Molecular mechanism of propane oxidative dehydrogenation on surface oxygen radical sites of VO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/TiO<Subscript>2</Subscript> catalysts

Minimum energy pathways of propane oxidative dehydrogenation to propene and propanol on supported vanadium oxide catalyst VO _x /TiO₂ were studied by periodic discrete Fourier transform (DFT) using a surface oxygen radical as the active site. The propene formation pathway was shown to consist of two consecutive hydrogen abstraction steps. The first step includes C_β–H bond activation of propane followed by the formation of a surface hydroxyl group V–O _t H and a propyl radical n-C₃H₇. This step with the activation energy E* = 0.56 eV (54.1 kJ/mol) appears to be rate-determining. The second step involves the reaction of the bridging O _b oxygen atom with the methylene C–H bond of propyl radical n-C₃H₇ followed by the formation of a hydroxylated surface site HO _t –V⁴⁺–O _b H and propene. The initial steps of the C–H bond activation during propane conversion to propanol and propene by ODH on V⁵⁺–(O _t O _b )^? active sites are identical. The obtained results demonstrate that participation of surface oxygen radicals as the active sites of propane ODH makes it possible to explain relatively low activation energies observed for this reaction on the most active catalysts. The presence of very active radical species in low concentration seems to be the key factor for obtaining high selectivity.     

Chemical composition optimization and characterization of the NiO/B<Subscript>2</Subscript>O<Subscript>3</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system as a catalyst for ethylene oligomerization

The samples of the NiO/B₂O₃-Al₂O₃ system with NiO contents from 0.48 to 38.30 wt % were synthesized by the impregnation of borate-containing alumina (20 wt % B₂O₃). It was found that nickel oxide occurred in an X-ray amorphous state in the samples containing to 23.20 wt % NiO. At a NiO content of 4.86 wt % or higher, the support was blocked by the modifier to cause a decrease in the specific surface area from 234 to 176 m²/g and in the amount of acid sites from 409–424 to 333 μmol/g. An extremal character of the dependence of catalyst activity in ethylene oligomerization on NiO content was found with a maximum in the range of 4.86–9.31 wt %. Based on spectroscopic data, it was found that ethylene activation on the NiO/B₂O₃-Al₂O₃ catalyst can be associated with the presence of Ni²⁺ cations, which chemically interact with the support. The catalyst containing 4.86 wt % NiO at 200°C, a pressure of 4 MPa, and an ethylene supply rate of 1.1 h⁻¹ provided almost complete ethylene conversion at the yield of liquid oligomerization products to 90.0 wt %; the total concentration of C⁸⁺ alkenes in these products was 89.0 wt %.     

Monolithic FeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts for ammonia oxidation and nitrous oxide decomposition

(1.2–8.3)%FeO_х/Al₂O₃ monolith catalysts have been prepared by impregnating alumina with aqueous solutions of iron(III) nitrate and oxalate and have been tested in NH₃ oxidation and in the selective decomposition of N₂O in mixtures resulting from ammonia oxidation over a Pt–Rh gauze pack under conditions of nitric acid synthesis (800–900°C). In the case of the support calcined at 1200°C, the catalyst is dominated by bulk Fe₂O₃ particles localized on the Al₂O₃ surface. The activity of these samples in both reactions decreases with a decreasing active component content, thus limiting the potential of Fe₂(C₂O₄)₃ · 5H₂O, an environmentally friendlier but poorly soluble compound, as a substitute for Fe(NO₃)₃ · 9H₂O. Decreasing the support calcination temperature to 1000°C or below leads to the formation of a highly defective Fe–Al–O solid solution in the (1.2–2.7)%FeO_х/Al₂O₃ catalysts. The surface layers of the solid solution are enriched with iron ions or stabilize ultrafine FeO_х particles. The catalytic activity of these samples in both reactions is close to the activities measured for ~8%FeO_х/Al₂O₃ samples prepared using iron nitrate.     

