Mechanism Of N2O Decomposition Research Articles

N2O Decomposition on Singly and Doubly (K and Li)-Doped Co3O4 Nanocubes─Establishing Key Factors Governing Redox Behavior of Catalysts.

The intimate mechanism of N2O decomposition on bare and redox-tuned Co3O4 nanocubes (achieved by single (Li or K) and double (Li and K) doping) was elucidated. The catalysts synthesized by the hydrothermal method were characterized by X-ray electron absorption fine structure measurements, X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and Kelvin Probe techniques. TPSR and steady-state isothermal catalytic tests reveal that the N2O turnover frequencies are critically sensitive to the work function of the catalysts, adjusted purposely by doping. For the catalysts obtained by one-pot hydrothermal synthesis, lithiation of the Co3O4 nanocubes leads to the formation of {Li'8a, Co·16d} species, decreasing steadily the work function and the activity, while for the catalysts prepared by postsynthesis impregnation, formation of {Li'8a, Co'16d, Co··16c} species leads to a volcano-type dependence of the catalytic activity and the work function in parallel. The beneficial effect of potassium was discussed in terms of mitigation of surface potential buildup due to the accumulation of ionosorbed oxygen intermediates (surface electrostatics), which hinders the interfacial electron transfer. Analysis of the catalytic activity response to the redox tuning of Co3O4, substantiated by DFT calculations, allowed for a straightforward conceptualization of the redox nature of the N2O decomposition in terms of the lineup of frontier orbitals of the N2O/N2O- and O2-/O2 reactants with the surface DOS structure and the resultant molecular orbital interactions. The positions of the virtual bonding 3πg0(N2O)-α-3dz2 and the occupied 2πg1(O2-)-α-3dz2 states relative to the Fermi energy level play a crucial role in the regulation of the forward and backward interfacial electron transfer events, which drive the redox process.

Boosting N2O Decomposition by Fabricating the Cs-O-Co Structure over Co3O4 with Single-Layer Atoms of Cs.

Developing effective catalysts for N2O decomposition at low temperatures is challenging. Herein, the Cs-O-Co structure, as the active species fabricated by single-layer atoms of Cs over pure Co3O4, originally exhibited great catalytic activity of N2O decomposition in simulated vehicle exhaust and flue gas from nitric acid plants. A similar catalytic performance was also observed for Na, K, and Rb alkali metals over Co3O4 catalysts for N2O decomposition, illustrating the prevalence of alkali-metal-promotion over Co3O4 in practical applications. The catalytic results indicated that the TOF of Co3O4 catalysts loaded by 4 wt% Cs was nearly 2 orders of magnitude higher than that of pure Co3O4 catalysts at 300 °C. Interestingly, the conversions of N2O decomposition over Co3O4 catalysts doped by the same Cs loadings were significantly inhibited. Characterization results indicated that the primary active Cs-O-Co structure was formed by highly orbital hybridization between the Cs 6s and the O 2p orbital over the supported Co3O4 catalysts, where Cs could donate electrons to Co3+ and produce much more Co2+. In contrast, the doped Co3O4 catalysts were dominated by Cs2O2 species; meanwhile, CsOH species was generated by adsorbed water vapor led to a significant decrease in catalytic activity. In situ DRIFTS, rigorous kinetics, and DFT results elaborated the reaction mechanism of N2O decomposition, where the direct decomposition of adsorbed N2O was the kinetically relevant step over supported catalysts in the absence of O2. Meanwhile, the assistance of adsorbed N2O decomposition by activated oxygen was observed as the kinetically relevant step in the presence of O2. The results may pave a promising path toward developing alkali-metal-promotion catalysts for efficient N2O decomposition.

Nature of active Fe species and reaction mechanism over high-efficiency Fe/CHA catalysts in catalytic decomposition of N2O

Fe/CHA catalysts for N2O decomposition with various Fe loadings (Fe/SAPO-34 and Fe/SSZ-13) were synthesized by a solid-state ion exchange method. Under the same reaction conditions, obviously superior activities of Fe/CHA catalysts were observed in comparison with the current state-of-the-art Fe/zeolite, i.e., Fe/FER. Meanwhile, Fe/CHA catalysts exhibited a robust high-temperature hydrothermal stability. The characterization results and density functional theory (DFT) calculations indicated that the dispersion of Fe species over Fe/CHA catalysts was strongly influenced by the nature of the support, although SAPO-34 and SSZ-13 have similar morphology. Combined with the catalytic performance, dinuclear [HO–Fe–O–Fe–OH]2+ species were identified as the most active Fe species for Fe/CHA catalysts in the N2O decomposition reaction. Furthermore, the dimeric Fe species in SSZ-13 were more active than those in SAPO-34, due to lower Fe–O binding energies. Finally, DFT calculations provided a comprehensive understanding of the N2O decomposition mechanism over Fe/CHA catalysts.

