121,827 research outputs found
Graphitised carbon as support for Ru/C ammonia synthesis catalyst
No full textIn the present work, we compared the catalytic activity and mainly the stability under the usual ammonia synthesis conditions, of some carbon supports, differing as for their nature, purity and temperature of pretreatment. The effect of catalyst composition (metal and promoters loading) on stability was also investigated. XRD and N-2 adsorption/desorption analysis helped in elucidating the effect of carbon treatment. It was found that only after the support has been heated at least at 1900 degrees C the stability of the tri-promoted catalyst becomes fully satisfactory and virtually independent of Ru loading. For practical purposes such high temperature treatment must be compatible with the preservation of a sufficiently high surface area
Carbon-supported promoted Ru catalyst for ammonia synthesis
No full textA series of alkali- and/or alkali-earth-promoted, carbon-supported Ru catalysts have been prepared by impregnation from aqueous solutions of the precursors. The supports have been pretreated by heating in inert atmosphere at various temperatures, followed by partial oxidation in air at 425 degrees C and then by hydrogen treatment at 900 degrees C. The catalyst samples, diluted 1/22 with quartz powder, have been studied in ammonia synthesis by means of a bench-scale, downflow, continuous, tubular reactor, under standard reaction conditions (430 degrees C, 100 bar total pressure, H-2/N-2 = 1.5/1 feeding ratio). Caesium and Barium proved to be much more effective promoters than Potassium as promoters and the optimal temperature range for support pretreatment was found to be about 1900 degrees C. The ammonia productivity, on a catalyst volume basis, of our best Ru catalyst was about twice higher than that of the most widely used Fe-based commercial catalyst
Promoters effect in Ru/C ammonia synthesis catalyst
No full textA 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
Characterisation of Ru/C catalysts for ammonia synthesis by oxygen chemisorption
No full textA standard chemisorption procedure has been set up for the determination of Ru dispersion in Ru/C catalysts. Pulse chemisorption of oxygen was carried out at 0 °C, after having proved that no corrosive chemisorption phenomena are present. An average chemisorption stoichiometry was experimentally determined through measurements on Ru black. The procedure was applied to the investigation of promoted and unpromoted Ru/C catalysts for ammonia synthesis, supported on two different carbon supports. The main factor influencing Ru dispersion showed to be Ru loading, while the addition of even large amounts of Ba–Cs–K promoters has practically no influence. It is also briefly discussed how such results can help in elucidating several aspects of the behaviour of Ru/C as catalyst for ammonia synthesis
Ru loading effect on Ru/C ammonia synthesis catalyst
No full textThis work deepens the study of the formulation of Ru/C catalysts for ammonia synthesis. The focus is posed on the optimisation of the content of Ru in order to achieve a competitive cost of the catalyst
Influence of preparation procedure on physical and catalytic properties of carbon-supported Pd-Au catalysts
No full textThe influence of preparation variables on the physical and catalytic properties of the Pd-Au/C system, taking benzaldehyde hydrogenation as a probe reaction, has been investigated. Catalysts were prepared by wet impregnation (simultaneous and consecutive). TPR, CO chemisorption and WAXS were used for catalyst characterization, in addition to catalytic tests. The chemisorption data point to the absence of Au surface enrichment in the fresh samples. The metal dispersion decreases upon heating at temperatures up to 500°C, independently on Au presence. WAXS data show that coimpregnation gives rise to a Pd-Au alloy, while consecutive impregnation gives progressive alloying only after heat treatment. An appreciable amount of very small Pd particles is likely to be present in all the samples. For monometallic Pd samples the turnover frequency appears to increase with Pd particle size, but this does not occur in the presence of Au. Au addition to Pd catalysts does not increase their activity in the hydrogenation of aromatic aldehydes
Detecting Palladium nanoparticles in Pd/C catalysts using X-Ray Rietveld method
No full textThis study uses X-ray diffraction (XRD) techniques to investigate the nanostructural features of a series of four Pd/C catalysts, which had the same load, 0.51 +/- 0.02 wt%, as palladium, with significantly different dispersions, obtained by applying different temperature ageings up to 873 K. By means of a Rietveld refinement, performed using a special fitting procedure, which takes into account the various contributions to the background scattering, the palladium fraction due to nanoparticles or clusters smaller than about 20-25 Angstrom could be determined. We have compared this Rietveld (absolute) quantitative method with a simpler, but less precise, single-peak (relative) XRD analysis, interesting for fast industrial applications. The Pd fractions due to nanoparticles, as determined by the two methods, are close each other for all samples investigated, apart from one for which the disagreement is near 20%
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