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Computational modeling of transition metals carbides with relevant to nanotechnology and catalysis

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Universitat de Barcelona. Facultat de Química

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[eng] The work carried out in this thesis is based on the study of transition metal carbides (TMCs) as catalysts study from a theoretical point of view, using the techniques of Computational Chemistry. TMC materials have been proposed as candidates to replace the catalysts currently used in industry which are based on noble metals, which are much more expensive than TMCs and have a low abundance in the earth. The physical and chemical properties of these materials make them promising candidates for replacing the industrial catalysts. Recent studies have reported TMCs as catalysing hydrogen dissociation reactions, hydrogenation reactions, adsorption and dissociation of carbon dioxide, carbon monoxide and methane, among others. Nowadays, these materials could be of great relevance to capture CO2 from the environment, and further, to be able to transform it into other more useful chemical compounds, such as methanol. TMCs have also been shown to be good materials for capturing CO2 and CH4, the two main gases that cause global warming.
In this Thesis, the dissociation of hydrogen on seven different TMCs has been studied: TMCs containing metals of group IV, V, and a polymorphic form of molybdenum carbide (δ-MoC). The results have made it possible to determine that only the TMCs of group IV are likely candidates to carry out this dissociation reaction. We have mainly focused our studies on titanium carbide (TiC), where the computational studies have been accompanied by experimental studies. The studies in this thesis have shown how the TiC surface is covered when the hydrogen atoms are adsorbed. This study has allowed us to better understand and explain the experimental results. In addition, the dissociation of methane on TiC has also been studied. Our calculations have been made using clean surfaces and surfaces doped with Ni clusters. This studies have also been accompanied by experiments, where the computational studies have permitted us to propose a mechanism to understand how CH4 dissociation occurs on Ni clusters surfaces at room temperature.
On the other hand, in this Doctoral thesis several TiC nanoparticles have also been studied. In this part of the work, the stability of different nanoparticle sizes has been investigated. Previously, in the literature some IR spectra were available for some specific nanoparticle sizes. One of our goals in this Thesis was to find the structure of the nanoparticles that had been characterized with these spectra. Therefore, different studies were carried out where either four C atoms or one Ti atom were removed from the stoichiometric nanoparticle – to match the composition and mass of the nanoparticles in experiment. These studies focused on size (TiC)18, where from the cubic structure, on the one hand 4C have been removed giving rise to the composition Ti18C14 and on the other hand, a Ti atom has been removed, obtaining the composition Ti17C18. For both compositions, a set of isomers have been studied where the most energetically stable isomer has been sought and extrapolated to larger sizes. Then, the IR spectra for these compounds was simulated. In an attempt to obtain the spectrum that matches with the experimental IR, the spectra were simulated at different temperature. The use of temperature to carry out the simulation of IR spectra, has made it possible match our simulated spectra with the experimental spectra and thus to identify the structure of the experimental nanoparticles. Finally, molecular hydrogen adsorption and dissociation on stoichiometric TiC nanoparticles of different sizes was also studied. For these studies, the results show that TiC nanoparticles are a good candidate to perform the hydrogen dissociation reaction.

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[cat] El treball realitzat en aquesta Tesi es basa en l'estudi, des del punt de vista teòric i emprant les tècniques de la Química Computacional, del ús dels carburs de metalls de transició (TMCs) com a catalitzadors. Aquests materials han estat proposats com a candidats a substituir els catalitzadors que actualment s'utilitzen a la indústria que solen ser metalls nobles, molt més cars que els TMC i amb una abundància escassa a la terra.
Els estudis recents duts a terme han proposat els TMCs com a bons candidats per dur a terme reaccions de dissociació d'hidrogen, reaccions d'hidrogenació, adsorció i dissociació de diòxid de carboni, monòxid de carboni, i metà, entre d'altres. Aquestes reaccions són possibles degut a les propietats físiques i químiques que presenten aquests materials. Durant aquest treball, s'ha estudiat la dissociació d'hidrogen en un conjunt de TMC diferents. Aquests TMC corresponen als metalls del grups IV,V, i una forma polimòrfica del carbur de molibdè (MoC). Els resultats han permès determinar que els TMC són bons candidats per dur aquesta reacció de dissociació. En el carbur de Titani (TiC) els estudis computacionals s'han acompanyat amb estudis experimentals. Els nostres estudis computacionals han servit per entendre i explicar els resultats obtinguts als experiments. A més, en el TiC, també s'ha estudiat la dissociació de metà que també ha estat acompanyada d'experiments.
D'altra banda, en aquesta Tesi Doctoral també s'han estudiat diverses nanopartícules de TiC. En aquest treball s'han investigat l'estabilitat de diferents mides de nanopartícules. Per algunes mides concretes de nanopartícules, es disposava d'espectres IR a la literatura.  El nostre objectiu va ser trobar l'estructura de les nanopartícules que havien sigut caracteritzades amb aquests IR. Per això, s'ha dut a terme diferents estudis on s'eliminaven o be quatre àtoms de C o be un àtom de Ti de la nanopartícula estequiomètrica. En una primera etapa, s'ha buscat l'isòmer més estable energèticament d'aquests compostos, i en una segona etapa,  s'han caracteritzat mitjançant l'espectroscòpia IR. La caracterització d'aquestes nanopartícules ha permès identificar l'estructura de les nanopartícules d'on s'havien aconseguit els espectres IR experimentals.
Finalment, també s'ha estudiat l'adsorció d'hidrogen nanopartícules de TiC estequiomètriques de diferents mides.

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Computational modeling of transition metals carbides with relevant to nanotechnology and catalysis