Search

Environmental regulations concerning fuel‐related sulfur emissions are currently experiencing worldwide expansion. This is motivating the research and development of improved hydrotreating (HDT) catalysts with carefully engineered synthesis methods that maximize activity as ultimate goal. In this contribution, the effect of phosphorus doping and triethylene glycol (TEG) incorporation on the genesis of the CoMoS phase of model HDT catalysts is assessed. Following a surface‐science approach, α‐Al2O3 single crystals with four orientations: C(0001), A(112‾ 0), M(101‾ 0), and R(11‾ 02) are used as model supports of the traditional γ‐Al2O3 support for the study of active phase‐support interactions at the molecular scale. The model catalysts are prepared by traditional impregnation of a solution containing metal and phosphorus precursors and TEG. The catalysts are characterized in the sulfide phase by XPS, XAS, TEM, and AFM, revealing that the calcination step is key in the emergence of distinctive metal‐support interactions, which are directly related to the nature of alumina surface sorption sites. In addition, TEG incorporation on dried catalysts increases metal sulfidation and enhances promotion with respect to non‐additivated systems, especially at mild sulfidation temperatures. The catalytic activity of the model catalysts is tested in a thiophene hydrodesulfurization reaction, revealing the following activity trend with respect to alumina surface orientations: A112‾0 ≫ M101‾0 >R11‾02 , C(0001). This trend is identical to the one observed for non‐phosphorus doped non‐TEG additivated model systems, confirming the predominance of surface effects on the catalytic activity over those exerted by P and TEG. By transposing these results to the industrial γ‐Al2O3 support, the (100) facet would provide surface sites that lead to better performing catalysts. This could result in the development of novel supports, engineered to expose surface sites that maximize hydrotreating activity. [ABSTRACT FROM AUTHOR]

[Display omitted] • Co promotion do not alter metal-support interactions compared to non-promoted systems. • Co promotion is not surface-dependent but contingent to the formation of MoS 2. • Highest catalytic activity for metal-support interactions of intermediate strength. The current environmental and industrial contexts require constant revision and improvement of hydrotreating processes for obtaining cleaner (bio)fuels through the development of supported transition metal sulfide (TMS) catalysts. The present work proposes a surface-science approach to describe the surface-dependent genesis of the cobalt-promoted active phase (CoMoS) supported on α-Al 2 O 3 single crystals used as surrogates of the industrial γ-Al 2 O 3 support. Co-Mo precursor solutions are deposited by aqueous-phase impregnation onto four α-Al 2 O 3 single crystals of C(0001), A (11 2 ¯ 0) , M (10 1 ¯ 0) , and R (1 1 ¯ 02) orientations, which display specific OH sorption sites. The model catalysts are characterized in the oxide and sulfide states by XPS, AFM, and TEM revealing a surface dependent sulfidation and dispersion as a result of metal-support interactions of diverse nature. The surfaces can be ranked following metal-support interactions of increasing strength C(0001) < A (11 2 ¯ 0) , M (10 1 ¯ 0) < R (1 1 ¯ 02) in good agreement with model non-promoted catalysts. However, AFM and TEM imaging also reveal that cobalt incorporation leads to an increase in the stacking of CoMoS particles with respect to non-promoted catalysts. The activity of the model catalysts in thiophene hydrodesulfurization cannot be simply related to the common features of the active phase (sulfidation, dispersion, promotion extent) but a volcano-shape curve is obtained when plotting the activity as a function of metal-support interactions. These results suggest that high activity is obtained on surfaces with intermediate metal-support interactions that lead to an optimal metal-sulfur bond energy in agreement with the Sabatier principle. Transposing these results to the traditional γ-Al 2 O 3 support suggests an enhanced activity of the (1 0 0) facet, which can lead to the development of improved supports for hydrotreating catalysts. [ABSTRACT FROM AUTHOR]

