Heterogeneous catalysis of green chemistry reactions on molybdenum carbide based catalysts

[eng] Our society has a problem with the use of fossil fuels, due to the vast and exceeding emissions derived from human activities. Two ways could be consider to mitigate these harmful effects. On the one hand, the capture, activation, and conversion of these hazardous gases towards valuable compounds, and on the other hand, the substitution of fossil fuels for renewable energies. This thesis encompasses the study of two different green chemistry reactions to convert the most abundant greenhouse gas in Earth's atmosphere and the production of a new environmental friendly fuel, the hydrogen. In the current search for new catalysts, Transition Metal Carbides (TMCs) have arisen as an appealing alternative, because their exhibit broad and amazing physical and chemical properties and their low cost. In particular, titanium carbide (001) was proposed from experimental and theoretical points of view as active catalyst and support of small metal particles for CO2 hydrogenation to methanol and water gas shift reaction. However, given that titanium carbide is a cumbersome support to be used in applications due to the difficulty of obtaining nanoparticles on working conditions, we have carried out these reactions on cubic δ-MoC (001) and orthorhombic β-Mo2C (001) surfaces. The adsorption and activation of a CO2 molecule on cubic δ-MoC (001) and orthorhombic β-Mo2C (001) surfaces have been investigated by means of periodic density functional theory based calculations using the Perdew-Burke-Ernzerhof exchange-correlation functional showing that both surface are promising catalyst for CO2 conversion because they are able to activate and bend the CO2 molecule. The β- Mo2C (001) surface is able to dissociate the CO2 molecule easily, with a low energy barrier, whereas δ-MoC (001) surface activates CO2 but it does not promote its direct dissociation. Experiments accomplished by the group of Dr. Jose Rodriguez revealed that CO and methane are the main products of the CO2 hydrogenation using β-Mo2C (001) as catalyst, and the amount of methanol is lower. On the other hand, only CO and methanol are produced using δ-MoC (001). Experiments revealed that the deposition of small copper particles on the carbide surfaces increase drastically the catalysts' activity and selectivity, which was demonstrated by theoretical calculations. On β-Mo2C, the amount of CO and methanol increase whilst the amount of methane decrease, since copper blocks reactive sites on surface. This is a positive fact since copper avoid the excessive oxygen deposition, which deactivated the catalysts. On the other hand, the deposition of copper on δ-MoC (001) increases a lot the amount of CO and methanol. In summary, our combining DFT- experimental study proposed the Cu/δ-MoC as promising catalyst for CO2 hydrogenation due to its activity (the amount of products is superior than other TMCS, metals, and the model of commercial catalysts), selectivity (only CO and methanol are produced), and stability ( this catalysts is not deactivated by the oxygen deposition). The results obtained in the first part of the thesis were used to study the water gas shift reaction. Given that the excellent features, experiments proposed Au supported on δ-MoC (001) as catalysts. Our theoretical calculations demonstrated that clean δ-MoC (001) is not a good catalysts for WGS, due to the fact that the reverse reactions are favorable respect the direct ones, which implies that the amount of products is lower. Nevertheless, the deposition of Au clusters change the reaction mechanism, favoring the direct barriers instead of reverse ones, and increasing the amount of produced H2. In summary, this thesis has displayed the prominent role of molybdenum carbides as support of small metal particles to catalyze green chemistry reactions.[cat] En aquesta tesi es mostra un treball computacional sobre l'ús de catalitzadors econòmics per a la conversió de CO2, un perillós gas d'efecte hivernacle i també per a la producció d'hidrogen, el combustible del futur. En la recerca actual de nous catalitzadors, els carburs de metalls de transició (TMC) han sorgit com una alternativa atractiva pel el seu baix cost i per exhibir excel·lents propietats físiques i químiques. En aquest treball utilitzarem com a catalitzadors les superfícies cúbica δ-MoC (001) i ortoròmbica β-Mo2C (001). L'adsorció de la molècula de CO2 mostra que ambdues superfícies són capaces d'activar i doblegar la molècula. La superfície β-Mo2C (001) és capaç de dissociar fàcilment la molècula de CO2, mentre que la superfície δ-MoC (001) activa CO2 però no la dissocia. Els experiments realitzats pel grup del Dr. Jose Rodriguez van revelar que el CO i el metà són els principals productes de la hidrogenació de CO2 utilitzant β-Mo2C (001) com a catalitzador, i la quantitat de metanol és menor. D'altra banda, només es produeixen CO i metanol utilitzant δ-MoC (001). La deposició de partícules de coure a les superfícies del carbur augmenta dràsticament l'activitat dels catalitzadors, cosa que es va demostrar mitjançant càlculs teòrics. A la superfície β-Mo2C, la quantitat de CO i metanol augmenten mentre que la quantitat de metà disminueix. D'altra banda, la deposició de coure a δ-MoC (001) augmenta molt la quantitat de CO i metanol. En resum, el nostre estudi proposa el Cu/δ-MoC com a prometedor catalitzador de la hidrogenació de CO2 a causa de la seva activitat (la quantitat de productes és superior a la resta de TMCS, metalls i el model de catalitzadors comercials), selectivitat (només el CO i el metanol es produeixen) i l'estabilitat (aquests catalitzadors no es desactiven per la deposició d'oxigen). Tenint en compte els resultats previs, es va proposar la deposició d'or en la superfície δ-MoC per a la producció d'hidrogen. Els càlculs teòrics demostren que la superfície δ-MoC (001) no és un bon catalitzador per WGS, però la deposició dels clústers d'or canvia el mecanisme de reacció i augmenta la quantitat d'H2 produïda

