Investigation of the sulfidation of Mo/TiO2-Al2O3 catalysts by TPS and LRS

A series of Mo/Al₂O₃ and Mo/TiO₂-Al₂O₃ catalysts were investigated by temperature programmed sulfiding (TPS) and laser Raman spectroscopy (LRS). The effect of TiO₂ on the sulfidability of molybdena was studied in detail. It is found that Mo/Al₂O₃ catalysts can be partially sulfided by O-S exchange at low temperature, forming molybdenum oxysulfide. The Mo-S bond subsequently ruptures in the presence of H₂ to produce H₂S. At 530–550 K deep sulfiding of molybdenum oxysulfide occurs forming crystalline MoS₂. When the surface of Al₂O₃ was covered by a monolayer of TiO₂, the sulfiding rate of molybdena at low temperature was not only greatly increased, but H₂S produced in the reduction of Mo-S species caused deep sulfiding of the catalyst which resulted in a decrease of the TPS peak temperature by 80–100 K. The results indicate that this promotion of the sulfiding of molybdena is enhanced with TiO₂ loading. The function of TiO₂ is explained by the weakened interaction between MoO₃ and Al₂O₃ due to the coverage of the Al₂O₃ surface by TiO₂.     

Morphology and Atomic-Scale Structure of MoS2 Nanoclusters Synthesized with Different Sulfiding Agents

We use scanning tunneling microscopy to investigate the morphology and atomic-scale edge structure of MoS₂ nanoclusters synthesized on a gold single crystal as a model system for hydrodesulfurization catalysts using three different sulfur sources for sulfiding. Crystalline and triangularly shaped MoS₂ nanoclusters are predominantly produced from sulfiding with H₂S, dimethyl disulfide (DMDS) and dimethyl sulfide (DMS), but the detailed dispersion, stacking and distribution of active edge sites is sensitive to the selection of the sulfiding agent. The main effect of varying the sulfiding agent seems to be related to the variation in the reactivity of the sulfur, i.e. changes in the sulfur chemical potential. H₂S and DMDS both yield fully sulfided edge structures, but lower sulfur content is obtained on the edges of MoS₂ sulfided with DMS reflected by Mo-edges terminated by sulfur monomers. The present model studies demonstrate that MoS₂ morphology and hereby also the dispersion, type and amounts of active edge sites can be manipulated by control of sulfiding conditions and choice of sulfiding agent. The findings are in line with previous reports on activity variations on technical catalysts which have undergone different sulfidation procedures.     

Selective oxidation of H<Subscript>2</Subscript>S to sulfur over CeO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> catalyst

The catalytic oxidation of hydrogen sulfide (H₂S) to elemental sulfur was studied over CeO₂-TiO₂ catalysts. The synthesized catalysts were characterized by various techniques such as X-ray diffraction, BET, X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption of ammonia, and scanning electron microscopy (SEM). Catalytic performance studies of the CeO₂-TiO₂ catalysts showed that H₂S was successfully converted to elemental sulfur without considerable emission of sulfur dioxide. CeO₂-TiO₂ catalysts with Ce/Ti=1/5 and 1/3 exhibited the highest H₂S conversion, possibly due to the uniform dispersion of metal oxides, high surface area, and high amount of acid sites.     

Characterization of pure and sulfided NiMoO4 catalysts using synchrotron-based X-ray absorption spectroscopy (XAS) and temperature-programmed reduction (TPR)

This study aims at characterizing the properties of pure and sulfided NiMoO₄ catalysts using synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) and temperature-programmed reduction (TPR). Mo L_II-edge and M_III-edge NEXAFS spectra indicate that on reaction with H₂S, the Mo component of NiMoO₄ gets partially reduced with the formation of MoS₂ type species. For the β-phase of NiMoO₄, the sulfidation of Mo is more extensive than for the α-phase, making the former a better precursor for catalysts of hydrodesulfurization (HDS) reactions. The Ni L_II-edge features are relatively insensitive to the changes accompanying the partial sulfidation of NiMoO₄. The sulfidation of the Ni component is confirmed by analysis of the Ni K-edge extended X-ray absorption fine structure (EXAFS) spectra which show the formation of Ni–S bonds (bond length ～2.48 Å) and a NiMoS_x phase. The S K-edge NEXAFS spectra show the presence of at least two types of sulfur species, one associated with a formal oxidation state of 2- and another associated with a formal oxidation state of 6+. We attribute the former to the presence of metal–sulfur bonds (MoS_x and NiS_y). The latter is associated with the formation of S–O bonds (SO ₄ ^2- ). The formation of sulfates is also supported by the O K-edge NEXAFS spectra. The partially sulfided NiMoO₄ catalysts (both α- and β-isomorphs) have a much lower thermal stability in a reducing environment than pure NiMoO₄ and MoS₂. The sulfided molybdates react with H₂ in TPR producing H₂O and H₂S at temperatures above 400 K.     

