The effect of nickel on propane oxidation and sulfur resistance of Pt/Ce0.4Zr0.6O2 catalyst

The effect of nickel on propane oxidation and sulfur resistance of Pt/Ce_0.4Zr_0.6O₂ catalyst has been studied. Samples were characterized by X-ray diffraction, BET area, H₂ temperature programmed reduction. It has been found that the introduction of nickel not only enhances the surface area of the catalyst, but also decreases its reduction temperature. The nickel promoted catalyst is more active in complete oxidation of C₃H₈. Furthermore, the addition of nickel to the catalyst is able to improve the desorption amount of sulfur species under reducing atmosphere, which could decrease the accumulation of sulfur species in the catalyst. Consequently, the sulfur resistance of Pt/Ce_0.4Zr_0.6O₂ catalyst has been improved.     

Surface and bulk changes of a Pt 1%/Ce0.6Zr0.4O2 catalyst during CO oxidation in the absence of O2

Topics in Catalysis - The reduction of a Pt 1%/Ce0.6Zr0.4O2 catalyst by CO in the absence of gaseous oxygen was studied by transient reactivity tests, temperature programmed surface reaction with...     

CuO-modified LaNi0.4Al0.6O3−δ with improvement performance in MSR at low temperature

The novel CuO/LaNi_0.4Al_0.6O_3−δ catalyst was synthesized by the impregnation method and characterized by X-ray diffraction (XRD), TEM, BET, and X-ray photoelectron spectroscopy (XPS) techniques. The effects of CuO loading amount on the microstructure and the hydrogen production performance from methanol steam reforming (MSR) were investigated. Results demonstrate that the sample with 15 wt% CuO loading has the best hydrogen production performance. Furthermore, compared with the unloaded LaNi_0.4Al_0.6O_3−δ perovskite catalysts, the methanol conversion of 15 wt% CuO/LaNi_0.4Al_0.6O_3−δ was improved, and the catalytic temperature of MSR was reduced. LaNi_0.4Al_0.6O_3−δ as support will form a strong interaction with Cu particles exposed on or around its surface and improve the catalytic performance. The XPS results show that the structure of LaNi_0.4Al_0.6O_3−δ perovskite oxide is not destroyed after reduction treatment, whereas CuO is reduced to elemental Cu, which is consistent with the XRD results. The reduced catalyst has a porous structure, which helps to increase the contact area between the reaction medium and the active components. This improves the efficiency of the reaction. In addition, the optimal catalytic temperature, water–methanol ratio, and liquid hourly space velocity were determined to be 350°C, 2.5:1, and 15 h⁻¹, respectively. Therefore, the CuO/LaNi_0.4Al_0.6O_3−δ developed in this study is a promising catalyst for MSR applications.     

Detailed Mechanism for Reduction of N2O over Rhodium by CO in Automotive Exhaust

Elementary-steps based mechanisms of CO–O₂ and CO–N₂O over rhodium catalyst were proposed and utilized to simulate experimental data from literature. The results showed that the mechanisms possess good prediction capability. It was found that the dissociation of adsorbed N₂O is the rate limiting step of N₂O reduction under conditions characterized by high CO coverages. The rather high light-off temperature (50 % conversion) of CO–N₂O (638 K) compared to that of CO–O₂ (453 K) is explained by the high temperature to initiate N₂O dissociation to offer surface oxygen needed for CO oxidation. Removing CO out of the reaction system, the oxygen generated via the dissociation of adsorbed N₂O accumulates on the surface of Rh, and finally leads to a poisoned catalyst and termination of the N₂O reduction process. However, increasing the inlet CO concentration inhibits the adsorption of N₂O to some extent, thus the reduction rate of N₂O is lowered on the contrary. Analysis of kinetic parameters showed that facilitating CO desorption or the decomposition of adsorbed N₂O leads to higher conversion of N₂O, with the latter having larger influence.     