Effect of Water Molecule on Photo-Assisted Nitrous Oxide Decomposition over Oxotitanium Porphyrin: A Theoretical Study

Water vapor has generally been recognized as an inhibitor of catalysts in nitrous oxide (N2O) decomposition because it limits the lifetime of catalytic reactors. Oxygen produced in reactions also deactivates the catalytic performance of bulk surface catalysts. Herein, we propose a potential catalyst that is tolerant of water and oxygen in the process of N2O decomposition. By applying density functional theory calculations, we investigated the reaction mechanism of N2O decomposition into N2 and O2 catalyzed by oxotitanium(IV) porphyrin (TiO-por) with interfacially bonded water. The activation energies of reaction Path A and B are compared under thermal and photo-assisted conditions. The obtained calculation results show that the photo-assisted reaction in Path B is highly exothermic and proceeds smoothly with the low activation barrier of 27.57 kcal/mol at the rate determining step. The produced O2 is easily desorbed from the surface of the catalyst, requiring only 4.96 kcal/mol, indicating the suppression of catalyst deactivation. Therefore, TiO-por is theoretically proved to have the potential to be a desirable catalyst for N2O decomposition with photo-irradiation because of its low activation barrier, water resistance, and ease of regeneration.

Catalytic N2O decomposition over La(Sr)FeO3 perovskites

In this study, La1−xSrxFeO3 perovskites with x = 0, 0.25, 0.50, 0.75 and 1.0 have been synthesized via a sol gel auto-combustion method using glycine as fuel. All materials were characterized with x-ray powder diffraction (XRD), N2 porosimetry, scanning electron microscopy (SEM) and O2 Temperature programmed adsorption- desorption (O2/TPA-D). The catalytic activity of perovskites was tested for the decomposition of N2O to N2 and O2 as a probe reaction in a bench scale plug flow reactor. The most active catalyst was La0.75Sr0.25FeO3 and the catalytic activity is strongly correlated to the amount of the reverse oxygen uptake from the perovskite structure. According to the N2O decomposition mechanism the slow and rate determining step was found to be the dissociation of N2O adsorbed on the surface of catalyst as $${\text{N}}_{ 2} {\text{O}}^{ - }_{{({\text{ads}})}}$$ .

N2O decomposition over CuO/CeO2 catalyst: New insights into reaction mechanism and inhibiting action of H2O and NO by operando techniques

In this work, a combination of ex situ (STEM-EELS, STEM-EDX, H2-TPR and XPS), in situ (CO-DRIFTS) and operando (DR UV–vis and DRIFTS) approaches was used to probe the active sites and determine the mechanism of N2O decomposition over highly active 4wt.% Cu/CeO2 catalyst. In addition, reaction pathways of catalyst deactivation in the presence of NO and H2O were identified. The results of operando DR UV–vis spectroscopic tests suggest that [Cu−O−Cu]2+ sites play a crucial role in catalytic N2O decomposition pathway. Due to exposure of {100} and {110} high-energy surface planes, nanorod-shaped CeO2 support simultaneously exhibits enhancement of CuO/CeO2 redox properties through the presence of Ce3+/Ce4+ redox pair. Its dominant role of binuclear Cu+ site regeneration through the recombination and desorption of molecular oxygen is accompanied by its minor active participation in direct N2O decomposition. NO and H2O have completely different inhibiting action on the N2O decomposition reaction. Water molecules strongly and dissociatively bind to oxygen vacancy sites of CeO2 and block further oxygen transfer as well as regeneration of catalyst active sites. On the other hand, the effect of NO is expressed through competitive oxidation to NO2, which consumes labile oxygen from CeO2 and decelerates [Cu+ Cu+] active site regeneration.

