Cobalt catalysts derived from hydrotalcite-type precursors applied to steam reforming of ethanol

Catalysts derived from Co/Mg/Al hydrotalcite-type precursors modified with La and Ce were characterized by XANES and tested in ethanol steam reforming. The reaction data showed that, with a molar ratio of water:ethanol = 3:1 in the feed, addition of Ce and La favored acetaldehyde production. Increasing the water content (water:ethanol = 5:1) decreased the acetaldehyde formation by favoring the adsorption of water molecules on these samples, enhancing the acetaldehyde conversion.     

Nanosize cobalt oxide-containing catalysts obtained through microwave-assisted methods

Nanosized cobalt mixed oxide, with high stability of Co(II), were obtained by controlled thermal decomposition of layered double hydroxides (LDHs) with Co, Zn and Al in the brucite-type sheets. The effect of the microwave ageing treatment of the LDHs into the properties of the calcined products was studied. For this reason, the precursors were submitted to increasing periods of time to microwave-hydrothermal treatment. The use of microwave ageing yields to well-crystallised compounds without oxidation of Co²⁺ species to Co³⁺. The structure features of oxides remain during whole ageing process; the microwave-hydrothermal treatment yield, in all cases, spinel-like compounds. Studies of surface properties, N₂ adsorption/desorption and the TEM micrographs, show that the textural properties are strongly modified depending on the irradiation time were submitted to, altering both the pore size distribution and the particle size of the nano-oxides.     

Effect of porous structure on the performance of bidispersive catalysts for model reforming reactions

Making use of experimentally determined structural and kinetic parameters of the catalyst, a method was proposed, which makes it possible to assess the specific activity and the performance of the catalyst in the model reaction of reforming. Three reforming catalysts differing in activity were tested. The contribution of catalytic activity, porous structure and process temperature to the efficiency factors defined in this paper was analysed for each catalyst. The advantage of using bidispersive catalysts was substantiated, and the usefulness of the presence of macropores in the catalyst structure was determined numerically. Of the structural parameters affecting the performance of the porous bidispersive catalysts, macropore volume and macropore radius were found to be the most important. In the course of the study, the parameters of the porous structure were optimised for a catalyst of choice, which was used in the dehydrocyclisation of n-heptane and in the dehydrogenation of methylcyclohexane at different temperatures.     

Zn, Al, Rh-mixed oxides derived from hydrotalcite-like compound and their catalytic properties for N2O decomposition

The catalytic decomposition of nitrous oxide was studied over Zn, Al, Rh-mixed oxides derived from hydrotalcite-like compounds (HTlc) as a precursor. The study showed that, when the Zn/Al atomic ratio was 3, the rate of ZnAlRh-HTlc for N₂O decomposition increased with Rh loading up to 1.4 wt% (4.55 × 10⁴ μmol g⁻¹ h⁻¹ at 400°C), and levels off with further increase in Rh loading. Analogous behavior was found in the presence of NO₂ and water. In the presence of 0.5% water, the activity decreased with decreasing Zn/Al ratio, whereas the activity for N₂O decomposition in the presence of 0.1% NO₂ reached a maximum when the Zn/Al ratio was 3. The observed catalytic activities were comparable to those of the reported catalysts in the presence of NO₂ and water.     

Effect of Mn/Al ratio in Co–Mn–Al mixed oxide catalysts prepared from hydrotalcite-like precursors on catalytic decomposition of N2O

The Co–Mn–Al mixed oxide catalysts were prepared by thermal decomposition of hydrotalcite-like precursors with Co/(Mn + Al) molar ratio of 2 and Mn/Al molar ratio varying from 0 to 2. The obtained catalysts were characterized by powder XRD, XPS, BET surface area and TPR measurements and tested in N₂O decomposition. The most active Co4MnAl catalyst exhibited both the optimum Mn/Al molar ratio and the optimum amount of components reducible in the temperature region in which the catalytic reaction proceeds (350–450 °C).     

