Rare earth oxides as carbon oxidation catalysts

The behavior of a number of rare earth oxides as catalysts for the oxidation of graphite in air has been investigated by the methods of thermal analysis. Of the oxides studied, only CeO₂ showed significant activity in accelerating the gasification of graphite by oxygen between 500 and 1000°C. Cerium salts, which decompose to a finely dispersed oxide phase at low temperatures, e.g. Ce (III) nitrate and ammonium Ce (IV) nitrate, were found to be very active catalysts. The catalytic effect may be due to a redox process involving the cyclic conversion of the oxide from the Ce (IV) to the Ce (III) oxidation state, or the oxide particles may provide sites for the dissociative chemisorption of oxygen.     

Effect of alkaline earth promoters (MgO,CaO, and BaO) on the activity and coke formation of Ni catalysts supported on nanocrystalline Al2O3 in dry reforming of methane

Ni/Al₂O₃ promoted catalysts with alkaline earth metal oxides (MgO, CaO, and BaO) were prepared and employed in dry reforming of methane (DRM). The catalysts were prepared by impregnation method and characterized by XRD, BET, TPR, TPO, and SEM techniques. The obtained results showed that the addition of MgO, CaO, and BaO as promoter decreased the surface area of catalysts (S_BET). The catalysis results exhibited that adding alkaline earth promoters (MgO, CaO, and BaO) enhanced the catalytic activity and the highest activity was observed for the MgO promoted catalyst. TPR analysis showed that addition of MgO increased the reducibility of nickel catalyst and decreased the reduction temperature of NiO species. The TPO analysis revealed that addition of promoters decreased the amount of deposited coke; and among the studied promoters, MgO has the most promotional effect for suppressing the carbon formation. SEM analysis confirmed the formation of whisker type carbon over the spent catalysts.     

Characterization and catalytic activity of unpromoted and alkali (earth)-promoted Au/Al2O3 catalysts for low-temperature CO oxidation

Various unpromoted and alkali (earth) promoted gold catalysts were characterized by means of XRD, HRTEM, DR/UV–Vis and TPR. Based on the results we conclude that metallic Au is the active species in CO oxidation and that the reduction of Au³⁺ to Au⁰ proceeds below 200 °C. Pretreatment at mild temperatures, viz. 200 °C, results in the highest catalytic performance of Au/Al₂O₃ in low-temperature CO oxidation. Alkali (earth) metal oxide additives are most probably structural promoters. The best promoting effect is found for BaO.     

Direct Oxidation of Propylene to Propylene Oxide with Molecular Oxygen: A Review

The direct oxidation of propylene to propylene oxide (PO) using molecular oxygen has many advantages over existing chlorohydrin and hydroperoxide process, which produce side products and require complex purification schemes. Recent advances in liquid-phase and gas-phase catalytic oxidation of propylene in the presence of only molecular oxygen as oxidant and in absence of reducing agents are summarized. Liquid-phase PO processes involving soluble or insoluble Mo, W, or V catalysts have been reported which provide moderate conversions and selectivities, but these likely involve autoxidation by homogeneous chain reactions. Gas-phase PO catalysts have been mostly Ag-, Cu-, or TiO₂-based substances, although other compositions such as Au-, MoO₃-, Bi-based catalysts and photocatalysts have also been suggested as possibilities. The Ag catalysts differ from those used for ethylene oxide production in having high Ag contents and numerous additives. The additives are solid-phase alkali metals, alkaline earth metals, and halogens, with the most common substances being NaCl and CaCO₃. Nitrogen oxides in the form of gas-phase species or nitrates have also been found to be effective in enhancing PO production. Direct epoxidation by surface nitrates is a possibility. Titania catalysts supported on silicates have also been reported. These have higher PO selectivities at high conversion than silver catalysts.     

Characterization and electrocatalytic properties of PtRu/C catalysts prepared by impregnation-reduction method using Nd2O3 as dispersing reagent

A simple impregnation-reduction method introducing Nd₂O₃ as dispersing reagent has been used to synthesize PtRu/C catalysts with uniform Pt-Ru spherical nanoparticles. X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analysis have been used to characterize the composition, particle size and crystallinity of the catalysts. Well-dispersed catalysts with average particle size about 2 nm are achieved. The electrochemically active surface area of the different PtRu/C catalysts is determined by the CO_ad-stripping voltammetry experiment. The electrocatalytic activities of these catalysts towards methanol electrooxidation are investigated by cyclic voltammetry measurements and ac impedance spectroscopy. The in-house prepared PtRu/C catalyst (PtRu/C-03) in 0.5 M H₂SO₄ + 1.0 M CH₃OH at 30 °C display a higher catalytic activity and lower charge-transfer resistance (R_t) than that of the standard PtRu/C catalyst (PtRu/C-C). It is mainly due to enhanced electrochemically active specific surface, higher alloying extent of Ru and the abundant Pt⁰ and Ru oxides on the surface of the PtRu/C catalyst.     

