Selective Hydrogenation Catalysts For Environmental Processes: Nitrate And Chlorocompounds Removal

En el presente trabajo se han diseñado, sintetizado y caracterizado catalizadores heterogéneos. Se ha estudiado la actividad y selectividad, de estos materiales, en el tratamiento de compuestos que comportan problemas medioambientales. Para la preparación de los catalizadores se han utilizado diferentes técnicas tales como impregnación, proceso redox y combustión. A su vez, se han sintetizado nanopartículas metálicas, depositándolas en diferentes materiales y testado su actividad catalítica. La actividad de estos catalizadores se ha comparado con catalizadores comunes en diferentes procesos. Por otro lado, se han utilizado diferentes materiales, alumina, carbón activo, ceria e hidrotalcitas, con el propósito de estudiar el efecto del soporte.La contaminación por nitratos en las aguas subterráneas es un problema importante en determinadas regiones. Esta problemática ha impulsado el estudio de la eliminación catalítica de los nitratos, en la que se enfoca la primera parte de la tesis. Este estudio se realizó en un reactor en continuo de lecho fijo. Se sintetizaron, caracterizaron y ensayaron, varios catalizadores monometálicos, bimetálicos y de nanopartículas soportadas. Una vez determinada la actividad y la estabilidad de los catalizadores se procedió a optimizar los materiales con el fin de mejorar la selectividad de éstos hacia nitrógeno. El presente trabajo ha sido el punto de partida para la puesta en marcha de una planta piloto para el tratamiento de aguas subterráneas reales contaminadas con nitratos. Próximamente será inaugurada una planta de eliminación catalítica de nitratos de aguas subterráneas con capacidad 500m3/día en las proximidades de El Morell (Tarragona).La segunda parte de la tesis se enfoca en el proceso de hidrodecloración catalítica de compuestos orgánicos clorados como el tricloroetileno. Los experimentos se llevaron a cabo en fase gas utilizando diferentes tipos de catalizadores, los cuales fueron sintetizados y caracterizados. Se emplearon diferentes catalizadores, mono y bimetálicos, para la transformación del tricloroetileno a un producto de valor añadido, como es el etileno.El presente trabajo, además de aportar conocimientos fundamentales en la catálisis heterogénea, contribuye a forjar soluciones a problemas medioambientales reales que afectan a la salud humana.In the present study have been designed, synthesised and characterised heterogeneous catalysts. Their activities and selectivity have been studied in the treatment of compounds that present environmental problems. For the catalysts preparation different protocols such as incipient-wetness impregnation, co-impregnation, redox and combustion have been followed. In addition, metal nanoparticles were synthesized and deposited on several materials as well as tested their catalytic behaviour. The activity and the selectivity of nanoparticle catalysts with that of other catalysts in different processes were compared. On the other hand, different materials such as alumina, active carbon, ceria and hydrotalcites were used to study the role of them as catalyst supports.In the light of the current demand for drinking water, the first part of this thesis studies the catalytic hydrogenation of nitrates. The study is carried out in a fixed bed continuous reactor. Several bimetallic, monometallic and supported nanoparticle catalysts have been synthesised, characterized and tested in order to improve their activity and stability and to optimise their selectivity to nitrogen. The present study was further extended and some of the catalysts have been used as a starting point for a Pilot Plant investigation into eliminating nitrate in real ground water. At this moment, a plant is being constructed in the vicinity of El Morell, Tarragona, which can use catalysis to reduce nitrates in 500 m3/day of groundwater.The second part of the thesis deals with the catalytic hydrodechlorination of chlorinated organic compounds such as TCE. The experiments were done in gas phase. Different types of catalysts have been synthesised, characterized and tested and the final objective is to obtain an active and stable catalyst that is selective toward valuable products such as ethylene. This research aims to use catalytic technologies to contribute to solving real environmental problems that affect human life

Structural evolution after oxidative pretreatment and CO oxidation of Au nanoclusters with different ligand shell composition: a view on the Au core

The reactivity of supported monolayer protected Au nanoclusters is directly affected by their structural dynamics under pretreatment and reaction conditions. The effect of different types of ligands of Au clusters supported on CeO$_2$ on their core structure evolution, under oxidative pretreatment and CO oxidation reaction, was investigated. X-ray absorption and X-ray photoelectron spectroscopy studies revealed that the clusters evolve to a similar core structure above 250 °C in all the cases, indicating the active role of the ligand–support interaction in the reaction

Directing Intrinsic Chirality in Gold Nanoclusters: Preferential Formation of Stable Enantiopure Clusters in High Yield and Experimentally Unveiling the “Super” Chirality of Au144_{144}

