Probing topological electronic effects in catalysis: thiophene adsorption on NiMoS and CoMoS clusters

Abstract

Um método teórico geral em duas etapas para estudar redistribuições eletrônicas em processos catalíticos é apresentado. Na primeira etapa, teoria do funcional da densidade (DFT) é usada para otimizar completamente duas geometrias: o aglomerado que representa o catalisador e o sistema aglomerado mais molécula adsorvida. Na segunda etapa, a densidade eletrônica molecular convergida é dividida em multipolos centrados em sítios atômicos obtidos segundo uma análise de multipolos distribuídos, que fornece informação topológica detalhada da redistribuição de carga do catalisador e da molécula, antes e depois da adsorção. Esta abordagem é aplicada para a adsorção de tiofeno sobre a borda metálica 10 -10 dos catalisadores Ni(Co)MoS e comparada com a mesma reação em MoS 2 não-substituído. Energias de adsorção, geometrias e a análise de multipolos calculada indicam uma fraca adsorção do tiofeno em ambas as situações. Um modelo coulombiano para a ligação química mostra que a magnitude da ligação metal-enxofre na superfície dos catalisadores de Ni(Co)MoS promovidos é consideravelmente menor do que em MoS 2 não substituído, desta forma confirmando a origem do aumento da atividade de hidrodessulfurização (HDS) nestes catalisadores.A general two-step theoretical approach to study electronic redistributions in catalytic processes is presented. In the first step, density functional theory (DFT) is used to fully optimize two geometries: the cluster representing the catalyst and the cluster plus adsorbed molecule system. In the second step, the converged electron density is divided into multipoles centered on atomic sites according to a distributed multipole analysis which provides detailed topological information on the charge redistribution of catalyst and molecule before and after adsorption. This approach is applied to thiophene adsorption on the 10 -10 metal edge of Ni(Co)MoS catalysts and compared to the same reaction on bare MoS 2 . Calculated adsorption energies, geometries and multipole analysis indicate weak thiophene chemisorption on both cases. A Coulombic bond model showed that surface metal-sulfur bond strengths in Ni(Co)MoS promoted catalysts are considerably smaller than in bare MoS 2 , thus confirming the origin of the enhancement of hydrodesulfurization (HDS) activity in these catalysts.

On the molecular origin of the sensitivity to impact of cyclic nitramines

“…According to the proposed Coulombic bonding model, the nature of the nitro group bonded to distinct atoms in the strained nitramines can be rationalized. For instance, TNAZ is the only molecule of the strained ring set that has nitro groups attached to both nitrogen and carbon ring atoms.…”

“…To examine the nature of bond strengths in the molecules, a Coulombic bonding model previously used was employed in the following way. It was considered that (a) bond strengths are dominated by Coulombic interactions between DMA charges and thus are ion‐ion like and (b) only nearest‐neighbor atoms contribute to the bond strength.…”

“…[24] For this purpose, electric multipole moments derived from the atomcentered Distributed Multipole Analysis (DMA) [49,50] partition scheme of the molecular charge densities were used. DMA was successfully used before to rationalize very distinct chemical phenomena such as molecular adsorption in catalysis, including microwave effects, [51][52][53] and energetic materials, including sensitivity properties. [38,39] 2 | THEORETICAL AND COMPUTATIONAL METHODS…”

Nature of cobalt species during the in situ sulfurization of Co(Ni)Mo/Al2O3 hydrodesulfurization catalysts

“…The details of the fits including all numbers are given in Tables S2 and S3 of the supporting information. The increase in pre-edge intensity is assigned to a reduction in symmetry of Co 3+ O h and Co 2+ O h in the cobalt oxide species to a Co 2+ ion in a square-pyramidal cobalt sulfide species for the fully sulfided catalyst sample (Borges et al, 2012;Orita et al, 2004;Paul et al, 2008;Ma & Schobert, 2000). In this case, the crystal field splits the 3d orbitals partly belonging to the same irreducible representation of the point group as metal 4p orbitals, leading to partial mixing and hybridization (Griffith, 1964;Figgis, 1966).…”

Estudio computacional del efecto promocional del cobalto en agregados aislados de Mo y Co como modelo de catalizadores de hidrodesulfuración

“…Borges y Silva utilizaron DFT y un modelo de Coulomb para estimar las fuerzas de enlace con la intención de dilucidar el efecto de Co y Ni en los clústers aislados de MoS frente a la adsorción de tiofeno. Descubrieron que el efecto de los promotores se debe, fundamentalmente, a el debilitamiento de los enlaces metal-azufre superficiales [9]. Costa y colaboradores utilizaron una combinación de XPS, TEM y pruebas catalíticas con cálculos DFT con PBC para estudiar la hidrogenación de tolueno en catalizadores de (Co)MoS soportados en γ-alumina [10].…”