Xosé L. Seoane

NO reaction at surface oxygen vacancies generated in cerium oxide

NO adsorption on CeO2 pre-outgassed at different temperatures (Tv) has been studied by EPR and FTIR. The results are compared with those obtained by measuring the products of the reaction, performed at 323 K, between NO and CeO2 samples activated by previous heating under N2 flow. EPR results indicate that, for Tv 573 K, NO molecules are adsorbed in radical form at 77 K and react at 298 K with surface oxygen vacancies generating diamagnetic products. FTIR spectra show correspondingly the formation of hyponitrites, nitrites and nitrates upon NO adsorption at 298 K on CeO2 outgassed at different temperatures. The hyponitrites can decompose at ambient temperatures to produce N2O; the process leads to modification (probably reoxidation) of the surface centres initially present. This can be followed by O2 adsorption and EPR spectroscopy, which shows that O2– adsorbed on the modified vacancies is less stable than on CeO2 without NO adsorbed, its reaction leading in some cases to the formation of peroxynitrate-type radicals. The reaction at 323 K of NO on the pretreated CeO2 surface produced N2O for pretreatment temperatures TF 573 K, in amounts increasing with TF. The results indicate that surface centres containing associated oxygen vacancies are active sites for this process.

Role of the countercations on the molecular sieve properties of a clinoptilolite

Abstract Samples of clinoptilolite enriched with Na + , K + , Cs + , NH 4 + , Ca 2+ , Mg 2+ , and Ba 2+ cations were prepared by ion-exchange. The degree of exchange for the different samples is discussed in terms of location and diffusivity of the cations. The thermal stability of natural clinoptilolite and of the enriched forms, as determined by XRD, is related to the nature and location of the major countercation in the zeolite framework. The specific retention volumes of O 2 , N 2 , CO and CH 4 on each sample and the adsorption enthalpies of these gases were measured by a chromatographic method in the range of 298–393 K. The results are explained on the basis of the strength of the interaction molecule-adsorption sites of the zeolite, taking into account the diffusivity of gas molecules through the zeolite channels, and their accessibility to the adsorption sites, according to the more probable location of the cation in the lattice.

On the deactivation of supported palladium hydrogenation catalysts by thiophene poisoning

The deactivation of supported palladium catalysts for ethylbenzene hydrogenation by thiophene was studied. The presence of Pdn+ species on the catalyst surface, in samples prepared from acid solutions of PdCl2, and reduced at temperatures below 450 °C, was evidenced by XPS. A correlation between the concentration of electron-deficient Pd species and the sulfur resistance was found. Thus, the higher the value of the Pdn+/Pd0 ratio, the higher the sulfur resistance. Catalysts prepared from Pd(NO3)2 or from PdCl2 reduced at 450 °C, which essentially contain Pd0, are more quickly deactivated. Furthermore, our results also suggest that while the influence of the metal dispersion on the stability of the palladium catalysts toward sulfur is not significant, the nature of the support, on the other hand, plays an important role.

Comparative Study of the Deactivation of Group Viii Metal Catalysts by Thiophene Poisoning in Ethylbenzene Hydrogenation

Summary The deactivation of Group VIII metal catalysts by thiophene poisoning in ethylbenzene hydrogenation has been studied. With the exception of Pt, the sequence of sulfur resistance found, Pt

Effect of Ba and rare earths cations on the properties and dehydrocyclization activity of Pt/K‐LTL catalysts

Several Pt/L‐zeolite catalysts were prepared by the impregnation method from L‐zeolite samples previously exchanged with Ba2+, La3+, Pr3+, Nd3+ and Sm3+. Lanthanides (Ln3+) increase the overall dispersion of platinum, measured by H2–O2 titration. TPR and CO/FTIR measurements indicate that these ions modify the distribution of Pt in the zeolite lattice, increasing the fraction of metal on the external surface. Additionally, both CO/FTIR and competitive hydrogenation of toluene–benzene mixtures indicate that, in the presence of Ln3+, the electron density of the platinum decreases in comparison with Pt/KL‐zeolite. On the other hand, Ba2+ does not substantially modify neither the distribution nor the electronic state of Pt. The reactivity measurements in the hydroconversion of n‐heptane show that Pt/BaKL and Pt/KL exhibit similar catalytic behaviour with a high dehydrocyclization activity. However, the exchange of K+ by Ln3+ increases the production of heptane isomers, while selectivities to aromatic and terminal hydrogenolysis products significantly decrease.

