Elsevier

Catalysis Today

Volume 130, Issue 1, 15 January 2008, Pages 86-96
Catalysis Today

Recent STM, DFT and HAADF-STEM studies of sulfide-based hydrotreating catalysts: Insight into mechanistic, structural and particle size effects

https://doi.org/10.1016/j.cattod.2007.08.009Get rights and content

Abstract

The present article will highlight some recent experimental and theoretical studies of both unpromoted MoS2 and promoted Co–Mo–S and Ni–Mo–S nanostructures. Particular emphasis will be given to discussion of our scanning tunnelling microscopy (STM), density functional theory (DFT), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) studies which have provided insight into the detailed atomic structure. In accordance with earlier theoretical studies, the experimental studies show that the Ni–Mo–S structures may in some instances differ from the Co–Mo–S analogues. In fact, the Co–Mo–S and Ni–Mo–S structures may be even more complex than previously anticipated, since completely new high index terminated structures have also been observed. New insight into the HDS mechanism has also been obtained and complete hydrogenation and hydrogenolysis pathways for thiophene hydrodesulfurization (HDS) have been calculated on the type of structures that prevail under reaction conditions. It is seen that important reaction steps may not involve vacancies, and special brim sites are seen to play an important role. Such studies have also provided insight into inhibition and support effects which play an important role in practical HDS. Recent STM studies have shown that MoS2 clusters below 2–3 nm may exhibit new structural and electronic properties, and a large variety of size-dependent structures have been identified. In view of the large structure sensitivity of hydrotreating reactions this is expected to give rise to large effects on the catalysis.

Introduction

In recent years, new legislation regarding the sulfur content in transport fuels has resulted in the demand for ultra low sulfur diesel (ULSD), and this has introduced new challenges for hydrodesulfurization (HDS) in the refining industry [1], [2], [3], [4], [5], [6], [7], [8], [9]. In addition, the demand for diesel fuels is increasing, and as the availability of light petroleum resources decreases, increasingly heavy feedstocks have to be refined. In order to achieve the higher sulfur conversion, very refractory sulfur compounds, like dialkylated dibenzothiophenes (DBT), need to be removed [1], [3], [4], [5], [6], [10]. It has been known for some time that the conversion of the sterically hindered DBTs mainly proceeds via a pre-hydrogenation route (HYD) rather than the direct desulfurization route (DDS), which dominates for molecules like DBT [3]. However, under industrial conditions, the presence of other compounds in the feed often changes the relative role of the HYD and the DDS pathways. In particular, specific basic nitrogen-containing compounds inhibit HDS, and these compounds are observed to mainly inhibit the HYD pathway [11], [12], [13]. Furthermore, H2S is an HDS inhibitor, and interestingly, it mainly inhibits the DDS rather than the HYD pathway [1]. To improve HDS catalysts and gear them to the increasingly heavy feedstocks, detailed understanding of their mechanistic action is necessary so that targeted modifications can be made.

In order to elucidate the HDS reaction and the two different HYD and DDS pathways in detail, it is important to characterize the active nanostructures and in particular to identify the active sites for the two pathways. Until the early 1980s, very little information was available on the structure of active hydrotreating catalysts. A key discovery was the identification of the MoS2 and Co–Mo–S structures by EXAFS, Mössbauer and infrared techniques, and it was shown that the Co–Mo–S structure was responsible for the promotion of catalytic activity [14], [15], [16], [17], [18]. These results revealed that Co–Mo–S (and also Ni–Mo–S) structures are small MoS2-like nanocrystals, where the promoter atoms are located at the edges of the MoS2 layers. The results furthermore suggested that Co atoms are located in the same plane as Mo, but that their local coordination is different. In spite of the significant progress, it was for a long-time difficult to address the issue of the detailed edge structure of unpromoted and promoted MoS2, as no atomic-resolved structures could be obtained. As a consequence, it has also been difficult to understand the nature of HYD and DDS pathways and sites. Recently, we have achieved a large breakthrough in the structural studies of the active nanostructures in HDS using scanning tunnelling microscopy (STM) to image the real-space structure of MoS2 nanoclusters grown on flat model substrates. With the STM, it was possible for the first time to reveal the equilibrium morphology of the nanoclusters. Furthermore, atomic-resolution STM images made it possible to elucidate the detailed structure of the catalytically important edges, the sulfur coverage and the location of sulfur vacancies, which are normally considered to be active sites [19]. In further studies, we have also managed to synthesize and characterize the atomic-scale structure of Co–Mo–S and Ni–Mo–S and thereby it has been possible to obtain information on the location of the Co and Ni promoter atoms [20], [21]. Recently, MoS2, WS2 and promoted structures were studied by another new technique, high-angle annular dark-field scanning tunnelling electron microscopy (HAADF-STEM), and additional information on the morphology of MoS2- and WS2-based nanostructures could be obtained [22], [23], [24]. In most of the STM studies, gold was used as a support of the sulfide nanostructures. Since gold is a weakly interacting support, the studies have provided important insight into the intrinsic properties of the nanostructures. In industrial HDS catalysts, the support usually plays a significant role and the STM studies have recently been extended to carbon-supported systems [25]. Many earlier studies have indicated that hydrotreating reactions are extremely sensitive structure [1]. One may therefore expect that the reactions will depend strongly on the particle size, but not much has been known about such effects. Recently, STM has for the first time provided us with atom resolved images of MoS2 clusters of different sizes [26]. Many size-dependent structural and electronic changes were observed and such effects must clearly also be taken into account when addressing the catalysis.

