Sulfidation Step Research Articles

Efficient electrocatalysts for OER: Amorphous cerium-doped cobalt sulfide with enhanced performance and durability

Developing highly efficient electrocatalysts is essential for advancing the oxygen evolution reaction (OER), a key step in water splitting. In this study, a novel approach for the synthesis of an amorphous sulfide structure has been presented. First, a cerium-doped zeolitic imidazolate framework-67 (Ce-ZIF-67) using a co-precipitation method, followed by a multi-step transformation process. This process includes oxidation to form cerium-doped cobalt oxide (Ce-CO) and a subsequent sulfidation step to produce an amorphous cerium-doped cobalt sulfide (Ce-CS) structure. The introduction of cerium and the formation of an amorphous sulfide structure result in a significantly enhanced OER performance due to increased atomic disorder, improved electron mobility, and an expanded active surface area. Remarkably, the Ce-CS structure achieved a reduction in overpotential from 352 mV for Ce-CO to 291 mV at 100 mA cm−2 in 1.0 M KOH, alongside a Tafel slope reduction from 86.2 mA decade−1 to 67.2 mV decade−1. These enhancements underline the importance of cerium doping and amorphization in optimizing electrocatalytic efficiency. Furthermore, the Ce-CS catalyst demonstrated exceptional durability, with no observable degradation in performance or structural integrity after 10 h of continuous operation. This work presents a pioneering strategy for designing and synthesizing highly effective OER electrocatalysts, contributing a significant advancement to the field.

ZIF-67-derived N-enriched porous carbon doped with Co, Fe and CoS for electrocatalytic hydrogen evolution reaction

Hybrid composites based on transition metal-doped materials exhibit excellent performance and stability as electrocatalysts for the hydrogen evolution reaction (HER). Thus, they could easily replace HER catalysts based on noble metals. To demonstrate this, we fabricated Co, Fe, and CoS doped N-enriched porous carbon materials (CoFeS/NC) using a simple, straightforward and quick method (involving absorption, pyrolysis and sulfidation steps), which used ZIF-67 metal-organic framework (MOF) material as a precursor. The fabricated CoFeS/NC showed excellent HER performance and long-term stability: it achieved a low potential (equal to 176 mV) at 10 mA cm−2 current density and a small Tafel slope (equal to 67.8 mV dec−1) in 1.0 KOH. Such outstanding HER performance was attributed to the synergistic effect of the CoFeS/NC components, including unique mesoporosity. All these properties ensured the presence of numerous active sites and high conductivity provided by the carbon matrix. The excellent CoFeS/NC electrocatalytic activity makes it a promising material for H2 production on an industrial scale. Our work demonstrated a simple way of its preparation, which could be applied to other material groups.

Synthesis and characterization of WS2/SiO2 microfibers

Tungsten disulfide polycrystalline microfibers were successfully synthesized by a process involving electrospinning, calcination, and sulfidation steps. We used an aqueous solution of silicotungstic acid (H4SiW12O40) and polyvinyl alcohol as precursors for the synthesis of composite fibers by the needle-less electrospinning technique. The obtained green composite fibers (av. diam. 460 nm) were converted by calcination in air to tungsten oxide WO3 fibers with traces of SiO2 and a smaller diameter (av. diam. 335 nm). The heat treatment of the WO3 fibers under flowing H2/H2S/N2 stream led to conversion to tungsten disulfide WS2 with retention of the fibrous morphology (av. diam. 196 nm). Characterization of the intermediate and final fibers was performed by the XRD, SEM, TEM, HAADF STEM EDS, elemental analyses ICP-OES, and IR spectroscopy methods.

