In situ studies of catalytic processes by Near Ambient X-ray Photoelectron Spectroscopy

A a portada logotip del Sincrotò "Alba"(English) Labels misplacement in the bottles are currently causing problems in wine cellar's labelling lines. According to data provided by Codorníu and Torres cellars, 1 % of the bottles present this issue. Once the wine or cava have been bottled, the bottles pass through the labeling station and the scanner separates the correctly labeled bottles from those with imperfections. When a considerable number of bottles are involved, the wine is emptied and rebottled, while the labeled bottle is discarded. However, if few bottles are mislabeled, a worker manually removes the labels with a scraper and retums the bottles to the labeling line. Of course, this is a problem. In this thesis, a system has been developed consisting of a pressure-sensitive adhesive anda detaching station that allows the adhesive to be deactivated for easy removal of misplaced labels. The adhesive had to meet the current market requirements and to be easily peeled off easily in the detaching station to leave the bottle completely clean to it can be relabelled without being taken out of the labelling lines. As bottles are full when they pass through the detaching station, a technology that could quickly remove the labels from the bottles at 25ºC was required to avoid affecting the quality of the product. lt was designed to include a cleaning solution bath, a thermostatic, and a mechanical agitator. Several chemical components with different weight ratios were tested for the cleaning solution formulation to determine the most effective solution. The design of the adhesive was sub-divided into 5 parts. First, the influence of the soft monomers: n-butyl acrylate and 2- ethylhexyl acrylate influence was studied via semi-batch emulsion polymerization taking as a starting point a formulation recipe composed of n-butyl acrylate and acrylic acid. In the second part, the influence of including a hard monomer, such as acrylonitrile, was studied. In both studies it was observed that the adhesives showed poor water resistance. For it, in the third part, different polymerizable surfactants were studied as stabilizer of the emulsion and were compared with the conventional surfactant used in the base formulation. The use a polymerizable surfactant clear1y increased the water resistance of the adhesives. Although the incorporation of 2-ethylhexyl acrylate showed better adhesive performance, the acrylonitrile showed better results in the detaching bath. For it, it was decided to continue the experiments introducing in the formulation the weight ratio of acrylonitrile that showed the best balance between the adhesive properties and its performance in the detaching bath and the polymerizable surfactant that showed the highest water resistance. In the fourth part, the optimization of the adhesive properties balance was carried out by the study of the influence of acrylic acid as functional monomer and the tert-dodecyl mercaptan as chain transfer agent. Clearly, the acrylic acid improved all adhesive properties. However, the maximum amount of acrylic acid testad only improved the shear resistance since with the gel content increase, the peel resistance and tack properties deceased. On the other hand, the chain transfer agent only improved the peel resistance and tack. Therefore, it was not possible to achieve a good performance among the three adhesive properties. Finally, the influence of adhesiva preparation process was investigated. In this case an improvement in the balance of adhesive properties was observed. Several of the pressure-sensitive adhesives carne close to current market requirements, as well as showing excellent results in the detaching station.