Pulse Reaction Studies Research Articles

LaFe0.9Ni0.1O3 perovskite catalyst with enhanced activity and coke-resistance for dry reforming of ethane

A LaFe0.9Ni0.1O3 perovskite catalyst was evaluated for dry reforming of ethane (DRE), with two conventional oxide supported Ni catalysts (Ni/La2O3 and NiFe/La2O3) being used as references. LaFe0.9Ni0.1O3 showed the best activity and high coke-/sintering-resistance. TEM, TGA, and Raman characterizations confirmed that the deactivation of Ni/La2O3 was owing to the growth of Ni particles and the accumulation of coke, although the formation of La2O2CO3 was able to remove part of the coke during the reaction. The introduction of Fe-related species inhibited the coke formation while decreased the activity due to the loss of active sites. A portion of Ni ions in the perovskite lattice could be reduced to form highly dispersed and stable Ni nanoparticles on the surface during the reaction and oxygen vacancies were left in the perovskite lattice. Pulse reactor studies revealed that the oxygen vacancies in the perovskite could facilitate the activation and dissociation of CO2 to form CO and reactive oxygen species. Moreover, C2H6 was activated with the assistance of oxygen from the surface or subsurface of LaFe0.9Ni0.1O3 to form CO, rather than directly dissociated to surface carbon species as observed over Ni/La2O3.

Methane Oxidation on Pd@ZrO2/Si–Al2O3Is Enhanced by Surface Reduction of ZrO2

The catalytic properties of Pd@ZrO2 core–shell catalysts supported on Si-modified alumina were studied for application to methane oxidation and compared to the analogous Pd@CeO2 catalysts. In the absence of water (dry conditions), both Pd@ZrO2 and Pd@CeO2 were highly active and showed nearly identical reaction rates and thermal stabilities. However, unlike catalysts based on Pd@CeO2, the Pd@ZrO2 catalysts were also very stable in the presence of high concentrations of water vapor. By means of Coulometric titration and pulse-reactor studies, we demonstrate that ZrO2 in contact with Pd can be reduced. Additionally, Coulometric titration showed that the Pd-PdO equilibrium at 600 °C is shifted to much lower P(O2) in the Pd@ZrO2 catalyst compared to conventional Pd/ZrO2 or Pd/Al2O3 catalysts. Because PdO is more active for methane oxidation, this observation provides a possible explanation for the superior performance of the Pd@ZrO2 catalyst.

Structure and reactivity of copper iron pyrophosphate catalysts for selective oxidation of methane to formaldehyde and methanol

The structure of copper iron pyrophosphate catalysts prepared with various atomic ratios of metals and different pretreatment conditions was studied by using XRD, FT-IR, DR UV–vis and H 2-TPR techniques. The preparation methods modify the phase composition, oxidation state and reducibility of the patterns. The catalytic properties in the selective oxidation of methane to formaldehyde and methanol were examined in both, pulse and flow reactors, using O 2 and/or N 2O oxidizing agents. The oxidants used impact the onset of methane conversion, the primary oxidation products formation and the methane conversion. The highest single-pass yield of useful oxygenates (1.8%) was achieved with N 2O over the Cu/Fe ≅ 1:2 pyrophosphate catalyst which consists mainly of the crystalline Fe IIFe III 2(P 2O 7) 2 and copper containing nanodomains. Enhanced interaction and cooperation between these phases leading to a synergic effect of Cu and Fe has been emphasized. The Cu II and Fe III species of the above structure can be reduced more easily but at similar temperatures than those in the crystalline CuFe 2(P 2O 7) 2. Pulse reaction studies indicate that the lattice oxygen of the catalyst could react with methane molecules producing formaldehyde and methanol and replenishment of the lattice oxygen by N 2O takes place rather readily and rapidly. The nature of active species, methane conversion and oxygenates selectivity remained almost unchanged during the time on stream.

