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1.
The kinetics of CO oxidation and NO reduction reactions over alumina and alumina-ceria supported Pt, Rh and bimetallic Pt/Rh catalysts coated on metallic monoliths were investigated using the step response technique at atmospheric pressure and at temperatures 30–350°C. The feed step change experiments from an inert flow to a flow of a reagent (O 2, CO, NO and H 2) showed that the ceria promoted catalysts had higher adsorption capacities, higher reaction rates and promoting effects by preventing the inhibitory effects of reactants, than the alumina supported noble metal catalysts. The effect of ceria was explained with adsorbate spillover from the noble metal sites to ceria. The step change experiments CO/O 2 and O 2/CO also revealed the enhancing effect of ceria. The step change experiments NO/H 2 and H 2/NO gave nitrogen as a main reduction product and N 2O as a by-product. Preadsorption of NO on the catalyst surface decreased the catalyst activity in the reduction of NO with H 2. The CO oxidation transients were modeled with a mechanism which consistent of CO and O 2 adsorption and a surface reaction step. The NO reduction experiments with H 2 revealed the role of N 2O as a surface intermediate in the formation of N 2. The formation of NN bonding was assumed to take place prior to, partly prior to or totally following to the NO bond breakage. High NO coverage favors N 2O formation. Pt was shown to be more efficient than Rh for NO reduction by H 2. 相似文献
2.
The interactions between Pd/TiO 2 catalyst and the reactants and potential reaction intermediates present during aqueous nitrate reduction, including NO 3−, NO 2− and NO in the presence of H 2 and H 2O were studied by infrared spectroscopy. Adsorbed forms of NO, nitrite and nitrate could all be detected in the presence of water. In the presence of water/H 2, nitrate was the most stable surface species followed by nitrite and then highly reactive NO, suggesting that the reduction of nitrate to nitrite is the rate-limiting step. High concentrations of adsorbed nitrite appear to be linked to the detection of gaseous N 2O while the formation of ammonia is related to reactions on the Pd surface and the extent of formation is linked to high levels of adsorbed NO in addition to the surface hydrogen availability and the presence of water. 相似文献
3.
The reduction of NO and N 2O by CO over a silica-supported iron oxide catalyst was investigated by the transient response method, with different initial oxidation states of the catalyst, i.e. completely reduced (Fe 3O 4), or oxidised (Fe 2O 3). The influence of CO pre-adsorption was also studied. From the material balance on the gas phase species, it was shown that the composition of the catalyst changes during relaxation to steady-state. The degree of reduction of the catalyst at steady-state could thus be estimated. During the transient period, CO was shown to inhibit N 2O as well as NO reductions by adsorption on reduced sites. The activity of the reduced catalyst was found to be substantially higher as compared to the oxidised catalyst for both reactions. On this basis, it was attempted to keep the catalyst in a reduced state by periodically reducing it with CO. As a result, a significant increase in the performance of the reactor with respect to steady-state operation could be achieved for N 2O reduction by CO. Finally, the dynamic behaviour of the N 2O–CO and NO–CO reactions made it possible to evidence reaction steps, the occurrence of which could not be shown during our previous investigations on the separate interactions of the reactants with the catalyst. 相似文献
4.
Preliminary activity tests show a synergic effect on the yield of the N 2O+CO reaction by the addition of small quantities of rhodium in a Ag/Al 2O 3 catalyst. An analytical comparative kinetic study over Rh/Al 2O 3, Ag/Al 2O 3 and Rh-Ag/Al 2O 3 was performed in order to explain this effect. The reaction kinetics seems to follow a L–H mechanism with competitive adsorption of N 2O and CO over the rhodium catalyst, where a strong CO inhibition effect was obvious. On the two other catalysts (silver and mixed) a L–H mechanism with the reactants adsorbed in different active sites seems to be followed. The kinetic, adsorption and thermodynamic constants were calculated and compared. From the results it seems that the synergic effect is connected with an increase of the activity of rhodium since silver offers more active sites for CO adsorption and removes the inhibition effect, while rhodium offers more active sites for both reactants to be adsorbed. The above findings show that there are no strong interactions (e.g. alloying) between silver and rhodium for the catalyst prepared by the method and active constituents concentrations of this study. 相似文献
5.