EXAFS study on the dispersion of molybdenum sulfide catalysts on γ-Al2O3

The dispersion of molybdenum sulfide catalysts was characterized based on the lateral dimensions of MoS₂ crystallites estimated by EXAFS. A new index ofN(Mo)/N(S), instead ofN(Mo), was used to estimate the average MoS₂ size to minimize the contribution of the coexisting oxide or oxisulfide phase in the catalysts. EXAFS showed some advantages over other techniques, such as TEM or XPS.     

An EXAFS study on oxidic and sulfided K-MoO3/γ-Al2O3 catalysts

By analyzing the extended X-ray absorption fine structure (EXAFS) of the Mo K-absorption edge, structural information for both oxidic and sulfided K-MoO₃/-Al₂O₃ catalysts with different potassium content was obtained. The oxidic samples show two backscatterer peaks in the radial distribution function (RDF), which correspond to the Mo-O coordinations in the nearest Mo-O shell. The nearest oxygen atoms are present with large configurational disorder. The RDF for the K/Mo = 0 sample is significantly different from that for crystalline MoO₃ and ammonium heptamolybdate. The RDFs for potassium promoted samples are, in some extent, similar to that for ammonium heptamolybdate. The sample with K/Mo = 0.8 and that with K/Mo=1.5 do not show obvious difference in their local Mo-O structures. The EXAFS results support the earlier conclusions from Raman spectroscopy studies on identical samples [7]. When the samples are sulfided, a rearrangement of the local neighbors around Mo atoms takes place, to form small MoS₂-like crystallites. The Mo-S and Mo-Mo coordination distances on these catalysts are the same as those in crystalline MoS₂, but the coordination numbers are significantly lower than in MoS₂. The EXAFS results indicate that Mo species on the K/Mo=0 catalyst mainly consist of Mo-S-Mo units (the basic building units of MoS₂), which are highly dispersed and show a higher level of disorder than in MoS₂. With the modification by the potassium promoter, Mo species are significantly aggregated and their local neighbors are more similar to those in MoS₂, but the Mo species still exist in a state of high dispersion.     

Effect of sulfidation process on catalytic performance over unsupported Ni-Mo-W hydrotreating catalysts

Oxidic unsupported Ni-Mo-W catalysts were chosen to elucidate the effect of sulfidation conditions on the catalytic performance. The catalysts were sulfided in situ by using dimethyl sulfide as sulfiding agent. The relationships between the time needed for sulfidation and the sulfiding conditions were studied by GC-analysis method. Straight-run gas oil with high sulfur and nitrogen content was used to evaluate the hydrotreating performance. The oxidic catalyst precursors and sulfided catalysts were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectroscopy (XPS). With increasing sulfiding liquid hourly space velocity (LHSV) of the sulfiding agent, the reaction time necessary for the sulfidation decreased, while the activity of the catalyst increased significantly. The higher catalytic activity might be due to the MoS₂/WS₂ slabs with shorter length and higher stacking number, which might contribute to the catalyst with more active sites. Sulfiding at 330 °C took the longest sulfidation time, while the catalytic activity was also the highest after sulfiding at this temperature. Furthermore, within a certain range, the sulfidation pressure had no evident effect on the catalytic behavior or activity. The purpose of this work was to provide a basis for actual production and a reference for further research.     

