1173020-41-1Relevant articles and documents
The mechanism of reduction of NO with H2 in strongly oxidizing conditions (H2-SCR) on a novel Pt/MgO-CeO2 catalyst: Effects of reaction temperature
Savva,Costa,Efstathiou
, p. 743 - 747 (2008)
Steady State Isotopic Transient Kinetic Analysis (SSITKA) experiments using on-line Mass Spectrometry (MS) and in situ Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS) have been performed to study essential mechanistic aspects of the Selective Catalytic Reduction of NO by H2 under strongly oxidizing conditions (H2-SCR) in the 120-300°C range over a novel 0.1 wt % Pt/MgO-CeO2 catalyst. The N-path of reaction from NO to the N2 gas product was probed by following the 14NO/H2O2 → 15NO/H 2/O2 switch (SSITKA-MS and SSITKA-DRIFTS) at 1 bar total pressure. It was found that the N-pathway of reaction involves the formation of two active NO x species different in structure, one present on MgO and the other one on the CeO2 support surface. Inactive adsorbed NO x species were also found on both the MgO-CeO2 support and the Pt metal surfaces. The concentration (mol/g cat) of active NO x leading to N2 was found to change only slightly with reaction temperature in the 120-300°C range. This leads to the conclusion that other intrinsic kinetic reasons are responsible for the volcano-type conversion of NO versus the reaction temperature profile observed.
Model car-exhaust catalyst studied by TPD and TP-RAIRS: Surface reactions of NO on clean and O-covered Ir{1 0 0}
Khatua, Sabyasachi,Held, Georg,King, David A.
, p. 1 - 14 (2005)
The adsorption of NO on Ir{1 0 0} has been studied as a function of NO coverage and temperature using temperature programmed reflection absorption infrared spectroscopy (TP-RAIRS), low energy electron diffraction (LEED) and temperature programmed desorption (TPD). After saturating the clean (1 × 5)-reconstructed surface with NO at 95 K, two N2 desorption peaks are observed upon heating. The first N2 peak at 346 K results from the decomposition of bridge-bonded NO, and the second at 475 K from the decomposition of atop-bonded NO molecules. NO decomposition is proposed to be the rate limiting step for both N2 desorption states. For high NO coverages on the (1 × 5) surface, the narrow width of the first N 2 desorption peak is indicative of an autocatalytic process for which the parallel formation of N2O appears to be the crucial step. When NO is adsorbed on the metastable unreconstructed (1 × 1) phase of clean Ir{1 0 0} N2 desorption starts at lower temperatures, indicating that this surface modification is more reactive. When a high coverage of oxygen, near 0.5 ML, is pre-adsorbed on the surface, the decomposition of NO is inhibited and mainly desorption of intact NO is observed.
Kinetics and mechanism of the N2O reduction by NH3 on a Fe-zeolite-beta catalyst
Coq, Bernard,Mauvezin, Mathias,Delahay, Gerard,Kieger, Stephane
, p. 298 - 303 (2000)
In the context of decreasing the emissions of greenhouse gases, a Fe-exchanged zeolite-be (Fe-VEA) catalyst was shown to be very active in the reduction of N2O by NH3 in the presence of O2. NH3 accelerated the reduction of N2O to N2 on Fe-BEA. In the presence of O2, it was proposed that N2O conversion occurred through the redox cycle FeIII ? FeII with NN-O splitting mainly. N2O oxidized FeII to lead FeIII-oxocations, which were regenerated back to FeII by NH3. However, significant N-NO splitting occurred in the absence of O2. There was no inhibiting effect of O2 for the reduction of N2O by NH3, following a modified Mars and van Krevelen oxido-reduction kinetics, considering an inhibiting term of NH3.
Reactions of NH Radicals. I. Photolysis of NH3 Vapor at 313 nm
Kodama, Sukeya
, p. 2348 - 2354 (1983)
Photolysis of HN3 vapor was studied at 313 nm as a function of HN3 and Xe pressures, light intensity, and temperature.The photolysis of hydrazoic acid labeled with 15N was also studied.The quantum yields of N2, H2, and NH4N3 as a product were 4.85, 0.494, and 0.842 at 30 deg C and 6.7 kPa of HN3, respectively.The mechanism for the main reactions was postulated as follows: HN3+hv(313 nm) -> N2+NH(a1Δ); NH(a1Δ)+HN3 -> 2N2+2H (2); NH(a1Δ)+HN3 -> N3+NH2 (3); NH(a1Δ)+HN3 -> N2+N2H2* (4).The rate constant ratios of k3/k2=0.746 and k4/k2=1.23 were obtained at 30 deg C. (k3+k4)/k2 decrease drastically with rising temperature.Xe is effective for the collisional deactivation, NH(a1Δ)+Xe -> NH(X3Σ-)+Xe (15), and k15/(k2+k3+k4)=0.187 was obtained at 30 deg C.