Cobalt sulfides nanoparticles encapsulated in N, S co-doped carbon substrate for highly efficient oxygen reduction

A facile and eco-friendly synthetic method of oxygen reduction reaction (ORR) electrocatalysts is one of crucial steps in the development of large-scale and commercial regenerative energy devices. Herein, a simple and template-free approach is proposed to design CoSx (Co1-xS and Co9S8) nanoparticles encapsulated in N, S-doped carbon substrate (CoSx/NSCS). Interestingly, the carbon substrates are consisted of carbon nanotubes and carbon nanosheets. Moreover, the high-temperature sulfidation step is adopted to form cobalt sulfides and sulfur doping into carbon simultaneously. Benefiting from the N, S doped carbon bedframe and the synergistic effect between cobalt sulfide species and carbon substrate, the optimal CoSx/NSCS-800 (obtained at sulfidation temperature of 800 °C) exhibits robust and excellent ORR electrocatalytic activity in terms of half-wave potential of 0.85 V in alkaline electrolyte. Most importantly, this work, offers great potential to prepare transition metal sulfides nanoparticles integrated with N, S-doped carbon-based electrocatalysts toward ORR.

Evaluation of dry acid gas removal process on bench scale test facility coupled with syngas produced by O2/CO2-blown gasifier

Dry acid gas removal test facility that comprises of halide and sulfur control units was coupled with O2/CO2-blown gasifier to demonstrate performance and operability of the dry gas purification process for the oxy-fuel IGCC power generation with efficient carbon dioxide separation. The facility was operated for initial five desulfurization cycles of sequential operation of sulfidation, regeneration, and reduction. The duration of each step was fixed to three hours which includes the purge operation to change the gas condition from syngas to oxidative gas, and vice versa. The test facility has the sulfur removal unit equipped with three fixed-bed reactors. Honeycomb shaped desulfurization sorbent that was improved to prevent harmful carbon deposition was installed in those reactors. The cyclic operation results revealed that the unit could reasonably remove sulfur compounds in the sulfidation steps and released the absorbed sulfur during succeeding regeneration steps for the initial five cycles of operation. The spent sorbent did not suffered with carbon deposition and retained their performance on sulfur removal. The absorbed sulfur in the spent sorbent was adequately released in regeneration steps for further operation of desulfurization cycles. The performance and operability of the dry acid gas removal process was successfully demonstrated in the actual syngas operation test.

Activation of regenerated CoMo/Al2O3 hydrotreating catalysts by organic additives – The particular case of maleic acid

Activation of hydrodesulfurization (HDS) catalysts through sulfidation of oxide precursors is a critical procedure required to achieve preparation of the Co(Ni)-promoted Mo(W)S2 sulfides commonly used for performing sulfur removal from transportation fuels. Better optimized preparation of HDS catalysts can be reached if organic additives, glycol-type compounds or chelating agents, are used during the activation process. Moreover, comprehension of the role of the organic additives during activation has become even more essential with the increasing necessity to regenerate satisfactorily the last generation of HDS catalysts, more sensitive to the harsh conditions used during the oxidative regeneration treatment. In this respect, maleic acid (MA) was recently found to be an efficient additive for the activation of regenerated HDS catalysts.In the present study, deeper understanding of the role played by maleic acid for restoring the HDS activity was determined at different steps of the preparation of a regenerated CoMo-Al2O3 (here called CoMo-R) catalyst. Comparison was first performed to two other additives, a chelating agent, ethylenediaminetetraacetic acid (EDTA) and a glycol-type compound, triethyleneglycol (TEG), after impregnation and drying steps, using UV–vis diffuse reflectance spectroscopy, ex situ and in situ X-ray diffraction and thermal analysis. Results emphasized that maleic acid is the most efficient additive for extracting cobalt from cobalt molybdate species leading after sulfidation to much better HDS catalytic properties in the HDS of 4,6-dimethyldibenzothiophene (4,6-DMDBT) and of a straight run gasoil.The sulfidation step was also carefully analyzed during the activation of CoMo-R in the presence of MA using inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray photoelectron, Raman and electron paramagnetic resonance spectroscopies, mass spectrometry analysis and transmission electron microscopy. This detailed analysis allows proposing a complete description of the sulfidation mechanism in the presence of MA leading to a better understanding of the key aspects to be reached during the activation process.

