Temperature-programmed Sulfidation Research Articles

TPS, XPS, and QEXAFS Investigation of the Sulfidation Behavior of Tungsten on Fluorine-Promoted Alumina

The sulfidation behavior of alumina-supported W catalysts was investigated by means of temperature-programmed sulfidation, quick extended X-ray adsorption fine structure measurements, and X-ray photoelectron spectroscopy of two series of tungsten catalysts, one made from ammonium metatungstate and the other from ammonium tetrathiotungstate. The effect of the fluorination of the alumina support on the sulfidation behavior of W on these two series of catalysts was also studied. The sulfidation of catalysts prepared with ammonium metatungstate passes through intermediates of W oxysulfides; the sulfided catalysts are mixtures of W oxysulfides and WS2. After sulfidation at 400°C and atmospheric pressure for 4 h, the degree of sulfidation is only about 50%. Fluorination slightly increases the degree of sulfidation. When ammonium tetrathiotungstate is used as the precursor, fully sulfided catalysts can be obtained. Fluorination accelerates the transformation of WS3 sulfide to WS2.

Stability of highly dispersed Ni/Al 2O 3 catalysts: Effects of pretreatment

Oxidic and reduced and passivated Al2O3-supported nickel catalysts with loading up to 19.3 wt% were characterized by temperature-programmed sulfiding (TPS), temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy. The oxidic catalyst precursors contain a highly disperse nickel species consisting of either an oxidic core and ahydroxide outer layer or a hydroxide as a whole, dependent on the Ni loading. The crystallite size of the active phase varies between 0.4 and 1.6 nm. This high dispersion was maintained during sulfidation. From TPR and TPS, it is inferred that no nickel aluminate is present. During reduction and passivation the crystallite size increases to 1.3–2.5 nm. From TPR of the oxidic catalyst it was concluded that the active phase contained more oxygen than that corresponding to the stoichiometry of NiO since an excess of hydrogen of 20–50% was consumed for the removal of the reactive oxygen species.

Effect of the fluorine-addition order on the hydrodesulfurization activity of fluorinated NiW/Al2O3 catalysts

Fluorinated NiW/Al2O3 catalysts with different orders of fluorine addition have been prepared, tested for hydrodesulfurization (HDS) of thiophene, and characterized using nitric oxide chemisorption and temperature-programmed sulfidation. The catalyst surface area has been affected by fluorine addition but not by the order of fluorination. The fluorine addition-order does not affect the amount of fluorine retained in the catalysts after the calcination and the reaction steps, either. On the other hand, the order of fluorine addition changes the dispersion of the nickel and the tungsten species, incorporation of nickel with the tungsten edge sites, and consequently the HDS activity of the catalysts. The catalyst fluorinated in the last step, i.e., after addition of both tungsten and nickel, shows the highest activity in thiophene HDS, which is supported by other experimental results indicating the most nitric oxide chemisorption and the largest incorporation of nickel with the tungsten species. Accordingly, enhancement of the catalyst activity by fluorination is due to the repartition of the metal species rather than to partial solubilization of alumina in the fluorine-addition step.

The Evolution of Surface Species in NiW/Al2O3Catalysts in Various Stages of Sulfidation: A Quasiin-SituHigh Resolution Transmission Electron Microscopic Investigation

The effect of the calcination temperature on the evolution of the active phase in NiW/γ-Al2O3catalysts during sulfidation has been studied using high resolution transmission electron microscopy (HREM) and temperature programmed sulfidation (TPS). The evolution of the active phase is discussed in view of the performance of the catalysts in the hydrodesulfurization (HDS) of thiophene. Dependent on the pretreatment, three different species were observed with HREM: (i) subnanometer clusters (about 0.5 nm diameter), (ii) nanometer size particles (1 to 2 nm diameter), and (iii) WS2-like slabs (2–3 nm length). The clusters and particles predominantly appear after low temperature sulfidation of catalysts calcined at 673 and 823 K. Hence, it is concluded that they are initially formed from the oxidic catalyst precursor. WS2-like slabs are predominantly formed during high temperature sulfidation, independent of the calcination temperature. Activity measurements for the HDS of thiophene show that all phases exhibit a significant catalytic activity. HREM and activity measurements on various freshly sulfided and spent catalysts showed no significant differences in the catalysts prior to and after the reaction.

