Presence Of Oxygen In Gas Phase Research Articles

Exploring the rate-control step of toluene oxidation over the novel octahedral Pt/Mn3O4 catalyst with stable low-temperature catalytic performance via in situ DRIFTS

The high efficiency and stability of catalysts are crucial for their practical application in toluene oxidation. Herein, a novel octahedral Pt/Mn3O4-110 catalyst with excellent structural stability was synthesized and 100 % toluene conversion can be achieved at low-temperature 160 °C. XRD and Raman results indicated that the structure of Pt/Mn3O4-110 can be well maintained after 120 h on-stream reaction, the resistance to H2O (3 or 5 vol%) and high concentration toluene (3000 ppm)/CO2 (5 vol%) tests. In situ DRIFTS comparative studies between Pt/Mn3O4-110 and Pt/Mn3O4-100 (30 % toluene conversion at 160 °C) samples demonstrated that the rate-control steps of toluene oxidation on Pt/Mn3O4-110 and Pt/Mn3O4-100 were both the further oxidation of benzoate species in the presence of gas-phase oxygen, while the transformation of benzaldehyde to benzoate species was the rate-control step on Pt/Mn3O4-100 in the absence of gas-phase oxygen. The weaker Mn-O bonds, richer oxygen vacancies and higher mobility of oxygen species on Pt/Mn3O4-110 sample than that of Pt/Mn3O4-100 are beneficial for the easier release of lattice oxygen from the surface of catalyst and then participated in toluene oxidation via Mars-van Krevelen mechanism, contributing to easier oxidation of benzaldehyde to benzoate species and formation of formic acid and bicarbonate species.

Electrocatalyticaly-Assisted Oxidative Dehydrogenation of Lower Alkanes to Olefins

Introduction Conversion of short-chain alkanes to olefins is a very important process for the chemical industry. The large availability of lower alkanes from refineries as well as their abundance in natural gas reserves and oil shale makes these reactions economically very relevant. A major challenge in the dehydrogenation of alkanes is that these reactions are highly endothermic. At lower temperatures, thermodynamic equilibrium limits the conversion, making use of high temperatures a necessity. When they are exposed to high temperatures in an oxygen-free environment, however, cracking/decomposition of alkanes becomes dominant, leading to coking, and hence deactivation of the catalyst, as well as feedstock loss through coke formation. Oxidative dehydrogenation (ODH) has been studied as an alternative to direct dehydrogenation. In this reaction, hydrogen abstraction is achieved oxidatively, hence removing the thermodynamic limitation. Also, ODH reactions are exothermic and do not require additional energy input to the reaction. In ODH, the major challenge is controlling the selectivity at high conversion levels. The presence of gas phase oxygen at high temperatures leads to complete oxidation products, mostly through further oxidation of the olefin product, which is often more reactive than the alkane. Electrocatalytic reactors using an oxide ion-conducting membrane offer a potential solution to control the selectivity to the desired olefins by regulating the availability of oxygen to the alkane. In this scheme, the hydrocarbons and gas-phase oxygen never come in contact, and the selectivity can be controlled by tuning the adsorption/desorption characteristics of the anode electro-catalyst and by controlling the oxide ion transfer rate. We have been working with Ti-based perovskites as anode electro-catalysts for this application. Lanthanum strontium titanates, LaxSr1-xTiO3-δ or LSTs, as well as chlorine-doped perovskites of the same form (LST-Cls) have been examined for potential anode catalysts in electrocatalytic ODH reactions. Materials and Methods LST catalysts, with varied La doping from 10 to 50 mol% at the A-site, were prepared via a modified Pechini route. Cl-doped LST catalysts were also synthesized using a similar method. These catalysts were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy, diffuse reflectance infrared Fourier Transform spectroscopy, electrical conductivity measurements, and other techniques such as surface area analysis and scanning electron microscopy. Electrocatalytically-asssisted ODH tests were performed in an electrochemical reactor consisting of an yttria-stabilized zirconia (8 mole% doped YSZ) membrane, an LSM (Lanthanum strontium manganite)-YSZ composite cathode and titanate-perovskite type catalyst on the anode. The cell was sealed using SCHOTT GM31107 glass sealant. A Keithley 6220 current source was used to apply current and the reactor effluent was monitored using a Shimadzu GC 2014. Oxygen flux tests were conducted in the 550-700°C temperature range. Results and Discussion XRD confirmed cubic perovskite structure of the LST and LST-Cl catalysts after calcination. At lanthanum-doping levels of 40% or higher at the A-site of the perovskite, other phases begin to precipitate out of the structure. Because of this, lanthanum-dopant of 0, 10, 20, and 30% were of most interest. Although these materials are similar to each other in behavior and surface features, characterization confirmed 20% doping as an optimal value in this particular application. Oxygen flux tests displayed a good match between the actual and predicted oxygen flux values (by Faraday’s Law), for all temperatures. This confirms the reactor’s ability to regulate oxygen amount as a function of applied current. Ethane ODH tests using the same reactor showed the most promising results for LST28-Cl (La0.2Sr0.8TiO3-δClσ). Although activity improvement is needed, these studies showed that the anode electrocatalyst characteristics can be fine tuned by dopant levels, and oxygen flux can be controlled by the current applied. Acknowledgement The financial support provided by the National Science Foundation through the Grant CHE-1213443 and the Ohio Development Services Agency is gratefully acknowledged.

