Henrik Topsøe

In situ Mössbauer emission spectroscopy studies of unsupported and supported sulfided CoMo hydrodesulfurization catalysts: Evidence for and nature of a CoMoS phase

Information regarding the structure and type of phases present in sulfided alumina-supported, as well as unsupported, Coue5f8Mo catalysts is obtained from in situ Mossbauer emission spectroscopy (MES) studies. The results give the first direct evidence of the presence of a Coue5f8Moue5f8S phase in alumina-supported and unsupported catalysts with similar CoMo ratios. Information regarding the nature of the Coue5f8Moue5f8S phase is obtained from a detailed study of the Mossbauer parameters, their temperature dependence and their sensitivity to changes in the gaseous environment. Previously proposed structural models cannot explain all the observed features of the Coue5f8Moue5f8S phase. It is proposed that in alumina-supported catalysts the Coue5f8Moue5f8S phase is present as single Sue5f8Moue5f8S slabs (i.e., one layer of the MoS2 structure) with cobalt most likely present at molybdenum sites. For unsupported catalysts the Coue5f8Moue5f8S phase consists of several slabs with bulk MoS2-like structure. The present observations suggest that the previously observed similarities between supported and unsupported catalysts are associated with the presence of the Coue5f8Moue5f8S phase in both catalyst systems. A possible reaction mechanism for hydrodesulfurization involving cobalt in the Coue5f8Moue5f8S phase is proposed. Information regarding other phases in the catalysts is obtained by MES studies of CoAl2O3, CoMo2S4, Co9S8, CoS2, Co3S4, and CoS1 + x samples. It is observed that for a sulfided Coue5f8MoAl2O3 catalyst with a composition typical of those used industrially, part of the cobalt is located in the alumina. For unsupported catalysts the effect of changing the cobalt concentration and preparation method is investigated and it is observed that under certain conditions also the thermodynamically stable cobalt sulfide, Co9S8, may be formed.

Characterization of the structures and active sites in sulfided CoMoAl2O3 and NiMoAl2O3 catalysts by NO chemisorption

Infrared and volumetric studies of NO adsorption have been used to elucidate the type of surface structures present in sulfided CoAl2O3, NiAl2O3, MoAl2O3, Coue5f8MoAl2O3, and Niue5f8MoAl2O3 catalysts. Catalytic implications were obtained from measurements of thiophene hydrodesulfurization (HDS) activities. The content of active material in the catalysts and calcination temperature were varied in these studies. The ir bands were observed to be very different for NO adsorbed on Co, Ni, and Mo atoms and are sensitive to the surface concentration of the element, the nature of the surface phase and the extent of sulfiding or reduction. For MoAl2O3 catalysts, the NO most probably adsorbs on edge or corner sites of MoS2-like structures and the adsorption therefore reflects the edge dispersion of these structures. In the case of the promoted Coue5f8MoAl2O3 and Niue5f8MoAl2O3 catalysts, the ir studies give simultaneous information on the NO adsorption occurring on the Co or Ni promoter atoms and that occurring on the Mo atoms. It is seen that the addition of promoter atoms results in a decreased adsorption on the Mo atoms. This indicates that the promoter atoms occupy edge positions of the MoS2 “support”. The Co atoms located in such positions are found to be related to those present in the so-called Coue5f8Moue5f8S structure identified previously by Mossbauer emission spectroscopy. Evidence for similar Ni-Mo-S type structures is found. The HDS activity correlated neither with the total amount of chemisorbed NO nor with the amount of NO adsorbed on the Mo atoms. However, for all the catalysts a good correlation was observed between the HDS activity and the amount of NO adsorbed on the Co or Ni promoter atoms. This further supports the “Coue5f8Moue5f8S model” in which the primary role of the promoter atoms is to create new sites (associated with the promoter atoms) with higher intrinsic HDS activity than that of the unpromoted sites. Pyridine, which is known to be a partial HDS poison, was observed to block a large fraction of the NO adsorption sites.

Catalyst and process technologies for ultra low sulfur diesel

Abstract The production of clean diesel by hydrotreating and deep hydrodesulfurization (HDS) has attracted increased attention recently due to the introduction of new environmental legislation regarding fuel specifications. In order to meet the specifications there is a need to modify and improve existing reactors and processes and to introduce more active and selective catalysts. The removal of sterically hindered sulfur-containing molecules is observed to be a key issue for deep HDS. Also the choice of operation conditions and reactor internals play an important role for deep HDS. The present article will focus on key hydrotreating options available to obtain ultra low sulfur diesel levels and some of the theoretical and experimental structure–activity relationships which may aid catalyst developments.