MoS2 Domains on TiO2-Based Nanostructures: Role of Titanate/TiO2 Transformation and Sulfur Doping on the Interaction with the Support

MoS2/TiO2-based hybrid structures have been synthesized, via a bottom-up approach, by sulfidation, with CS2 as sulfiding molecule, of molybdenum oxide precursor supported on high surface area hydrogen titanate nanotubes (HTNTs). The evolution of morphology and structure of the support, moving from titanates to TiO2 phases together with the simultaneous formation of MoS2 nanosheets have been imaged by means of AFM, HRTEM microscopies, while the vibrational and the optical properties have been investigated by FTIR and UV–visible techniques after each step of the process. More in detail, the different stacking degrees, the size distribution of the MoS2 nanosheets, decorating the heterogeneous supports, have been carefully obtained by means of HRTEM. In order to explore the nature of the surface sites on the exposed faces, in situ FTIR spectra of adsorbed CO as probe molecule have been carried out. It was shown that the sulfidation steps are affecting not only the structure of MoS2 nanosheets, including their curvature, surface defects, and stacking order, but are involving the support, too, and then in turn the MoS2/support interaction, so helping to control and preserve the size of the particles. Lastly, to elucidate the nature of the hybrid composites, a simple scheme summarizing the reaction pathways has been proposed.

Conversion of guaiacol over different Re active phases supported on CeO2-Al2O3

The effect of Al2O3 modification by CeO2 addition (14wt.% to obtain a maximum monolayer coverage) on the catalytic activity of three supported Re active phases (Re, ReOx and ReSx) for guaiacol conversion was studied. A 10wt.% loading of Re was impregnated into CeO2-Al2O3 support and used to prepare the three rhenium active phases by calcination, reduction or sulfidation steps. The catalysts were characterized by different techniques in order to explain the catalytic behavior of the solids studied. The catalysts were then evaluated in a batch reactor at 300°C and 5MPa of H2. The characterization results showed that the addition of CeO2 modified the acidity of the support as well as the metal-support interaction, and consequently the catalytic activity. In fact, CeO2 addition doubled the activity of Re and ReSx catalysts in comparison to the corresponding γ-Al2O3-supported catalysts. These results were attributed to a higher dispersion of active sites in the case of metallic Re catalyst and higher concentration of non-stoichiometric sulfur species (ReS2-x) in the case of ReSx catalyst. On the other hand, a similar catalytic activity was observed over the two supported ReOx catalysts, indicating that the role of ReOx active phase on guaiacol conversion outweighs structural and surface modification induced by CeO2 addition. In relation to products distribution, it was observed that CeO2 addition enhanced phenol formation and generally led to higher degree of deoxygenation which is most likely related to decrease in acidity.

Influence of the sulfidation temperature in a NiMoW catalyst derived from layered structure (NH4)Ni2OH(H2O)(MoO4)2

NiMoW-O mixed oxide was obtained by calcination of the ϕy layered precursor (NH4)Ni2OH(H2O)(MoO4)2 (NiMoW-L). Unsupported catalysts were obtained from porous NiMoW-O activated ex-situ under a H2S/H2 (15% v/v) mixture at temperatures in the range of 673–823K. Activation temperature was crucial in catalytic activity for dibenzothiophene (DBT) hydrodesulfurization (HDS). The best temperature of activation for the trimetallic oxide was in the upper limit, with DBT conversion of 98%. X-ray diffraction (XRD), confirmed the formation of ϕy, α oxide phase and metal sulfided phases. Hydrogen temperature-programmed reduction (H2-TPR) provided useful information about the oxidation states of metallic species for NiMoW-O, which was related to the sulfidation step and the catalytic activity in HDS reaction. Sulfided catalysts were also characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Additional characterization was carried out using BET surface area and elemental analysis. All catalysts were tested in the HDS of DBT at 593K and 5.5MPa. The results indicated that the activity of the catalysts depended on the activation temperature, where the NiMoWS-823 catalyst presented the highest activity and hydrogenation capacity with values equal to 98% and 5.0, respectively.