Chiral gold nanoclusters offer significant potential for exploring chirality at a fundamental level and for exploiting their applications in sensing and catalysis. However, their widespread use is impeded by low yields in synthesis, tedious separation procedures of their enantiomeric forms, and limited thermal stability. In this study, we investigated the direct synthesis of enantiopure chiral nanoclusters using the chiral ligand 2-MeBuSH in the fabrication of Au$_{25}$, Au$_{38}$, and Au$_{144}$ nanoclusters. Notably, this approach leads to the unexpected formation of intrinsically chiral clusters with high yields for chiral Au$_{38}$ and Au$_{144}$ nanoclusters. Experimental evaluation of chiral activity by circular dichroism (CD) spectroscopy corroborates previous theoretical calculations, highlighting the stronger CD signal exhibited by Au$_{144}$ compared to Au$_{38}$ or Au$_{25}$. Furthermore, the formation of a single enantiomeric form is experimentally confirmed by comparing it with intrinsically chiral Au$_{38}$(2-PET)$_{24}$ (2-PET: 2-phenylethanethiol) and is supported theoretically for both Au$_{38}$ and Au$_{144}$. Moreover, the prepared chiral clusters show stability against diastereoisomerization, up to temperatures of 80°C. Thus, our findings not only demonstrate the selective preparation of enantiopure, intrinsically chiral, and highly stable thiolate-protected Au nanoclusters through careful ligand design but also support the predicted “super” chirality in the Au$_{144}$ cluster, encompassing hierarchical chirality in ligands, staple configuration, and core structure

Catalytic reduction of nitrates in water on Pt promoted Cu hydrotalcite-derived catalysts: Effect of the Pt-Cu alloy formation

International audienceIn this work, the role of the Pt-Cu interaction and the influence of the Pt metal loading in the catalytic reduction of nitrates in water have been studied, using Pt supported on CuMgAl mixed oxides catalysts in a continuous reactor. Following three Pt impregnation protocols different surface chemistries were obtained, as confirmed by BET, TPR, HRTEM, XRD and FTIR spectroscopy. In the first protocol, the presence of Cu and Pt-Cu alloy formation has been promoted, whereas the second protocol leads to separated Pt and Cu particles in close contact. The third protocol leads to the presence of Cu, Pt and Pt-Cu alloy particles. The different catalytic behaviors were related to the differences in the surface metal chemistry of the samples. Low ammonium formation was detected in all cases but nitrite concentrations need to be improved to fulfill the maximum admissible concentration of the EU legislation. It is concluded that: (i) the presence of Pt-Cu alloy particles leads to an increase in nitrogen selectivity, enhancing the nitrite reduction but showing lower nitrate conversion, and (ii) to maximize the nitrate conversion it is necessary to obtain mainly Pt particles interacting with Cu and Pt-Cu alloy particles

Chemically Selective Imaging of Individual Bonds through Scanning Electron Energy-Loss Spectroscopy : Disulfide Bridges Linking Gold Nanoclusters

As proof-of-principle of chemically selective, spatially resolved imaging of individual bonds, we carry out electron energy-loss spectroscopy in a scanning transmission electron microscopy instrument on atomically precise, thiolate-coated gold nanoclusters linked with 5,5′-bis(mercaptomethyl)-2,2′-bipyridine dithiol ligands. The images allow the identification of bridging disulfide bonds (R–S–S–R) between clusters, and X-ray photoelectron spectra support the finding.peerReviewe

Doped metal clusters as bimetallic AuCo nanocatalysts: insights into structural dynamics and correlation with catalytic activity by in situ spectroscopy.

Acknowledgements: The authors acknowledge the support by the Austrian Science Fund (FWF) via grant Elise Richter (V831-N). ALBA synchrotron is acknowledged for the XAFS experiments at the CLAESS beamline (2017092492) with the collaboration of ALBA staff. Support during the experiments by Vera Truttmann, Clara Garcia and Stephan Pollitt are acknowledged.Co-doped Au25 nanoclusters with different numbers of doping atoms were synthesized and supported on CeO2. The catalytic properties were studied in the CO oxidation reaction. In all cases, an enhancement in catalytic activity was observed compared to the pure Au25 nanocluster catalyst. Interestingly, a different catalytic performance was obtained depending on the number of Co atoms within the cluster. This was related to the mobility of atoms within the cluster's structure under pretreatment and reaction conditions, resulting in active CoAu nanoalloy sites. The evolution of the doped Au clusters into nanoalloys with well-distributed Co atoms within the Au cluster structure was revealed by combined XAFS, DRIFTS, and XPS studies. Overall, these studies contribute to a better understanding of the dynamics of doped nanoclusters on supports upon pretreatment and reaction, which is key information for the future development and application of bimetallic nanocluster (nanoalloy) catalysts