Theoretical study of N2O decomposition mechanism over binuclear Cu-ZSM-5 zeolites

The mechanism of N2O decomposition over binuclear Cu-ZSM-5 zeolites is investigated using density functional theory (DFT). It is found that the activation energy of two N2O molecules dissociation are 47.19 and 63.89kcal/mol, and the desorption energy of O2 from Cu2O2 is 14.70kcal/mol. The DFT results show that the NO splitting step of N2O molecule controls the whole catalytic cycle, and the binuclear Cu-ZSM-5 has not suffered from O2 inhibition. Moreover, some experimental work reports O2 inhibition is not observed at high Cu loading, which is in agreement with the present calculation. These findings suggest that the catalytic behavior of binuclear Cu-ZSM-5 is different from that of mononuclear. Furthermore, the kinetic analysis confirms the rate-limiting step in the binuclear Cu-ZSM-5 system is the N2O dissociation step, and the rate constant indicates that high temperature could accelerate the N2O dissociation. These results could allow us to understand the structure of the active sites and reveal the active sites controlling catalytic activity. The DFT calculation results combined with the kinetic study would provide us a comprehensive understanding on the NOx decomposition mechanism over the Cu-ZSM-5 zeolites.

Small CuO clusters on CeO2 nanospheres as active species for catalytic N2O decomposition

High surface area CeO2 nanospheres as an active catalyst support were synthesized using glycothermal approach. Different loadings of copper (4, 6, 10 and 15wt.%) were supported by wet impregnation method. Prepared materials were characterized by means of TEM, SEM-EDX, XRD, UV-Vis diffuse reflectance, N2 adsorption/desorption, DRIFT and H2-TPR techniques, and tested for the catalytic reaction of nitrous oxide decomposition. The best activity in the N2O degradation was found for the sample containing 10wt.% of Cu that can be attributed to the highest number of small CuO clusters on the catalyst surface. Further increase of copper content strongly affects the dispersion and leads to the formation of less active segregated CuO phase, which was confirmed by XRD, UV-Vis and H2-TPR results. Accordingly to UV–Vis examination and DRIFT analysis using CO as a probe molecule, all solids contain Cu+1 ions which play a crucial role in the N2O decomposition mechanism. The synthesized catalysts were also tested in wet or NO containing atmospheres, where an inhibiting effect takes place and leads to shifting of conversion profiles to higher temperature by 65 and 10°C, correspondingly. It was found out that the formation of a new, crystalline CuO·3H2O phase occurs in water vapour containing atmosphere, which can result in catalyst deactivation. However, this effect is fully reversible and the catalyst is able to replenish initial activity in dry atmosphere. Potentiality of CuO/CeO2 materials in catalytic N2O decomposition in industrial processes was confirmed by long-term stability tests performed in the period of 50h in the presence of inhibiting gas components.

Ordered mesoporous CuO–CeO2 mixed oxides as an effective catalyst for N2O decomposition

Ordered mesoporous CuO–CeO2 mixed oxides with different Cu loadings were synthesized by means of hard template replication approach using mesoporous silica KIT-6 as a template. Prepared materials were characterized by SEM-EDX, XRD, UV–Vis DR, N2 adsorption/desorption, and H2-TPR techniques. The catalytic decomposition of N2O was studied in a fixed-bed reactor in the temperature range from 300 °C to 600 °C and GHSV = 45,000 h−1. Solids prepared by replication approach showed superior catalytic activity in comparison to materials synthesized by conventional preparation methods. Among the solids tested, the highest conversion of N2O was observed in the presence of a catalyst containing 40 mol.% Cu. Reduction of prepared samples occurs at much lower temperatures in comparison to individual CuO and CeO2 oxides due to synergetic effect present in mixed oxides. N2O decomposition tests revealed very good agreement between the catalytic activity and material reducibility, which increased with increasing Cu content from 25 to 40 mol.%. Formation of segregated CuO phase was observed for samples with Cu content above 40 mol.%. Accordingly to UV–Vis examination, all solids contain Cu+1 and Ce+3, which play a crucial role in the N2O decomposition mechanism. In wet (1.5 vol.% H2O) or NO (1.5 vol.%) containing atmospheres, where inhibiting effect due to competitive adsorption of water and NO molecules on active sites takes place, resulting conversion profiles were shifted to higher temperature by 70 °C and 25 °C, respectively. Results of long-term stability test performed in the period of 50 h confirmed good catalytic stability of an investigated material, which can be attributed to catalyst ability to regenerate active sites by desorption of formed oxygen from the catalyst surface.