Characterization of CoMo-Al2O3 Catalyst by Combination Technique of Temperature-programmed Sulfiding and Electron Spin Resonance(TPS-ESR).

CoMo-Al2O3 catalyst was characterized with a combination technique of temperature-programmed sulfiding (TPS) and electron spin resonance (ESR). The behavior of the MoO3 loaded over the surface of Al2O3 obtained from TPS has been compared with that of Mo5+ observed with ESR at isothermal and elevated temperatures. The Mo6+ in the MoO3 supported is easily reduced with H2S+H2 under very mild conditions and transformed into Mo5+ as an intermediate. This Mo5+ peak drastically decreased around 200°C with H2S desorption, and H2 consumption appeared simultaneously in the TPS profile. These results suggest that the Mo5+ species observed with ESR associates with sulfur from H2S.

Effect of cobalt on the sulfiding temperature of CoO [sbnd]MoO 3/Al 2O 3 studied by temperature programmed sulfiding

The effect of cobalt on the sulfiding temperature of CoO MoO 3/Al 2O 3 was investigated by means of temperature programmed sulfiding (TPS). MoO 3 supported on alumina transforms into MoS 2− x by sulfidation via formation and subsequent hydrogenation of Mo oxi-sulfide or MoS 3. The temperature of which the Mo oxi-sulfide or MoS 3 phase are hydrogenated (Ps temperature) decreased with increasing amount of cobalt in the CoO MoO 3/Al 2O 3 catalysts. A decrease in the Ps temperature was also observed in the case of ‘ Co/Al+ Mo/Al catalysts’ which were prepared by mechanical mixing of CoO/Al 2O 3 and MoO 3/Al 2O 3. On the other hand, the Ps temperature did not change by mechanical mixing of bulk CoO and the MoO 3/Al 2O 3 catalyst. The cobalt adsorption test suggested that the cobalt species preferably interact with the molybdenum species during impregnation. It was concluded that the well dispersed cobalt species have a capability to provide spillover hydrogen to accelerate the hydrogenation of the Mo oxi-sulfide. Furthermore, a correlation between the Ps temperature and HDS activity for light gas oil was obtained. This result suggests that the cobalt species facilitate the hydrogenation of sulfur atoms adsorbed on the HDS active sites (coordinatively unsaturated Mo sites) by supplying spillover hydrogen and consequently accelerate the HDS reaction.

The Sulfidation of γ-Alumina and Titania Supported (Cobalt)Molybdenum Oxide Catalysts Monitored by EXAFS

The sulfidation of γ-alumina- and titania-supported(cobalt)molybdenum oxide catalysts has been studied with X-rayabsorption spectroscopy and temperature programmed sulfidation (TPS).The catalysts were stepwise sulfided at temperatures between 298 and673 K and their structure was determined with EXAFS spectroscopy. Onalumina oxygen–sulfur exchange starts at a temperature just aboveroom temperature resulting in the formation of monomeric and dimericmolybdenum sulfide species containing disulfide ligands. Between 448and 523 K the disulfide ligands are reduced with hydrogen and onlydimeric molybdenum sulfide species with a Mo–Mo distance of 2.77 Åremain. Above 523 K these dimers aggregate to larger MoS2particles.In contrast, on titania molybdenum oxide is sulfided to yieldisolated molybdenum sulfide monomers at 523 K, which similarlyaggregate to MoS2particles between 523 and 623 K. Nomolybdenum sulfide dimers are observed as intermediates. Both onalumina and titania all intermediates are anchored to the support viaMo–O linkages with a bond distance of 2.0 Å. The addition of Co toMo/Al2O3accelerates the rate of sulfidation, demonstrated by the TPS patterns. However, on titania a lowering of the sulfidation rate is observed next to the presence of molybdenum sulfide dimers, that are not present in the unpromoted catalyst. These observations can be ascribed to the presence of CoMoO4in the oxidic catalyst.

