Herman Pieter Charles Eduard Kuipers

Quantitative photoelectron spectroscopy as applied to non-ideal surfaces

Abstract X-ray photoelectron spectroscopy (XPS) is an increasingly popular tool for the quantitative characterization of solid surfaces. For this purpose model expressions are required which explicitly take into account the geometric distribution of the various surface phases. Such expressions can be (and have been) established for “ideal” cases such as single crystals and other flat, well-defined and well-prepared samples. Practical samples, such as fracture surfaces, scales, granules, powders and in particular heterogeneous catalysts seem to have ill-defined surfaces. However, the randomness of the surface layer of such samples will be shown to be beneficial rather than disastrous for the quantification. This is due to the fact that complete randomness is frequent and - in a sense-ideal, as it can be used as a mathematical key to the description of XPS signals. It will be shown that one simple assumption, namely that all angles of orientation in space of a differential surface element are of equal probability, leads to model expressions that are only a little more inconvenient than those for “ideal” cases. Furthermore, the dependence of XPS signals on the shape of interfacial particles is scrutinized. This leads to the convenient finding that, for random samples, XPS signals are proportional to the surface-to-volume ratio of the dispersed phases. Thanks to the increase in the absolute accuracy of XPS-derived surface quantities which this represents, it becomes possible to determine the locations of the various compounds with respect to each other. To this end the results of XPS have to be linked to those of other independent techniques. The usefulness of this approach will be illustrated with an example.

Shape and Size of Cobalt Nanoislands Formed Spontaneously on Cobalt Terraces during Fischer–Tropsch Synthesis

Cobalt-based catalysts undergo a massive and spontaneous reconstruction to form uniform triangular nanoislands under Fischer-Tropsch (FT) conditions. This reconstruction is driven by the unusual and synergistic adsorption of square-planar carbon and CO at the 4-fold edge sites of the nanoislands, driving the formation of triangular islands. The size of the nanoislands is determined by the balance between energy gain from creating C/CO-covered edges and energy penalty to create C/CO-covered corners. For carbon chemical potentials corresponding to FT conditions, triangular Co islands with 45 Co atoms (about 2 nm) are the most stable surface structure. Decreasing the carbon chemical potential and hence the stability of square-planar carbon favors the formation of larger islands, until reconstruction becomes unfavorable and CO-covered terraces are thermodynamically the most stable. The predicted structure of the islands is consistent with in situ scanning tunneling microscopy images obtained for the first time under realistic FT reaction conditions on a Co(0001) surface.