Structure of Pt–Co/Al2O3 and Pt–Co/NaY Bimetallic Catalysts: Characterization by In Situ EXAFS,TPR, XPS and by Activity in Co (Carbon Monoxide) Hydrogenation

Temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and in situ extended X-ray absorption fine structure (EXAFS) studies were performed to investigate Pt-Co/NaY and Pt-Co/Al₂O₃ bimetallic catalysts. The EXAFS experiments were carried out at the Pt L_III and Co K edges of the same sample. This particular approach allows a precise determination of the electronic and structural characteristics of the metallic part of the catalyst. For both systems in situ reduction under pure H₂ results in the formation of nanometer-scale metallic clusters. For both Co and Pt, nearest neighbors are Co atoms. The complete set of parameters implies the presence of two families of nanometer-scale metallic clusters: monometallic Co nanosized particles and Pt-Co bimetallic clusters, in which only Pt-Co bonds exist (no Pt-Pt bonds). TPR and XPS results indicating a reduction of Co²⁺ ions in Pt-Co/NaY to a greater extent than in Pt-Co/Al₂O₃ give evidence of a facilitated reduction. XRD also shows the presence of nanometer-scale particles with only a very small fraction of larger bimetallic particles. In subsequent mild oxidation of the reduced systems the Co nanoparticles are still present inside the supercage of NaY zeolite in bimetallic form and the oxidation of the metallic particles is slowed down. Catalytic behavior is in good agreement with the structure of the Pt-Co bimetallic system.     

Thiophene hydrodesulfurization over modified alumina-supported molybdenum sulfide catalysts

Methanethiol has been synthesized by one‐step catalytic reaction from H₂S‐content syngas on K₂MoS₄/SiO₂ catalyst with selectivity over 95% under the optimum reaction conditions of 563 K, 2.0–3.0 MPa and 5–6% H₂S content in the feed syngas. The results of XRD and XPS showed that Mo–S–K phase on the surface of the catalyst K₂MoS₄/SiO₂ was responsible for the high activity and selectivity to methanethiol, and which may be restrained by the existence of (S–S)^2- species. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis of metal-oxide pillared montmorillonite clay for the selective catalytic oxidation of H2S

In the present work, titania-pillared clay (Ti-PILC) and zirconia-pillared clay (Zr-PILC) were synthesized, and their catalytic performance was studied for the selective oxidation of H₂S to elemental sulfur. The obtained pillared clays (PILCs) were characterized by X-ray fluorescence spectroscopy (XRF), infrared (FTIR) spectroscopy, BET surface area measurements, X-ray photoelectron spectroscopy (XPS), UV–vis-diffuse reflectance spectroscopy (UV–vis-DRS), temperature-programmed desorption of ammonia (NH₃-TPD), and thermogravimetric (TG) analysis. The reaction tests were carried out in a continuous flow fixed-bed reactor at temperature ranging from 220 to 300 °C. The conversion of H₂S increased with increasing temperature for all PILCs while the selectivity to SO₂ remained almost constant. The good catalytic performance of the metal-oxide pillared clays may be due to the high surface area and the presence of Brönsted and Lewis acid sites.     

Ga-Promoted Tungstated Zirconia Catalyst for n-Butane Isomerization

Ga-promoted tungstated zirconia (GWZ) was prepared by a slurry impregnation method. The textural properties as well as the acidities of the Ga-promoted catalysts were characterized by X-ray powder diffraction (XRD), N₂ adsorption, NH₃ temperature-programmed desorption (NH₃ TPD), microcalorimetry and H₂ temperature-programmed reduction (H₂ TPR). The catalytic behavior of GWZ for n-butane isomerization was studied in the presence of hydrogen. In comparison to tungstated zirconia (WZ), the catalytic activity of the Ga-promoted catalyst was greatly improved. The reason proposed for the higher activity of the Ga-promoted catalysts was that Ga enhances the oxidizing ability of the catalysts.     

Catalytic Wet Oxidation of H<Subscript>2</Subscript>S to Sulfur on V/MgO Catalyst