Catalytic properties of Pt/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts in the aqueous-phase reforming of ethylene glycol: Effect of the alumina support

The influence of the alumina support on the catalytic activity of Pt/Al₂O₃ catalysts in aqueous phase reforming of ethylene glycol to hydrogen was studied. The catalysts were prepared by impregnation of γ-, δ-, and α-alumina with H₂PtCl₆. The highest rate of hydrogen production (452 μmol min⁻¹ g⁻¹) obtained with the Pt/α-Al₂O₃ catalyst can be related to the highest extent of dispersion of Pt on α-Al₂O₃. XPS, TEM-EDX and TPR-H₂ measurements showed the absence of chloride-containing surface complexes in the Pt/α-Al₂O₃ catalyst. However, chloride-containing entities were found on the surface of Pt/γ-Al₂O₃ and Pr/δ-Al₂O₃ catalysts. When chloride ions are removed chlorinated Pt species facilitate the sintering of Pt crystallites and in this way affect the extent of Pt dispersion. Moreover, depending upon the particular crystalline form, alumina atoms have different coordination and alumina surfaces contain varying amounts of OH groups of different nature which affect the interaction between Pt and the support.     

Hydrothermal synthesis of nanosized and amorphous alumina in the ZrO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-H<Subscript>2</Subscript>O system

The reciprocal influence of components on the chemical and structural transformations in the ZrO₂-Al₂O₃-H₂O system during nanoparticle formation under hydrothermal conditions is considered. The possibility of formation of amorphous aluminum oxide under hydrothermal conditions is found as a result of the influence of zirconia nanoparticles on the crystallization.     

High-temperature X-ray study of the formation and delamination of manganese-alumina spinel Mn<Subscript>1.5</Subscript>Al<Subscript>1.5</Subscript>O<Subscript>4</Subscript>

The behavior of the manganese-alumina system with Mn:Al = 1:1 on heating in air and vacuum was studied. The starting samples were mixtures of β-Mn₃O₄, α-Mn₂O₃, and γ-Al₂O₃. On heating to 950°C in air, the samples were partially oxidized into α-Mn₂O₃, and corundum α-Al₂O₃ formed along with mixed manganese-alumina cubic spinel, whose composition was close to Mn₂AlO₄. In vacuum at 1200°C, the starting sample with a ratio of Mn:Al = 1:1 transformed into the manganese-alumina spinel Mn_1.5Al_1.5O₄, which retained its cubic structure after slow cooling in vacuum. When cooled in air, this solid solution delaminated, and a nanocrystalline Mn_2.8Al_0.2O₄ phase formed, whose structure was β-Mn₃O₄ type tetragonal spinel.     

Structure and Catalytic Properties of Ceria-based Nickel Catalysts for CO<Subscript>2</Subscript> Reforming of Methane

Carbon dioxide reforming (CDR) of methane to synthesis gas over supported nickel catalysts has been reviewed. The present review mainly focuses on the advantage of ceria based nickel catalysts for the CDR of methane. Nickel catalysts supported on ceria–zirconia showed the highest activity for CDR than nickel supported on other oxides such as zirconia, ceria and alumina. The addition of zirconia to ceria enhances the catalytic activity as well as the catalyst stability. The catalytic performance also depends on the crystal structure of Ni–Ce–ZrO₂. For example, nickel catalysts co-precipitated with Ce_0.8Zr_0.2O₂ having cubic phase gave synthesis gas with CH₄ conversion more than 97% at 800 °C and the activity was maintained for 100 h during the reaction. On the contrary, Ni–Ce–ZrO₂ having tetragonal phase (Ce_0.8Zr_0.2O₂) or mixed oxide phase (Ce_0.5Zr_0.5O₂) deactivated during the reaction due to carbon formation. The enhanced catalytic performance of co-precipitated catalyst is attributed to a combination effect of nano-crystalline nature of cubic Ce_0.8Zr_0.2O₂ support and the finely dispersed nano size NiO _x crystallites, resulting in the intimate contact between Ni and Ce_0.8Zr_0.2O₂ particles. The Ni/Ce–ZrO₂/θ–Al₂O₃ also exhibited high catalytic activity during CDR with a synthesis gas conversion more than 97% at 800 °C without significant deactivation for more than 40 h. The high stability of the catalyst is mainly ascribed to the beneficial pre-coating of Ce–ZrO₂ resulting in the existence of stable NiO _x species, a strong interaction between Ni and the support, and an abundance of mobile oxygen species in itself. TPR results further confirmed that NiO _x formation was more favorable than NiO or NiAl₂O₄ formation and further results suggested the existence of strong metal-support interaction (SMSI) between Ni and the support. Some of the important factors to optimize the CDR of methane such as reaction temperature, space velocity, feed CO₂/CH₄ ratio and H₂O and/or O₂ addition were also examined.     