The activation of hydrogen and CO was examined using D 2+H 2 equilibration at room temperature and by 13CO+C 18O scrambling at 170°C, respectively, the adsorption of NO at room temperature and its TPD were used to characterise the activation of NO. These reactions were compared with the NO reduction by carbon monoxide and by hydrogen. It appeared that the M/NaX clusters (M=Pt, Rh or Pd) exhibit opposite behaviour in the NO reduction by these two reductants: with CO the sequence was RhPdPt, while platinum (Pt/NaX) was the most active catalyst (PtPd>Rh) when hydrogen was employed. The CO scrambling was found to be most rapid over Pt, while the adsorption and dissociation of NO was most extensive over Rh; in the NO reduction by CO the weak CO activation over Rh was overwhelmed by the strong NO dissociation. On the other hand, the extensive NO adsorption and dissociation over Rh hindered the dissociation of hydrogen, which resulted in the lowest activity in the NO reduction by H2 accompanied by an intermediate formation of N2O. This was not the case with Pt, over which easily dissociated hydrogen reacted with probably molecularly adsorbed NO. The reduction of N2O by hydrogen proceeded readily over all metallic clusters at room temperature, being thus, either of the same activity as that of NO+H2 reaction, or even of higher activity over Pd and especially over Rh. The easy reduction of N2O by hydrogen does not agree with the reduction by CO, which was found to proceed worse than that of NO. In some cases, also bimetallic species (PtRh/NaX, PtPd/NaX, PdRh/NaX) were employed, as well as oxidized M clusters. 相似文献
6.
The reaction of NO + CO was studied over Pt/NaX prepared by the decomposition of [Pt(NH 3) 4] 2+. The decomposition was carried out via calcination followed by reduction, by vacuum decomposition, and by decomposition in hydrogen, by ways which are known to lead to the formation of Pt clusters of different sizes and location. The NO reduction by CO was studied under static conditions for longer (20–30 min) and shorter (100 s) time intervals, and the reaction was followed by temperature programmed decomposition (TPD) of species adsorbed during the preceding isothermal reactions. The effect of various NO/CO ratios and of added oxygen was examined. The reactions of N 2O + CO were compared with those of NO + CO. The increasing size of Pt clusters enhances the reduction of NO by CO, but it is complicated at lower reaction temperatures (below 230°C) by the poisoning of active Pt centres, especially by adsorbed CO. Smaller Pt clusters exhibit higher preference towards NO adsorption from NO + CO mixtures than the larger Pt clusters. The incomplete reduction of NO to N 2O proceeds under our experimental conditions below 230°C, and is accompanied by the formation of adsorbed species. N 2O formation is enhanced by the increased NO/CO ratio and by the addition of oxygen. The reduction of nitrous oxide occurs much slower than that of nitric oxide, and therefore N 2O could play a role only as a surface intermediate in the CO + NO reaction. 相似文献
7.
The catalytic reduction of N 2O by CH 4, CO, and their mixtures has been comparatively investigated over steam-activated FeZSM-5 zeolite. The influence of the molar feed ratio between N 2O and the reducing agents, the gas-hourly space velocity, and the presence of O 2 on the catalytic performance were studied in the temperature range of 475–850 K. The CH 4 is more efficient than CO for N 2O reduction, achieving the same degree of conversion at significantly lower temperatures. The apparent activation energy for N 2O reduction by CH 4 was very similar to that of direct N 2O decomposition (140 kJ mol −1), being much lower for the N 2O reduction by CO (60 kJ mol −1). This suggests that the reactions have a markedly different mechanism. Addition of CO using equimolar mixtures in the ternary N 2O + CH 4 + CO system did not affect the N 2O conversion with respect to the binary N 2O + CH 4 system, indicating that CO does not interfere in the low-temperature reduction of N 2O by CH 4. In the ternary system, CO contributed to N 2O reduction when methane was the limiting reactant. The conversion and selectivity of the reactions of N 2O with CH 4, CO, and their mixtures were not altered upon adding excess O 2 in the feed. 相似文献
8.