Molecular level insights to the interaction of toluene with ZrO2-based biomass gasification gas clean-up catalysts

Gasification of biomass, followed by ZrO2-catalyzed hot gas clean-up at 600–900°C for the oxidation of impurities (such as tar), is an environmentally attractive way to produce heat and power or synthesis gas. The interaction of toluene (as a model compound for tar) with ZrO2-based gasification gas clean-up catalysts was studied by in situ DRIFTS and temperature-programmed gas-phase analysis. Toluene was found to interact in four ways with ZrO2 surfaces: forming molecularly adsorbed toluene, surface benzoate species, carbonaceous deposits and surface benzyl species. The adsorption of toluene in the absence of gas-phase oxygen at ambient temperature on ZrO2-based catalysts resulted in weakly adsorbed molecular toluene, while the adsorption of toluene at higher temperatures yielded carbonaceous deposits and surface benzoate species. Combined analysis of infrared and TP data showed that some of the benzoate species decomposed, producing benzene and carbon dioxide. Surface benzyl species, on the other hand, were detected on the surface of ZrO2-based catalysts only in the presence of gas-phase oxygen at a temperature where toluene and oxygen started to convert. Therefore, it is suggested that benzyl species are the active intermediates from toluene to carbon oxides during the gasification gas cleaning over ZrO2-based catalysts.

Resolving kinetics and dynamics of a catalytic reaction inside a fixed bed reactor by combined kinetic and spectroscopic profiling

The oxidative dehydrogenation of ethane to ethylene was studied using a MoO3 based catalyst supported on γ-alumina spheres. The measurement of species and temperature profiles through a fixed bed reactor shows for the first time the reaction pathways inside the catalyst bed directly. Oxidative dehydrogenation of ethane to ethylene and water occurs on the redox sites of MoO3 only in the presence of gas phase oxygen. Further oxidation of the product ethylene to carbon dioxide occurs as a subsequent reaction step by lattice oxygen of MoO3. Deep oxidation of ethylene to CO2 is the only existing reaction in the absence of gas phase oxygen reducing MoO3 to MoO2. Oxidation of CO and C2H6 by lattice oxygen does not occur. The reduction of the catalyst can be followed by in situ fiber Raman spectroscopy as a function of the oxygen partial pressure. The in situ Raman measurements are complemented by ex situ micro-Raman spectroscopy and X-ray diffraction. The combined measurement of kinetic and spectroscopic reactor profiles presents a novel approach in in situ catalysis research to establish catalyst structure–function relationships under technically relevant conditions of temperature and pressure.