(Català) Els nanomaterials han rebut malta atracció com a catalitzadors no només perla seva elevada relació superfície-volum, sinó també perla capacitat de sintetitzar-los en diferents dimensions i formes, les quals poden influir en els processos catalítics tot exposant plans cristal·logràfics específics. Degut al caràcter superficial de la catàlisi, l'ús de tècniques sensibles a la superfície és clau, però la investigació de sistemes catalítics s'ha de fer en les condicions més realistes possibles (en condicions de reacció) per poder identificar i comprendre el comportament de les espècies actives en condicions de treball. Aquest tipus d’estudis són possibles mitjançant l’espectroscòpia fotoelectrònica de raigs-X en pressions properes a l’ambient (NAP-XPS). En aquest context, les principals línies d'investigació d'aquesta tesi, dividida entres capítols, s'han centrat en el disseny, síntesi i caracterització fisicoquímica de catalitzadors basats en cèria i deis seus centres actius, especialment mitjançant espectroscòpia fotoelectrònica de raigs-X en condicions “operando”, en dues reaccions especifiques: la oxidació del sutge i la oxidació del CO. La primera part esta dedicada a la síntesi de múltiples catalitzadors diòxid de ceri amb diferent nanoforma (cubs, bastonets, octaedres i poliedres, així com cubs i bastonets microestructurats amb plans {111}) per estudiar la influència deis plans exposats en la seva activitat catalítica per la reacció d’oxidació del sutge. S'ha avaluat la seva activitat catalítica per l'oxidació del sutge, i els nanocubs i bastonets de cèria (amb domini de cares {100} i {110}) mostren la millar activitat específica d'oxidació del sutge. Les mesures NAP-XPS han demostrat que les cares {100} i {11O} presenten la quantitat més elevada de Celll i de vacants d'oxigen durant el seu escalfament en atmosfera d'argó, i que la presència de sutge ha promogut altament la formació de Celll, confirmat també mitjançant ex-situ i “operando” Raman. La segona partes focalitza en l'estudi de la reacció d'oxidació de CO sobre un catalitzador policristal·lí de Pd/CeO2 per investigar la interacció entre el pal·ladi i el suport de céria i determinar la seva morfologia i la composició química i estructura electrònica deis diferents elements en la superfície de la mostra. S'ha preparat i caracteritzat fisicoquímicament un catalitzador convencional en pols (Pd/CeO2) abans d'estudiar la seva activitat per l'oxidació de CO amb NAP-XPS. Les mesures “operando” han permès la identificació de dues especies de Pd diferents en condicions d'oxidació de CO, essent PdxCe1-xO2--0 la fase activa. Aquesta part també inclou una anàlisi no destructiu del perfil de profunditat tot utilitzant energies d'excitació de sincrotró variables, dut a terme per esclarir la ubicació i el paper d'aquestes espècies de pal·ladi en la reacció d'estudi. La tercera i última partes basa en el disseny i preparació de catalitzadors inversos cèria/Pd mitjançant la deposició de capa atòmica (ALD) per investigar la reacció d'oxidació de CO en condicions “operando”. S'ha preparat i caracteritzat morfològicament un sistema model Pd/CeO2/Si i diversos sistemes inversos CeO2/Pd per investigar el rol de les diferents interfícies formades en l'activitat de la reacció esmentada. La optimització del disseny del sistema invers cèria/Pd ha comportat un estudi de la influència de la temperatura i del nombre de cicles de deposició sobre l'estructura i l'activitat d'aquests sistemes, i s'ha establert una relació entre la rugositat superficial deis sistemes i el nombre de cicles de deposició. També s'ha fet un seguiment de l'efecte de la temperatura sobre la uniformitat i continuïtat de les capes fines deis sistemes. Les mesures de NAP-XPS han permès concloure que 100 cicles d'ALD són el valor òptim que permet estudiar les fases de Ce i Pd deis sistemes, i que la presència de la fase activa PdxCe1-xOPostprint (published version