Temperature-Programmed Sulfiding of Vanadium Oxides and Alumina-Supported Vanadium Oxide Catalysts

Sulfiding of bulk and alumina-supported vanadium oxides has been studied using temperature-programmed sulfiding and reduction techniques. Bulk compounds (V 2O 5, V 2O 3) and V/Al 2O 3 catalysts are sulfided via a similar mechanism. For bulk V 2O 5, two major sulfiding steps have been identified. At temperatures up to 673 K, V 2O 5 is reduced to V 2O 3 by O-S exchange and subsequent rupture of V-S bonds where H 2S acts as reducing agent. Sulfiding to V 2S 3 takes place above 673 K. The catalysts are sulfided more easily than the bulk oxides due to the higher dispersion of the vanadium species. In catalysts sulfided at 673 K which are still partially oxidic, four types of sulfur have been observed, viz. adsorbed H 2S, stoichiometric sulfur, S-H groups, and nonstoichiometric (excess) sulfur (S x ). There are indications that (isothermal) room temperature H 2S adsorption can be used to determine the dispersion of the supported microcrystallites at higher vanadium loadings. From the present results it is inferred that alumina-supported vanadium-based catalysts, when sulfided at temperatures commonly applied in hydrotreating operations, essentially consist of an oxide, the outer surface of which is sulfided.

HDS Activity and Characterization of Zeolite-Supported Nickel Sulfide Catalysts

Catalysts of nickel sulfide supported on zeolite Y have been prepared (by impregnation or ion exchange) and characterized by means of thiophene hydrodesulfurization (HDS), sulfur analysis, temperature-programmed sulfiding, 129Xe-NMR, HREM and dynamic oxygen chemisorption. The catalysts show large differences in catalytic behavior dependent on the preparation method (impregnation vs ion exchange) and the pretreatment conditions (method of sulfidation). Especially the ion-exchanged catalysts show a high initial activity, but due to the presence of acid sites deactivation is very strong. The initial activity of these catalysts can be improved significantly by drying prior to sulfidation. In all cases sulfidation results in quantitative formation of nickel sulfide, with Ni3S2 being the main product. Occasionally, also some NiS appears to be present. The major part of the nickel sulfide phase is invariably located on the outside of the zeolite particles. The fraction of nickel sulfide in the zeolite pores depends on the preparation method and the pretreatment conditions. The differences in catalytic activity are ascribed not only to variations in overall nickel sulfide dispersion but also to the acidity of the support, and the presence of very active small nickel sulfide clusters in the pores of the zeolite can have a strong influence on the thiophene HDS activity.

The Structure of Highly Dispersed SiO2-Supported Molybdenum Oxide Catalysts during Sulfidation

