Fil: Costilla, Ignacio Oscar. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Bahia Blanca; Argentina. Universidad Nacional del Sur. Departamento de Ciencias e Ingenieria de la Computacion. Laboratorio de Investigacion y Desarrollo en Computacion Cientifica; Argentina

Low loaded Pd/alpha-Al2O3 catalysts (<0.5 wt% Pd) were characterized and tested for CH4 reforming with CO2 at 650 degrees C. The catalysts were prepared by a recharging procedure, using an organometallic precursor, followed by intermediate washing and calcination steps. FTIR spectra of adsorbed CO showed that the Pd surface structure and the particle size were dependent on the number of post-impregnation washing steps. A catalyst sample with a metal dispersion of 33% showing well defined low-index planes (by FTIR) and nearly spherical particles (by TEM) was obtained using two-washing steps. In the reaction, it exhibited a high initial activity followed by a pronounced deactivation due to carbon nanofiber's formation and sintering. TEM analysis of the used catalyst revealed the presence of spherical Pd particles at the end of the fibers that were not attached to the support surface. On the other hand, a high dispersion sample (78%) with a large fraction of Pd atoms with low coordination was obtained by applying three washing steps after impregnation. The presence of small hemispherical particles and larger nearly-flat ones attached to the support were found by TEM. In this case, the catalyst initially showed a very low activity that increased slowly up to a steady value. Although sintering also occurred and the surface structure of the used catalyst resembled the one of the low dispersion catalyst, the amount of carbon formed was quite low. The observed activation under reaction conditions was associated with the slow development of a surface structure that exhibited mainly the (100) plane favoring methane dissociation. However, the initial interaction of the particles with the support suggested by TEM micrographs seems to remain unaltered despite the particle size increase. Consequently, the process of nanofiber's formation and particle separation was inhibited. (C) 2014 Elsevier B.V. All rights reserved.

The activity, selectivity and stability of Pd/H-MOR and Ce–Pd/H-MOR catalysts for the selective catalytic reduction of NO with CH4 (SCR-CH4) were studied in O2 excess (4.1%) in the absence and presence of H2O (5%). The reaction was carried out in a 300–650°C temperature range with a S.V. of ≅33,000h−1. The characterization of fresh and used catalysts by FTIR spectroscopy of adsorbed CO and NO showed that the loss of NO conversion in H2O presence was due to a decreasing concentration of Pd2+ sites and to the parallel formation of PdO particles that enhanced CH4 combustion under dry conditions. This behavior was modified by Ce addition. Under wet conditions the Ce–Pd/H-MOR catalyst was very active (XNO=76% at 550°C), selective (100% of N2) and stable. The FTIR spectrum of CO adsorbed on used samples demonstrated that Ce inhibited the transformation of active Pd2+ into PdO particles. However, Ce presence also enhanced CH4 combustion under wet conditions. At low temperatures (<500°C) H2O presence inhibited N2O formation and 90% selectivity to N2, which improved with Ce addition, was obtained.

The aim of this study was to characterize Pd catalysts supported on three different mordenite zeolites (a commercial sample and two laboratory preparations) previously tested for the NO decomposition reaction. H2 chemisorption, TPR, FTIR spectroscopy of adsorbed CO and NO, DRS UV–vis and TPD of NO were used to characterize the catalysts. After reduction at 573 K a large fraction of Pdo particles and some Pd2+ ions were found in the commercial sample and one laboratory preparation. In contrast a sample prepared with a mordenite synthesized using a natural clay was found to exhibit mainly ionic Pd. The amount of Pdo (PdO) relative to Pd2+ depends on the oxidation temperature and the metal loading. The catalysts exhibited similar levels of activity for the NO decomposition reaction at 673 K but a different selectivity to N2O. This behavior was related to differences in metal dispersion and other surface conditions that influence the formation of PdO particles.

We have investigated the interaction between Pd and Ce in a (0.47 wt %) CeOx–Pd(1 wt %)/α-Al2O3 catalyst that is used in the reforming reaction of CH4 with CO2. The freshly reduced catalyst was characterized by various electron microscopy techniques, such as elemental mapping, Z-contrast imaging, and electron energy-loss spectroscopy to understand the role of Ce on a microscopic scale. The high spatial resolution elemental mapping indicates that CeOx is located in close proximity of the palladium nanoparticles. High-resolution lattice images and energy-loss spectra obtained in the vicinity of the Pd particles show an anisotropic distribution of CeOx crystallites limited to the interface region between Pd and the substrate but not covering the surface of the Pd nanoparticles. Energy-loss near edge fine structure of Pd M edges reveals that the Pd nanoparticles are not oxidized.

In this work, Pd and Mo-Pd catalysts supported on Al2O3 modified SiO2 and Si-MCM-41 materials were prepared, characterized by H-2 chemisorption and FTIR spectroscopy of adsorbed CO and NO and tested for the reduction of NO with CO. The characterization results indicated that in the bimetallic catalysts, Mo that is loaded first, migrates over the I'd atoms and reduced the fraction of exposed metal atoms. The reduction of NO with CO at 523 K showed that the specific activity for NO conversion increased as a result of the Pd-Mo interaction. In the 523-573 K temperature range NH3 was produced in addition to CO2, N-2 and N2O. This result suggests the reaction of NCO species formed on the Pd surface with OH groups of the support materials to produce isocyanic acid (HNCO) that is hydrolyzed to NH3. The catalysts exhibited a decreasing selectivity to N2O and a parallel increase in the production of N-2 and NH3 as the conversion increases. The effect of Mo on the reaction selectivity was clear for Pd/Al2O3/SiO2; at similar NO conversion level the selectivity to N2O and N-2 decreased with increasing NH3 formation. FTIR spectra of adsorbed species under reaction conditions demonstrated the absence of isocyanate species and the presence of NH4+ ions in accordance with the high Bronsted acidity of the support materials. (c) 2008 Elsevier B.V. All rights reserved.

We have studied carbon nanofibers (CNF) formed in the reforming reaction of CH4 with CO2 over a Pd (0, 42%)/α-Al2O3 catalyst at 973K. The obtained nanostructured carbon material was characterized by high-resolution transmission electron microscopy (HRTEM). The results showed CNF of well-defined structures with diameters between 8 and 18nm. A fraction of these fibers showed branched structures. Our results suggest that the fiber grows taking the metallic particle off the substrate, that the type of fiber obtained is related to the particle shape and that the origin of fiber growth is the formation of graphitic sheets on the metallic particle surface.

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

The Pd–Ce interaction was studied over CeO2 (0.3–2.5wt.%)–Pd (1wt.%)/α-Al2O3 catalysts used in the reforming reaction of CH4 with CO2. The samples were characterized by using high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The activity and selectivity behavior was in good agreement with that of other supported metal catalysts (Ni and Pd) modified with different promoters. The preliminary results of HRTEM would indicate that the CeOx forms small crystallites around the Pd particle. The XPS analysis for the regions of Ce 3d and Pd 3d, gives an account of Ce being present mostly as Ce3+ and a high binding energy for Pd 3d5/2 (335.3eV), an evidence of Pd–Ce chemical interaction. The Pd/Al XPS intensity ratios vs. the Pd average particle size, determined by TEM, show an excellent correlation for fresh and used catalyst. These results indicate that the diminution of the Pd/Al ratios was due to Pd sintering. Consequently, the small amounts of CeOx species do not cover the Pd particle, in agreement with the HRTEM results. The overall results stand for the promoter action mechanism of the CeOx for the reforming reaction with CO2.