Temperature-programmed Sulfidation Research Articles

Sulfur Tolerance Properties of Rhodium Phosphide Evaluated by a New Temperature-programmed Sulfidation Technique

Sulfur Tolerance Properties of Rhodium Phosphide Evaluated by a New Temperature-programmed Sulfidation Technique

Effects of Citric Acid Addition Method on the Activity of NiMo/γ-Al2O3 Catalysts in Simultaneous Hydrodesulfurization and Hydrodenitrogenation Reactions

Two NiMo catalysts were prepared by different methods using citric acid to assess the effects of preparation method on dibenzothiophene hydrodesulfurization (HDS) and simultaneous HDS and hydrodenitrogenation: co-impregnation and post-treatment. Characterization of oxide catalysts was performed by nitrogen physisorption, X-ray diffraction, and temperature-programmed sulfidation, while sulfided samples were characterized by temperature-programmed reduction (TPR-S) and NO chemisorption. The catalytic evaluation for HDS reaction was performed in a three-phase reactor, with and without quinoline. The direct desulfurization is the preferred route for the HDS of dibenzothiophene. The post-treatment method was more efficient in achieving higher catalytic activity than co-impregnation. Furthermore, TPR-S and NO chemisorption results show an enhancement in the density of active sites on the catalyst prepared by post-treatment, being more hydrogenating than the one synthesized by co-impregnation. Although the catalyst prepared by post-treatment was more inhibited by quinoline, it remained more active than the one synthesized by co-impregnation.

Effects of Doping Rare Earth Elements (Y, La, and Ce) on Catalytic Performances of CoMo/MgAlM for Water Gas Shift Reaction

Rare earth element (La, Y, and Ce) modified MgAl-hydrotalcites of MgAlM were synthesized from coprecipitation and calcination, and further loaded with CoMo active species to give CoMo/MgAlM catalysts. X-ray powder diffraction, inductively coupled plasma, and N2 adsorption isotherms indicate that MgAlM possess large BET surface areas (58–91 m2/g), and rare earth elements were successfully introduced into samples. CO2-TPD (temperature-programmed desorption), NH3-TPD, H2-TPR (temperature-programmed reduction), H2S-TPS (temperature-programmed sulfidation), and Raman spectra indicate the presence of unique interactions between rare earth elements and Mo active species, which strongly affect the reduction and sulfidation behaviors of these catalysts. High resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) analysis suggest that the addition of rare earth elements decreases the slab length and stacking numbers of MoS2 and promotes the sulfidation degree of Mo oxides. Th...

Effect of pH on Activity of NiMo/Al2O3 Catalysts Prepared with Citric Acid in Simultaneous Hydrodesulfurization and Hydrodenitrogenation Reactions

NiMo/alumina catalysts were prepared by impregnation of metal precursors in the presence of citric acid (CA) at different pHs. Catalysts were characterized by nitrogen physisorption, X-ray diffraction, temperature programmed reduction of sulfided catalysts (TPR-S), temperature programmed sulfidation (TPS) and NO chemisorption of sulfided catalysts. Catalytic activity for dibenzothiophene (DBT) hydrodesulfurization (HDS) was studied in a trickle-bed reactor, with and without quinoline in the reaction feed. Results suggest that HDS of DBT occurs mainly by direct desulfurization route and CA addition at pH 3 was detrimental, resulting in a less active catalyst, which was confirmed by TOF calculations. Furthermore, TPR-S results show that the catalyst prepared at pH 3 presented less coordinatively unsaturated sites (CUS), being less active than the one synthesized at pH 1. Besides that, the catalyst prepared at pH 1 was less inhibited at higher quinoline concentrations.

Self-complexation: the nature for high hydrodesulfurization activity of NiWO4 nanoparticles

NiWO4 nanoparticles and two kinds of the precursors were synthesized by a hydrothermal approach and tested for the hydrodesulfurization of sulfur containing organics. Results show that the hydrodesulfurization activity of sulfided NiWO4 nanoparticles is highest among them. The investigation about the nature for different hydrodesulfurization activities of the catalysts reveals that a simultaneous sulfidation i.e. self-complexation of nickel and tungsten can be observed on NiWO4 nanoparticles. Furthermore, both temperature-programmed sulfidation and TEM images demonstrated that the self-complexation is originated from the special structure and size of NiWO4 nanoparticles. These results show a promising potential to develop efficient hydrotreatment catalyst.