THE SELECTIVE CATALYTIC REDUCTION OF NITRIC OXIDE WITH METHANE OVER NONZEOLITIC CATALYSTS

This article deals with recent progress in the development of nonzeolitic catalysts for the selective catalytic reduction (SCR) of nitrogen oxides (NO_x) with methane. Although metal-exchanged zeolitic catalysts exhibit high activities for this reaction, most of these metastable structures suffer from deactivation problems when exposed to wet exhaust streams at elevated temperatures. Nonzeolitic oxide catalysts have the potential for improved durability because many of these are thermodynamically stable structures.

The NO_x reduction activity and poisoning resistance of lanthanide oxides, Group IIA oxides, Group IIIB oxides, as well as gallium-, tin-, and palladium-based catalysts are reviewed. Current opinions on mechanistic aspects of the SCR reaction are discussed in relation to the development of more active catalytic systems.

A detailed experimental study of the SCR of NO_x over rare earth oxides has also been performed. The role of absorbed oxygen in the activation of CH₄ and consumption of CH₃ radicals is discussed and related to the catalytic properties of the rare earth oxides. The presence of highly basic centers is shown to promote catalytic activity, and the presence of weakly absorbed oxygen species is found to be detrimental to catalytic selectivity. Oxides with lower electronic conductivities are found to be more selective than catalysts with higher conductivities. The selectivities of rare earth oxides for the SCR of NO_x with methane and the oxidative coupling of methane are also compared and contrasted.     

Effects of promoters on catalytic activity and carbon deposition of Ni/γ‐Al2O3 catalysts in CO2 reforming of CH4

Catalytic reforming of methane with carbon dioxide was studied in a fixed‐bed reactor using unpromoted and promoted Ni/γ‐Al₂O₃ catalysts. The effects of promoters, such as alkali metal oxide (Na₂O), alkaline‐earth metal oxides (MgO, CaO) and rare‐earth metal oxides (La₂O₃, CeO₂), on the catalytic activity and stability in terms of coking resistance and coke reactivity were systematically examined. CaO‐, La₂O₃‐ and CeO₂‐promoted Ni/γ‐Al₂O₃ catalysts exhibited higher stability whereas MgO‐ and Na₂O‐promoted catalysts demonstrated lower activity and significant deactivation. Metal‐oxide promoters (Na₂O, MgO, La₂O₃, and CeO₂) suppressed the carbon deposition, primarily due to the enhanced basicities of the supports and highly reactive carbon species formed during the reaction. In contrast, CaO increased the carbon deposition; however, it promoted the carbon reactivity. © 2000 Society of Chemical Industry     

Synthesis of Biodiesel from Canola Oil Using Heterogeneous Base Catalyst

A series of alkali metal (Li, Na, K) promoted alkali earth oxides (CaO, BaO, MgO), as well as K₂CO₃ supported on alumina (Al₂O₃), were prepared and used as catalysts for transesterification of canola oil with methanol. Four catalysts such as K₂CO₃/Al₂O₃ and alkali metal (Li, Na, K) promoted BaO were effective for transesterification with >85 wt% of methyl esters. ICP-MS analysis revealed that leaching of barium in ester phase was too high (~1,000 ppm) when BaO based catalysts were used. As barium is highly toxic, these catalysts were not used further for transesterification of canola oil. Optimization of reaction conditions such as molar ratio of alcohol to oil (6:1–12:1), reaction temperature (40–60 °C) and catalyst loading (1–3 wt%) was performed for most efficient and environmentally friendly K₂CO₃/Al₂O₃ catalyst to maximize ester yield using response surface methodology (RSM). The RSM suggested that a molar ratio of alcohol to oil 11.48:1, a reaction temperature of 60 °C, and catalyst loading 3.16 wt% were optimum for the production of ester from canola oil. The predicted value of ester yield was 96.3 wt% in 2 h, which was in agreement with the experimental results within 1.28%.     

