Sorbents for SO2 retention at low temperature consisting of CaO supported on activated carbons have been prepared by different methods (physical mixing, incipient wetness impregnation, rotary ...evaporator impregnation, complex formation and ionic exchange). Ca(OH)2, CaCO3, CaO, Ca(CH3COO)2 and Ca(C2H5COO)2 have been used as calcium precursors. The preparation method affects calcium loading as well as calcium dispersion. SO2 retention depends both on the calcium loading and on the calcium dispersion. The best preparation method, among those tested, was impregnation of the activated carbon with calcium acetate. Similar calcium loading can be obtained by the physical mixing method but the resulting calcium dispersion is low. On the contrary, samples with higher calcium dispersion were obtained by the complex formation method, presenting the disadvantage of the low calcium loading reached. The molar calcium conversion results are much higher than values reported for unsupported calcium compounds.
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► Carbon NT’s of different dimensions are used for the heterogenization of a Rh complex. ► The hybrid catalysts are, in general, more active than the homogeneous Rh complex. ► An ...effect of the nanotubes diameter in the catalytic properties has been found. ► It seems that there is an optimum inner diameter of the nanotubes support.
Hybrid catalysts have been prepared by the immobilization of a Rh diamine complex on four multiwall carbon nanotubes of different inner diameter (average inner diameter ranging from 4 to 10
nm). The complex has been tethered to the support by the reaction of the –Si(OCH
3)
3 functionality in the rhodium complex with phenol type –OH groups on the support surface, that have been created by oxidation with air. The hybrid catalysts have been tested in the hydrogenation of cyclohexene and carvone. In general terms, the obtained hybrid catalysts are more active than the Rh complex in homogeneous phase and they are recyclable. It has been found that the nanotubes dimensions have an influence in the catalytic properties. In the case of the thinner nanotubes (inner Ø
∼
4
nm) the support has not a positive effect while the catalysts prepared with nanotubes of inner diameter around 7
nm give the best results.
The Selective Catalytic Reduction (SCR) of NOx has been performed in a real diesel exhaust stream with commercial diesel fuel by using a full size home-made Pt/beta zeolite/honeycomb prototype ...catalyst. Fuel was injected upstream of the catalyst to achieve total hydrocarbon concentrations between 1000 and 5000ppm, and the SCR behavior observed was similar to that typically reported in laboratory experiments performed with model hydrocarbons. Typical NOx removal volcano-shape profiles, with maxima at 250°C for all THC inlet concentrations, were obtained, with an optimum THC concentration of 3000ppm.
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•SCR of NOx has been performed in a real diesel exhaust with commercial diesel fuel.•A full size Pt/beta zeolite/honeycomb monolith catalyst has been prepared and used.•The SCR behavior was similar to that reported for model hydrocarbons at laboratory.•NOx removal volcano-shape profiles, with maxima at 250°C, were obtained.•The optimum THC concentration was 3000ppm.
The retention of SO2 on CaO/activated carbon sorbents is studied. The effect of several variables such as the reaction temperature, partial pressure of SO2 for different calcium loads, and O2 ...presence are analysed. Additionally, the regeneration and reutilization of spent sorbents is investigated. In all cases presence of well-dispersed CaO in the sorbents improves SO2 retention in comparison with the activated carbon. In absence of O2 in the gas mixture, the amount of SO2 retained does not depend on the SO2 partial pressure in the range of partial pressures studied and, as expected, SO2 physisorption on the activated carbon support occurs at room temperature. SO2 retention occurs in surface CaO between 100 deg C and 250 deg C, and in bulk CaO above 300 deg C. The total calcium conversion is reached at 500 deg C. Above 550 deg C calcium-catalysed carbon gasification by SO2 occurs. In presence of O2 in the gas mixture, the studied sorbents are very effective for SO2 removal. However, the SO2 retention process in presence of oxygen must be carried out at temperatures lower than 300 deg C to avoid carbon gasification by O2. The thermal regeneration of the spent sorbents can be done under inert atmosphere (880 deg C) with only 20% activity loss after the first regeneration cycle due to sintering and formation of CaS. No additional activity loss is detected in the subsequent cycles.
The SCR of NOx by C3H6 using a 0.1 wt.% Pt/Al2O3 catalyst has been successfully performed under real diesel gas streams in a power bench. The influence of some experimental variables affecting NOx, ...CO, and THC conversions, such as hydrocarbon concentration, reaction temperature and space velocity effect, are discussed. 96% NOx conversion has been reached under optimum conditions. The highest NOx conversion level is maintained in the range 250-325 deg C but slightly higher temperature (350 deg C) is desired to ensure a stable temperature and complete CO and THC removal. An optimum THC concentration is found (1500 ppm THC), higher THC concentration having a negative effect in the NOx conversion. From experiments performed at different GHSV, the reactor volume required for total NOx removal in a real exhaust stream has been estimated to be 6.5 L. Additionally, the effect of CeO2 on platinum behaviour has been studied, concluding that it enhances N2 formation as NOx reduction product with respect to N2O, but lowers the platinum activity for NOx reduction and CO and THC oxidation.
