ba/MeO (where MeO = Al 2O 3, CeO 2, SiO 2 and ZrO 2) NO
x
storage-reduction catalysts with Ba-loading varying from 0 wt.% to 28 wt.% were investigated concerning stability of Ba phases
and NO
x
storage-reduction efficiency. For Pt–Ba/Al 2O 3 three different Ba-containing phases with different thermal stability are distinguished based on their interaction with the
support. The relative concentration of these phases varies with the Ba-loading and NO
x
storage tests indicated that the BaCO 3 phase decomposing between 400 °C and 800 °C (LT-BaCO 3) is the most efficient Ba containing phase for NO
x
storage. Similar investigations of Pt–Ba catalysts supported on CeO 2, SiO 2 and ZrO 2 showed that the relative amount of LT-BaCO 3 phase depends also on the support material. NO
x
storage measurements confirmed a correlation between the concentration of LT-BaCO 3 and NO
x
storage efficiency. Basicity and textural properties of the support are identified as crucial parameters for efficient NO
x
storage catalysts. 相似文献
18.
Effect of additives, Ce and Mn, on the catalytic performance of Sn/Al 2O 3 catalyst prepared by sol–gel method for the selective reduction of NO x with propene under lean conditions was studied. Sn–Ce/Al 2O 3 catalysts exhibited higher activity than Sn/Al 2O 3 catalyst and the optimum Ce loading is 0.5–1%. The promoting effect of Ce is to enhance the oxidation of NO to NO 2 and facilitate the activation of propene, both of which are important steps for the NO x reduction. The presence of oxygen contributes to the oxidation of NO and shows a promoting effect. 相似文献
19.
The gas phase hydrogenation of acrolein over supported silver catalysts has been investigated with a focus on the influence of the support acidity. Acidity has been varied by preparing silver catalysts supported on silica/alumina supports with varying SiO 2/Al 2O 3 ratio. After the catalytic experiments the Ag catalysts exhibit similar particle sizes, as revealed with TEM (transmission electron microscopy). The acidity of the samples was estimated using TPD of adsorbed ammonia which gives the total acidity of the samples, furthermore by IR of adsorbed pyridine to identify the Brønsted and Lewis acidity. No Brønsted acidity was found, and the Lewis acidity showed a clear dependence on the support composition. It is shown that a high total acidity and a high amount of strong Lewis acid sites on the catalysts cause a low conversion of acrolein and low selectivity to allyl alcohol. The interaction of silver with the support or effects of the metal–support perimeter are discussed as possible reasons for this behaviour. 相似文献
20.
A new Ag/Al 2O 3 catalyst for removing NO x in diesel engine exhaust gas was developed. The influence of SO 2 on the reduction of lean NO x by ethanol over the Ag/Al 2O 3 catalyst was evaluated in simulated diesel exhaust and characterized using TPD, XRD, XPS, SEM and BET measurements. The Ag/Al 2O 3 catalyst was highly active for the reduction of NO x with ethanol in the presence of SO 2 although the reduction of NO x is suppressed at lower temperatures. The activity for NO x reduction is high even on the Ag/Al 2O 3 catalyst exposed to a SO 2 (200 ppm)/O 2 (10%)/H 2O (10%) flow for 20 h at 723 K and comparable to that on the fresh Ag/Al 2O 3 catalyst. No crystallized Ag metal and Ag compounds were formed by the SO 2/O 2/H 2O exposure. On the other hand, crystallized Ag 2SO 4 was easily formed when the Ag/Al 2O 3 catalyst was exposed to a SO 2 (200 ppm)/O 2 (10%)/NO (800 ppm)/H 2O (10%) flow for 10 h at 723 K. XRD, SEM and XPS studies showed that the formation of crystallized Ag 2SO 4 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/Al 2O 3 catalyst decreases from 221 to 193 m 2/g. Although the dispersion of Ag particles was decreased by the formation of Ag 2SO 4, the activity for the reduction of lean NO x was, remarkably, not affected. This suggests that the Ag–alumina sites created by the Ag 2SO 4 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. 相似文献
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