Transient Redox Activity of Vanadyl Pyrophosphate at Ambient and Elevated Pressure

Abstract The transient catalytic activity of vanadyl pyrophosphate (VPP) catalyst was studied at ambient and elevated pressure (4.1 bar) and a wide range of operating conditions. The range included the commercial operating conditions typical of fixed bed, fluidized bed and circulating fluidized beds (CFB) for the partial oxidation of n-butane to maleic anhydride (MA). The maleic anhydride yield improved by increasing the feed oxygen molar fraction, temperature and pressure. When the catalyst was cycled between an oxidizing (synthetic air) and a reducing environment; yield increased with an increase in the catalyst residence time in the oxidizing environment. This effect was more pronounced at higher pressure. At ambient pressure, MA selectivity varied between 50-73 % while it decreased to about 48-54 % at a pressure of 4.1 bar. A strong MA selectivity dependency on feed composition was observed when the oxidation time was in the range of actual industrial reactors (< 1 minute). Selectivity data suggested that different oxygen species might be responsible for CO formation compared to other products such as CO2 and MA. Under oxidizing feed conditions (oxygen/n-butane ≥ than 3.7), an increase in n butane conversion was the main contributor to improved MA yield: n-butane conversion increased by about 70 % when the catalyst oxidation time extended from 0.3 to 10 minute. While, under fuel rich feed conditions, typical of industrial CFB operations, both MA selectivity and n-butane conversion contributed to enhancement in MA yield. Depending on the feed composition, MA selectivity increases by about 16-30 % and n butane conversion increases by about 32-55 % by extending the catalyst oxidation time. These results show the critical importance of catalyst oxidation time on reaction yield improvement especially when operating under fuel rich feed conditions. The surface adsorbed or surface lattice oxygen species were suggested to be the main responsible for n butane activation. While, the contribution of catalyst’s sub-surface lattice oxygen was believed to be very limited at fuel rich feed conditions. Under these conditions, catalyst over-reduction cannot be effectively compensated even after excessive catalyst regeneration and presence of gas phase oxygen is critical to maintain a high catalytic activity. As the reactor pressure increased to 4.1 bar, up to 60 % increase in n butane conversion accompanied by 100 % increase in oxygen conversion was observed. MA selectivity decreased by about 20 % on average but the increase in n-butane conversion resulted in an overall yield improvement of up to 30 %. Data show that the catalytic performance could be enhanced at certain combination of reactor pressure and temperature.

Activity Comparison of MCM-41 and V-MCM-4 Catalysts for Ethanol Selective Oxidation and DRIFTS Analysis

The activity of a V-MCM-41 catalyst with a V/Si molar ratio of 0.04 was compared with the activity of a pure MCM-41 catalyst and some DRIFTS studies were performed in order to understand the mechanism of selective oxidation of ethanol. V-MCM-41 was found to be much more active than pure MCM-41, favouring the formation of high amounts of ethylene over 300°C. MCM-41, on the other hand, was found to favour the formation of acetaldehyde over ethylene at all studied temperatures. It was shown by reaction findings that over V-MCM-41, ethylene was produced mainly in the presence of gas phase oxygen whereas acetaldehyde was produced using the lattice oxygen even in the absence of gas phase oxygen. DRIFTS studies indicated the formation of acetaldehyde molecules from acetate-like surface species which were formed by the removal of a hydrogen atom from the ?-carbon of chemisorbed ethoxy species by the help of the lattice oxygen. On the other hand, removal of a hydrogen atom from the ?-carbon of the ethoxy species, in the presence of gas phase oxygen, resulted in the formation of vinyl-like surface species which then yielded ethylene molecules.