Solid solutions in the MgO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> system

Coexisting solid solutions with spinel and corundum structure were synthesized at 1773 K and two pressures, 1 bar and 25 kbar. Samples were analyzed by electron microprobe analysis and X-ray powder diffraction. Pressure and temperature were shown to affect the properties of the solid solutions in different ways. Pressure governs the composition of the defect spinel Mg_1−xAl₂O₄, and temperature changes the cation distribution between coexisting phases. This allows one to separate the effects of cation exchange and magnetic contribution to the heat capacity in thermodynamic modeling. The defect spinel itself can form only because γ-Al₂O₃ exists, polymorph with spinel structure.     

Catalytic properties of the VO<Subscript><Emphasis Type="Italic">х</Emphasis></Subscript>/Ce<Subscript>0.46</Subscript>Zr<Subscript>0.54</Subscript>O<Subscript>2</Subscript> oxide system in the oxidative dehydrogenation of propane

Ce_0.46Zr_0.54O₂ solid solution prepared using a cellulose template was employed as a carrier for vanadium catalysts of the oxidative dehydrogenation of propane. The properties of VO _х /Ce_0.46Zr_0.54O₂ catalyst (5 wt % vanadium) are compared with the properties of the neat support. The carrier and catalyst are studied by means of BET, SEM, DTA, XRD, and Raman spectroscopy. It is shown that the CeVO₄ phase responsible for the ODH process is formed upon interaction between vanadate ions and cerium ions on the surface of the solid solution. The catalytic properties of the catalyst and the support are studied in the propane oxidation reaction at temperatures of 450 and 500°C with pulse feeding of the reagent. It is found that the complete oxidation of propane occurs on the support with formation of CO₂ and H₂O. Three products (propene, CO₂, and H₂O) form in the presence of the vanadium catalyst. It is suggested that there are two types of catalytic centers on the catalyst’s surface. It is concluded that the centers responsible for the complete oxidation of propane are concentrated mainly on the carrier, while the centers responsible for propane ODH are on the CeVO₄.     

Kinetics and mechanism of carbon monoxide oxidation on the supported metal complex catalyst PdCl<Subscript>2</Subscript>-CuCl<Subscript>2</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>

The kinetics of carbon monoxide oxidation with atmospheric oxygen on a PdCl₂-CuCl₂/γ-Al₂O₃ catalyst was studied at T = 27°C and an N₂-O₂-CO mixture pressure of 1 atm. The catalyst was prepared by cold impregnation. Three groups of mechanistic hypotheses are considered, and two of them are demonstrated to be consistent with kinetic data, although they differ in the roles of water and oxygen in carbon monoxide oxidation.     

Sol–gel synthesis,characterization and photocatalytic activity of mesoporous TiO<Subscript>2</Subscript>/γ-Al<Subscript>2</Subscript>O<Subscript>3 </Subscript>granules

Mesoporous TiO₂/γ-Al₂O₃ composite granules were prepared by combining sol–gel/oil-drop method, using various titania solution. The product granules can be used as a photocatalyst or adsorbent in moving, fluidized bed reactors. The phase composition and pore structure of the granules can be controlled by calcination temperature and using different titania solution. In the photocatalysis of NH₃ decomposition, TiO₂/γ-Al₂O₃ granules using Degussa P25 powder treated thermally at 450 °C showed the highest catalytic ability. However, TiO₂/γ-Al₂O₃ granules using titania made by hydrothermal method had comparable performance in NH₃ decomposition.     

Facile synthesis of Fe@Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> core-shell nanowires as O<Subscript>2</Subscript> electrode for high-energy Li-O<Subscript>2</Subscript> batteries

Fe@Fe₂O₃ core-shell nanowires were synthesized via the reduction of Fe³⁺ ions by sodium borohydride in an aqueous solution with a subsequent heat treatment to form Fe₂O₃ shell and employed as a cathode catalyst for non aqueous Li-air batteries. The synthesized core-shell nanowires with an average diameter of 50–100 nm manifest superior catalytic activity for oxygen evolution reaction (OER) in Li-O₂ batteries with the charge voltage plateau reduced to ～3.8 V. An outstanding performance of cycling stability was also achieved with a cutoff specific capacity of 1000 milliampere hour per gram over 40 cycles at a current density of 100 mA g^?1. The excellent electrochemical properties of Fe@Fe₂O₃ as an O₂ electrode are ascribed to the high surface area of the nanowires’ structure and high electron conductivity. This study indicates that the resulting iron-containing nanostructures are promising catalyst in Li-O₂ batteries.     

