Cryptomelane-type manganese oxide with cesium and lithium incorporated in its tunnel structure is a highly active catalyst for the oxidation of ethyl acetate. Display omitted
► Li and Cs were ...successfully incorporated in the cryptomelane structure. ► Alkali metals doping enhances both the reducibility and the basicity of cryptomelane. ► These parameters have a significant effect on the catalytic properties. ► Li and especially Cs-doped catalysts are highly active for ethyl acetate oxidation. ► The amounts of Cs or Li incorporated do not influence the performance (pore mouth activity).
Different amounts of cesium (0.8–1.3at.%) and lithium (up to 0.5at.%) were introduced in the tunnel structure of cryptomelane in order to tailor its physical and chemical properties. The mechanism by which cryptomelane accommodates these alkali cations is different; Li occupies mainly empty sites by a redox-type reaction, while Cs is ion-exchanged by H3O+ cations. The adsorption of these alkali metals into cryptomelane was highly selective and is dictated by the solvation energy and ionic radius. Extrinsic defects (e.g. positive holes and oxygen vacancies) are created upon doping, enhancing the redox-properties of cryptomelane. The basicity of the framework also increases when Cs and Li are incorporated. The catalytic properties of this material were tested in the oxidation of ethyl acetate, and greatly improved after doping. The observed activity increase is explained by the redox and basic properties of the modified materials. The amount of cesium loaded has no effect on the activity, as a result of pore mouth catalysis.
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Cobalt-cerium mixed oxides were prepared by the wet impregnation method and evaluated for volatile organic compounds (VOCs) abatement, using ethyl acetate (EtAc) as model molecule. ...The impact of Co content on the physicochemical characteristics of catalysts and EtAc conversion was investigated. The materials were characterized by various techniques, including N2 adsorption at −196°C, scanning electron microscopy (SEM), X-ray diffraction (XRD), H2-temperature programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) to reveal the structure–activity relationship. The obtained results showed the superiority of mixed oxides compared to bare CeO2 and Co3O4, demonstrating a synergistic effect. The optimum oxidation performance was achieved with the sample containing 20wt.% Co (Co/Ce atomic ratio of ca. 0.75), in which complete conversion of EtAc was attained at 260°C. In contrast, temperatures above 300°C were required to achieve 100% conversion over the single oxides. Notably, a strong relationship between both the: (i) relative population, and (ii) facile reduction of lattice oxygen with the ethyl acetate oxidation activity was found, highlighting the key role of loosely bound oxygen species on VOCs oxidation. A synergistic Co-Ce interaction can be accounted for the enhanced reducibility of mixed oxides, linked with the increased mobility of lattice oxygen.
Characterization of Active Sites on Carbon Catalysts Figueiredo, José L; Pereira, Manuel F. R; Freitas, Maria M. A ...
Industrial & engineering chemistry research,
06/2007, Letnik:
46, Številka:
12
Journal Article
Recenzirano
Odprti dostop
A method based on the deconvolution of TPD spectra is proposed for the characterization of surface oxygen groups, which can act as the active sites on carbon catalysts. The method, which was ...previously used to characterize activated carbons oxidized in the gas phase, has been extended and applied to other materials, carbons oxidized in the liquid phase. It is shown that this method fits quite well the TPD experimental data of the original activated carbon as well as the gas-phase and liquid-phase oxidized materials and is suitable to estimate the amounts of each type of oxygen surface groups.
•Ce–Co and La–Co oxides were prepared by exotemplating and evaporation methods.•The materials obtained were tested as catalysts for toluene oxidation to CO2.•Ce–Co materials prepared by exotemplating ...were the most active.•A high concentration of weak acid sites is favourable for catalytic activity.•A Mars–van Krevelen mechanism can explain the acidity–activity correlation.
Ce–Co and La–Co mixed oxides were synthesized by two different methods: exotemplating and evaporation. The obtained catalysts were evaluated for volatile organic compounds (VOCs) abatement, using toluene as model molecule. The materials were characterized by N2 adsorption at −196°C, X-ray diffraction (XRD), scanning electron microscopy (SEM), H2 temperature-programmed reduction (H2-TPR) and NH3 temperature-programmed desorption (NH3-TPD) in order to reveal the structure–activity relationship. The results obtained showed the superiority of mixed oxides compared to single oxides in toluene oxidation. Ce–Co mixed oxides were more active than La–Co samples. For Ce–Co materials, the exotemplating method produced catalysts which were more active than those prepared by the evaporation procedure. The former showed the best catalytic performances, with full conversion of toluene into CO2 at about 250°C. Temperatures higher than 320°C were required with single oxides. Characterization studies revealed strong interactions between Ce (or La) and Co, leading to a fine dispersion of oxide phases in binary systems. As a result, both the surface area and reducibility of the catalysts increase, which can be accounted for the higher performance of the mixed oxides. Furthermore, NH3-TPD studies showed a linear relationship between acidity and VOC oxidation activity. In fact, a high concentration of weak acid sites is required for high toluene oxidation activity. The results can be explained in terms of a Mars–van Krevelen type of mechanism, involving the adsorption of toluene and its subsequent oxidation by lattice and/or surface oxygen.
