Evidence of new Ni-O-K catalytic sites with superior stability for methane dry reforming

Liquid fuels produced via Fischer-Tropsch synthesis from biomass-derived syngas constitute an attractive and sustainable energy vector for the transportation sector. This study focuses on the role of potassium as a promoter in Ni-based catalysts for reducing coke deposition during catalytic dry reforming. The study provides a new structural link between catalytic performance and physicochemical properties. We identify new Ni-O-K chemical states associated with high stability in the reforming process, evidenced by different characterization techniques. The nickel particles form a core surrounded by a Ni-O-K phase layer (Ni@Ni-O-K) during the reduction of the catalyst. This phase likely presents an alkali-nickelate-type structure, in which nickel is stabilized in oxidation state + 3. The Ni-O-K formation induces essential changes in the electronic, physical, structural, and morphological properties of the catalysts, notably enhancing their long-term stability in dry reforming. This work thus provides new directions for designing more efficient catalysts for sustainable gas-to-liquids processes.Ministerio de Economía y Competitividad RTI2018-096294-B-C33Universidad de Sevilla US-126328

Reforming of biomass-derived stream: evidence of an active Ni-O-K phase

The dependence of our current energetic model based on fossil fuels and their harmful effects on the environment are strengthen the development of renewable energy sources. Liquid transportation fuels produced via Fischer-Tropsch synthesis (FTS) from biomass derived syngas promises an attractive and sustainable energy source for the transportation sector of our society. This work mainly focuses on producing liquid fuels from lignocellulosic biomass, emphasising the catalytic aspects that involve the generation of the syngas through the reforming of biogas and cleaning-up of producer gas for the subsequent FTS reaction. In this context, the motivation of the present PhD thesis is to understand the fundamental role of potassium as a promoter in Ni-based catalysts for reducing carbon formation during catalytic biogas and producer gas model reforming reactions. So far, important discrepancies about the real effect of potassium in steam reforming are found in the numerous works reported in the literature. Several questions remain still unsolved: Where is the potassium located on the catalyst? What kind of properties, structural, electronic and/or physical, are affected? or Is it potassium a carbon gasifier? Hence, a series of Ni supported on MgAl2O4 catalysts promoted with different loadings of potassium (0, 1, 3 and 5 wt.% of K2O) were synthesised by coimpregnation and characterised using an extended variety of techniques. The catalytic performance was evaluated in several reforming reactions for producing valuable syngas: dry reforming of methane (DRM) reaction, bi-reforming of methane (BRM) and tar reforming reactions with toluene as tar model. This PhD dissertation has been divided into six chapters. Chapter I address a global view of the problem statement and describes the objectives. Chapter II contains all the experimental methods and techniques as well as the reaction systems used in the lab. Chapter III provides a detailed discussion of the physicochemical properties of the synthesised K-Ni /MgAl2O4 catalysts. Chapter IV includes an exhaustive study of the catalytic performance in DRM and BRM reactions and the postmortem analysis of the catalysts. Furthermore, operando spectroscopic methods are used to gain insights into the reaction mechanism. Chapter V is focused on the reactions of toluene reforming and its influence in a producer gas model reforming reaction, as well as the regeneration capability of the catalysts. Finally, Chapter VI included the general conclusions. In general, all chapters include a brief introduction at the beginning and the most relevant partial findings at the end. Successful Ni-K catalysts were developed, showing optimal activity and long-term stability under severe reaction conditions. Furthermore, it was demonstrated that the presence of steam in the feed reactant mixtures has a positive effect since it decreases notably the formation of carbon. In addition, it was successfully demonstrated that Ni-K catalysts are excellent candidates for obtaining clean syngas from producer gas reforming. Although coke deposition is unavoidable in the presence of tars in the stream, the catalyst presents a high capacity to be regenerated. This study provides a new structural approach to establish the correlation between catalytic performance and physicochemical properties from a fundamental perspective. Primarily, the addition of potassium affects the surface acid-base properties directly since hydroxyls become neutralised, favouring the formation of potassium carbonates that influence the reaction pathway. On the other hand, it has been found that potassium may be located on the support generating strong basic Mg-O-K sites and also covering the nickel particles to form a surface Ni-O-K composite layer. It must be stated that this work is a pioneer proposing the presence of the Ni-O-K phase for steam reforming processes. The presence of the Ni-O-K phase has been experimentally evidenced through different characterisation techniques. Indeed, XPS results showed the formation of species in which the oxidation state of nickel is Ni3+ and pointed out the possible formation of potassium nickelate structural phases. Consequently, these results reveal that potassium presents electronic, physical, structural and morphological effects on Ni-based catalysts. Finally, it has been demonstrated the great potential of these catalysts in other catalytic processes such as the low-temperature Reverse Water Gas Shift (RWGS) reaction. The formation of a complex nickel-layered potassium structure enhances notably the selectivity into CO due to its low CO adsorption capacity and inhibition of the competitive methanation reaction. These results expand the horizon for designing more efficient catalysts for low-carbon energy processes.El uso de la biomasa como fuente alternativa de carbono en los procesos Biomass-to-Liquid (BTL) y Bio Gas-to Liquid (Bio-GTL) para producir biocombustibles mediante la síntesis de Fischer-Tropsch (FTS) contribuye al importante reto de conseguir una sociedad neutra en carbono en 2050. Sin embargo, aunque esta tecnología es muy atractiva, cabe destacar que aún presenta algunas limitaciones para la obtención de un gas de síntesis óptimo para las diferentes reacciones de FTS. El reformado seco de metano (DRM) se utiliza para producir gas de síntesis a partir del producer gas o del biogás obtenido de la biomasa. A pesar del impacto medioambiental positivo que supone el consumo de dos importantes gases de efecto invernadero (CH4 y CO2), la ingeniería de catalizadores del proceso para la reacción de reformado aún requiere superar algunos inconvenientes. Aunque está bien establecido el uso de catalizadores de Ni /MgAl2O4 en grandes procesos industriales, su desactivación por deposición de carbono es acelerada debido a la presencia de alquitranes en los gases efluentes, siendo una cuestión de suma importancia que debe ser resuelta. La presencia de alquitranes en los gases de salida obliga a incluir pasos de acondicionamiento o mejora para obtener un gas de síntesis óptimo, así como el perfeccionamiento de catalizadores avanzados. En este sentido numerosos estudios se han centrado en el desarrollo de catalizadores resistentes al coque. Uno de estos estudios consiste en el uso del potasio como promotor de la gasificación del carbono, siendo ampliamente aceptado por la comunidad científica. Aun así, a día de hoy hay distintas controversias sobre la contribución real del potasio. Por lo tanto, es fundamental proporcionar nuevos conocimientos sobre el efecto de este promotor y comprender cómo se mitiga la deposición de carbono. Dentro de esta perspectiva, esta tesis tiene como objetivo obtener una comprensión completa del papel del potasio en los catalizadores de Ni/ MgAl2O4 para las reacciones de DRM y establecer una relación entre las propiedades de los catalizadores y el efecto de resistencia al coque bajo diferentes condiciones de reacción. Para ello, se sintetizaron una serie de catalizadores Ni/ MgAl2O4 con diferentes cargas de K mediante coimpregnación sobre espinela de aluminato de magnesio. Este estudio proporciona un nuevo enfoque para estos materiales en el que se establece una correlación estructural entre el rendimiento catalítico y las propiedades fisicoquímicas desde una perspectiva fundamental. Principalmente, la adición de potasio afecta directamente a las propiedades ácido-base de la superficie ya que los hidroxilos se neutralizan, favoreciendo la formación de carbonatos de potasio que influyen en la ruta de reacción. Por otro lado, se ha encontrado que el potasio puede localizarse en el soporte generando fuertes sitios básicos Mg-O-K y también cubriendo las partículas de níquel formando una capa Ni-O-K sobre la superficie metálica. En consecuencia, estos resultados revelan que el potasio presenta efectos electrónicos, físicos, estructurales y morfológicos sobre los catalizadores basados en Ni. Finalmente, se ha demostrado el gran potencial de estos catalizadores en otros procesos catalíticos como la reacción de Reverse Water Gas Shift (RWGS) a baja temperatura. La formación de una estructura compleja de níquel y potasio mejora notablemente la selectividad en CO debido a su baja capacidad de adsorción y a la inhibición de la reacción de metanación competitiva. Estos resultados amplían el horizonte para el diseño de catalizadores más eficientes para los procesos energéticos de baja emisión de carbono

