Elucidating the K‐Edge X‐Ray Absorption Near‐Edge Structure of Cobalt Carbide

Abstract

The cobalt K‐edge X‐ray absorption near‐edge structure of a cobalt carbide compound, synthetized under in‐situ conditions, has been determined by using laboratory‐based and synchrotron‐based X‐ray absorption spectroscopy. We have carburized pure cobalt metal in‐situ, avoiding all adverse effects of metal‐support interactions as well as any degradation of the cobalt carbide formed. A non‐negative matrix factorization was applied to determine the features of the spectrum of cobalt carbide, highly needed to identify this phase in an unambiguous manner in cobalt‐based Fischer‐Tropsch catalysts.

Synthesis, characterisation and water–gas shift activity of nano-particulate mixed-metal (Al, Ti) cobalt oxides

“…The mixed-metal oxides Co1+xAl2-xO4 and CoTiO3 were further characterised by XANES at the Co K-edge. The spectra obtained strongly resemble published spectra of cobalt aluminate 17,48,49 and cobalt titanate 15 confirming the successful preparation of the particular mixed-metal cobalt oxides (Figure 4). The white line characteristics for single phase and mixed-metal cobalt oxides dominate the near-edge structure, while unique features at energies above the white line allow for a clear distinction between the cobaltous species.…”

“…3,10 Hence, a potential WGS activity has been associated to oxidic cobalt species, 4,5,[7][8][9] which may form upon phase transformation of the metallic FT active Co 0 phase during intrinsic exposure to H2O-rich high conversion environment. [11][12][13][14][15] In particular, the formation of CoO and metal-support compounds have been suggested to drive this change in catalytic WGS activity. 4,5,[7][8][9] In general, the formation of CoO 4,12,13,[16][17][18][19] and mixed-metal cobalt oxides [11][12][13][14][15] is expected at high FT conversion levels due to the increased concentration of H2O, which may induce oxidation and/or a solid state reaction between the active metallic Co 0 phase and widely applied metal oxide carriers (Al2O3, SiO2, TiO2) resulting in metal-support compounds (MSCs).…”

“…11,13,17 Recently, the formation of CoAl2O4 and CoTiO3 has been directly monitored by means of in situ XANES during FTS over a Re-Co/Al2O3 and a Co/TiO3 catalyst, respectively. 13,15 2H2 + CO → (CH)2 + H2O…”

On isothermality in some commonly used plug flow reactors for X-ray based investigations of catalysts

“…A question that obviously arises is: to what degree these fundamentals are achieved within the various experimental solutions developed for operando study of catalysis using Xrays? In this respect some very laudable and often detailed studies have appeared [6][7][8][9][10][11][12][13][14][15] regarding how appropriate some reactor designs are for reliably obtaining different types of information, and to what degree some reactors might be deemed to conform to the required starting point. In the current case we shall specifically address the notion of isothermality, how well it may be achieved in two types of commonly used tubular microreactors.…”

“…Further, and for the most part, detailed assessments of whether subsequently developed sample environments are indeed radially and axially isothermal are not as common as perhaps they ought to be, though examples do exist where this sort of exercise has been undertaken. [8][9][10][11][12][13][14][15] There are also some elegant studies using similar systems that make use of infrared thermographic imaging to follow catalyst light off and the migration of reactive wavefronts through catalyst beds for suitably exothermic reactions. 22 In these cases, as the main method used is thermal imaging, any pre-existing thermal gradients that might exist should be immediately apparent.…”