Mesoporous silica–alumina as support for Pt and Pt–Mo sulfide catalysts: Effect of Pt loading on activity and selectivity in HDS and HDN of model compounds

The potential of mesoporous silica–alumina (MSA) material as support for the preparation of sulfided Pt and Pt–Mo catalysts of varying Pt loadings was studied. The catalysts were characterized by their texture, hydrogen adsorption, transmission electron microscopy, temperature programmed reduction (TPR) and by activity in simultaneous hydrodesulfurization (HDS) of thiophene and hydrodenitrogenation (HDN) of pyridine. Sulfided Pt/MSA catalysts with 1.3 and 2 wt.% Pt showed almost the same HDS and higher HDN activities per weight amounts as conventional CoMo and NiMo/Al₂O₃, respectively. The addition of Pt to sulfided Mo/MSA led to promotion in HDS and HDN with an optimal promoter content close to 0.5 wt.%. The results of TPR showed strong positive effect of Pt on reducibility of the MoS₂ phase which obviously reflects in higher activity of the promoted catalysts. The activity of the MSA-supported Pt–Mo catalyst containing 0.5 wt.% Pt was significantly higher than the activity of alumina-supported Pt–Mo catalyst. Generally, Pt–Mo/MSA catalysts promoted by 0.3–2.3 wt.% Pt showed lower HDS and much higher HDN activities as compared to weight amounts of CoMo and NiMo/Al₂O₃. It is proposed that thiophene HDS and pyridine hydrogenation proceed over Pt/MSA and the majority of Pt–Mo/MSA catalysts on the same type of catalytic sites, which are associated with sulfided Pt and MoS₂ phases. On the contrary, piperidine hydrogenolysis takes place on different sites, most likely on metallic Pt fraction or sites created by abstraction of sulfur from MoS₂ in the presence of Pt.     

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.     

Characterization of hydrodesulfurization catalyst prepared by impregnating cobalt nitrate solution onto the sulfided MoO3/Al2O3 catalyst

CoMo/Al₂O₃ catalysts were prepared by impregnating Cobalt nitrate solution into oxidic or sulfided Mo/Al₂O₃. The properties of CoMo/Al₂O₃ catalysts were characterized by XRD, TPS, oxygen chemisorption and ESR. Catalytic activity of CoMo/Al₂O₃ catalyst was evaluated by thiophene HDS as a probe reaction. When CoMo/Al₂O₃ catalyst was prepared by impregnating Cobalt nitrate solution into sulfided Mo/Al₂O₃, the interaction between Mo and alumina became weaker and the formation of synergic phase was facilitated. These structural changes may explain higher HDS activity of CoMo/Al₂O₃ catalyst prepared by impregnating Cobalt nitrate solution into sulfided Mo/Al₂O₃.     

Elucidation of hydrodesulfurization mechanism using S radioisotope pulse tracer methods

The sulfidation state in a series of Co-promoted Mo/Al₂O₃ catalysts was investigated using a ³⁵S pulse tracer method. ³⁵S-labeled H₂S ([³⁵S]H₂S) pulses were introduced into catalysts in a nitrogen stream until the radioactivity in the recovered pulse approached the radioactivity of the introduced pulse. From the amount of introduced [³⁵S]H₂S, the amount of sulfur accumulated on the catalyst was estimated. The result indicated that the amounts of sulfur accumulated on the catalysts increased with increasing temperature for all catalysts. Only molybdenum was sulfided in both Co–Mo/Al₂O₃ and Mo/Al₂O₃ catalysts below 300°C, but the sulfided states of the catalysts at 400°C were very close to the stoichiometric states where Co and Mo are present as Co₉S₈ and MoS₂. Further, hydrodesulfurization (HDS) reactions of radioactive ³⁵S labeled dibenzothiophene were carried out over the series of Co-promoted Mo/Al₂O₃ catalysts. The amount of labile sulfur and the release rate constant of H₂S were determined. The promotion effect of cobalt on activity of the molybdenum catalyst was attributed to the formation of more active sites. Moreover, the increase in the catalytic activity with Co/Mo ratio among the promoted catalysts was due to increase in the number of the sites with the same activity.     

