F. Rodríguez-Reinoso

Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)

Abstract Gas adsorption is an important tool for the characterisation of porous solids and fine powders. Major advances in recent years have made it necessary to update the 1985 IUPAC manual on Reporting Physisorption Data for Gas/Solid Systems. The aims of the present document are to clarify and standardise the presentation, nomenclature and methodology associated with the application of physisorption for surface area assessment and pore size analysis and to draw attention to remaining problems in the interpretation of physisorption data.

The role of carbon materials in heterogeneous catalysis

The increasing importance of carbon materials in catalytic processes is analyzed in terms of the most important characteristics of these materials when acting as catalysts or catalyst supports. Thus, surface area, porosity, chemical inertness and oxygen surface groups affect not only the preparation, but also influence the resistance to sintering and the catalytic activity and selectivity of the catalyst. Several series of catalysts, mainly carbon-supported catalysts, are used to show the possible advantages of carbon as support and the series of variables to be taken into account when selecting a carbon as catalyst support for a given reaction. The role of carbon properties when acting as a catalyst in its own right is also analyzed.

Activated carbons from lignocellulosic materials by chemical and/or physical activation: an overview

Abstract Four series of activated carbons prepared from lignocellulosic materials (almond shells and olive and peach stones) by either physical activation—gasification (uncatalysed and iron catalysed) in CO2 or in a water-nitrogen mixture—of chars or direct chemical activation with ZnCl2 of the precursor were selected to show the comparative behavior of the activation procedures. Activation with CO2 opens and widens the microporosity of the char with even a shift to meso- and macroporosity, the ablation of the exterior of the particle being very important at high burn-off; the final activated carbon has a well developed micro- and macroporosity, with a relatively small contribution of mesoporosity. The iron catalysed CO2 gasification and gasification with water-nitrogen mixture produce carbons with a well developed macroporosity, although the latter has the advantage of maintaining a well developed micro- and mesoporosity. Direct chemical activation of the precursor with ZnCl2 produces, in only one step, a larger yield of activated carbon having microporosity as well developed as in the CO2 gasification of the char, with the advantage of producing a much larger mesopore volume.

The use of steam and CO2 as activating agents in the preparation of activated carbons

Four series of activated carbon have been prepared from carbonized olive stones. One of them, series D, was prepared using carbon dioxide as activating agent, and the other three, series AV, W, and H, with water vapor under different experimental conditions. Two of the series, D and H, were prepared in such a way that the gasification rate for both reactants was identical, in an attempt to reduce the effect of the relative differences in diffusion and accessibility of both gases to the interior of the particles. The changes in porosity of the original char during activation have been studied by adsorption of N2 at 77 K and CO2 at 273 K, as well as by mercury porosimetry. The results obtained show that carbon dioxide produces an opening, followed by widening, of narrow microporosity, whereas water vapor widens the microporosity from the early stages of the process, the resulting activated carbon exhibiting lower micropore volume. However, dilution of water vapor and high activation temperatures approach the development of total microporosity by steam to that of carbon dioxide, although there is a more important role of narrow microporosity widening in the former.

Preparation of activated carbon by chemical activation with ZnCl2

Abstract Several series of activated carbons have been prepared by chemical activation of peach stones with ZnCl 2 in order to show the effect of variables such as a precursor particle size, extent of impregnation, impregnation method, and carbonization temperature on surface area, porosity, and bulk density of the resulting activated carbons. The adsorption isotherms of n-butane at 273 K on all carbons prepared are of type I, with a defined plateau, the extent of which is a function of the preparation conditions. The main factor affecting the surface area and the micropore size distribution is the amount of Zn introduced in the precursor during impregnation. Partial gasification in CO 2 of the carbons produces a considerable developing of surface area and porosity, maximum for burn-offs around 60–70%. In this way, it is possible to prepare activated carbons with very high surface area (larger than 3000 m 2 /g) compatible with a granular form and reasonable bulk density.

Porosity in granular carbons activated with phosphoric acid

Abstract Three series of activated carbon have been prepared by heat treatment of peach stones impregnated with solutions of phosphoric acid, in order to analyze the effect of phosphoric acid on the yield, bulk density and porosity of the resultant activated carbons. The analysis of the adsorption isotherms of N2 at 77 K. CO2 at 273 K and n-C4H10, at 273 K shows that the amount of phosphorus introduced into the material. Xp. is the main factor conditioning the porosity and pore size distribution of the activated carbon. In general terms, the increase in Xp leads to an increase in the volumes of micro and mesopores of the carbon. Furthermore, there is a noticeable similarity between the volume of micropores and the volume occupied by the P2O5·.xH2O in the interior of the peach stones. This suggests that the microporosity is mainly caused by the phosphoric acid remaining in the impregnated material, which inhibits the contraction of the material during carbonization. On the other hand. it is suggested that the mesoporosity of the activated carbon, which is large only when the concentration of phosphoric acid is high, is mainly caused by the hydrolysis of the lignocellulosic material and subsequent partial extraction of some of its components during impregnation.