Review of the Decomposition of Ammonia to Generate Hydrogen

Because of the problems associated with the generation and storage of hydrogen in portable applications, the use of ammonia has been proposed for on-site production of hydrogen through ammonia decomposition. First, an analysis of the existing systems for ammonia decomposition and the challenges for this technology are presented. Then, the state of the art of the catalysts used to date for ammonia decomposition is described considering the catalysts composed of noble and non-noble metals and their combinations, as well as novel materials such as alkali metal amides and imides. The effect of the supports and promoters used is analyzed in detail, and the catalytic activity obtained is compared. An analysis of the kinetics of the reaction obtained with different catalysts is also presented and discussed, including the reaction mechanism, the determining step of the reaction, and the apparent activation energy. Finally, the structured reactors used to date for the decomposition reaction of ammonia are explored, as well as the possibilities offered by catalytic membrane reactors, which allow the on-site simultaneous production and separation of hydrogen.Peer ReviewedPostprint (author's final draft

Operando NAP-XPS studies of a ceria-supported Pd catalyst for CO oxidation

Supported Pd/CeO2 catalytic systems have been widely investigated in the low-temperature oxidation of CO (LTO CO) due to the unique oxygen storage capacity and redox properties of the ceria support, which highly influence the structural, chemical and electronic state of Pd species. Herein, operando near-ambient pressure XPS (NAP-XPS) technique has allowed the study of a conventional Pd/CeO2 catalyst surface during the CO oxidation reaction under experimental conditions closer to the actual catalytic reaction, unfeasible with other surface science techniques that demand UHV conditions. SEM, HRTEM and XRD analyses of the powder catalyst, prepared by conventional incipient wetness impregnation, reveal uniformly CeO2-loaded Pd NPs of less than 2 nm size, which generated an increase in oxygen vacancies with concomitant ceria reduction, as indicated by H2-TPR and Raman measurements. Adsorbed peroxide (O22-) species on the catalyst surface could also be detected by Raman spectra. Operando NAP-XPS results obtained at the ALBA Synchrotron Light Source revealed two kinds of Pd species under reaction conditions, namely PdOx and PdII ions in a PdxCe1-xO2-d solution, the latter one appearing to be crucial for the CO oxidation. By means of a non-destructive depth profile analysis using variable synchrotron excitation energies, the location and the role of these palladium species in the CO oxidation reaction could be clarified: PdOx was found to prevail on the upper surface layers of the metallic Pd supported NPs under CO, while under reaction mixture it was rapidly depleted from the surface, leaving a greater amount in the subsurface layers (7% vs. 12%, respectively). On the contrary, the PdxCe1-xO2-d phase, which was created at the Pd–CeO2 interface in contact with the gas environment, appeared to be predominant on the surface of the catalyst. Its presence was crucial for CO oxidation evolution, acting as a route through which active oxygen species could be transferred from ceria to Pd species for CO oxidation.Peer ReviewedPostprint (published version

Investigation of the evolution of Pd-Pt supported on ceria for dry and wet methane oxidation

Efficiently treating methane emissions in transportation remains a challenge. Here, we investigate palladium and platinum mono- and bimetallic ceria-supported catalysts synthesized by mechanical milling and by traditional impregnation for methane total oxidation under dry and wet conditions, reproducing those present in the exhaust of natural gas vehicles. By applying a toolkit of in situ synchrotron techniques (X-ray diffraction, X-ray absorption and ambient pressure photoelectron spectroscopies), together with transmission electron microscopy, we show that the synthesis method greatly influences the interaction and structure at the nanoscale. Our results reveal that the components of milled catalysts have a higher ability to transform metallic Pd into Pd oxide species strongly interacting with the support, and achieve a modulated PdO/Pd ratio than traditionally-synthesized catalysts. We demonstrate that the unique structures attained by milling are key for the catalytic activity and correlate with higher methane conversion and longer stability in the wet feed.Peer ReviewedPostprint (published version

X-ray photoelectron and Raman spectroscopy of nanostructured ceria in soot oxidation under operando conditions

Ceria nanoshapes exhibiting different amounts of {100}, {110} and {111} facets (cubes, rods, octahedra and polyhedra) were prepared, characterized, tested in the carbon soot oxidation reaction, and studied by operando near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and operando Raman spectroscopy up to 550 °C. The specific soot oxidation reaction rate (mgC·m-2·min-1) clearly indicated that the {110} and {100} crystallographic planes of ceria were more active than the {111} ones for the oxidation of carbon soot. As deduced from the Ce 3d and O 1s signals recorded using different photon energies, all ceria nanoshapes experienced progressive reduction upon increasing the temperature under Ar, which was accompanied by the formation of oxygen vacancies. Raman studies revealed that, at this stage, isolated graphene layers in carbon soot reacted with ceria lattice oxygen atoms following a Mars-Van Krevelen mechanism. After exposure to O2 at 550 °C, a broad signal in the O 1s region at 531.2–532.5 eV was ascribed to surface active oxygen species, such as peroxide (O22-) and superoxide (O2-) species, generated from the interaction of molecular O2 with oxygen vacancies, which proved to be highly reactive to oxidize the graphitic structures of carbon soot.Peer ReviewedPostprint (published version

Facing seawater splitting challenges by regeneration with Ni-Mo-Fe bifunctional electrocatalyst for hydrogen and oxygen evolution

