Evaluating the Structure of Catalysts Using Core-Level Binding Energies Calculated from First Principles

X-ray photoelectron spectroscopy (XPS) is a powerful and popular surface characterization technique, and the measured shifts in the core electron binding energies are sensitive to the chemical structure and local environment of the surface species. C Is binding energies were calculated with density functional theory (DFT) for 17 structures including eight well-characterized structures on a Co(0001) surface and nine on a Pt(111) surface, while B Is binding energies were calculated for six well-characterized structures and compared with experimental values. DFT calculations describe the 2.8 eV variation in the C Is binding energies on Co surfaces, the 4.2 eV variation in the C Is binding energies on Pt surfaces, and the 5.5 eV variation in the B Is binding energies in the test sets with average deviations of 85, 73, and 53 meV, respectively. The shift in the C Is and the B Is binding energies can be correlated with the calculated charges, though only within homologous series. To illustrate how binding energy calculations can help elucidate catalyst structures, the nature of the resilient carbon species deposited during Fischer Tropsch synthesis (FTS) over Co/gamma-Al2O3 catalysts was studied. The catalysts were investigated using XPS after reaction, and the measured C Is binding energies were compared with DFT calculations for various stable structures. The XPS peak at 283.0 eV is attributed to a surface carbide, while the peak at 284.6 eV is proposed to correspond to remaining waxes or polyaromatic carbon species. Boron promotion has been reported to enhance the stability of Co FTS catalysts. Again, the combination of XPS with DFT B Is binding energy calculations helped identify the nature and location of the boron promoter on the Co/gamma-Al2O3 catalyst.