Fe-saponite pillared and impregnated catalysts: II. Nature of the iron species active for the reduction of NOx with propene

A study of the NO_x reduction with propene in the absence/presence of oxygen over Fe-saponite clay catalysts with different iron loadings (ca. 1–28 wt.%) has been performed. The catalysts were prepared by pillaring and impregnation methods and the iron active phases were characterized by using X-ray absorption spectroscopies and electron paramagnetic resonance. The samples display significant activity in the NO_x reduction in the absence of oxygen with a maximum for a Fe content close to 10 wt.%. A strong decay of catalytic activity was observed with the introduction of increasing quantities of oxygen in the feed. Iron is mostly present in all catalysts as Fe(III) with a slightly distorted local octahedral symmetry. The catalytic behavior was explained in terms of the nature and properties of the Fe existing phases.     

Er/β-zeolite-catalyzed one-pot conversion of cellulose to lactic acid

Various Er/β-zeolite catalysts were prepared by grafting erbium species in aqueous solutions of erbium chloride onto β-zeolites. The catalysts were characterized using X-ray fluorescence spectroscopy, inductively coupled plasma optical emission spectroscopy, N₂ physical adsorption, powder X-ray diffraction, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and NH₃ temperature-programmed desorption. These catalysts were evaluated in the one-pot hydrothermal conversion of cellulose to lactic acid. Lactic acid yields of approximately 58 % were obtained using an erbium grafted on de-aluminated β zeolite (Er/deAlβ-2) with an erbium content of 12.4 wt% and a Si/Al ratio of 159:1. In catalyst recycling tests, the lactic acid yield decreased from 57.9 % in the first cycle to 51.2, 49.8 and 49.6 % in the second, third and fourth cycles, respectively. The decreases in catalytic activity during recycling are proposed to arise mainly from a combination of erbium ion leaching, deposition of carbon species in the zeolite pores and the partial structure collapse.     

Controllable Fe/HCS catalysts in the Fischer-Tropsch synthesis: Effects of crystallization time

The Fischer–Tropsch synthesis (FTS) continues to be an attractive alternative for producing a broad range of fuels and chemicals through the conversion of syngas (H₂ and CO), which can be derived from various sources, such as coal, natural gas, and biomass. Among iron carbides, Fe₂C, as an active phase, has barely been studied due to its thermodynamic instability. Here, we fabricated a series of Fe₂C embedded in hollow carbon sphere (HCS) catalysts. By varying the crystallization time, the shell thickness of the HCS was manipulated, which significantly influenced the catalytic performance in the FTS. To investigate the relationship between the geometric structure of the HCS and the physic-chemical properties of Fe species, transmission electron microscopy, X-ray diffraction, N₂ physical adsorption, X-ray photoelectron spectroscopy, hydrogen temperature-programmed reduction, Raman spectroscopy, and Mössbauer spectroscopy techniques were employed to characterize the catalysts before and after the reaction. Evidently, a suitable thickness of the carbon layer was beneficial for enhancing the catalytic activity in the FTS due to its high porosity, appropriate electronic environment, and relatively high Fe₂C content.     

Screening of Fe–BEA catalysts for wet hydrogen peroxide oxidation of crude olive mill wastewater under mild conditions

A series of Fe–BEA catalysts, differing in the amount of iron have been characterized by XRD, BET surface area, UV–vis spectroscopy and chemical analysis. The zeolite samples have been tested as heterogeneous catalysts for the wet hydrogen peroxide oxidation of crude olive mill wastewaters (OMW) under very mild conditions (at 28 °C and atmospheric pressure). All experiments were performed on a laboratory scale set-up.BSE-1/3 catalyst with a moderate Fe content (Fe/Al = 1.19) showed the best results in terms of catalytic activity and loss of active species into the aqueous solutions. The stability of Fe species has been shown to be strongly dependent on the Fe environment into the zeolite framework.Over the selected catalyst, application of catalytic procedure on diluted OMW solution permitted high removal efficiencies of pollutants. The process produces a removal capacity of 28% of total organic carbon (TOC), 40% of total phenols, 30% of chemical oxygen demand (COD) and 59% of colour, just after 12 h. 5-Day biochemical oxygen demand (BOD₅), and toxicity towards the bioluminescent bacteria Vibrio fischeri were selected to follow the performance of this process in terms of reducing the ecotoxicity of OMW. Results showed an increase in the biodegradability of the treated sample and a decrease of the microtoxicity from 100% to 70% load towards V. fischeri.Occurrence of small catalyst deactivation by carbonaeous during the oxidation reaction was observed through scanning electron microscopy (SEM) and elemental analysis.     