Exploring the impact of culturally and non culturally relevant texts on english reading comprehension: Case studies of two undergraduate students of an english language teaching program at a public university

Esta investigación se enfocó principalmente en explorar el impacto que tienen textos culturalmente familiares y no familiares en la comprensión de lectura de dos estudiantes adscritos a un programa de enseñanza del inglés en una universidad pública Colombiana. Estos dos participantes pasaron por un proceso de lectura de 3 textos culturalmente relevantes y 3 textos culturalmente no relevantes acompañados de un cuestionario de comprensión de lectura cada uno y un ejercicio oral previo y posterior a la lectura. Luego los participantes fueron entrevistados acerca de sus procesos de lectura y acerca de las impresiones que los textos y los cuestionarios les dejaron. Este análisis mostró que el hecho de que los textos incluyeran información culturalmente relevante influyó positivamente en la comprensión de lectura y en la actitud frente al ejercicio de lectura de los participantes. Esta investigación también generó algunos hallazgos relevantes acerca de la importancia de usar estrategias de lectura. A su vez, el análisis y los resultados de este estudio mostraron formas de evaluar la comprensión lectora de una manera no tradicional que generó una reflexión hacía una evaluación lectora alternativa.The present study aims mainly to investigate the impact that texts with cultural and non cultural relevant information have on the reading comprehension of two undergraduate learners of an English language teaching program at a Colombian public university. The two participants of this study went through a process of reading three culturally relevant texts (CRTs) and three non culturally relevant texts (NCRTs). They were assessed through some previous and post exercises along with some reading comprehension tests. Likewise, they were interviewed about some processes they applied when reading and they were asked about their impressions from the texts and the questionnaires. Data analysis showed that the participants’ reading comprehension and attitudes were positively affected by the fact that they were familiar with the information presented in the CRTs. This study also generated some relevant insights about the importance of using reading strategies. Moreover, this research also showed ways to assess reading comprehension in a non traditional way and still get valid results

Interacciones y percepciones en procesos de mediación tecnológica Caso: experiencia de aprendizaje mediada a través de un grupo en whatsapp como complemento para el desarrollo de habilidades comunicativas en un curso básico de inglés

La presencia de las tecnologías de la información y la comunicación en la educación y su consideración en diferentes contextos de aprendizaje se ha vuelto un tema ineludible que merece una mirada más cercana para analizar su pertinencia y uso dentro de un ambiente de estudio formal. Con el auge de las redes social y en especial el WhatsApp, se tuvo como objetivo general en la presente investigación, de carácter cualitativo, comprender las relaciones que se dieron entre las interacciones y las percepciones de los participantes de un curso básico de inglés a través de la implementación de un grupo en la red social como herramienta de apoyo al proceso de aprendizaje del inglés de un grupo de estudiantes de nivel básico del SENA (Servicio Nacional de Aprendizaje) en Colombia..