Manganese-promoted cobalt oxide as efficient and stable non-noble metal catalyst for preferential oxidation of CO in H2 stream

The modification by MnOx significantly enhanced the activity and stability of Co3O4 for the preferential oxidation of CO in H2-rich stream. The MnOx-promoted Co3O4 catalyst with a Mn/Co molar ratio of 1/8 exhibited the best performance; CO conversion was 56% at 40°C and increased to >90% as the temperature was raised to >75°C. This catalyst could provide 100% CO conversion at 125–175°C. CO conversions of 97% and 100% remained unchanged for 100h at 100 and 125°C, respectively, whereas the deactivation was observed over Co3O4 alone. Our characterizations using XRD, XPS and TEM indicated that the modification of Co3O4 with MnOx caused the formation of smaller MnxCo3−xO4 solid-solution particles. The O2-TPD, CO-TPD, and pulse reaction studies suggest that the incorporation of MnOx into Co3O4 increased the amounts of reactive oxygen species and adsorbed CO species over catalyst surfaces and enhanced the regeneration ability of the reduced catalyst by O2. All these enhancements are proposed to be responsible for the improved catalytic performance of the MnOx-promoted Co3O4.

Copper-Catalyzed Selective Oxidation of Methane by Oxygen: Studies on Catalytic Behavior and Functioning Mechanism of CuOx/SBA-15

While copper is the active center of particulate methane monooxygenase in methanotrophic bacteria, there are few studies to utilize synthetic copper catalysts for the selective oxidation of methane by oxygen. In this work, we have found that the copper ions attached on mesoporous silica SBA-15 with high dispersion can catalyze the selective oxidation of methane to formaldehyde by oxygen efficiently. The catalyst with a copper content of 0.008 wt % (Si/Cu = 13200) exhibits the best catalytic performance, and the specific site rate for formaldehyde formation can reach 5.6 mol (mol Cu)−1 s−1, significantly higher than those reported to date for other catalysts. We have elucidated that the oxidation of methane produces formaldehyde as a major primary product together with a small amount of carbon dioxide, while carbon monoxide is formed mainly via the consecutive oxidation of formaldehyde over our copper-based catalyst. Pulse reaction studies have indicated that methane molecules can react with the lattice oxygen of the catalyst, producing carbon oxides, and CuII in the catalyst is reduced at the same time. Detailed pulse reaction studies combined with EPR characterizations suggest that the reduced copper (probably CuI) sites generated by methane molecules during the reaction account for the activation of molecular oxygen, forming active oxygen species for the selective oxidation of methane to formaldehyde.

In situ IR and pulse reaction studies on the active oxygen species over SrF 2/Nd 2O 3 catalyst for oxidative coupling of methane

Pulse reaction method and in situ IR spectroscopy were used to characterize the active oxygen species for oxidative coupling of methane (OCM) over SrF 2/Nd 2O 3 catalyst. It was found that OCM activity of the catalyst was very low in the absence of gas phase oxygen, which indicated that lattice oxygen species contributed little to the yield of C 2 hydrocarbons. IR band of superoxide species (O 2 −) was detected on the O 2-preadsorbed SrF 2/Nd 2O 3. The substitution of 18O 2 isotope for 16O 2 caused the IR band of O 2 − at 1128 cm −1 to shift to lower wavenumbers (1094 and 1062 cm −1), consistent with the assignment of the spectra to the O 2 − species. A good correlation between the rate of disappearance of surface O 2 − and the rate of formation of gas phase C 2H 4 was observed upon interaction of CH 4 with O 2-preadsorbed catalyst at 700 °C. The O 2 − species was also observed on the catalyst under working condition. These results suggest that O 2 − species is the active oxygen species for OCM reaction on SrF 2/Nd 2O 3 catalyst.

