In-situ studies of catalysts by XAFS and Mössbauer spectroscopy

An EXAFS study on the so-called "cobalt-molybdenum-sulfur" phase in cobalt/carbon and cobalt-molybdenum/carbon, compared with a Moessbauer emission spectroscopy study

“…This is substantially higher than the value (2.19-2.21 Á) reported by Niemann et al25 and Bouwens et al23 It is remarkable that Bouwens et al found a much higher Co-S coordination number than Niemann et al, but the same coordination distance. One would expect as also discussed by Niemann et al25 that a coordination number of 6 is more probable for a Co-S distance of 2.26 A than for one as short as 2 21. A, which distance corresponds with that of the nearest-neighbor value of the tetrahedrally surrounded Co in Co9S8.…”

A DFT Study of CoMoS and NiMoS Catalysts: from Nano-Crystallite Morphology to Selective Hydrodesulfurization

“…Up to now, numerous experimental techniques have been employed to better characterize the catalytically "CoMoS" or "NiMoS" active phases [2,3]. Experimental techniques involved in the characterization of these complex phases consist of transition electron microscopy (TEM) [4,5], X-ray photoelectron spectroscopy (XPS) [6][7][8][9], Mössbauer spectroscopy [10][11][12], laser Raman spectroscopy [5,13], extended X-ray absorption fine structure (EXAFS) [14][15][16][17][18], and IR spectroscopy [19,20]. These techniques furnished many detailed features of the so-called Co(Ni)MoS active phase made of MoS 2 layers with a stacking close to 1 [21] and nanometer sizes (less than 30 Å), "decorated" by Co or Ni at the edges.…”

“…These techniques furnished many detailed features of the so-called Co(Ni)MoS active phase made of MoS 2 layers with a stacking close to 1 [21] and nanometer sizes (less than 30 Å), "decorated" by Co or Ni at the edges. While Mössbauer spectroscopy [10][11][12] and XPS [6][7][8][9] revealed the specific signature of Co atoms engaged in the new CoMoS phase, EXAFS gave relevant insights into the local environment of the promoter atoms (Co or Ni) in the structure [14][15][16].…”

Hydrogenolysis and β–elimination mechanisms for C S bond scission of dibenzothiophene on CoMoS edge sites

“…The active phase of HDS catalysts is made of Co or Ni promoted MoS 2 [1][2][3], and it was historically identified as a so-called "mixed CoMoS phase" [4,5]. Since these earlier findings, many experimental [2,[6][7][8][9][10] and theoretical [10][11][12][13] investigations have provided an ever deeper atomistic description of the potentially active sites located on the edges of the Co(Ni)MoS nanocrystallites. Nevertheless, challenging questions remain about the mechanism of transformation of sulfur containing polycyclic aromatic molecules such as derivatives of dibenzothiophene (DBT) on these edge-sites.…”