IR spectroscopic insights into the coking-resistance effect of potassium on nickel-based catalyst during dry reforming of methane

Dry reforming of methane (DRM) is an effective catalytic route for transforming CO and CH into valuable syngas and thus potentially attractive for mitigating the emission of environmental harmful gases. Therefore, it is crucial to develop rationally Ni-based catalysts highly resistant to coking and sintering. In this scenario, the addition of small amounts of potassium to nickel catalyst increases their resistance to coking during dry reforming of methane. Nonetheless, the specific role of potassium in these catalysts not have been fully understood and there are still important discrepancies between the different reported studies. This work provides a new approach on the anticoking nature of a K-promoted Ni catalyst by means of a combined IR spectroscopic study of in situ characterization by CO adsorption under static conditions and operando DRIFTS measurements under dynamic conditions of DRM reaction. The involved surface species formed during the reaction were elucidated by transient and steady-state operando DRIFTS studies. It was revealed that the existence of Ni-K interfacial sites favours the gasification of carbonaceous deposits towards reverse Boudouard reaction and reduces the sticking probability of CO dissociative adsorption. Moreover, the presence of strongly Mg-O-K basic sites leads to the formation of carbonate intermediates that are subsequently reduced into CO gaseous towards the associative mechanism by RWGS reaction. These results provide a fundamental understanding of the relevant anticoking effect of potassium on Ni-based catalysts

