Suprafacial Mechanism Research Articles

Unraveling mechanistic aspects of the total oxidation of methane over Mn, Ni and Cu spinel cobaltites via in situ electrical conductivity measurements

Mn, Ni and Cu cobaltite spinels were synthesized by coprecipitation, characterized and evaluated in the total oxidation of methane. To gain a deeper insight into their catalytic behavior, their electrical conductivity was studied as a function of temperature and oxygen partial pressure, and was followed with time during sequential exposures to different gases in conditions close to those used for the catalytic tests. The Ni cobaltite is in a metallic conductivity state and cannot be reduced in the reaction conditions, suggesting that it functions via a suprafacial mechanism. The Cu cobaltite behaves as a p-type semiconductor, but it is not reduced under the reaction mixture at the reaction temperature of 350 °C, suggesting that, at least in these conditions, the suprafacial mechanism is operating. The Mn cobaltite has an n-type semiconducting character and is reduced in the reaction conditions, suggesting that a heterogeneous redox mechanism is involved.

Total oxidation of lean methane over cobalt spinel nanocubes—Mechanistic vistas gained from DFT modeling and catalytic isotopic investigations

Abstract Mechanistic investigations based on combination of catalytic isotopic experiments with DFT modeling were used to probe the pathways by which CH4 in transformed into H2O and CO2 over cobalt spinel nanocubes exposing the (100) surface exclusively. The catalyst synthetized by the hydrothermal method was thoroughly characterized by means of XRD, XRF, Raman and SEM techniques. Combustion of methane was performed in a TPSR mode using sole 16O2 or a 1:1 mixture of 16O2 and 18O2. Two distinct mechanistic pathways of the C H bond activation, and subsequent oxidation of the consecutive intermediates involve a suprafacial mechanism operating in the oxygen covered surface and an interfacial mechanism operating on the oxygen depleted/bare surface. It was found that depending on the conditions they may act separately in the early and late stages of the reaction, and simultaneously in the middle phase, in response to differences in the nature and accessibility of the surface and lattice oxygen species, however the activation of the first C– bond remains the kinetically relevant step. The cobalt-oxo CoT2c–O species exhibit the highest activity in the suprafacial C H bond breaking, and the peroxo O22– species are engaged in the intermediate reaction steps leading to formation of final H2O and CO2 products. On the oxygen exhausted surface, where the interfacial mechanism operates, dual Co–O sites constituted by CoO5c–O3O and CoT2c–O2O,1T pairs may activate the C H bond in a concerted way, with the former being more active. For all the mechanistic steps the transition and stable states of the involved intermediates and products were identified and quantified in terms of their structure and the associated free enthalpy profiles. The three dimensional (log(kintra/ksupra) versus (ΔGasupra-ΔGasupra) and ΘO) diagrams were constructed to discriminate between the suprafacial (Eley-Rideal/Langmuir-Hinshelwood) and interfacial (Mars-van Krevelen) mechanism of CH4 combustion. It is believed that the main features of the established mechanism are also applicable in the case of the methane oxidation over other electron rich transition metal oxides.

Preparation of LaMnO3 for catalytic combustion of vinyl chloride

LaMnO3 was prepared by citrate sol-gel, coprecipitation, hard template, and hydrothermal methods, respectively, and its catalytic performance for the combustion of vinyl chloride was investigated. N2 adsorption-desorption, X-ray diffraction (XRD), Raman spectroscopy (Raman), O2 temperature programmed desorption (O2-TPD), H2 temperature programmed surface reaction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) were used to characterize the physicochemical properties of the LaMnO3 samples. The preparation methods had obvious effects on the distribution of oxygen and manganese species on the catalyst surface. The reaction followed the suprafacial mechanism; the activity corresponded with the high amount of Mn4+ and adsorbed oxygen species. LaMnO3 prepared by the citrate sol-gel method had the best performance for vinyl chloride combustion with T90 of 182 °C. The optimal activity was attributed to the improved redox capability of Mn4+/Mn3+. More available adsorbed oxygen and Mn4+ species on the surface were mainly responsible for the remarkable enhancement of the catalytic activity.

