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3D-printed high-surface carbon monoliths have been fabricated and tested as catalyst supports of CO2 metha nation active phases (NiO-CeO2, 12 wt% Ni). The carbon carriers show a developed microporosity and good adherence to the catalytic phases of NiO-CeO2, showing great stability and cyclability. Two monolith designs were used: a conventional parallel-channeled structure (honeycomb) and a complex 3D network of non-linear channels built upon interconnected circular sections (circles), where flow turbulences along the reactant gas path are spurred. The effect of the active phases particle size on the catalyst distribution and the overall per formance has been assessed by comparing NiO-CeO2 nanoparticles of 7 nm verage (Np), with a reference counterpart of uncontrolled structure (Ref). The improved radial gases diffusion in the circles monolith design is confirmed, and nanoparticles show enhanced CO2 methanation activity than the uncontrolled-size active phase at low temperatures (< 300 ºC). On the contrary, the Ref catalysts achieve higher CH4 production at higher temperatures, where the reaction kinetics is controlled by mass transfer limitations (T > 300 ºC). SEM and Hg porosimetry evidence that nanoparticles are deposited at deeper penetration through the narrow micropores of the carbon matrix of the monolithic supports, which tend to accumulate on the channels surface remaining more accessible to the reactant molecules. Altogether, this study examines the impact of the channel tortuosity and the active phase sizing on the CO2 methanation activity, serving as ground knowledge for the further rational and scalable fabrication of carbon monolith for catalytic applications.

The present research exploits an innovative methodology for producing auto-pressurized carbon microreactors with a precise and controlled structure analyzing the influence of their design on the fluid dynamics and their catalytic performance. Carbon monoliths with Tesla-valve shape channels (Tesla, T, and modified Tesla, Tm) are synthesized through the combination of 3D printing and sol–gel process and further probed as Ni/CeO2 supports on CO2 methanation. The experimental results and mathematical modeling corroborated the improved performance obtained through the complex design compared to a conventional one. In addition to chaotic fluid flow induced by the deviation in flow direction, which improves the reagents-active phase interaction, local pressure increases due to convergence of flows may enhance the Sabatier reaction according to Le Châtelier's principle. Conversely to straight channels, T and Tm are not affected by flow rate and presented chemical control. Tesla-valve with curved angle (Tm) improved the mass transfer, achieving higher conversion and ≈30% reaction rate increase regarding right angle (T). Thus, this auto-pressurized multi-stage Tesla-valve monolith opens the gate to design specific and advanced functional materials for multitude chemical reactions where not only the reactant-active phase contact can be maximized but also the reaction conditions can be controlled to maximize the reaction kinetics.

In this study, the effect of the cell density of monolithic catalysts was investigated and further mathematically modeled on cordierite supports used in CO2 methanation. Commercial cordierite monoliths with 200, 400, and 500 cpsi cell densities were coated by immersion into an ethanolic suspension of Ni/CeO2 active phase. SEM–EDS analysis confirmed that, owing to the low porosity of cordierite (surface area < 1 m2 g−1), the Ni/CeO2 diffusion into the walls was limited, especially in the case of low and intermediate cell density monoliths; thus, active phase was predominantly loaded onto the channels’ external surface. Nevertheless, despite the larger exposed surface area in the monolith with high cell density, which would allow for better distribution and accessibility of Ni/CeO2, its higher macro-pore volume resulted in some introduction of the active phase into the walls. As a result, the catalytic evaluation showed that it was more influenced by increments in volumetric flow rates. The low cell density monolith displayed diffusional control at flow rates below 500 mL min−1. In contrast, intermediate and high cell density monoliths presented this behavior up to 300 mL min−1. These findings suggest that the interaction reactants-catalyst is considerably more affected by a forced non-uniform flow when increasing the injection rate. This condition reduced the transport of reactants and products within the catalyst channels and, in turn, increased the minimum temperature required for the reaction. Moreover, a slight diminution of selectivity to CH4 was observed and ascribed to the possible formation of hot spots that activate the reverse water–gas shift reaction. Finally, a mathematical model based on fundamental momentum and mass transfer equations coupled with the kinetics of CO2 methanation was successfully derived and solved to analyze the fluid dynamics of the monolithic support. The results showed a radial profile with maximum fluid velocity located at the cente

