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The ability to gain access to the speciation of hydrodesulfurization (HDS) catalysts used for removing sulfur and nitrogen from transportation fuels is becoming a key feature in the understanding and improvement of their catalytic activity with the aim of reducing greenhouse gas emissions of fossil fuels. A novel coupling methodology between Raman spectroscopy and X‐ray absorption spectroscopy (XAS) was developed, which allows for precise qualitative and quantitative analysis of the oxide phase of Mo‐based catalysts. This enabled the detailed study of various impregnation parameters and the characterization of HDS catalysts before sulfidation. To illustrate this methodology, the effects of two additives (triethylene glycol and citric acid) were studied. The results demonstrated that organic additives exert a profound influence on the speciation of oxide precursors of molybdenum‐based catalysts. The presence of these additives enhances the dispersion of Keggin or Dawson heteropolyanions (HPAs) on the support by maintaining a high phosphorus‐to‐molybdenum ratio in solution. This is achieved by two strategies: triethylene glycol (TEG) exhibits a pronounced affinity for alumina hydroxyls, while citric acid compensates for phosphate adsorption through the formation of a stable Mo‐citrate complex. [ABSTRACT FROM AUTHOR]

This paper aims to describe and validate two cells designed to monitor, by in situ quick‐EXAFS, the transformation of Mo based oxidic species occurring during maturation and drying preparation steps of a single hydrodesulfurization catalyst. These transformations occur firstly upon impregnation/maturation, due to the contact of the acidic impregnation solution with the basic alumina support, and furthermore upon drying. The Mo K edge in situ quick‐EXAFS monitoring of these two elementary preparation steps is hereby reported for the first time, providing quantitative results of the Mo‐speciation. Ex situ Raman spectroscopy is used effectively as a complement. It considers an impregnation solution with a Mo concentration of 0.75 M and molar ratios of P/Mo 0.56 and Co/Mo 0.4. A two‐step drying mechanism is proposed to explain the evolution of the Mo‐speciation during in situ drying where Strandberg's, Anderson's and Dawson's HPAs are transformed into lacunary Keggin's HPA, octa‐ and monomolybdates. [ABSTRACT FROM AUTHOR]

To study the influence of ageing and drying steps onto the multiscale Mo active phase arrangement (i. e. MoS2 slabs and slab aggregates), a series of highly loaded CoMoP/γ‐Al2O3 catalysts aged, unaged, dried or freeze‐dried were prepared and tested in toluene hydrogenation. Electron Probe Micro Analyses (EPMA) and Anomalous Small Angle X‐Ray Scattering (ASAXS) permit to highlight the influence of each preparation steps. A growth of Mo entities occurs during ageing along with metal diffusion inside the support and leads to longer slabs. The same occurs during drying with metal redistribution due to solvent evaporation. Concomitantly, a fragmentation of slab aggregates takes place and leads to numerous small slabs aggregates. Slab length and aggregates amount were found to drive the catalytic activity, which translates the relationship between activity, number of active sites and their accessibility. Conversely, macroscopic metal distribution into the extrudates appears to have no impact on the catalytic performance. [ABSTRACT FROM AUTHOR]

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]

Requirements for improved catalytic formulations is continuously driving research in hydrotreating (HDT) catalysis for biomass upgrading and heteroatom removal for cleaner fuels. The present work proposes a surface‐science approach for the understanding of the genesis of the active (sulfide) phase in model P‐doped MoS2 hydrotreating catalysts supported on α‐Al2O3 single crystals. This approach allows one to obtain a surface‐dependent insight by varying the crystal orientations of the support. Model phosphorus‐doped catalysts are prepared via spin‐coating of Mo–P precursor solutions onto four α‐Al2O3 crystal orientations, C(0001), A(112‾ 0), M(101‾ 0) and R(11‾ 02) that exhibit different speciations of surface ‐OH. 31P and 95Mo liquid‐state NMR are used to give a comprehensive description of the Mo and P speciation of the phospho‐molybdic precursor solution. The speciation of the deposition solution is then correlated with the genesis of the active MoS2 phase. XPS quantification of the surface P/Mo ratio reveal a surface‐dependent phosphate aggregation driven by the amount of free phosphates in solution. Phosphates aggregation decreases in the following order C(0001)≫M(101‾ 0)>A(112‾ 0), R(11‾ 02). This evolution can be rationalized by an increasing strength of phosphate/surface interactions on the different α‐Al2O3 surface orientations from the C(0001) to the R(11‾ 02) plane. Retardation of the sulfidation with temperature is observed for model catalysts with the highest phosphate dispersion on the surface (A(112‾ 0), R(11‾ 02)), suggesting that phosphorus strongly intervene in the genesis of the active phase through a close intimacy between phosphates and molybdates. The surface P/Mo ratio appears as a key descriptor to quantify this retarding effect. It is proposed that retardation of sulfidation is driven by two effects: i) a chemical inhibition through formation of hardly reducible mixed molybdo‐phosphate structures and ii) a physical inhibition with phosphate clusters inhibiting the growth of MoS2. The surface‐dependent phosphorus doping on model α‐Al2O3 supports can be used as a guide for the rational design of more efficient HDT catalysts on industrial γ‐Al2O3 carrier. [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]

