Selective Reduction of NO by HNCO over Pt Promoted Al2O3☆ (original) (raw)
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Journal of environmental chemical engineering, 2016
The impact of H 2 as additional reducing agent on the SCR of NO with C 3 H 6 in excess oxygen, was comparatively explored over low noble metal loading (0.5 wt%), Pt/g-Al 2 O 3 , Pd/g-Al 2 O 3 , Ir/g-Al 2 O 3 catalysts. To gain insight into the role of H 2 , the reactions NO + C 3 H 6 + O 2 (R#1), NO + C 3 H 6 + O 2 + H 2 (R#2), NO + H 2 + O 2 (R#3) were employed. In respect to propene oxidation, the Pd > Pt > Ir sequence was obtained under R#1, since they exhibit complete conversion at 220, 250, 325 C, respectively; all metals exhibit moderate deNOx performances (X NO, <40%). H 2 co-presence (R#2) promotes both the NO and C 3 H 6 conversions, which is valid in the whole temperature interval investigated (50-400 C), being more substantial for Pt/g-Al 2 O 3 and Ir/g-Al 2 O 3 , less beneficial for Pd/g-Al 2 O 3. A two-maxima feature is obtained on X NO pattern (at 100and100 and 100and230 C) of Pt and Pd during R#2. The low temperature maximum Àattributed to NO reduction by H 2-is substantially more pronounced on Pt than Pd, offering X NO 9090% and S N2 9085%; the high temperature maximum-attributed to NO reduction by C 3 H 6-is higher by $15% on both Pt and Pd, in respect to the values obtained during R#1, while S N2 remained unaffected. Different X NO pattern with one maximum is obtained over Ir, implying a synergistic interaction between H 2 and C 3 H 6. This synergy is accompanied by a substantial widening of the NO reduction window toward lower temperatures and a considerable increase on both X NO,max and S N2
A series of Pt/Ce x Zr 1-x O 2-(x = 0.4–0.6) solids were synthesized and evaluated for the SCR of NO under lean burn conditions (2.5 vol% O 2) using C 3 H 6 and H 2 as reducing agents. SSITKA-Mass Spectrometry, SSITKA-DRIFTS and other in situ DRIFTS experiments were conducted for the first time to gather fundamental information in explaining the remarkable H 2 /C 3 H 6 synergy effect towards steady-state selective reduction of NO into N 2 at T > 400 • C. In particular, the chemical structure of adsorbed active and inactive (spectator) NO x species formed under C 3 H 6-SCR, H 2-SCR and H 2 /C 3 H 6-SCR of NO and the surface coverage and site formation of active NO x were probed. The Pt/Ce 1-x Zr x O 2-catalysts present significant differences in their H 2-SCR performance (NO conversion and N 2-selectivity) in the low-temperature range of 120–180 • C but practically the same catalytic behavior at higher temperatures. It was proved that the active NO x of the H 2-SCR path reside within a reactive zone around each Pt nanoparticle which extends to less than one lattice constant within the support surface. The chemical structure of the active intermediate was proved to be the chelating nitrite, whereas nitrosyl, monodentate and bidentate nitrates were considered as inactive species (spectators). It was illustrated for the first time that the presence of 15 vol% H 2 O in the H 2-SCR feed stream applied over the 0.1 wt% Pt/Ce 0.5 Zr 0.5 O 2 catalyst results in a 25% decrease in the concentration of active NO x , thus partly explaining the drop in activity observed when compared to the H 2-SCR in the absence of H 2 O. A remarkable activity and N 2-selectivity enhancement was observed at T > 400 • C when both H 2 and C 3 H 6 reducing agents were used compared to H 2-SCR or C 3 H 6-SCR alone. This synergy effect was explained to arise mainly because of the increase of H by the presence of –CH x species derived from adsorbed propylene decomposition on Pt, which block sites of oxygen chemisorption, and of the increase of surface oxygen vacant sites that promote the formation of a more active chelating nitrite (NO 2 −) species compared to the case of H 2-SCR.
Journal of Catalysis, 2002
The selective reduction of NO with propene has been investigated on Al 2 O 3 and Ga 2 O 3-Al 2 O 3 , using in situ diffuse reflectance Fourier transform infrared (FT-IR) spectroscopy combined with on-line mass spectrometry and IR gas analysis. During the NO-C 3 H 6-O 2 reaction, the main surface species detectable by IR were adsorbed nitrate, acetate, formate, cyanide (-CN), and isocyanate (-NCO). Ga 2 O 3 was found to promote the formation of these surface species, especially nitrate,-CN, and-NCO. From the experiments focusing on the reactivity of surface NO x (ads) (nitrite and nitrate) species, we found a very high reactivity of those compounds toward C 3 H 6 , leading to the rapid formation of-CN and-NCO species. On the other hand, acetate and formate species were deduced to be spectator species in this reaction. An appearance of ν(NH) bands ascribed to the formation of the surface complexes including amido groups (-NH complexes) was observed simultaneously with a decrease of the-NCO band, suggesting that the-NCO species is hydrolyzed to the-NH complexes by the reaction with some traces of water. A reaction mechanism has been proposed: NO x (ads) species formed by NO-O 2 reaction on the catalyst surface react with C 3 H 6derived species to generate the-NCO species via acrylic species and organic nitro compounds, and then the surface-NH complexes generated by hydrolysis of the-NCO species react with NO x (ads) species or NO 2 to produce N 2 .