Activated carbon materials are widely used as high-performance electrodes for supercapacitors because they allow improving the electrochemical performance of these devices. The development of high surface areas and high porosity contributes to the energy storage by double electric layer, whereas the content of different functional groups on their surface to the pseudocapacitive storage. In addition, they can be obtained from lignocellulosic wastes providing a novel and economic perspective for the development of carbon-based electrodes. In this work, activated carbons were synthesized from cocoa husks using KOH as activating agent. The materials were obtained at three impregnation ratios (1:1, 3:1, 5:1) and three carbonization temperatures (500, 650, and 800 °C), optimizing the best conditions to obtain materials with high surface areas and porosity together with surface functional groups to improve energy storage. The results show that the 800 °C synthesized material with 3:1 impregnation ratio presents a capacitance of 120 F/g at 1 A/g, with a retention capacity of 93 % after 12,000 cycles, being the best electrochemical behavior among the carbons prepared. This is attributed to the adequate micro-mesoporosity, content of surface oxygenated functional groups and high surface area (1444 m2/g).

Los microplásticos son pequeñas partículas de material plástico de tamaño inferior a 5 mm que ya se encuentran en la totalidad del medio ambiente, incrementándose cada vez más las cantidades que se vierten a los océanos, lo que ha generado gran preocupación científica y social en los últimos años, despertando el interés por conocer el impacto que estas partículas pueden tener en la salud y el medio ambiente. En la actualidad no existe un método estandarizado para el análisis de microplásticos, con lo que cada laboratorio utiliza una técnica diferente, que van desde las técnicas espectroscópicas como Espectroscopía infrarroja con transformada de Fourier (FTIR) o espectroscopia Raman hasta las técnicas cromatográficas como la pirólisis acoplada a cromatografía de gases-masas (Pyr-CG-MS) o la termoextracción-desorción acoplada a cromatografía de gases-masas (TED-CG-MS). Todas estas técnicas han probado ser útiles para la detección e identificación de microplásticos, cada una de ellas con sus ventajas y desventajas, pero es necesario optimizarlas y encontrar un método más eficiente, robusto y a la vez que sea rápido y económico para que pueda estar al alcance desde el laboratorio más avanzado hasta el más modesto y que se puedan comparar los estudios realizados por los diferentes grupos. Se han estudiado y optimizado algunas de estas técnicas, como es la Calorimetría Diferencial de Barrido (DSC), donde se ha conseguido cuantificar, no solo los microplásticos formados por polímeros semicristalinos, sino también los constituidos por polímeros amorfos utilizando su temperatura de transición vítrea. Por otro lado, las técnicas cromatográficas son muy útiles en el análisis, tanto cualitativo como cuantitativo, de microplásticos, ya que se trata de técnicas muy robustas que proporcionan cromatogramas que pueden considerarse huellas dactilares de los compuestos que se analizan, no presentando interferencias entre los diferentes polímeros y siendo técnicas mucho más selectivas