The structure of sulfided Mo-catalysts and their oxidic precursors has been abundantly studied, but the genesis of the active phase has remained much less investigated. The sulfidation (in H2S/H2 atmosphere) of a series of MoO3/SiO2 catalysts has been examined by means of temperature-programmed sulfidation, X-ray absorption fine structure, and transmission electron microscopy.The oxidic, oligomeric clusters in a 5.6 wt 5% MoO3/Si02 catalyst are transformed into partly sulfided particles (MOOS,,) by 0-S exchange at room temperature. A molybdenum sulfide species the structure of which resembles the MoS, structure is formed during sulfidation at 423 K. The MoS2 phase is formed at temperatures between 523 and 573 K, depending on the dispersion of the initial Moo3 phase. The transition of MoS3 into MoS2 can be monitored by the evolution of H2S from the catalyst with a simultaneous consumption of H2. The two-dimensional size of the MoS2 slabs can be derived from the EXAFS Mo-Mo coordination number by means of a theoretical model. TEM is required to elucidate the stacking height of the slabs. The application of supported molybdena-based catalysts in industrial processes, such as partial oxidation’-5 and hydrotreating,+lB has rendered supported molybdenum oxide catalysts a main subject of investigations. Many studies are dedicated to the preparation and the structure of Moo3 catalysts on different supports. The sulfded counterpart of the oxidic precursors of Co(Ni)/Mo(W) catalysts is one of the most thoroughly investigated catalyst systems. Conversion of the oxidic catalyst into the active sulfide occurs under reaction conditions, due to presulfiding or the presence of sulfur-containing feedstocks. It is industrial practice to “spike” the feedstock with agents, such as CS2, dimethyl sulfide (DMS), or dimethyl disulfide (DMDS), to accelerate the sulfidation of the catalyst.I9-20 As an alternative, thecatalyst can besulfided exsitu by impregnation with a sulfurcontaining liquid. In situ heating of the catalyst subsequently leads to sulfidation.2’ In most of the laboratory tests the catalyst is sulfided by a mixture of H2S/H2 at high temperat~re~~v~~ or in a temperature program. A number of models have been proposed to explain the activity of the Co(Ni)/Mo(W) catalysts. Consensus exists on the structure of the molybdenum sulfide phase: under typical hydrotreating conditions (i.e., p = 10-200 bar, T = 573-723 K, excess H2, and a sulfur-rich feedstock”) MoS2 is present, in which molybdenum is trigonal prismatically coordinated by sulfur However, the position and the role of the cobalt atoms in the catalysts are still debated. The earlier proposed models are based on the assumptionof a monolayer,34~ontactsynergy,’~J~ inter~alation,3S33~ and remote control.37 The most popular model is based on publications by Topsoe et al.30 and by Wive1 et a1.38 The latter reported a linear relationship between the activity in thiophene HDS and the concentration of Co present in the “Co-Mo-S” species. This phase had been designated as ‘COMOS”~~ because its Mbssbauer spectrum did not resemble any of the known cobalt sulfide or cobalt molybdenum sulfide spectra. The CoMoS species is envisioned as a MoS2 layer (slab), with cobalt atoms decorating the edges.30J9 Since the introduction of this model, some

Temperature programmed sulfiding of commercial cobalt oxide-molybdenum oxide (CoO-MoO3)/alumina catalysts

The potential of the temperature programmed sulfiding (TPS) technique is enlightened using commercial type of CoO-MoO 3 /Al 2 O 3 catalysts. A modified TPS procedure is proposed to simulate more closely industrial sulfiding procedures. A relation is found between the amount of sulfidable cobalt ions and the hydrodesulfurization activity of CoO-MoO 3 /Al 2 O 3 catalysts, calcined between 823 and 873 K and containing at most 4 wt % CoO. Provided (i) typically industrial calcination temperatures are used and (ii) the catalysts do not contain phosphorus, TPS is a direct method to estimate the amount of cobalt which shows hardly any catalytic activity

Reduction and Sulfidation Properties of Iron Species in Iron-Supported Y-Zeolite by Temperature-Programmed Reduction and Sulfiding

Temperature-programmed reduction (TPR) and temperature-programmed sulfiding (TPS) were used to characterize reduction and sulfiding properties of Fe-exchanged Y-zeolites and Fe-treated Y-zeolites, which were prepared by treating NH4Y-zeolite with an aqueous ferric nitrate solution (Fe-treatment). By considering their unique TPR and TPS patterns, it was confirmed that the Fe2+-species in the Fe-exchanged Y-zeolites are stabilized inside the sodalite cages and the hexagonal prisms. On the basis of the TPR and TPS characterizations, it was demonstrated that three types of the Fe-species are present in the Fe-treated Y-zeolites: ion-exchanged species, small Fe oxide clusters, and Fe oxide without interaction with the zeolite framework (including aggregated ferric oxide), the proportion of which is dependent on the extent of the Fe-treatment. Prolonged Fe-treatment weakens the interaction between the Fe-species and the framework oxygen atoms by hydrolysis, and leads to the aggregation of the Fe oxides and to the formation of bulk ferric oxide. The small Fe-oxide clusters, which are probably situated inside the supercages through a coordination with the framework oxygen atoms, are responsible for the high activity for toluene disproportionation in the presence of H2S.

