Steady‐state N2O decomposition on Pt and Rh surfaces using a free‐jet molecular‐beam excited by nozzle heating

Steady‐state N₂O decomposition reaction on polycrystalline Pt and Rh surfaces has been studied using a supersonic free‐jet molecular beam (2.1 × 10¹⁸ molecules/cm² s). The energy of the incident N₂O beam was controlled by a nozzle heating technique in conjunction with a seeding technique. The decomposition rate shows both translational and vibrational energy dependence on the Pt surface. However, there is also the surface temperature dependence of the decomposition rate even varying the incident beam energy, indicating precursor‐mediated dissociation of N₂O on the Pt surface. On the other hand, no energy dependence was observed on the Rh surface, suggesting that the decomposition dynamics are different between Pt and Rh surfaces. This revised version was published online in August 2006 with corrections to the Cover Date.     

Rh–Sr/Al2O3 Catalyst for N2O Decomposition in the Presence of O2

The improving effect of Sr in the catalytic activity of Rh for N₂O decomposition has been studied under 1,000 ppm N₂O/He and 1,000 ppm N₂O/5% O₂/He (GHSV = 10,000 h^?1). Different techniques have been used for catalysts characterization: TEM, SEM-EDX, XRD, N₂ adsorption at ?196 °C and in situ XPS. Sr favours the Rh dispersion and reduction under reaction conditions, and allows the low temperature removal of N₂O in the presence of O₂ (100% decomposition at 350 °C).     

Electrodeposition of Co–Pd alloys from ammonia solutions and their catalytic activity for hydrogen evolution reaction

Investigations of the influence of electrolysis parameters such as the concentration of metal ammonia complexes, working electrode potential and temperature on the composition, structure and catalytic activity of synthesized alloys for water molecule reduction reaction in 2 M NaOH (T = 25 °C) were conducted. The alloys were deposited under potentiostatic conditions within potential range from ?0.7 to ?1.1 V in electrolytes of pH 9.5, containing ammonia complexes of cobalt(III) and palladium(II), [Co(NH₃)₆]³⁺ and [Pd(NH₃)₄]²⁺, of different concentration ratio. Structural changes in electrodeposited alloys were discussed based on results of X-ray diffraction measurements. An elemental analysis was performed using the energy-dispersive X-ray spectroscopy technique. Finally, based on results of galvanostatic measurements, the Tafel slope within the range of activation control for hydrogen evolution reaction was determined and mechanism of the process was discussed. The alloys presented low Tafel slope value, from 25.4 to 88.7 mV dec^?1. The alloy of the highest activity for hydrogen evolution reaction contained 31.2 at.% of Pd.     

Thermal stability of Pt particles of Pt/Al2O3 catalysts prepared using microemulsion and catalytic activity in NO–CO reaction

Sintering behaviors of the Pt particles of Pt/Al₂O₃ catalyst prepared using different preparation methods (microemulsion, sol–gel, and impregnation methods) were investigated. It was found that the catalyst prepared by microemulsion had a higher resistance to sintering than did the sol–gel and impregnation catalysts. To limit the sintering even more, the catalysts were pressed. The resistance to sintering in all the catalysts was improved by pressing. The pressed microemulsion catalyst was little deactivated in the NO–CO reaction by thermal treatment at 700 °C for 12 h, and had a high activity relative to that of the sol–gel and impregnation catalysts.     

Simultaneous removal of N2O and CH4 as the strong greenhouse‐effect gases over Fe‐BEA zeolite in the presence of excess O2

Simultaneous catalytic removal of N₂O and CH₄ as the strong greenhouse‐effect gases was found to be possible over an Fe‐ion‐exchanged BEA zeolite (Fe‐BEA) by the selective catalytic reduction (SCR) of N₂O with CH₄. The direct decomposition of N₂O (2N₂O → 2N₂ + O₂) and the oxidation of CH₄ (CH₄ + 2O₂ → CO₂ + 2H₂O) over Fe‐BEA zeolite required high temperature above 400 and 450 °C, respectively. Nevertheless, the catalytic reduction of N₂O by adding CH₄ over Fe‐BEA zeolite readily occurred at much lower temperatures (ca. 250–350 °C) whether in the presence of O₂ or not. No oxidation of CH₄ by O₂ took place at these temperatures. On the basis of these results and the kinetic studies, it was concluded that CH₄ reacted selectively with N₂O to produce N₂, CO₂ and H₂O over Fe‐BEA zeolite even in the presence of excess O₂. Overall stoichiometry of the SCR of N₂O with CH₄ was determined as follows: 4N₂O + CH₄ → 4N₂ + CO₂ + 2H₂O. This revised version was published online in August 2006 with corrections to the Cover Date.     