An in situ electrical conductivity study of LaCoFe perovskite-based catalysts in correlation with the total oxidation of methane

LaCo1−yO3 (y=0 and 0.2) and LaCo1−xFexO3 (x=0.6 and 1) perovskites were used as catalysts for the total oxidation of methane in the temperature range 300–800°C and characterized by in situ electrical conductivity measurements. Their electrical conductivity was studied as a function of temperature and oxygen partial pressure and temporal responses during sequential exposures to air, methane–air mixture (reaction mixture) and pure methane in conditions similar to those of catalysis were analyzed. The intrinsic activity of the perovskites at 550°C followed the order: LaFeO3<LaCo0.8O3<LaCo0.4Fe0.6O3<LaCoO3. LaCo1−yO3 perovskites were in a metallic conductivity state in the reaction temperature range while LaCo1−xFexO3 perovskites appeared to be p-type semiconductors both under air and under reaction mixture with positive holes as the main charge carriers. A transition from semiconducting state to the metallic state was observed for LaCo0.4Fe0.6O3 in the reaction temperature range. Their electrical conductivity at 550°C increased following the order: LaFeO3<LaCo0.8O3<LaCo0.4Fe0.6O3<LaCoO3. Correlations between their conducting behavior and redox properties, on one hand, and their catalytic behavior, on the other hand, have been evidenced. It was demonstrated that the reaction mechanism involves surface lattice O− species and can be assimilated to a Mars and van Krevelen mechanism for LaFeO3, while it involves only adsorbed oxygen species for LaCo1−yO3, being assimilated to a suprafacial mechanism. Both suprafacial and redox mechanisms take place on LaCo0.4Fe0.6O3 perovskite.

Structure–activity relation of spinel-type Co–Fe oxides for low-temperature CO oxidation

A series of cobalt ferrite thin films was prepared via pulsed spray evaporation chemical vapour deposition (PSE-CVD). The samples were comprehensively characterised in terms of structure, surface, morphology, and optical and redox properties. Both X-ray diffraction (XRD) and Raman analysis show that all samples exhibited an inverse spinel structure. X-ray photoelectron spectroscopy (XPS) results indicate that the films were mainly composed of Co, Fe and O species, and an increase in the Co : Fe ratio with Fe substitution by Co was observed. Helium ion microscopy (HIM) images show the film morphology to be dependent on the Co : Fe ratio. The investigation of the optical property using ultraviolet-visible spectroscopy reveals that the increase in the Co content results in an increase in the band gap energy. In situ emission FTIR spectroscopy was used to evaluate the redox properties of the samples, and a shift of the redox temperature to higher values was observed upon increase in the Co content. The effect of Fe substitution by Co in the mixed oxide systems on their catalytic performance for CO oxidation was investigated. Co–Fe oxides exhibit substantially better catalytic performance than the single α-Fe2O3. The catalytic performance of the Co–Fe oxides towards CO oxidation was discussed with respect to the participation of surface and lattice oxygen in the oxidation process. According to XPS and temperature-programmed reduction/oxidation (TPR/TPO) results, a suprafacial mechanism where CO molecules react with surface-adsorbed oxygen functions to form CO2 was proposed as the dominant mechanism.

Identification of the Active Sites for Low Temperature CO Oxidation over Nanocrystalline Co3O4 Catalysts

AbstractThe microstructural properties of dry‐grinding derived Co3O4 catalysts pretreated under different atmospheres, in relation to the activities on CO oxidation were investigated. The Co3O4 synthesized by soft reactive grinding and pretreated with O2 resulted in the best activity, with 100% conversion of CO at −52 °C, superior to that of Co3O4 pretreated with He. To find out the active sites on Co3O4 for low temperature CO oxidation, the characterizations of the cobalt oxides had been investigated by means of N2 physisorption, XRD, TEM, H2‐TPR, CO‐titration, XPS and O2‐TPD technologies. XPS of Co2p results show that it is difficult to ascribe the difference in catalytic performance to the surface concentration of active Co3+ sites. A correlation between the activity and the CO‐titration and O2‐TPD results for Co3O4 reveals that a high abundance of readily accessible superficial electrophilic oxygen (O−) species is important for achieving a high activity. Therefore, CO oxidation takes place on the surface active oxygen sites in Co3O4 crystallites via the suprafacial mechanism.

