Hydration effects of Al2(MoO4)3 and AlPO4 phases in hydrotreating catalysts studied by solid state nuclear magnetic resonance spectroscopy

Solid state nuclear magnetic resonance spectroscopy of²⁷Al,³¹P, and⁹⁵Mo nuclei was used to investigate species on a NiPMo/Al₂O₃ hydrotreating catalyst calcined at 750 °C before and after rehydration. The Al₂(MoO₄)₃ phase disappeared after rehydration in 100% relative humidity at room temperature. However, almost none of the observed crystalline AlPO₄ reacted with adsorbed water. The results show that the Al₂(MoO₄)₃ phase on an alumina surface reacts easily with moisture unlike Al₂(MoO₄)₃ in the bulk phase. It also suggests that the crystalline AlPO₄ phase is formed under the Al₂(MoO₄)₃ phase or that both species are formed as neighboring islands on the surface of the alumina but have different reactivities toward moisture.     

Fluoride treatment of AlPO4-Al2O3 catalysts. II. Poisoning experiments by bases for cyclohexene conversion and cumene cracking

Brønsted acid sites on fluoride-modified AlPO₄-Al₂O₃ (2.5 wt% F; APAl-P-2.5F) catalyst are poisoned by the presence of 2,6-dimethylpyridine (DMPY) and hexamethyldisilazane (HMDS), thus decreasing the catalytic activity for cyclohexene and cumene reaction processes, while the effect of pyridine (PY) was scarce. Besides, the drop in activity for cyclohexene conversion was accompanied by a change in reaction selectivity so that hydrogen transfer sites are much more sensitive to base poisoning (getting greater as the poisoning effect increased) than isomerization sites. Moreover, surface trimethylsilyl (TMS) complexes (formed by covalent reaction of HMDS with surface hydroxyls) decomposed and thus, the activity progressively increased at increasing time intervals, thus reaching greater values (at ca. 4 h) than the unpoisoned APAl-P-2.5F catalyst. So, DMPY was more suitable than PY and HMDS for the poisoning of Brønsted acid sites on APAl-P-F catalyst.     

Continuous flow toluene methylation over AlPO4 and AlPO4-Al2O3 catalysts

Toluene methylation with methanol over AlPO₄ (AP) and AlPO₄-Al₂O₃ (APAl) catalysts, obtained through different methods, was carried out in a continuous down-flow fixed bed reactor. The main products were xylenes (XYL), although trimethylbenzenes (TMB) were also found over APAl catalysts. The benzene and ethylbenzene selectivities increased slightly with time on stream at the expense of XYL and TMB selectivities. Isomer distribution was approximately 50, 24 and 26 mol% foro-, m- andp-XYL, and 72, 27 and 0 mol% for 1,2,3-, 1,2,4- and 1,3,5-TMB. The initial reaction rate constants were higher on APA1 catalysts and, furthermore, APAl catalysts exhibited similar catalytic activities, although those obtained in ethylene or propylene oxide are the most active ones. The same occurs on AP catalysts. Moreover, the changes in catalytic activity are similar to the changes in the acidic characteristics measured, in gas phase, versus pyridine. Furthermore, the activity decreased with time on stream due to coke deposition according to the expressionk = k₀ exp(-t). The rate of deactivation, evaluated from the deactivation coefficients (), was greater for APAl than for AP catalysts.     

Partial oxidation of methane over nickel catalysts supported on various aluminas

Partial oxidation of methane (POM) was systematically investigated in a fixed bed reactor over 12 wt% Ni catalysts supported on α-A1₂O₃, γ-A1₂O₃ and θ-A1₂O₃ which were prepared at different conditions. Results indicate that the catalytic activity toward POM strongly depends on the BET surface area of the support. All the Ni/ θ-Al₂O₃ catalysts showed high activity toward POM due to the less formation of inactive NiAl₂O₄ species, the existence of NiO, species and stable θ-Al₂O₃ phase. Although Ni/γ-Al₂O₃ showed the highest activity toward POM, long-time stability cannot be expected as a result of the deterioration of the support at higher temperature, which is revealed from BET results. From the reaction and characterization results, it is inferred that the optimal conditions for the preparation of θ-Al₂O₃ are 1,173 K and 12 h.     