Microstructure and microwave-frequency electromagnetic properties of Ni0.4Zn0.6Fe2O4/Ba0.6Sr0.4TiO3 composites

(x)Ni_0.4Zn_0.6Fe₂O₄+(1−x)Ba_0.6Sr_0.4TiO₃ composite ceramics with x=0.6, 0.7, 0.8, 0.9 and 1 were synthesized by solid state reaction method. The high dense composites have only two phases, i.e., Ni_0.4Zn_0.6Fe₂O₄ and Ba_0.6Sr_0.4TiO₃. The permittivity ε′ of the composites decreases slightly with the frequency increasing from 3 MHz to 1 GHz. The permittivity ε′′ of the composites also shows a little increase with frequency in the 3 MHz–1 GHz range. The permeability displays a relaxation resonance within the 3 MHz–1 GHz frequency range. The permeability μ′ increases while the cut-off frequency decreases with the Ni_0.4Zn_0.6Fe₂O₄ concentration, obeying the Snoek's law μ_if_r=constant. The permittivity ε′ of the composites decreases with Ni_0.4Zn_0.6Fe₂O₄ concentration. The composites have a relatively higher ε′ than the pure Ni_0.4Zn_0.6Fe₂O₄ at 1–10 GHz. In the frequency range of 1–10 GHz, the magnetic permeability μ′ reaches its maximum and μ′′ shows a minimum for the composite with x=0.6 in all ceramics. The permeability μ′ of the composites decreases with dc magnetic field at 1–10 GHz. The permeability shows a domain wall resonance, and the resonance frequency shifts to high frequency with the dc magnetic field. The permittivity was also influenced by the dc magnetic field due to a magnetodielectric effect.     

Hydrothermally Stable WO3/ZrO2–Ce0.6Zr0.4O2 Catalyst for the Selective Catalytic Reduction of NO with NH3

A new catalyst WO₃/ZrO₂–Ce_0.6Zr_0.4O₂ (15 wt % WO₃/ZrO₂:Ce_0.6Zr_0.4O₂ = 50:50) has been developed for the selective catalytic reduction of NO with NH₃. The redox component Ce_0.6Zr_0.4O₂ was dispersed on the surface of acidic WO₃/ZrO₂ by the solution combustion method showing the best NO _x reduction efficiency among the catalysts prepared by various modes of mixing of the components. The catalyst has been characterized by XRD, Raman spectroscopy and NH₃-TPD. A NO _x reduction efficiency of more than 90 % was obtained between 300 and 500 °C at α = NH_3,in/NO _x,in = 1. The catalyst showed stable NO _x reduction efficiency after hydrothermal ageing at 700 °C. Sulfur poisoning promoted the NO _x reduction efficiency at high temperatures at the expense of a reduced activity at lower temperatures, but the catalyst could be fully regenerated by heating in O₂ at 650 °C.     

Role of the surface state of Ni/Al2O3 in partial oxidation of CH4

The effect of gas phase O₂ and reversibly adsorbed oxygen on the decomposition of CH₄ and the surface state of a Ni/Al₂O₃ catalyst during partial oxidation of CH₄ were studied using the transient response technique at atmospheric pressure and 700°C. The results show that, when the catalyst surface is completely oxidized under experimental conditions, only a small amount of CO and H₂ can be produced from non‐selective oxidation of CH₄ by reversibly adsorbed oxygen which is more active in oxidizing CH₄ completely than NiO via the Rideal–Eley mechanism and both the conversions of CH₄ and O₂ and the selectivities to CO and H₂ are very low. Therefore, keeping the catalyst surface in the reduced state is the precondition of high conversion of CH₄ and high selectivities to CO and H₂. The surface state of the catalyst decides the reaction mechanism and plays a very important role in the conversions and selectivities of partial oxidation of CH₄. During partial oxidation of CH₄, no oxygen species but a small amount of carbon exists on the catalyst surface, which is favorable for maintaining the catalyst in the reduced state and the selectivity of CO. The results also indicate that direct oxidation is the main route for partial oxidation of CH₄, and the indirect oxidation mechanism is not able to gain dominance in the reaction under the experimental conditions. This revised version was published online in July 2006 with corrections to the Cover Date.     

CO adsorption and correlation between CO surface coverage and activity/selectivity of preferential CO oxidation over supported Ag catalyst: an in situ FTIR study

In situ IR measurements for CO adsorption and preferential CO oxidation in H₂-rich gases over Ag/SiO₂ catalysts are presented in this paper. CO adsorbed on the Ag/SiO₂ pretreated with oxygen shows a band centered around 2169 cm^–1, which is assigned to CO linearly bonded to Ag⁺ sites. The amount of adsorbed CO on the silver particles (manifested by an IR band at 2169 cm^–1) depends strongly on the CO partial pressure and the temperature. The steady-state coverage on the Ag surface is shown to be significantly below saturation, and the oxidation of CO with surface oxygen species is probably via a non-competitive Langmuir–Hinshelwood mechanism on the silver catalyst which occurs in the high-rate branch on a surface covered with CO below saturation. A low reactant concentration on the Ag surface indicates that the reaction order with respect to Pco is positive, and the selectivity towards CO₂ decreases with the decrease of Pco. On the other hand, the decrease of the selectivity with the reaction temperature also reflects the higher apparent activation energy for H₂ oxidation than that for CO oxidation.     

