Abstract: The present work reports on the production of H-2 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 degrees 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 (Rosl-B1 a commercial CNT) presented the highest activity in terms of H2 production (296 mol H-2/g(Ni)) and carbon capacity (3552 g(C)/g(Ni)). In terms of transition metal used as active catalytic phase, the activity (moles H-2 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 degrees 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 degrees 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. (C) 2009 Elsevier B.V. All rights reserved.
Abstract: Steady-state isotopic transient kinetic analysis (SSITKA), transient isothermal, and temperature-programmed surface reaction in H-2 (H-2-TPSR) techniques coupled with online mass spectroscopy (MS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were used to study essential mechanistic and kinetic aspects of the selective catalytic reduction (SCR) of NO with the use of H-2 under strongly oxidizing conditions (H-2-SCR) over a novel Pt/MgO-CeO2 catalyst. The main focus was to study and report for the first time the effects of reaction temperature on the chemical structure and surface concentration of the active NOx intermediate species thereby formed. The information obtained is essential to understanding the volcano-type profile of the catalyst activity versus reaction temperature observed here and also reported previously. In the present work, two active NOx intermediate species identified by SSITKA-DRIFTS were found in the nitrogen-reaction path toward N-2 and N2O formation one, species located in the vicinity of the Pt-CeO2 Support interface region (nitrosyl [NO+] coadsorbed with a nitrate [NO3-] species on an adjacent Ce4+-O2- site pair) and the second located in the vicinity of the Pt-MgO support interface region. The chemical structure of the second kind of active NOx species was found to depend on reaction temperature. In particular, the chemical structure was that of bidentate or monodentate nitrate (NO3-) at T < 200 degrees C and that of chelating nitrite (NO2-) at T > 200 degrees C. The concentration of the active NOx intermediates that lead to N-2 formation was found to be practically independent of reaction temperature (120-300 degrees C) and significantly larger than 1 equivalent monolayer of surface Pt (theta(NOx) = 2.4-2.6). The former result cannot be used to explain the volcano-type behavior of the catalytic activity versus the reaction temperature observed; alternative explanations are explored. The H-spillover process involved in the H-2-SCR mechanism was found to be limited within a support region of about a 4-5 angstrom radius around the Pt nanoparticles (d(pt) = 1.2-1.5 nm). (c) 2008 Elsevier Inc. All rights reserved.
Abstract: Nickel supported on alumina catalysts have been prepared using various synthesis methods (dry impregnation, co-precipitation, sol-gel) and evaluated for the hydrogenation of benzene contained in gasoline. The evaluation was carried out in a laboratory scale high pressure fixed bed reactor fed with a stream of surrogated reformate gasoline consisted by a mixture of hexane, benzene and toluene. All catalysts have been characterized by the joint use of various physicochemical characterization methods (XRF, BET, TGA, SEM, XRD, UV-vis DRS and XPS) in order to correlate their catalytic performances to their physicochemical properties. The results obtained revealed that sol-gel procedure, especially when it is followed by supercritical drying (aerogel), produced the most promising catalysts for the aforementioned catalytic process. Sol-gel methodology ensured the best compromise between dispersion of the nickel phase and its interaction with the support surface. Co-precipitated catalysts exhibited important activities but lower than those of the sol-gel catalysts. The catalyst prepared by dry impregnation proved to be the less active. Calcination of the catalysts before their activation by reduction decreased their activities. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: Steady State Isotopic Transient Kinetic Analysis (SSITKA) experiments using on-line Mass Spectrometry (MS) and in situ Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS) have been performed to study essential mechanistic aspects of the Selective Catalytic Reduction of NO by H-2 under strongly oxidizing conditions (H-2-SCR) in the 120-300 degrees C range over a novel 0.1 wt % Pt/MgO-CeO2 catalyst. The N-path of reaction from NO to the N-2 gas product was probed by following the (NO)-N-14/H2O2 -> (NO)-N-15/H-2/O-2 switch (SSITKA-MS and SSITKA-DRIFTS) at 1 bar total pressure. It was found that the N-pathway of reaction involves the formation of two active NO (x) species different in structure, one present on MgO and the other one on the CeO2 support surface. Inactive adsorbed NO (x) species were also found on both the MgO-CeO2 support and the Pt metal surfaces. The concentration (mol/g cat) of active NO (x) leading to N-2 was found to change only slightly with reaction temperature in the 120-300 degrees C range. This leads to the conclusion that other intrinsic kinetic reasons are responsible for the volcano-type conversion of NO versus the reaction temperature profile observed.