Mechanism of Iron Catalysis of Carbon Monoxide Decomposition in Refractories

The catalytic effects of selected iron phases (metals, oxides, sulfides, and carbides) on the Boudouard reaction (2CO ⇄ CO₂+ C) studied in an effort to more fully understand the disintegration of refractories when exposed to CO for long periods of time. It was found that active Fe atoms generated from the reduction of the iron oxides, especially α-Fe₂O₃, are the actual catalysts for the Boudouard reaction. The catalytic process, confirmed by thermodynamic calculations, kinetic data, and X-ray diffraction data, consists of adsorption and decomposition of CO simultaneously forming carbides of iron. The chemisorption and subsequent decomposition of the iron carbides, rather than diffusion, constitute the rate-controlling process for carbon deposition.     

Characterization and catalytic properties of perovskites with nominal compositions La1-xSrxAl1-2yCuyRuyO3

Nine different metal oxide catalysts were prepared by impregnating alumina washcoats with water solutions containing La³⁺, Sr²⁺, Cu²⁺ and Ru³⁺ ions and calcining them at 900°C. The produced samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) studies combined with energy-dispersive spectroscopy (EDS) analysis, X-ray powder diffraction and specific surface area measurements. A perovskite phase of the nominal composition La_1-xSr_xAl_1-2yCu_yRu_yO₃ was found in all samples, in increasing amount in the samples with increasing contents of strontium and ruthenium. The catalysts were evaluated with respect to light-off temperatures and redox characteristics using two gas mixtures, one containing NO/CO/C₃H₆/O₂/N₂ and the other NO/CO/N₂. The light-off temperatures for nitric oxide reduction decreased from 534 to 333°C for the catalysts without and with strontium and ruthenium, respectively. In the presence of oxygen the conversion of nitric oxide declined rapidly under oxidative conditions whereas in absence of oxygen this decline was less pronounced and found to be linear over the entire redox interval studied. These studies suggest that the perovskite phase takes an active part in the conversion of nitric oxide and carbon monoxide to nitrogen and carbon dioxide.     

Catalytic performance of various mesoporous silicas modified with copper or iron oxides introduced by different ways in the selective reduction of NO by ammonia

Mesoporous silicas (MCM-48, SBA-15, MCF), reflecting various porous structures, were modified with copper and iron oxides by two different methods. For a first series of the samples the molecular designed dispersion (MDD) method using acetylacetonate complexes of copper and iron was applied for the deposition of transition metal oxides on the silica supports. A second series of the catalysts was obtained by the incipient impregnation technique using aqueous solutions of the suitable metal nitrates. The modified materials were characterized with respect to the texture (BET), composition (electron microprobe analysis), coordination of the transition metals (UV–vis–DRS) and surface acidity (NH₃-TPD, FTIR). The mesoporous silica supports modified with transition metal oxides were tested as catalysts of the selective reduction of NO with ammonia. The catalytic performance of the studied samples depended on the method used for the deposition of transition metal oxide as well as the kind of mesoporous silica used as a catalytic support. In general, the Cu-containing mesoporous samples effectively operated at lower temperatures than silicas modified with iron. The samples obtained by the MDD method have been found to be more active and selective compared to the analogous samples prepared by the impregnation technique. An introduction of water vapor into the reaction mixture only slightly decreased the NO conversion and selectivity towards N₂ over the MCF mesoporous silica modified with copper or iron oxide.     

