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1.
Ammonium nitrate is thermally stable below 250 °C and could potentially deactivate low temperature NOx reduction catalysts by blocking active sites. It is shown that NO reduces neat NH4NO3 above its 170 °C melting point, while acidic solids catalyze this reaction even at temperatures below 100 °C. NO2, a product of the reduction, can dimerize and then dissociate in molten NH4NO3 to NO+ + NO3, and may be stabilized within the melt as either an adduct or as HNO2 formed from the hydrolysis of NO+ or N2O4. The other product of reduction, NH4NO2, readily decomposes at ≤100 °C to N2 and H2O, the desired end products of DeNOx catalysis. A mechanism for the acid catalyzed reduction of NH4NO3 by NO is proposed, with HNO3 as an intermediate. These findings indicate that the use of acidic catalysts or promoters in DeNOx systems could help mitigate catalyst deactivation at low operating temperatures (<150 °C).  相似文献   

2.
The catalytic activities of ceria–zirconia mixed oxides CexZr1−xO2 (x = 0.17, 0.62 and 0.8) rhodium catalysts were determined by isothermal steady-state experiments using a representative mixture of exhaust gases of coal combustion. Results show that all supports are active in deNOx reaction in the presence of the mentioned gas mixture. However, their catalytic activity varies with the content of cerium and goes through a maximum for x = 0.62, leading to 27% NOx consumption. The effect of rhodium on Ce0.62Zr0.38O2 considerably improves the catalytic activity during the deNOx process assisted by hydrocarbons. The rhodium addition decreases by about 34 °C the temperature of NOx consumption, which goes up to 57%. A mechanism of hydrocarbon (HC) assisted reduction of NO is proposed on ceria–zirconia-supported rhodium catalysts. This mechanism is divided in three catalytic cycles involving (i) the oxidation of NO into NO2, (ii) the reaction of NO2 and the hydrocarbons leading to RNOx species and CxHyOz, and finally (iii) the decomposition of NO assisted by these latter CxHyOz species.  相似文献   

3.
The performances of different promoters (CeO2, ZrO2 and Ce0.5Zr0.5O2 solid solution) modified Pd/SiC catalysts for methane combustion are studied. XRD and XPS results showed that Zr4+ could be incorporated into the CeO2 lattice to form Zr0.5Ce0.5O2 solid solution. The catalytic activities of Pd/CeO2/SiC and Pd/ZrO2/SiC are lower than that of Pd/Zr0.5Ce0.5O2/SiC. The Pd/Zr0.5Ce0.5O2/SiC catalyst can ignite the reaction at 240 °C and obtain a methane conversion of 100% at 340 °C, and keep 100% methane conversion after 10 reaction cycles. These results indicate that active metallic nanoparticles are well stabilized on the SiC surface while the promoters serve as oxygen reservoir and retain good redox properties.  相似文献   

4.
The objective of this work is the study of fundamental common aspects of NOx catalytic reduction over a Co/Pd-HFER zeolite catalyst, using methanol or methane as reducing agent. Temperature Programmed Surface Reaction (TPSR) studies were performed with reactant mixtures comprising NO2 and one of the reducing agents.The formation of formaldehyde was detected in both studied reactions (NO2–CH4 and NO2–CH3OH) in the temperature range between 100 and 220 °C. At higher temperature, when the NOx reduction process effectively begins, formaldehyde starts to be consumed.Using methanol as reducing agent, nitromethane and nitrosomethane, are detected. At 300 °C these species are consumed and cyanides and iso-cyanides formation occurs. On the contrary, with methane, these last species were not detected; however, there are strong evidences for CH3NO and CH3NO2 formation.Thus, using methanol or methane, similar phenomena were detected. In both cases, common intermediary species seem to play an important role in the NOx reduction process to N2.These results suggest that methanol can be considered as a reaction intermediate species in the mechanism of the reduction of NO2 with methane, over cobalt/palladium-based ferrierite catalysts.  相似文献   

5.
Gas–solid interactions and surface intermediates evolution after NO adsorption onto calcined Ce0.62Zr0.38O2 were investigated. The results of adsorption and temperature-programmed desorption of NO were explained using diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) coupled with temperature-programmed experiments in environmental cell. Surface NO-containing species such as nitrites and nitrates were identified during evolution of NO on the surface of Ce0.62Zr0.38O2 solid solution at low and high temperature. The ceria–zirconia solid solution was found to be active in deNOx reaction in the presence of a “toluene, propene and propane” mixture.  相似文献   

