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[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

[ES] Dado su tamaño subnanométrico, los clusters metálicos están regidos por el confinamiento cuántico, lo que les hace más "moleculares" y menos "metálicos". En consecuencia, manifiestan propiedades que difieren con respecto a las de partículas más grandes del mismo elemento, y que a menudo son ventajosas para la catálisis de reacciones específicas. Además, su menor tamaño los hace más económicos, con una mayor superficie expuesta. Todo ello hace que los clusters sean opciones muy interesantes en catálisis, y su estudio, síntesis y aplicación ha crecido continuamente desde su descubrimiento en los años 90. Esta tesis se ha centrado principalmente en el cobre, del que se presenta, en primer lugar, un estudio fundamental sobre la disociación de oxígeno por clusters de diferentes tamaños. Después, se explora computacionalmente la catálisis de las oxidaciones de CO y propeno, confirmando que los clusters de Cu5 (o inferior) son prometedores para reacciones de oxidación. Las dos reacciones utilizadas son buenos ejemplos de la aplicación potencial en industria, sea para reducir emisiones de CO o para producir epóxido de propeno, que es un intermedio importante en la producción de plásticos y adhesivos, entre otros. Además, también se estudió la influencia de dos soportes en los clusters de cobre y su capacidad de oxidación: N-grafeno como un sistema más inerte y ceria como uno que puede participar activamente en reacciones de oxidación. Finalmente, se incluyen otros dos estudios más específicos, sobre la capacidad de los clusters de Pt3 y Pd3 para catalizar reaciones de acoplamiento C-C como la reacción de Heck, importante para la síntesis de productos de la química fina, y sobre la reacción CO + NO en clusters de Pt, motivado por su uso potencial como catalizadores para la conversión de esas especies en los menos perjudiciales CO2 y N2 en motores de combustión interna., [CA] Atès que són de grandària subnanomètrica, els clusters metàl·lics estan regits pel confinament quàntic, el qual els fa més "moleculars" i menys "metàl·lics". En conseqüència, manifesten propietats que són diferents a les de partícules més grans del mateix element, i que sovint són avantatjoses per a la catàlisi de reaccions específiques. A més a més, la seua menor grandària fa que siguen més econòmics, amb una major superfície exposada. Així, els clusters són una opció molt interesant en catàlisi, i el seu estudi, síntesi i aplicació ha cres-cut contínuament des del seu descobriment als anys 90. Aquesta tesi s'ha centrat principalment en el coure, del qual es presenta, en primer lloc, un estudi fonamental sobre la dissociació de l'oxígen per clusters de diferents grandàries. Després, s'explora computacionalment la catàlisi de les oxidacions de CO i de propè, confirmant que els clusters de Cu5 (o inferior) són prometedors per a reaccions d'oxidació. Les dues reaccions utilitzades són bons exemples de l'aplicació potencial en indústria, siga per reduir emissions de CO o per produir epòxid de propè, que és un intermedi important en la producció de plàstics i adhesius, entre altres. A més, també es va estudiar la influència de dos suports en els clusters de coure i la seua capacitat d'oxidació: N-grafè com a un sistema més inert i cèria com a un que pot participar activament en reaccions d'oxidació. Finalment, s'inclouen altres dos estudis més específics, sobre la capacitat dels clusters de Pt3 y Pd3 per catalitzar reaccions d'acoblament C-C com la reacció de Heck, important per a la síntesi de productes de la química fina, i sobre la reacció CO + NO als clusters de Pt, motivat pel seu ús potencial com a catalitzadors per a la conversió d'eixes espècies en els menys perjudicials CO2 i N2 als motors de combustió interna., [EN] Due to their subnanometric size, metal clusters belong to the regime affected by quantum confinement, which makes them more "molecular" and less "metallic". As a result, they exhibit properties that differ with respect to those of larger particles of the same element, and which are often advantageous in the catalysis of specific reactions. Besides, their smaller size makes them more economic and with a higher surface exposed. All of this renders metal clusters very interesting options for catalysis, and their study, synthesis and application has steadily increased since their discovery in the 90s. In this work we have largely focused on copper, of which a fundamental study on the oxygen dissociation by clusters of different sizes is first presented. Then, the catalysis of the CO and propene oxidation reactions is theoretically explored, confirming that Cu5 (or smaller) clusters are promising systems for oxidation reactions. The two reactions used are good examples of the potential application in industry, either to reduce CO emissions or to produce propene epoxide, an important intermediate in the production of plastics and adhesives, among others. In addition, the influence of two supports in the copper clusters and their oxidation capability is explored: on N-graphene as a more inert system and on ceria as one that can actively participate in oxidation reactions. Finally, two other more specific studies are included, regarding the capability of Pt3 and Pd3 clusters to undergo C-C coupling reactions such as the Heck reaction, important for the synthesis of many products of fine chemistry, and regarding the CO + NO reaction on Pt clusters, motivated by their potential use as catalysts for the conversion of those species in less harmful CO2 and N2 in internal combustion engines.

[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.

[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.

[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.