TiO2-Supported Mo Model Catalysts: Ti as Promoter for Thiophene HDS?

The combination of thiophene hydrodesulfurization (HDS) activity measurements and X-ray photoelectron spectroscopy on flat model systems of sulfided HDS Mo catalysts showed that sulfided Ti-species can act as a promoter in the same way as Co and Ni, although less effectively. This explains the higher thiophene HDS activity and hydrogenation selectivity of Mo/TiO₂ compared with Mo/Al₂O₃, while for Ni-promoted Mo catalysts the difference between the two supports is negligible.     

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.     

Catalytic Synthesis of Thiophene from the Reaction of Furan and Hydrogen Sulfide

The catalysts were prepared from pseudo-boehmite mixed with dilute nitric acid and calcined at different temperatures. The vapour-phase reaction of furan and hydrogen sulfide was performed in a fixed-bed flow in the presence of catalyst. The catalysts were characterized by XRD, N₂ adsorption, FT-IR techniques. The Al₂O₃ calcined at 550 °C has large surface areas which resulted in high yield of thiophene under the conditions: at atmosphere, reaction temperature 500 °C, the ratio of H₂S to Furan about 10 (mol) and LHSV 0.2 h⁻¹. The reaction mechanism was proposed for the synthesis of thiophene from furan and hydrogen sulfide over Al₂O₃.     

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.     

A comparative study on the effect of promoter content of hydrodesulfurization catalysts at different evaluation scales

CoMo and NiMo supported Al₂O₃ catalysts have been investigated for hydrotreating of model molecule as well as industrial feedstock. Activity studies were carried out for thiophene and SRGO hydrodesulfurization (HDS) in an atmospheric pressure and batch reactor respectively. These activities on sulfided catalysts were evaluated as a function of promoter content [M/(M + Mo) = 0.30, 0.34, 0.39; M = Co or Ni] using fixed (ca. 8 wt.%) molybdenum content. The promoted catalysts were characterized by textural properties, XRD, and temperature programmed reduction (TPR). TPR spectra of the Co and Ni promoter catalysts showed that Ni promotes the easy reduction of Mo species compared with Co. With the variation of promoter content NiMo catalyst was found to be superior to CoMo catalyst for gas oil HDS, while at low-promoter content the opposite trend was observed for HDS of thiophene. The behavior was attributed to the several reaction mechanisms involved for gas oil HDS. A nice relationship was obtained for hydrodesulfurized gas oil refractive index (RI) and aromatic content, which corresponds to the Ni hydrogenation property.     

Temperature programmed desorption studies on hydrodesulfurization catalysts

The temperature programmed-desorption (TPD) of butane, butene, butadiene and thiophene over a series of Co-Mo/-Al₂O₃ catalysts with varying Co to Mo ratio has been investigated. The TPD of butane, butene and butadiene over catalysts containing no Co showed a single desorption profile while incorporation of Co created an additional site without significantly affecting desorption from the original site. The TPD of thiophene over a series of catalysts with varying Co content showed identical desorption temperature as well as heat of desorption. It was concluded that thiophene was adsorbed on the Mo-S component of the catalyst and was unaffected by the presence of Co.     

Reactive and Non-reactive Interactions of Thiophene with WS2 Fullerene-like Nanoparticles: An Ultra-high Vacuum Surface Chemistry Study

The adsorption kinetics of thiophene on WS₂ nanoparticles with fullerene-like (onion-like) structure has been studied at ultra-high vacuum conditions by sample temperature ramping techniques. At low temperatures, thiophene adsorbs molecularly. The formation of H₂S and alkanes is evident at greater temperatures on fully sulfided as well as reduced and oxidized WS₂ nanoparticles.     