Methane oxidation behavior over La0.08Sr0.92Fe0.20Ti0.80O3−δ perovskite oxide for SOFC anode

Abstract A single phase perovskite-type La 0.08 Sr 0.92 Fe 0.20 Ti 0.80 O 3− δ catalyst was fabricated in air at 800 °C using the Pechini method. The methane conversion rate and hydrogen selectivity over the La 0.08 Sr 0.92 Fe 0.20 Ti 0.80 O 3− δ catalyst were evaluated over the temperature range, 300–900 °C. Complete and partial methane oxidation were predominant at the low and high temperature regions, respectively. The complete oxidation of methane was attributed to a suprafacial mechanism. On the other hand, an intrafacial mechanism might be responsible for the partial oxidation of methane at high temperatures. XPS showed that Fe ions were active sites for methane oxidation reactions over the La 0.08 Sr 0.92 Fe 0.20 Ti 0.80 O 3− δ catalyst. The methane conversion rate and hydrogen selectivity at 900 °C were 52% and 66%, respectively.

Low-temperature CO oxidation over nanosized Fe–Co mixed oxide catalysts: Effect of calcination temperature and operational conditions

In this paper, low temperature CO oxidation over iron–cobalt mixed oxide nanocatalysts with molar ratio of Co/Fe=1/1 was investigated. The nanocatalysts were prepared with a facile co-precipitation method. The prepared catalysts were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), temperature programmed desorption of oxygen (O2-TPD), N2 adsorption/desorption and transmission electron microscopy (TEM) techniques. The BET results revealed that the samples showed a mesoporous structure and the TEM analysis showed a nanostructure for all samples with crystallite sizes smaller than 30nm. The results revealed that inexpensive iron–cobalt mixed metal oxide nanoparticles have a high potential as catalyst in low temperature CO oxidation. The results also showed that increasing in calcination temperature increased the crystallite and particle size and decreased the specific surface area, which caused a decrease in catalytic activity of prepared catalysts. O2-TPD showed that the suprafacial mechanism is dominant in CO oxidation over Fe–Co catalyst. In addition, catalyst pretreated under oxidative atmosphere (O2-pretreat) showed the higher activity than those pretreated under reductive (H2-pretreat) and inert atmospheres (He-pretreat) and showed no obvious deactivation with repetition used in the reaction atmosphere.

Reactivity of LaNi1−y Co y O3−δ Perovskite Systems in the Deep Oxidation of Toluene

In the present work we have evaluated the oxidation of toluene over different lanthanum perovskites with a general composition of LaNi1−y Co y O3−δ. These catalysts, prepared by a spray pyrolysis method, have been characterised by XRD, BET and FE-SEM techniques. Additional experiments of temperature programmed desorption of O2, reduction in H2 and X-ray absorption spectroscopy were also performed in order to identify the main surface oxygen species and the reducibility of the different perovskites. The catalytic behaviour toward the oxidation of toluene (as a model for VOCs compounds) was evaluated in the range 100–600 °C, detecting a total conversion for all the samples below 400 °C and higher activities for the cobalt-containing perovskites. The catalytic behaviour of these samples is consistent with a suprafacial mechanism, with the α-type oxygen playing an active role in the oxidation reaction.

Influence of the Support Treatment on the Behavior of MnOx/Al2O3 Catalysts used in VOC Combustion

Alumina was treated with water and diluted nitric acid and then was used to prepare supported MnO x /Al2O3 catalysts with two different loadings. The influence of the support treatment on the catalytic behavior in ethanol and toluene combustion was studied. The treatments modified the alumina physicochemical properties (porosity, surface area, isoelectric point, and surface acidity). The modification of these properties affected the interaction of the manganese oxide species with the support and increased the dispersion of the active phase. Catalysts prepared from treated supports showed the best catalytic performance in ethanol combustion. At high manganese loading, this better catalytic performance was related to the high capacity for adsorbing oxygen. While at low manganese loading, the great amount of dispersed surface manganese oxide species and/or the existence of surface defects were relevant in the catalytic activity. On the other hand, the reactivity of the catalysts in toluene combustion was roughly correlated with the reducibility of the surface manganese oxide species. On the basis of these observations, we conclude that the ethanol combustion occurs by a suprafacial mechanism whereas the toluene combustion proceeds through an intrafacial mechanism.