Molecular engineering of microporous crystals: (III) The influence of water content on the crystallization of microporous aluminophosphate AlPO4-11

The influence of water content on the crystallization of the microporous aluminophosphate AlPO₄-11 was investigated by a combination of in situ UV Raman and ex situ XRD and NMR characterizations. Under high water content conditions (Al₂O₃:H₂O = 1:20-69), an AlPO₄-5 intermediated formed first and later co-existed with AlPO₄-11. In the last stage of crystallization, the crystalline AlPO₄-5 completely disappeared and well-crystallized AlPO₄-11 was obtained. The position of the three signals observed in the ³¹P MAS NMR spectra of the isolated solid samples did not change during the hydrothermal treatment, suggesting that the fragments containing the P site in the junction of 6- and 4-membered rings of both AlPO₄-5 and AlPO₄-11 formed first and that the environments of the P sites were very similar to those in the final structure. A significant increase of Al concentration in liquid media was observed before the appearance of XRD detectable AlPO₄-11. Under low water content conditions (Al₂O₃:H₂O = 1:15), the AlPO₄-11 was directly crystallized from the initial mixture. In situ Raman spectra showed that the 10-membered rings were complete by the time the transformation of octahedral Al to tetrahedral Al was finished and that the conformation of the protonated di-(i-propyl)amine changed little during the crystallization. The present study showed that the composition of the liquid phase is critical to the formation of a specific structure.     

Deterioration Mode of Barium-Containing NOx Storage Catalyst

Barium-containing NO _x storage catalyst showed serious deactivation under thermal exposure at high temperatures. To elucidate the thermal deterioration of the NO _x storage catalyst, four types of model catalyst, Pt/Al₂O₃, Ba/Al₂O₃, Pt–Ba/Al₂O₃, and a physical mixture of Pt/Al₂O₃ + Ba/Al₂O₃ were prepared and their physicochemical properties such as BET, NO TPD, TGA/DSC, XRD, and XPS were evaluated while the thermal aging temperature was increased from 550 to 1050°C. The fresh Pt–Ba/Al₂O₃ showed a sorption capacity of 3.35 wt%/g-cat. but the aged one revealed a reduced capacity of 2.28 wt%/g-cat. corresponding to 68% of the fresh one. It was found that this reduced sorption capacity was directly related to the deterioration of the NO _x storage catalyst by thermal aging. The Ba on Ba/Al₂O₃ and Pt–Ba/Al₂O₃ catalysts began to interact with alumina to form Ba–Al solid alloy above 600°C and then transformed into stable BaAl₂O₄ having a spinel structure. However, no phase transition was observed in the Pt/Al₂O₃ catalyst having no barium component, even after aging at 1050°C.     

Acid-Base Properties of Alumina-Supported M2O3 (M=B, Ga, In) Catalysts

The acid-base properties of -Al₂O₃ and alumina-supported B₂O₃, Ga₂O₃ and In₂O₃ have been determined by microcalorimetry of ammonia and sulfur dioxide adsorption. From the adsorption of NH₃, it was found that the addition of B₂O₃ on alumina leads to an increase of the number of acid sites, while Ga₂O₃ and In₂O₃ additives caused a decrease in the acidity of alumina. Using SO₂ as a probe molecule to study the basicity, the number of surface basic sites on alumina was found to be strongly decreased by the addition of boron oxide, while it was only slightly affected by the addition of gallium oxide and decreased by the addition of indium oxide. The differential heats of adsorption are discussed as a function of the coverage by the probe molecules. The electronic properties of the oxides are examined in order to explain the acid-base properties of the supported oxides.     

Effect of peptiser species on crystallisation of alumina gel produced by sol–gel process

The present work reports a study on the effect that a peptiser species has on the crystallisation of alumina gel produced by a sol–gel process to help develop a method for producing α-Al₂O₃ at low temperature. The white precipitate of aluminium hydroxide, which was prepared with a homogeneous precipitation method using aluminium nitrate and urea in an aqueous solution, was peptised using various peptisers at room temperature to form a transparent alumina sol. The alumina gel obtained from the alumina sol, which was produced using formic acid as the peptiser, was most dominantly crystallised into α-Al₂O₃ by annealing at 900°C. The optimal [peptiser]/[Al³⁺] (P/A) molar ratio for the crystallisation into α-Al₂O₃ was 0.2. The alumina gel began to crystallise into α-Al₂O₃ with annealing at as low as 500°C when formic acid and a P/A ratio of 0.2 were used.     

