Concepción Salinas-Martínez de Lecea

Calcium-catalysed carbon gasification in CO2 and steam

Abstract Isothermal CO 2 (0.1 MPa) and steam (19.7 KPa) reactivities on pure polymer carbons have been studied at different temperatures as a function of their calcium contents. Ion-exchange and impregnation methods have been used to load the carbons from calcium acetate solutions. Calcium appears to be a very active catalyst for both reactions, its effectivity being much higher in CO 2 than in steam. Reactivities in both atmospheres increase linearly with Ca content up to a loading saturation level (LSL) of about 4 wt%. For comparable calcium loading, the catalyst addition method, ion-exchange or impregnation, does not produce any significant difference. Selective CO 2 chemisorption has been used to determine the Ca atoms on the surface of the catalyst (and hence its external surface area or dispersion) as functions of both the calcium loading and the preparation method. The results obtained show that CO 2 chemisorption is much more suitable than XRD. This allows interpretation of the catalytic activity of Ca in the carbon-gas reactions. Briefly, gasification rates are related to the amount of CO 2 chemisorbed. The different catalytic behaviour of calcium catalyst above and below the LSL has been interpreted as being due to catalyst dispersion conditioned by carbon carboxyl groups. Only the exchanged calcium has a catalytic activity, whereas the excess calcium has low dispersion, its mean particle size increases considerably, and does not have additional catalytic activity.

Potassium-containing briquetted coal for the reduction of NO

Abstract The reduction of NO by potassium-containing activated carbons and briquettes prepared from a bituminous coal was investigated. Partial washing of KOH activated carbons produced samples with different potassium contents. Briquetting with a binder containing potassium, at different binder/coal ratios, also allowed carbons with different potassium contents to be obtained. The NO-carbon reaction was studied with a fixed-bed flow reactor at atmospheric pressure in two types of experiment: (1) isothermal reaction at 300–600°C; (2) temperature-programmed reaction (TPR) in an NO-He mixture. The reaction products were monitored in both cases, allowing detailed oxygen and nitrogen balances to be determined. Both the potassium remaining in the activated carbons and that present in the briquettes act as a catalyst for the NO-carbon reaction. Therefore, the briquettes are active for NO reduction with the advantage of being produced by a much simpler process with no KOH consumption and no washing process. Furthermore, the briquettes can be moulded in the desired form with appreciable mechanical strength. The final potassium loading controls the capacity of the potassium-carbon samples for NO reduction. The reduction of NO by the potassium-containing briquettes is enhanced by the presence of oxygen.

Nitrogen complexes formation during NO–C reaction at low temperature in presence of O2 and H2O

Oxygen and nitrogen complexes formation in coal char due to its reaction with NO at 100 °C in presence or absence of O2 and/or H2O was studied by XPS. Formation of NO2 and pyridinic complexes and a considerable increment of oxygen complex was observed. The NO2 complexes can be formed in all cases, even when there is no molecular oxygen present in the gas mixture. The presence of O2 and/or H2O promotes their formation. Apparently, this complex can be formed by the presence of C(O) complexes next to the site capable of chemisorbing NO.

Thermal treatment effect on NO reduction by potassium-containing coal-briquettes and coal-chars

This communication reports the NO reduction activity of potassium containing coal-briquettes prepared at different pyrolysis temperatures. For comparative purposes coal-chars prepared at the same temperatures have also been analyzed. NO reduction experiments were performed in a fix-bed flow reactor at atmospheric pressure using two types of experiment: (i) temperature programmed reaction in a NO/He mixture; and (ii) isothermal reactions at 600°C. The reaction products were monitored in both cases, thus allowing detailed oxygen and nitrogen balances to be determined. It has to be pointed out that a previous in situ heat treatment in He before reaction is necessary for the low temperature activity of potassium containing coal-briquettes. For fresh briquettes, NO direct attack in addition to NO conversion by the catalytic action of potassium is observed at high temperatures. The NO direct attack, both in potassium containing samples and coal-chars, is clearly manifested by a nitrogen imbalance.