Catalytic combustion of methane over mixed oxides derived from Co-Mg/Al ternary hydrotalcites

Co_xMg_3 − x /Al composite oxides (xCoMAO-800) were prepared by calcination of Co_xMg_3 − x/Al hydrotalcites (x = 0.0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, respectively) at 800 °C. The materials were characterized using XRD, TG-DSC, N₂ adsorption-desorption and TPR. The methane catalytic combustion over the xCoMAO-800 was assessed in a fixed bed micro-reactor. The results revealed that cobalt can be homogenously dispersed into the matrices of the hydrotalcites and determines the structure, specific surface areas and porosity of the derived xCoMAO-800 oxide catalysts. The thermal stability and homogeneity of the hydrotalcites markedly depends on the cobalt concentration in the hydrotalcites. The Co-based hydrotalcite-derived oxides exhibit good activity in the catalytic combustion of methane. The catalytic activity over the xCoMAO-800 oxides enhances with increasing x up to 1.5, but subsequently decreases dramatically as cobalt loadings are further increased. The 1.5CoMAO-800 catalyst shows the best methane combustion activity, igniting methane at 450 °C and completing methane combustion around 600 °C. The catalytic combustion activity over the xCoMAO-800 oxides are closely related to the strong Co-Mg/Al interaction within the mixed oxides according to the TG-DSC, TPR and activity characteristics.     

Synthesis of Ti-containing mesoporous silicates from inorganic titanium sources

Titanium-containing mesoporous molecular sieves including periodic mesoporous silicas (SBA-15-type) and organosilicas (PMO-type) can be assembled by using mixed inorganic acid–base pairs (TiCl₄ and tetrabutyl titanate) or a single inorganic TiCl₃ as the titanium sources and tetraethoxysilane and/or 1,2-bis(triethoxysilyl)ethane as the silica sources and triblock copolymer as the structure-directing agent in acidic media through the hydrothermal method. Characterization using XRD, nitrogen sorption isotherms, UV–vis, FT-IR and NMR techniques reveals that the Ti-containing mesoporous materials possess ordered 2D hexagonal mesostructures, high surface areas (421–1070 m²/g), uniform pore sizes (5.1–8.0 nm), large pore volumes (0.5–1.3 cm³/g), and tetrahedrally incorporated titanium (IV) species in the silica network. The maximum incorporated Ti content is about 0.34 wt% for the ordered mesostructure regardless of the titania and silica sources and the initial Si/Ti ratio.     

Biogas reforming over La-NiMgAl catalysts derived from hydrotalcite-like structure: Influence of calcination temperature

Hydrotalcite-like compound with general formula [M(II)_1 − xM(III)_x(OH)₂]^x+(Aⁿ⁻_x/n) ^· mH₂O, where Aⁿ⁻ is the compensation anion, has been used as precursor of active catalysts for biogas reforming. This precursor was calcined at six different temperatures between 250 and 750 °C and the resulting catalysts were tested in order to evaluate the influence of the calcination temperature on the catalytic activity and stability. XRD characterization showed that from 250 °C the hydrotalcite structure is no longer detected, leading to Mg(Ni,Al)O solid solutions, where no peaks related to lanthanum appear. An increase on the calcination temperature increased the grain size and cell parameter value. 50 h-catalytic tests were carried out at 700 °C, CH₄:CO₂ molar ratio of 1:1 and a mass/feed alimentation ratio (W/F) of 0.4 mg min cm^− 3. Used catalysts were characterized by temperature programmed oxidation (TPO), scanning electron microscopy (SEM) and Raman spectroscopy in order to obtain information about coke deposition. Catalytic tests highlighted the great influence of calcination temperature over catalytic activity and stability, having found that, as a general trend, calcination temperatures below 750 °C decrease both the stability and catalytic activity, with the exception of the catalyst calcined at 550 °C, where a higher activity was achieved but with a comparatively low stability.     

Deactivation of the metal and acidic functions for Pt, Pt-Sn and Pt-Sn-K using physically mixed catalysts

The effect of K addition on the amount and dispersion of carbon deposition on metal sites and support sites was investigated on a physical mixture for hexane dehydrogenation. TPO, BET and ESR experiments were used for characterization. The K addition significantly decreases catalyst deactivation involving the amount of coke deposits and the density of carbon radicals on the metal and support sites because of ensemble and electronic effects, especially on the metal sites. Coke on the metal sites associated with carbonaceous species rich in hydrogen is less polymerized than coke on the support sites, corresponding to a more graphitic-like carbon.     