Nanocrystalline Brookite-Type Titanium(IV) Oxide Photocatalysts Prepared by a Solvothermal Method: Correlation Between Their Physical Properties and Photocatalytic Activities

Mesoporous MSU-X alumina molecular sieves were synthesized through the neutral N⁰I⁰ assembly pathway using aluminum sec-butoxide as the precursor and a triblock poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol) as the structure-directing agent. Their surface acidity and basicity as well as textural properties were characterized by NH₃-TPD, CO₂-TPD, IR spectroscopy and N₂ adsorption methods. The materials are abundant in surface weak acid sites and weak-medium basic sites. They are more active than the commercial -Al₂O₃ catalyst for the CS₂ hydrolysis reaction, and the temperature of 90 conversion of CS₂ on the mesoporous alumina is 130K lower than that of -Al₂O₃.     

AlPO4-coated mullite/alumina fiber reinforced reaction-bonded mullite composites

A precursor for reaction-bonded mullite (RBM) is formulated by premixing Al₂O₃, Si, mullite seeds and mixed-rare-earth-oxides (MREO). An ethanol suspension thereof is stabilized with polyethyleneimine protonated by acetic acid. The solid in the suspension is infiltrated into unidirectional mullite/alumina fiber-preforms by electrophoretic infiltration deposition to produce fiber-reinforced, RBM green bodies. Crack-free composites with ≤25% porosity were achieved after pressureless sintering at 1300 °C. Pre-coating the fibers with AlPO₄ as a weak intervening layer facilitates significant fiber pullout on composite fracture and confers superior damage tolerance. The bend strength is 170 MPa at 25 °C ≤ T ≤ 1100 °C. At 1200 °C, the composite fails in shear due to MREO-based, glassy phase formation. However, the AlPO₄ coating acts as a weak layer even after thermal aging at 1300 °C for 100 h.     

Influence of the Storage Material on the Storage of NOx at Low Temperatures

The NO _x storage performance at low temperature (100–200 °C) has been studied for model NO _x storage catalysts. The catalysts were prepared by sequentially depositing support, metal oxide and platinum on ceramic monoliths. The support material consisted of acidic aluminium silicate, alumina or basic aluminium magnesium oxide, and the added metal oxide was either ceria or barium oxide. The NO _x conversion was evaluated under net-oxidising conditions with transients between lean and rich gas composition and the NO _x storage performance was studied by isothermal adsorption of NO₂ followed by temperature programmed desorption of adsorbed species. The maximum in NO _x storage capacity was observed at 100 °C for all samples studied. The Pt/BaO/Al₂O₃ catalyst stored about twice the amount of NO _x compared with the Pt/Al₂O₃ and Pt/CeO₂/Al₂O₃ samples. The storage capacity increased with increasing basicity of the support material, i.e. Pt/Al₂O₃·SiO₂ < Pt/Al₂O₃ < Pt/Al₂O₃ · MgO. Water did not significantly affect the NO _x storage performance for Pt/Al₂O₃ or Pt/BaO/Al₂O₃.     

Characterization and reactivity of MoO3 supported on AlPO4

A series of MoO₃/AlPO₄ catalysts with molybdena content varying from 2 to 16 wt% were prepared and characterized by low temperature oxygen chemisorption (LTOC), ammonia chemisorption, X-ray diffraction (XRD) and electron spin resonance (ESR). Maximum O₂ uptake was observed at 6 wt% MoO₃ loading indicating the completion of monolayer. The ESR results are in conformity with LTOC and XRD data. The activities of the catalysts were tested in methanol partial oxidation and are correlated with their surface characteristics wherever possible.IICT Communication No. 3188.     

The effect of steam treatment on MeO/Al2O3 model catalysts (Me = Ni,Co, Cu,Fe)

To study the influence of steam on the solid state reaction between MeO (Me = Ni, Co, Cu or Fe) and Al₂O3, MeO/-Al₂O₃ and MeO/-Al₂O₃ model catalysts were kept in either N₂/20% O₂ or N₂/O₂/30% H₂O at 500–1000°C. The samples were subsequently analyzed with RBS and FTIR. Surprisingly, nickel, cobalt and copper volatilized when MeO/-Al₂O₃ or MeAl₂U₄/-Al₂O₃ samples were annealed in the presence of 0.3 atm steam at 1000°C. Especially copper was found to volatilize very rapidly in the presence of steam, even at a temperature as low as 800°C. FTIR spectra of steam-treated NiO/-Al₂O₃ samples showed the incorporation of hydroxyl groups in the nickel oxide layer. This observation and an excellent agreement with thermochemical calculations support our conclusion that the volatile species are metal hydroxides. The solid state reaction of MeO with-Al₂O₃ was found to proceed at a much higher rate in the presence of 0.3 atm steam at 500–800°C, presumably as a result of an enhanced surface mobility of Me and Al ions along the grain boundaries and the surfaces of the internal pores of the-Al₂O₃ support, when steam is present.     