Effect of stepwise sulfidation on a MoO3/CeO2–Al2O3 catalyst for sulfur-resistant methanation

The effect of stepwise sulfidation on a MoO3/CeO2–Al2O3 catalyst for sulfur-resistant methanation was studied. The prepared catalysts were characterized using N2-physisorption (BET), X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), and temperature-programmed sulfidation (TPS). The methanation activities and stability of the one-step-sulfided MoO3/CeO2–Al2O3 catalysts were enhanced by stepwise sulfidation processes. The XRD, TEM, RS, and XPS results indicated that the enhancement in the stability of MoO3/CeO2–Al2O3 was caused by the reduction and sulfidation of the CeO2 particles, eliminating unstable CeO2 and driving the migration of Ce3+ and Al3+ ions. The catalyst subjected to stepwise sulfidation exhibited improved methanation activity because the stepwise sulfidation reduced the degree of CeO2 sulfidation and mitigated textural structure damage to the catalyst. Finally, a sulfidation mechanism for CeO2 and MoO3 was proposed.

Effect of sulfidation temperature on CoO–MoO3/γ-Al2O3 catalyst for sulfur-resistant methanation

The effect of sulfidation temperature on the catalytic activity of CoO–MoO3/γ-Al2O3 catalyst toward sulfur-resistant methanation was studied. The prepared catalysts were characterized by N2-physisorption (BET), Temperature-programmed sulfidation (TPS), X-ray diffraction (XRD), Raman spectroscopy (RS), and Transmission electron microscope (TEM). The combined results of TPS, XRD, and RS indicated that CoMoO4 species in CoO–MoO3/γ-Al2O3 catalyst would be sulfided when the sulfidation temperature was at or higher than 300 °C. The catalyst sulfided at 400 °C exhibited the highest catalytic activity because there were the most CoMoS structures in the catalyst. When the sulfidation temperature was higher than 400 °C, the catalytic activity decreased with increasing sulfidation temperature. This decline was primarily caused by the formation of Co9S8 and MoS2 crystals. The CoMoS(II) formation did not favor the catalytic activity, because the formation of this species resulted in the occurrence of Co9S8 and crystalline MoS2 in catalysts.

Effect of calcination temperature on the physicochemical and catalytic properties of FeSO4/SiO2 in hydrogen sulfide oxidation

The effect of calcination temperature on the state of the active component of iron-containing catalysts prepared by the impregnation of silica gel with a solution of FeSO4 and on their catalytic properties in selective H2S oxidation to sulfur was studied. With the use of thermal analysis, XPS, and Mossbauer spectroscopy, it was found that an X-ray amorphous iron-containing compound of complex composition was formed on the catalyst surface after thermal treatment in the temperature range of 400–500°C. This compound contained Fe3+ cations in three nonequivalent positions characteristic of various oxy and hydroxy sulfates and oxide and sulfate groups as anions. Calcination at 600°C led to the almost complete removal of sulfate groups; as a result, the formation of an oxide structure came into play, and it was completed by the production of finely dispersed iron oxide in the ɛ-Fe2O3 modification (the average particle size of 3.2 nm) after treatment at 900°C. As the calcination temperature was increased from 500 to 700°C, an increase in the catalyst activity in hydrogen sulfide selective oxidation was observed because of a change in the state of the active component. A comparative study of the samples by temperature-programmed sulfidation made it possible to establish that an increase in the calcination temperature leads to an increase in the stability of the iron-containing catalysts to the action of a reaction atmosphere.

