In situ IR Spectroscopic and XPS Study of Surface Complexes and Their Transformations during Ammonia Oxidation to Nitrous Oxide over an Mn-Bi-O/α-Al 2 O 3 … (original) (raw)
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Kinetics and Catalysis, 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.
Studies of the mechanism of ammonia oxidation into nitrous oxide over MnBiO/α-Al2O3 catalyst
Journal of Catalysis, 2004
A complex of kinetic and physicochemical methods: temperature-programmed surface reaction (TPSR), pulsing NH 3 or NH 3 / 16 O 2 ( 18 O 2 ) reaction mixture, and infrared and photoelectron spectroscopies, was used for characterization of the highly selective supported manganesebismuth oxide catalysts and for the study of the mechanism of the ammonia oxidation. Ammonia oxidation was demonstrated to proceed via alternating reduction and reoxidation of the catalyst surface with participation of the lattice oxygen. NH 3 interacts with weakly bonded oxygen species through hydrogen atom abstraction to form adsorbed [N] species, which are localized on Mn 2+ and Mn δ+ (2 < δ < 3). Manganese ions with different oxidation degrees (Mn 3+ (Mn 4+ ) and Mn δ+ ) serve as active sites of the catalyst surface. The correlation between the selectivity toward N 2 O and the portion of manganese in the Mn 3+ (Mn 4+ ) state was established. Bismuth oxide plays an important role by increasing the quantity, mobility, and thermal stability of the subsurface oxygen. The reaction kinetic scheme is suggested based on the experimental results. Numerical simulation of TPSR data confirms the reliability of the proposed reaction mechanism.
Applied Magnetic Resonance, 2000
EPR spectroscopy has been used to investigate changes occurring at the molecular level during coordination and activation of NO and N 2 0 small pollutant molecules on Cu/ZSM-5 and Mo/Sí02 catalysts, respectively. It is shown that the activation of nitric oxide occurs by formation of a bent r) {CuNO}" surface adduct with rehybridization of the NO orbitals. The spin Hamiltonian parameters of this species have been analyzed in detail, leading to a molecular and electronic picture of the complex. The mechanism along which N 20 is decomposed upon interaction with surface Mos+ species involves dissociative metal-to-ligand electron transfer. The reaction was monitored by EPR using naturally abundant and 95 Mo-enriched molybdenum. The dynamics of the NO bond cleavage is discussed in terms of two-dimensional Morse potential energy surfaces. From the EPR parameters of the resultant Oradical, the spin density distribution and the stabilization energy were calculated.
Catalysis Today, 2006
The reaction mechanism for the selective catalytic reduction (SCR) of nitric oxide by ammonia on (0 1 0) V 2 O 5 surface represented by a V 2 O 9 H 8 cluster was simulated by means of density functional theory (DFT) calculations performed at B3LYP/6-31G ** level. The computations indicated that SCR reaction consisted of three main parts. For the first part, ammonia activation on V 2 O 5 was investigated. Ammonia was adsorbed on Brønsted acidic V-OH site as NH 4 + species by a non-activated process with an exothermic relative energy difference of 28.65 kcal/mol. Lewis acidic ammonia interactions were also considered and they were found to be energetically unfavorable. Therefore, it is concluded that the SCR reaction on (0 1 0) vanadium oxide surface is initiated favorably by the Brønsted acidic ammonia adsorption. The second part of the SCR reaction consists of the interaction of nitric oxide with the pre-adsorbed ammonia species to eventually form nitrosamide (NH 2 NO) species. The rate limiting step for this part as well as for the total SCR reaction can be identified as NH 3 NHO formation with a high activation barrier of 43.99 kcal/ mol; however, it must be cautioned that only an approximate transition state was obtained for this step. For the last part, gas phase decomposition of NH 2 NO and decomposition of this species on catalyst surface were both considered. Gas phase decomposition of NH 2 NO was found to have high activation barriers when compared with the NH 2 NO decomposition on V 2 O 9 H 8 cluster surface. NH 2 NO decomposition on this cluster was achieved by means of a push-pull hydrogen transfer mechanism between the active V O and V-OH groups. #
Surface reactivity of NH3 on oxidised nickel surfaces
Surface and Interface Analysis
The aim of the present study was to investigate the reaction between ammonia and pre-adsorbed oxygen on Ni at 600 K using metastable impact electron spectroscopy (MIES) and UPS techniques. For a given oxygen-pretreated nickel surface, the changes in the MIES spectra along with ammonia adsorption can be decomposed into three different steps: a first domain without significant changes, a second domain corresponding to drastic changes in the intensity of a signal at 15.8 eV; and a third domain where no more change is detected. The first domain may correspond to the ammonia chemisorption on the surface, whereas the changes in the second domain would be attributed to surface dehydroxylation. The existence of NHx groups on the surface has been detected by UPS; they result from ammonia chemisorption. The changes in the MIES spectra have also been studied versus the initial oxygen uptake on Ni. It emerged that the first domain is larger with increasing initial oxygen exposure: this may be e...