EXAFS Study of Ni–Mo/Al2O3 Hydrodesulfurization Catalysts

Ni promoter effect on the structure and the sulfiding state of Mo phase in Ni-Mo/Al2O3 catalysts were studied using Temperature Programmed Sulfiding (TPS) and XAFS. The results for TPS and Mo-K edge EXAFS suggest that Ni addition promotes the sulfiding of Mo phase and the growth of MoS2-like species in the horizontal direction. Ni-K edge XAFS results for sulfided catalysts show that most of Ni atoms may exist as oxidic state.

Comparison of the sulfiding rate and mechanism of supported NiO and Ni 0 particles

The sulfiding rate and mechanism of (i) Al 2O 3-supported NiO and (ii) Al 2O 3-supported Ni 0 particles have been studied using the temperature-programmed sulfiding technique. Both the surface and the core of Ni 0 particles are easier to sulfide than those of NiO particles of comparable size. The Ni 0 surface sulfides at 210 K and the inner layers sulfide between 270 and 500 K; simultaneously with the H 2S uptake H 2 is produced. The surface layer of NiO particles starts sulfiding at 300 K, while the core sulfides between 370 and 650 K. The sulfiding of Ni 0 and Nil surface species takes place at various temperatures because of differences in the elementary steps involved in H 2S dissociation. Compared to NiO particles, the surface of Ni 0 particles sulfides at lower temperatures due to a higher dissociation rate of H 2S on these surfaces. The sulfiding rate of the core of the particles is determined by solid-state diffusion. Sulfiding of the core of the Ni 0 particles is determined by the diffusion rate of the Ni °+ and to a lesser extent by the S δ− ions, while sulfiding of the core of the NiO particles is most likely limited by diffusion of the O 2− ions. Surface nickel sulfide species are relatively more stable than their bulk analogues, possibly due to a relatively high entropy content of surface layers.

Temperature-programmed sulfiding of precursor cobalt oxide genesis of highly active sites on sulfided cobalt catalyst for hydrogenation and isomerization

It was found that the method of sulfidation of cobalt oxide strongly affects the catalytic activities and selectivities of the resultant cobalt sulfide catalyst, as well as the calcination temperature of the cobalt oxide. When cobalt oxide was sulfided at 673 K by a temperature-programmed sulfiding method (a heating rate of 6 K min −1), catalytic activities for the hydrogenation of butadiene and the isomerization of 1-butene were considerably enhanced compared with those for cobalt sulfide prepared by isothermal sulfidation at 673 K. Results of temperature-programmed sulfiding (TPS), temperature-programmed reduction (TPR), and X-ray diffraction (XRD) suggest that the catalysts showing high catalytic activities after sulfidation are partially sulfided at 673 K and consist of the unsulfided cobalt core phases (COO or metallic Co). The sulfidation property of precursor cobalt oxides has been studied using TPS, simulating the sulfidation process of the cobalt sulfide catalysts. Two distinctly different kinds of sulfidation process are estimated by TPS measurements of the cobalt oxides. The calcination temperature of the precursor cobalt oxides strongly affects the sulfidation paths. They are differentiated in terms of the presence of a metallic Co intermediate. The relationship of the mechanism of sulfidation of the cobalt oxides to the generation of highly active sites is discussed.

Silica-Supported MoO3 Catalyst Prepared by Alkoxide Method

Abstract Highly dispersed silica-supported molybdenum catalysts were prepared by an alkoxide method, using ethylsilicate and ammonium paramolybdate dissolved in ethylene glycol. By comparing temperature-programmed sulfiding(TPS) spectra of the catalysts with those of alumina or silica-supported catalysts formed by an impregnation method, the structures and the dispersion states of molybdenum oxide in the catalysts prepared by the alkoxide method were discussed.

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.