Fundamentals of methanol synthesis on metal carbide based catalysts: activation of CO2 and H2

CO2 hydrogenation to methanol and other alcohols constitutes an appealing route to recycle the large amount accumulated in the atmosphere through fossil-derived fuels burning. However, CO2 high chemical stability makes the overall process difficult and appropriate catalysts are needed. Transition metal carbides, either as active phase or as a support for noble metal clusters, have been shown to be able to activate CO2. Here, the mechanism involved in the decomposition of H2 and CO2 on many early transition metal carbides (TMC) surfaces is analyzed with the help of density functional theory (DFT) based calculations complemented by key experiments. Results show that H2 dissociation on VC and δ-MoC is unlikely, that TiC and ZrC are more reactive leading to an exothermic but activated process and that the C:Mo ratio is determinant factor since H2 dissociation on β-Mo2C(001) surface is even more exothermic. The DFT based calculations also show that CO2 adsorption on TMC results in an activated species with TMC→CO2 charge transfer, C-O bond elongations and OCO bending. Supporting Cu4 and Au4 clusters on TMC(001) surfaces leads to more active catalysts due to the induced charge polarization. For H2 dissociation, TiC appears to be the best support, enhancing H2 both thermodynamics and kinetics. CO2 is strongly adsorbed on supported Cu4 and Au4 clusters, and the adsorption energy strength correlates with the methanol formation rate: Cu4/TiC(001) > Au4/TiC(001) > Cu/ZnO(001) >> Cu(111), thus providing potential alternative catalysts for methanol synthesis, in principle dozens of times better than commercial Cu/ZnO based catalysts

Methane capture at room temperature: adsorption on cubic d-MoC and orthorhombic b-Mo2C molybdenum carbide (001) surfaces

Based on periodic Density Functional Theory (DFT) calculations, carried out using a standard generalized gradient approximation type exchange-correlation functional including or not a van der Waals dispersive forces, the ability of the cubic δ-MoC(001) surface to capture methane at room temperature is suggested. Adsorption on the orthorhombic β-Mo2C(001) surfaces, with two possible terminations, has been also considered and, in each case, several molecular orientations have been tested with one, two, or three hydrogen atoms pointing towards the surface on all high-symmetry adsorption sites. The DFT results indicate that the δ-MoC(001) surface shows a better affinity towards CH4 than β-Mo2C(001). The calculated adsorption energy values on δ-MoC(001) surface are larger, and hence better, than on other methane capturing materials such as metal organic frameworks. Besides, the theoretical desorption temperature values estimated from the Redhead equation indicate that methane would desorbs at 330 K when adsorbed on the δ-MoC(001) surface, whereas this temperature is lower than 150 K when the adsorption involves β-Mo2C(001). Despite of this, adsorbed methane presents a very similar structure compared to the isolated molecule, due to a weak molecular interaction between the adsorbate and the surface. Therefore, the activation of methane molecule is not observed, so these surfaces are, in principle, not recommended as possible methane dry reforming catalysts

Adsorption and dissociation of molecular hydrogen on orthorhombic β- Mo2C and cubic δ-MoC (001) surfaces

Molybdenum carbides are increasingly used in heterogeneously catalyzed hydrogenation reactions, which imply the adsorption and dissociation of molecular hydrogen. Here a systematic density functional theory based study, including or excluding dispersion terms, concerning the interaction and stability of H2 with cubic δ-MoC(001) and orthorhombic β-Mo2C(001) surfaces is presented. In the latter case the two possible C or Mo terminations are considered. In addition, different situations for the H covered surfaces are examined. Computational results including dispersive forces predict as essentially spontaneous dissociation of H2 on β-Mo2C(001) independently of the surface termination, whereas on δ-MoC(001) molecular hydrogen dissociation implies a small but noticeable energy barrier. Furthermore, the ab initio thermodynamics formalism has been used to compare the stability of different H coverages. Finally, core level binding energies and vibrational frequencies are presented with the aim to assist the interpretation of yet unavailable data from X-ray photoelectron and infrared spectroscopies