On the catalytic significance of a CoMoS phase in CoMoAl2O3 hydrodesulfurization catalysts: Combined in situ Mössbauer emission spectroscopy and activity studies

Abstract A series of sulfided Coue5f8Mo Al 2 O 3 catalysts with different Co Mo ratios but with constant molybdenum content is investigated. The catalysts are characterized by in situ Mossbauer emission spectroscopy (MES) and investigated for their thiophene hydrodesulfurization activity. The catalytic activity shows a pronounced maximum at a Co Mo ratio of about 1.0. The MES spectra reveal that cobalt may be present in three distinctly different phases: cobalt located in the alumina lattice (Co:Al 2 O 3 ), cobalt in Co 9 S 8 , and cobalt located in the Coue5f8Moue5f8S surface phase discussed in the preceding paper. It is found that the relative amounts of the three phases depend strongly on the Co Mo ratio. The Co:Al 2 O 3 phase and the Coue5f8Moue5f8S phase are observed in all catalysts studied, whereas Co 9 S 8 is observed only in catalysts with Co Mo ≳ 0.4 . It is shown that the presence of Co 9 S 8 cannot explain the promoting role of cobalt in the Coue5f8Mo Al 2 O 3 catalysts. However, a linear relation between the catalytic activity and the amount of Co in the Coue5f8Moue5f8S phase leads to the conclusion that the promoting effect of cobalt is associated with the presence of this phase.

Active sites and support effects in hydrodesulfurization catalysts

In the present article we will both review some of the current research on hydrodesulfurization (HDS) catalysts and present some new results. Five topics will be given special attention: (i) the nature of active sites in unpromoted and promoted HDS catalysts; (ii) the origin of promotional behaviors; (iii) studies of thermal ageing processes using high temperature sulfiding procedures; (iv) the nature of the phases present in catalysts after use under industrial conditions; and (v) support effects in carbon- and alumina-supported Co and Co-Mo catalysts. Regarding the first two topics, EXAFS results on Mo/Al2O3 catalysts show that the HDS activity is related to the concentration of MoS2 edge sites. For most promoted catalysts the activity behavior can be related to the concentration of promoted edge sites (related to Co-Mo-S) as revealed by Mossbauer emission spectroscopy (MES) or NO adsorption using infrared spectroscopy. High temperature sulfiding studies have been found useful to elucidate the structural changes occurring during industrial operation. For alumina-supported Co-Mo catalysts these high temperature sulfiding studies reveal the existence of a “low-temperature” (Type I) and a “high-temperature” (Type II) CoMoS structure. The differences in the activity of Type I and Type II CoMoS and different carbon- and alumina-supported catalysts are explained in terms of the nature of the interactions with the support. The interactions may lead to changes in the electronic properties of the active phase. In general, the highest activity is observed for systems exhibiting the highest “metallic” character (e.g., least oxygen in the structure).

A combined X-Ray photoelectron and Mössbauer emission spectroscopy study of the state of cobalt in sulfided, supported, and unsupported CoMo catalysts

Abstract Alumina-supported and unsupported Co-Mo catalysts, as well as Co metal, Co 9 S 8 , and CoMo 2 S 4 samples, have been studied using X-ray photoelectron spectroscopy (XPS) and Mossbauer emission spectroscopy (MES). The main aim of the study was to examine the feasibility of using XPS to characterize the different Co-containing phases which may be present in sulfided Co-Mo catalysts. The Co phase distributions in the catalyst samples studied by XPS were determined by means of MES. The different cobalt phases observed in the catalysts were Co-Mo-S and Co 9 S 8 , and for the supported catalysts cobalt in the alumina lattice was also observed. Although Co metal, Co 9 S 8 , Co-Mo-S, and CoMo 2 S 4 are structurally and chemically different and give rise to very different MES spectra, the Co 2p spectra of these compounds are similar. It is shown, however, that by a combination of accurate determinations of binding energy differences and comparisons of peak shapes it is possible to distinguish the different Co phases in the catalysts by XPS. The Co 2p binding energies of Co 9 S 8 are about 0.5 eV smaller than those of Co-Mo-S, and also the Co 2p peak shapes are different. Using XPS, Co 9 S 8 can only be distinguished from Co metal by a detailed comparison of the Co 2p peak shapes. The close similarity between the Co 2p spectra of Co-Mo-S and CoMo 2 S 4 suggests that the electronic state of Co in Co-Mo-S is similar to that in CoMo 2 S 4 . However, the MES results show that the two phases are structurally different.