In order to meet the demand for cleaner fuels, it is necessary to develop high performing hydrotreating catalysts. Although organic additives are often used to improve the performances of hydrotreating catalysts, bio-polymer additives have not yet been thoroughly investigated. We recently showed that dopamine, a neurotransmitter molecule, which is also frequently found in adhesive bio-polymers, efficiently coats the surface of an alumina carrier material and modifies the interaction with active components of hydrotreating catalysts, i.e . cobalt and molybdenum. The catalytic performance of the CoMoS/Al 2 O 3 catalysts could, thus, be improved. In the present paper we extend the strategy to other carrier materials (silica and titania). The catalysts prepared on (poly)dopamine (Pdop) coated silica showed enhanced activity for the hydrogenation of toluene reaction and the selective HDS of FCC gasoline model feed. Pdop coating lowers the MoS 2 nanocrystallites' stacking on silica, thereby improving the accessibility of active edge sites. The origin of this increase in catalytic activity may be found in the higher capacity of Pdop's functional groups to interact with metallic precursors leading to a better dispersion of the active phase. On the contrary, when the interaction between the dopamine and the support is too strong (as observed for TiO 2 ), the functional groups of Pdop are not available for interaction with Co and Mo species, resulting in decreased catalytic activity. [ABSTRACT FROM AUTHOR]

The constant improvement of hydrotreating (HDT) catalysts, driven by industrial and environmental needs, requires a better understanding of the interactions between the oxide support (mostly alumina) and the MoS 2 active phase. Hence, this work addresses the support-dependent genesis of MoS 2 on four planar single-crystal α-Al 2 O 3 surfaces with different crystal orientations (C (0 0 0 1), R ( 1 1 ¯ 0 2 ) , M ( 1 0 1 ¯ 0 ) , and A ( 1 1 2 ¯ 0 ) ). In contrast to classical surface science techniques, which often rely on UHV-type deposition methods, Mo is introduced by impregnation from an aqueous solution in order to mimic standard incipient wetness impregnation. Comparison between different preparation routes, impregnation vs. equilibrium adsorption (selective adsorption), is also considered. Atomic force microscopy, X-ray absorption spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy show that the α-Al 2 O 3 orientation has a clear impact on the strength of metal–support interactions in the oxide state with consequences for the sulfidation, size, stacking, and orientation of MoS 2 slabs. Aggregation of molybdenum oxide particles is observed on the C (0 0 0 1) plane, suggesting weak metal–support interactions leading to a high sulfidation degree with large slabs. Conversely, the presence of well-dispersed individual oxide particles on the R ( 1 1 ¯ 0 2 ) plane implies stronger metal–support interactions, leading to a low sulfidation degree and shorter MoS 2 slabs. Both A ( 1 1 2 ¯ 0 ) and M ( 1 0 1 ¯ 0 ) facets, of similar crystallographic structure, display intermediate behavior in terms of sulfidation rate and MoS 2 size, in line with intermediate metal–support interactions. Polarization-dependent grazing-incidence-EXAFS experiments, as well as high-angle annular dark-field (HAADF) scanning transmission electron microscopy analysis, allow us to demonstrate a surface-dependent orientation of MoS 2 slabs. Predominant basal bonding is suggested on the C (0 0 0 1) plane, in agreement with the existence of weak metal–support interactions. Conversely, a random orientation (edge and basal bonding) is observed for the other planes. Generalization of these conclusions to industrial catalysts is proposed, based on comparison of the surface structure of the various model α-Al 2 O 3 orientations used in this work and the predominantly exposed γ-Al 2 O 3 surfaces ((1 1 0), (1 0 0), and (1 1 1)). [ABSTRACT FROM AUTHOR]

New developments and breakthroughs in the design of hydrotreating (HDT) catalysts for complying with fuel specifications and reduced sulfur contents ask for a continuous improvement in the molecular scale description of the active phase both at the oxide and sulfide states. HDT catalysts are mainly based on alumina-supported molybdenum nanophases and the present contribution is dedicated to review the surface science approaches that have helped gain a molecular-scale insight into the structure of the active-phase (mainly molybdenum) and the role of metal-support (mainly alumina) interactions through the use of well-defined surfaces and surface sensitive analysis techniques. The main focus of this Review is placed on model systems that can bridge the gap with industrial catalysts, that is, oxide-supported active phases prepared in aqueous conditions, as opposed to more traditional surface-science studies that consider metal-supported nanophases prepared in UHV conditions. The synthesis and structure of well-defined alumina surfaces (thin films and single crystal wafers) is first presented before discussing aqueous-deposition of Mo precursors on flat surfaces. The detailed role of the oxide support is then addressed with an emphasis on the genesis and structure of the MoS2 phase namely sulfidation rate, stacking, and orientation. [ABSTRACT FROM AUTHOR]