Influence of PdO content and pathway of its formation on methane combustion activity

The methane oxidation reaction is known to induce changes in the surface structure and composition of Pd catalysts; making it extremely arduous to relate the methane oxidation activity to specific catalyst properties by conventional methods (continuous flow reactor studies). To circumvent this, methodical pulse reactor studies have been undertaken to obtain correlations between the initial methane combustion activity and the catalyst properties (Pd 0/PdO content and path of PdO formation). While the initial methane combustion activity (at 160–280 °C) continuously increased with increasing PdO concentration (0–100%) in the catalyst, it continuously decreased with increasing Pd 0 content (0–100%). Controlled studies were undertaken to obtain catalysts with identical PdO content by two pathways: (i) by controlled partial oxidization of Pd 0/Al 2O 3 and (ii) by controlled partial reduction of PdO/Al 2O 3. Interestingly, for a given PdO content, the catalysts obtained by partial oxidation of Pd 0/Al 2O 3 showed a significantly superior performance to the catalyst obtained by partial reduction of PdO/Al 2O 3 for all the temperatures investigated. These studies unambiguously show that along with the relative concentration of PdO, the PdO formation pathway is also critical in deciding the methane combustion activity of the catalyst.

Characterization of SO2-poisoned ceria-zirconia mixed oxides

CeO2, ZrO2, and a series of CexZr1−xO2 catalysts with 1wt.% Pd were exposed to fixed exposures of SO2 under oxidizing environments and then characterized by FTIR, pulse–reactor studies with CO and O2, and temperature-programmed desorption (TPD). For exposures above 473K, sulfates were formed on all of the materials; however, the results are consistent with the formation of bulk sulfates on CeO2 and only surface sulfates on ZrO2. For the mixed oxides, the quantity of sulfates formed at 673 K increased linearly with the Ce content. In TPD, the sulfates on ZrO2 were stable to higher temperatures than those formed on CeO2, which decomposed in a well-defined peak between 900 and 1050K. The sulfates on both oxides were reduced by CO above 750K. Even though XRD patterns for the mixed oxide were significantly different from that of the physical mixture, the TPD and pulse–reactor results were similar to what would be expected for physical mixtures of CeO2 and ZrO2, suggesting that sulfate species are associated with individual metal cations. Finally, pulse–reactor studies with CO and O2 at 873K show that the sulfates can be reversibly reduced and oxidized on both CeO2 and ZrO2, so that sulfur poisoning gives rise to an apparent increase in oxygen storage, demonstrating that this method is not acceptable for measurement of this quantity.

A Mechanistic Study of Sulfur Poisoning of the Water-Gas-Shift Reaction over Pd/Ceria

The effect of sulfur on the water-gas-shift (WGS) activity of Pd/ceria catalysts has been studied using steady-state rate measurements, pulse-reactor studies, and FTIR. After exposing Pd/ceria to SO2 at 673 K in an oxidizing environment, the WGS rates dropped to a value close to that observed on Pd/alumina. Both pulse-reactor and FTIR measurements showed that cerium sulfates can be readily reduced by CO and re-oxidized by O2 at 723 K; however, unlike reduced ceria, the Ce2O2S formed by reduction of the sulfates cannot be re-oxidized by H2O or CO2. The implications of these measurements for understanding the oxygen-storage capacity (OSC) of three-way catalysts are discussed.