Promoting effect of Co addition on the catalytic partial oxidation of methane at short contact time over a Rh/MgO catalyst

In catalytic partial oxidation of methane under isothermal conditions, a 0.3 wt% Rh/MgO catalyst modified with Co at a molar ratio of Co/Rh = 1 gave greater CH 4 conversion and selectivity to CO and H 2 than unmodified 0.3 wt% Rh/MgO. Characterization results using temperature-programmed reduction, extended X-ray absorption fine structure, and transmission electron microscopy demonstrated alloy formation between Rh and Co. In catalytic partial oxidation of methane without N 2 dilution, the Rh–Co/MgO catalyst with Co/Rh = 1 suppressed the temperature increase near the catalyst bed inlet and yielded a flat temperature profile. This behavior can be interpreted as greater selectivity in the direct partial oxidation route in the presence of gas-phase oxygen and lower activity in the steam reforming of methane in the absence of gas-phase oxygen. The better performance of Rh–Co/MgO (Co/Rh = 1) compared with Rh/MgO may result from greater methane dissociation and reduction degree during the partial oxidation of methane.

In situ FTIR study of the adsorption and reaction of ortho-dichlorobenzene on Pd–Co sulfated zirconia catalysts

The adsorption and oxidation of ortho-dichlorobenzene ( o-DCB) over sulfated zirconia (SZ), Co–SZ, and Pd/Co–SZ were studied by means of in situ FTIR spectroscopy to determine the nature of surface intermediates formed on these catalysts. The results indicate that o-DCB adsorbs on Lewis acid sites, forming surface intermediates that are strongly held on the surface. These intermediates can be subsequently oxidized to yield various partial oxidation products in either the presence or absence of gas-phase oxygen, confirming that surface oxygen is involved in the oxidation process. On Pd/Co–SZ samples, the oxidation is faster in the presence of gas-phase oxygen, yielding higher concentrations of partial oxidation products on the catalyst surface.

In situ FTIR study of the adsorption and reaction of ortho-dichlorobenzene over Pd-promoted Co-HMOR

The adsorption and oxidation of 1,2-dichlorobenzene ( o-DCB) with 8% O 2/He over Co-HMOR, Pd/Co-HMOR and H-MOR catalyst samples were studied by means of in situ FTIR spectroscopy. Strong o-DCB adsorption takes place on both Co-HMOR and Pd/Co-HMOR at 250 °C on Co 2+ Lewis acid sites, as indicated by the appearance of a characteristic IR band centered at 1663 cm −1. o-DCB can also adsorb on these materials through weaker interactions with hydroxyl (Brønsted) acid sites. o-DCB adsorption on such Brønsted acid sites is the only mode of interaction with the parent H-MOR material. Under reaction conditions the 1663 cm −1 band is no longer detectable in the spectra due to the low concentration of Co 2+ sites – which were converted to (Co-OH) + and CoO x due to interactions with the water generated by the reaction – and/or the reactivity of the corresponding adsorbed o-DCB species. In the case of Co-HMOR, the presence of gas phase oxygen is necessary for any reaction products to appear, while on Pd/Co-HMOR the onset of the reaction is observed even in the absence of gas phase oxygen, presumably due to the presence of PdO in the sample.

Methane catalytic partial oxidation on autothermal Rh and Pt foam catalysts: Oxidation and reforming zones, transport effects, and approach to thermodynamic equilibrium