The CO2 methanation mechanism over the highly active (TOF=75.1 h−1), selective (>92%) and stable 10% LaNiO3/CeO2-derived catalyst is still unresolved. The surface of the catalyst is monitored under hydrogenation (H2), oxidizing (CO2) and CO2 methanation (H2 +CO2) conditions by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) using synchrotron radiation. Meanwhile, the main reaction intermediates are identified by in situ FTIR analysis. NAP-XPS experiments confirm that LaNiO3 perovskite reduction leads to the ex-solution of Ni0 nanoparticles and Ni2+single bondCeO2−x and Ni2+single bondLa2O3 interfaces conformation, favouring the CO2 adsorption and the H2 dissociation/transfer. In situ FTIR experiments combined with the C1s spectra (NAP-XPS) suggest that the CO2 activation occurs on CeO2−x (oxygen vacancies and OH–) at low temperatures, in the form of bicarbonates; whereas, mono-/bidentate carbonates are formed on different strength La2O3 sites at increasing temperatures. These species are consecutively reduced to formates, as the main reaction intermediate, and methane by the H spilled from Ni0 nanoparticles near to NiOsingle bondCeO2−x and NiOsingle bondLa2O3 interfaces.

The reaction kinetics of CO2 methanation over a highly active 8.5% Ni/CeO2 catalyst was determined in a fixed-bed reactor, in the absence of heat- and mass-transfer limitations. Once the catalyst activity was stabilized, more than 120 kinetic experiments (with varying values of reaction temperature, total pressure, space velocity (GHSV), and partial pressure of products and reactants) were performed. From initial reaction rates, an apparent activation energy of 103.9 kJ mol–1 was determined, as well as the effect of reactants (positive) and water partial pressures (negative) on CO2 methanation rate. Three mechanistic models reported in the literature, in which CO2 is adsorbed dissociatively (carbon and formyl routes) or directly (formate route), were explored for modeling the entire reaction kinetics. For that, the corresponding rate equations were developed through the Langmuir–Hinshelwood–Hougen–Watson (LHHW) approach. In agreement with DRIFTS experiments, formate route, in which the hydrogenation of bicarbonate to formate is considered to be the rate-determining step, reflects the kinetic data accurately, operating from differential conversion to thermodynamic equilibrium. In fact, this mechanism results in a mean deviation (D) of 10.38%. Based on previous own mechanistic studies, the participation of two different active sites has been also considered. Formate route on two active sites maintains a high fitting quality of experimental data, providing kinetics parameters with a higher physical significance. Thus, the LHHW mechanism, in which Ni0 sites as well as oxygen vacant near to Ni-CeO2 interface participate in CO2 methanation, is able to predict the kinetics of Ni/CeO2 catalyst accurately for a wide range of operational conditions.

The mechanism and kinetic of CO2 methanation reaction of 9.5 % Ni/Al2O3 catalyst is analysed under a wide range of operating conditions. Once the catalyst activity is stabilized, the influence of temperature, total pressure and space velocity is studied for kinetic characterisation. A data set comprising of 153 experimental runs has been used to develop a kinetic model capable to accurately predict the reaction rate. Ni/Al2O3 catalyst shows an apparent activation energy of 80.1 kJ mol−1 in CO2 hydrogenation. Data obtained under differential mode adjust quite precisely to a power-law model with H2O inhibition, with a water adsorption constant of 3.1 atm−1 and apparent orders of 0.24 and 0.27 for H2 and CO2, respectively. Based on DRIFTS results, we propose for the first time the H-assisted CO formation route, which is compared with the more conventionally reported carbonyl route, and describe the corresponding reaction rate LHHW equation, resulting in notable improvement for mean deviation (D) of 7.0 % in our model related to that based on the carbonyl route (D = 20.1 %) usually suggested for catalysts with higher Ni loads around 20 %. The H-assisted CO formation route considers the formate species decomposition into carbonyls via H-assisted CO formation mechanism and further carbonyls hydrogenation into CHO as the rate determining step. Thus, the LHHW mechanism, in which carbonyls as well as formate species participate in CO2 methanation, is capable to reflect the kinetics of lowly-loaded Ni/Al2O 3 catalyst with high accuracy under relevant process conditions (315−430 °C, 1−6 bar, H2 to CO2 molar ratios between 1–16 and, different reagents and products partial pressures).