Adsorption of carbon dioxide on hydrotalcite-like compounds of different compositions

The adsorption of carbon dioxide on hydrotalcite-like compounds was investigated. Two different powdered hydrotalcites were used containing the cations nickel and iron. The powdered materials were screened for carbon dioxide adsorption using a thermogravimetric method and it was found that NiMgAl (Sample 1) hydrotalcite has the largest capacity for CO₂, adsorbing 1.58 mmol g⁻¹ at 20 °C, and highest rate of adsorption of up to 0.17 mmol g⁻¹ min⁻¹. This represented an increase of 53% in adsorption capacity, compared with NiMgAlFe (Sample 2). In order to improve the rheological behaviour of hydrotalcite paste for extrusion, hydrotalcite powders were combined with boehmite alumina (70:30 and 50:50 ratios of hydrotalcite:boehmite) before extrusion into pellets suitable for use in a fixed bed adsorber. These pellets were then re-crushed and further tested by thermogravimetric methods. The effects of temperature, composition and pre-treatment of the hydrotalcites on the adsorption of carbon dioxide and nitrogen are reported. At 20 °C, the amount of carbon dioxide adsorbed was between 2.0 and 2.5 mmol g⁻¹ for all the hydrotalcite/alumina samples in this study, although this decayed rapidly with increasing temperature. The results are compared with silica gel as a common sorbent reference, and with literature values. Hydrotalcite/alumina samples have thermal stability and a high adsorption capacity for carbon dioxide over a wide range of temperatures. The composition of the hydrotalcite/alumina pellets investigated in this study has less effect upon the adsorption behaviour compared with the non-calcined hydrotalcite powder, thus allowing a wide choice of pellet compositions to be used.     

Mn-containing catalytic materials for the total combustion of toluene: The role of Mn localisation in the structure of LDH precursor

Mg/Mn/Al mixed oxide systems of similar atomic ratios close to 2/0.5/1 were obtained by calcination of Mg,Al layered double hydroxide (LDH) type precursors containing Mn either in the cationic form within the brucite layer or as permanganate anions in the interlayer. The materials were characterised with PXRD, thermal analysis, XPS, ESR, TPR and BET. In mixed oxides derived from interlayer-doped precursor PXRD identified MgO–MnO solid solution and poorly crystalline Al-rich spinel phase, while those obtained from layer-doped LDH contained a better crystalline Mn-rich spinel. Surface of both materials was covered with poorly crystalline and/or amorphous Mn⁴⁺-containing phases. Higher reducibility of this surface coat in calcined layer-doped catalyst, as compared to interlayer-doped one, was attributed to the differences in the nature of underlying crystalline phases, and was considered the chief reason for the higher catalytic activity of this catalyst in the total oxidation of toluene.     

Transesterification of tributyrin with methanol over calcium oxide catalysts prepared from various precursors

Calcium oxide catalysts were prepared by calcining various precursors such as calcium acetate, carbonate, hydroxide, nitrate and oxalate and their catalytic activities were examined in the transesterification of tributyrin with methanol. The prepared calcium oxide catalysts were characterized using thermogravimetry (TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), N₂ adsorption and temperature programmed desorption (TPD) of CO₂. The calcium oxide catalyst obtained by calcining calcium hydroxide at 600–800 °C showed the highest tributyrin conversion and methyl butyrate yield. The large desorption peak of CO₂ TPD confirmed that its numerous basic sites were responsible for its high activity. The low-temperature decomposition of calcium hydroxide provided many nano-sized pores with strong basic sites. Although the activity of the calcium oxide catalyst prepared from calcium hydroxide was high, its activity was one order of magnitude less than that of sodium hydroxide catalyst. The dissolution of calcium oxide catalysts in products and their repeated uses were also investigated to discuss their advantages as heterogeneous catalysts in the production of biodiesel.     