We compare Rh and Pt as catalysts for the partial oxidation of methane to syngas at millisecond contact times. The basis for the comparison are species and temperature profiles, with a spatial resolution of about 300 μm measured along the centerline of an adiabatically operated metal-coated α-Al2O3 foam using a capillary sampling technique with mass spectrometric species measurement. Gas temperature profiles are measured with a thermocouple. Investigated stoichiometries range from C/O=0.6 to 2.6 at constant flow rate of 4.7 slpm and atmospheric pressure. Rh and Pt are compared with respect to (i) profile development at syngas stoichiometry, (ii) profile development at varying stoichiometries from C/O=0.6–2.6, (iii) product selectivities and yields in the oxidation zone, (iv) contribution of partial oxidation and steam reforming to the final syngas yield, (v) mass transport limitations, and (vi) approach to thermodynamic equilibrium. Independent of C/O and metal, all profiles show an oxidation zone and a steam-reforming zone. H2 and CO are formed in the presence of gas-phase oxygen by partial oxidation and in the absence of gas-phase oxygen by steam reforming. CO2 reforming is not observed. At the same C/O, H2 and CO selectivities and yields are higher in the oxidation zone on Rh than on Pt. As the C/O ratio increases, the catalyst temperature decreases and selectivities to H2 and CO in the oxidation zone decrease. The decrease is larger on Pt than on Rh. Because Rh is also the better steam-reforming catalyst, H2 and CO yields are generally higher on Rh than on Pt. The rate of O2 conversion at the catalyst entrance is largely mass transport-controlled on Rh but not on Pt. In the oxidation zone on Pt, the methane CPO is kinetically controlled with a constant reaction rate. An average O2 mass transport coefficient is calculated and compared with literature values on foam catalysts. Finally, exit species flow rates and temperatures are compared with thermodynamic calculations at constant pressure and enthalpy. Rh brings the methane oxidation close to equilibrium if C/O⩽1.0, whereas Pt reaches equilibrium only at very high catalyst temperatures if C/O⩽0.7. At higher C/O, deviations from equilibrium are observed mainly because steam-reforming slows, but also because water–gas shift equilibrium is not established.

Selective methanation of carbon oxides in a microchannel reactor—Primary screening and impact of gas additives

Selective removal of CO by methanation of carbon oxides has been performed over wash-coated supported metal catalysts in a microchannel reactor with simulated reformate feeding. The primary screening over a series of Ru and Ni-based catalysts shows that methanation activity is markedly dependent on the type of metal, the metal loadings and the promoter. Ni/CaO/Al 2O 3 catalyst exhibits the highest methanation activity with CO conversion higher than 93% and a relatively low conversion of CO 2 into methane among investigated catalysts at 300 °C under the operating condition. The appropriate temperature for selective methanation was identified to be 250 °C below which CO methanation preferentially and exclusively occurs. The effect of reaction temperature and gas additives such as steam and gaseous oxygen on the methanation performance is presented in terms of catalytic activity, selectivity, and methane yield. The regulation of carbon oxides methanation on Ni/CaO/Al 2O 3 catalyst by gaseous oxygen was realized by a switchover of the reaction pathway from combustion reaction in the presence of gas phase oxygen to the methanation upon the complete consumption of oxygen and the dynamic behaviour of catalyst surface induced by the interplay of the catalyst surface and the gas phase reactants.

Methyl Chloride Production from Methane over Lanthanum-Based Catalysts

The mechanism of selective production of methyl chloride by a reaction of methane, hydrogen chloride, and oxygen over lanthanum-based catalysts was studied. The results suggest that methane activation proceeds through oxidation-reduction reactions on the surface of catalysts with an irreducible metal-lanthanum, which is significantly different from known mechanisms for oxidative chlorination. Activity and spectroscopic measurements show that lanthanum oxychloride (LaOCl), lanthanum trichloride (LaCl3), and lanthanum phases with an intermediate extent of chlorination are all active for this reaction. The catalyst is stable with no noticeable deactivation after three weeks of testing. Kinetic measurements suggest that methane activation proceeds on the surface of the catalyst. Flow and pulse experiments indicate that the presence of hydrogen chloride is not required for activity, and its role appears to be limited to maintaining the extent of catalyst chlorination. In contrast, the presence of gas-phase oxygen is essential for catalytic activity. Density-functional theory calculations suggest that oxygen can activate surface chlorine species by adsorbing dissociatively and forming OCl surface species, which can serve as an active site for methane activation. The proposed mechanism, thus, involves changing of the formal oxidation state of surface chlorine from -1 to +1 without any changes in the oxidation state of the underlying metal.

