In Situ Detection of Active Edge Sites in Single-Layer MoS2_2 Catalysts

MoS2 nanoparticles are proven catalysts for processes such as hydrodesulphurization and hydrogen evolution, but unravelling their atomic-scale structure under catalytic working conditions has remained significantly challenging. Ambient pressure X-ray Photoelectron Spectroscopy (AP-XPS) allows us to follow in-situ the formation of the catalytically relevant MoS2 edge sites in their active state. The XPS fingerprint is described by independent contributions to the Mo3d core level spectrum whose relative intensity is sensitive to the thermodynamic conditions. Density Functional Theory (DFT) is used to model the triangular MoS2 particles on Au(111) and identify the particular sulphidation state of the edge sites. A consistent picture emerges in which the core level shifts for the edge Mo atoms evolve counter-intuitively towards higher binding energies when the active edges are reduced. The shift is explained by a surprising alteration in the metallic character of the edge sites, which is a distinct spectroscopic signature of the MoS2 edges under working conditions

Universal coding for distributions over co-trees

For an arbitrary ordered set, we consider the model of all distributions P for which an element which precedes another element is considered the more significant one in the sense that the implication a ≤ b ⇒ P(a) ≥ P(b) holds. It will be shown that if the ordered set is a finite co-tree, then the universal predictor for the indicated model or, equivalently, the corresponding universal code, can be determined exactly via an algorithm of reasonably low complexity

In-situ studies of catalysts by XAFS and Mössbauer spectroscopy

The unique advantages of XAFS and Mössbauer spectioscopy applied to studies of catalysts are discussed. These advantages include the importantin situ capability, as well as the possibility to provide information on X-ray amorphous materials. The successful use of the two techniques is demonstrated by examples of two important technical catalyst systems, hydrotreating and ammonia synthesis catalysts, representing supported and unsupported catalysts

X-Ray absorption studies of the Ni environment in Ni-Mo-S

The local environment of the Ni atoms in the Ni-Mo-S phase has been elucidated by Ni K-edge EXAFS and XANES measurements of carbon-supported sulfided Ni-Mo catalysts. The results show that the Ni atoms have a low sulfur coordination number (less than six). This is contrary to recent reports in which an octahedral-like sulfur coordination have been suggested. It is proposed that the Ni atoms in Ni-Mo-S are located at the(1010)(1010) edges of the MoS$_2$ structure in square and tetragonal pyramidal type sites. These two types of sites can easily interconvert during sulfur addition/extraction and may as such be involved in the catalytic cycle