The catalytic performance of different promoted iron catalysts on combined supports Al<Subscript>2</Subscript>O<Subscript>3</Subscript> for carbon dioxide hydrogenation

Global warming, fossil fuel depletion and fuel price increases have motivated scientists to search for methods for the storage and reduction of the amount of greenhouse gases, especially CO₂. The hydrogenation process has been introduced as an emerging method of CO₂ capture and convertion into value-added products. In this study, new types of catalysts are introduced for CO₂ hydrogenation and are compared based on catalytic activity and product selectivity. The physical properties of the samples are specified using BET. Iron catalysts supported on γ-Al₂O₃ with different metal promoters (X = Ni, K, Mn, Cu) are prepared through the impregnation method. Moreover, Fe–Ni catalysts supported on HZSM5-Al₂O₃ and Ce–Al₂O₃ are synthesized. Samples are reduced by pure H₂ and involved in hydrogenation reaction in a fixed bed reactor (H₂/CO₂ = 3, total pressure = 10 MPa, temperature = 523 K, GHSV = 2000, 1250 nml/min). All catalysts provide high conversion in hydrogenation reactions and the results illustrate that the selectivity of light hydrocarbons is higher than that of methane and CO. It is found that Ni has a promoting effect on the conversion fluctuations throughout the reaction with 66.13% conversion. Using combined supported catalysts leads to enhancing catalytic performance. When Fe–Ni/γ–Al₂O₃—HZSM5 is utilized, CO₂ conversion is 81.66% and the stability of the Fe–Ni catalyst supported on Al₂O₃ and Ce–Al₂O₃ furthey improves.     

ZrO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> binary oxides as promising supports for palladium catalysts of hydrodechlorination

Deposited palladium catalysts of the hydrodechlorination of 1,3,5-trichlorobenzene were studied. Pure zirconium and aluminum oxides and ZrO₂-Al₂O₃ mixtures with 1, 5, and 10 mol % Al₂O₃ prepared by coprecipitation were used as supports. Palladium was deposited by the precipitation of its hydroxide on supports. Catalysts on binary supports (ZrO₂ + 1% Al₂O₃ and ZrO₂ + 5% Al₂O₃) exhibited higher activity and stability in hydrodechlorination compared with catalysts on pure supports. The suggestion was made that the high activity and stability of these systems in hydrodechlorination was related to the formation of binary oxide in the interaction of ZrO₂ with palladium oxide at the stage of annealing of the catalyst precursor. Binary oxide, which was a center of the activation of the C-Cl bond, was simultaneously a source of active hydrogen. The presence of various palladium states in catalysts was substantiated by the temperature programmed reduction method.     

Preparation and performance of potassium-based sorbent using SnO<Subscript>2</Subscript> for post-combustion CO<Subscript>2</Subscript> capture

Potassium-based sorbents using γ-Al₂O₃ or TiO₂ as a support or an additive material have disadvantages in terms of their thermal stability and cyclic CO₂ capture. To overcome the shortcomings of these sorbents, a novel potassium-based sorbent (KSnI30) using SnO₂ was developed in this study. The KSnI30 sorbent formed only K₂CO₃ and SnO₂ phases without any inactive alloy species even after calcination at high temperatures (500–700 °C), indicating the good thermal stability of the KSnI30 sorbent regardless of the calcination temperature. Furthermore, the KSnI30 sorbent has an excellent regeneration property (above 98 %), as well as high CO₂ capture capacities (89–94 mg CO₂/g sorbent). Its excellent regeneration property is due to the formation of a KHCO₃ phase without by-products during CO₂ sorption. These results of the present study demonstrate that the SnO₂ shows promise as a new support or an additive material to replace TiO₂ and γ-Al₂O₃ in the preparation of a regenerable potassium-based sorbent for post-combustion CO₂ capture with good thermal stability and excellent regeneration property.     