The V/MgO catalysts with different V₂O₅ loadings were prepared by impregnating MgO with aqueous vanadyl sulfate solution. All of the catalysts were characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). It was observed that the H₂S removal capacity with respect to vanadia content increased up to 6 wt%, and then decreased with further increase in vanadia loading. The prepared catalysts had BET surface areas of 11.3 ~ 95.9 m²/g and surface coverages of V₂O₅ of 0.1 ~ 2.97. The surface coverage calculation of V₂O₅ suggested that a vanadia addition up to a monomolecular layer on MgO support increased the H₂S removal capacity of V/MgO, but the further increase of VO _x surface coverage rather decreased that. Raman spectroscopy showed that the small domains of Mg₃(VO₄)₂ could be present on V/MgO with less than 6 wt% vanadia loading. The crystallites of bulk Mg₃(VO₄)₂ and Mg₂(V₂O₇) became evident on V/MgO catalysts with vanadia loading above 15 wt%, which were confirmed by a XRD. The TPR experiments showed that V/MgO catalysts with the loading below 6 wt% V₂O₅ were more reducible than those above 15 wt% V₂O₅. It indicated that tetrahedrally coordinated V⁵⁺ in well-dispersed Mg₃(VO₄)₂ domains could be the active species in the H₂S wet oxidation. The XPS studies indicated that the H₂S oxidation with V/MgO could proceed from the redox mechanism (V⁵⁺ V⁴⁺) and that V³⁺ formation, deep reduction, was responsible for the deactivation of V/MgO.     

MoS2 morphology and promoter segregation in commercial Type 2 Ni–Mo/Al2O3 and Co–Mo/Al2O3 hydroprocessing catalysts

A series of commercial liquid-phase-sulfided Type 2 Ni–Mo/Al₂O₃ and Co–Mo/Al₂O₃ catalysts used in different trickle-phase reactions has been characterized by (scanning) transmission electron microscopy combined with energy-dispersive X-ray analysis ((S)TEM-EDX) and X-ray diffraction (XRD) analysis. In contrast to what is reported for H₂S/H₂ sulfided Type 2 model catalysts, MoS₂ stacking is not prominent in liquid-phase sulfided Type 2 commercial catalysts. The presence or absence of MoS₂ stacking has been shown to be highly dependent on the sulfidation procedure applied. Although Type 2 commercial Ni–Mo catalysts have high MoS₂ dispersion, there is a significant Ni sulfide segregation. This is not the case for their Co-containing counterparts.     

Hydrothermal synthesis of amorphous MoS2 nanofiber bundles via acidification of ammonium heptamolybdate tetrahydrate

MoS₂ nanofiber bundles have been prepared by hydrothermal method using ammonium molybdate with sulfur source in acidic medium and maintained at 180 °C for several hours. The obtained black crystalline products are characterized by powder X-ray diffraction (PXRD), Fourier transform infrared spectrometer (FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The PXRD pattern of the sample can be readily indexed as hexagonal 2H-MoS₂. FTIR spectrum of the MoS₂ shows the band at 480 cm⁻¹ corresponds to the γ_as(Mo-S). SEM/TEM images of the samples exhibit that the MoS₂ nanofiber exist in bundles of 120–300 nm in diameter and 20–25 μm in length. The effects of temperature, duration and other experimental parameters on the morphology of the products are investigated.     

Direct conversion of ethane to ethylene oxide over NiAgYO catalyst

NiAgYO catalyst prepared using sol–gel method exhibited better catalytic behavior than NiAgO for the direct conversion of ethane to ethylene oxide (EO). An optimal EO yield of 7.6% with 19.8% selectivity was obtained at 290 °C. The catalysts were characterized by X-ray diffraction (XRD), H₂ temperature-programmed reduction (H₂-TPR), O₂ temperature-programmed desorption (O₂-TPD), and X-ray photoelectron spectroscopy (XPS). The results showed that the interaction between Y and Ag made the absorbed oxygen species hold proper electrophilic character, thereby improving the performance of the NiAgYO catalyst.     

Characterization by temperature-programmed reduction of non-conventional catalysts for hydrotreatment

A temperature-programmed reduction (TPR) study of three series of biphasic hydrotreating (HDT) catalysts - PtS/γ-Al₂O₃/(PtS/ γ-Al₂O₃+MoS₂); Rh₂S₃/γ-AlOOH/(Rh₂S₃/γ-AlOOH+WS₂) and PdS/γ-Al₂O₃/(PdS/ γ-Al₂O₃+WS₂) - is presented. In all cases, the quantity of H₂S produced is larger than that which would correspond to the arithmetic addition of the quantities produced by each of the sulfide phases alone. This fact suggests that the noble metal sulfide dissociates the hydrogen molecule to spill-over hydrogen (H_SO), the latter producing a deeper reduction of molybdenum sulfide or tungsten sulfide. The similitude of catalytic effect in hydrogenation (HYD) and hydrodesulfurization (HDS) observed with the same mechanical mixture strongly suggests that H_SO also plays a role in the synergy between MoS₂ or WS₂ and the noble metal sulfides. This revised version was published online in July 2006 with corrections to the Cover Date.     