Role of Support in Lean DeNOx Catalysis

FTIR and pulse thermal analysis were applied to investigate catalysts containing Pt (1 wt%)/Ba (17 wt%) supported on -Al₂O₃, SiO₂ and ZrO₂. The aim was to learn how the support material affects the thermal stability of barium carbonate and its activity in the reaction to bulk Ba(NO₃)₂. The lower thermal stability of BaCO₃ in alumina supported samples was found to influence the formation of barium nitrate during the NO _x storage process. Quantification of Ba(NO₃)₂ formed during NO _x storage indicated that for alumina supported catalysts only ca. 30% of barium present in the sample is involved in the storage process. The low thermal stability found for alumina supported barium nitrite excludes its role in the formation of barium nitrate during interaction of NO _x with the catalyst at 300 °C. The studies indicate that -Al₂O₃ plays a major role in influencing the thermal stability of BaCO₃ and Ba(NO₃)₂. This finding seems to be relevant for the higher activity of -Al₂O₃-supported catalysts in NO _x storage reduction reactions.     

Low temperature sintering of Si3N4 ceramics by spark plasma sintering technique

Abstract

Abstract

Low temperature sintering of α‐Si₃N₄ matrix ceramics was developed in the present study using 4?wt‐%MgO together with Al₂O₃ or AlPO₄ as the sintering additives and spark plasma sintering technique. The results suggested that α‐Si₃N₄ ceramics could be densified at low sintering temperature by adjusting both the sintering temperature and sintering additive content. For low temperature sintered α‐Si₃N₄ ceramics, using MgO and Al₂O₃ as the sintering additives, the densification is not complete at a temperature lower than 1600°C, and the mechanical strength is <200?MPa. When MgO and AlPO₄ were used as the sintering additives, the increase in AlPO₄ content not only declines the sintering temperature but also promotes the mechanical property of the sintered Si₃N₄ ceramics. It was the AlPO₄ phosphate binder that played a significant role in low temperature sintering of Si₃N₄ ceramics.     

Preparation and characterization of NiO/MgO/Al2O3 supported CoPcS catalyst and its application to mercaptan oxidation

MgO/Al₂O₃ and NiO/MgO/Al₂O₃ solid bases were prepared by mixing method. The samples were characterized by X-ray diffraction (XRD), CO₂ temperature-programmed desorption (CO₂-TPD) and surface area measurements. After supported sulfonated cobalt phthalocyanine (CoPcS) the catalytic performance of these catalysts was evaluated in the mercaptan oxidation reaction. The effect of Mg/Al mole ratios on activity, crystal structure, basicity and stability in air was discussed. And the mechanism of the effect of NiO was identified. Results show that the base amount of MgO/Al₂O₃ increases with increasing Mg/Al mole ratio and catalyst with high Mg/Al mole ratio has a higher initial activity. NiO/MgO/Al₂O₃–CoPcS shows a higher initial activity and a much longer lifetime than MgO/Al₂O₃–CoPcS. When nickel oxide is doped into the MgO/Al₂O₃ support more crystal defects are generated, which increases the amount and types of basic sites.     

Preparation of Ni/Al2O3 catalysts from vapor phase by atomic layer epitaxy

Ni/Al₂O₃ catalysts were prepared by saturating gas-solid reactions as an atomic layer epitaxy (ALE) process. Vaporized Ni(acac)₂ was chemisorbed on a porous alumina support, and the produced surface complex was then air treated to remove the ligand residues. The nickel content could be precisely controlled by repeating this reactor cycle. On alumina preheated at 800°C, the nickel content varied from 3 to 21 wt%, when the number of reaction cycles was increased from one to ten. The performance of the Ni-catalysts was evaluated in the gas-phase hydrogenation of toluene. The preheat temperature of alumina influenced the activity of the catalyst, and a maximum in the activity was observed for catalysts prepared from alumina preheated at 875°C. Catalysts prepared by four reaction cycles, containing about 10 wt% nickel, gave the highest utilization of nickel.     