The formation and study of sensor thin layers based on zirconium and rare earth metal (Ce,Y, and Tb) oxides and the preparation of metal-oxide-semiconductor structures based on them

Low-temperature synthetic methods (the molecular layering and pyrolysis of a liquid aerosol) for the nanostructured oxide films for the gas sensors based on zirconium dioxides and rare earth element (Ce, Y, and Tb) oxides sensitive to the oxygen-impoverished media and ozone have been suggested. It has been shown that the films based on the CeO₂-ZrO₂ and TbO _x -YO_1.5 systems possess the largest sensitivity and a strong response rate (5–6 s) to the reducing media (CO₂ + CO), while the film sensors based on CeO₂, to ozone. Metal-oxide-semiconductor (MOS) structures that find wide application in semiconductor microelectronics have been obtained based on the synthesized films of rare earth element oxides.     

Dehydration of 1,4-butanediol over supported rare earth oxide catalysts

Vapor-phase catalytic dehydration of 1,4-butanediol was investigated over rare earth oxides supported on ZrO₂. In the dehydration of 1,4-butanediol, 3-buten-1-ol was mainly produced, together with tetrahydrofuran (THF) and γ-butyrolactone. The heavy group of rare earth oxides, such as Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, and Lu₂O₃, supported on monoclinic ZrO₂ showed higher selectivity to 3-buten-1-ol than pure monoclinic ZrO₂ and supported light rare earth oxides, such as La₂O₃ and Pr₆O₁₁. Supported Yb₂O₃ catalysts dispersed on other oxides, such as alumina, silica, and tetragonal ZrO₂ catalyze the formation of THF. X-ray diffraction (XRD) measurements reveal that cubic Yb₂O₃ crystallites dispersed on monoclinic ZrO₂ provide active sites in the dehydration of 1,4-butanediol to produce 3-buten-1-ol.     

Preparation and characterization of branched polyesteramide/mix rare earth oxides composites

Branched polyester amide containing various amount of mix rare earth oxides (RE₂O₃) were synthesized by in-situ solution condensation of AB₂ monomer synthesized based on the reaction between maleic anhydride and diethanolamine (DEA). The effect of AB₂ monomer self-polycondensation on the RE₂O₃ surface and the properties of synthesized composites were investigated by using Fourier-transform infrared spectra (FT-IR), ultraviolet–visible absorption spectra (UV–vis), X-ray photoelectron spectroscopy, scanning electron microscopy, thermogravimetric analysis and viscosity determination. AB₂ monomer can envelop RE₂O₃ particles as an organic matrix (HBPEA′) by self-polycondensation under the proposed condition. Ionization of surface of RE₂O₃ aggregates can result in the rare earth ionic (RE³⁺) and the resulting RE³⁺ might coordinate with the organic matrix. Thermal stability of branched polyesteramide can be significantly improved in presence of 5 wt% RE₂O₃, possibly due to the coordination reaction between RE₂O₃ and the active group of the organic matrix.     

Esterification over rare earth oxide and alumina promoted SO4/ZrO2

Solid superacid catalysts including SO₄²⁻/ZrO₂ (SZ), rare earth (RE) oxide-promoted SZ and RE oxides together with alumina-promoted SZ were prepared. Their catalytic performances in the esterification reaction of ethanol and acetic acid were investigated. The textural property, crystalline phase and surface acidity of the prepared catalysts were characterized by using nitrogen adsorption–desorption isotherms, X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy of pyridine adsorption techniques, respectively. Effects of the reaction time and catalyst reuse cycle as well as catalyst regeneration on the catalytic behaviors were studied. Experimental results showed that Yb₂O₃–Al₂O₃ promoted SZ (designated as SZAY) catalyst exhibited an optimal esterification performance; the Lewis acid sites with moderate and super strong strength could mainly be responsible for the esterification reaction; and doping both Yb₂O₃ and Al₂O₃ on SZ not only boosted the esterification activity but also alleviated catalyst deactivation resulted from the surface sulfur loss by solvation.     

Modification of Al2O3-based catalyst by rare earth elements for steam reforming of methane

Modification of rare earth elements (Ln) often contributes to the improved performance in many applications compared with the monometallic catalysts, but the current research is only limited in the individual light rare earth elements. In this work, a series of Ln elements involving Gd, Sm, Nd, Pr, and La was added into monometallic Ni/Al₂O₃ system to focus how the microstructure of active Ni centers depended on the presence and contents of Ln, and thus improved the catalytic behavior in the steam reforming of methane. Enhanced catalytic behavior could be observed over Ln-modified Ni catalysts, among which Ni-Gd/Al₂O₃ was predicted to be more high-efficient no matter supported on power or spherical Al₂O₃ catalysts. A combined investigation of XPS with STEM and TPR was carried out over the candidates to reveal the structure-performance relationship. Furthermore, this was further applied to noble metal systems, realizing the benign universality of the modification strategy.     