Importance of the sulfidation step in the preparation of highly active NiMo/SiO2/Al2O3 hydrodesulfurization catalysts

The influence of the heating rate on the degree of sulfidation and hydrodesulfurization activity of NiMo/SiO2/Al2O3 catalysts prepared with and without citric acid was analyzed. The temperature programmed sulfidation patterns of the catalysts were obtained by following the consumption of H2S at different rates of heating (1, 3 and 10K/min). The resulting dispersion of MoS2 crystallites was observed by transmission electron microscopy, and the reducibility of the different metal species was analyzed by temperature programmed experiments. The results of catalytic activity during the HDS of 4,6-dimethyldibenzothiophene show that the use of a slow rate of heating during sulfidation (1K/min) can increase twice the activity compared to a fast heating rate (10K/min). The use of citric acid during catalyst preparation produces smaller MoS2 crystallites without altering the stacking, and a greater number of promoted sites, detected by FT-IR of adsorbed CO. The temperature programmed reduction of the NiMo catalyst with citric acid showed the formation of a Mo-citrate complex.

Amorphous unsupported Ni–Mo sulfide prepared by one step hydrothermal method for phenol hydrodeoxygenation

Unsupported sulfide catalysts are a potentially promising approach towards furthering the understanding and development of a better heterogeneous catalytic system capable of performing the hydrodeoxygenation (HDO) of bio-oil proficiently under mild and short reaction conditions and times, respectively. Amorphous unsupported Ni–Mo sulfide, prepared from ammonium tetrathiomolybdate (ATTM) by a one step hydrothermal method, is already sulfided and so does not need a sulfidation step. The addition of the Ni promoter prevents the growth of Mo sulfide particles and causes a reduction in the surface area and a change in the pore characteristics as the amount of added Ni was increased. Ni sulfide alone (no Mo) showed a completely different morphology and properties compared to those of the Mo-containing sulfides, with or without the copresence of Ni. The activity and selectivity of catalysts was investigated using phenol as a model substrate in the direct-deoxygenation (DDO) and hydrogenation (HYD) reactions in a HDO system. The Ni–Mo sulfide catalyst with optimal Ni amount had a significantly higher phenol conversion efficiency (96.2mol%), and favored a HYD pathway, than that seen for the Mo sulfide one (71.0mol%) that favored a DDO pathway. H2-temperature programmed desorption (TPD) suggested that this synergy was mainly derived from a change in the quality and not the number of the active sites. The synergetic effect was a function of the stoichiometric composition with the maximum synergetic effect being obtained at a Ni/(Mo+Ni) ratio of 0.3. This could result from the high dispersion of the active species and the generation of a more active Ni–Mo–S phase.

Nanoscale ZnO/CdS heterostructures with engineered interfaces for high photocatalytic activity under solar radiation

Semiconductor based nanoscale heterostructures are promising candidates for photocatalytic and photovoltaic applications with the sensitization of a wide bandgap semiconductor with a narrow bandgap material being the most viable strategy to maximize the utilization of the solar spectrum. Here, we present a simple wet chemical route to obtain nanoscale heterostructures of ZnO/CdS without using any molecular linker. Our method involves the nucleation of a Cd-precursor on ZnO nanorods with a subsequent sulfidation step leading to the formation of the ZnO/CdS nanoscale heterostructures. Excellent control over the loading of CdS and the microstructure is realized by merely changing the initial concentration of the sulfiding agent. We show that the heterostructures with the lowest CdS loading exhibit an exceptionally high activity for the degradation of methylene blue (MB) under solar irradiation conditions; microstructural and surface analysis reveals that the higher activity in this case is related to the dispersion of the CdS nanoparticles on the ZnO nanorod surface and to the higher concentration of surface hydroxyl species. Detailed analysis of the mechanism of formation of the nanoscale heterostructures reveals that it is possible to obtain deterministic control over the nature of the interfaces. Our synthesis method is general and applicable for other heterostructures where the interfaces need to be engineered for optimal properties. In particular, the absence of any molecular linker at the interface makes our method appealing for photovoltaic applications where faster rates of electron transfer at the heterojunctions are highly desirable.