Propane gas sensing with high-density SrTi0.6Fe0.4O(3−δ) ceramics evaluated by thermogravimetric analysis

Semi-conducting SrTi_0.6Fe_0.4O_(3−δ) in the form of porous thick films might be of interest as a novel material for resistive hydrocarbon sensor applications. As the underlying gas-sensing mechanism is still a matter of question, thermogravimetric analysis was performed on high-density (≥99%) SrTi_0.6Fe_0.4O_(3−δ) ceramic specimens in order to investigate whether bulk properties are affected by the catalytic oxidation of propane at the material surface. At temperatures 250–400 °C, bulk oxygen content was found to significantly decrease upon exposure to traces of propane (0–3000 ppm) in a background of 20% O₂/80% Ar. Lattice oxygen is sufficiently mobile in this compound so that reducing surface reactions are followed by a bulk redox reaction even at low or moderate temperatures. Concomitant changes in the electrical resistance of dense ceramics are consistent with this picture. The hydrocarbon sensitivity of SrTi_0.6Fe_0.4O_(3−δ) artifacts seems thus to a large extent to be correlated to modifications in the bulk defect chemistry of the material.     

Ni0.4Co x Cd0.6 − x Fe2O4 ferrites as prepared by autocombustion synthesis

Ni_0.4Co _x Cd_0.6?x Fe₂O₄ ferrites (x = 0.0, 0.2, 0.4, and 0.6) were prepared by autocombustion synthesis and characterized by XRD, SEM, and other physical methods. XRD reveals the formation of spinel structure without any impurity phase. With increasing x, the lattice parameter was found to decrease from 8.60 to 8.37 Å. The FTIR spectra show the presence of tetrahedral site (at 571.12 cm^?1) and octahedral site (at 407.07 cm^?1). SEM images indicated the formation of agglomerated grains with a size of about 4.3 μm. The resistivity of the ferrites was found to decrease from 24.53 to 10.02 × 10⁸ Ω cm while the drift mobility to increase from 2.30 to 4.41 × 10⁸ cm² V^?1 s^?1 with increasing x.     

Decomposition and oxidation of CH3 13CH2OH on A12O3, Pd/Al2O3, and PdO/Al2O3 catalysts

Temperature-programmed desorption (TPD) and oxidation (TPO) were used to investigate the decomposition and oxidation of ethanol on Al₂O₃, Pd/Al₂O₃, and PdO/Al₂O₃. Ethyl--¹³C alcohol (CH₃ ¹³CH₂OH) was adsorbed on the catalysts so that reaction pathways of the two carbons could be distinguished. Alumina was mainly a dehydration catalyst, but dehydrogenation was also observed and some carbon remained on the surface. In the presence of O₂, A1₂O₃ oxidized the decomposition products and the-carbon was oxidized faster. Ethanol, which was adsorbed on A1₂O₃, decomposed much faster on Pd/A1₂O₃ by diffusing to Pd and undergoing CO elimination to form CH₄,¹³CO, H₂, and surface carbon. On PdO/A1₂O₃, the decomposition was slower than on Pd/A1₂O₃ until lattice oxygen was extracted above 450 K; the decomposition products were oxidized by lattice oxygen. In the presence of gas phase O₂, Pd/Al₂O₃ was an active oxidation catalyst at low temperature, but lattice oxygen had to be extracted from PdO/A1₂O₃ before it had significant oxidation activity.     

Poisoning of the adsorption of CO, O2 and D2 on Pt(111) by preadsorbed H2O

We have measured the influence of adsorbed H₂O on the sticking coefficients and saturation coverages of CO, O₂ and D₂ on Pt(111) at ∼100 K. Strong poisoning is observed for all three gases. For O₂ and D₂, the surface is essentially totally poisoned at 1 monolayer (ML) water coverage. For CO, the effect is weaker, with some CO adsorption still occurring at 2–3 ML H₂O. The influence of these results on the kinetics of the CO and H₂ oxidation reactions are discussed briefly. It is concluded that the influence of water must be included in kinetics simulations, at least at low temperatures, when significant humidity levels are present in inlet gas mixtures, or produced by the reactions themselves. This revised version was published online in July 2006 with corrections to the Cover Date.     

