Crivoi, D.; Miranda, R.A.; Finocchio, E.; Llorca, J.; Ramis, G.; Sueiras, J.E.; Segarra, A.; Medina, F. Applied catalysis A. General Vol. 519, p. 116-129 DOI: 10.1016/j.apcata.2016.03.018 Data de publicació: 2016-06-05 Article en revista
Nanohybrid materials based on L-leucine (L-Leu) and hydrotalcites (HT) were prepared by the ion exchange and reconstruction method, under mild synthesis conditions. The location, amount and the form of the immobilized L-Leu are affected not only by the time of synthesis, but also by temperature and ultrasound treatment. The XRD results demonstrate that the immobilization occurs in either a vertical or oblique orientation with respect to the HT layers. The catalytic activity of these materials was tested in the aldol addition reaction of cyclohexanone with different aromatic aldehydes, affording mainly the syndiastereomer.. Furthermore, the present study demonstrates that both diastereo- and enantioselectivity can be easily modulated by the appropriate combination of nanohybrid catalyst, solvent and reaction time. (C) 2016 Elsevier B.V. All rights reserved.
In situ X-ray photoelectron spectroscopy (XPS) was carried out over model Rh0.5Pd0.5 nanoparticles and Rh0.5Pd0.5 nanoparticles supported on CeO2 following exposure to oxygen at 573-823 K, to hydrogen at 573 K (activation of the catalyst), to a mixture of ethanol and water at 823 K simulating ethanol steam reforming (ESR) conditions, and to hydrogen at 823 K. The presence of the CeO2 support had a strong influence on the atomic rearrangement and on the oxidation state of Rh0.5Pd0.5 nanoparticles. CeO2 exerted a quenching effect on the metal nanoparticles and limited atomic rearrangement under the different atmospheres tested except for ESR, where a partial segregation of Rh toward the surface of the nanoparticles was observed. When supported on CeO2, Rh0.5Pd0.5 nanoparticles were significantly more oxidized due to metal-support interaction. By comparing these in situ XPS results with a previous operando near ambient pressure XPS study conducted in a synchrotron facility at 0.05 mbar over the same samples, it is concluded that the information obtained for the unsupported Rh0.5Pd0.5 nanoparticles is similar in both cases, whereas Rh0.5Pd0.5 nanoparticles supported on CeO2 are clearly more oxidized and enriched in Pd in the volume sampled under operando ESR conditions compared to that obtained in the in situ experiments. The study of catalytic systems under operando conditions appears essential to identify the active species at work during ESR, as the restructuring driven by the reforming environment induces strong changes in their architecture. (C) 2015 Elsevier B.V. All rights reserved.
Barbato, P.; Colussi, S.; Di Benedetto, A.; Landi, G.; Lisi, L.; Llorca, J.; Trovarelli, A. Applied catalysis A. General Vol. 506, p. 268-277 DOI: 10.1016/j.apcata.2015.09.018 Data de publicació: 2015-10-05 Article en revista
In this work the effect of Fe addition to copper/ceria catalysts on the catalytic activity toward CO-PROX reaction has been studied and related to the modification of textural and redox properties induced by iron. Catalysts have been characterized by the use of BET surface area measure, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and TPR with different reducing mixtures (CO/N-2 and/or H-2/N-2). Results show that iron promotes a superficial enrichment of highly dispersed copper sites improving catalyst reducibility, especially under CO-containing mixture, thus resulting in an enhanced selectivity of CO-PROX reaction. (C) 2015 Elsevier B.V. All rights reserved.