A series of CeO 2 promoted cobalt spinel catalysts were prepared by the co-precipitation method and tested for the decomposition of nitrous oxide (N 2O). Addition of CeO 2 to Co 3O 4 led to an improvement in the catalytic activity for N 2O decomposition. The catalyst was most active when the molar ratio of Ce/Co was around 0.05. Complete N 2O conversion could be attained over the CoCe0.05 catalyst below 400 °C even in the presence of O 2, H 2O or NO. Methods of XRD, FE-SEM, BET, XPS, H 2-TPR and O 2-TPD were used to characterize these catalysts. The analytical results indicated that the addition of CeO 2 could increase the surface area of Co 3O 4, and then improve the reduction of Co 3+ to Co 2+ by facilitating the desorption of adsorbed oxygen species, which is the rate-determining step of the N 2O decomposition over cobalt spinel catalyst. We conclude that these effects, caused by the addition of CeO 2, are responsible for the enhancement of catalytic activity of Co 3O 4. 相似文献
9.
This study aims at synthesizing a new by substituting 1 atom% Pd 2+ in ionic state in TiO 2 in the form of Ti 0.99Pd 0.01O 1.99 with oxide-ion vacancy. The catalyst was synthesized by solution combustion method and was characterized by XRD and XPS. The catalytic activity was investigated by performing CO oxidation, hydrocarbon oxidation and NO reduction. A reaction mechanism for CO oxidation by O 2 and NO reduction by CO was proposed. The model based on CO adsorption on Pd 2+ and dissociative chemisorption of O 2 in the oxide-ion vacancy for CO oxidation reaction fitted the experimental for CO oxidation. For NO reduction in presence of CO, the model based on competitive adsorption of NO and CO on Pd 2+, NO chemisorption and dissociation on oxide-ion vacancy fitted the experimental data. The rate parameters obtained from the model indicated that the reactions were much faster over this catalyst compared to other catalysts reported in the literature. The selectivity of N 2, defined as the ratio of the formation of N 2 and formation of N 2 and N 2O, was very high compared to other catalysts and 100% selectivity was reached at temperature of 350 °C and above. As the N 2O + CO reaction is an intermediate reaction for NO + CO reaction, it was also studied as an isolated reaction and the rate of the isolated reaction was less than that of intermediate reaction. 相似文献
10.
The kinetics of the reaction of NO, N 2O and CO 2 with activated carbon without catalyst and impregnated with a precursor salt of vanadium (ammonium monovanadate) was investigated. The conversion of NO, N 2O and CO 2 was studied (450–900°C) using a TGA apparatus and a fixed bed reactor. The reactor effluents were analysed using a GC/MS on line. The addition of vanadium increased carbon reactivity and adsorption at lower temperatures. For NO and N 2O conversion the main products obtained were N 2, N 2O, CO and CO 2 but for CO 2 conversion only CO was detected. In situ XRD was a useful tool for interpreting catalyst behaviour and identifying phases present during reaction conditions. The catalytic effect of vanadium can be explained by the occurrence of redox processes in which the catalyst is reduced to lower oxidation states such as V 2O 5/V 6O 13. 相似文献
11.
The reaction pathways of N 2 and N 2O formation in the direct decomposition and reduction of NO by NH 3 were investigated over a polycrystalline Pt catalyst between 323 and 973 K by transient experiments using the temporal analysis of products (TAP-2) reactor. The interaction between nitric oxide and ammonia was studied in the sequential pulse mode applying 15NO. Differently labelled nitrogen and nitrous oxide molecules were detected. In both, direct NO decomposition and NH 3–NO interaction, N 2O formation was most marked between 573 and 673 K, whereas N 2 formation dominated at higher temperatures. An unusual interruption of nitrogen formation in the 15NO pulse at 473 K was caused by an inhibiting effect of adsorbed NO species. The detailed analysis of the product distribution at this temperature clearly indicates different reaction pathways leading to the product formation. Nitrogen formation occurs via recombination of nitrogen atoms formed by dissociation of nitric oxide or/and complete dehydrogenation of ammonia. N 2O is formed via recombination of adsorbed NO molecules. Additionally, both products are formed via interactions between adsorbed ammonia fragments and nitric oxide. 相似文献
12.