Assessment of Fe2O3/SiO2 catalysts for the continuous treatment of phenol aqueous solutions in a fixed bed reactor

Different iron-containing catalysts have been tested for the oxidation of phenol aqueous solutions in a catalytic fixed bed reactor in the presence of hydrogen peroxide. All the catalysts consist of iron oxide, mainly crystalline hematite particles, over different silica supports (mesostructured SBA-15 silica and non-ordered mesoporous silica). The immobilization of iron species over different silica supports was addressed by direct incorporation of metal during the synthesis or post-synthesis impregnation. The synthesis conditions were tuned up to yield agglomerated catalysts with iron loadings between 10 and 15 wt.%. The influence of the preparation method and the type of silica support was evaluated in a catalytic fixed bed reactor for the continuous oxidation of phenol in terms of catalysts activity (phenol and total organic carbon degradation) as well as their stability (catalyst deactivation by iron leaching). Those catalysts prepared by direct synthesis, either in presence of a structure-directing agent (Fe₂O₃/SBA-15(DS)) or in absence (Fe₂O₃/SiO₂(DS)), achieved high catalytic performances (TOC reduction of 65% and 52%, respectively) with remarkable low iron leaching in comparison with their silica-based iron counterparts prepared by impregnation. Catalytic results have demonstrated that the synthesis method plays a crucial role in the dispersion and stability of active species and hence resulting in superior catalytic performances.     

Electrochemically induced self-assembly of alkanethiolate adlayers on carbon steel in aqueous solutions

The electrochemical behavior of carbon steel was studied in near neutral buffered aqueous solutions saturated with mercaptoundecanoic acid (MUA) and dodecanethiol (DT) by using electrochemical techniques combined with X-ray photoelectron spectroscopy (XPS). MUA and DT adsorb on the Fe surface by cathodization in the potential range corresponding to the hydrogen evolution reaction by forming thiolate bonds. The presence of the adsorbate layer in contact with an extremely low amount of thiols in the neutral solutions hinders the hydrogen evolution reaction and prevents the formation of the iron oxides in a wide potential range.     

Study of Methane Decomposition on Fe/MgO-Based Catalyst Modified by Ni,Co, and Mn Additives

Catalytic decomposition of methane (CDM) generates clean hydrogen and carbon nanomaterials. In this study, methane decomposition to hydrogen and carbon was investigated over Ni-, Co-, or Mn-doped Fe/MgO catalysts. The doping effect of different metals, varying from 3 to 10?wt%, was investigated. The catalytic performance of the obtained materials (noted 15%Fe+x%metal/MgO) revealed that the doping effect of Ni, Co, and Mn significantly improved the activity of Fe/MgO. Among the Ni-doped catalyst series, the 15%Fe+3%Ni/MgO catalyst performed better than the rest of the Ni catalysts. The 6%Co-containing catalyst remained the best in terms of activity in the Co-doped catalyst series and the 15%Fe+6%Mn/MgO solid showed better methane conversion for the Mn-doped series. Overall, 3%Ni-containing catalyst displayed the best catalytic performance among all Ni-, Co-, and Mn-doped catalysts. XRD, N₂ sorption, and H₂ temperature-programmed reduction (TPR), Laser–Raman spectroscopy, thermogravimetric analysis (TGA) under air, and temperature-programmed oxidation (TPO) were used for catalyst characterization. The results revealed that all the doped catalysts exhibited better metallic active site distribution than 15%Fe/MgO and proved that metal doping played a crucial role in catalytic performance.     

Fe-ZSM-5 Catalysts: Preparation in Organic Media, Fe-particle Morphology and NO x Reduction Activity

Fe-ZSM5 catalysts were prepared at room temperature. Iron was incorporated into H-ZSM5 in organic media (toluene) using iron acetyl acetonate. In order to elucidate the catalytic performance of this Fe-ZSM5, reference catalysts were prepared by chemical vapour deposition (CVD) and impregnation in aqueous media (IMW). The various catalysts were characterized structurally and morphologically. Materials prepared in organic media and CVD showed fibrous iron particles, with radius between 10 and 40 Å, highly dispersed on the external zeolite surface. Fractal dimension values suggest that iron particles grow dendritically when iron is incorporated into zeolite by impregnation in organic media. Nuclear magnetic resonance of ¹²⁹Xe adsorbed on catalysts shows that dendrites can penetrate into the zeolite channels. The dispersion of the iron species on the external surface of the zeolite and the presence of iron inside the channels explain the good catalytic performance in the Selective Catalytic Reduction (SCR) of NO. Ammonia and n-decane were employed as reducing agents. The highest NO to N₂ conversion (≈100% at 430 °C) was obtained when ammonia was used. In contrast, n-decane is unable to reach Fe species in channels, and the inner active surface is thus lost.     