Influence of Potassium on the Synthesis of Methanethiol from Carbonyl Sulfide on Sulfided Mo/Al2O3 Catalyst

AbstractPotassium‐doped MoS2 catalysts supported on Al2O3 were synthesized, characterized by using atomic absorption spectroscopy, N2 physisorption, NO adsorption, X‐ray diffraction, temperature‐programmed sulfidation, and Raman spectroscopy, and tested in the synthesis of methanethiol from carbonyl sulfide (COS) and H2. The results revealed that two phases, pure MoS2 and potassium‐decorated MoS2 (formed at high potassium loadings), were present in the active catalysts. The main effect of potassium during sulfidation and during the catalytic reaction was to increase the mobility of surface oxygen or sulfur atoms. Thus, potassium promoted the disproportionation of COS to CO2 and CS2 and the production of CO from CO2. Additionally, potassium cations hindered the reductive decomposition of COS to CO and H2S and the hydrogenolysis of methanethiol to methane. Mars–van Krevelen‐type mechanisms were proposed to explain the disproportionation of COS on alumina and on the MoS2 phases. The catalytic site in the potassium‐decorated MoS2 phase was proposed to include a potassium cation as adsorption site.

Relationship Between Sulfidation and HDS Catalytic Activity of Activated Carbon Supported Mo, Fe–Mo, Co–Mo and Ni–Mo Carbides

Temperature-programmed sulfiding (TPS) and temperature-programmed reduction of the sulfided catalysts (TPR-S) have been employed to evaluate Mo, Fe–Mo, Co–Mo and Ni–Mo carbide catalysts supported on activated carbon (AC). The carbides were prepared by carbothermal hydrogen reduction from co-impregnated precursors, and submitted to presulfiding prior to HDS. Samples of passivated catalysts prepared from the sulfates of Fe and Co contained variable amounts of sulfur, as shown by XPS and elemental analysis, while all samples showed increased sulfur contents after HDS. The TPS traces (up to 400 °C) of the Mo and Co–Mo samples exhibited two H2S consumption peaks, the first one starting at 100 °C and the second within the 400 °C isothermal region, located immediately after a H2S production peak centered at 400 °C for Mo and at 340 °C for Co–Mo. The Fe–Mo and Ni–Mo carbides exhibited a broad H2S consumption signal between 100 and 400 °C. The low temperature signals can be attributed to reduction of Mo(VI) oxide into Mo(V) or Mo(IV) sulfides, while the broad, higher temperature band is assigned to further reduction to the fully sulfided surface species. The TPR-S spectra showed three H2S evolution peaks, where the area of the first peak (assigned to reduction of sulfur species adsorbed on coordinatively unsaturated edge/corner sites) follows the order Co–Mo ≫ Ni–Mo ≈ Mo > Fe–Mo, which fully agrees with the order of HDS activities at steady state. Elemental analysis of the pre-sulfided AC support suggest that a large part of the sulfur consumed in sulfiding the catalysts is retained as elemental sulfur within the microporous structure of the support, although HRTEM observations confirm that MoS2-like structures are present in these samples. These results reinforce the importance of sulfided surface phases in HDS on carbides of transition metal catalysts. Transmission electron microscopy shows that activated carbon supported (M-)Mo carbide catalysts (M = Fe, Co, Ni) present MoS2-like fringes on top of bimetallic carbide crystals. Temperature-programmed techniques and elemental analysis prove that a large amount of sulfur is retained in the catalyst, likely within the slit-shaped microporous structure of the support. A good correlation was established between thiophene HDS activity at steady state and the size of a TPR peak of sulfided catalyst samples.

Influence of tungsten sources on the synthesis and properties of ammonium dioxothiotungstate

AbstractAmmonium dioxothiotungstate was synthesized using different tungsten sources and characterized in detail by powder X-ray diffraction, energy dispersive X-ray spectrometry, transmission electron microscopy, nitrogen adsorption, and temperature-programmed sulfidation. It was found that tungsten oxide nanobelts are superior to ammonium metatungstate as tungsten source for the synthesis of ammonium dioxothiotungstate due to a time-consuming aging step being excluded from the synthesis route. Moreover, detailed characteristic data reveal that, when tungsten oxide nanobelts are used, the physical and chemical properties of the resulting ammonium dioxothiotungstate including particles size, specific surface area, and sulfidation pattern were improved. Also, the hydrodesulfurization measurements showed higher catalytic activity and balanced selectivity of the resulting ammonium dioxothiotungstate.