Activation of methane to syngas over a Ni/TiO 2 catalyst

Partial oxidation of methane (POM) to syngas and CH 4/CO 2 reforming have been investigated over a Ni/TiO 2 catalyst in a fixed-bed reactor. The Ni/TiO 2 catalyst has high initial activity but undergoes significant deactivation during the partial oxidation of methane reaction. Deactivation is due largely to the oxidation of Ni(0) to NiTiO 3. After the partial oxidation of methane at 700 °C the catalyst was pale yellow. XRD confirmed that Ni(0) had been converted to NiTiO 3 and oxygen pulse reactions found only traces of carbon were present after the POM reaction. The Ni/TiO 2 catalyst has a high activity and a long term stability in the CO 2 reforming reaction. XRD found Ni(0) was present after the reforming reaction but NiO and NiTiO 3 were absent. Activation of methane over Ni/TiO 2 was also investigated using pulse reaction techniques in the absence of gas phase oxygen. Methane pulse reactions demonstrated that the mechanism of methane oxidation changes as the oxidation state of nickel changes. CH 4 may have been oxidized by oxygen from solid NiO or by active oxygen within the TiO 2 support via the non-selective Rideal–Eley mechanism over the oxidized Ni/TiO 2 catalyst surface. In contrast, CH 4 is efficiently converted to CO and H 2 via a direct oxidation mechanism when Ni/TiO 2 is reduced. Pulse reaction studies provide evidence that the oxidation state of nickel controls the methane activation mechanism and the product distribution.

Evidence for Oxidation of Ceria by CO2

The oxidation and reduction of Pd/ceria and Pd/zirconia catalysts were investigated using pulse-reactor studies in which products were monitored following step changes in the reactant composition. The results demonstrate that both ceria and zirconia can be partially reduced by CO and partially reoxidized by CO2. Steady-state rates for methane reforming by CO2 were also measured on Pd/ceria, Pd/zirconia, and Pd/silica. The Pd/ceria catalyst was slightly more active than Pd/zirconia and ∼104 times as active as Pd/silica. The high activities of Pd/ceria and Pd/zirconia are explained as resulting from oxidation and reduction of the supports.

Active and Spectator Adsorbates during NO Decomposition over Cu–ZSM-5: Transient IR, Site-Poisoning, and Site-Promotion Studies

A site-poisoning and -promotion technique was employed to investigate the role of adsorbates and to formulate a reaction pathway for the NO decomposition reaction over Cu–ZSM-5. Transient infrared and mass spectrometer studies of pulse NO reaction on under- and over-exchanged Cu–ZSM-5 reveal Cu2+(NO), Cu+(NO), bridging Cu2+(NO−3), and NO+ as the major adsorbates and N2, N2O, O2, and NO2 as the products. SiH4 and H2O moderately inhibited Cu+(NO) and N2 formation, but severely inhibited Cu2+(NO−3) and O2 formation. Addition of CO as a reducing agent led to the promotion of Cu2+ reduction to Cu+, depletion of Cu2+(NO−3), increase in NO conversion and O2 formation as well as formation of CO2. These results revealed that N2 and O2 formation proceeds through separate rate-limiting steps; O2 formation occurs via both autoreduction of Cu2+ to Cu+ and decomposition of Cu2+(NO−3). The results from pulse reaction studies on deactivated Cu–ZSM-5 reveal that O2 formation is more sensitive to poisoning than NO dissociation and N2 formation. Although silanation may improve the hydrothermal stability of the catalyst, silanation severely inhibited O2 and Cu2+(NO−3) formation through inhibition of adsorbed O migration. Promotion of O2 desorption by addition of a small amount of reducing agent such as CO in the presence of NO (NO/CO>3.5) can greatly enhance NO decomposition activity. Further investigation is needed to study such an effect on the NO decomposition activity under the O2 environment.