Mechanism of low-temperature CO oxidation on a model Pd/Fe2O3 catalyst

A model Pd/Fe₂O₃ catalyst prepared by the vacuum technique has been studied in the carbon monoxide oxidation in the temperature range of 300–550 K at reagent pressures P(CO)=16 Torr, P(O₂)= 4 Torr. It has been shown that the activity of the fresh catalysts is determined by palladium. According to the XPS data, the reduction with carbon monoxide results in the formation of Fe²⁺ (formally Fe₃O₄) and appearance of the catalytic activity in this reaction at low temperatures (350 K). High low-temperature activity of the catalyst is supposed to be connected with the reaction between oxygen adsorbed on the reduced sites of the support (Fe²⁺) and CO adsorbed on palladium (CO_ads) at the metal–oxide interface.     

A Fourier-transform infrared study of CO2 and CO2/H2 interactions with caesium-doped copper catalysts

FTIR spectra are reported of CO₂ and CO₂/H₂ on a silica-supported caesium-doped copper catalyst. Adsorption of CO₂ on a “caesium”/silica surface resulted in the formation of CO₂ ⁻ and complexed CO species. Exposure of CO₂ to a caesium-doped reduced copper catalyst produced not only these species but also two forms of adsorbed carboxylate giving bands at 1550, 1510, 1365 and 1345 cm⁻¹. Reaction of carboxylate species with hydrogen at 388 K gave formate species on copper and caesium oxide in addition to methoxy groups associated with caesium oxide. Methoxy species were not detected on undoped copper catalyst suggesting that caesium may be a promoter for the methanol synthesis reaction. Methanol decomposition on a caesium-doped copper catalyst produced a small number of formate species on copper and caesium oxide. Methoxy groups on caesium oxide decomposed to CO and H₂, and subsequent reaction between CO and adsorbed oxygen resulted in carboxylate formation. Methoxy species located at interfacial sites appeared to exhibit unusual adsorption properties.     

On the synthesis and stability of La0.6Sr0.4Ga0.3Fe0.7O3

La_0.6Sr_0.4Ga_0.3Fe_0.7O₃ is a MIEC perovskite with great chemical and thermal stability, considered a promising material for oxygen separation dense membranes and as an electrode in SOFCs. An easy, economic and scalable wet chemistry synthesis of La_0.6Sr_0.4Ga_0.3Fe_0.7O₃ (LSGF) was studied step by step, investigating and optimizing the most important aspects and parameters of the procedure (chemicals, pH, calcination temperature…). The obtained powders were carefully characterized with XRD, XPS, SEM/EDX and TPR. Once optimized the synthesis procedure, the stability in reducing condition and the reversibility of changes were tested submitting the samples to reduction/oxidation cycles at temperatures between 800/1000 °C. The influence of the synthesis parameters on stability/reversibility was investigated. The material is completely stable up to 800 °C even in aggressive reducing atmospheres. Reduction occurring at higher temperatures is reversible: a simple treatment in oxygen is enough to entirely re-absorb the side phases formed during the reduction and obtain the starting material.     

Effect of CeO2 Doping on Structure and Catalytic Performance of Co3O4 Catalyst for Low-Temperature CO Oxidation

The effect of CeO₂ doping on structure and catalytic performance of Co₃O₄ catalyst was studied for low-temperature CO oxidation. The Co₃O₄ catalyst was prepared by a precipitation method and the CeO₂/Co₃O₄ catalyst was prepared by an impregnation method. Their catalytic performance had been studied with a continuous flowing micro-reactor. The results reveal that the CeO₂/Co₃O₄ catalyst exhibits much better resistance to water vapor poisoning than the Co₃O₄ catalyst for CO oxidation. The CeO₂/Co₃O₄ catalyst can maintain CO complete conversion at least 8,400 min at 110 °C with 0.6% water vapor in the feed gas, while the Co₃O₄ catalyst can maintain at 100% for only 100 min. Characterizations with XRD, TEM and TPR suggest that the CeO₂/Co₃O₄ catalyst possesses higher dispersion degree, smaller particles and larger S_BET, due to the doping of Ceria, and exists the interaction between CeO₂ and Co₃O₄, which may contribute to the excellent water resistance for low-temperature CO oxidation. Furthermore, the H₂ detected in the reactor outlet gas seems to indicate that the water–gas shift reaction is the more direct reason.     