Resolving catalytic phenomena with scanning tunnelling microscopy

Scanning tunnelling microscopy (STM) has proved to be an invaluable tool for investigating surface reactions and catalysis at the atomic scale on model catalysts. We show that good models of nanoparticulate catalyst surfaces can now be fabricated and imaged by the use of surface science methodology. In this paper the application of STM to some particular problems in catalysis is addressed. Specifically, these are: (i) Sintering. It is shown that sintering is a complex process, and, at least for separated nanoparticles anchored to a support, it occurs in a surface-mediated Ostwald ripening manner. That is, a reduction in particle number density and an increase in average particle size occurs by loss of atoms from the edge of nanoparticles, which then diffuse across the support to another particle. The net effect of such diffusion is that big particles get bigger at the expense of small ones, which eventually disappear. (ii) Spillover. By imaging individual nanoparticles, at elevated temperature, in the presence of gas phase oxygen, spillover of oxygen to the support is seen to occur directly. It happens because the dissociation probability of oxygen on Pd is much higher than that on titania, and spillover occurs by reaction with reduced titanium cations to grow new layers of titania around the metal nanoparticle, eventually totally encapsulating it. (iii) The so-called 'strong metal-support interaction' (SMSI). By the use of atomically-resolving STM this is shown to be due to the formation of an alloy-like mixed layer of Pd and Ti, which results in a surface of much reduced reactivity.

Mechanism of [formula omitted] and CO formation in the catalytic partial oxidation of [formula omitted] on Rh probed by steady-state spatial profiles and spatially resolved transients

Spatially resolved species and temperature profiles have been measured for the catalytic partial oxidation of CH 4 on autothermally operated Rh-coated α - Al 2 O 3 foams in both steady state and transient mode. A probe consisting of a thin quartz capillary and a thermocouple was moved with sub-mm resolution through the catalyst/heat shield stack to sample the gases into a mass spectrometer and measure temperature. The probe was also used to follow the relaxation of the system upon perturbation by stepping the stoichiometry of the reactants periodically. The spatial profiles in steady state show that H 2 and CO are formed in presence of gas phase oxygen (oxidation zone) and after total oxygen conversion by steam reforming (reforming zone). The stepwise change of the reactant stoichiometry in the transient experiments allowed decoupling chemistry (ms timescale) and temperature (s timescale). By exposing a catalyst, initially operating very hot at C / O = 0.6 , suddenly to a CH 4 rich feed ( C / O = 1.4 ) , it was possible to follow how the integral H 2 and CO production rates decrease with decreasing temperature of the catalyst. The reverse switch from C / O = 1.4 to 0.6 showed how the integral H 2 and CO production rates increased with increasing temperature of the catalyst. Differential reaction rates were obtained by performing these transients spatially resolved at two adjacent points in the catalyst. For C / O = 0.6 → 1.4 , H 2 and CO formation show a strict linear Arrhenius behavior over the entire temperature range from ∼ 1100 to ∼ 600 ∘ C . For C / O = 1.4 → 0.6 , the Arrhenius plots show two straight segments, a steep increase in the formation rate from ∼ 630 to ∼ 710 ∘ C and a weak increase from ∼ 725 to ∼ 1000 ∘ C . In both cases, H 2 formation is more activated than CO formation indicating two different rate determining steps.