Highly active Au/d-MoC and Cu/d-MoC catalysts for the conversion of CO2: The metal/C ratio as a key factor defining activity, selectivity and stability

The ever growing increase of CO2 concentration in the atmosphere is one of the main causes of global warming. Thus, CO2 activation and conversion towards valuable added compounds is a major scientific challenge. A new set of Au/δ-MoC and Cu/δ-MoC catalysts exhibits high activity, selectivity, and stability for the reduction of CO2 to CO with some subsequent selective hydrogenation towards methanol. Sophisticated experiments under controlled conditions and calculations based on density functional theory have been used to study the unique behavior of these systems. A detailed comparison of the behavior of Au/β-Mo2C and Au/δ-MoC catalysts provides evidence of the impact of the metal/carbon ratio in the carbide on the performance of the catalysts. The present results show that this ratio governs the chemical behavior of the carbide and the properties of the admetal, up to the point of being able to switch the rate and mechanism of the process for CO2 conversion. A control of the metal/carbon ratio paves the road for an efficient reutilization of this environmental harmful greenhouse gas

The conversion of CO2 to methanol on orthorhombic β-Mo2C and Cu/β-Mo2C catalysts: mechanism for admetal induced change in the selectivity and activity

The conversion of CO2 into methanol catalyzed by β-Mo2C and Cu/β-Mo2C surfaces has been investigated by means of a combined experimental and theoretical study. Experiments have shown the direct activation and dissociation of the CO2 molecule on bare β-Mo2C, whereas on Cu/β-Mo2C, CO2 must be assisted by hydrogen for its conversion. Methane and CO are the main products on the clean surface and methanol production is lower. However, the deposition of Cu clusters avoids methane formation and increases methanol production even above that corresponding to a model of the technical catalyst. DFT calculations on surface models of both possible C- and Mo-terminations, corroborate the experimental observations. Calculations for the clean Mo-terminated surface reveal the existence of two possible routes for methane production (C + 4H → CH4; CH3O + 3H → CH4 + H2O) which are competitive with methanol synthesis, displaying slightly lower energy barriers. On the other hand, a model for Cu deposited clusters on the Mo- terminated surface points towards a new route for methanol and CO production avoiding methane formation. The new route is a direct consequence of the generation of a Mo2C-Cu interface. The present experimental and theoretical results entail the interesting catalytic properties of Mo2C as an active support of metallic nanoparticles, and also illustrate how the deposition of a metal can drastically change the activity and selectivity of a carbide substrate for CO2 hydrogenation

The Bending Machine: CO2 Activation and Hydrogenation on d-MoC(001) and b-Mo2C(001) Surfaces.

The adsorption and activation of a CO2 molecule on cubic d-MoC(001) and orthorhombic b-Mo2C(001) surfaces have been investigated by means of periodic density functional theory based calculations using the Perdew-Burke-Ernzerhof exchange-correlation functional and explicitly accounting for (or neglecting) the dispersive force term description as proposed by Grimme. The DFT results indicate that an orthorhombic b-Mo2C(001) Mo-terminated polar surface provokes the spontaneous cleavage of a C-O bond in CO2 and carbon monoxide formation, whereas on a b-Mo2C(001) C-terminated polar surface or on a d-MoC(001) nonpolar surface the CO2 molecule is activated yet the C-O bond prevails. Experimental tests showed that Mo-terminated b-Mo2C(001) easily adsorbs and decomposes the CO2 molecule. This surface is an active catalyst for the hydrogenation of CO2 to methanol and methane. Although MoC does not dissociate C-O bonds on its own, it binds CO2 better than transition metal surfaces and is an active and selective catalyst for the CO2+3H2-> CH3OH + H2O reaction. Our theoretical and experimental results illustrate the tremendous impact that the carbon/metal ratio has on the chemical and catalytic properties of molybdenum carbides. This ratio must be taken into consideration when designing catalysts for the activation and conversion of CO2.