The effect of oxygen surface groups of the support on platinum dispersion in Pt/carbon catalysts

Abstract A series of Pt catalysts supported on carbon black has been prepared; the original support was treated in such a way as to ensure carbons with the same porous texture but with an increasing amount of oxygen surface groups. Pt catalysts (about 1% loading) were prepared with these carbons using the same experimental procedure and the metal dispersion was calculated from hydrogen chemisorption measurements. Both dispersion and resistance to sintering were found to be a function of the number of oxygen surface groups of the support.

High‐Surface‐Area Carbon Molecular Sieves for Selective CO2 Adsorption

A series of carbon molecular sieves (CMSs) has been prepared, either as powders or monoliths, from petroleum pitch using potassium hydroxide as the activating agent. The CMS monoliths are prepared without the use of a binder based on the self-sintering ability of the mesophase pitch. Characterization results show that these CMSs combine a large apparent surface area (up to ca. 3100 m(2) g(-1)) together with a well-developed narrow microporosity (V(n) up to ca. 1.4 cm(3) g(-1)). The materials exhibit high adsorption capacities for CO(2) at 1 bar and 273 K (up to ca. 380 mg CO(2) g sorbent(-1)). To our knowledge, this is the best result obtained for CO(2) adsorption using carbon-based materials. Furthermore, although the CO(2) adsorption capacity for activated carbons has usually been considered lower than that of zeolites, the reported values exceed the total amount adsorbed on traditional 13X and 5A zeolites (ca. 230 mg and 180 mg CO(2) g sorbent(-1), respectively), under identical experimental conditions. Additionally, the narrow pore openings found in the CMS samples (ca. 0.4 nm) allows for the selective adsorption of CO(2) from molecules of similar dimensions (e.g., CH(4) and N(2)).

Effect of steam and carbon dioxide activation in the micropore size distribution of activated carbon

Abstract This work presents the different effects of steam and carbon dioxide activation of a char in both the development of microporosity and the micropore size distribution using immersion microcalorimetry of liquids with different molecular size (benzene, 2,2 dimethylbutane, iso-octane and α-pinene). The study has been carried out with three series of carbons, two of them prepared by steam activation and the third one by carbon dioxide activation, covering a wide range of burn-off (8–70%). The experimental results show that carbon dioxide activation mainly causes the creation of microporosity. However, steam activation widens the microporosity as from the early stages of the activation process, the resulting activated carbons exhibiting a lower micropore volume. The different porous structures produced by both activating agents is related to the oxygen surface groups in the carbon, as measured by temperature programmed desorption (TPD). Activation by carbon dioxide creates not only a larger number of groups evolving as CO but also these groups are thermally more stable than those produced by steam activation.

Crotonaldehyde hydrogenation over bimetallic PtSn catalysts supported on pregraphitized carbon black. Effect of the preparation method

Three bimetallic PtSn/C catalysts have been prepared by successive impregnation of pregraphitized carbon black with an aqueous solutions of hexachloroplatinic acid and tin(II) chloride. One monometallic Pt/C sample was also prepared and studied for comparison. All catalysts were characterized by hydrogen and carbon monoxide chemisorption at room temperature and X-ray photoelectron spectroscopy and their behaviour in the gas phase hydrogenation of crotonaldehyde, at atmospheric pressure, determined. The amount of surface platinum is greatly reduced by the addition of tin, as deduced from chemisorption measurements and XPS. Both Pt0 and PtII are detected by XPS in the fresh bimetallic catalysts; after reduction in flowing hydrogen at 623 K platinum is completely reduced to the metallic state and, although a high proportion of tin remains in an oxidized state, a relatively important amount is reduced to Sn0, this allowing the possibility of PtSn alloys formation. The catalytic activity in the gas phase hydrogenation of crotonaldehyde is greatly improved by the presence of tin, in spite of the fact that the amount of surface platinum is reduced. Tin has also a very important effect on the selectivity towards the hydrogenation of the CO bond, increasing the production of crotyl alcohol in respect to the hydrogenation of the CC bond that would lead to the production of butyraldehyde. The observed results are explained on the basis of a promoting effect of oxidized tin species for the hydrogenation of the CO group, whereas the formation of a PtSn alloy or the dilution of surface platinum by metallic tin would hinder the hydrogenation of the olefinic CC bond.