Ligand Migration from Cluster to Support : A Crucial Factor for Catalysis by Thiolate-protected Gold Clusters

Thiolate protected metal clusters are valuable precursors for the design of tailored nanosized catalysts. Their performance can be tuned precisely at atomic level, e. g. by the configuration/type of ligands or by partial/complete removal of the ligand shell through controlled pre-treatment steps. However, the interaction between the ligand shell and the oxide support, as well as ligand removal by oxidative pre-treatment, are still poorly understood. Typically, it was assumed that the thiolate ligands are simply converted into SO, CO and HO. Herein, we report the first detailed observation of sulfur ligand migration from Au to the oxide support upon deposition and oxidative pre-treatment, employing mainly S K-edge XANES. Consequently, thiolate ligand migration not only produces clean Au cluster surfaces but also the surrounding oxide support is modified by sulfur-containing species, with pronounced effects on catalytic properties

On the mechanism of rapid metal exchange between thiolate-protected gold and gold/silver clusters: a time-resolved in situ XAFS study

The final publication is available via https://doi.org/10.1039/C7CP08272J.The fast metal exchange reaction between Au38 and AgxAu38-x nanoclusters in solution at -20˚C has been studied by in situ X-ray absorption spectroscopy (time resolved quick EXAFS) in transmission mode. A cell was designed for this purpose consisting of a cooling system, remote injection and mixing devices. The capability of the set-up is demonstrated for second and minute time scale measurements of the metal exchange reaction upon mixing Au38/toluene and AgxAu38-x /toluene solutions at both Ag K-edge and Au L3-edge. It has been proposed that the exchange of gold and silver atoms between the clusters occurs via the SR(-M-SR)n (n=1,2; M=Au, Ag) staple units in the surface of the reacting clusters during their collision. However, at no point during the reaction (before, during, after) evidence is found for cationic silver atoms within the staples. This means that either the exchange occurs directly between the cores of the involved clusters or the residence time of the silver atoms in the staples is very short in a mechanism involving the metal exchange within the staples.Austrian Science Funds (FWF)Swiss National Science Foundatio

Dynamic behaviour of platinum and copper dopants in gold nanoclusters supported on ceria catalysts

Abstract Understanding the behaviour of active catalyst sites at the atomic level is crucial for optimizing catalytic performance. Here, the evolution of Pt and Cu dopants in Au25 clusters on CeO2 supports is investigated in the water-gas shift (WGS) reaction, using operando XAFS and DRIFTS. Different behaviour is observed for the Cu and Pt dopants during the pretreatment and reaction. The Cu migrates and builds clusters on the support, whereas the Pt creates single-atom active sites on the surface of the cluster, leading to better performance. Doping with both metals induces strong interactions and pretreatment and reaction conditions lead to the growth of the Au clusters, thereby affecting their catalytic behaviour. This highlights importance of understanding the behaviour of atoms at different stages of catalyst evolution. These insights into the atomic dynamics at the different stages are crucial for the precise optimisation of catalysts, which ultimately enables improved catalytic performance

Directing Intrinsic Chirality in Gold Nanoclusters : Preferential Formation of Stable Enantiopure Clusters in High Yield and Experimentally Unveiling the “Super” Chirality of Au144

Chiral gold nanoclusters offer significant potential for exploring chirality at a fundamental level and for exploiting their applications in sensing and catalysis. However, their widespread use is impeded by low yields in synthesis, tedious separation procedures of their enantiomeric forms, and limited thermal stability. In this study, we investigated the direct synthesis of enantiopure chiral nanoclusters using the chiral ligand 2-MeBuSH in the fabrication of Au25, Au38, and Au144 nanoclusters. Notably, this approach leads to the unexpected formation of intrinsically chiral clusters with high yields for chiral Au38 and Au144 nanoclusters. Experimental evaluation of chiral activity by circular dichroism (CD) spectroscopy corroborates previous theoretical calculations, highlighting the stronger CD signal exhibited by Au144 compared to Au38 or Au25. Furthermore, the formation of a single enantiomeric form is experimentally confirmed by comparing it with intrinsically chiral Au38(2-PET)24 (2-PET: 2-phenylethanethiol) and is supported theoretically for both Au38 and Au144. Moreover, the prepared chiral clusters show stability against diastereoisomerization, up to temperatures of 80 °C. Thus, our findings not only demonstrate the selective preparation of enantiopure, intrinsically chiral, and highly stable thiolate-protected Au nanoclusters through careful ligand design but also support the predicted “super” chirality in the Au144 cluster, encompassing hierarchical chirality in ligands, staple configuration, and core structure.peerReviewe