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1.
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. 相似文献
2.
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. 相似文献
3.
4.
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. 相似文献
5.
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. 相似文献
6.
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. 相似文献
7.
N. A. Sadokhina D. E. Doronkin P. V. Pributkov V. I. Bukhtiyarov R. I. Kvon A. Yu. Stakheev 《Topics in Catalysis》2011,54(16-18):1190-1196
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. 相似文献
8.
The influence of silver loading on the lean NOx 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 NOx reduction towards lower temperatures. In particular Ag–Al2O3 with 3 wt% silver is active for NOx 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 NOx. However, too high C/N ratios results in poor selectivity to N2. 相似文献
9.
Asima Sultana Masaaki Haneda Tadahiro Fujitani Hideaki Hamada 《Catalysis Letters》2007,114(1-2):96-102
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 Agnδ+ clusters and acidity of Al2O3 support was found to be one of the important parameter in promoting the formation and stabilization of Agnδ+ 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. 相似文献
10.
N. A. Sadokhina D. E. Doronkin G. N. Baeva S. Dahl A. Yu. Stakheev 《Topics in Catalysis》2013,56(9-10):737-744
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. 相似文献
11.
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. 相似文献
12.
Iglesias-Juez A. Fernández-García M. Martínez-Arias A. Schay Z. Koppány Zs. Hungría A. B. Fuerte A. Anderson J. A. Conesa J. C. Soria J. 《Topics in Catalysis》2004,30(1-4):65-70
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... 相似文献
13.
Yoshinori Saito Kenichi Shimizu Takeshi Nobukawa Atsuhshi Satsuma 《Topics in Catalysis》2010,53(7-10):584-590
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%. 相似文献
14.
Ja Hun Kwak Do Heui Kim Tamás Szailer Charles H. F. Peden János Szanyi 《Catalysis Letters》2006,111(3-4):119-126
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. 相似文献
15.
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. 相似文献
16.
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. 相似文献
17.
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. 相似文献
18.
Burch R. Breen J. P. Hill C. J. Krutzsch B. Konrad B. Jobson E. Cider L. Eränen K. Klingstedt F. Lindfors L.-E. 《Topics in Catalysis》2004,30(1-4):19-25
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... 相似文献
19.
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. 相似文献