Mechanistic Aspects of N2O Decomposition Over Carbon Films and Carbon-Film-Supported Catalysts

The mechanism of N2O decomposition on the platinum and copper loaded carbon film catalysts was studied using in situ FTIR spectroscopy at the temperature range 200–600 °C. It was experimentally proved that on these catalysts, carbon becomes not only a support but also a reactant due to the formation of the surface oxygen complexes. The oxidation and dissociation of surface oxides play a significant role in the overall process due to the formation of the new active centres. It is experimentally proved that –NCO formation is a major step in the overall mechanism at the low temperature range.

Rationales for the selection of the best precursor for potassium doping of cobalt spinel based deN2O catalyst

Cobalt spinel Co3O4 catalysts promoted with different potassium precursors (K2CO3, KNO3, CH3COOK, KOH) were prepared and characterized by surface (XPS, work function, SEM) and structure (Raman, XRD, H2-TPR) sensitive techniques. The stability of potassium promoter was determined by means of Species Resolved-Thermal Alkali Desorption method (SR-TAD). The N2O decomposition mechanism on series of synthesized catalysts was studied by temperature programmed reaction and pulse experiments of isotopically labeled 15N218O. The role of the octahedral cobalt ions as catalytic active sites was revealed. The most dispersed state of alkali promoter, obtained from K2CO3 precursor, results in its highest stability and lowest work function, leading to the highest catalytic activity in N2O decomposition in the investigated K-Co3O4 series. The beneficial effect of potassium is discussed in terms of the facile activation of N2O via dissociative electron transfer (N2O +e−=N2+O−) and promotion of the preferred fast suprafacial recombination of oxygen intermediates (revealed by the isotopic experiments).

Density Functional Theory Study of Mechanism of N2O Decomposition over Cu-ZSM-5 Zeolites

The N2O decomposition mechanism is investigated over Cu-ZSM-5 using density functional theory (DFT). Though the mechanism is extended from Fe/Co-ZSM-5, the results show that a different step may be rate-determining over Cu-ZSM-5 compared to the Fe/Co-ZSM-5 system. In the beginning, Z[Cu] as active center decomposes the first N2O and generates Z[CuO] (process 1), and the energy barrier of N2O dissociation is 35.18 kcal/mol. Then Z[CuO] could decompose the second N2O and generate Z[CuOO] (process 2), and the energy barrier of N2O dissociation is 28.07 kcal/mol. In process 2, oxygen could desorb from Z[CuOO], and the desorption energy is 39.48 kcal/mol, which is only higher 4.30 kcal/mol than 35.18 kcal/mol in the process 1. However the corresponding rate constants show approximately that the rate-limiting step is O2 desorption in process 2 and not the N2O dissociation in process 1. Next, if Z[CuOO] could not desorb O2, it could decompose the third N2O and generate Z[CuO(O2)] (process 3). In this process, the energy barrier for N2O dissociation and the O2 desorption energy from Z[CuO(O2)] are 42.10 and 63.42 kcal/mol, respectively, which are much higher than the former processes. It indicates the presence of O2 could inhibit the N2O decomposition over Cu-ZSM-5, which is in line with the kinetic experiment. The results suggest the process 1 and 2 are the main catalytic cycle in N2O decomposition. Importantly, O2 desorption from Z[CuOO] shows that the mechanism over Cu-ZSM-5 is different from that over Fe/Co-ZSM-5 system.

Decomposition of nitrous oxide over a low‐cost catalyst

AbstractNitrous oxide (N2O) is a significant greenhouse gas and its emissions from fluidized bed combustion systems can be controlled using catalytic conversion and decomposition. The catalytic decomposition of N2O was studied in a quartz fixed‐bed reactor at 450–700 °C. The low‐cost catalyst was prepared from fly ash and iron oxides, and was characterized in terms of the BET specific surface area and XRD. This study showed that both the Fe content on the catalyst surface and the calcination temperature during the preparation of the catalyst should be optimized in order to significantly improve the catalyst performance when used for N2O decomposition. The influences of O2, H2O, CO2 and SO2 on the N2O decomposition were investigated at the most reactive temperature. Experimental results showed that O2 had a small negative effect on the catalytic activity, while CO2 had no obvious effect on the N2O decomposition. The N2O conversion was still high even when H2O was present. However, the N2O conversion dropped sharply when H2O and O2 were both present. The catalytic activity was reduced by SO2 because of its reaction with the catalyst. The difference between the N2O conversion with H2O present and with both H2O and O2 present was analyzed in order to determine the N2O decomposition mechanism. Two mechanisms for the N2O catalytic decomposition mechanism have been considered, of which the one with formation of oxygen atoms at two sites with migration and recombination shows better agreement with our experimental results. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd.