The effect of modifying alumina with sulfate and phosphate on the catalytic properties of Mo/Al 2O 3 in HDS reaction

The properties of alumina modified with sulfate (AlS) and phosphate (AIP) and Mo catalysts supported on modified alumina (Mo/AIP and Mo/AIS) were characterized by IR, XRD, DRS, ESR, temperature-programmed sulfidation (TPS), and the oxygen chemisorption. Catalytic activities of Mo/AIP and Mo/AlS were evaluated in the thiophene HDS reaction. The point of zero charge (PZC) of the modified alumina and the amount of Mo adsorbed on modified alumina decreased with increasing modifier content. The bonding mode of sulfate on alumina changed with increasing sulfate content. The decrease in the PZC of AlP and in the number of the adsorption sites for Mo anions on AlP facilitated the formation of polymolybdate on Mo/AlP. The sulfidation of Mo catalyst was also promoted in Mo/AlP. The catalytic activity in the thiophene HDS reaction of phosphate-modified catalysts was higher than that of Mo/Al 2O 3 when the phosphate content was less than 5 wt%. Bulk MoO 3 was formed more easily in Mo/AlS than in Mo/AIP. The suppression of the sulfidation of Mo/AlS may be explained by the formation of bulk MoO 3 and the presence of the S=0 group. The HDS activity of sulfate-modified catalysts was lower than that of Mo/Al 2O 3, regardless of the sulfate content.

Influence of phosphate on the structure of sulfided alumina supported cobalt-molybdenum catalysts

The influence of phosphate on the sulfiding rate of alumina-supported cobalt, molybdenum and cobalt-molybdenum catalysts has been studied by means of Temperature Programmed Sulfiding (TPS). Combination of the TPS results with recently published Temperature Programmed Reduction (TPR), Mössbauer spectrometry and nitrogen oxide adsorption measurements results in a detailed picture of the sulfidic catalyst. The sulfiding of cobalt ions in phosphorus-containing alumina-supported cobalt catalysts shifted to somewhat higher temperatures with increasing phosphorus content, most likely due to the presence of “Co-AlPO4” phases. In the temperature region from 1000-1270 K, part of the cobalt sulfide species reacted with AlPO4 to cobalt phosphides. The amount of cobalt which reduced to phosphides increased strongly with increasing phosphorus content and became about 70% of the total amount of cobalt present. The sulfiding pattern of phosphorus-containing Co-Mo/Al catalysts is only slightly influenced by increasing phosphorus content below 1000 K. All components including the Co-Mo-O-P phase, present in the oxidic precursor, are relatively easy to sulfide. After sulfiding at 673 K, the Co-Mo-S phase is most likely still to be associated with the AlPO4 species. Analogous to cobalt catalysts, the major part of the cobalt in Co-Mo-P/Al catalysts ultimately reacted to cobalt phosphides above about 1000 K. The sulfiding rate of the molybdenum ions is not changed by the presence of phosphates. MoS2 does not react with the phosphates to molybdenum phosphide probably because this reaction is thermodynamically restricted. Based on the surface structures, deduced from a combination of TPR and TPS data, the striking difference between the effect of phosphates on the hydrodenitrogenation (HDN) and on the hydrodesulphurisation (HDS) reactions is discussed.

Temperature-programmed sulfidation and oxidation of Ni-Mo/alumina catalysts and reaction with ammonia

Temperature-programmed sulfiding (TPS) of an oxidic Ni-Mo-P/Al 2O 3 catalyst and subsequent temperature-programmed oxidation (TPO) of the sulfided catalyst were performed. Treatments of the sulfided catalysts with ammonia-hydrogen, followed by TPO were also carried out, as well as reaction with n-butylamine-hydrogen at elevated pressure. The TPS results were in agreement with previous studies, showing three general stages of sulfiding: (1) initial exchange of catalyst surface oxygen for sulfur; (2) reduction of molybdenum accompanied by release of hydrogen sulfide; and (3) slow continued sulfiding of the residual oxygen. The TPO runs revealed the presence of hydrogen on the sulfided catalyst. Reaction of ammonia or butylamine with the sulfided catalysts provided evidence for strongly adsorbed nitrogen species, probably as NH or NH 2, on the catalyst.