A. Guerrero-Ruiz

Platinum catalysts supported on activated carbons: I. Preparation and characterization

Several Pt catalysts supported on activated carbons (manufactured from olive stones and almond shells) have been prepared with both H2PtCl6 · 6H2O and [Pt(NH3)4]Cl2 as metal precursor and using different methods. Once reduced, the supported catalysts were characterized by H2 and CO chemisorption as well as X-ray diffraction and transmission electron microscopy. The effect of reduction conditions on metal dispersion has been studied and correlated with the surface properties of the supports. The results show that porosity with sizes ranging from 9 to 11 nm is decisive for obtaining a high Pt dispersion. To increase the dispersion of the catalysts prepared from [Pt(NH3)4]Cl2 a treatment with He prior to reduction of the catalysts in H2 seems to be essential in order to avoid the formation of an unstable hydride which leads to agglomeration of the Pt particles.

The role of nitrogen and oxygen surface groups in the behavior of carbon-supported iron and ruthenium catalysts

Abstract A carbon prepared from a copolymer Saran, and the products resulting from its reaction with NH3 and HNO3 under different experimental conditions to introduce nitrogen and oxygen surface groups, have been used as supports for Fe and Ru catalysts. The catalysts have been characterized by H2 and CO chemisorption, X-ray line broadening and transmission electron microscopy were used to estimate metal particle sizes. There is, for Fe catalysts, an increase in hydrogen uptake and a decrease in CO uptake with increasing nitrogen content of the support and a decrease of both uptakes with increasing oxygen content. There is, however, in the case of Ru, an increase in both H2 and CO uptake with increasing nitrogen content of the support. The results obtained in the hydrogenation of CO at 523 K and 101 KPa do not, in all cases, follow the sequence observed in the chemisorption experiments; it seems as if the functional groups introduced on the carbon are not sufficient to produce sensible changes in activity or selectivity of the catalysts in this reaction. Some effect is observed only in highly dispersed Fe catalysts supported on carbons treated with ammonia. Moreover, the hydrogenolysis of n-butane at 573 K (Fe catalysts) or 413 K (Ru catalysts) seems to be clearly affected by the nitrogen content of the support, especially in catalysts prepared from Fe(CO)5.

Adsorption capacity of Saran carbons at high temperatures and under dynamic conditions

Abstract Several activated carbons were prepared by carbonizing Saran 872 at different temperatures. The capacity of these carbons to adsorb several hydrocarbons was studied using a gas-chromatographic technique at high temperatures. All the carbons had pores of less than 0.62 nm in size. The sample obtained at 1573K in a He atmosphere had slit-shaped pores which acted as a molecular sieve for cyclohexane. These pores were opened by mild gasification of the sample, resulting in the loss of molecular sieve properties. Heats of adsorption of the hydrocarbons were calculated from the specific net retention volumes. The effect of oxygen surface complexes on hydrocarbon specific net retention volumes was also studied.

Platinum catalysts supported on activated carbons: II. Isomerization and hydrogenolysis of n-butane

Abstract A series of carbon-supported Pt catalysts prepared using different precursors and methods and covering a wide range of metal dispersion (8–51) have been studied in the reaction of n-butane with hydrogen. These are basically hydrogenolysis catalysts in which the splitting of the terminal CC bond predominates. The hydrogenolysis of n-butane is a reaction sensitive to the structure of the catalysts, whereas this is not the case for isomerization. The selectivity for hydrogenolysis increases with decreasing mean Pt particle size. The apparent activation energies for the two reactions (larger for isomerization) are independent of particle size.

The effect of inorganic constituents of the support on the characteristics of carbon-supported platinum catalysts

Abstract Two commercial activated carbons and the corresponding demineralized samples have been used as catalyst supports for platinum, in order to learn about the role played by the inorganic constituents of the carbons on the catalyst characteristics. CO and H2 chemisorption and X-ray line broadening have been used to determine metal dispersion and mean particle size. The results indicate that the inorganic matter of the carbon plays an important role in the stabilization of the average platinum particle size by making sintering more difficult. H2-C2H4 titrations have also been used to determine the platinum particle size; the results found are in good agreement with H2 chemisorption, especially for catalysts supported on carbons with larger micropore volume and lower meso and macropore volume.

Hydrogenolysis of n-butane and hydrogenation of carbon monoxide on Ni and Co catalysts supported on saran carbons

Abstract Ni and Co catalysts were supported on Saran carbons and characterized by CO and H 2 chemisorption and X-ray diffraction. For Ni catalysts, the mean Hydrogenation of CO is not a demanding reaction either for Ni or Co catalysts in the dispersion range we have studied. Methane was the main product obt The formation of olefins is suppressed in the supported Co catalysts as compared with the bulk one.

Effect of hydrogen reduction on the surface characteristics of carbon-supported iron and ruthenium catalysts

Abstract Carbons prepared by carbonization and subsequent activation of the copolymer Saran and later treated with NH 3 and HNO 3 to introduce nitrogen and oxygen surface complexes have been used as supports of Fe and Ru catalysts (5% metal loading). The effect of reduction in H 2 at 673 and 723 K on the surface properties of the catalysts has been studied by means of N 2 (77 K) and CO 2 (298 K) adsorption and mercury porosimetry. The different degrees of gasification undergone by the various types of support and the subsequent widening of microporosity are reflected in the adsorption data. An in-depth discussion of the changes in microporosity and their different effects on the S N 2 and S CO 2 values is presented, based on different adsorption mechanisms of the two adsorbates.

High temperature adsorption of hydrocarbons by activated carbons prepared from olive stones

The evolution of microporosity of activated carbons prepared from carbonized olive stones as a function of burn-off in carbon dioxide (activating agent) has been studied at high temperature using a gas chromatographic technique. The specific net retention volumes have been used to determine the possible molecular sieve properties of the carbons towards several hydra-carbons. This molecular sieving is thought to be due to the existence of slit-shaped pores in the carbons with well-defined dimensions. The retention data have also been used to calculate the heats of adsorption and their dependence on the porosity of the carbons.

INFLUENCE OF THE PARTICLE SIZE OF METAL IN THE HYDROGENOLYSIS OF N-BUTANE ON CARBON suPPORTED IRON CATALYSTS

The hydrogenolysis of n-butane on different carbon supported iron catalysts has been studied. Changes in activity, product distribution, apparent activation energy and frequency factor were found to be a function of particle size. This behavior is explained as a change in the reaction mechanism, which also leads to the isomerization of n-butane in the case of catalysts with higher dispersity.AbstractБыл изучен гидрогенолиз н-бутана на различных железных катализаторах на углеродных носителях. Изменения в активности, распределеление продуктов, кажущаяся энергия активации и частотный фактор находятся в зависимости от размеров частиц. Это поведение обьяснялось изменением механизма реакции, что также приводит к изомеризации н-бутаха в случае катализатора с высокой дисперсностью.

Characterization of carbon-supported iron catalysts prepared from Fe(CO)5

Iron catalysts have been prepared by adsorption of iron pentacarbonyl on activated carbon. After reduction in H2 flow at different temperatures, metal dispersions and average crystallite sizes have been studied. The catalysts have been characterized by H2 and CO chemisorption at 373 K, X-ray diffraction line broadening and transmission electron microscopy. The results obtained show that the iron crystallites prepared by this method present a narrow particle size distribution and that the iron catalysts have high metal dispersions. It may be concluded that CO chemisorption at 373 K can be a good technique to study iron supported catalysts.