6.
Dry reforming of methane was studied over Ni catalysts supported on γAl2O3, CeO2, ZrO2 and MgAl2O4 (670 °C, 1.5 bar, 16–20 l CH4 mlcatalyst−1 h−1). It is shown that MgAl2O4 supported Ni catalysts promoted with both CeO2 and ZrO2 are promising catalysts for dry reforming of methane with carbon dioxide. Within a certain composition range, the simultaneous promotion with CeO2 and ZrO2 has great influence on the amount of coke and the catalyst service time. XRD analyses indicate that formation of crystalline CexZr1−xO2 mixed oxide phases occurs on double promotion. In particular, incorporation of low amounts of Zr in the CeO2 fluorite structure provides stable dry reforming catalysis. As shown with TPR, promotion leads to a higher reduced state of Ni. SEM, XRD and TPR analyses demonstrate that highly dispersed, doubly promoted Ni catalysts with a strong metal-support interaction are essential for stable dry reforming and suppression of the formation of carbon filaments.  相似文献   

7.
The stabilization of Co-mordenite catalysts through lanthanum exchange is reported here. The effect of exchange order and calcination conditions upon the reduction of NOx to N2 at 500 °C was tracked during 400 h on a stream containing NOx, CH4, O2 and 10% H2O. Both the fresh and used catalysts were characterized through TPR, Raman spectroscopy, FTIR spectroscopy using CO as probe molecule, and XPS. These techniques revealed that the CoLa-mordenite catalysts which were not affected by the severe hydrothermal treatment showed no sign of Co or La migration out of the exchange positions. Instead, those that rapidly deactivated showed the formation of cobalt oxides and, in some cases, the migration of the cations to other exchange positions. The presence of exchanged lanthanum seems to preserve the integrity of the zeolite structure preventing the migration of cobalt ions with the subsequent formation of cobalt oxides which favors the reaction of methane with O2, thus decreasing N2 production.  相似文献   

8.
9.
Oxygen storage capacity (OSC) of CeO2–ZrO2 solid solution, CexZr(1−x)O4, is one of the most contributing factors to control the performance of an automotive catalyst. To improve the OSC, heat treatments were employed on a nanoscaled composite of Al2O3 and CeZrO4 (ACZ). Reductive treatments from 700 to 1000 °C significantly improved the complete oxygen storage capacity (OSC-c) of ACZ. In particular, the OSC-c measured at 300 °C reached the theoretical maximum with a sufficient specific surface area (SSA) (35 m2/g) after reductive treatment at 1000 °C. The introduced Al2O3 facilitated the regular rearrangement of Ce and Zr ions in CeZrO4 as well as helped in maintaining the sufficient SSA. Reductive treatments also enhanced the oxygen release rate (OSC-r); however, the OSC-r variation against the evaluation temperature and the reduction temperature differed from that of OSC-c. OSC-r measured below 200 °C reached its maximum against the reduction temperature at 800 °C, while those evaluated at 300 °C increased with the reduction temperature in the same manner as OSC-c.  相似文献   

10.
In this work we report results of NOx adsorption and diesel soot combustion on a noble metal promoted K/La2O3 catalyst. The fresh-unpromoted solid is a complex mixture of hydroxide and carbonate compounds, but the addition of Rh favors the preferential formation of lanthanum oxycarbonate during the calcination step. K/La2O3 adsorbs NOx through the formation of La and K nitrate species when the solid is treated in NO + O2 between 70 and 490 °C. Nitrates are stable in the same temperature range under helium flow. However, they become unstable at ca. 360 °C when either Rh and/or Pt are present, the effect of Rh being more pronounced. Nitrates decompose under different atmospheres: NO + O2, He and H2. The effect of Rh might be to form a thermally unstable complex (Rh–NO+) which takes part both in the formation of the nitrates when the catalyst is exposed to NOx and in the nitrates decomposition at higher temperatures. Regarding soot combustion, nitrates react with soot with a temperature of maximun reaction rate of ca. 370 °C, under tight contact conditions. This temperature is not affected by the presence of Rh, which indicates that the stability of nitrates has little effect on their reaction with soot.  相似文献   