The inhibition effect of H 2O on V 2O 5/AC catalyst for NO reduction with NH 3 is studied at temperatures up to 250 °C through TPD, elemental analyses, temperature-programmed surface reaction (TPSR) and FT-IR analyses. The results show that H 2O does not reduce NO and NH 3 adsorption on V 2O 5/AC catalyst surface, but promotes NH 3 adsorption due to increases in Brønsted acid sites. Many kinds of NH 3 forms present on the catalyst surface, but only NH 4+ on Brønsted acid sites and a small portion of NH 3 on Lewis acid sites are reactive with NO at 250 °C or below, and most of the NH 3 on Lewis acid sites does not react with NO, regardless the presence of H 2O in the feed gas. H 2O inhibits the SCR reaction between the NH 3 on the Lewis acid sites and NO, and the inhibition effect increases with increasing H 2O content. The inhibition effect is reversible and H 2O does not poison the V 2O 5/AC catalyst. 相似文献
13.
In this paper, the effect of CO 2 and H 2O on NO x storage and reduction over a Pt–Ba/γ-Al 2O 3 (1 wt.% Pt and 30 wt.% Ba) catalyst is shown. The experimental results reveal that in the presence of CO 2 and H 2O, NO x is stored on BaCO 3 sites only. Moreover, H 2O inhibits the NO oxidation capability of the catalyst and no NO 2 formation is observed. Only 16% of the total barium is utilized in NO storage. The rich phase shows 95% selectivity towards N 2 as well as complete regeneration of stored NO. In the presence of CO 2, NO is oxidized into NO 2 and more NO x is stored as in the presence of H 2O, resulting in 30% barium utilization. Bulk barium sites are inactive in NO x trapping in the presence of CO 2·NH 3 formation is seen in the rich phase and the selectivity towards N 2 is 83%. Ba(NO 3) 2 is always completely regenerated during the subsequent rich phase. In the absence of CO 2 and H 2O, both surface and bulk barium sites are active in NO x storage. As lean/rich cycling proceeds, the selectivity towards N 2 in the rich phase decreases from 82% to 47% and the N balance for successive lean/rich cycles shows incomplete regeneration of the catalyst. This incomplete regeneration along with a 40% decrease in the Pt dispersion and BET surface area, explains the observed decrease in NO x storage. 相似文献
14.
The effect of acidic treatments on N 2O reduction over Ni catalysts supported on activated carbon was systematically studied. The catalysts were characterized by N 2 adsorption, mass titration, temperature-programmed desorption (TPD), and X-ray photoelectron spectrometry (XPS). It is found that surface chemistry plays an important role in N 2O-carbon reaction catalyzed by Ni catalyst. HNO 3 treatment produces more active acidic surface groups such as carboxyl and lactone, resulting in a more uniform catalyst dispersion and higher catalytic activity. However, HCl treatment decreases active acidic groups and increases the inactive groups, playing an opposite role in the catalyst dispersion and catalytic activity. A thorough discussion of the mechanism of the N 2O catalytic reduction is made based upon results from isothermal reactions, temperature-programmed reactions (TPR) and characterization of catalysts. The effect of acidic treatment on pore structure is also discussed. 相似文献
15.
Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) has been used to study NH 3 and NO adsorption over a 15% w/w vanadia/titania catalyst. NH 3 is adsorbed as coordinate NH 3 and NH 4+ species over the oxidised catalyst, leading to the reduction of the vanadia surface. At 300°C, adsorbed nitrosyls species are detected, suggesting that the oxidation of gaseous or adsorbed ammonia species takes place over the V=O sites. Coadsorption experiments show that NO is able to reoxidise about the 57% of the reduced V=O groups, resulting in N 2, according to a NO+V □→1/2N 2+V=O reaction. On the other hand, NO is only adsorbed over vanadia reduced surfaces. The measure of the area of the 2 ν(V=O) bands results in an estimate of the oxidation state of vanadium. From this estimate it can be concluded that nitrosyls species are adsorbed on the catalyst surface for vanadium atoms having an oxidation state ranging from +4 to +3.1. 相似文献
16.