Structure sensitivity of supported ruthenium catalysts for ammonia synthesis

Abstract The catalytic ammonia synthesis activities of four supported ruthenium catalysts are reported. It is seen that Ru/MgAl2O4 is more active than two similar Ru/C catalysts, which are significantly more active than Ru/Si3N4. The activity differences cannot be satisfactorily explained solely by the differences in dispersion. Recent results from single crystal studies and DFT calculations have shown that ammonia synthesis over ruthenium catalysts is a very structure sensitive reaction, more so than on iron catalysts. It is suggested that special B5-type sites are primarily responsible for the catalytic activity of the present supported Ru catalysts. It is shown how the number of such B5-type sites depends on the Ru crystal size for a given crystal morphology. We have found that the activity of the Ru/MgAl2O4 catalyst increases significantly during the initial part of a test run. This activity increase is paralleled by the disappearance of crystals smaller than ca. 1.0xa0nm due to sintering and a resulting formation of larger crystals. We conclude that there exists a lower limit to the desired crystal size of Ru in supported ammonia synthesis catalysts. This is in agreement with a low number of B5-type sites expected for such crystal sizes. Furthermore, we suggest that the support plays a decisive role in controlling the morphology of the Ru crystals and the resulting change in abundance of B5-type sites is the main cause for the significant activity variations observed for Ru catalysts with different supports. Finally, the support may also influence the electronic and catalytic properties of neighboring B5-type sites.

Adsorption studies on hydrodesulfurization catalysts: I. Infrared and volumetric study of NO adsorption on alumina-supported Co, Mo, and Co-Mo catalysts in their calcined state

Abstract Infrared and volumetric studies were made of NO adsorbed on Co Al 2 O 3 , Mo Al 2 O 3 , and Co-Mo Al 2 O 3 oxide catalysts in order to elucidate the nature and amount of exposed Co and Mo ions. The effect of varying the cobalt loading, calcination temperature, and sample pretreatment conditions was studied. Bulk samples of Co 3 O 4 and CoAl 2 O 4 were also investigated. All Co-containing samples and catalysts give rise to two main infrared bands around 1780–1805 and 1850–1890 cm −1 . The details of the adsorption vary from system to system but in all cases NO seems to be predominantly adsorbed as a dinitrosyl (or dimeric) species. Low-loading Co Al 2 O 3 catalysts have a majority of the cobalt atoms located at the surface, in octahedral coordination. The remaining cobalt atoms are probably in tetrahedral coordination inside the alumina. As the cobalt loading increases the total number of cobalt atoms capable of adsorbing NO passes through a maximum, at which point formation of a separate Co 3 O 4 phase is observed. This is accompanied by a decrease in the amount of cobalt atoms at the alumina surface. Upon increasing the calcination temperature some cobalt atoms move from octahedral sites at the surface to tetrahedral positions inside the alumina. Adsorption of NO on the Co-Mo Al 2 O 3 catalyst leads to an adsorption which differs from that on all the Co Al 2 O 3 catalysts. The results give evidence for formation of a Co-Mo interaction phase which is different from well-crystallized CoMoO 4 as well as the interaction phase observed for silica-supported catalysts. Increasing the calcination temperature causes a decrease in the number of cobalt atoms associated with the Co-Mo interaction phase. In contrast to previous studies, it is observed that CoAl 2 O 4 adsorbs NO and that this is related to the spinel structure not being completely normal. Calcined Mo Al 2 O 3 catalysts only adsorb minor amounts of NO depending on the degree of reduction which has occurred during the sample pretreatment. The dominant adsorption seems to be in the form of dinitrosyls with bands around 1690–1710 and 1800–1810 cm −1 .

Sulfur bonding in MoS2 and Co-Mo-S structures

The structure and bonding in small MoS2 structures with and without Co is studied theoretically using self-consistent density functional theory with a non-local exchange-correlation energy. The structures model the catalysts used extensively in hydrotreating. We study in detail the structure and binding energies as a function of the amount of sulfur. The calculations show that extensive reconstructions occur at the two types of MoS2 edges where the sulfur dimerizes and occupies non-lattice positions. These structures are shown to be in good agreement with available experimental data. We also study the energy required to form sulfur vacancies, which are believed to be the active sites for many hydrotreating reactions. The presence of Co atoms at the edges is shown to lead to a significant lowering of the metal-sulfur binding energy. This imposes an increase in the concentration of active sites for the reactions and may thus explain the promoting effect of Co.

Chemistry of one-dimensional metallic edge states in MoS2 nanoclusters

Nanostructures often have unusual properties that are linked to their small size. We report here on extraordinary chemical properties associated with the edges of two-dimensional MoS2 nanoclusters, which we show to be able to hydrogenate and break up thiophene (C4H4S) molecules. By combining atomically resolved scanning tunnelling microscopy images of single-layer MoS2 nanoclusters and density functional theory calculations of the reaction energetics, we show that the chemistry of the MoS2 nanoclusters can be associated with one-dimensional metallic states located at the perimeter of the otherwise insulating nanoclusters. The new chemistry identified in this work has significant implications for an important catalytic reaction, since MoS2 nanoclusters constitute the basis of hydrotreating catalysts used to clean up sulfur-containing molecules from oil products in the hydrodesulfurization process.