First principles modelling techniques, like density functional theory (DFT), have over the last decade provided increasing insight into atomic structure and reactivity of the active phases of HDS catalysts. DFT can often provide information that is complementary to the multitude of experimental information, and the synergy of theoretical and experimental approaches can thus give a very detailed picture of catalyst structure and reactivity. After our initial studies of Co–Mo–S [27], [28], [29], we have recently used this approach in a series of studies, where STM and DFT were combined to obtain insight into the structure of unpromoted and promoted MoS2 under different conditions [30], [31], [32], [33], [21]. The general approach of combining DFT with the chemical potential of the gas phase can be used to connect DFT calculations performed at 0 K and in vacuum to reaction conditions with relevant temperatures and pressures [34], [35], [36], [37], [38], [39].

The very powerful combination of STM experiment and DFT calculations has led to several important findings in our studies, and one of the most significant results was the discovery of the so-called brim states and their role in HDS catalysis. It was found that the Mo edge exhibits a special electronic edge state, which can easily be identified in STM images of the nanoclusters as a very bright brim extending along the edges (see e.g. Fig. 1). These brim states arise from a perturbation of the electronic structure near the edges relative to the interior part of the clusters. Detailed analysis using DFT revealed the presence of edge states, which are metallic states that are localized at the cluster edges and give rise to the brim states [30]. Quite surprisingly, it was observed that these states possess reactivity towards the hydrogenation of thiophene, which could be observed using STM [31], [32]. Thus, insight into these sites is essential for understanding hydrotreating reactions.

The ever increasing computational power makes it possible to study increasingly complex systems, and in recent years a number of reports on catalyst-support interactions have been published [40], [41], [42], [43], [44]. Also, the reaction pathway of thiophene and thiophene derivatives on MoS2 have been studied by us [45] and several other groups [46], [47], [48], [49] and thus, we can begin to understand reaction pathways and find descriptors for catalytic activity.

In this review, we will highlight some of the above-mentioned developments. In Section 2, we discuss the recent STM and DFT studies of promoted CoMoS and NiMoS structures as well as HAADF-STEM studies on unpromoted and promoted MoS2 and WS2. In Section 3, we summarize the results concerning support interactions and in Section 4 we discuss recent developments concerning reaction pathways and inhibition. In Section 5, recent STM results regarding size effects are described.

Section snippets

Structure of MoS2, Co–Mo–S and Ni–Mo–S

According to the now well-accepted Co–Mo–S model for the promoted MoS2 hydrotreating catalysts, the Co and Ni promoter atoms are located at edge positions of MoS2 nanostructures. Their substitution of Mo at edge sites is believed to enhance vacancy formation and the creation of new and more active sites. Several studies have been carried out to correlate the structure of the active promoted phases to the activity [1], [15], [17], but the exact location and coordination of the promoter atoms

Support interactions

The role of support interactions has been an important topic in catalysis research for many years, since the catalytic properties of MoS2 are significantly influenced by the support [1]. The most common support is high-surface area alumina, since it allows for the production of small stable nanoclusters of MoS2. Preparation conditions influence the activity significantly, e.g. it has been observed that an increase in sulfidation temperature resulted in the formation of modified Co–Mo–S

Hydrogenation and direct desulfurization reaction routes

As discussed in Section 2, STM images have clearly revealed that MoS2, Co–Mo–S, and Ni–Mo–S expose bright brims at the edges [19], [20], [21]. Using DFT, these brims have been shown to be the result of one or more metallic edge states [30], [38]. Combined STM and DFT studies have investigated thiophene HDS over MoS2 particles at STM conditions, and it was found that fully sulfided MoS2 particles which have a bright brim are able to hydrogenate thiophene and make 2,5-dihydrothiophene.

Size effects

It is well-known that materials scaled down to particles in the nanometer regime may adopt new and interesting structural and electronic properties that are significantly different from those observed in bulk systems [98]. Numerous studies have shown that this also may lead to unique catalytic properties. For example, catalysts based on gold nanoparticles supported on a metal oxide have indeed been shown to exhibit interesting size-dependent activities for low temperature oxidation reactions

Conclusions and outlook

Using a combination of novel experimental and theoretical techniques like STM, DFT and HAADF-STEM, we have recently gained further insight into structure, support, size and reactivity effects in hydrotreating catalysis. One picture which emerges from these studies is the important concept of the special “brim sites”, which we have shown to exhibit catalytic activity for hydrogenation reactions. This is quite contrary to the common belief that vacancy sites are the key active sites, since the

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