The effect of rare earth oxides on the pressureless liquid phase sintering of α-SiC

Silicon carbide (SiC) ceramics have been fabricated by pressureless liquid phase sintering with Al₂O₃ and rare-earth oxides (Lu₂O₃, Er₂O₃ and CeO₂) as sintering additives. The effect was investigated of the different types of rare earth oxides on the mechanical property, thermal conductivity and microstructure of pressureless liquid phase sintered SiC ceramics. The room temperature mechanical properties of the ceramics were affected by the type of rare earth oxides. The high temperature performances of the ceramics were influenced by the triple junction grain boundary phases. With well crystallized triple junction grain boundary phase, the SiC ceramic with Al₂O₃–Lu₂O₃ as sintering additive showed good high temperature (1300 °C) performance. With clean SiC grain boundary, the SiC ceramic with Al₂O₃–CeO₂ as sintering additive showed good room temperature thermal conductivity. By using appropriate rare earth oxide, targeted tailoring of the demanding properties of pressureless liquid phase sintered SiC ceramics can be achieved.     

Recent Advances on TiO2‐ZrO2 Mixed Oxides as Catalysts and Catalyst Supports

This is the first review of titanium dioxide‐zirconium dioxide (TiO₂‐ZrO₂) mixed oxides, which are frequently employed as catalysts and catalyst supports. In this review many details pertaining to the synthesis of these mixed oxides by various conventional and nonconventional methods and their characterization by several techniques, as reported in the literature, are assessed. These mixed oxides have been synthesized by different preparative analogies and were extensively characterized by employing various spectroscopic and nonspectroscopic techniques. The TiO₂‐ZrO₂ mixed oxides are also extensively used as supports with metals, nonmetals, and metal oxides for various catalytic applications. These supported catalysts have also been thoroughly investigated by different techniques. The influence of TiO₂‐ZrO₂ on the dispersion and surface structure of the supported active components as examined by various techniques in the literature has been contemplated. A variety of reactions catalyzed by TiO₂‐ZrO₂ and supported titania‐zirconia mixed oxides, namely; dehydrogenation, decomposition of chlorofluoro carbons (CFCs), alcohols from epoxides, synthesis of ?‐caprolactam, partial oxidation, deep oxidation, hydrogenation, hydroprocessing, organic transformations, NO_x abatement, and photo catalytic VOC oxidations that have been pursued in the literature are presented with relevant references.     

Highly selective oxidative dehydrogenation of ethane to ethene over layered complex metal chloride oxide catalysts

Complex metal chloride oxides consisting of bismuth, alkali, alkaline earth, chlorine, and oxygen, were synthesized, characterized structurally, and tested as catalysts for the oxidative dehydrogenation of ethane to ethene with molecular oxygen. The catalysts were prepared by high-temperature solid-state reaction of appropriate mixtures of bismuth chloride oxide, bismuth oxide, alkali chloride, and alkaline earth chloride. We found that the catalysts containing strontium as the alkaline earth constituent and potassium as the alkali constituent were highly active and selective for the oxidative dehydrogenation of ethane. SrBi₃O₄Cl₃, which is a fundamental phase of the catalysts, was characterized by single and double chlorine sheets in its layer structure. The catalyst having the composition of KSr₂Bi₃O₄Cl₆ showed an extremely high ethene selectivity, more than 90%, even under high oxygen partial pressure conditions and at high molar ratios of oxygen to ethane, and gave 70% yield of ethene at 640°C under an optimized feed gas composition. This revised version was published online in July 2006 with corrections to the Cover Date.     

Oxidative dehydrogenation of propane over a series of low‐temperature rare earth orthovanadate catalysts prepared by the nitrate method

A series of high‐purity rare earth orthovanadates were prepared by the nitrate method and found to be effective low‐temperature catalysts for the oxydehydrogenation of propane at 320°C, at which no reactions occurred over the catalysts reported in the literature, and, thus, may be of practical significance. The catalytic performances of LnVO₄ (Ln = Y, Ce–Yb) at 500°C were much better than those of rare earth orthovanadate catalysts and also slightly exceeded that of magnesium orthovanadate Mg₃(VO₄)₂ reported in the literature. LnVO₄ (Ln = Y, Ce–Yb) materials were tetragonal active phases which could stabilize the existence of active sites for the oxydehydrogenation of propane. Some catalysts with a certain amount of LnVO₃ reduced from LnVO₄ (Ln = Ho–Yb) under reaction atmosphere exhibited better redox properties and catalytic performances possibly due to the existence of biphasic catalytic synergy. LaVO₄ was a monoclinic unstable active phase, although its bulk structure did not change after reaction. The remarkable deactivation of the LaVO₄ catalyst was probably due to that LaVO₄ could not stabilize the existence of surface active sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Struktur-Aktivitäts/Selektivitäts-Beziehungen von Oxidationskatalysatoren