The influence of ammonia and nitric oxide oxidation on the selective catalytic reduction (SCR) of NO by ammonia with copper/nickel and vanadium oxide catalysts, supported on titania or alumina have been investigated, paying special attention to N 2O formation. In the SCR reaction, the VTi catalyst had a higher activity than VAl at low temperatures, while the CuNiAl catalyst had a higher activity than CuNiTi. A linear relationship between the reaction rate of ammonia oxidation and the initial reduction temperature of the catalysts obtained by H 2-TPR showed that the formation rate of NH species in copper/nickel catalysts would be higher than in vanadia catalysts. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that copper/nickel catalysts presented ammonia coordinated on Lewis acid sites, whereas ammonium ion adsorbed on Brønsted acid sites dominated on vanadia catalysts. The NO oxidation experiments revealed that copper/nickel catalysts had an increase of the NO 2 and N 2O concentrations with the temperature. NO could be adsorbed on copper/nickel catalysts and the NO 2 intermediate species could play an important role in the reaction mechanism. It was suggested that the presence of adsorbed NO 2 species could be related to the N 2O formation. 相似文献
17.
Nitric oxide and nitric dioxide compounds (NO x) present in stack gases from nitric acid plants are usually eliminated by selective catalytic reduction (SCR) with ammonia. In this process, small quantities of nitrous oxide (N 2O) are produced. This undesirable molecule has a high greenhouse gas potential and a long lifetime in the atmosphere, where it can contribute to stratospheric ozone depletion. The influence of catalyst composition and some operating variables were evaluated in terms of N 2O formation, using V 2O 5/TiO 2 catalysts. High vanadia catalyst loading, nitric oxide inlet concentration and reaction temperature increase the generation of this undesirable compound. The results suggest that adsorbed ammonia not only reacts with NO via SCR, but also with small quantities of oxygen activated by the presence of NO. The mechanism proposed for N 2O generation at low temperature is based on the formation of surface V–ON species which may be produced by the partial oxidation of dissociatively adsorbed ammonia species with NO + O 2 (eventually NO 2). When these active sites are in close proximity they can interact to form an N 2O molecule. This mechanism seems to be affected by changes in the active site density produced by increasing the catalyst vanadia loading. 相似文献
18.
An effect of oxygen species formed from O 2, N 2O and NO on the selectivity of the catalytic oxidation of ammonia was studied over a polycrystalline Pt catalyst using the temporal analysis of products (TAP) reactor. The transient experiments were performed in the temperature range between 773 and 1073 K in a sequential pulse mode with a time interval of 0.2 s between the pulses of the oxidant (O 2, N 2O and NO) and NH 3. In contrast to adsorbed oxygen species formed from NO, those from O 2 and N 2O reacted with ammonia yielding NO. It is suggested that the difference between these oxidising agents may be related to the different active sites for dissociation of O 2, N 2O and NO, where oxygen species of various Pt-O strength are formed. Weaker bound oxygen species, which are active for NO formation, originate from O 2 and N 2O rather than from NO. These species may be of bi-atomic nature. 相似文献
19.
Cu-AlTS-1 catalyst was prepared by solid state ion exchange and studied in the NO and N 2O decomposition. Oscillation was observed in a wide range of experimental conditions during the decomposition of N 2O. At full N 2O conversion, oscillations were observed only in the O 2 and NO concentrations the latter being out of phase with respect to O 2 and being originated from the decomposition of an excess oxygen containing nitrito–nitrato-like surface complex. Traces of NO extinguished the oscillations and increased the N 2O conversion if it was below 100%. The NO also plays a key role in the feed back and synchronisation. 相似文献
20.
N 2O是一种重要的温室气体,且对臭氧层有很大的破坏作用,而直接催化分解法是除去N 2O最经济有效的方法之一。针对目前报道较多的钴氧化物催化剂活性较差的问题,将包覆型Co 3O 4核壳材料引入N 2O直接催化分解反应,利用核壳结构的限域特性与壳层的多孔孔道使Co 3O 4分散性增加,粒径减小,金属载体相互作用与接触反应界面增强,从而提高了催化剂在N 2O直接催化分解反应中的低温活性。此外,还制备了一系列不同金属含量的Co 3O 4@SiO 2球形核壳催化剂来研究包覆结构对催化剂性能的影响,通过X射线荧光光谱(XRF)、透射电镜(TEM)、X射线衍射(XRD)、N 2物理吸附、H 2-程序升温还原(H 2-TPR)等表征,证实在保证稳定单分散核壳结构的前提下,活性Co 3O 4位点越多,催化剂反应活性越好。 相似文献
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