11.
Chaoquan Hu   《Catalysis communications》2009,10(15):2008-2012
Ultrafine Cu0.1Ce0.5Zr0.4O2−δ catalyst operated in a fluidized bed reactor was found to be very effective for complete oxidation of dilute benzene in air. The complete conversion of benzene could be achieved at reaction temperature as low as 220 °C. The mechanism of benzene oxidation over the Cu0.1Ce0.5Zr0.4O2−δ catalyst was investigated by conducting pulse reaction of pure benzene in the absence of O2 over the catalyst and the results indicated the involvement of lattice oxygen from the catalyst in benzene oxidation.  相似文献   

12.
The 1-propanol assisted-reduction of NO x was investigated over Ir/Ce0.6Zr0.4O2. The catalytic performances of such a catalyst, the associated FTIR characterizations, and transient experiments suggest the formation of adsorbed R-NO x species as intermediates of the deNO x process; they provide the partially oxidized species required by the deNO x model.  相似文献   

13.
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 range over which NO x could be reduced during the SCR reaction with octane. Reduction of NO x was initiated at as low a temperature as 100 °C and >95% NO x conversion was achieved over a temperature range of 200–450 °C. Hydrogen has the effect of activating octane at lower temperatures and also promotes the oxidation of NO to NO2 in the absence of hydrocarbon. Transient kinetic and in situ DRIFTS measurements indicated that hydrogen has a direct role in the reaction mechanism by either promoting the formation and storage of an organic C = N species which can then readily reduce NO x and/or removing a species which acts as a poison to the SCR reaction at low temperatures.  相似文献   

14.
In this study, the nature of surface intermediates generated by adsorption of NO and NO2 on a commercial ceria–zirconia powder of composition Ce0.69Zr0.31O2 was investigated using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The conditions of occurrence of the main adsorbed species, i.e. nitrites and nitrates, are studied semi-quantitatively as a function of catalyst pre-treatment and/or type of adsorbed NOx molecule. On the partially reduced ceria–zirconia, the primary role of NOx is to re-oxidize the surface via adsorption/decomposition on reduced sites. By contrast, the formation of nitrites and nitrates readily occurs on oxidized surfaces, the latter kind of species being strongly promoted in the case of NO2 adsorption only.  相似文献   

15.

Abstract  

A series of Ce1−x Zr x O2 (x = 0, 0.1, 0.2, 0.3) solid solution supported lean-burn NO x trap (LNT) catalysts K/LaCoO3/Ce1−x Zr x O2 were prepared by successive impregnation. After sulfation the supported perovsikte LaCoO3 was well maintained; reducing treatment partly destroyed the perovsikte, but it can be well recovered by re-oxidation treatment. Based on NO x storage and sulfur-resisting performance of the catalysts, the optimal atomic ratio of Zr in Ce1−x Zr x O2 support is x = 0.2. The catalyst K/LaCoO3/Ce0.8Zr0.2O2 exhibits much better NO x storage capacity than the Pt-based catalyst Pt/K/Ce0.8Zr0.2O2, which is highly related to its stronger capability for NO to NO2 oxidation. During NO x storage much larger amounts of nitrate and nitrite species were identified by in situ DRIFTS over perovskite-based catalysts than over Pt-based one. The H2-TPR results reveal that after deep sulfation little sulfur species were deposited on the catalyst K/LaCoO3/Ce1−x Zr x O2, showing strong sulfur-resisting ability. As a result, it is thought that the full replacement of Pt by perovskite LaCoO3 in the corresponding LNT catalysts is feasible.  相似文献   

16.
Granger  P.  Lamonier  J.F.  Sergent  N.  Aboukais  A.  Leclercq  L.  Leclercq  G. 《Topics in Catalysis》2001,16(1-4):89-94
The intrinsic activity of various Zr x Ce1–x O2 mixed oxides and after a Pd deposition has been investigated in the CO + NO reactions from temperature-programmed experiments performed under stoichiometric conditions. It has been found that the activity of Zr x Ce1–x O2 depends on either the specific surface area or the number of Ce cations and their intrinsic activity, Zr0.5Ce0.5O2 being the most active support. The addition of palladium strongly enhances the catalytic activity of the supports probably due to a synergistic effect between CeO2 and the metal since the initial activity of palladium-based catalysts is directly related to their Ce content. Such a catalytic enhancement has been explained by a bifunctional mechanism involving active sites probably composed of Pd and ceria. A strong deactivation operates leading to the disappearance of the beneficial effect of ceria. Such a deactivation seems to be dependent on the support composition, Pd supported Zr0.25Ce0.75O2 being the most resistant to deactivation.  相似文献   