The reactivity of a series of amines with various structures and different numbers of hydrogen atoms on the carbon atoms in the a and ß position was used to evidence the C-N bond scission mechanism in the presence of H2S on Pt based catalysts (deposited on alumina, zirconia and silica-alumina) and compare it with the mechanism on a NiMoP on alumina sulfide catalyst. The effect of the H2S partial pressure was also checked. Catalytic activities (amine overall conversion, C5 hydrocarbon formation, and amine disproportionation) deeply depend on the structure of the N-containing molecule.Tert-pentylamine is the most reactive molecule for sulfide catalysts whereas, in the case ofn-pentylamine, Pt on zirconia was found to be the most efficient for C-N bond breaking. Such properties cannot be related to the acidic properties of the support but to a unique support-metal interaction, since alumina or silica-alumina supported platinum catalysts do not present this behaviour. [ABSTRACT FROM AUTHOR]

This study describes the phenomena involved during the regeneration of an aged industrial hydrodesulfurization catalyst (CoMoP/Al2O3) using a non-thermal plasma at a low temperature (200 °C). The changes occurring during regeneration were studied by characterizing spent, partially, and fully regenerated catalysts by XRD, Raman, TEM spectroscopy, and the coke deposited on the catalyst surface by Laser desorption/ionization time-of-flight mass spectrometry (LDI TOF/MS). The coke is a mixture of several polycyclic molecules, the heaviest with a coronene backbone, containing up to seven sulfur atoms. This kinetic study shows that the oxidation rate depends on the nature of the coke. Hence, explaining the formation of VOCs from heavy polycyclic carbon molecules without complete oxidation to CO2. However, XRD and Raman spectroscopies evidence CoMoO4 formation after a long treatment time, indicating hot spots during the regeneration. [ABSTRACT FROM AUTHOR]

The removal of coke from an aged industrial hydrodesulfurization catalyst, using dielectric barrier discharge (DBD) non-thermal plasma with a pin to plate geometry, was investigated. The aged catalyst was introduced into the plasma reactor as a thin wafer. After 130 minutes of plasma treatment, with P = 30 W, 70% of the coke was removed while more than 40% of the sulfur was still present. Characterization of catalyst at different locations of the wafer showed that the coke was more easily removed at the center, close to the pin electrode where the electric field was more intense. The formation of an unexpected phase, under the plasma discharge, was highlighted, it corresponded to the family of Keggin HPA PMo12O403−, which could be an interesting precursor of catalyst for the hydrodesulfurization (HDS) process. Compared with a coked zeolite, the rate of regeneration is lower for the HDS catalyst under plasma discharge, while a lower temperature is required under conventional thermal oxidation. This is explained by the presence of metal particles, which could be responsible for the limitation in O-atom formation under plasma. [ABSTRACT FROM AUTHOR]

Review: 158 refs. [ABSTRACT FROM AUTHOR]

Requirements for improved catalytic formulations is continuously driving research in hydrotreating (HDT) catalysis for biomass upgrading and heteroatom removal for cleaner fuels. The present work proposes a surface-science approach for the understanding of the genesis of the active (sulfide) phase in model P-doped MoS 2 hydrotreating catalysts supported on α-Al 2 O 3 single crystals. This approach allows one to obtain a surface-dependent insight by varying the crystal orientations of the support. Model phosphorus-doped catalysts are prepared via spin-coating of Mo-P precursor solutions onto four α-Al 2 O 3 crystal orientations, C(0001), A(11 2 ‾ 0), M(10 1 ‾ 0) and R(1 1 ‾ 02) that exhibit different speciations of surface -OH. 31 P and 95 Mo liquid-state NMR are used to give a comprehensive description of the Mo and P speciation of the phospho-molybdic precursor solution. The speciation of the deposition solution is then correlated with the genesis of the active MoS 2 phase. XPS quantification of the surface P/Mo ratio reveal a surface-dependent phosphate aggregation driven by the amount of free phosphates in solution. Phosphates aggregation decreases in the following order C(0001)≫M(10 1 ‾ 0)>A(11 2 ‾ 0), R(1 1 ‾ 02). This evolution can be rationalized by an increasing strength of phosphate/surface interactions on the different α-Al 2 O 3 surface orientations from the C(0001) to the R(1 1 ‾ 02) plane. Retardation of the sulfidation with temperature is observed for model catalysts with the highest phosphate dispersion on the surface (A(11 2 ‾ 0), R(1 1 ‾ 02)), suggesting that phosphorus strongly intervene in the genesis of the active phase through a close intimacy between phosphates and molybdates. The surface P/Mo ratio appears as a key descriptor to quantify this retarding effect. It is proposed that retardation of sulfidation is driven by two effects: i) a chemical inhibition through formation of hardly reducible mixed molybdo-phosphate structures and ii) a physical inhibition with phosphate clusters inhibiting the growth of MoS 2 . The surface-dependent phosphorus doping on model α-Al 2 O 3 supports can be used as a guide for the rational design of more efficient HDT catalysts on industrial γ-Al 2 O 3 carrier., (© 2020 Wiley-VCH GmbH.)