The preparation and use of Guefoams as heterogeneous catalyst is reported. The Guefoam catalyst consists of an open-pore Al-Si foam that accommodates a freely mobile guest phase (Ni/CeO2/Al2O3 particles) in its cavities, with neither a physical nor a chemical matrix-guest bond. A eutectic Al-12Si alloy was used as a low-melting matrix precursor to prevent thermal sintering of the active phase during liquid metal infiltration. CO2 methanation was chosen as the reaction test. The activity and CH4 selectivity (close to 100%) achieved with the Guefoam catalyst were similar to those obtained with a packed bed of the same active phase particles, but with the advantages of a structured reactor such as robustness and ease of handling. The thermal conductivity of the Guefoam catalyst is significantly improved with regard to the packed bed of active phase particles, which reduces the temperature gradients in the catalytic reactor, as demonstrated by computational fluid dynamic modelling. Since the permeability of the Guefoam catalyst is 2.7 times that of the packed bed, the pressure drop caused by the passage of a fluid through the novel material is reduced, resulting in a significantly higher catalytic performance index than the packed bed.

Ni/CeO2 catalysts are very active and selective for total hydrogenation of CO2 to methane, but the nature of the active sites is still unclear. The surface of a Ni/CeO2 catalyst has been monitored under CO2 methanation conditions by Near Ambient Pressure-XPS (NAP-XPS) using synchrotron radiation, and has been concluded that the species involved in the redox processes taking place during the CO2 methanation mechanism are the Ni2+-CeO2/Ni0 and Ce4+/Ce3+ pairs. In addition, a small fraction of nickel is present on the catalyst surface forming NiO and Ni2+-carbonates/hydroxyls (around 20% of the total surface nickel), but these species do not participate in the redox processes of the methanation mechanism. Under CO2 methanation conditions the H2 reduction rate of the Ni2+-CeO2/Ni0 and Ce4+/Ce3+ couples is much faster than their CO2 reoxidation rate (2 times faster, at least, at 300ºC), but a certain proportion of nickel always remains oxidized under reaction conditions. The high activity of Ni/CeO2 catalysts for CO2 methanation is tentatively attributed to the simultaneous presence of Ni2+-CeO2 and Ni0 active sites where CO2 and H2 are expected to be efficiently dissociated, respectively.

A Ce0.5Pr0.5Ox soot combustion catalyst has been shaped combining this material prepared in two different structures (nanoparticles and MS-macroporous) in a single active phase. Nanoparticles, prepared by reversed microemulsion method, provide high activity to the hybrid material, and the MS structure, obtained by a hard template synthesis method, provides high thermal stability and improved soot-catalyst contact. Ce0.5Pr0.5Ox nanoparticles present high surface area (148 m2/g; 5 nm) and improved redox properties with regard to other Ce0.5Pr0.5Ox structures, and the macropores size of MS Ce0.5Pr0.5Ox are large enough (around 100 nm) to properly disperse Ce0.5Pr0.5Ox nanoparticles inside and also to host soot particles, creating a high reactive catalyst environment. As a result, the as prepared Ce0.5Pr0.5Ox hybrid catalyst, mixed in loose-contact mode with soot, decreased soot combustion by ca. 100 °C with regard to the uncatalyzed reaction, and thermal aging at 800 °C only decreased this difference to ca. 75 °C. The as-prepared counterpart hybrid catalyst obtained dispersing pure ceria nanoparticles on pure ceria MS support was also active for soot combustion, but the activity was almost lost completely upon thermal aging due to ceria sintering.