Abstract: We describe here the performance of a novel MgO-CeO2-supported Pt (0.1 wt%) catalyst towards the selective conversion of NO into N-2 (S-N2 > 80%) by using H-2 (H-2-SCR) under process conditions similar to those encountered in the NH3-SCR in the low-temperature range of 150200 degrees C. At 200 degrees C, 100% conversion of NO and 85% N-2-selectivity were obtained with a feed stream containing 1000 ppm NO, 5% O-2, 5% H2O, 10% CO2,0-0.5% CO, and using 1.5% H-2 in the feed as reducing agent (GHSV = 40,000 h(-1)). Thus, a N-2-yield of 85% similar to that obtained in most NH3-SCR applications could make H-2-SCR as the most environmentally friendly NO, control catalytic technology with great potential to replace the existing NH3-SCR technology. The latter is currently used industrially mainly in power and nitric acid plants, gas turbines, furnaces, boilers, and waste incinerators for the elimination of No,. However, this technology faces several problems such as catalyst deterioration, emissions of non-reacted toxic NH3 (ammonia slip), ash odor, air-heaters fouling, and a high running cost. (c) 2007 Elsevier B.V. All rights reserved.
Abstract: 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 H-2-Instead it was found that iron improves significantly the T-50 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 H-2. 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. (C) 2007 Elsevier B.V. All rights reserved.
Abstract: 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 H-2/mol Ni), following consecutive reaction (complete deactivation) -> regeneration (20% O-2/He, 400 degrees 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 degrees C, result that reduced 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 H-2 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.%). (c) 2005 Elsevier B.V. All rights reserved.
Abstract: A 0.1 wt% Pt supported on La0.7Sr0.2Ce0.1FeO3 solid (mixed oxide containing LaFeO3, SrFeO3-x, CeO2, and Fe2O3 phases) has been studied for the NO/H-2/O-2 reaction in the 100-400degreesC range. For a critical comparison, 0.1 wt% Pt was supported on SiO2, CeO2, and Fe2O3 and tested under the same reaction conditions. For the Pt/La0.7Sr0.2Ce0.1FeO3 catalyst a maximum in the NO conversion (83%) has been observed at 150degreesC with a N-2 selectivity value of 93%, while for the Pt/SiO2 catalyst at 120degreesC (82% conversion) with a N-2 selectivity value of 65% using a GHSV of 80,000 h(-)1 Low N-2 selectivity values, less than 45%, were obtained with the Pt/CeO2 and Pt/Fe2O3 catalysts in the 100-400degreesC range. For the Pt/La0.7Sr0.2Ce0.1FeO3 catalyst, addition of 5% H2O in the feed stream at 140degreesC resulted in a widening of the operating temperature window with appreciable NO conversion and no negative effect on the stability of the catalyst during 20 h on stream. In addition, a remarkable N-2 yield (93%) after 20 h on 0.25% NO/1% H-2/5% O-2/5% H2O/He gas stream at 140degreesC has been observed. Remarkable N-2 selectivity values in the range of 80-90% have also been observed in the 100-200degreesC low-temperature range either in the absence or in the presence of water in the feed stream. A maximum specific integral reaction rate of 443.5 mumol N-2/S . 9 Of Pt metal was measured at 160degreesC during reaction with a 0.25% NO/1% H-2/5% O-2/5% H2O/He gas mixture. This value is higher by 90% than the corresponding one observed on the 0.1 wt% Pt/SiO2 catalyst at 120degreesC and it is the highest value ever reported for the reaction at hand in the 100-200degreesC low-temperature range on Pt-based catalysts. A TOF value of 13.4 x 10(-2) s(-1) for N-2 formation was calculated at 110degrees'C for the Pt/La0.7Sr0.2Ce0.1FeO3 catalyst. Temperature-programmed desorption (TPD) of NO and transient titration experiments of the catalyst surface following NO/H-2/O-2 reaction have revealed important information concerning the amount and chemical composition of active and inactive (spectator) adsorbed N-containing species present under reaction conditions. (C) 2002 Elsevier Science (USA).