Thermal activation effect on catalytic systems MnO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> for deep oxidation of hydrocarbons

The effect of thermal activation, sharp increase in the catalytic activity of the system MnO _x -Al₂O₃ in reactions of deep oxidation of CO and hydrocarbons after calcination of the catalyst at 900–1000°C was discovered and investigated. With the use of X-ray phase analysis, X-ray electron spectroscopy, EXAFS, IR spectroscopy, electronic spectroscopy of diffuse reflections, electron microscopy etc. it was established that the effect of thermal activation is related to reversible phase transitions in the system at heating and cooling. On cooling from 1100°C to 650°C disperse particles of cubic spinel of composition Mn_{2.1 ? x} · Al_{0.9 + x} O₄ are conserved on the corundum surface. On further cooling the spinel decomposes and finally the nanocristalline species of β-Mn₃O*₄ containing up to 15 at% of Al³⁺ form and govern the activity.The thermal activation effect was implemented in an industrial catalyst IK-12-40. Joint Stocks Co “KATALIZATOR” produced and supplied to customers hundreds of tons of this catalyst. The catalyst was awarded with a silver medal of the International exhibition EUREKA in Brussels (1995).     

No-Glycerol rapid heterogeneous biodiesel production on K<Subscript>2</Subscript>CO<Subscript>3</Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst by coupling process

Biodiesel containing almost no glycerol has been produced by coupling reaction carried out over K₂CO₃ supported by calcium oxide as solid base catalysts. The solid base catalysts synthesized by wet impregnation exhibit an exceedingly high activity in biodiesel production. It was found that the reaction time required for the highest yield of biodiesel, 99.2%, can be shortened to 30 min over K₂CO₃/Al₂O₃ under the optimum reaction conditions: 8: 1: 1 molar ratio of methanol/DMC/oil, 30 wt % K₂CO₃/Al₂O₃ catalyst, and 65°C reaction temperature. Solid basic catalysts examined in the study were characterized by BET surface area, XRD, CO₂-TPD, and SEM techniques. The strong interaction between K₂CO₃ and the support yields a new basic active site, which can be probably responsible for the high activity of K₂CO₃/Al₂O₃.     

Catalytic performances and correlations with metal oxide band gaps of metal-tungsten mixed oxide catalysts in propane oxydehydrogenation

Summary Tungsten-based catalysts of the form X wt.% Na₂WO₄/support (where X = 5, 10, 50 or 75 and the support is either γ-Al₂O₃ or SiO₂) were prepared by impregnation method and tested for oxidative dehydrogenation of propane. The effects of Na₂WO₄ loadings on catalyst performances were more pronounced on γ-Al₂O₃ - supported sample. Individual first-row transition metals (V, Cr, Mn, Co, Ni or Zn) were doped on NaWSi. CoWSi exhibited the best improvement, with conversion of 10.7% and selectivity to propylene of 52.9% at 450^oC. The metal oxide band gaps were correlated with catalyst performances. At a constant propane conversion, strong correlations were observed between the band gaps and respective CO and CO₂ yields. This offers insight into relationship between the species conductivities and the products distributions.     

Electric Conductivity of α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Suspensions in Carbonate and Carbonate-Chloride Melts

The temperature dependence of the specific electric conductivity of suspensions based on carbonate and carbonate-chloride melts immobilized with α-Al₂O₃ powder was studied. For carbonate-chloride melts immobilized with α-Al₂O₃, the decrease in the specific electric conductivity considerably exceeded the effect associated with a decrease in the volume fraction of charge carriers. The Raman spectroscopy and Xray diffraction analysis showed that there was no chemical interaction of α-Al₂O₃ with the carbonate and carbonate-chloride melt that could lead to the formation of additional nonconducting phases in these systems in the temperature range under study.     

Specific features of the interaction of α- and γ-modifications of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> with carbonate and carbonate-chloride melts

Data on reactivities of α- and γ-Al₂O₃ finely dispersed powders in a melted carbonate eutectic (Li₂CO₃–Na₂CO₃–K₂CO₃)_eut and carbonate-chloride mixture 0.72(Li₂CO₃–Na₂CO₃–K₂CO₃)_eut–0.28NaCl were obtained. The methods of synchronous thermal and X-ray phase analyses and Raman spectroscopy confirmed that, in contrast to γ-Al₂O₃, α-Al₂O₃ does not chemically interact with the melted carbonate eutectic and carbonate-chloride mixture (Li₂CO₃–Na₂CO₃–K₂CO₃)_eut–NaCl can be recommended as a thickening agent for a carbonate fuel cell.