The mechanism and kinetic of CO2 methanation reaction of 9.5 % Ni/Al2O3 catalyst is analysed under a wide range of operating conditions. Once the catalyst activity is stabilized, the influence of temperature, total pressure and space velocity is studied for kinetic characterisation. A data set comprising of 153 experimental runs has been used to develop a kinetic model capable to accurately predict the reaction rate. Ni/Al2O3 catalyst shows an apparent activation energy of 80.1 kJ mol(-1) in CO2 hydrogenation. Data obtained under differential mode adjust quite precisely to a power-law model with H 2 O inhibition, with a water adsorption constant of 3.1 atm(-1) and apparent orders of 0.24 and 0.27 for H-2 and CO2, respectively. Based on DRIFTS results, we propose for the first time the H-assisted CO formation route, which is compared with the more conventionally reported carbonyl route, and describe the corresponding reaction rate LHHW equation, resulting in notable improvement for mean deviation (D) of 7.0 % in our model related to that based on the carbonyl route (D = 20.1 %) usually suggested for catalysts with higher Ni loads around 20 %. The H-assisted CO formation route considers the formate species decomposition into carbonyls via H-assisted CO formation mechanism and further carbonyls hydrogenation into CHO as the rate determining step. Thus, the LHHW mechanism, in which carbonyls as well as formate species participate in CO2 methanation, is capable to reflect the kinetics of lowly-loaded Ni/Al2O3 catalyst with high accuracy under relevant process conditions (315-430 degrees C, 1-6 bar, H-2 to CO2 molar ratios between 1-16 and, different reagents and products partial pressures).

La transformación del CO2 ha suscitado gran interés a la comunidad científica en el área de la catálisis heterogénea, no solo por su potencial aplicación y escalado, sino también por estimular la creatividad de la comunidad científica en el diseño de las formulaciones y generación de conocimiento. En este sentido, la conversión (electro/foto) catalítica de CO2 ha experimentado un avance significativo en el desarrollo de nuevos combustibles, productos químicos, farmacéuticos y building-blocks entre otros. La hidrogenación catalítica de CO2 a productos valiosos se considera prometedora en la economía circular y el reciclado de CO2. El uso de CO2 como materia prima para producir componentes básicos ha surgido como un tema candente en la comunidad científica. Desde productos químicos C1 hasta combustibles de cadena larga, la hidrogenación catalítica de CO2 ofrece actualmente una amplia gama de tecnologías escalables para disminuir las emisiones de gases de efecto invernadero. Principalmente, la conversión de CO2 en metano se puede lograr mediante catalizadores funcionales duales a presión ambiente. Bien en sabido que metales como Ni o Ru soportados sobre óxidos reducibles exhiben una alta actividad y selectividad hacia la producción de metano. Es por ello que, este proyecto de tesis doctoral consiste en el diseño integral de catalizadores de Ni y Ru soportados sobre sistemas basados en óxido de cerio para el estudio de la valorización sostenible de CO2. Esto comprende desde la síntesis y optimización de los nuevos materiales, el estudio de sus propiedades físico-químicas mediante técnicas de caracterización convencional, técnicas avanzadas focalizadas en estudios mecanísiticos, el estudio in-situ de la evolución de los estados de oxidación, el estudio de agentes dopantes trivalentes y tetravalentes, el papel de los centros activos implicados en el mecanismo de reacción y la influencia de la morfología cuando se obtienen diferentes interacciones metal/óxido sobre cataliz

Carbon sorbents have been tested in this study to improve the retention of microplastics thermodecomposition compounds in Thermoextraction-Desorption Gas Chromatography-Mass Spectrometry (TED-GC-MS) technique and has been demonstrated an optimal behavior for this application, highlighting two main advantages with regard to conventional PDMS sorbents; On the one hand, carbons present slit-type pores, being optimal for retaining the aromatic compounds released from most polymers. In addition, those materials have shown to have a greater retention capacity than PDMS being between 1.25 and 25 times more efficient. Combination of PDMS+Carbon sorbents increases retention of the most representative compounds for each polymer up to 1.7 times with regard to only carbon, increasing sensitivity, but with an experimental procedure more complicated, because two desorption process must be done. On the other hand, carbon materials can retain the chlorinated compounds released by thermodecomposition of PVC, while conventional PDMS sorbent is not effective for these compounds.