Bjerne S. Clausen

Catalytic activity of Au nanoparticles

Au is usually viewed as an inert metal, but surprisingly it has been found that Au nanoparticles less than 3–5 nm in diameter are catalytically active for several chemical reactions. We discuss the origin of this effect, focusing on the way in which the chemical activity of Au may change with particle size. We find that the fraction of low-coordinated Au atoms scales approximately with the catalytic activity, suggesting that atoms on the corners and edges of Au nanoparticles are the active sites. This effect is explained using density functional calculations.

Size-dependent structure of MoS2 nanocrystals

Molybdenum disulphide nanostructures are of interest for a wide variety of nanotechnological applications ranging from the potential use of inorganic nanotubes in nanoelectronics to the active use of nanoparticles in heterogeneous catalysis. Here, we use atom-resolved scanning tunnelling microscopy to systematically map and classify the atomic-scale structure of triangular MoS2 nanocrystals as a function of size. Instead of a smooth variation as expected from the bulk structure of MoS2, we observe a very strong size dependence for the cluster morphology and electronic structure driven by the tendency to optimize the sulphur excess present at the cluster edges. By analysing of the atomic-scale structure of clusters, we identify the origin of the structural transitions occurring at unique cluster sizes. The novel findings suggest that good size control during the synthesis of MoS2 nanostructures may be used for the production of chemically or optically active MoS2 nanomaterials with superior performance.

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 CoMo 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 CoMoS 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 CoMoS 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 CoMo Al 2 O 3 catalysts. However, a linear relation between the catalytic activity and the amount of Co in the CoMoS 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.

In situ cell for combined XRD and on-line catalysis tests: Studies of Cu-based water gas shift and methanol catalysts

A newly developed in situ X-ray diffraction (XRD) cell has been used to obtain information on the structure of binary CuZn and ternary CuZnAl catalysts during reduction and water gas shift and methanol synthesis. A major advantage of the cell is that it also serves as an ideal plug flow catalytic reactor such that realistic catalytic and structural information can be obtained simultaneously on the same sample. The cell can be operated both at high temperatures and high pressures. Direct methanol activity tests confirmed the suitability of the cell. By use of X rays from a synchrotron source, dynamic studies on the time scale of seconds have been demonstrated. This feature was used to study the phase transformation occurring during the activation of the calcined catalysts. In the active catalysts, Cu metal is the only crystalline Cu phase observed, and the formation of this phase is seen to be closely related to the disappearance of CuO in the calcined catalyst. The XRD results provide detailed information on the nucleation and growth processes. The variation in the water gas shift activity appears to correlate with the changes in the copper surface area.

A combined QEXAFS/XRD method for on-line, in situ studies of catalysts: Examples of dynamic measurements of Cu-based methanol catalysts

The development of a new in situ method which allows high quality XRD and EXAFS to be obtained on-line under ideal catalytic conditions is discussed. The possibility to obtain both types of information simultaneously enables a much better structural description of catalytic materials which typically contain both crystalline and X-ray amorphous structures. The system employs a capillary tube as the microreactor/in situ cell. The high degree of temperature uniformity of the cell and the possibilities of fast changes in reaction conditions make the system ideal for dynamic studies. For this purpose, the EXAFS measurements are carried out in the newly developed quick scanning mode (QEXAFS) which also allows high quality data to be obtained. The XRD is acquired using a position sensitive detector. The application of the setup for time resolved measurements is demonstrated in a study of the calcination and reduction of Cu-based methanol catalysts where the changes take place over a few degrees. The high quality of the data made it possible to obtain important new insight regarding the presence of intermediate phases during these processes. Studies of Cu/SiO2 catalysts show the advantages of a newly developed theory for a better estimation of coordination numbers (and thus particle sizes) from EXAFS.

Combining XRD and EXAFS with on-Line Catalytic Studies for in situ Characterization of Catalysts

Advances in the study of catalysts by in situ structural characterization, using X-ray absorption spectroscopy (XAFS) and X-ray diffraction (XRD), have recently been achieved and they are illustrated by reference to several examples. Emphasis in the present overview is laid on the study of catalysts under realistic working conditions and therefore in particular on-line gas analysis during activation and/or during catalysis. The examples encompass (a) activation of copper-based methanol catalysts, (b) dynamic changes of copper-based catalysts during methanol synthesis conditions, (c) catalytic partial oxidation on Rh/Al2O3 catalysts, and (d) reduction (activation) of copper-promoted high temperature shift catalysts.

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.

Mössbauer emission studies of calcined CoMoAl2O3 catalysts: Catalytic significance of Co precursors

The nature of Co in calcined CoMoAl2O3 catalysts and the changes that occur upon sulfiding have been investigated by Mossbauer emission spectroscopy (MES). The studies of CoMoAl2O3 catalysts show that changes in the Co loading, calcination temperature, and impregnation procedure significantly alter the Co phase distribution. In catalysts with low Co loading, two different types of Co species are generally present: one is identified as tetrahedrally coordinated Co (Cotet) and another is ascribed to cobalt atoms located in octahedral-like coordination (Cooct). The Cooct species, which seem to be located at or close to the surface of the alumina, dominate at low calcination temperatures, whereas calcination at high temperatures favors formation of the Cotet species which appear to be predominantly located in the interior of the alumina. The Cotet species have MES parameters different from those of CoAl2O4. Catalysts with high Co loadings also contain Co3O4. The concentrations of the different Co species in the various calcined CoMoAl2O3 catalysts have been compared with the corresponding data for the same catalysts after sulfiding. It is found that the fraction of Co which is present in the alumina after sulfiding is related to the amount of Cotet, the amount of Co9S8 is related to Co3O4, and Co in the catalytically active CoMoS phase is related to COoct. As a consequence of the latter, the thiophene hydrodesulfurization activity is related to the amount of Cooct in the calcined catalysts.