An FT-IR study of ammonia adsorption and oxidation over anatase-supported metal oxides
Applied Catalysis B-environmental, 1997
The adsorption and the oxidation of ammonia over sub-monolayer Ti02-anatase supported chromium, manganese, iron, cobalt, nickel and copper oxides, has been investigated using FT-IR spectroscopy. These materials are models of catalysts active in the Selective Catalytic Reduction of NO, by ammonia (SCR process) and in the Selective Catalytic Oxidation of ammonia to dinitrogen (SC0 process). For comparison, the adsorption of ammonia and hydrazine over the TiOz-anatase support has also been studied. CrO-Ti02 adsorbs ammonia both in a co-ordinated form over Lewis acid sites and in a protonated form over Bronsted acid sites, involving high-valence chromium (chromyl species). However, simple outgassing at r.t. causes the desorption of ammonia from Bronsted acid sites showing that they are very weak. All other catalysts do not present any Bronsted acidity. Co-ordinated ammonia gives rise to several oxidation products over Fe#-TiOz, CrO,-Ti02, Coo,-Ti02 and CuO-Ti02, among which hydrazine is likely present. Other species have been tentatively identified as imido species, NH, nitroxyl species, HNO, and nitrogen anions, NT. NiO,-Ti02 and MnO,-Ti02 appear to be even more active in ammonia oxidation, because the adsorbed species disappeared completely at lower temperature (473 K) than in the other cases. However, possibly just due to their excessive activity, no adsorbed species different from co-ordinated ammonia can be found in significant amounts over these surfaces. Based on these data, the mechanism of the SCR and SC0 processes over these catalytic materials is discussed. In particular, it is concluded that Bronsted acidity is not a requirement for SCR and SC0 activity.
Journal of Catalysis, 2004
In this work a complete mechanism for describing the low-temperature (125 • C) selective catalytic reduction of NO with NH 3 over carbon-supported Mn 3 O 4 is discussed. This study sets out to explain for the first time certain specific interactions among NH 3 , NO, O 2 , and a manganese-based catalyst. A set of SCR reactions was obtained through a detailed TPD analysis of the surface NH 3 species and by taking into account the conclusions of a previous study on the role of NO species [Phys. Chem. Chem. Phys. 6 (2004) 453]. The SCR reactions proceed via an Eley-Rideal mechanism, in which NO 2 , and to a lesser extent NO, reacts from the gas phase with surface-active NH 3 species. The overall reaction path involves the simultaneous occurrence of two different SCR mechanisms in which either aminooxy groups or ammonium ions react with NO/NO 2 . These NH 3 -based species are related to the local phases that coexist in Mn 3 O 4 : (a) SCR by aminooxy groups (steady-state mechanism). Aminooxy groups formed on the locally octahedral environment of Mn 3 O 4 (Mn 2 O 3 ) react with gaseous NO 2 . O 2 cannot dissociate on this phase in order to reoxidize the reduced catalyst and therefore the overall SCR process is 6NO + 4NH 3 → 5N 2 + 6H 2 O. (b) SCR by ammonium ions (pseudo-steady-state mechanism). This mechanism occurs on the locally tetrahedral environment of Mn 3 O 4 (MnO) and initially accounts for ∼ 60% of the total NO reduction. However, it is gradually deactivated by the nitrates formed on those same hydroxyl groups that are available for ammonium formation. The ammonium ions formed on the hydroxyl groups of this tetrahedral environment react with gas-phase NO 2 . The overall SCR process is 4NO + 4NH 3 + O 2 → 4N 2 + 6H 2 O. 2004 Elsevier Inc. All rights reserved.
Catalysis Today, 1996
The adsorption and transformation of ammonia over V,O,, V,O,/TiO,, V,O,-WOJTiO, and CuO/TiO, systems has been investigated by FT-IR spectroscopy. In all cases ammonia is first coordinated over Lewis acid sites and later undergoes hydrogen abstraction giving rise either to NH, amide species or to its dimeric form N,H,, hydrazine. Other species, tentatively identified as imide NH, nitroxyl HNO, nitrogen anions N; and azide anions N; are further observed over CuO/TiO,.
Selective catalytic oxidation of ammonia by nitrogen oxides in a model synthesis gas
Fuel, 2013
Synthesis gas generated by the gasification of nitrogen-containing hydrocarbons will contain ammonia. This is a catalyst poison and elevated levels of nitrogen oxides (NO X) will be produced if the synthesis gas is combusted. This paper presents a study of the selective oxidation of ammonia in reducing environments. The concept is the same as in traditional selective catalytic reduction, where NO X are removed from flue gas by reaction with injected ammonia over a catalyst. Here, a new concept for the removal of ammonia is demonstrated by reaction with injected NO X over a catalyst. The experiments were carried out in a model synthesis gas consisting of CO, CO 2 , H 2 , N 2 and NH 3 /NO X. The performance of two catalysts, V 2 O 5 /WO 3 /TiO 2 and H-mordenite, were evaluated. On-site generation of NO X by nitric acid decomposition was also investigated and tested. The results show good conversion of ammonia under the conditions studied for both catalysts, and with on-site generated NO X .