17.
V.R. Choudhary  K.C. Mondal  T.V. Choudhary 《Fuel》2006,85(17-18):2484-2488
The oxy-CO2 methane reforming reaction (OCRM) has been investigated over CoOx supported on a MgO precoated highly macroporous silica–alumina catalyst carrier (SA-5205) at different reaction temperatures (700–900 °C), O2/CH4 ratios (0.3–0.45) and space velocites (20,000–100,000 cc/g/h). The reaction temperature had a profound influence on the OCRM performance over the CoO/MgO/SA-5205 catalyst; the methane conversion, CO2 conversion and H2 selectivity increased while the H2/CO ratio decreased markedly with increasing reaction temperature. While the O2/CH4 ratio did not strongly affect the CH4 and CO2 conversion and H2 selectivity, it had an intense influence on the H2/CO ratio. The CH4 and CO2 conversion and the H2 selectivity decreased while the H2/CO increased with increasing space velocity. The O2/CH4 ratio and the reaction temperature could be used to manipulate the heat of the reaction for the OCRM process. Depending on the O2/CH4 ratio and temperature the OCRM process could be operated in a mildly exothermic, thermal neutral or mildly endothermic mode. The OCRM reaction became almost thermoneutral at an OCRM reaction temperature of 850 °C, O2/CH4 ratio of 0.45 and space velocity of 46,000 cc/g/h. The CH4 conversion and H2 selectivity over the CoO/MgO/SA-5205 catalyst corresponding to thermoneutral conditions were excellent: 95% and 97%, respectively with a H2/CO ratio of 1.8.  相似文献   

18.
The selective catalytic reduction (SCR) of NO x over zeolite H-ZSM-5 with ammonia was investigated using in situ FTIR spectroscopy and flow reactor measurements. The adsorption of ammonia and the reaction between NO x , O2 and either pre-adsorbed ammonia or transiently supplied ammonia were investigated for either NO or equimolar amounts of NO and NO2. With transient ammonia supply the total NO reduction increased and the selectivity to N2O formation decreased compared to continuous supply. The FTIR experiments revealed that NO x reacts with ammonia adsorbed on Brønsted acid sites as NH4 + ions. These experiments further indicated that adsorbed -NO2 is formed during the SCR reaction over H-ZSM-5.  相似文献   

19.
SmYb1−xMgxZr2O7−x/2 (0 ≤ x ≤ 0.15) ceramics are pressureless-sintered at 1973 K for 10 h in air. The structure and electrical conductivity of SmYb1−xMgxZr2O7−x/2 ceramics are investigated by the X-ray diffraction, scanning electron microscopy and impedance spectroscopy measurements. SmYb1−xMgxZr2O7−x/2 ceramics exhibit a defect fluorite-type structure. The measured electrical conductivities of SmYb1−xMgxZr2O7−x/2 ceramics obey the Arrhenius relation, and electrical conductivity of each composition increases with increasing temperature from 673 to 1173 K. At identical temperature levels, the electrical conductivity of SmYb1−xMgxZr2O7−x/2 ceramics gradually increases with increasing magnesia content. SmYb1−xMgxZr2O7−x/2 ceramics are oxide-ion conductors in the oxygen partial pressure range of 1.0 × 10−4 to 1.0 atm at all test temperature levels. The electrical conductivity obtained in SmYb1−xMgxZr2O7−x/2 ceramics reaches the highest value of 2.72 × 10−3 S cm−1 at 1173 K for the SmYb0.85Mg0.15Zr2O6.925 ceramic.  相似文献   

20.
The interaction of CO, C2H4, O2, and NO reaction gas compounds over the metallic Pd/Al2O3 and Pd/OSC/Al2O3 monoliths was investigated in order to understand the behaviour of OSC material in the oxidation and reduction reactions. FT-IR gas analyser was used for the analysis of the product gas composition. Several activity experiments carried out with dissimilar feedstreams have revealed that the Ce x Zr1–x O2 mixed oxide is an oxygen storage compound, which promotes CO and C2H4 oxidation as well as NO reduction in particular at low temperatures.  相似文献   

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