Bimetallic Au-Cu/CeO2 catalyst: Synthesis,structure, and catalytic properties in the CO preferential oxidation

The preparation of bimetallic Au-Cu catalysts via the decomposition of the double complex salt [Au(en)₂]₂[Cu(C₂O₄)₂]₃ · 8H₂O is considered. It is found that this method of preparation allows us to selectively obtain Au_0.4Cu_0.6 solid solution nanoparticles on the surface of a support. The composition of the particles corresponds to the stoichiometry of the double complex salt. The properties of bimetallic Au-Cu/CeO₂ catalyst and monometallic Au/CeO₂ and Cu/CeO₂ catalysts were studied during the preferential oxidation of CO in a mixture containing CO₂ and H₂O. The experiments were performed in a catalytic flow system within a temperature range of 50–250°C with a mixture of the following composition, vol %: CO, 1; O₂, 0.6; H₂O, 10; CO₂, 20; H₂, 60; and the balance, He. The weight hourly space velocity (WHSV) was 276000 cm³/(g h). The bimetallic catalyst made it possible to oxidize a considerably larger amount of CO with higher selectivity with CO₂ and H₂O in the mixture, relative to the monometallic catalysts. The preferential oxidation of carbon monoxide in the presence of hydrogen is a promising method for the deep purification of hydrogen-containing gas mixtures in order to remove carbon monoxide. The purified hydrogen-containing gas can be used to feed portable power units based on low-temperature proton-exchange membrane fuel cells, for the synthesis of ammonia, and for hydrogenation in fine organic synthesis.     

FT-IR study of Au/Fe2O3 catalysts for CO oxidation at low temperature

Coprecipitated Au/Fe₂O₃ catalysts used for low-temperature catalytic oxidation of carbon monoxide have been studied by FT-IR spectroscopy of adsorbed CO. The FT-IR results have shown that after preparation and exposure to a CO/O₂ mixture gold is present on the surface mainly as Au¹⁺ and Au⁰ species. It has been found that in the CO oxidation Au¹⁺ is more active and less stable than Au⁰. This revised version was published online in July 2006 with corrections to the Cover Date.     

La0.6Sr0.4Co0.8Ga0.2O3‐δ (LSCG) hollow fiber membrane reactor: Partial oxidation of methane at medium temperature

In this study, La_0.6Sr_0.4Co_0.8Ga_0.2O_3‐δ (LSCG) hollow fiber membrane reactor was integrated with Ni/LaAlO₃‐Al₂O₃ catalyst for the catalytic partial oxidation of methane (POM) reaction. The process was successfully carried out in the medium temperature range (600–800°C) for reaction of blank POM with bare membrane, catalytic POM reaction and swept with H₂:CO gas mixture. For the catalytic POM reaction, enhancement in selectivity to H₂ and CO is obtained between 650–750°C when O₂:CH₄ <1. High CH₄ conversion of 97% is achieved at 750°C with corresponding H₂ and CO selectivity of about 74 and 91%. The oxygen flux of the membranes also increased with the increase in oxygen partial pressure gradient across the membrane. The postreacted membranes were tested via XRD and FESEM‐EDX for their crystallinity and surface morphology. XPS analysis was further used to investigate the O1s, Co 2p and Sr 3d binding energies of the segregated elements from the reducing reaction environment. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3874–3885, 2013     

Low temperature CO oxidation over Au/TiO2 and Au/SiO2 catalysts

After a high-temperature reduction (HTR) at 773 K, TiO₂-supported Au became very active for CO oxidation at 313 K and was an order of magnitude more active than SiO₂-supported Au, whereas a low-temperature reduction (LTR) at 473 K produced a Au/TiO₂ catalyst with very low activity. A HTR step followed by calcination at 673 K and a LTR step gave the most active Au/TiO₂ catalyst of all, which was 100-fold more active at 313 K than a typical 2% Pd/Al₂O₃ catalyst and was stable above 400 K whereas a sharp decrease in activity occurred with the other Au/TiO₂ (HTR) sample. With a feed of 5% CO, 5% O₂ in He, almost 40% of the CO was converted at 313 K and essentially all the CO was oxidized at 413 K over the best Au/TiO₂ catalyst at a space velocity of 333 h^–1 based on CO + O₂. Half the chloride in the Au precursor was retained in the Au/TiO₂ (LTR) sample whereas only 16% was retained in the other three catalysts; this may be one reason for the low activity of the Au/TiO₂ (LTR) sample. The reaction order on O₂ was approximately 0.4 between 310 and 360 K, while that on CO varied from 0.2 to 0.6. The chemistry associated with this high activity is not yet known but is presently attributed to a synergistic interaction between gold and titania.