Methane decomposition over ceria modified iron catalysts

The catalytic behaviour of ceria supported iron catalysts (Fe–CeO₂) was investigated for methane decomposition. The Fe–CeO₂ catalysts were found to be more active than catalysts based on iron alone. A catalyst composed of 60 wt.% Fe₂O₃ and 40 wt.% CeO₂ gave optimal catalytic activity, and the highest iron metal surface area. The well-dispersed Fe state helped to maintain the active surface area for the reaction. Methane conversion increased when the reaction temperature was increased from 600 to 650 °C. Continuous formation of trace amounts of carbon monoxide was observed during the reaction due to the oxidation of carbonaceous species by high mobility lattice oxygen in the solid solution formed within the catalyst. This could minimise catalyst deactivation caused by carbon deposits and maintain catalyst activity over a longer period of time. The catalyst also produced filamentous carbon that helped to extend the catalyst life.     

Nickel-iron catalysts prepared by homogeneous deposition-precipitation of cyanide complexes on a titania support

Nickel-iron catalysts have been prepared by homogeneous deposition-precipitation of complex nickel-iron cyanides on a titania support. The nickel-iron alloys obtained after calcination and reduction of the cyanide precursors were characterized by Mössbauer spectroscopy, high-temperature X-ray diffraction, and magnetic measurements. Fischer-Tropsch experiments show remarkable results. Catalysts prepared from K₃Fe(CN)₆ and Na₂Fe(CN)₅NO cyanide precursors exhibit a high activity and selectivity, whereas catalysts prepared from K₄Fe(CN)₆ do not show any activity. This lack of activity is caused by the presence of potassium in catalysts prepared from K₄Fe(CN)₆. Potassium or iron titanate inhibits the adsorption of CO on the nickel-iron surface. Deactivation of active nickel-iron catalysts was caused by the deposition of inactive carbon during Fischer-Tropsch synthesis.     

Heteroatom-doped carbon nanorods with improved electrocatalytic activity toward oxygen reduction in an acidic medium

High-performance heteroatom-doped carbon catalysts with large surface areas were prepared by pyrolyzing nanorod precursors that had been synthesized by polymerizing a mixture of aniline (An) and β-naphthalene sulfonic acid (NSA). The catalysts were characterized by scanning and transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, N₂ adsorption/desorption isotherms, and elemental analysis. We intensively investigated how the catalysts’ structure and catalytic performance were affected by (i) the ratio of NSA to An and (ii) the addition of Fe. The catalysts retained their nanorod morphology after pyrolysis. The optimal NSA/An ratio was 3/2 and the optimal Fe content was 3 wt%. The catalysts showed excellent activity toward oxygen reduction in an acidic medium, with the onset potential, half-wave potential, and limiting current density values reaching 0.86, 0.73 V (vs. reversible hydrogen electrode), and 5.28 mA cm⁻², respectively. We suggest that the catalysts’ high performance may be due to the co-doping effects of nitrogen, sulfur, and iron, as well as the large surface area created by the nanorod structures.     

Metal dispersion and distribution in Pd-based PTA catalysts

The influence of the pH of the impregnating H₂PdCl₄ solution was studied in the preparation of Pd catalysts on granular active carbon. All catalysts were tested in the purification of crude terephthalic acid (4-CBA hydrogenation), in conditions strictly similar to those of industrial operation. The pH of the impregnating H₂PdCl₄ solution was found to strongly influence both Pd surface area and activity (best results with pH 1.5–2.0), while has a relatively small influence on Pd distribution. A linear relationship between catalytic activity for 4-CBA hydrogenation and Pd surface area was confirmed.     