Preparation of high-performance MoP hydrodesulfurization catalysts via a sulfidation–reduction procedure

MoP/MCM-41 was prepared from its oxidic precursors by sulfidation at 400 °C for 3 h using 10% H 2S/H 2, followed by temperature-programed reduction in H 2. The resulting catalyst, MoP/MCM-41(SR), showed considerably a higher catalytic activity in the hydrodesulfurization (HDS) of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) than a catalyst prepared by temperature-programed reduction (MoP/MCM-41(R)). 4,6-DMDBT exhibited much higher activity than DBT both in independent and in simultaneous HDS. Comparison of the HDS performance of MoP/MCM-41(SR) with mixtures of MoP/MCM-41(R) and MoS 2/MCM-41 excluded the synergetic effect of separate MoS 2 and MoP phases in MoP/MCM-41(SR). XRD characterization indicated that smaller molybdenum phosphide crystals were generated in MoP/MCM-41(SR). A shift of the Mo 3d 5/2 binding energy from 228.6 to 229.1 eV was observed in the XPS spectra, which might be related with the incorporation of sulfur in the MoP structure. Moreover, the sulfidation step facilitated the formation of MoP crystals and lowered the threshold reduction temperature.

Hydrodesulfurization catalysts: Promoters, promoting methods and support effect on catalytic activities

CoMoS phases supported on γ-Al2O3, TiO2 and SiO2 were prepared using various synthesis methods and Co(NO3)2·6H2O or Co(acac)2·nH2O as precursors of the promoting agent. The catalysts obtained were tested in the thiophene hydrodesulfurization (HDS) reaction at atmospheric pressure. These experiments showed that the catalysts prepared by the impregnation of supported MoS2 with cobalt precursors exhibit better HDS properties than the catalysts prepared by successive impregnation of Mo and Co salts followed by a sulfidation step. Furthermore, with γ-Al2O3 and SiO2, the use of Co(acac)2·nH2O, without any further calcination after the impregnation, allowed even better catalytic activities. With TiO2, the promoting effect is identical whatever the Co precursor used. Des catalyseurs ≪CoMoS≫ supportés sur γ-Al2O3, TiO2et SiO2 ont été préparés en utilisant différentes méthodes de synthèse et Co(NO3)2·6H2O ou Co(acac)2·nH2O comme précurseur de l'agent promoteur. Les catalyseurs obtenus ont été testés en hydrodésulfuration (HDS) du thiophène sous pression atmosphérique. Ces expériences ont montré que les catalyseurs préparés par imprégnation de MoS2 supporté avec les précurseurs de cobalt conduisaient à une meilleure activité catalytique en HDS que ceux qui étaient préparés par imprégnation successive des sels de Mo et de Co suivie d'une étape de sulfuration. Sur γ-Al2O3et SiO2, l'utilisation du Co(acac)2·nH2O, sans calcination supplémentaire après l'imprégnation, conduit aux meilleures activités catalytiques. Avec TiO2 par contre, l'effet de promotion est le même quel que soit le précurseur de Co utilisé.

Calculation of the UV-visible spectra and the stability of Mo and Re oxysulfides in aqueous solution

