Heterogeneous catalysis of green chemistry reactions on molybdenum carbide based catalysts

Abstract

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.

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.