Controlling carbon microporosity: the structure of carbons obtained from different phenolic resin precursors

Carbons were prepared from resins synthesised using the phenolic precursors phenol, para methylphenol, para ethylphenol, para n-propylphenol, para isopropylphenol and 3,5-dimethylphenol. Loss of phenolic OH from these materials was followed using solid-state nuclear magnetic resonance. The surface areas of the carbons were determined using N₂ and CO₂ adsorption. No significant differences in the loss of phenolic OH were observed. Under the same carbonisation conditions, the para alkyl phenols gave carbons with wide micropores, while the phenol and 3,5-dimethylphenol gave carbons with narrow micropores. Grinding the cured resins prior to carbonisation was found to significantly increase the surface area of the carbons obtained, with the microporous surface area increasing rapidly with a fall in particle size, without a significant increase in burn-off. Higher carbonisation temperatures widened the micropore size distribution, as shown by fitting the CO₂ adsorption isotherm with the Dubinin-Astakhov equation. The ability to change the carbon micropore structure obtained from a simple, well defined precursor, has many potential applications in carbon molecular sieves, catalyst supports and the investigation of adsorption processes.     

Decomposition of nitrous oxide over supported rhodium catalysts and dependency on feed gas composition

The catalytic decomposition of nitrous oxide to nitrogen and oxygen was studied overRh/ZnO, Rh/CeO₂, Rh/ZSM-5, CuZSM-5 and CoAlCO₃HT (hydrotalcite). The effects of metal loading and calcination conditions upon the catalytic performance were examined on Rh/ZnO. A 0.5 wt.% Rh/ZnO catalyst was found to be the most active catalyst, whose reaction rate was 4.0 × 10⁴ μmol(N₂O) · g⁻¹ · h⁻¹ under the conditions of 950 ppm N₂O and 5% O₂ at 300°C. The oxidized Rh/ZnO showed a higher activity than that calcined in a reducing atmosphere. The TEM and EDX observations revealed the formation of particles of ca. 50Åin diameter. They consisted of rhodium and zinc oxides as major and minor components, respectively. The activities of all these catalysts decreased when NO₂ and H₂O were added to the feed.     

High stability of Ce-promoted Ni/Mg-Al catalysts derived from hydrotalcites in dry reforming of methane

Ce-promoted Ni/Mg-Al catalysts were synthesized by means of a methodology that involves the doping of Ni-Mg-Al mixed oxides derived from hydrotalcites with [Ce(EDTA)]⁻ and subsequent thermal decomposition. The effect of the nominal load of Ce in the catalytic performance of the materials was studied. The solids were characterized by means of XRD, BET area, TPR-H₂, TPD-CO₂, chemical analysis by ICPs, TGA, SEM and TEM and were evaluated in CO₂ reforming of methane at 700 °C. The results indicate the partial reconstruction of the periclase phase during the doping with [Ce(EDTA)]⁻ and the formation of a mixture of crystalline periclase and fluorite phases after the calcination. Catalysts with particle sizes of Ni⁰ between 5 and 9 nm were obtained. Ce presents a promote effect in the degree of reduction of Ni and the amount and strength of the basic sites. It was evident a beneficial effect of cerium in the catalytic activity and selectivity of the doped materials. The increase of the nominal Ce load between 1 and 10% causes no considerable effect in the catalytic activity and selectivity or in the size of crystallite in these materials but in the inhibition of the coke formation. The catalysts show excellent catalytic performance under drastic conditions of reaction and long operation times. The Ce-doped Ni/Mg-Al catalyst is stable up to 100 h of reaction using a feed mixture of CH₄/CO₂/He 10/10/80 at 24 L g⁻¹ h⁻¹, up to 20 h of reaction using CO₂/CH₄ 20/20 at 48 L g⁻¹ h⁻¹ and up to 15 h of reaction using CO₂/CH₄ 40/40 at 96 L g⁻¹ h⁻¹. The filamentous coke formation is demonstrated on the surface of the catalyst when gas of dilution in the reactants is not used. The developed method of synthesis becomes an interesting methodology for obtaining catalysts for CO₂ reforming of methane.     

Titania supported Co-Mn-Al oxide catalysts in total oxidation of ethanol

Catalytic activity of the Co-Mn-Al mixed oxide catalysts (Co:Mn:Al molar ratio of 4:1:1) supported over titania was examined in total oxidation of ethanol. The prepared catalysts were characterized by chemical analysis (AAS), surface area measurements, and temperature programmed techniques (TPR, TPD). In ethanol oxidation, the catalysts activity gradually increased with increasing active phase content. Low concentration of Co-Mn-Al oxides in the catalyst negatively affected formation of reaction byproducts: carbon monoxide production steeply increased when Co + Mn metals concentration were lower than 5 wt.%. On the other hand, formation of the second main reaction intermediate, acetaldehyde was limited, when acidity of the catalyst was not high, i.e. concentration of Co-Mn metals over titania was low.     

