. Display omitted
► A simple temperature-programmed procedure was used. ► Both CH4/CO2 reforming and coke formation via CH4 or CO decomposition were studied. ► Bimetallic Pd(or Pt)-Ni catalysts on ...zirconia and alumina were investigated. ► The co-impregnated Ni-Pd/ZrO2 catalyst looks the most promising for an industrial application.
Catalytic reforming of CH4 with CO2 was investigated on mono and bimetallic Pd(or Pt)-Ni based samples supported on Al2O3 and ZrO2. A simple temperature-programmed procedure was used to study both reforming and coke formation via CH4 or CO decomposition. Catalysts supported on zirconia are more active than alumina-supported samples. The addition of Pt or Pd to Ni/ZrO2, preferably by co-impregnation, prevents coke formation. The co-impregnated Ni-Pd/ZrO2 catalyst looks the most promising for a possible industrial application.
Quantitative measurements of CO chemisorption in the range 140–180 K, supported by FTIR data on adsorbed CO, were performed on Au/TiO
2, Au/Fe
2O
3, and Au/CeO
2 catalysts. On the first two samples, ...which had similar particle size distributions, an average Au/CO chemisorption stoichiometry of about 3, referred to step-edge Au atoms, was found. On Au/CeO
2, where very small clusters and quite large particles are present, the CO-chemisorbed volume was much higher than expected, due to the prevailing contribution of very small Au clusters. On the same sample, a change in the IR absorption coefficient was observed and was reasonably explained.
In his heyday, Carlo Tresca ranked among the most important radicals and labour activists in the United States, often sharing the spotlight with Elizabeth Gurley Flynn, 'Big Bill' Haywood, and Emma ...Goldman. A charismatic Italian anarchist who became a folk hero to immigrant and native-born workers alike, Tresca was described by comrades as a 'freelance revolutionary' because of his independent spirit and militant activism. During his wild and adventurous career spanning nearly forty years (1904-1943), Tresca pursued a range of activities unmatched by any of his radical contemporaries: independent newspaper editor, labour agitator and organizer, civil libertarian, foremost leader of the Italian American anti-fascist resistance, and an indomitable foe of Stalinism. Culminating over a decade of research, this fast-paced and vivid biography brings to life the volatile world of radical politics in early twentieth-century America through one of its foremost figures.
Pd catalysts on different supports (active carbon, silica, alumina) were studied for the selective hydrogenation of benzaldehyde to benzyl alcohol. They were characterized by TPR, CO chemisorption ...and XRD. Batch hydrogenation tests were performed before and after sulfur poisoning. Strong metal–support interaction was found for Pd/alumina, giving high Pd dispersion. Chemisorption stoichiometry Pd/CO=2 was confirmed again. While Pd/C has the highest activity for benzaldehyde hydrogenation, a satisfactory selectivity to benzyl alcohol requires an oxidic carrier or proper sulfur poisoning of Pd/C.
Contradictory results about the best oxidic precursor of Fe ammonia synthesis catalyst prompted the present comparative investigation on wustite- and magnetite-based catalysts. Many physical ...(density, porous texture, crystalline phases, reduction rate, metal surface, abrasion loss) and catalytic (kinetic constants, thermoresistancy) properties have been determined on both catalysts. The wustite-based catalyst proved to be much more active, especially at lower temperatures, approaching the performances of Ru/C catalyst, except at high conversion. Possible reasons for such a behavior of the wustite-based catalyst are discussed, suggesting that a reconsideration of the present consolidated knowledge on Fe ammonia synthesis catalyst might be convenient.
In this work we have determined, on a series of 0.5% Pd/C catalysts, the palladium particle sizes by the following physical techniques: (i) X-ray diffraction (XRD) line broadening (LB) method, ...associated with the Rietveld method, (ii) small-angle X-ray scattering, and (iii) transmission electron microscopy. The catalysts, suitably aged at different temperatures (673, 773, 873, and 973 K), had significantly different metal dispersions. Since the XRD-LB technique is not able to measure directly very small metal particles or clusters (roughly ≤25 Å in size), because they give diffuse X-ray scattering spreading out into the background, we have tackled this problem by means of a suitably tailored Rietveld quantitative analysis. This analysis allowed determination of the Pd fraction “visible” in the Voigtian XRD peaks and its average crystallite size using the LB method. As to the nanoparticle size of the undetectable fraction, an average value of 20 Å was assumed, corresponding to the size of a cubooctahedral perfect cluster of the fourth order. Combining all these data, real effective Pd average particle sizes could be calculated and compared with the corresponding values found by CO chemisorption. It was found that a surface Pd/CO stoichiometry of 2 must be assumed, irrespective of Pd dispersion, to get correct values of the average Pd particle size.
Due to the multiplicity of problems recently evidenced by the widely used N
2O decomposition technique, the oxygen chemisorption technique for the measurement of Cu surface area was updated and ...optimized. A standard procedure was defined, which allows to obtain reliable and reproducible data. In particular, no arbitrary assumption was made for the Cu/O chemisorption stoichiometry and for the degree of surface coverage. Conversely an experimental value of the average Cu/O surface ratio, including the degree of coverage, was determined by XRD–WAXS data. The value obtained (2.35) corresponds to a chemisorption stoichiometry Cu/O=2 and to about 15% unoccupied surface Cu atoms. The proposed technique has been successfully used for the characterization of several commercial low-temperature water gas shift catalysts (Cu-ZnO-Al
2O
3).
A polydomain internal structure was evidenced by means of High Resolution Transmission Electron Microscopy (HRTEM) in palladium nanoparticles of two 5% Pd/SiO2 catalysts prepared by impregnation ...technique. Such poly-domain structure biased the line broadening analysis of the XRPD patterns, from which highly underestimated average diameters were obtained in comparison to those measured by means of Transmission Electron Microscopy (TEM).In both the investigated catalysts, a Pd/CO average chemisorption stoichiometry close to 2 was found, in good agreement with previous results obtained for Pd/C and Pd/γ-Al2O3 catalysts. The consistency of this result, in cases with different particle sizes and different support materials, suggests that such average stoichiometry is of general validity for palladium, when chemisorption measurements of CO are made using pulse flow technique.
A series of carbon-supported, ruthenium-based catalysts, variously promoted with alkali and alkali-earth compounds, were prepared, aiming at investigating the effect of such promoters on catalyst ...activity and stability. It was found that a simultaneous action of three promoters (K+Ba+Cs) maximised both activity and thermal resistance of the catalyst. In particular, Ba is very effective in providing catalyst activity and resistance to methanation, while Cs strongly improves resistance to metal sintering. A further activity improvement is given by K as third promoter. Through a XPS study, the promoter effect was confirmed to be essentially of electronic nature. The optimal catalyst composition, in terms of activity, thermal resistance in the reaction environment and cost, corresponds to Ru ca. 5
wt.%, Ba/Ru=0.6, Cs/Ru=1 and K/Ru=3.5 atomic ratios.
PDF