The liquid-phase oxidation of glycerol was studied to obtain value-added oxidation products such as dihydroxyacetone or glyceric acid. The effect of the reaction conditions was studied for carbon ...nanotube supported Pd and Pt catalysts. Under strongly alkaline conditions, conversions close to 90% were reached after 5 h of reaction, with selectivities to glyceric acid of approximately 60–70%. Carbon nanotube supported bimetallic Pt–Cu catalysts were prepared and tested in the reaction under base-free conditions, where, besides glyceric acid, there was the formation of dihydroxyacetone and glyceraldehyde. Bimetallic Pt–Cu/CNT was found to be more effective than monometallic Pt/CNT toward the selective glycerol oxidation.
N-doped carbon materials are promising metal-free catalysts for a number of applications. In this work, a cost effective and easy method to prepare N-doped carbon nanotubes by ball milling was ...developed, which avoids the use of solvents and production of wastes. Melamine and urea were used as nitrogen precursors. The textural and chemical properties of the N-doped materials were characterized by N2 adsorption at −196°C, temperature programmed desorption, elemental analysis, X-ray photoelectron spectroscopy and thermogravimetric analysis. The highest efficiency of N-doping was obtained with melamine. This method leads to the incorporation of large amounts of N-groups, namely quaternary nitrogen, pyridinic, and pyrrolic groups.
Different types of activated carbon were prepared by chemical activation of brewer’s spent grain (BSG) lignin using H
3PO
4 at various acid/lignin ratios (1, 2, or 3
g/g) and carbonization ...temperatures (300, 450, or 600
°C), according to a 2
2 full-factorial design. The resulting materials were characterized with regard to their surface area, pore volume, and pore size distribution, and used for detoxification of BSG hemicellulosic hydrolysate (a mixture of sugars, phenolic compounds, metallic ions, among other compounds). BSG carbons presented BET surface areas between 33 and 692
m
2/g, and micro- and mesopores with volumes between 0.058 and 0.453
cm
3/g. The carbons showed high capacity for adsorption of metallic ions, mainly nickel, iron, chromium, and silicon. The concentration of phenolic compounds and color were also reduced by these sorbents. These results suggest that activated carbons with characteristics similar to those commercially found and high adsorption capacity can be produced from BSG lignin.
The catalytic conversion of lignocellulosic biomass to renewable and valuable chemicals has attracted global interest. Given the abundance of this renewable raw material and its reduced impact on the ...food chain, it is an attractive source for replacing fossil fuels and obtaining chemicals or fuels in the context of a sustainable economy. In this work, a catalyst (Ru/AC) was developed to perform, in a single step, hydrolysis and hydrogenation of cellulose to sorbitol. An activated carbon supported ruthenium catalyst was examined for the one-pot hydrolytic hydrogenation of cellulose and it has shown to be very active and selective for the conversion of cellulose into sorbitol. When microcrystalline cellulose was used, a conversion of 36% was reached after 5 hours of reaction, with a selectivity to sorbitol of 40%. On the other hand, ball-milled cellulose allowed attaining conversions close to 90%, with a selectivity to sorbitol of 50%. Moreover, if the catalyst was ball-milled together with cellulose, the selectivity to sorbitol could be further increased to almost 80%. The catalyst showed excellent stability after repeated use. In this work we combined hydrolysis and hydrogenation in one-pot (using heterogeneous catalysts instead of homogeneous), in the presence of a Ru/AC catalyst (without any support pre-treatment with acids) and pre-treated cellulose just by ball-milling (instead of using acids). For this reason, the results obtained in this work are one of the best values achieved when using supported metal catalysts to convert cellulose by an environmentally friendly process.
Promising progress has been made towards the catalytic conversion of cellulose into sorbitol with high selectivity by an environmentally friendly process.
Multi-walled carbon nanotubes were submitted to three activation procedures: nitric acid oxidation, ball-milling and air oxidation. The influence of these treatments on nanotubes surface chemistry ...and morphology was evaluated by XPS, Raman and infrared spectroscopy, TGA, TPD, nitrogen adsorption and TEM. The three activated materials were used to prepare Pt supported catalysts from the organometallic precursor Pt(CH
3)
2(COD). The influence of the activation treatments, together with that of a post-reduction thermal treatment, on the performances of the catalytic systems in the selective hydrogenation of cinnamaldehyde was investigated. It was shown that the best compromise between catalyst activity and selectivity requires a low amount of oxygenated groups on the support surface of the final catalyst, typically less than 700
μmol/g CO
+
CO
2 evolved during TPD experiments, together with an optimized platinum particle size ranging between 10 and 20
nm.
The oxygen content of activated carbon surface plays a key role on the activity of Au/AC catalysts for the oxidation of glycerol. Oxygen-free supports promote electron mobility, which enhances the ...catalytic performance.
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► The sol immobilization method leads to efficient Au/AC catalysts for glycerol oxidation. ► The activated carbon surface chemistry plays a key role in catalysis by gold. ► Activated carbons with high content of acid groups lead to poor performances. ► Basic activated carbons with a high density of free electrons enhance the activity.
The main goal of this work is the study of the relationship between the surface chemical characteristics of activated carbon and the performance of the respective gold-supported catalysts in the oxidation of glycerol. For that purpose, a set of modified activated carbons with different levels of oxygenated functional groups on the surface, but with no major differences in their textural parameters, was prepared. A strong effect of the activated carbon surface chemistry on the catalytic activity was observed. Gold particles with similar average sizes resulted in different performances, being the surface oxygenated acid groups particularly prejudicial for the catalytic activity. Basic oxygen-free supports characterized by a high density of free π-electrons lead to more active catalysts; the observation was tentatively explained on the basis of a recent proposed mechanism by considering the capability to promote electron mobility. However, the presence of oxygenated groups on the support does not influence significantly the selectivities.
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