Theoretical evidence of the observed kinetic order dependence on temperature during the N2O decomposition over Fe-ZSM-5

The characterization of Fe/ZSM5 zeolite materials, the nature of Fe-sites active in N(2)O direct decomposition, as well as the rate limiting step are still a matter of debate. The mechanism of N(2)O decomposition on the binuclear oxo-hydroxo bridged extraframework iron core site [Fe(II)(mu-O)(mu-OH)Fe(II)](+) inside the ZSM-5 zeolite has been studied by combining theoretical and experimental approaches. The overall calculated path of N(2)O decomposition involves the oxidation of binuclear Fe(II) core sites by N(2)O (atomic alpha-oxygen formation) and the recombination of two surface alpha-oxygen atoms leading to the formation of molecular oxygen. Rate parameters computed using standard statistical mechanics and transition state theory reveal that elementary catalytic steps involved into N(2)O decomposition are strongly dependent on the temperature. This theoretical result was compared to the experimentally observed steady state kinetics of the N(2)O decomposition and temperature-programmed desorption (TPD) experiments. A switch of the reaction order with respect to N(2)O pressure from zero to one occurs at around 800 K suggesting a change of the rate determining step from the alpha-oxygen recombination to alpha-oxygen formation. The TPD results on the molecular oxygen desorption confirmed the mechanism proposed.

N2O Decomposition over Perovskite Catalysts

Four perovskite catalysts LaBO3 (where B = Cr, Mn, Fe, and Co) were prepared via a highly exothermic and self-sustaining reaction, the so-called “solution combustion synthesis (SCS)”, and characterized by means of X-ray diffraction, BET, field-emission scanning electron microscopy−energy-dispersive spectrometry, and H2-temperature-programmed reduction (TPR) analyses. The performance of these catalysts toward the decomposition of N2O to N2 and O2 was evaluated in a temperature programmed reaction (TPRe) apparatus in the absence and the presence of different oxygen concentrations. Among the catalysts screened, LaCoO3 showed the best performance, with 50% conversion of N2O at 455 °C and 490 °C in the absence and presence of 5% of oxygen, respectively. The LaCoO3 catalyst was deposited by in situ SCS directly over a ceramic honeycomb monolith and then tested in a lab-scale test rig. The coated ceramic monolith gave 50% N2O conversion performance similar to that obtained on powder for GHSV values of industrial interest (10 000−30 000 h-1). The correlation between the observed oxygen inhibition and the proposed N2O decomposition mechanism as well as the relationship between the observed activity and the reducibility of the B site, determined from TPR experiments, is discussed.

Transient multi pulse method for the determination of N2O - interaction with ZSM-5 type zeolites

Multipulse experiments reveal qualitative and quantitative, time-resolved information about the interaction of N2O with ZSM-5 type zeolites under conditions of catalytic applications, like the mechanism of N2O decomposition, amount and reactivity of an atomic surface oxygen species.

The Mechanism of N2O Decomposition on Fe-ZSM-5: An Isotope Labeling Study

The mechanism of N2O decomposition over an 18O-labeled Fe-ZSM-5 catalyst was studied. The isotope distribution of the O2 produced during N2O decomposition was statistical. This shows that O2 was formed from a pool of equilibrated surface oxygen atoms. The size of the pool was 1.0 O/Fe, i.e. most or all of the iron clusters took part in the reaction. The reactivity of the oxygen atoms in the pool was low. They did not exchange with 18O2 to a significant extent.

Catalytic Decomposition of N2OON ZrO2and Co/Ni/ZrO2Systems

AbstractMonoclinic ZrO2and its supported catalysts Co/Ni/ZrO2for catalytic decomposition of N2O have been studied with FTIR, EDAX, XPS, and the evaluation of activity of the catalysts. It is found that monoclinic ZrO2alone has the catalytic effect for N2O decomposition although the gas decomposes on Co/Ni/ZrO2more efficiently. The XPS study shows that only Co exists in the surface region of ZrO2. In evaluation experiments, it is found that when Co/Ni exceeds a threshold concentration, the conversion of N2O is no longer accelerated with the increase of Co/Ni content. The gas decomposition on Co/Ni/ZrO2can be described as first order with respect to partial pressure of N2O. No nitrogen N(ls) photoelectrons were detected for the catalysts after N2O decomposition. Surface reactions on ZrO2and Co/Ni/ZrO2, including the behaviour of retained carbon (Cls) and N2O decomposition mechanism, will also be addressed.