Hydrogen peroxide decomposition over Ln 1− xA xMnO 3 (Ln = La or Nd and A = K or Sr) perovskites

The following perovskites La 1− x K x MnO 3 ( x=0–0.15), La 1− x Sr x MnO 3 ( x=0–0.3), Nd 1− x Sr x MnO 3 ( x=0–0.4) and Nd 1− x K x MnO 3 ( x=0–0.15) have been prepared by the freeze-drying method. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy spectra (XPS) analyses showed the presence of the single crystalline perovskite phases with an enrichment of the surface with potassium for the Ln 1− x K x MnO 3 samples. δ-Oxygen non-stoichiometries of the perovskites were studied by chemical analyses, temperature programmed reduction (TPR) and oxygen desorption (TPD) experiments. The results obtained suggest that the catalytic H 2O 2 decomposition over these perovskites is not determined by the surface K, Ln and Mn concentration nor by the Mn 4+/Mn 3+ surface ratio. On the other hand, a good correlation between the catalytic H 2O 2 decomposition and the δ-oxygen non-stoichiometries was found, especially for La perovskites. A suprafacial mechanism where the oxygen vacancies participate in the H 2O 2 decomposition is suggested.

La[sbnd]Ce[sbnd]Co perovskites as catalysts for exhaust gas depollution

A series of La1 − xCexCoO3 + δ perovskite-type catalysts, with x ranging from 0 to 0.20, showed to be quite active for reduction of NO by CO (reaction 1) and for oxidation of CO by air oxygen (reaction 2) at temperatures ranging from 373 to 723 K. Analysis by X-ray diffraction, electron probe microanalysis, temperature-programmed desorption-temperature-programmed reaction and electron paramagnetic resonance, coupled with catalytic activity data, showed that the active sites on these catalysts are very likely localised onto Co ions, which coordinate O2− ions as intermediates for reaction 2, the latter taking place essentially through a suprafacial mechanism. For reaction 1, taking place through both a suprafacial and an intrafacial mechanism, the Co-based sites play also an additional role, favouring the electron transfer from site to site and so enhancing the transfer of oxygen species from surface to bulk and vice versa. Ce ions seem to act only as a stabiliser of O2− ions, helping in keeping them at the catalyst surface.

An ab initio study on the mechanism of the alkene–isocyanate cycloaddition reaction to form β-lactams

Ab initio calculations predict that the [2 + 2] reaction between isocyanates and alkenes to form β-lactams is a cycloaddition which takes place through a concerted suprafacial mechanism.

Novel cycloaddition of tetrafluoroethylene to the tetrafluoroethylene radical anion at 95 K: direct observation by EPR studies

The novel cycloaddition reaction C 2F − 4 + C 2F 4 ⇁ c-C 4F − 8 has been observed directly by EPR spectroscopy in organic glasses at 80–95 K. According to the frontier orbital theory, the concerted reaction is symmetry allowed irrespective of whether the interaction is SOMOLUMO or SOMOHOMO, the SOMO of C 2F − 4 being the 5b 1u(σ*) orbital suggested by EPR studies. The assignment of C 2F − 4 as a σ radical also suggests that a concerted [2 + 2] suprafacial mechanism should be considered for the thermal cycloaddition reaction between two neutral C 2F 4 molecules.

Reaction of singlet oxygen with conformationally fixed cyclohexylidenecyclohexanes. Failure of an all suprafacial mechanism

Reaction of singlet oxygen with conformationally fixed cyclohexylidenecyclohexanes. Failure of an all suprafacial mechanism