Influence of H2, CO and CO2 co-feeding on the catalytic activity of Rh/Ti–SiO2 during the partial oxidation of methane

The influence of the addition of 1, 2 or 5 vol.% of CO, H₂ or CO₂ to the feed during the partial oxidation of methane (POM) was studied over a Rh/Ti–SiO₂ catalyst. The addition of H₂ or CO decreases the conversion and syngas selectivity. This decrease of performance seems to be related to a higher reduction of the catalyst due to the co-feeding of H₂ or CO. The addition of CO₂ also appears unfavourable to the production of hydrogen but increases the CO yield. A combination of the dry reforming and the reverse water–gas shift reactions is suggested to explain the observed modifications in the product yields.     

Comparison of the catalytic properties of Al‐ZSM‐22 and Fe‐ZSM‐22 in the skeletal isomerization of 1‐butene

The catalytic activities of Al‐ZSM‐22 (Al in the framework) and Fe‐ZSM‐22 (Fe in the framework) were compared in the skeletal isomerization of 1‐butene to isobutene. The catalysts synthesized in the laboratory were characterized by means of XRD, FTIR spectroscopy, SEM and surface area measurements. The activity of the zeolites was investigated using a fixed‐bed microreactor system. Al‐ZSM‐22 demonstrated higher activity in 1‐butene transformation compared to Fe‐ZSM‐22, while the selectivity to isobutene, on the other hand, was higher over Fe‐ZSM‐22. Coke formation was monitored using a microbalance and the results showed that the weight gain of Fe‐ZSM‐22 was slightly higher compared to Al‐ZSM‐22. This revised version was published online in July 2006 with corrections to the Cover Date.     

Catalytic properties in N2O decomposition of mixed cobalt–iron spinels

The effect of introduction of iron in the Co_3 − xFe_xO₄ on catalytic activity in N₂O decomposition was investigated. The spinel catalysts were characterized by XRD, SEM, RS, BET methods, work function measurements and Mössbauer spectroscopy. Introduction of iron in the Co_3 − xFe_xO₄ spinel catalysts at the level of x < 1 leads to preferential substitution of Fe³⁺ in tetrahedral sites, whereas for x > 1 also octahedral ones are substituted. A strong correlation between deN₂O activity (T_50%) and work function was observed showing that electronic factor controls the catalytic reactivity of Co–Fe spinels. The results revealed that the active centers for N₂O decomposition are cobalt ions and thus even a low level of their substitution by iron leads to substantial decrease of the deN₂O activity of the cobalt spinel.     

Effects of pretreatment atmospheres on the catalytic performance of Pd/γ-Al2O3 catalyst in benzene degradation

In the current paper, a strategy for catalytic degradation of benzene over Pd/γ-Al₂O₃ catalysts via different atmospheres (H₂, N₂, He and air) pretreatment was carried out in a fixed bed reactor. The experimental results indicated that H₂, N₂, and He pretreatments have a significant positive effect on the initial activity of the catalyst compared to air. We have also investigated the effects of pretreatment atmospheres on the catalytic performance for benzene degradation through the information on the chemical state and crystal structure of the catalysts using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), and CO chemisorption measurements techniques. The chemical state of Pd species decreased via H₂ pretreatment, leading to the increase of the initial catalytic activity, while chemical state increased accompanying with a decrease in degradation benzene via air pretreatment. There is no change in the chemical state of Pd species using inert atmosphere (N₂ and/or He) pretreatment, but the initial activity of the catalyst improved significantly due to the modified crystal structure of Pd species in the catalysts, with the crystalline PdO being transformed to amorphous state.     

Synthesis and Metal‐ion Binding Properties of New N2S4O2‐ and N2S5O2‐Donor Macrocycles

Abstract

Synthetic procedures for new mixed‐donor macrocycle compounds were reported. The macrocyclic compounds were used in solvent extraction metal picrates such as Ag⁺, Hg²⁺, Cd²⁺, Zn²⁺, Cu²⁺, Ni²⁺, Mn²⁺, Pb²⁺, and Co²⁺. The metal picrate extractions were investigated at 25±0.1°C with the aid of UV‐visible spectrometry. It was found that 6,7,9,10,12,13,23,24‐octahydro‐19H,26Hdibenzo[h,t](1,4,7,13,16,22,10,19) dioxatetrathiadiazasiclotetracosine‐20,27(21H,28H)‐dione showed selectivity towards Ag⁺, Hg²⁺, and Cd²⁺ among the other metals. The extraction constants (Log K_ex) and complex compositions were determined for the Ag⁺ and Hg²⁺ complexes for this compound and 9,10,12,13,23,24,26,27,29,30‐decahydro‐5H,15H‐dibenzo‐[h,w][1,4,7,13,16,19,25‐,10,22] dioxapentathiadiazacycloheptacosine‐6,16(7H,17H)‐dione.     