Mesoporous molecular sieve MCM-41 supported Co–Mo catalyst for hydrodesulfurization of dibenzothiophene in distillate fuels

A mesoporous aluminosilicate molecular sieve with MCM-41 type structure was synthesized using aluminum isopropoxide as the Al source. Supported Co–Mo/MCM-41 catalysts were prepared by co-impregnation of Co(NO₃)₂·6H₂O and (NH₄)₆Mo₇O₂₄ followed by calcination and sulfidation. For comparison, conventional Al₂O₃-supported sulfided Co–Mo catalysts were also prepared using the same procedure. These two types of catalysts were examined at two different metal loading levels in hydrodesulfurization of a model fuel containing 3.5 wt% sulfur as dibenzothiophene in n-tridecane. At 350–375°C under higher H₂ pressure (6.9 MPa), sulfided Co–Mo/MCM-41 catalysts show higher hydrogenation and hydrocracking activities at both normal and high metal loading levels, whereas Co–Mo/Al₂O₃ catalysts show higher selectivity to desulfurization. Co–Mo/MCM-41 catalyst at high metal loading level is substantially more active than the Co–Mo/Al₂O₃ catalysts.     

Catalytic combustion of diesel soot: Deactivation by SO2 of copper and potassium nitrate catalysts supported on alumina

The SO₂ poisoning of Cu KNO₃/Al₂O₃ catalysts used in the catalytic combustion of diesel soot with NO/O₂ feed has been studied in this work. Fresh catalysts show temperatures of maximum combustion rate (T_max) at about 470, 375 and 368 °C for Cu/Al₂O₃, KNO₃/Al₂O₃ and Cu KNO₃/Al₂O₃ respectively. Poisoned catalysts decrease the activity and techniques of vibrational spectroscopy (FTIR) and temperature programmed reduction (TPR) suggest the KNO₃ loss and the formation of potassium sulfate in the mentioned poisoned catalysts.     

High resolution electron microscopy characterization of sulfided palladium particles on amorphous SiO2

The microstructure of palladium particles deposited on a porous silica support was examined y high resolution microscopy following various sulfidation treatments. For catalysts sulfided by the reaction itself (hydrodesulfurization of thiophene), the micrographs show large particles where an amorphous sulfide layer surrounds the Pd core; in the case of small particles, the presence of PdS₂ was identified. For catalysts sulfided under 20% H₂S/H₂ at 623 K, the particles show lattice planes compatible with PdS₂ and PdS. By increasing the sulfidation temperature to 723 K, faceted PdS particles are mostly formed. Some structural defects related to the sulfidation treatment are also shown.     

The treatment of binary VOC mixtures by adsorption and oxidation using activated carbon and a palladium catalyst

The adsorption of binary thiophene‐containing mixtures on activated carbon and their subsequent desorption and oxidation over a Pd/CeO₂/Al₂O₃ catalyst has been studied as a model for the treatment of dilute VOC streams by intermittent accumulation and combustion. When adsorbed together, cyclohexene and thiophene have similar breakthrough times with little tendency for one to displace the other. However the adsorption of thiophene continues beyond that of diethylamine in mixtures while thiophene is itself displaced by methylmethacrylate. Oxidation of the binary mixtures is controlled by that of thiophene which requires temperatures somewhat above 300 °C for complete removal. Combustion of the other VOCs proceeds in parallel with thiophene although they react at much lower temperatures when alone. Both components of the mixtures can be removed by a system in which the VOCs are accumulated on carbon and then undergo temperature programmed desorption in air onto Pd/CeO₂/Al₂O₃ held at 350 °C. However it is not possible to operate with the adsorbent and catalyst together since desorption then occurs before the catalyst reaches working temperature. © 2002 Society of Chemical Industry     

Enhancement of hydrogen spillover by surface labile oxygen species on oxidized Pt/TiO2 catalyst

TPD, H₂-O₂ titration and TPR techniques have been used to study the effect of the surface oxygen species on hydrogen adsorption in an oxidized Pt/TiO₂ catalyst system. Oxygen desorption peaks in the temperature range of 610–730 K were observed in the O₂-TPD profiles of the Pt/TiO₂ samples oxidized in oxygen at temperatures 573, 673 and 773 K, respectively. They reveal that labile oxygen species were formed on the surfaces of these oxidized catalysts. Much more hydrogen spillover on the oxidized Pt/TiO₂ catalysts was observed at room temperature using H2-O₂ titration. The results from TPR and H₂-TPD experiments further support the proposal that the existence of labile oxygen species enhances hydrogen spillover in the system studied.     