Hydrogen, produced by water splitting, has been proposed as one of the main green energy vectors of the future if produced from renewable energy sources. However, to substitute fossil fuels, large amounts of pure water are necessary, scarce in many world regions. In this work, we fabricate efficient and earth-abundant electrodes, study the challenges of using real seawater, and propose an electrode regeneration method to face undesired salt deposition. Ni-Mo-Fe trimetallic electrocatalyst is deposited on non-expensive graphitic carbon felts both for hydrogen (HER) and oxygen evolution reactions (OER) in seawater and alkaline seawater. Cl- pitting and the chlorine oxidation reaction are suppressed on these substrates and alkalinized electrolyte. Precipitations on the electrodes, mainly CaCO3, originating from seawater-dissolved components have been studied, and a simple regeneration technique is proposed to rapidly dissolve undesired deposited CaCO3 in acidified seawater. Under alkaline conditions, Ni-Mo-Fe-based catalyst is found to reconfigure, under cathodic bias, into Ni-Mo-Fe alloy with a cubic crystalline structure and Ni¿:¿Fe(OH)2 redeposits whereas, under anodic bias, it is transformed into a follicular Ni:FeOOH structure. High productivities over 300 mA¿cm-2 and voltages down to 1.59 V@10 mA¿cm-2 for the overall water splitting reaction have been shown, and electrodes are found stable for over 24 h without decay in alkaline seawater conditions and with energy efficiency higher than 61.5¿% which makes seawater splitting promising and economically feasible. Supporting InformationPeer ReviewedPostprint (author's final draft

Water-mediated photo-induced reduction of platinum films

Platinum thin films activated ex situ by oxygen plasma become reduced by the combined effect of an intense soft X-ray photon beam and condensed water. The evolution of the electronic structure of the surface has been characterized by near-ambient-pressure photoemission and mimics the inverse two-step sequence observed in the electro-oxidation of platinum, i.e. the surface-oxidized platinum species are reduced first and then the adsorbed species desorb in a second step leading to a surface dominated by metallic platinum. The comparison with measurements performed under high-vacuum conditions suggests that the reduction process is mainly induced by the reactive species generated by the radiolysis of water. When the photon flux is decreased, then the reduction process becomes slower

Water-mediated photo-induced reduction of platinum films

Platinum thin films activated ex situ by oxygen plasma become reduced by the combined effect of an intense soft X-ray photon beam and condensed water. The evolution of the electronic structure of the surface has been characterized by near-ambient-pressure photoemission and mimics the inverse two-step sequence observed in the electro-oxidation of platinum, i.e. the surface-oxidized platinum species are reduced first and then the adsorbed species desorb in a second step leading to a surface dominated by metallic platinum. The comparison with measurements performed under high-vacuum conditions suggests that the reduction process is mainly induced by the reactive species generated by the radiolysis of water. When the photon flux is decreased, then the reduction process becomes slower.Postprint (author's final draft

Water-mediated photo-induced reduction of platinum films

Platinum thin films activated ex situ by oxygen plasma become reduced by the combined effect of an intense soft X-ray photon beam and condensed water. The evolution of the electronic structure of the surface has been characterized by near-ambient-pressure photoemission and mimics the inverse two-step sequence observed in the electro-oxidation of platinum, i.e. the surface-oxidized platinum species are reduced first and then the adsorbed species desorb in a second step leading to a surface dominated by metallic platinum. The comparison with measurements performed under high-vacuum conditions suggests that the reduction process is mainly induced by the reactive species generated by the radiolysis of water. When the photon flux is decreased, then the reduction process becomes slower

Ceria-based catalysts studied by near ambient pressure x-ray photoelectron spectroscopy: A review

The development of better catalysts is a passionate topic at the forefront of modern science, where operando techniques are necessary to identify the nature of the active sites. The surface of a solid catalyst is dynamic and dependent on the reaction environment and, therefore, the catalytic active sites may only be formed under specific reaction conditions and may not be stable either in air or under high vacuum conditions. The identification of the active sites and the understanding of their behaviour are essential information towards a rational catalyst design. One of the most powerful operando techniques for the study of active sites is near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), which is particularly sensitive to the surface and sub-surface of solids. Here we review the use of NAP-XPS for the study of ceria-based catalysts, widely used in a large number of industrial processes due to their excellent oxygen storage capacity and well-established redox propertiesPostprint (published version