Dynamic Behavior of Adsorbed NO and CO under Transient Conditions on Pd/Al2O3

The dynamic behavior of adsorbed NO and CO under transient NO–CO reaction conditions on Pd/Al2O3has been studied byin situinfrared (IR) spectroscopy coupled with TPR and pulse reaction techniques in the 303–673 K range. Below the light-off temperature (i.e., 561 K), Pd0–NO and Pd0–CO are the dominant adsorbates on the Pd surface. Pd0–NO competes favorably over Pd0–CO for the same reduced Pd0site when the temperature is increased. Pulse reaction studies at 473 K suggest that Pd0–NO dissociates to form adsorbed nitrogen and adsorbed oxygen. Adsorbed oxygen further reacts with Pd0–CO to produce CO2. Concentration profiles of CO2and Pd0–CO during the pulse reaction studies indicate that removal of adsorbed oxygen from the Pd surface to produce CO2is the rate-limiting step. Prolonged exposure of the catalyst to the NO flow at 473 K results in oxidation of Pd0to Pd+and produces Pd–NO+; the presence of gaseous CO reduces Pd+to Pd0and increases the surface coverage of Pd0–NO. Above the light-off temperature, Pd–NO+, Al–NCO, nitrate, and carbonate species are the dominant adsorbates. The presence of Pd–NO+indicates that the process for Pd0oxidation to Pd+by NO is faster than that of Pd+reduction to Pd0by CO. This study demonstrates that careful selection of transient IR techniques allows (i) determination of the modes of adsorbed NO and CO participating in the reaction and (ii) development of a comprehensive mechanism for the NO–CO reaction on Pd/Al2O3catalyst.

Characterization and Catalytic Activity of CuCl2-Al2O3Ethylene Oxychlorination Catalysts

CuCl2/Al2O3ethylene oxychlorination catalysts have been characterized by using IR spectroscopy of the surface hydroxy-groups and of adsorbed pyridine and CO2. The ethylene oxychlorination reaction to 1,2-dichloroethane has been investigated by using pulse reactor studies andin-situIR experiments in static conditions. The results allowed emphasizing the negative role of uncovered alumina surface, evidenced by IR experiments, considered to be responsible for the dehydrochlorination of 1,2-dichloroethane with a consequent loss in selectivity that is very important in low Cu-loading samples.

Mechanistic Studies of the NO–CO Reaction onRh/Al2O3under Net-Oxidizing Conditions

The dynamic behavior of NO and CO adsorbates over a 0.2 wt% Rh/Al2O3catalyst was studied under net-oxidizing conditions byin situinfrared spectroscopy combined with pulse transient techniques. The observed sequence of adsorbate and CO2concentration profiles during the pulse reaction studies reveals that Rh+(CO)2is an active adsorbate which reacts with adsorbed oxygen to form CO2. The observed transient behaviors of Rh+(CO)2, Rh–NO−, and CO2during the pulse reactions can be explained by the proposed NO–CO reaction mechanism in which (i) Rh–NO−dissociates to form adsorbed nitrogen and oxygen and (ii) adsorbed oxygen further reacts with Rh+(CO)2to produce CO2. Addition of air to the NO–CO reactant stream produces a net-oxidizing environment for the NO–CO reaction. Adsorbed oxygen from air reacts with Rh–NO+producing the nitrato species and providing more Rh sites for the NO–CO reaction at 473 K. At 573 K and above, adsorbed oxygen reacts with Rh+(CO)2producing CO2, blocks the reduced Rh0sites for Rh–NO−, oxidizes Rh0/Rh+to Rh2+sites, and inhibits the NO conversion and N2O formation. Rh sites are readily reduced and return to their initial state and the catalyst returns to its initial activity when air is withdrawn from the reactant stream. Preservation of the reduced Rh sites for Rh–NO−is required to maintain the catalyst activity for the NO–CO reaction under net-oxidizing conditions.

Pulse Reaction Studies on Variations of Initial Activity/Selectivity of O2and H2Pretreated Ga-Modified ZSM-5 Type Zeolite Catalysts in Propane Aromatization

Influence of O2and H2pretreatments (at 600°C for 10 h) on the initial activity and product selectivity and their variation with time on stream (0–180 min) of H–gallosilicate (H–GaMFI), H–galloaluminosilicate (H–GaAlMFI), and Ga/H–ZSM-5 catalysts (with bulk Si/(Al + Ga) = 34.5 ± 2.5) in the propane aromatization (at 600°C) has been investigated, using a microreactor operated in both the pulse and continuous reaction modes. The catalysts have been characterized for their acidity (measured in terms of pyridine chemisorbed at 400°C), framework and nonframework Ga and Al, and surface Si/Ga ratio. The initial propane conversion/aromatization activity and aromatics selectivity of the H–GaMFI are decreased but that of the H–GaAlMFI and Ga/H–ZSM-5 are increased significantly due to the H2pretreatment for all the Ga-modified zeolite catalysts. The variation of the activity and product selectivity of all the three catalysts with the time on stream is found to be strongly dependent upon the pretreatment. Among the catalysts, H–GaAlMFI shows superior performance in the propane aromatization.