The oxygen-assisted transformation of propane to CO x/H 2 through combined oxidation and WGS reactions catalyzed by vanadium oxide-based catalysts

This paper reports about the gas-phase oxidation of propane catalyzed by bulk vanadium oxide and by alumina- and silica-supported vanadium oxide. The reaction was studied with the aim of finding conditions at which the formation of H 2 and CO 2 is preferred over that of CO, H 2O and of products of alkane partial oxidation. It was found that with bulk V 2O 5 considerable amounts of H 2 are produced above 400 °C, the temperature at which the limiting reactant, oxygen, is totally consumed. The formation of H 2 derived from the combination of: (i) oxidation reactions, with generation of CO, CO 2, oxygenates (mainly acetic acid), propylene and H 2O, all occurring in the fraction of catalytic bed that operated in the presence of gas-phase oxygen, and (ii) WGS reaction, propane dehydrogenation and coke formation, that instead occurred in the fraction of bed operating under anaerobic conditions. This combination of different reactions in a single catalytic bed was possible because of the reduction of V 2O 5 to V 2O 3 at high temperature, in the absence of gas-phase oxygen. In fact, vanadium sesquioxide was found to be an effective catalyst for the WGS, while V 2O 5 was inactive in this reaction. The same combination of reactions was not possible when vanadium oxide was supported over high-surface area silica or alumina; this was attributed to the fact that in these catalysts vanadium was not reduced below the oxidation state V 4+, even under reaction conditions leading to total oxygen conversion. In consequence, these catalysts produced less H 2 than bulk vanadium oxide.

In Situ Investigations of Structureactivity correlations of Mixed Molybdenum Oxide Catalysts

Molybdenum oxide based catalysts promoted with other transition metals are active for the selective oxidation of propene in the presence of gas phase oxygen at temperatures above ~ 600 K. Crystalline (Mo,V,W)5O14 constitutes a three-dimensional model system for the more complex molybdenum based mixed oxide catalysts that are employed industrially for various partial oxidation reactions. In the mixed oxide system little is known about the cooperation of the different metal centers and their role for the stabilization of the active phase [1,2,3]. Here we report in situ X-ray absorption spectroscopy (XAS) and in situ X-ray diffraction (XRD) studies on the structural evolution of a precursor for bulk (MoVW)5O14 during calcination and under propene oxidation conditions

In situ FTIR studies of the mechanism of NO x storage and reduction on Pt/Ba/Al 2O 3 catalysts

The mechanism of NO x adsorption and reaction on a typical 1 wt.% Pt/20 wt.% BaO/Al 2O 3 NO x storage reduction (NSR) catalyst was studied by the use of in situ Fourier Transform InfraRed (FTIR) spectroscopy. Parallel studies were also conducted with the individual components of the NSR system (i.e., 1 wt.% Pt/Al 2O 3 and 20 wt.% BaO/Al 2O 3) in an effort to identify the role and importance of each component in the NSR process. Finally, the role of SO 2 was also investigated, since it has been reported that it represents a poison for the NSR process. The results indicate that at 350 °C NO x is stored on the NSR catalyst primarily in the form of barium nitrite and nitrate species. In the presence of Pt a conversion of the surface barium nitrite to bulk barium nitrate is taking place during the storage phase. This step is accelerated in the presence of gas phase oxygen and at elevated temperatures. Both barium nitrite and nitrate species are reduced in the presence of propylene, although at different rates. These reactions are catalyzed by Pt and do not proceed at any significant rate over the BaO/Al 2O 3 sample. Finally, the formation of surface and bulk barium sulfates was observed when SO 2 was introduced as a component of the NO-containing gas phase mixture. These species are stable on the catalyst surface under the reduction conditions studied and inhibit the NO x storage process.

On the role of monomeric vanadyl species in toluene oxidation over V2O5/TiO2 catalysts: a kinetic study using the TAP reactor