Effect of potassium loading on basic properties of Ni/MgAl2O4 catalyst for CO2 reforming of methane

. Coke deposition is one of the key issues in the dry reforming of methane on Ni catalysts. In the present work, we investigate the effect of potassium addition for suppressing carbon deposition in the Dry Reforming of Methane. The results obtained demonstrated that potassium contents above 3 wt% promote carbon gasification, favouring both Reverse Water Gas Shift and Boudouard reaction. Strong basic Mg-O-K sites are responsible for these reactions allowing the suppression of carbon deposits and allowing the stability of the catalystPeer reviewe

Potentialization of bentonite properties as support in acid catalysts

Enhancement of the main physicochemical properties of a natural bentonite was carried out by means of modifications using surfactant, reflux, microwave treatment and, subsequently, the incorporation of AlZr and AlCe species. The evolution of the main changes in each modification stage was evaluated by means of X-ray diffraction, N sortometry, scanning microscopy (SEM), NH-TPD, NH-DRIFTS and CO adsorption at low temperature. For the evaluation of the catalytic behavior, the dehydration-dehydrogenation reactions of 2-propanol and hydro-conversion of decane were used; both of which generate, in addition, information regarding the acidic properties of the materials. The correlation of the number, type and acid strength with the catalytic behavior, allowed establishing the effect produced by both the delamination method and the nature of the incorporated cation. This generated tools that allow controlling the physicochemical properties, and more specifically, the enhancement of the acidity of new supports based on this type of natural clay mineral

Modulation of the acidity of a vermiculite and its potential use as a catalytic support

The modulation and characterization of the acidity of a vermiculite were carried out, which was modified by delamination by means of hydrothermal and acid treatments with the subsequent incorporation of AlZr and AlCe species to modulate the acidity. The effect of these species was evaluated regarding the structural (XRD, XPS and IR), textural (N sortometry) and acidity properties (NH-TPD, NH-DRIFTS and CO adsorption at low temperature). The catalytic performance was studied in the dehydration–dehydrogenation reactions of 2-propanol and the hydroconversion of decane, which generate important information about the acidity properties such as the type, number and strength of acidic sites. The correlation between the number, type and acid strength with the catalytic behavior allowed to establish the important effect regarding the nature of the mineral, its method of delamination and the nature of the incorporated cation, thus generating tools for controlled processes for the potentiation of the acidity of new supports from raw vermiculite

Influence of the preparation method in the metal-support interaction and reducibility of Ni-Mg-Al based catalysts for methane steam reforming

Ni-Mg-Al based catalysts were prepared using different preparation methods (impregnation, impregnation-coprecipitation and coprecipitation) and tested in steam reforming of methane. The differences observed in catalytic activity were directly correlated to the physicochemical properties and the different degree of Ni-Mg-Al interaction. The reducibility results showed that the catalyst prepared by the impregnation-coprecipitation method presented the most optimal metal-support interaction to reduce the NiO preserving the Ni particles highly dispersed on the support surface. These results demonstrate that the structure and catalytic performance of Ni-Mg-Al based catalysts can be tuned by controlling the metal-support interaction through of the preparation method

Modulation of the Acidity of a Vermiculite and its Potential Use as a Catalytic Support

The modulation and characterization of the acidity of a vermiculite were carried out, which was modified by delamination by means of hydrothermal and acid treatments with the subsequent incorporation of AlZr and AlCe species to modulate the acidity. The effect of these species was evaluated regarding the structural (XRD, XPS and IR), textural (N2 sortometry) and acidity properties (NH3-TPD, NH3-DRIFTS and CO adsorption at low temperature). The catalytic performance was studied in the dehydration–dehydrogenation reactions of 2-propanol and the hydroconversion of decane, which generate important information about the acidity properties such as the type, number and strength of acidic sites. The correlation between the number, type and acid strength with the catalytic behavior allowed to establish the important effect regarding the nature of the mineral, its method of delamination and the nature of the incorporated cation, thus generating tools for controlled processes for the potentiation of the acidity of new supports from raw vermiculite.National University of Colombia 0405-201

Insights on Guerbet Reaction: Production of Biobutanol From Bioethanol Over a Mg–Al Spinel Catalyst

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.The production of biobutanol from bioethanol by the Guerbet reaction is an alternative pathway to renewable sources. The commercial viability of this green route requires improvements in the process development. This study experimentally examines the influence of operating conditions on the performance of a Mg–Al spinel catalyst prepared from hydrotalcite precursors. This catalyst demonstrates an exceptional performance in the Guerbet reaction with a promising activity/butanol selectivity balance, excellent long-term stability, and very-low-carbon footprint (CO2 generation as by-products is minimal). This study showcases a systematic strategy to optimize the reaction parameters in the Guerbet reaction for biobutanol production using an advanced spinel catalyst. Upon carefully adjusting temperature, pressure, space velocity, and reactants co-feeding, very promising conversion (35%) and butanol selectivity values (48%) were obtained