Pulse Reaction Studies of Transient Nature of Adsorbates during NO-CO Reaction over Rh/SiO

Pulse Reaction Studies of Transient Nature of Adsorbates during NO-CO Reaction over Rh/SiO

Catalytic Oxidation of Methane to Methanol with H2-O2 Gas Mixture at Atmospheric Pressure

Oxidation of methane with oxygen in the presence of hydrogen has been performed at atmospheric pressure over a large variety of catalysts. Among the catalysts tested, iron phosphate showed a very unique catalytic property. The presence of hydrogen initiates the selective oxidation of methane to methanol at ≥623 K over FePO4 catalyst. Formaldehyde as a secondary product is produced through methanol at higher reaction temperatures, while, in the absence of hydrogen, no reaction occurs at ≤673 K on the same FePO4 catalyst. The conversion of methane is notably low at >673 K in the absence of hydrogen and only formaldehyde and carbon oxides are formed. The comparison of the kinetic results in the presence and absence of hydrogen has suggested the generation of an active oxygen species on the catalyst surface in the presence of hydrogen. The pulse reaction studies show that methanol is formed only when methane, hydrogen, and oxygen are cofed. The formation of methanol is enhanced by pretreatment of the catalyst surface with hydrogen. The lattice oxygen of the surface is suggested to be responsible for the oxidation of methane to formaldehyde in the absence of hydrogen at relatively high temperatures (>673 K), while gaseous oxygen is indispensable for the selective oxidation of methane to methanol in the presence of hydrogen. The comparison of the results for the methane oxidation using H2O2 and H2-O2 gas mixture suggests that the active species generated from H2-O2 gas mixture is the same as that from H2O2. X-ray photoelectron spectroscopy studies have indicated that Fe(II) exists on the surface after treatment in a stream of CH4-O2-H2 suggesting the interconversion of Fe(III) ⇄ Fe(II) during the reaction. The reductive activation of dioxygen by hydrogen on an iron site must generate the active and selective oxygen species for the conversion of methane to methanol.

Pulse reactor studies of catalysts for the oxidative coupling of methane

The behaviour of the catalysts MgO, Li/MgO, Li/ZnO and Mn/Na 4P 2O 7/SiO 2 are compared using a pulse reactor:. The catalysts are exposed to pulses of different gases, such as air, methane and a methane:air mixture (1:1). There is more interaction from lattice oxygen with the Mn/Na 4P 2O 7/SiO 2 and Li/MgO catalysts than with the pure MgO or Li/ZnO.

Kinetics of Dehydrogenation of 2-Propanol on La2−xSrxCuO4−y

Abstract The catalytic properties of mixed oxides La2−xSrxCuO4−y (0.0≤x≤1.0 and y=non-stoichiometry) having K2NiF4 structure were investigated with regard to the effect of changes in the lattice parameters, the oxidation state of copper and oxygen nonstoichiometry in order to correlate the catalytic activity with physicochemical properties. The decomposition of 2-propanol was studied by pulse and flow reactor techniques. The flow reactor data was analyzed by statistical methods to deduce the mechanism on the basis of Langmuir–Hinshelwood and Hougen–Watson models. The reaction followed the first order kinetics with dual site surface reaction as the rate controlling step. The kinetic and thermodynamic parameters obtained from flow reactor data are comparable to those obtained from pulse reactor studies. The higher catalytic activity as indicated by low activation energy and higher conductivity value of La1.8Sr0.2CuO3.96 among the series have been attributed to the presence of higher concentration of Cu(III).