Promoter Action of Alkali Nitrate in Raney Ruthenium Catalyst for Activation of Dinitrogen
Hikita, Tokihisa,Kadowaki, Yasushi,Aika, Ken-ichi
, p. 9396 - 9402 (1991)
Alkali nitrate promoted Raney Ru catalysts were prepared by decomposition of alkali nitrates (CsNO3, RbNO3, KNO3, and NaNO3) with hydrogen over Raney Ru.These catalysts were as active as Raney Ru promoted with metallic potassium at 573 K in N2 activation (ammonia synthesis and especially isotopic equilibration reaction (IERR) of N2).The promotional behavior of alkali nitrates on Raney Ru was different from that on the supported Ru catalysts.The alkali was estimated to work as a metallic on Raney Ru, whereas it was estimated to be hydroxide on supported Ru.The more reduced form on Raney Ru-CsNO3 was considered to give a higher turnover frequency of IER of N2 than that over alumina-supported Ru-CsNO3.Since the rate of IER of N2 is a rate of tracer atom moving from N2 to adsorbed N under the condition of adsorption equilibrium, it should be slower than the rate of ammonia synthesis whose adsorption step is rate-determing in a dynamic condition.To the contrary, the rates of ammonia synthesis were slower than IER rates of N2 over Raney Ru-CsNO3, suggesting hydrogen inhibition in the N2 activation process.Indeed, the IER of N2 over Raney Ru-CsNO3 was proved to be retarded by the presence of hydrogen.A kinetic analysis disclosed that N(a) and H(a) compete with each other on the Ru surface where H(a) adsorption is stronger than N(a) adsorption at 473-523 K.The heats of adsorption of N2 and H2 were estimated from the kinetics.
Promoting effect of CeO2 on the catalytic activity of Ba-Y2O3for direct decomposition of NO
Doi, Yasuyuki,Haneda, Masaaki,Ozawa, Masakuni
, p. 117 - 123 (2015)
The effect of CeO2 additive on the catalytic performance of Ba-Y2O3 prepared by coprecipitaion for the direct decomposition of NO was investigated. Although Ba-Y2O3 effectively catalyzed NO decomposition, its activity was clearly increased by addition of CeO2. The optimum CeO2 content was 10 mol%. CO2-TPD measurement revealed that the addition of CeO2 into Ba-Y2O3 caused an increase in the CO2 desorption peak in the temperature range of 473 and 723K derived from highly dispersed Ba species. The predominant role of CeO2 additive was suspected to effectively create the highly dispersed Ba species as catalytically active sites. Kinetic studies of NO decomposition on Ba-CeO2(10)-Y2O3 suggested that coexisting O2 suppresses the NO decomposition reaction by competitive adsorption. Isotopic transient kinetic analysis suggested a reaction pathway in which the surface NOx adspecies act as reaction intermediates for the formation of N2 in NO decomposition over Ba-CeO2-Y2O3. We concluded that CeO2 additive does not directly participate in the NO decomposition reaction as catalytically active species.
Isotopic transient kinetic analysis of Cs-promoted Ru/MgO during ammonia synthesis
Davis,McClaine
, p. 387 - 396 (2002)
Steady-state isotopic transient kinetic analysis was utilized to analyze a Cs-promoted Ru/MgO catalyst (1.75 wt% Ru) for NH3 synthesis. For comparison, an unpromoted Ru[SiO2 (22 wt % Ru) catalyst was also studied. Steady-state and isotopic transient measurements were carried out at 603-673 K with a stoichiometric ratio of N2 and H2 at a total pressure of 3 atm. The fractional surface coverage of nitrogen-containing intermediates based on total H2 chemisorption was 0.02-0.05 on both catalysts under the conditions used. However, the intrinsic turnover frequency of NH3 synthesis over Cs-Ru/MgO was two orders of magnitude greater than that of the unpromoted Ru/SiO2. The combination of the Cs promoter and MgO support improved the intrinsic activity of ruthenium, presumably by lowering the barrier for dinitrogen dissociation.