Heptane dehydrocyclization over Pt/KL catalysts doped with barium or lanthanum

Several 1 wt% Pt/KL catalysts doped with different concentrations of either BaO or La2O3 were prepared by successive impregnation of a zeolite KL and then characterized by H2-O2 titration, TPR, CO–FTIR, TEM-XEDS and XPS. Catalytic activity measurements in the hydroconversion of n-heptane showed that barium highly enhances the aromatization activity of Pt/KL, the more so the higher the barium concentration. The yield to aromatics increases to a lesser degree by adding 1 wt% La, but it is little modified at higher La concentrations. The CO–FTIR results suggest that the promoter effect of barium is related to an electron enrichment of Pt produced by the BaO → Pt0 interaction which, according to the TEM–XEDS and XPS results, is more favored than the La2O3 → Pt interaction.

Effect of Iron on the Deactivation of Ni/Clinoptilolite Catalysts by Thiophene Poisoning

Deactivation of several Ni catalysts supported on a K-clinoptilolite containing iron species was analyzed in the hydrogenation of ethylbenzene in the presence of thiophene. The location and coordination of iron in the zeolite was modified by different thermal treatments given to the catalysts. Studies by ESR showed that the state of iron affects the nature and strength of the iron–nickel interaction. Experimental results indicated the existence of a correlation between that interaction and the resistance of the catalysts to thiophene poisoning, in such a way that the stronger the iron–nickel interaction the higher the sulfur resistance. An interpretation of the differences in sulfur resistance of the catalysts based on the formation of surface electron-deficient Ni species by interaction of Ni particles with Fe3+ of the zeolite is discussed.

Effect of reduction temperature and support on the surface electronic state of supported Pd catalysts

The effect of metal precursor, support and reduction temperature on the surface reduction of Pd on α-Al2O3 and SiO2 has been studied. For catalysts prepared from PdCl2, temperatures as high as 450 °C are necessary to obtain surface Pd0.AbstractИсследовали влияние металлического прекурсора, носителя и температуры на поверхностное восстановление Pd на α-Al2O3 и SiO2. Найдено, что для катализаторов, приготовленных из PdCl2, для образования Pdo на поверхности необходима температура по-крайней мере 450°C.

Effect of the CeO2 dispersion on alumina on its reactivity for co and no conversion

Abstract The reactivity of CeO 2 , pure or supported on alumina, in NO adsorption and catalytic activity for the CO+NO+O 2 reaction (in the absence of metal), has been tested using IR, ESR and temperature-programmed desorption/reactivity techniques. Significant differences are found in alumina-supported ceria in comparison with pure CeO 2 ; its lower activity can be connected to the difficulty of generating on well-dispersed CeO 2 /Al 2 O 3 the associated oxygen vacancy centers observed on CeO 2 , which are able to activate reductively NO to give N 2 O. The same centers might play also a role in enhancing the reduction of surface nitrite species by CO simultaneously with the combustion of the latter molecule.

Hydrodechlorination of carbon tetrachloride to chloroform in the liquid phase with a Pd/carbon catalyst. Study of the mass transfer steps

The e7ect of the resistance to interface mass transfer on the overall rate of the hydrodechlorination of carbon tetrachloride has been analyzed with a 1% Pd=carbon catalyst, in a bubble column slurry reactor. The reaction was carried out at 393 K and 3 MPa. The e7ect of catalyst concentration on the external di7usion was analyzed for 7:96; 19:09; 39:77 and 63:64 g l −1 TTCM (0:5; 1:2; 2:5 and 4 wt%, respectively) with an H2 <ow F(H2 )=2 4× 10 3 Nc m 3 h −1 and an average particle size of the catalyst, dp =7 5� m. The importance of the gas–liquid resistance for F(H2 )=3 × 10 3 ,1 2× 10 3 ,1 8× 10 3 ,2 4× 10 3 and 30 × 10 3 Nc m 3 h −1 was investigated with a catalyst concentration of 1:2 wt%. The e7ect of dp for 25, 75 and 130 � m was similarlystudied for F(H2 )=2 4× 10 3 Nc m 3 h −1 and 1:2 wt% of catalyst. The