Preparation of Highly Active Alumina-Pillared Clay Montmorillonite-Supported Platinum Catalyst for Hydrodesulfurization

Effect of Pt precursor and pretreatment on hydrodesulfurization (HDS) activity of Pt/Al-PILM catalyst was examined to prepare highly active Pt-supported HDS catalyst. The order of HDS activities of Pt/alumina-pillared clay montmorillonite (Al-PILM) catalysts prepared by various Pt precursors was Pt(C5H7O2)2 > H2PtCl6 · 6H2O > [Pt(NH3)4](NO3)2 > [Pt(NH3)4]Cl2 · H2O > H2Pt(OH)6. This order was in accordance with that of Pt dispersion. Thus, high Pt dispersion is essential factor to prepare highly active Pt/Al-PILM catalyst for HDS reaction. On the other hand, the effect of pretreatment on the HDS activities of Pt/Al-PILM catalysts prepared by various Pt precursors was also evaluated. The UC-TPS Pt/Al-PILM catalyst showed the highest HDS activity among various pretreated Pt/Al-PILM catalysts, in which uncalcined catalyst was sulfided by temperature programmed sulfidation (TPS). We assumed that high HDS activity of UC-TPS Pt/Al-PILM catalyst is caused by partly sulfided Pt particle with high dispersion. It is concluded that the highly active Pt/Al-PILM catalyst for the HDS reaction could be prepared by using Pt(C5H7O2)2 as a precursor and UC-TPS treatment.

Feasibility study on high-temperature sorption of hydrogen sulfide by natural soils

In this study, seven natural soils were tested for the sorption of hydrogen sulfide from coal gasification gas at high temperature. Results indicate that the LP natural soil has the best performance and the highest sulfur sorption capacity. After extracting free iron oxides, most natural soils have no sorption efficiency. The free iron oxides, therefore, proved to be the major components that react with hydrogen sulfide to form iron sulfides. The sulfur sorption capacity, either determined by EA or breakthrough time, is very close to the theoretical value based on the stoichiometric calculation with the content of free iron oxides. Moreover, the presence of CO is a positive effect while H 2 is a negative effect. This can be explained via the water-shift reaction. On the basis of the results of temperature-programmed sulfidation (TPS), the starting temperature for the sorption of hydrogen sulfide is between 623–673 K. From the analyses of temperature-programmed oxidation (TPO) and XPS, the iron polysulfides are the major products and approximately 90% regeneration efficiency can be theoretically achieved while the temperature is controlled higher than 813 K. In the regeneration tests, the results show that the LP natural soil can be regenerated and thus reused after the oxidation process. No significant degeneration occurs on the LP natural soil after five sorption/regeneration cycles. The sulfur sorption capacity of the tenth regenerated soil can be achieved at least 80% compared to the fresh one. The experimental analyzed SO 2 concentration from the regeneration process is almost identical to the theoretical calculated equilibrium concentration of the process. Maghemite is the main product after the regeneration process.

Resistance to sulfidation and catalytic performance of titanium–tin solid solutions in SO 2 + CO and NO + SO 2 + CO reactions

The resistance of TiO 2–SnO 2 solid solutions with different ratios to the sulfidation in the reduction of sulfur dioxide (SO 2) and/or nitric oxide (NO) by carbon monoxide (CO) has been studied by temperature-programmed sulfiding (TPS), X-ray powder diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The catalytic performance has been related to these resistance results. A surface chemisorbed sulfur species which can be written as SnS 2 is found rather than bulk tin sulfides in the solid solution catalysts after reactions. The resistance of TiO 2–SnO 2 solid solutions is rationalized by the formation of solid solutions (not mechanical mixtures) of TiO 2 and SnO 2, by the excellent stability of TiO 2 in sulfur-containing gases, and by the bigger relative electronegativity of Sn than Ti. This leads to an extended electronic effect together with the lower stability of tin sulfides than titanium sulfides. The Ti 0.88Sn 0.12O 2 catalyst can be sulfided neither by the SO 2 + CO reaction nor by TPS of H 2S. As the tin amount increased, TiO 2–SnO 2 solid solutions might separate into two microscopic agglomeration groups: one is rich in Ti 4+ which cannot be sulfided like TiO 2, the other is rich in Sn 4+ which can be easily sulfided like pure SnO 2. The higher tin content means that more Sn 4+-rich agglomerate is available, which would suggest that: (1) the solid solution is more oxidative; and (2) the solid solution is more prone to be sulfided. Under the two interactions, the highest SO 2 and NO conversions were obtained at a balanced composition of a weight ratio of 1:1 of TiO 2 and SnO 2 for the SO 2 + CO and NO + SO 2 + CO reactions. SO 2 in the flue gas is a great promoter to the latter reaction.