CaO dispersed on carbon as an SO2 sorbent

Calcium-containing carbons were tested as low-temperature sorbents for SO2. CaO particles dispersed on carbon were prepared by thermal decomposition of calcium acetate. Samples with calcium contents ranging from 1.5 to 8.0 wt% were analysed. CO2 chemisorption was used to characterize the area of the CaO particles available to interact with and retain SO2. CaO surface areas confirm that CaO dispersed on a high-surface-area carbon has a much larger available surface than unsupported CaO obtained by decomposition of a large variety of limestones. Isothermal reaction experiments on SO2 retention at 50–400°C were performed in a thermobalance. SO2 molar conversion at 300°C reached values from 0.3 to 0.89. SEM-EDX analysis, TPD and TPR experiments and conversion values at different temperatures showed that the SO2-CaO interaction is not restricted to the surface of the CaO, as was found with limestone-derived CaO. Of interest for practical application is the observation that bulk CaSO3 formation occurs to a significant extent in comparison with the behaviour found with unsupported CaO. The degree of regeneration of the sorbents by thermal treatment was found to depend on calcium content and dispersion.

Effect of the Support in de-NOx HC-SCR Over Transition Metal Catalysts

Several catalysts based on transition metals (Cu, Co, Fe) and different supports (ZSM-5, activated carbon, Al2O3) have been tested by Temperature-Programmed Reaction (TPR) experiments for the selective catalytic reduction of NOx with propene in the presence of excess oxygen, simulating lean-burn conditions. The activity order with respect to the metal was Cu∼Fe>Co for all supports used. ZSM-5 catalysts have a superior behavior over Al2O3, as observed for noble metal catalysts. Application of activated carbon as a support is not practical due to its consumption at the reaction temperatures. The selectivity to N2 of the catalysts was also independent of the support, being higher than 95% in all the cases.

‘Anomalous’ increase in CO2 reactivity, relative to steam and air on acid treatment of coals☆

Abstract The reactivities in steam, CO 2 and air of two high-ash Spanish coal chars (from a semi-anthracite and a high volatile bituminous coal) and the effects of previous acid demineralization on these reactivities have been studied. The different acid treatments used increased the reactivities of both chars in the three atmospheres, the increase being much larger for samples treated with HCl + HF , especially in the reaction with CO 2 . This apparently anomalous increase in CO 2 reactivity (relative to steam and air) for both chars, which can also be observed in published results, seems to be mainly related to the HF treatment and the remaining mineral matter, as deduced from a study undertaken on two very low-ash carbons.

Immobilization of homogeneous catalysts in nanostructured carbon xerogels

A Rh diamine complex has been successfully immobilized by anchorage on the surface of carbon xerogels. The catalysts are active and fully recyclable for cyclohexene hydrogenation, with conversion higher than 80% kept in four catalytic runs. TEM analysis reveals the presence of Rh particles in the used catalysts, meaning that partial reduction of the metal complex takes place under reaction conditions. The XPS data show that in the used catalysts Rh is present as Rh(I) (anchored complex) and Rh(0) metallic particles (about 30%).

Ligand Tethering by Ion-Exchange for the Immobilization of Homogeneous Catalysts

The authors thank the Spanish MICINN (project MAT2009-07150) and Generalitat Valenciana (PROMETEO/2009/047) and FEDER for financial support. I. Such thanks Ministerio de Educacion y Ciencia for a FPU grant.

New hybrid materials based on the grafting of Pd(II)-amino complexes on the graphitic surface of AC: preparation, structures and catalytic properties

A novel procedure for the preparation of solid Pd(II)-based catalysts consisting of the anchorage of designed Pd(II)-complexes on an activated carbon (AC) surface is reported. Two molecules of the Ar–S–F type (where Ar is a plane-pyrimidine moiety, F a Pd(II)-ligand and S an aliphatic linker) differing in F, were grafted on AC by π–π stacking of the Ar moiety and the graphene planes of the AC, thus favouring the retaining of the metal-complexing ability of F. Adsorption of Pd(II) by the AC/Ar–S–F hybrids occurs via Pd(II)-complexation by F. After deep characterization, the catalytic activities of the AC/Ar–S–F/Pd(II) hybrids on the hydrogenation of 1-octene in methanol as a catalytic test were evaluated. 100% conversion to n-octane at T = 323.1 K and P = 15 bar, was obtained with both catalysts and most of Pd(II) was reduced to Pd(0) nanoparticles, which remained on the AC surface. Reusing the catalysts in three additional cycles reveals that the catalyst bearing the F ligand with a larger Pd-complexing ability showed no loss of activity (100% conversion to n-octane) which is assigned to its larger structural stability. The catalyst with the weaker F ligand underwent a progressive loss of activity (from 100% to 79% in four cycles), due to the constant aggregation of the Pd(0) nanoparticles. Milder conditions, T = 303.1 K and P = 1.5 bar, prevent the aggregation of the Pd(0) nanoparticles in this catalyst allowing the retention of the high catalytic efficiency (100% conversion) in four reaction cycles.