Biomass samples have been studied as activated carbon precursors. Two, denoted as recalcitrant biomass, are ethanol production wastes, one from acid hydrolysis of starch-rich banana (Musa acuminate) ...fruit and one from enzymatic hydrolysis of banana pseudostem (lignocellulosic material) and a third one is raw banana tree pseudostem. The samples were characterized by proximate, ultimate, forage, morphological and structural analyses. They all have volatile matter contents above 50% but present some remarkable differences. The sample coming from the acid hydrolysis has too high ash content (mostly calcium sulfate) and low cellulose and lignin contents that prevents its use as an activated carbon precursor. Contrarily, the other two samples have lower ash contents, higher carbon percentages and higher cellulose and lignin contents. The activation with phosphoric acid at 450 °C has been investigated. The results are quite reproducible, showing good porosity developments and a strong dependence on the precursor, the H3PO4 concentration and the H3PO4/sample weight ratio used. The activated carbons have quite high apparent surface area, porosity and yield. Interestingly, the phosphoric acid activated carbons prepared in this work have quite well developed mesoporosity which is especially useful for some applications (e.g. for gasoline evaporative control). Our preliminary comparative study, carried out with a well known commercial activated carbon used to control automobile hydrocarbons emissions, has shown that these activated carbons perform very well, being their performance a function of their mesoporosity development. Their oxygenated surface complexes (assessed from TPD experiments) are considerably high.
The present paper analyses the role of the activated carbon (AC) properties on the SO2 uptake capacity of CaO/AC sorbents prepared by AC impregnation or ionic exchange with calcium acetate water ...solutions. Gas adsorption and mercury porosimetry have been used for textural characterization of the AC and surface oxygen groups have been characterized by temperature programmed desorption (TPD). Thermogravimetry has been used for SO2 retention tests and CO2 chemisorption at 300 deg C for CaO dispersion (d) determinations. The results show that the surface calcium on CaO/AC samples, determined as 'Ca loading * CaO dispersion' (parameter Ca(%) * d), governs the SO2 uptake. The surface oxygen content is the AC property that mainly controls both the calcium loading and surface calcium on CaO/AC samples, which could be explained by the fact that the surface oxygen lowers the hydrophobic character of the AC supports therefore favouring the interaction with the calcium acetate water solutions. The combination of high calcium loading and dispersion leads to SO2 uptakes up to 123 mg SO2/g. The textural properties of the supports have some influence in the calcium loading. However, the effect is masked by the blockage of AC porosity by the calcium loaded.
Rh/Ce0.9Pr0.1O2 active phase has been loaded on γ-Al2O3 particles and successfully tested for N2O decomposition in simulated nitric acid plants conditions. Reaction mechanisms have been studied by in ...situ DRIFTS-MS experiments. Display omitted
► A γ-Al2O3 particles-supported Rh/Ce0.9Pr0.1O2 catalyst has been prepared and characterised. ► N2O decomposition in simulated nitric acid plant conditions has been successfully performed. ► Ceria–alumina interaction slightly hinders rhodium–ceria interaction. ► Among inhibitors in nitric acid plant gasses (H2O, NOx and O2) NOx is the strongest and O2 the weakest.
A supported catalyst composed by the active phase Rh/Ce0.9Pr0.1O2 loaded on γ-Al2O3 particles was prepared and characterised by SEM, TEM, XRD, Raman spectroscopy, N2 adsorption at −196°C, H2-TPR and XPS. The catalytic decomposition of N2O was studied in simulated nitric acid plant streams (in the presence of O2, H2O and/or NOx) both in a fix-bed reactor and by in situ DRIFTS-MS experiments. The supported catalyst is able to decompose N2O from around 350–400°C in the presence of O2, H2O and/or NOx, and this temperature would be enough to decompose N2O in a nitric acid production plant after the expansion turbine, avoiding the use of an additional energy input. Among O2, H2O and NOx, NOx is the strongest inhibitor and O2 the weakest. The inhibiting effect of O2 is attributed to its reversible chemisorption on catalyst active sites, while the effect of H2O and NOx is mainly related with their irreversible chemisorption. The inhibition of H2O is not as high as that of NOx because the product of H2O chemisorption (Ce–OH surface groups) is suitable for N2O chemisorption and decomposition, while the surface nitrogen species created upon NOx chemisorption are not.
Activated carbon felts (ACFs) have been used as supports for Pt catalysts. The preparation was carried out by the impregnation method using chloroplatinic acid as metal precursor. The effect of ...impregnation time and surface chemistry of the support on the catalytic properties and the characteristics of the metallic phase have been investigated. Nitrobenzene (Nbz) hydrogenation in liquid phase at 25
°C and cyclohexane (CH) dehydrogenation in gas phase at 300
°C were used as catalytic tests. The state of platinum in reduced catalysts (at 100 and 350
°C) was studied by TPR and XPS. Oxygen surface groups only produce a slight effect on the catalytic properties. The use of low impregnation times (30
min) during the preparation of Pt/ACF leads to catalysts with Pt mainly deposited in the outer shell of the fibers, while at higher impregnation times, the metallic atoms seem to be deposited inside the pores. Pt(0) species appear in catalysts reduced at 100
°C by effect of the reducing properties of the carbon fiber exhibiting a considerable catalytic activity for Nbz hydrogenation.
The research work conducted in our group concerning the study of the potassium-catalyzed NOx reduction by carbon materials is presented. The importance of the different variables affecting the ...NOx-carbon reactions is discussed, e.g. carbon porosity, coal rank, potassium loading, influence of the binder used, and effect of the gas composition. The catalyst loading is the main feature affecting the selectivity for NOx reduction against O{sub 2} combustion. The NOx reduction without important combustion in O{sub 2} occurs between 350 and 475{sup o}C in the presence of the catalyst. The presence of H{sub 2}O in the gas mixture enhances NOx reduction at low carbon conversions, but as the reaction proceeds, it decreases as the selectivity does. The presence of CO{sub 2} diminishes the activity and selectivity of the catalyst. SO{sub 2} completely inhibits the catalytic activity of potassium due to sulfate formation.
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