Synthesis,microstructure, and catalytic properties of porous SiFeCO nanocomposites derived from MIL-101(Fe)-modified PCS

SiFeCO ceramic nanocomposites with porous structure have been successfully prepared via pyrolysis of novel iron-containing precursors, which were synthesized by modification of polycarbosilane with different contents of MIL-101(Fe), a kind of metal-organic framework made by our laboratory. Iron-containing precursors were pyrolyzed at relatively low temperatures ranging from 500 to 900°C under nitrogen atmosphere, the polymer-to-ceramic conversion process were investigated by Fourier transform infrared spectroscopy, X-ray diffraction (XRD), and thermo gravimetric analysis (TGA) measurements. SiFeCO ceramics with different iron contents were intensively studied with respect to their crystallization behavior, microstructure evolution, and porous morphology influenced by additive amount of MIL-101(Fe). The results of XRD and Transmission electron microscopy revealed that MIL-101(Fe)-modified precursors converted into amorphous SiFeCO ceramics when pyrolyzed below 700°C under nitrogen, extremely small Fe₃O₄ and α-Fe crystallites with grain size of only 2–5 nm were in situ generated and dispersed uniformly in the SiCO ceramic matrix. Simultaneously, interconnected porous structure was created in the matrix by the decomposition of MIL-101(Fe). The as-obtained SiFeCO ceramics could serve as heterogeneous Fenton-like catalysts for the degradation of xylene orange and exhibit outstanding catalytic properties.     

Selective catalytic reduction of nitric oxide by ammonia on Fe-promoted active carbon

Selective catalytic reduction of nitric oxide by ammonia on Fe³⁺-promoted active carbons was investigated. The catalysts were prepared by the impregnation of active carbon (N/m) preoxidised with concentrated nitric acid at different temperatures. The amount of oxygen-containing surface groups on the supports was determined by infrared spectroscopy and X-ray photoelectron spectroscopy (XPS) while the distribution of active material was investigated using XPS. Catalytic activity of Fe³⁺-active carbon systems depended on the degree of the oxidation of the supports and pretreatment of the catalysts (drying, calcination in helium). The presence of oxygen in the reaction mixture enhanced the nitric oxide conversion. The catalysts showed a long-term stability.     

Influences of surface functional groups on catalytic activity over activated carbon catalysts for sulfur dioxide removal from flue gases

The catalytic oxidation of sulfur dioxide in flue gases over activated carbon catalysts was studied. Catalysts made from different materials or by different methods were markedly different. The catalysts were characterized by BET isotherms and X-ray power diffraction. The acidic and basic sites and/or strength on the catalysts were determined by temperature-programmed desorption, while surface functional groups were determined by X-ray photoelectron spectroscopy. The catalytic activity depended on oxygen-containing surface functional groups with Brønsted basicity properties. These groups on a furfural residue activated carbon could be increased by a suitable treatment to enhance the catalytic activity but other catalysts in this study showed no activity enhancement with a similar treatment.     

Preparation of Pt/CeO2/HCSs anode electrocatalysts for direct methanol fuel cells

Hollow carbon spheres (HCSs) were prepared through a simple hydrothermal method using silica particles and glucose as the template and carbon precursor, respectively. HCSs used as supports for platinum catalysts deposited with cerium oxide (CeO₂) were prepared for application as anode catalysts in direct methanol fuel cells. The composition and structure of the samples were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). The electrocatalytic properties of the as-prepared catalysts for methanol oxidation were investigated by cyclic voltammetry (CV). The Pt/CeO₂/HCSs catalyst heated at 550 °C for 1 h exhibited the best catalytic activity for methanol oxidation.     

Selective oxidation of CO in hydrogen atmosphere on Pt–Fe catalysts supported on zeolite P-based materials

The catalytic properties of Pt supported on zeolite P (ZP)-based materials for the preferential CO oxidation in hydrogen atmosphere under mild conditions (from room temperature to 150 °C), have been investigated. Pt catalysts (1–4 wt%) supported on a zeolitized pumice support (Z-PM) have been prepared. A series of bimetallic Pt–Fe on ZP, having 2 wt% Pt and different Fe loading (0.5–4 wt%), have been also prepared and used as model catalysts. A detailed characterization of the catalysts has been carried out by means of surface area and porosity measurements, X-ray diffraction, scanning electron microscopy and transmission electron microscopy in order to investigate the morphological and microstructural properties of both support and catalytic system. Pt/Z-PM exhibited complete CO conversion with 55 % selectivity at temperatures as low as 50 °C, with no noticeable degradation of the catalytic performances, indicating that the Fe content present as an impurity in the zeolitized pumice support allows to obtain catalysts characterized by high activity and stability. On the basis of the characterization and kinetic tests, hypotheses on the role of Fe promoter have been formulated.