Effect of the reductant nature on the catalytic removal of N2O on Fe-zeolite-β catalysts

The catalytic reduction of N₂O by H₂, NH₃, CO, propene and n-decane, in the presence of O₂, has been studied on two Fe-zeolite-β (Fe-BEA) catalysts. In the following experimental conditions (GHSV=35 000 h⁻¹, 2000 ppm N₂O, 3% O₂), CO starts to reduce N₂O from 473 K and is the most efficient reductant in the low temperature domain. The absence of strong adsorption and the ability to reduce Fe^III oxo-cations can be put forward to explain this behaviour. At medium temperature range n-decane become very reactive to reduce N₂O. Finally, in the absence of NH₃ slip, this reductant can be considered as an excellent candidate owing to the harmless of its reduction products.     

Surface structure and metathesis activity of photoreduced allyl‐based Mo/SiO2 catalysts

XPS and metathesis activity studies were performed on oxidic allylbased Mo/SiO₂ catalysts (0.5 wt% Mo) and the oxidic catalyst following photoreduction to various extents. XPS results and average oxidation state measurements from the amount of CO₂ formed during photoreduction, indicated that controlled photoreduction of the oxidic catalyst, in CO atmosphere, produces a mixture of Mo⁶⁺ and Mo⁴⁺ oxidation states with Mo⁴⁺ in abundances ranging from 0 to 100%. Propene metathesis activity studies were performed on the oxidic and photoreduced allylbased Mo/SiO₂ catalysts. The results indicated that metathesis activity appears on photoreduction and increases linearly with increasing abundance of Mo⁴⁺ present on the catalyst. The linear relationship between the % propene conversion and the % Mo⁴⁺ present on the catalyst is consistent with the proposed uniformity of the Mo species and with the hypothesis that the active site (or active site precursor) is associated with Mo⁴⁺.     

Electronic nature of potassium promotion effect in Co–Mn–Al mixed oxide on the catalytic decomposition of N2O

A series of Co–Mn–Al mixed oxides was prepared by a thermal treatment of coprecipitated layered double hydroxide precursors modified with different amount of potassium (0–3 wt.%) and tested in N₂O catalytic decomposition. Chemical analysis, XRD, XPS, surface area measurements, SEM, and contact potential difference measurements were used for bulk and surface characterization of the catalysts. The Co–Mn–Al mixed oxide with 1.1–1.8 wt.% K exhibited the highest conversion of N₂O and minimum value of the catalyst surface work function. Direct correlation between the work function values and the activity of the catalysts demonstrates that N₂O decomposition over K-promoted Co–Mn–Al mixed oxides proceeds via the cationic redox mechanism and controlled modification of surface electronic properties provides the essential factor for catalyst optimization.     

Phase stability and equilibria in the La2O3–TiO2 system

XRD, electron probe wavelength and energy dispersive X-ray analyses were used to reexamine the phase relations in the La₂O₃–TiO₂ system. The diagram was redrawn to include the compound La₄Ti₃O₁₂ in addition to La₄Ti₉O₂₄, La₂Ti₂O₇ and La₂TiO₅. Above 1455°C a cation deficient perovskite La_2/3TiO₃ is stabilized by a small number of Ti³⁺ ions and remains stable on cooling in air. The proposed diagram represents a section through the system at the normal oxygen pressure in air at 1 atm, compositions being expressed in terms of the oxide components stable at room temperature.     

The influence of oxide–oxide interaction on the catalytic properties of Co/Al2O3 in CO hydrogenation

The influence of oxide–oxide interaction on the catalytic properties of cobalt in CO hydrogenation is investigated on the example of a Co/Al₂O₃ catalyst. Oxide–oxide interaction was prevented by modification of the alumina surface with magnesia. It has been shown that oxide–oxide interaction affects catalytic activity and the amount of carbon deposited on the catalyst surface. This revised version was published online in July 2006 with corrections to the Cover Date.     

Facile synthesis of α-Fe2O3 nanoparticles and their catalytic activity in oxidation of benzyl alcohols with periodic acid

α-Fe₂O₃ nanoparticles (NPs) were synthesized by homogeneous chemical precipitation followed by calcination. Size-tuning of NPs was achieved by using polyethylene glycol with a molecular weight of 400 and 4000 as surfactant. Synthesized α-Fe₂O₃ NPs were characterized by powder XRD, transmission electron microscopy, N₂ adsorption–desorption isotherm and vibrating sample magnetometry. These α-Fe₂O₃ NPs were used as magnetically recoverable catalyst in oxidation of benzyl alcohols to aldehydes with periodic acid. The catalyst showed higher efficiency with decrease in their sizes.     