Transient isotopic kinetic study of the NO/H2/O2 (lean de-NOx) reaction on Pt/SiO2 and Pt/La-Ce-Mn-O catalysts
Costa, Costas N.,Efstathiou, Angelos M.
, p. 2620 - 2630 (2004)
Steady-state isotopic transient kinetic analysis (SSITKA) coupled with temperature-programmed surface reaction (TPSR) methods and using in situ mass spectroscopy and DRIFTS have been applied for the first time to study essential mechanistic aspects of the NO/H2/O2 reaction at 140°C under strongly oxidizing conditions over 0.1 wt % Pt/SiO2 and 0.1 wt % Pt/La-Ce-Mn-O catalysts. The nitrogen-pathway of the reaction from NO to form N2 and N2O gas products was probed by following the 14NO/H2/O2 → 15NO/H2/O2 isotopic switch at 1 bar total pressure. It was found that the chemical structure of active intermediate NOx species strongly depends on support chemical composition. In the case of the Pt/SiO2 catalyst, the reaction route for N2 and N2O formation passes through the interaction of one reversibly and one irreversibly NOx species chemisorbed on the Pt surface. On the other hand, in the case of a Pt/La-Ce-Mn-O catalyst, the reaction route passes through the interaction of two different in structure irreversibly chemisorbed NOx species on the support. For the latter catalyst, the mechanism of the reaction must involve a hydrogen-spillover process from the Pt metal to the support surface. A surface coverage Θ = 1.8 (based on Pt metal surface) of active NOx intermediate species was found for the Pt/La-Ce-Mn-O catalyst. A large fraction of it (81.5%) participates in the reaction path for N2 formation, whereas in the case of Pt/SiO2, this fraction was found to be 68.4% (active NOx, Θ = 0.65). These important results provide an explanation for the lower N2 reaction selectivity values observed on Pt/SiO2 compared to Pt/La-Ce-Mn-O catalyst. Inactive adsorbed NOx species (spectators) were found to accumulate on both Pt and support surfaces. It was found via the NO/H2/16O2 → NO/H2/18O2 isotopic switch that the reaction path from NO to form N2O passes through the oxidation step of NO to NO2 with the participation of gaseous O2, where the extent of it depends on support chemical composition.
Stoicheiometric and Nitrogen-15 Labelling Studies on the Hyponitrous Acid- Nitrous Acid Reaction
Bonner, Francis T.,Donald, Caroline E.
, p. 527 - 532 (1989)
The stoicheiometry of the hyponitrous acid - nitrous acid reaction has been determined over a wide acidity range, up to 8.5 mol dm-3 HClO4.For approximately 1:1 reaction conditions, the major reaction pathway gives N2 and HNO3 as products, together with the production of N2O (by self decomposition of hyponitrous acid) and NO (by self decomposition of nitrous acid).In addition, (15)NO produced by self decomposition of H(15)NO2 reacts with H2(14)N2O2 to give some (14)NO and N2O of mixed isotopic composition.Reactions under other conditions gave products that may be accounted for by varying contributions from these reactions.
Selectivity-directing factors of ammonia oxidation over PGM gauzes in the Temporal Analysis of Products reactor: Secondary interactions of NH3 and NO
Perez-Ramirez,Kondratenko,Kondratenko,Baerns
, p. 303 - 313 (2005)
Factors that direct the selectivity of ammonia oxidation for NO were determined previously (J. Catal. 227 (2004) 90) by the investigation of primary NH3-O2 interactions over pure Pt and Pt-Rh (95-5) alloy gauzes at 973-1173 K in the temporal analysis of products (TAP) reactor. A solid mechanistic understanding of the processes leading to by-products (N 2O and N2) requires an analysis of secondary NH 3-NO interactions, which we investigated in the TAP reactor with isotopically labeled molecules. Our experiments under transient vacuum conditions indicate that these secondary processes determine the reaction selectivity for N2O and N2 in high-temperature ammonia oxidation over noble metal catalysts. Adsorbed oxygen species initiate the reaction of ammonia with nitric oxide. N2O originates from the coupling of ammonia intermediates (NHx) and nitric oxide. Different reaction pathways leading to N2 have been identified, including primary (NH3 oxidation) and secondary (NHx and H-assisted NO reduction) processes. The relative contributions of these routes depend on the surface coverage of nitrogen and hydrogen-containing species. A reaction scheme accounting for our experimental observations has been proposed, giving rise to an improved mechanistic description of the complex processes in ammonia burners.