Pt–Ni/γ-Al2O3 catalyst for the preferential CO oxidation in the hydrogen stream

The preferential CO oxidation (PROX) in the presence of excess hydrogen was studied over Pt–Ni/γ-Al₂O₃. CO chemisorption, X-ray diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy and temperature-programmed reduction were conducted to characterize active catalysts. The co-impregnated Pt–Ni/γ-Al₂O₃ was superior to Pt/Ni/γ-Al₂O₃ and Ni/Pt/γ-Al₂O₃ prepared by a sequential impregnation of each component on alumina support. The PROX activity was affected by the reductive pretreatment condition. The pre-reduction was essential for the low-temperature PROX activity. As the reduction temperature increased above 423 K, the CO₂ selectivity decreased and the atomic percent of Ni in the bimetallic phase of Pt–Ni increased. This catalyst exhibited the high CO conversion even in the presence of 2% H₂O and 20% CO₂ over a wide reaction temperature. The bimetallic phase of Pt–Ni seems to give rise to high catalytic activity for the PROX in H₂-rich stream.     

Formation and reactions of CH3 species over Mo2C/Mo(111) surface

The reaction pathways of adsorbed CH₃ on the Mo₂C/Mo(111) surface were investigated by means of temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS). CH₃ fragments were produced by the dissociation of the corresponding iodo-compound. CH₃I adsorbs molecularly on Mo₂C at 90 K and dissociates at and above 140 K. The main products of the reaction of adsorbed CH₃ are hydrogen, methane and ethylene. The coupling into ethane was not observed. The results are discussed in relevance to the conversion of methane into benzene on Mo₂C deposited on ZSM-5. This revised version was published online in August 2006 with corrections to the Cover Date.     

Partial oxidation of methane and the effect of sulfur on catalytic activity and selectivity

Partial oxidation of methane into syngas was conducted over fresh and sulfided catalysts at a temperature range of 450–750 °C. The temperature dependence of conversion, H₂/CO ratio, and the CO₂ concentration were measured for both fresh and sulfided catalysts. Regardless of metal type, metal loading, support type, and the methods of preparation it appears that all the fresh catalysts were very active and conversions of higher than 70% with H₂/CO ratio of about 2 were observed at 750 °C. Pulse sulfidation appears to be reversible for some of the catalysts but not for all. Under pulse sulfidation conditions, the Rh(0.5%)/Al₂O₃ and NiMg₂O_x-1100 °C (solid solution) catalysts were fully regenerated after reduction with hydrogen. Rh catalyst showed the best overall activity, less carbon deposition, both fresh and when it was exposed to pulses of H₂S. Sulfidation under steady-state conditions, flowing H₂S/Ar mixture over the catalysts, significantly reduce catalyst activity. The catalysts were characterized before and after reaction with H₂S using temperature-programmed oxidation (TPO) and reduction (TPR), X-ray diffraction, and XPS.     

Chemistry of hydrogen sulphide under coal liquefaction conditions: 1. Modelling hydrogen transfer catalysis

The work reported here represents initial attempts to develop a complete kinetic and mechanistic understanding of the reaction chemistry of H₂S under coal liquefaction conditions, using both model systems and coal. Hydrogen sulphide was found to promote/catalyse the transfer of hydrogen from tetralin to 2-hydroxyquinoline (2-HOQ). The presence of H₂S can increase the rate of hydrogen transfer from tetralin to 2-HOQ by a factor of 10 compared with the same reaction run in the absence of H₂S. The energy of activation for hydrogen transfer was found to decrease by ≈5 kcal mol⁻¹ in the presence of H₂S. The presence of H₂S was also found to promote loss of oxygen from 2-HOQ to form small amounts of quinoline. No evidence of CC or CN bond cleavage in 2-HOQ was noted under any of the reaction conditions studied. These results suggest that the presence of H₂S reduces the temperatures necessary to promote effective hydrogen transfer from tetralin by 50–75 °C. Moreover, they imply that similar effects occur in H₂S-promoted coal liquefaction.