Catalytic synthesis of methanethiol from hydrogen sulfide and carbon monoxide over vanadium-based catalysts

The direct synthesis of methanethiol, CH 3SH, from CO and H 2S was investigated using sulfided vanadium catalysts based on TiO 2 and Al 2O 3. These catalysts yield high activity and selectivity to methanethiol at an optimized temperature of 615 K. Carbonyl sulfide and hydrogen are predominant products below 615 K, whereas above this temperature methane becomes the preferred product. Methanethiol is formed by hydrogenation of COS, via surface thioformic acid and methylthiolate intermediates. Water produced in this reaction step is rapidly converted into CO 2 and H 2S by COS hydrolysis. Titania was found to be a good catalyst for methanethiol formation. The effect of vanadium addition was to increase CO and H 2S conversion at the expense of methanethiol selectivity. High activities and selectivities to methanethiol were obtained using a sulfided vanadium catalyst supported on Al 2O 3. The TiO 2, V 2O 5/TiO 2 and V 2O 5/Al 2O 3 catalysts have been characterized by temperature programmed sulfidation (TPS). TPS profiles suggest a role of V 2O 5 in the sulfur exchange reactions taking place in the reaction network of H 2S and CO.

Phosphorus promotion of Ni (Co)-containing Mo-free catalysts in thiophene hydrodesulfurization

Several unsupported nickel (cobalt)–phosphorus (sulfur) model compounds were prepared, identified and characterized by X-ray diffraction (XRD), electron microscope (EM)/energy dispersive X-ray analysis (EDAX), temperature programmed sulfidation (TPS), BET, thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). They were characterized again following pretreatments (in He, H 2 or H 2/H 2S) at different pressures, then by thiophene hydrodesulfurization (HDS) catalytic activity test at atmospheric pressure. The presence of sulfur (presulfidation by H 2S) was not required to convert the model compounds into active catalysts, a beneficial influence of H 2S on the activity was only to facilitate the reduction process into the most active Ni 2P or less active Co 2P phases. Ni(P)/SiO 2 catalysts were also prepared, characterized and tested in thiophene HDS. For sulfided P-free Ni/SiO 2 catalysts the HDS activity correlated with the Ni 3S 2 surface area determined by dynamic oxygen chemisorption (DOC). The addition of phosphorus causes a strong increase in HDS activity of silica supported catalysts, which are much higher than could be expected by their nickel sulfide surface area, therefore, the formation of Ni 2P (identified by XRD) again explains their high activity. The HDS activity of phosphorus promoted nickel (or cobalt) containing molybdenum-free catalysts is related to the amount and dispersion of Ni 2P or Co 2P present in the material.

Basic Reaction Steps in the Sulfidation of Crystalline Tungsten Oxides

The sulfidation of crystalline m-WO3 and WO3·H2O and the thermal decomposition of (NH4)2WO2S2 to an {WOS2} oxysulfide and (NH4)2WS4 to {WS3} were studied by means of temperature-programmed sulfidation (TPS), X-ray photoelectron spectroscopy (XPS), thermogravimetrical analysis (TGA), and laser Raman spectroscopy. Several basic steps of the sulfidation reaction could be resolved and explained in terms of the structures of crystalline m-WO3 and WO3·H2O. The sulfidation reaction starts at low temperatures with a reduction of the crystalline oxides by reaction with H2S without incorporation of sulfur. Only after this reduction does an exchange of terminal O2- ligands of the oxides for S2- take place. In subsequent W−S redox reactions S22- ligands form, which leads to oxysulfidic intermediates. The {WOS2} oxysulfide phase serves as a reference of structural features of oxysulfides. The presence of oxygen at bridging and terminal positions as well as of disulfide ligands seems to be a common structural property ...