Erickson and Helz (2000) established that molybdate, MoO 4 −2, is quickly and completely transformed to thiomolybdate, MoS 4 −2, by reaction with sulfide. They monitored the equilibria and kinetics of this process by measuring the UV-visible spectra of solutions containing the different oxythiomolybdate species. There is interest in the analogous reactions for the ReO 4 − species but little experimental data. We have therefore calculated quantum-chemically the equilibrium constants in solution for the sulfidation reactions of both MoO 4 −2 and ReO 4 −, as well as recalculating their UV-visible charge-transfer spectra. Calculations using configuration interaction singles and time-dependent hybrid Hartree-Fock density functional methods give good absolute values and trends in UV-vis energies for both series. For MoO 4 −2 the calculated equilibrium constants for the various sulfidation steps match reasonably well against the experimental values (within two log K units). For the ReO 4 − sulfidation reaction the first two steps are considerably less favorable than for MoO 4 −2, suggesting that the “geochemical switch” of Erickson and Helz, a rapid transformation of oxyanion to thioanion highly dependent on sulfide concentration, will be less effective in the Re case. However, both our calculations and experiment indicate that ReO 4 − and ReS 4 − are both easier to reduce than their Mo analogs, so that reduction of these Re(VII) species will be the preferred mechanism for their removal from seawater. A previous suggestion that the neutral species MoO 4H 2 is actually the hydrated octahedral Mo(OH) 6 species is found to be incorrect, but the MoO 3(OH 2) 3 species, a highly distorted six-coordinate complex, is almost competitive in energy with MoO 4H 2 plus two H 2O.

Breakthrough Analysis OF H 2 S removal on CU-V-MO, CU-V, and CU-MO mixed oxides

Kinetic studies carried out for the sorption of H 2 S in the presence and absence of hydrogen on Cu-V-Mo, Cu-V, and Cu-Mo mixed oxides gave detailed information about the reaction sequences. Formation of SO 2 even in the presence of hydrogen at the initial stages of the sorption process showed the partial reduction of the metal oxides prior to the sulfidation step. A sorption experiment carried out with Cu-Mo mixed oxide gave incomplete sulfidation of molybdenum, whereas with Cu-V mixed oxide complete sulfidation was obtained. Predictions of the breakthrough curves by the deactivation model showed good agreement with the experimental results.

Quasi in Situ Sequential Sulfidation of CoMo/Al2O3 Studied Using High-Resolution Electron Microscopy

Using a specially developed, quasi in situ vacuum-sample-transfer system, the stepwise sequential sulfidation of CoMo/γ-Al2O3 was studied with transmission electron microscopy without exposing the catalyst to air. In a first step, the sulfidation was performed at 400 °C at either 1 or 10 bar pressure. In a second step, the sulfidation was performed at 600 °C and 1 bar. As the second sulfidation step was performed on the sample on the transmission electron microscopy grid, the same areas of the samples that had been studied for the first sulfidation step could be studied once more, providing detailed data on the changes in the sulfided phase. The slab length and degree of stacking have been studied in detail from the TEM/HREM images. Sulfidation at higher pressure or higher temperature results in a higher average slab size. Detailed study of the same areas of the catalysts before and after the second, more severe, sulfidation step shows that although the dispersion of the sulfided phase decreases, the total amount visible in the TEM images remains unchanged. This indicates that practically all the sulfide present is actually imaged on the micrographs. Additionally, detailed HRTEM study shows increased crystallinity of the slabs after the second sulfidation step. XPS analysis shows that after sulfidation at 400 °C and 1 bar, the degree of sulfidation is 95%, whereas after sulfidation at 600 °C and 1 bar, sulfidation is complete. The catalytic performance for thiophene hydrodesulfurization is much higher after sulfidation at 10 bar and 400 °C as compared to 1 bar and 400 °C, and decreases after the second sulfidation step (1 bar 600 °C). The latter effect is more pronounced for the sample sulfided at 10 bar in the first step, indicating that a distorted yet fully sulfided structure is the most active phase for thiophene HDS.