Support modification to improve the sulphur tolerance of Ag/Al2O3 for SCR of NOx with propene under lean-burn conditions

Ag/Al₂O₃ catalysts with 1 wt% SiO₂ or TiO₂ doping in alumina support have been prepared by wet impregnation method and tested for sulphur tolerance during the selective catalytic reduction (SCR) of NO_x using propene under lean conditions. Ag/Al₂O₃ showed 44% NO_x conversion at 623 K, which was drastically reduced to 21% when exposed to 20 ppm SO₂. When Al₂O₃ support in Ag/Al₂O₃ was doped with 1 wt% SiO₂ or TiO₂ the NO_x conversion remained constant in presence of SO₂ showing the improved sulphur tolerance of these catalysts. Subsequent water addition does not induce significant deactivation. On the contrary, a slight promotional effect on the activity of NO conversion to nitrogen is observed after Si and Ti incorporation. FTIR study showed the sulphation of silver and aluminum sites of Ag/Al₂O₃ catalysts resulting in the decrease in the formation of reactive intermediate species such as –NCO, which in turn decreases NO_x conversion to N₂. In the case of Ag/Al₂O₃ doped with SiO₂ or TiO₂, formation of silver sulphate and aluminum sulphate was drastically reduced, which was evident in FTIR resulting in remarkable improvement in the sulphur tolerance of Ag/Al₂O₃ catalyst. These catalysts before and after the reaction have been characterized with various techniques (XRD, BET surface area, transmittance FTIR and pyridine adsorption) for physico-chemical properties.     

The catalytic performance in oxidative coupling of methane and the surface basicity of La2O3, Nd2O3, ZrO2 and Nb2O5

The catalytic performance in oxidative coupling of methane (OCM) of unmodified pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ has been investigated under various conditions. The results confirmed that the activity of La₂O₃ and Nd₂O₃ was always much higher than that of the remaining two. The surface basicity/base strength distribution of pure La₂O₃, Nd₂O₃, ZrO₂ and Nb₂O₅ was measured using a test reaction of transformation of 2-butanol and a temperature-programmed desorption of CO₂. Both methods showed that La₂O₃ and Nd₂O₃ had high basicity and contained medium and strong basic sites (lanthanum oxide more and neodymium oxide somewhat less). ZrO₂ had only negligible amount of weak basic sites and Nb₂O₅ was rather acidic. The confrontation of the basicity and catalytic performance indicated that in the case of investigated oxides, the basicity (especially strong basic sites) could be a decisive factor in determination of the catalytic activity in OCM. Only in the case of ZrO₂ it was observed a moderate catalytic performance in spite of negligible basicity. The influence of a gas atmosphere used in the calcination of oxides (flowing oxygen, helium and nitrogen) on their basicity and catalytic activity in OCM had been also investigated. Contrary to earlier observations with MgO, no effect of calcination atmosphere on the catalytic performance of investigated oxides in OCM and on their basicity was observed.     

Comparison of AlPO4- and Co3(PO4)2-coated LiNi0.8Co0.2O2 cathode materials for Li-ion battery

Electrochemical and thermal properties of Co₃(PO₄)₂- and AlPO₄-coated LiNi_0.8Co_0.2O₂ cathode materials were compared. AlPO₄-coated LiNi_0.8Co_0.2O₂ cathodes exhibited an original specific capacity of 170.8 mAh g⁻¹ and had a capacity retention (89.1% of its initial capacity) between 4.35 and 3.0 V after 60 cycles at 150 mA g⁻¹. Co₃(PO₄)₂-coated LiNi_0.8Co_0.2O₂ cathodes exhibited an original specific capacity of 177.6 mAh g⁻¹ and excellent capacity retention (91.8% of its initial capacity), which was attributed to a lithium-reactive Co₃(PO₄)₂ coating. The Co₃(PO₄)₂ coating material could react with LiOH and Li₂CO₃ impurities during annealing to form an olivine Li_xCoPO₄ phase on the bulk surface, which minimized any side reactions with electrolytes and the dissolution of Ni⁴⁺ ions compared to the AlPO₄-coated cathode. Differential scanning calorimetry results showed Co₃(PO₄)₂-coated LiNi_0.8Co_0.2O₂ cathode material had a much improved onset temperature of the oxygen evolution of about 218 °C, and a much lower amount of exothermic-heat release compared to the AlPO₄-coated sample.