Effect of fluorine addition on the formation of active species and hydrotreating activity of NiWS/Al 2O 3 catalysts

Fluorine-added Ni/Al 2O 3, W/Al 2O 3 and NiW/Al 2O 3 catalysts were prepared and their surface properties investigated using temperature-programmed reduction, diffuse reflectance spectroscopy and temperature-programmed sulfidation. The hydrodesulfurization (HDS) of thiophene was used as a model reaction to examine the activity of fluorinated NiW/Al 2O 3. Fluorination led to an increase in the formation of polymeric tungstates in W/Al 2O 3 and the surface nickel spinel species in the case of Ni/Al 2O 3. Fluorination promoted the sulfidation of the NiW/Al 2O 3 catalyst and the interaction of nickel with tungsten to produce the Ni–W–O species, a precursor of active catalytic sites. The activity of NiW/Al 2O 3 was increased by fluorination up to a fluorine content of 4 wt.%, and then decreased at higher fluorine contents. Two opposite effects of fluorination determine the characteristic changes in the activity of NiW/Al 2O 3: the enhanced sulfidation of the catalyst and incorporation of nickel with tungsten, both of which promote the activity, and a decrease in the dispersion of active species, which lowers the activity. The activity results obtained in this study are different from those reported previously [Appl. Catal. A 144 (1996) 343], suggesting that the extent of catalyst modification by fluorination is strongly dependent on the conditions of catalyst preparation.

TPS, XPS, QEXAFS, and XANES Investigation of the Sulfidation of NiW/Al2O3–F Catalysts

The sulfidation behavior of alumina-supported Ni–W catalysts was investigated by means of temperature-programmed sulfidation (TPS), X-ray photoelectron spectroscopy (XPS), quick extended X-ray absorption fine structure (QEXAFS), and X-ray absorption near-edge structure spectroscopy (XANES). Either ammonium tetrathiotungstate or ammonium metatungstate was used as the precursor of tungsten, and nickel nitrate was the source of nickel. The effect of fluorination of the alumina support on the sulfidation behavior of tungsten and nickel on these two series of catalysts was studied as well. The sulfidation of the catalysts prepared from ammonium metatungstate passes through W(VI) oxysulfide intermediates. Fluorination of the alumina support aids the sulfidation of tungsten and nickel at low temperature and promotes the transformation of the W(VI) oxysulfide intermediates to WS2. After sulfidation at 400°C and atmospheric pressure for 4 h, about 50% of tungsten and 60% of nickel in the catalysts prepared from ammonium metatungstate were sulfided. EXAFS showed that ammonium tetrathiotungstate supported on alumina decomposes to oxidic tungsten during the second impregnation with nickel nitrate. Nevertherless, sulfidation of the catalysts prepared from ammonium tetrathiotungstate is much easier. It also passes through W(VI) oxysulfide intermediates, and fluorination aids the formation WS2. In the sulfided catalysts prepared from ammonium tetrathiotungstate and nickel nitrate, 100% of tungsten and nickel is in the sulfided state, but a small amount of tungsten is in a {WS3} state, with fully sulfided W(VI), rather than in the WS2 state. The fluorine-containing catalyst contains a larger fraction of WS2 than the fluorine-free catalyst.

Combined TPS, XPS, EXAFS, and NO-TPD study of the sulfiding of Mo/Al2O3

The sulfiding of Mo/Al2O3 in H2S/Ar versus in H2S/H2 has been studied by temperature-programmed sulfiding (TPS), X-ray photon electron spectroscopy (XPS), extended X-ray absorption fine structure (EXAFS), and temperature-programmed desorption of NO (NO-TPD). All the applied techniques agree on the sulfur content in the sulfided catalysts and the findings are in accord with a model for the H2S production reaction. The nucleation and growth of well-ordered MoS2 clusters are probed by XPS during sulfiding with and without the presence of hydrogen. The resulting dispersion of the MoS2 phase is evaluated on the basis of XPS, EXAFS, and NO-TPD, and is found to be highest when the sulfiding occurs in the presence of hydrogen.