Model oxide supported MoS2 HDS catalysts: structure and surface properties

Abstract

Thisis an author version of the contribution published on:Questa è la versione dell'autore dell'opera: By Federico Cesano, Serena Bertarione, Andrea Piovano, Giovanni Agostini, Mohammed Mastabur Rahman, Elena Groppo, Francesca Bonino, Domenica Scarano, Carlo Lamberti, Silvia Bordiga, Luciano Montanari, Lucia Bonoldi, Roberto Millini and XRPD, HRTEM, Raman and UV-Vis characterization methods have been applied to obtain information on the morphology and the structure of the catalysts (including degree of staking and structural disorder) as well as on the vibrational and spectroscopic properties. It is shown that, when compared with HRTEM results, XRPD, Raman and UV-Vis data give a realistic information on the staking degree, on the particle size distribution and on the heterogeneity of supported MoS 2 particles on the various supports. EXAFS and XANES spectroscopies have been also used to set up the best sulfidation procedure on the MoS 2 /SiO 2 system, and the obtained recipe has been adopted for the MoS 2 /γ-Al 2 O 3 and MoS 2 /MgO systems. UV-vis analysis under controlled atmosphere has been performed to understand the effect of reductive and sulfiding treatments on the presence of sulfur vacancies and on the valence state of Mo ions associated with them. To explore the structure of coordinatively unsaturated Mo sites after reducing or sulfiding treatments (with CS 2 or, occasionally, with H 2 S), in situ FTIR of CO adsorbed at low-temperature has been performed on all samples. It is demonstrated that CO is a sensitive probe for coordinatively unsaturated sites and that the formation of sulfur vacancies on the MoS 2 surface upon reduction in pure H 2 at 673 K is accompanied by an increase of the coordinative unsaturation and a decrease of the valence state of a fraction of surface Mo cations, mainly located on corner and edge sites. Considering the non planarity of a consistent fraction of lammelae (as revealed in particular by HRTEM of MoS 2 on Al 2 O 3 and MgO), the presence of reducible Mo ions, located in defective positions on basal planes, cannot be excluded. Furthermore, it is demonstrated that this process can be reversed upon interaction with the sulfiding agent and that this reversible behavior is really mimicking some of the elementary acts occurring in the HDS process. Comparing the results obtained by all the adopted characterization techniques, it is concluded that the reductive effect of H 2 preferentially affects the particles characterized by the lowest staking degree. The complexity of the IR results suggests that the adopted reduction procedure in pure H 2 at 673 K induces the formation of several type of sulfur vacancies, presumably located in different crystallographic positions of the MoS 2 particles. In conclusion, the IR results obtained with the CO probe fully support the idea that the adopted reduction procedure in pure H 2 at 673 K is not only inducing the formation of several families of sulfur vacancies but that a continuity exists between the stages associated with the reducti...

MoS2 supported on P25 titania: A model system for the activation of a HDS catalyst

“…The pale yellow color of the MoO x /TiO 2 sample can be explained by considering the effect of the presence of a MoO x phase due to the impregnated species. No other signals, ascribable to any d-d transitions associated with the Mo centers, suggest that Mo species have a d 0 electronic configuration (i.e., Mo lies in the 6+ oxidation state), while in presence of Mo 5+ and/or Mo 4+ sites different d-d transitions falling in the visible region are expected [22,63,64].…”

“…Only after the sulfidation at 673 K, the signals of MoS 2 are clearly visible in the Raman spectrum, which shows a doublet of sharp peaks with maxima at 407 cm À1 and 383 cm À1 , ascribable to A 1g and E 1 2g vibrational modes, respectively [18,22,61,62]. The A 1g mode defines a vibration along the stacking of MoS 2 , while E 1 2g is a mode characteristic of the lateral extension of the MoS 2 sheets.…”

“…This exfoliation can be performed in several ways (exfoliation [17,18], sonication in a solvent till to obtain a suspension [19] or intercalation [20]). Conversely, following a bottom-up approach, MoS 2 can be grown in situ in a sulfiding atmosphere from a Mo oxide precursor (typically prepared via impregnation of a Mo salt) [21,22] or by direct decomposition of a Mo S containing precursor [23]. As sulfiding reagent, typically H 2 S in H 2 is used, but also CS 2 [22] and organosulfides [24] have been successfully applied to convert the precursor into MoS 2 .…”

“…Conversely, following a bottom-up approach, MoS 2 can be grown in situ in a sulfiding atmosphere from a Mo oxide precursor (typically prepared via impregnation of a Mo salt) [21,22] or by direct decomposition of a Mo S containing precursor [23]. As sulfiding reagent, typically H 2 S in H 2 is used, but also CS 2 [22] and organosulfides [24] have been successfully applied to convert the precursor into MoS 2 . The nature of the support, in terms of chemical composition and surface properties, plays a fundamental role during the growth of the sulfide phase [22,[25][26][27].…”

“…As sulfiding reagent, typically H 2 S in H 2 is used, but also CS 2 [22] and organosulfides [24] have been successfully applied to convert the precursor into MoS 2 . The nature of the support, in terms of chemical composition and surface properties, plays a fundamental role during the growth of the sulfide phase [22,[25][26][27]. As the process is proposed to occur at sulfur coordination vacancies placed on the edges of MoS 2 particles [8,28], usually a small particle size combined with a good dispersion is expected to lead to the exposure of a large number of active sites.…”