A. Linares-Solano

States of Pt in Pt/C catalyst precursors after impregnation, drying and reduction steps

Abstract The characterization of Pt/C catalysts after impregnation and activation steps has been carried out by using different techniques: TPD; TPR; XPS; EXAFS and H 2 chemisorption. Furthermore, the catalytic activity of the samples has been tested in the cyclohexane dehydrogenation. The catalysts were prepared from a purified peach pit derived carbon and two H 2 O 2 -functionalized supports obtained from it. Results showed that the interaction of H 2 PtCl 6 with the carbon implies a redox process in which after impregnation and drying steps, the metal complex is stabilized as Pt 2+ on the carbon surface. The functionalization treatment seems to have no effect on the resulting state of platinum. During the activation step (thermal treatment in H 2 ), apart from the reduction of Pt species to Pt 0 , catalyzed and non-catalyzed reactions involving CO and CO 2 desorbed from the carbon surface, take place. It was also found that the Pt 2+ species are reduced to zerovalent Pt even by thermal treatment of the dried samples with He. The electronic state of reduced platinum is not modified by the differences in the support surface chemistry.

The controlled reaction of active carbons with air at 350°C—I: Reactivity and changes in surface area

Abstract Two activated carbons prepared from almond shells and olive stones were reacted with air at 350°C to different percentages burn-off. The reactivity was studied in the temperature range 350–500°C where the reaction is relatively slow. The activated carbon from almond shells is more resistant to the reaction with air and the activation energy of that reaction is 101 kJ mol −1 . The adsorption of N 2 at 77 K has been used to characterize the adsorptive properties and surface area of all the obtained products, which have high surface areas (around or above 1000 m 2 g −1 ). The gas adsorption results, together with mercury porosimetry have allowed a study of the variation of surface area and porosity as a function of the burn-off. In any case, the exposure of the active carbons to air at 350°C for several days does not considerably affect their adsorptive properties even for a weight loss up to 50%.

Platinum catalysts supported on graphitized carbon black—I: Characterization of the platinum by titrations and differential calorimetry

Abstract A graphitized carbon black and samples resulting from its oxidation in air at 798 K over different periods of time have been used as supports for platinum catalysts. Platinum dispersion and mean diameter of platinum particles have been determined by O 2 -H 2 , H 2 -O 2 and O 2 -CO titrations and by differential calorimetry of the O 2 -H 2 reaction. Comparison of titration results does not permit an unambiguous choice of Pt O or Pt CO ratios to be used in the titration stoichiometries. However, results from the calorimetric study suggest that the Pt O ratio should be taken as one. Dispersion increases with increasing burn-off of the support, the surface area of the support being the principal factor conditioning the platinum dispersion, although there is also a smaller contribution due to the increase in surface heterogeneity.

The two-stage air-CO2 activation in the preparation of activated carbons. II: Characterization by adsorption from solution

Olive stones and almond shells have been used as raw materials to prepare activated carbons following three different experimental methods: (a) carbonization in N2 followed by activation in CO2, (b) direct activation in CO2 and (c) treatment in dry air at 573 K followed by activation in CO2. The carbons have been characterized by the adsorption of paranitrophenol, methylene blue, orange II, crystal violet and victoria blue, all in aqueous solution. Methods (a) and (b) yield carbons with very similar adsorptive capacities and carbons prepared by method (c) have larger adsorptive capacities for similar overall yields. On the other hand, carbons from almond shells (more microporous) would be more suitable for adsorption of small solutes and carbons from olive stones (more macroporous and consequently, with larger rate of adsorption) for larger dimension molecules.

Platinum catalysts supported on graphitized carbon black—II: Characterization of the platinum by Small Angle X-ray scattering and transmission electron microscopy

Abstract Particle sizes of platinum dispersed on a graphitized carbon black (Vulcan 3G) and samples resulting from its prior oxidation in air at 798 K have been measured by Small Angle X-ray Scattering (SAXS) and Transmission Electron Microscopy (TEM). Mean diameters of platinum particles decrease with increasing burn-off of the carbonaceous supports. The values of mean diameters as determined by these physical methods agree well with those estimated from gas titration procedures. Size distributions of the platinum particles are also described in terms of log-normal functions.

Development of porosity and surface heterogeneity upon air activation of graphitized carbon black

Abstract One graphitized carbon black (Vulcan 3G) and three samples prepared by its activation in air at 978K have been used as supports for platinum catalysts. Surface characteristics of the four supports have been studied by physical adsorption of N2 at 77 and 90K. Surface area increases smoothly with burn-off from 62 m2 g−1 (original V3G) to 121 m2g−1 (52.8% burn-off sample). Porosity and surface heterogeneity develop upon activation but all samples have a large degree of surface homogeneity and low porosity.

The Two-Stage Air–CO2 Activation in the Preparation of Activated Carbons. I. Characterization by Gas Adsorption

Three different methods for the preparation of activated carbons from almond shells and olive stones have been carried out in the temperature range 1073–1123 K: (a) carbonization in N2 followed by activation in CO2; (b) direct activation in CO2 and (c) treatment of the raw material in air at 573 K followed by activation in CO2. The characterization of the samples by gas adsorption and mercury porosimetry has shown that methods (a) and (b) yield similar results (and consequently it is preferable to use the direct activation) and that method (c) gives the best results, both in an adsorptive capacity and in the overall yield. On the other hand, the carbons from almond shells are more micro- and mesoporous whereas those from olive stones are more macroporous; in any case, the adsorptive capacity for gas adsorption of activated carbons prepared from both raw materials is very large.

Adsorption of Hydrocarbons on Air-Reacted Activated Carbons. I. Adsorption Isotherms

The adsorption of several hydrocarbons (benzene, cyclohexane, n-butane, n-hexane, 2,2-dimethylbutane and isooctane) on a series of carbons resulting from the reaction of an activated carbon prepared from almond shells with air at 623 K is discussed. The results are parallel to those obtained with N2 at 77 K (González-Vilchez et al. 1979) but very different to those of CO2 at 273 and 298 K (Rodríguez-Reinoso et al. 1984). A comparison of three different ways of expressing the adsorption data is used to discuss the possible molecular sieving effect of the carbons towards the hydrocarbons.

The n-nonane preadsorption method applied to activated carbons

The n-nonane preadsorption method has been applied to evaluate the microporosity of two series of activated carbons (prepared from olive stones and almond shells) reacted with air at 623 K to different degrees of burn-off. A detailed study of the experimental conditions needed for the successive removal of the n-nonane preadsorbed has been carried out, showing that the sample outgassed at 673 K is identical to the original. In general, the results obtained with the preadsorption of n-nonane are satisfactory only in carbons with a narrow microporosity. On the other hand, this method has been used to calculate the apparent surface area of the carbons from the n-nonane retained using an Am value of 0.844 nm2, the results being in good agreement with the N2 BET values. An analysis of the evolution with burn-off of the parameters C(BET) and D(DR) has also been carried out and their relationship calculated; both parameters describe reasonably well the evolution of the microporosity of the carbons.