V2O5/TiO2 catalysts with V2O5 loadings of 1wt.% (Eurocat EL10V1) and 8wt.% (Eurocat EL10V8) as well as the corresponding NH3-washed samples (EL10V1w, EL10V8w) exhibiting exclusively monomeric vanadyl species were used as model catalysts in order to elucidate the role of different vanadia species in toluene oxidation. The catalysts were characterized by elemental analysis and X-ray photoelectron spectroscopy. The catalytic performance in toluene oxidation over EL10V1 and EL10V1w was investigated using a temporal analysis of products (TAP) reactor system under high-vacuum conditions. When oxidizing toluene over the untreated sample EL10V1 the products benzaldehyde (BA), benzoic acid (BAc), maleic acid anhydride (MA), and COx were detected. BA was observed at low temperatures followed by BAc at temperatures higher than about 500K, and finally COx above 600K indicating a consecutive oxidation mechanism. However, only the products BA and COx were observed for the EL10V1w catalyst. Previous DRIFTS and TPR/TPO studies had shown that the monomeric vanadyl species are able to adsorb toluene dissociatively, and that in the absence of gas-phase oxygen the desorption of oxygenated products from these species cannot occur, since the capability of fast oxygen insertion into toluene and desorption of oxygenates is associated with polymeric and crystalline vanadia species. Thus, a concerted mechanism for the selective oxidation of toluene in the presence of gas-phase oxygen over the ammonia-washed catalysts is suggested, i.e., the desorption of BA and the reoxidation of the monomeric vanadyl species are assumed to occur simultaneously.

Selective oxidation of ethane over a VO x/γ-Al 2O 3 catalyst – investigation of the reaction network

The oxidation of ethane was studied over a VO x /γ-Al 2O 3 catalyst (1.4% V) in a laboratory fixed bed reactor. Ethylene, CO and CO 2 were observed as the main products. To analyze the reaction network the oxidation of ethylene and CO was investigated in the same range of operation conditions. Based on the results obtained, a reaction network for ethane oxidative dehydrogenation can be proposed consisting of five partial reactions. Ethane reacts in two parallel reactions to ethylene (1) and directly to CO 2 (2). From the formed ethylene two further parallel pathways lead to CO (3) and CO 2 (4). Thus, ethylene over-oxidation is the source for CO observed during ethane oxidation. Finally, the consecutive oxidation of CO to CO 2 (5) is a part of the network. To understand the role of the catalyst, experiments were carried out comparing the performance of the VO x /γ-Al 2O 3 catalyst with that of the pure γ-alumina support and a FeO x /γ-Al 2O 3 catalyst. From these experiments it can be concluded that ethane conversion is correlated with the Broenstedt basic properties of the catalyst. Ethylene selectivity reveals an opposite trend. Ethylene formation from ethane consumes lattice oxygen, which can only be provided by redox sites of the catalyst. In contrast, deep oxidation reactions of both hydrocarbons (reactions 2–4) do not depend on the presence of redox sites, but only on the presence of gas phase oxygen. These deep oxidation reactions can be considered as surface reactions. Finally, CO oxidation (reaction 5) occurs primarily again under consumption of lattice oxygen, but under oxygen excess conditions also through surface reactions. In addition, two sources of carbon formation were identified, ethylene pyrolysis under oxygen absence and Boudouard reactions. For the network of the five main reactions, kinetic parameters were estimated. Reaction 1 was described by a Mars van Krevelen mechanism, the reactions 2–5 by Langmuir Hinshelwood equations. Applying this model a satisfactory fit of all experimental data was obtained.

Adsorption and catalytic reduction of NO with methanol over carbon and carbon-supported catalysts

The effect of various metal additives on the catalytic performance of carbon during the selective reduction of NO with methanol has been studied in the absence and in the presence of gas-phase oxygen. The mechanism of this reaction in the use of carbon-supported catalysts was studied by means of in situ FTIR combined with the measurements of catalytic activity. In the absence of oxygen, NO is adsorbed in a very small amount on the surface of carbon. The reaction of NO with the products of the methanol decomposition, results in the formation of adsorbed isocyanate (NCO) species at 2229cm−1. Formation of gaseous NH3 as a product of the reaction between NO and methanol was observed. Oxygen-containing surface functional groups of carbon effectively promotes the reduction of NO with methanol. IR spectroscopic results of this study show that in the presence of O2 nitric oxide is catalytically oxidized to nitrogen dioxide, which is chemisorbed on the surface of carbon. The surface nitrogen species were identified by FTIR spectroscopy.