Structure-Activity/Selectivity Correlation of Oxidation Catalysts Various reactive forms of oxygen are active in the selective oxygen functionalization of olefins and aromatic compounds. These forms are different on different crystal faces of transition metal oxides as shown by their different cation-oxygen bond lengths. Therefore, face specifity of transition metal oxides as catalysts can be expected for the selective oxidation of hydrocarbons. Furthermore, the reactivity of the framework oxygen in the transition metal oxides for selective oxygen functionalization of hydrocarbons is dependent on the nature of the catalytic cycle (one- or two-electron-processes). Therefore, structure sensitivity of oxidation catalysts on the selectivity of transition metal oxides as oxidation catalysts is possible. Starting from this concept the following phenomena of the activity/selectivity relations are discussed for the most important industrial oxidation catalysts:

• —ensemble effects on the selectivity of supported Ag catalysts for the oxidation of ethylene to ethylenoxide.
• —face specifity of multicomponent Bi/Mo-oxide catalysts in the selective oxidation of propylene to acrolein and in the ammoxidation of propylene.
• —structure sensitivity of V₂O₅-containing catalysts in the oxidation of aromatic compounds.

     

Side chain alkylation of 2-picoline with formaldehyde over alkali modified zeolites

The catalytic side-chain alkylation of 2-picoline with formaldehyde (37 wt/v) was studied over alkali and alkaline earth metal ion modified zeolites in vapor phase conditions at atmospheric pressure, and at a reaction temperature of 300°C. A mixture of vinylpyridine and ethylpyridine were formed by the alkylation of the corresponding picoline over Li, Na, K, Rb, Cs, Mg, Ca, Sr and Ba metal ion modified zeolites. The catalytic activity of side-chain alkylation of 2-picoline was studied over various alkali modified zeolite molecular sieves like ZSM-5 (SiO₂/Al₂O₃ = 30), X, Y, Mordenite and MCM-41. Alkali modified ZSM-5 (30) catalyst was found more active in side-chain alkylation of 2-picoline when compared to other zeolites. Among all these catalysts studied K modified ZSM-5 (30) and K-Cs-ZSM-5 (30) gave best conversion of 2-picoline and selectivity to vinylpyridine. Cs-ZSM-5 (30) and K-ZSM-5 (30) were employed to study the reaction parameters like reaction temperature, weight hourly space velocity, molar ratio, and time on stream for 2-picoline independently. The effects of alkali metal ion content (K, Cs) and precursors of potassium ion on catalytic activity in side-chain alkylation was studied. An attempt has been made to correlate between the basicity with the activity of side-chain alkylation. The bifunctional catalyst is required containing medium or weak acidic centers and basic centers in the side-chain alkylation, which is understood through proposed reaction mechanism. The selectivities of 2-vinylpyridine were 81.7, 90.8, and 94.8% at 65.4, 62.1 and 57.2% conversions at 300°C from 2-picoline and formaldehyde over K-ZSM-5 (30), Rb-ZSM-5 (30) and K-Cs-ZSM-5 (30) respectively. Indian Institute of Chemical technology (IICT) communication no: 020707     

Potential rare-earth modified CeO2 catalysts for soot oxidation

The physico-chemical properties of ceria (CeO₂) and rare earth modified ceria (with La, Pr, Sm, Y) catalysts are studied and correlated with the soot oxidation activity with using O₂ and O₂ + NO. CeO₂ modified with La and Pr shows superior soot oxidation activity with O₂ compared with the unmodified catalyst. The improved soot oxidation activity of rare earth doped CeO₂ catalysts can be correlated to the increased meso/micro pore volume and the stabilisation of the external surface area. On the other hand, unreducible ions decrease the intrinsic soot oxidation activity of rare earth modified ceria with both O₂ and NO + O₂ due to the decreased amount of redox surface sites. The catalyst bulk oxygen storage capacity is not a critical parameter in determining the soot oxidation activity. The modification with Pr shows the best soot oxidation with both O₂ and O₂ + NO compared with all other catalysts.