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The present work attempts to address the issue whether iron (Fe) which is accumulated on the surface of "three-way" catalysts (TWCs) used in gasoline-driven cars is a true chemical poison of their catalytic activity. This important issue from a scientific and technological point of view is addressed via catalytic activity, temperature-programmed surface reaction (TPSR), and X-ray photoelectron spectroscopy (XPS) measurements over a model TWC (1 wt% Pd-Rh/20 wt% CeO2-Al2O3). It was found that deposition of Fe up to the level of 0.4 wt% (an average concentration found in aged commercial TWCs) on the model TWC does not deteriorate its activity towards CO and C3H6 oxidation, and reduction of NO by H2. Instead it was found that iron improves significantly the T50 parameter in the activity versus temperature profile. Small Fe clusters in contact with the noble metal (Pd and Rh) particles due to the lower work function of Fe compared to Pd and Rh act likely as a source of electron flow towards the noble metals (as evidenced by XPS measurements), thus altering their surface work function and adsorption energetics of reaction intermediates. The latter have increased significantly the activity of the model TWC towards oxidation of CO and propylene, and to a lesser extent the activity towards the reduction of NO by H2. The presence of Fe on the surface of the model TWC provided and/or created also new active catalytic sites for the reactions investigated. According to previous work from this laboratory, iron up to the level of 0.4 wt% was shown not to deteriorate the oxygen storage capacity (OSC) of the same model TWC used in the present work. Thus, it could be concluded that Fe when deposited on a commercial TWC at least up to the level of 0.4 wt% acts likely as a promoter than a poison of its catalytic activity. © 2007 Elsevier B.V. All rights reserved. 76 3-4 375 385 Cited By :29

The present work reports on the production of H2 and secondary carbon nanotubes (CNTs) during catalytic decomposition of ethylene over a novel catalytic system, namely, nickel supported on carbon nanotubes (Ni/CNTs) at remarkably low-temperatures, e.g. 400 °C. A number of catalyst parameters were investigated, namely the chemical nature of support, the Ni metal loading (0.1-10 wt%), the nature of nickel metal precursor (organometallic vs. inorganic) used during catalyst synthesis, and the nature of transition metal used (e.g. Co, Fe, Cu, Ni). Among the different Ni/CNT supported catalysts investigated, 0.5 wt% Ni/Ros1-B1 (Ros1-B1 a commercial CNT) presented the highest activity in terms of H2 production (296 mol H2/gNi) and carbon capacity (3552 gC/gNi). In terms of transition metal used as active catalytic phase, the activity (moles H2 per gram of metal) was found to decrease in the order Co ≫ Fe > Cu. The activity of supported Ni and Co catalysts was found to strongly depend on the metal loading. The structural and morphological features of primary (catalytic support) and secondary carbon nanotubes produced during ethylene decomposition at 400 °C were studied using X-ray Diffraction (XRD), scanning electron microscopy (SEM), High-resolution Transmission Electron Microscopy (HRTEM), and X-ray Photoelectron Spectroscopy (XPS). The production of secondary carbon nanotubes at 400 °C was confirmed after using HRTEM and after a comparison with the primary carbon nanotubes of catalyst support was made. Different regeneration conditions (use of oxygen or steam) were investigated in order to remove by gasification the amorphous carbon deposited under reaction conditions. Oxygen appeared to be a better regeneration reagent than steam, where after ten consecutive reaction/regeneration cycles the 0.5 wt% Ni/Ros1-B1 catalyst showed high and stable activity with time on stream. © 2009 Elsevier B.V. All rights reserved. 93 3-4 314 324 Cited By :14

Ethylene decomposition over Ni supported on novel carbon nanotubes (CNT) and nanofibers under consecutive reaction/regeneration cycles to form CO-free hydrogen and carbon deposits have been investigated. The present work highlights the effects of support chemical composition, catalyst synthesis method and Ni metal loading on the catalytic activity and stability of nickel. A novel 0.5 wt.% Ni/CNT catalyst which presents the highest value of a constant hydrogen product yield (17.5 mol H2/mol Ni), following consecutive reaction (complete deactivation) → regeneration (20% O2/He, 400 °C) cycles, ever reported in the open literature has been developed. Transmission electron microscopy (TEM) and XRD studies revealed that the 0.5 wt.% Ni/CNT catalyst promotes the formation of two types of carbon nanofibers during ethylene decomposition at 400 °C, result that reduces significantly the rate of Ni encapsulation during reaction. The opposite is true in the case of 0.3 wt.% Ni/SiO2 catalyst. In the case of Ni/CNT, there is a narrow range of Ni loading for which maximum H2 product yield is obtained, while in the case of Ni/SiO2, a monotonic increase in the H2 product yield is obtained (Ni loading in the range 0.3-7.0 wt.%). © 2005 Elsevier B.V. All rights reserved. 102-103 78 84 Cited By :19