窄分布聚醚合成用催化剂的研究

研究了不同碱催化剂催化合成聚氧乙烯(PEG)与聚氧乙烯-丙烯共聚醚(PEPG)的反应,考察了镁铝复合氧化物的制备条件对催化性能的影响,通过凝胶色谱仪(GPC)对产品分子量及分布情况进行了分析。结果表明,催化制备聚氧乙烯研究中,以KOH、镁铝复合氧化物的催化效果较好;催化制备氧化乙烯-氧化丙烯共聚醚研究中,以NaOH、镁铝复合氧化物的催化效果较好;制备镁铝复合氧化物催化剂时,当以Na2CO3,NaOH为共沉淀剂,镁铝物质的量比3∶1,煅烧温度550℃时,制备的镁铝复合氧化物催化剂催化性能最佳。     

NOx storage-reduction catalysts based on hydrotalcite: Effect of Cu in promoting resistance to deactivation

Novel NO_x storage-reduction (NO_xSR) catalysts prepared by Pt and/or Cu impregnation of Mg–Al (60:40) hydrotalcite (HT)-type compounds show better performances in NO_x storage than Pt–Ba/Al₂O₃ Toyota-type NO_xSR catalysts at reaction temperatures lower than 250 °C. The presence of Pt or Cu considerably enhances the activity, with the former more active. The nature of the HT source, however, also influences performance. The co-presence of Pt and Cu slightly worsens the low temperature activity, but considerably promotes the resistance to deactivation after severe hydrothermal treatment and in the presence of SO₂. This effect is attributed to both the possibility of formation of a Pt–Cu alloy after reduction, and the modification of the HT induced during the deposition of Cu. The overall Pt–Cu/HT performances are thus superior to those of the Pt–Ba/Al₂O₃ Toyota-type NO_xSR catalysts.     

Comparative study of Cu/ZnO catalysts derived from different precursors as a function of aging

Structural modifications of Cu/ZnO catalysts for methanol steam reforming (MSR) as a function of precipitate aging in catalysts preparation process has been investigated comparatively. Freshly precipitated Cu,Zn-hydroxycarbonate (HC) and Cu,Zn-hydroxynitrate (HN) were aged in their mother liquor for a period of 120 min followed by washing, drying, calcination and reduction. Pronounced effect of aging was found for aged HC precipitates while no significant effect of aging was observed for aged HN solids. The bulk structure of the Cu/ZnO catalysts was investigated by means of TG/MS, in situ XRD and ⁶³Cu NMR. The increase in the activity of the catalysts prepared by HC aging did not correlate linearly with the specific Cu surface area but coincides with an increase in the microstrain in the copper clusters presumably because of the improved interface between Cu and ZnO. Meanwhile, aging of HN precipitates results in large, separated and less strained Cu and ZnO particles with an inferior catalytic activity. Finally, both aged Cu/ZnO catalysts revealed smaller copper crystallite size compared to unaged samples.     

Effect of cobalt oxide on surface structure of alumina supported molybdena catalysts studied by in situ Raman spectroscopy

A set of Co promoted 10% Mo/Al₂O₃ samples have been characterized by means of Raman spectroscopy under ambient as well as in situ dehydrated conditions. Under ambient conditions, the degree of the polymerization of surface molybdenum oxide species decreases with increasing Co loading. Under dehydrated conditions, the polymeric molybdenum oxide species is absent with the addition of only 0.2% Co. At low Co loadings (2%), before the formation of CoMoO₄ compound, the spectral features are very similar under ambient conditions. Dehydration causes the upward shift of the Mo=O symmetric stretching mode. A broad band around 920–930 cm^–1 was thus observed. This band has been suggested to be associated with the Co-Mo interaction species. In contrast to crystalline CoMoO₄, this species shows a reversibility on H₂ reduction-O₂ reoxidation treatments. From the results obtained, it is proposed that cobalt oxide interacts with the most polymerized molybdenum oxide species to form Co-Mo interaction species and/or crystalline CoMoO₄; therefore, the amount of the surface molybdenum oxide species decreases with a change in the molecular structure as a function of the Co concentration.