Suchergebnisse

[EN] The selective oxidation of propene to propene oxide (PO) is an industrially relevant and still challenging reaction that requires the design of highly specific catalysts able to improve simultaneously activity and selectivity. Metallic copper exhibits high selectivity toward propene epoxidation that drops when the catalyst surface is oxidized under reaction conditions. On the basis of previous work showing that small planar Cu-5 clusters are more resistant to oxidation than 3D ones, we have performed a detailed theoretical study of the mechanism of propene oxidation with molecular O-2 and atomic O adsorbed on both planar and 3D Cu-5 clusters. The desired pathways leading to PO as well as the undesired routes producing propanal, acetone, or allyl intermediates that finally evolve to acrolein or CO2 have been considered, and the global analysis of all data indicates that planar Cu-5 clusters are promising candidates for the selective epoxidation of propene ; This work has been supported by the European Union through ERC-AdG-2014-671093 (SynCatMatch) and by the Spanish Government through "Severo Ochoa"(SEV-2016-0683, MINECO) and MAT2017-82288-C2-1-P (AEI/FEDER, UE) Projects. E.F.V. thanks Spanish MINECO for her fellowship SVP-2013-068146. ; Fernández Villanueva, E.; Boronat Zaragoza, M.; Corma Canós, A. (2020). The Crucial Role of Cluster Morphology on the Epoxidation of Propene Catalyzed by Cu-5: A DFT Study. The Journal of Physical Chemistry C. 124(39):21549-21558. https://doi.org/10.1021/acs.jpcc.0c06295 ; S ; 21549 ; 21558 ; 124 ; 39

[EN] By a combination of theoretical modeling and XPS and SERS spectroscopic studies, it has been found that it is possible to stabilize metallic copper species under oxidizing reaction conditions by adjusting the atomicity of subnanometer copper clusters. Small Cu-5 clusters display low reactivity toward O-2 dissociation, being less susceptible to oxidation than larger Cu-8 or Cu-20 systems. However, in the presence of water this reactivity is strongly enhanced, leading to oxidized Cu-5 clusters. In that case, the interaction of Cu-5 with atomic O oxygen is weak, favoring recombination and O-2 desorption, suggesting an easier transfer of O atoms to other reactant molecules. In contrast, copper clusters of higher atomicity or nanoparticles, such as Cu-5 and Cu-20, are easily oxidized in the presence of O-2, leading to very stable reactive O atoms, resulting in low reactivity and selectivity in many oxidation reactions. Altogether, Cu-5, clusters are proposed as promising catalysts for catalytic applications where stabilization of metallic copper species is strongly required. ; The authors thank the MINECO (Consolider Ingenio 2010-MULTICAT CSD2009-00050 and Severo Ochoa program SEV-2012-0267), Generalitat Valenciana (PROMETEOII/2013/011 Project), and European Union (ERC-AdG-2014-671093-SynCatMatch) for financial support. E.F. and S.G.-G. thank the MINECO for their fellowship SVP-2013-068146 and financial support through project MAT2011-28009, respectively. ; Concepción Heydorn, P.; Boronat Zaragoza, M.; García García, S.; Fernández-Villanueva, E.; Corma Canós, A. (2017). Enhanced Stability of Cu Clusters of Low Atomicity against Oxidation. Effect on the Catalytic Redox Process. ACS Catalysis. 7(5):3560-3568. https://doi.org/10.1021/acscatal.7b00778 ; S ; 3560 ; 3568 ; 7 ; 5

[EN] The catalytic subnanometric metal clusters with a few atoms can be regarded as an intermediate state between single atoms and metal nanoparticles (>1 nm). Their molecule-like electronic structures and flexible geometric structures bring rich chemistry and also a different catalytic behavior, in comparison with the single-atom or nanoparticulate counterparts. In this work, by combination of operando IR spectroscopy techniques and electronic structure calculations, we will show a comparative study on Pt catalysts for CO + NO reaction at a very low temperature range (140-200 K). It has been found that single Pt atoms immobilized on MCM-22 zeolite are not stable under reaction conditions and agglomerate into Pt nanoclusters and particles, which are the working active sites for CO + NO reaction. In the case of the catalyst containing Pt nanoparticles (similar to 2 nm), the oxidation of CO to CO2 occurs in a much lower extension, and Pt nanoparticles become poisoned under reaction conditions because of a strong interaction with CO and NO. Therefore, only subnanometric Pt clusters allow NO dissociation at a low temperature and CO oxidation to occur well on the surface, while CO interaction is weak enough to avoid catalyst poisoning, resulting in a good balance to achieve enhanced catalytic performance. ; This work has been supported by the European Union through the European Research Council (grant ERC-AdG-2014671093, SynCatMatch) and the Spanish government through the "Severo Ochoa Program" (SEV-2016-0683) and MAT2017-82288-C2-1-P projects. L.L. thanks ITQ for providing a contract. E.F. thanks MINECO for her fellowship SVP-2013-068146. The authors also thank Microscopy Service of UPV for the TEM and STEM measurements. The HRSTEM studies were conducted at the Laboratorio de Microscopias Avanzadas, Universidad de Zaragoza, Spain. RA. acknowledges support from Spanish MINECO grant MAT2016-79776-P (AEI/FEDER, UE), from the Government of Aragon and the European Social Fund (grant number E13_17R, FEDER, UE), and ...

[EN] We present a combined theoretical-experimental study aiming to provide information about the location and coordination environment of the Cu2+ species involved in the selective reduction of NOx with NH3 catalyzed by Cu-zeolites. From the experimental side, we show and discuss the EPR spectra of the three molecular sieves most widely used as catalysts for the NH3-SCR-NOx reaction, namely Cu-SSZ-13, Cu-SAPO-34 and Cu-ZSM-5 both in their hydrated state and after dehydration. Then, we investigate the EPR spectra of Cu-SSZ-13 and Cu-SAPO-34 under the following conditions: (i) after NH3 adsorption, (ii) after NO addition, and (iii) in the presence of a NO/O-2 mixture. As regards the theoretical part, an exhaustive computational study has been performed that includes geometry optimization and calculation of the EPR parameters of all the relevant systems involved in the NH3-SCR-NOx reaction. The influence of local geometry and Al/Si distribution in the zeolite framework on the EPR parameters and the most probable location of Cu2+ in each material are analyzed, and assignations of the EPR signals obtained under different reaction conditions are discussed. ; This work has been supported by the Spanish Government through Severo Ochoa Program (SEV 2012-0267), MAT2015-71261-R and CTQ2015-68951-C3-1-R, and by the European Union through ERC-AdG-2014-671093 (SynCatMatch). Red Española de Supercomputación (RES) and Centre de Càlcul de la Universitat de Valencia are gratefully acknowledged for computational resources and technical support. E.F.V. thanks MINECO for her fellowship SVP-2013-068146. ; Fernández-Villanueva, E.; Moreno González, M.; Moliner Marin, M.; Blasco Lanzuela, T.; Boronat Zaragoza, M.; Corma Canós, A. (2018). Modeling of EPR Parameters for Cu(II): Application to the Selective Reduction of NOx Catalyzed by Cu-Zeolites. Topics in Catalysis. 61(9-11):810-832. https://doi.org/10.1007/s11244-018-0929-y ; S ; 810 ; 832 ; 61 ; 9-11 ; Chen H-Y (2014) In: Nova I, Tronconi E (eds) Urea-SCR technology for deNOx after ...