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.     

Support effect on the formation of catalytic site precursors in the thiophene hydrodesulfurization

The TiO₂ and Al₂O₃ supported FeMo catalysts (6 and 12 wt.% Mo) have been prepared using (NH₄)₃Fe(OH)₆Mo₆O₁₈ salt (FeMo₆). They have been studied by the IR, TPR, XPS and MS methods and tested in thiophene conversion. The initial salt transforms in analog of the TiMo acid anion on the titania support and alumina heteropolymolybdate anion on the alumina support. Higher thiophene conversion is observed on the TiO₂ supported catalysts.     

Effect of Al2O3/SiO2 ratio on iron-based catalysts for Fischer–Tropsch synthesis

A systematic study was undertaken to investigate the effects of Al₂O₃/SiO₂ ratio on reduction, carburization and catalytic behavior of iron-based Fischer–Tropsch synthesis (FTS) catalysts promoted with potassium and copper. The catalysts were characterized by N₂ physical adsorption, CO₂ temperature-programmed desorption (TPD), H₂ temperature-programmed reduction (TPR) and Mössbauer effect spectroscopy (MES). CO₂-TPD indicated that Al₂O₃ binder has stronger acidity than SiO₂ binder and weakens the surface basicity of the catalysts. H₂-TPR profiles suggested that the lower Al₂O₃/SiO₂ ratio promotes the reduction of Fe₂O₃→Fe₃O₄. With further increasing Al₂O₃/SiO₂ ratio, the transformation of Fe₂O₃→Fe₃O₄ shifts to higher temperatures. The MES results showed that the increase of Al₂O₃/SiO₂ ratio leads to the relatively large crystallite size of α-Fe₂O₃ and inhibits carburization of the catalyst. During reaction tests in a fixed bed reactor it was found that a maximum in catalyst activity is noted at the Al₂O₃/SiO₂ ratio of 5/20 (weight basis). The selectivity to olefins shows a rapid decrease and the formations of methane and light hydrocarbons are promoted with increasing Al₂O₃/SiO₂ ratio. The oxygenate selectivity in total products increases with increasing Al₂O₃/SiO₂ ratio.     

Mechanisms of hydrodenitrogenation of alkylamines and hydrodesulfurization of alkanethiols on NiMo/Al2O3, CoMo/Al2O3, and Mo/Al2O3

The simultaneous hydrodenitrogenation (HDN) of alkylamines and hydrodesulfurization (HDS) of alkanethiols, with the NH₂ and SH groups attached to primary, secondary, and tertiary carbon atoms, were studied at 270–320 °C and 3 MPa over sulfided NiMo/Al₂O₃, CoMo/Al₂O₃, and Mo/Al₂O₃ catalysts. Pentylamine and 2-hexylamine reacted by substitution with H₂S to form alkanethiols and with another amine molecule to form dialkylamines. Alkenes and alkanes were not formed directly from pentylamine and 2-hexylamine, but indirectly by elimination and hydrogenolysis of the alkanethiol intermediates, as confirmed by their secondary behavior and the similar alkene/alkane ratios in the simultaneous reactions of amines and thiols. Only 2-methyl-2-butylamine, with the NH₂ group attached to a tertiary carbon atom, produced alkenes as primary products by E1 elimination. NiMo/Al₂O₃ and CoMo/Al₂O₃ have a higher activity for the HDS of alkanethiols than does Mo/Al₂O₃; H₂S has a negative influence. This shows that the thiols react on vacancies on the catalyst surface (Lewis acid sites). Mo/Al₂O₃ is the best HDN catalyst; H₂S has a positive influence on the HDN of amines with the NH₂ group attached to a secondary and a tertiary carbon atom. This indicates that the HDN of alkylamines occurs on Brønsted acid sites.