Synthesis of Active Hydrodesulfurization Carbon-Supported Co−Mo Catalysts. Relationships between Preparation Methods and Activity/Selectivity

Several series of Co−Mo-based carbon catalysts series were synthesized using three methods, successive impregnation, successive impregnation with sulfidation in between, and equilibrium adsorption methods. Co/C and Mo/C were also prepared by impregnation. The activities of sulfided catalysts for hydrodesulfurization of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) as models of the most refractory sulfur-containing compounds found in petroleum oil were studied using an autoclave reactor operating at 2.9 MPa hydrogen pressure. The activity results were found to be highly dependent on the method of preparation. The series that was prepared by impregnation but with the sulfidation step before the cobalt addition exhibited superior activities for hydrodesulfurization (HDS) of DBT and 4,6-DMDBT. The activity results were compared with that of the conventional commercial Co−Mo/Al2O3 catalyst. Co−Mo/C catalysts had about twice the activity of the commercial catalyst. It was found that the activity of HDS depends on the metal loading as well as the order of metal addition during preparation. The highest synergetic effect between Co and Mo for HDS activity was observed at a Co/Mo atomic ratio of 0.325.

97/02723 Recent advances and future prospects for direct coal liquefaction process development

Unsupported sulfide catalysts are a potentially promising approach towards furthering the understanding and development of a better heterogeneous catalytic system capable of performing the hydrodeoxygenation (HDO) of bio-oil proficiently under mild and short reaction conditions and times, respectively. Amorphous unsupported Ni–Mo sulfide, prepared from ammonium tetrathiomolybdate (ATTM) by a one step hydrothermal method, is already sulfided and so does not need a sulfidation step. The addition of the Ni promoter prevents the growth of Mo sulfide particles and causes a reduction in the surface area and a change in the pore characteristics as the amount of added Ni was increased. Ni sulfide alone (no Mo) showed a completely different morphology and properties compared to those of the Mo-containing sulfides, with or without the copresence of Ni. The activity and selectivity of catalysts was investigated using phenol as a model substrate in the direct-deoxygenation (DDO) and hydrogenation (HYD) reactions in a HDO system. The Ni–Mo sulfide catalyst with optimal Ni amount had a significantly higher phenol conversion efficiency (96.2 mol%), and favored a HYD pathway, than that seen for the Mo sulfide one (71.0 mol%) that favored a DDO pathway. H2-temperature programmed desorption (TPD) suggested that this synergy was mainly derived from a change in the quality and not the number of the active sites. The synergetic effect was a function of the stoichiometric composition with the maximum synergetic effect being obtained at a Ni/(Mo + Ni) ratio of 0.3. This could result from the high dispersion of the active species and the generation of a more active Ni–Mo–S phase.

The Effect of Passivation on the Activity and Structure of Sulfided Hydrotreating Catalysts

Air exposure (passivation) and subsequent resulfidation caused a substantial increase in the thiophene hydrodesulfurization activity of sulfided Co-Mo/A12O3 catalysts. Since no effect was observed for Mo/A12O3 and Co/A12O3 catalysts, the passivation effect must be related to the CoMoS structure. EXAFS, Mssbauer emission spectroscopy, and transmission electron microscopy measurements showed that the passivation effect is neither related to a change in the MoS2 dispersion, nor to a change in number and nature of the Co sites. A Co-Mo/TiO2, a Co-Mo/C, and a Co-Mo/A13O3 catalyst, which was prepared with the aid of nitrilotriacetic acid, hardly showed an increase in the catalytic activity after passivation. Because of this, and since the Mo-S coordination number for a Co-Mo/Al2O3 catalyst is increased by passivation, it is concluded that the activity increase originates from a decrease in Mo-support interactions. These interactions are broken when Co atoms migrate to MoOAl bonds during a passivation and resulfidation treatment and assist in the sulfidation of these bonds. Removal of steric hindrance is held responsible for the activity increase. The effect of passivation on the catalytic activity decreased with increasing Co/Mo ratio, probably because at high Co loadings most MoOAl bonds are already sulfided in the first sulfidation step. A maximum activity increase was obtained at a passivation temperature of 340 K. At lower temperatures the oxidation and migration of Co is too slow, whereas at higher temperatures new MoOAl bonds are formed in the passivation treatment. Ni-Mo/Al2O3 catalysts showed the same behavior as Co-Mo/Al2O3 catalysts.