Elucidating NH3 formation during NOx reduction by CO on Pt–BaO/Al2O3 in excess water
Catalysis Today, 2012
The reduction of NO was investigated with CO, H 2 , CO + H 2 O and CO + H 2 as the reducing species on a Pt-BaO/Al 2 O 3 monolith catalyst over an intermediate temperature range (200-300 • C). NH 3 is a major product of the NO + CO + H 2 O system under conditions of CO inhibition. The data are interpreted by the known NH 3 formation route in which the NO is reduced by H 2 formed by the water gas shift (WGS) reaction. In the absence of water, the strong adsorption of CO leads to sharp transitions between a high rate mass transport controlled regime and a lower rate kinetically controlled regime between 200 and 300 • C. Differential kinetics and integral experiments are reported at 270 • C. The intrinsic order with respect to CO in the latter regime is −1. When water is added with the CO feed, the regime transitions are more gradual and mitigated by the enhancement afforded by the hydrogen formed by the WGS reaction. Kinetic evidence for the effect of hydrogen is the much lower inhibition by CO during the WGS reaction (−0.23 order). When H 2 is added to the NO + CO mixture (without H 2 O) in the CO inhibited feed regime NH 3 and CO 2 are the major products even for low H 2 /NO feed ratio (∼1). Collectively, the steady-state findings are consistent with the major NH 3 formation pathway involving reaction of surface H (from WGS) and OH (from water) with adsorbed NO and N. The NH 3 formation route involving the hydrolysis of a surface isocyanate species formed from the reaction of NO and CO, is only of secondary importance.
Applied Catalysis B: Environmental, 2009
The NO + CO + H 2 reaction over CeO 2 , Au/CeO 2 (3 wt% Au), Au/CeO 2-Al 2 O 3 (2.9 wt% Au, 20 wt% Al 2 O 3) and CeO 2-Al 2 O 3 mixed support prepared by co-precipitation has been studied by FT-IR spectroscopy at elevated temperatures. Formation of NCO species has been detected on all of the samples. The presence of metallic gold is not necessary for the generation of the isocyanates on ceria and the mixed ceria-alumina support. The NCO species are produced by a process involving the dissociation of NO on the oxygen vacancies of the support, followed by the reaction between N atoms lying on the surface and CO molecules. Gold plays an important role in the modification of ceria leading to Ce 3+ and oxygen vacancies formation, and causes significant lowering of the reduction temperature of CeO 2 and CeO 2-Al 2 O 3 enhancing the reducibility of ceria surface layers. The role of H 2 is to keep the surface reduced during the course of the reaction. The onset temperature, at which the interaction between the surface isocyanates and NO begins, is low (100 8C). This explains the high activity of the Au/CeO 2-Al 2 O 3 catalyst with 100% selectivity in the reduction of NO by CO at low temperature (200 8C) and in the presence of H 2 .
Kinetics and …, 2005
Surface complexes resulting from the interaction between ammonia and a manganese-bismuth oxide catalyst were studied by IR spectroscopy and XPS. At the first stage, ammonia reacts with the catalyst to form the surface complexes [NH] and [NH 2 ] via abstraction of hydrogen atoms even at room temperature. Bringing the catalyst into contact with flowing air at room temperature or with helium under heating results in further hydrogen abstraction and simultaneous formation of [N] from [NH 2 ] and [NH]. The nitrogen atoms are localized on both reduced (Mn 2+) and oxidized (Mn δ + , 2 < δ < 3) sites. Atomic nitrogen is highly mobile and reacts readily with the weakly bound oxygen of the oxidized (Mn δ +-N) active site. The nitrogen atoms localized on oxidized sites play the key role in N 2 O formation. Nitrous oxide is readily formed through the interaction between two Mn δ +-N species. N 2 molecules result from the recombination of nitrogen atoms localized on reduced (Mn 2+-N) sites.
Catalytic Reduction of NO by CO over Rhodium Catalysts
Journal of Catalysis, 2000
techniques. It is found that, under the experimental conditions employed, four kinds of nitrogen oxide species may coexist in the adsorbed mode, namely, Rh-NO − (high), Rh-NO − (low), Rh(NO) 2 , and Rh-NO + , giving rise to IR bands located at 1770, 1660, 1830/1725, and 1908 cm −1 , respectively. Both negatively charged species readily dissociate on reduced surface sites, yielding nitride, and are mainly responsible for dinitrogen formation in the gas phase. The dinitrosyl species, the formation of which is favored over partially oxidized surfaces, is related to the production of nitrous oxide. The formation of both N 2 and N 2 O requires the presence of reduced surface sites. In the absence of Rh 0 , dissociative adsorption of NO stops and Rh-NO + species dominate the catalyst surface. Doping TiO 2 with W 6+ cations alters the electronic properties of supported Rh crystallites and, concomitantly, the chemisorptive behavior of the catalyst toward NO and CO. In particular, doping results in blue shifts in the stretching frequencies of N-O and C-O bonds contained in Rh-NO + , Rh(NO) 2 , Rh-CO, and Rh(CO) 2 species, indicating a weaker bonding of the adsorbed molecules with the surface. This is also evidenced by the significantly lower amounts of accumulated species, desorbed in TPD experiments. In contrast, the N-O bond of the Rh-NO − species is weakened by doping, resulting in higher rates of dissociation and, therefore, in higher transient yields of N 2 production in the gas phase, compared to the undoped catalyst.