NO presence effects on the reduction of N2O by CO over Al–Pd–Co oxide catalyst

Nitrous oxide (N₂O) is known as one of the greenhouse gases of which the emission levels are to be controlled and also an ozone-depleting material in the stratosphere. For more than a last couple of decades, various catalysts have been investigated for the reduction of N₂O emission. However, most of those catalysts require reaction temperatures as high as 450 °C even with the use of effective reducing agents. Furthermore, NO, which is usually present with N₂O, significantly interferes with the removal efficiency of N₂O. Al–Pd–Co oxide catalyst which is a type of mixed metal oxides (MMO) has shown 100% conversion of N₂O at temperatures as low as 200 °C using CO. This paper examines the effect of NO on the reduction of N₂O with CO over Al–Pd–Co oxide catalyst. The experiments were carried out in the temperature range of 100–500 °C with space velocity of 10,000–50,000 h^?1. Though the efficiency of N₂O reduction is decreased significantly when NO is present, the efficiency increases when sufficient CO is supplied. In this study, the reduction mechanisms of N₂O and NO by CO have been confirmed, and it was shown that MMO catalyst can simultaneously remove N₂O and NO using CO with high efficiency.     

Coke formation on the surface of α-Al2O3 in the catalytic pyrolysis of naphtha

The catalytic pyrolysis of naphtha has been carried out in a quartz reactor loaded with 5 mm α-A1₂0₃ spheres. The yields of ethylene and propylene exhibit about 10% and 5% higher values compared to the thermal pyrolysis in the absence of α-A1₂0₂ spheres at the same reaction conditions. The coke formation on α-A1₂0₃ spheres increased continuously along with the axial length of the reactor as well as with reaction time. Results suggest that the concentration of the coke precursors in the gas phase may increase along with the axial length of the reactor. Coke filled up completely the internal pore of the sphere near the exit of the reactor after reaction for 4 hr. The coke film on the external surface of the sphere grew continuously thicker. The coke formation was influenced strongly by the physical properties of the α-Al₂O₃ spheres. Coke deposition was the least on the α-A1₂0₃ sphere with the lowest surface area and pore volume among the tested α-A1₂0₃ spheres.     

Gold nanoparticles: effect of treatment on structure and catalytic activity of Au/Fe2O3 catalyst prepared by co‐precipitation

1 wt% Au/Fe₂O₃ catalyst was prepared by a co‐precipitation method. The structure of the sample in the as prepared, oxidized and reduced states was investigated by means of X‐ray photoelectron spectroscopy (XPS), transition electron microscopy (TEM), electron diffraction (ED) and X‐ray diffraction (XRD). The structure of the samples after various treatments and their activity in the CO oxidation were compared. The results show the stability of the gold particle size during the treatments. However, after oxidation, a slight shift in the Au 4f binding energy towards lower values points to the formation of an electron‐rich state of the metallic gold particles compared to that revealed in the as prepared sample. Simultaneously, a goethite phase in the Fe₂O₃ support is present, which is not observed in the “as prepared” and reduced samples. In the reduced sample the presence of a crystalline maghemite‐c phase indicates a change in the support morphology. In the CO oxidation the oxidized sample shows the highest activity and it might be the result of the cooperative effect of goethite, FeO and the electron‐rich metallic gold nanoparticles. We suggest that a structural transformation occurs along the gold/support perimeter during the treatments and we propose a possible mechanism for the effect of the oxidation treatment. This revised version was published online in July 2006 with corrections to the Cover Date.     

High‐activity catalyst of SO 4 2− /ZrO2 supported on γ‐Al2O3 for n‐butane isomerization

A series of Al₂O₃supported SO ₄ ^2– /ZrO₂ superacid catalysts (named SZ/Al₂O₃) were prepared by a precipitation method and their catalytic behavior for nbutane isomerization at low temperature in the absence of H₂ and at high temperature in the presence of H₂ was studied in this paper. The catalytic activities of some of these catalysts were enhanced significantly at both low and high temperatures. At 250°C after 6 h on stream, the steady activity of the most active sample, 60%SZ/Al₂O₃, is about two times higher than that of conventional SZ. The texture properties of catalysts were studied by the methods of XRD and the adsorption of N₂. Experimental evidence of IR of adsorbed pyridine indicates that the significant activity enhancements of SZ/Al₂O₃ catalysts are caused by the increasing of the amount of strong acid sites.