Mistri, R.; Maiti, S.; Llorca, J.; Dominguez, M.; Mandal, T.; Mohanty, P.; Ray, B.C.; Gayen, A. Applied catalysis A. General Vol. 485, p. 40-50 DOI: 10.1016/j.apcata.2014.07.027 Data de publicació: 2014-09-05 Article en revista
Copper ion substituted MAl2O4 (M = Mg, Mn, Fe, Ni and Zn) spinels, CuxM1-xAl2O4 (x = 0.03 and 0.05), have been synthesized by a single step solution combustion method. Of the various compositions studied the 3 at.% copper ion substituted hercynite, Cu0.03Fe0.97Al2O4, reported here for the first time, is shown to be much more active (similar to 92% conversion with similar to 99% selectivity) than other spinet analogues towards liquid phase oxidation of cyclohexane in acetonitrile with H2O2 as oxidant in air. Powder XRD analyses have revealed formation of pure hercynite phases. The least-square refined lattice parameters obtained from XRD data together with microstructural data by HRTEM have indicated copper ion substitution in the spinel lattice. The oxidation state of copper has been established as +2 from XPS analysis and it seem to be primarily substituted in the Fe-site of hercynite. Incorporation of the copper in the spinel structure of FeAl2O4 leading to an ionic interaction is explained to be responsible for the higher oxidation activity observed over the combustion synthesized catalyst than the corresponding impregnated catalyst which contains finely dispersed CuO crystallites. Effect of recycling (repeated thrice) has shown almost no degradation of activity of the copper ion substituted hercynite. In contrast, the analogous impregnated catalyst has shown appreciable loss of activity in the consecutive cycles due to the presence of dispersed CuO crystallites which can agglomerate with ease and subsequently leach out. (C) 2014 Elsevier B.V. All rights reserved.
Meshesha, B.; Barrabes, N.; Llorca, J.; Dafinov, A.; Medina, F.; Foettinger, K. Applied catalysis A. General Vol. 453, p. 130-141 DOI: 10.1016/j.apcata.2012.12.019 Data de publicació: 2013-02-26 Article en revista
PdCu bimetallic catalysts supported on alumina were prepared by different common impregnation protocols and by a polyol nanoparticles synthesis route. These materials were studied in order to establish the relationship between structure, particle size, Pd–Cu interaction and catalytic activity/selectivity in the hydrodechlorination of trichloroethylene (TCE) in gas phase. The surface properties and the interaction between Cu and Pd in the bimetallic particles, as well as their catalytic behaviour are strongly influenced by the preparation protocol. Depending on the synthesis, PdCu alloy formation or isolated phases were observed. The presence of isolated Pd particles leads to high ethane selectivity, whereas upon alloy formation or strong interaction between Pd and Cu a higher ethylene yield was obtained. By the nanoparticles polyol synthesis, Pd-Cu alloy formation was obtained, leading to total selectivity to ethylene in the TCE hydrodechlorination reaction. On the Pd monometallic catalysts larger particle size resulted in higher levels of ethylene formation.
Colussi, S.; Trovarelli, A.; Vesselli, E.; Baraldi, A.; Comelli, G.; Groppi, G.; Llorca, J. Applied catalysis A. General Vol. 390, num. 1-2, p. 1-10 DOI: 10.1016/j.apcata.2010.09.033 Data de publicació: 2010 Article en revista
Catalytic combustion of methane has been studied for many years due to its applications in power generation
and emissions clean-up. Among different catalysts, Pd-based materials are the most active for
the catalytic combustion of methane. In this work we have investigated the Pd–PdO transformation
process on Pd–alumina and Pd–ceria–alumina combustion catalysts by combining different analytical
techniques. Both decomposition of PdO and reoxidation of Pd take place via the formation of an intermediate,
which has been identified as a surface or interfacial PdOx on the basis of X-ray Photoelectron
Spectroscopy analysis. The results obtained by coupling temperature programmed oxidation with high
resolution transmission electron microscopy and X-ray photoelectron spectroscopy indicate that PdO
decomposition is a thermodynamically driven process, in which the support affects only the amount of
PdO species involved in each decomposition step. On the contrary, Pd reoxidation is a kinetically limited
process, which is strongly affected by the environment. In particular, the presence of promoters such
as CeO2 or residual PdO significantly speeds up Pd oxidation, thus reducing the characteristic Pd–PdO