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

Abstract

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 1s 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 1s 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 1s binding energies on Co surfaces, the 4.2 eV variation in the C 1s binding energies on Pt surfaces, and the 5.5 eV variation in the B 1s binding energies in the test sets with average deviations of 85, 73, and 53 meV, respectively. The shift in the C 1s and the B 1s 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/γ-Al2O3 catalysts was studied. The catalysts were investigated using XPS after reaction, and the measured C 1s 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 1s binding energy calculations helped identify the nature and location of the boron promoter on the Co/γ-Al2O3 catalyst.

Origin of the Formation of Nanoislands on Cobalt Catalysts during Fischer–Tropsch Synthesis

“…Since this frequency differs significantly from the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 range computed for bridge CO in the 2√3x2√3 structure, 1805-1815 cm -1 , and from the lowcoverage value of 1751 cm -1 , it might be an experimental fingerprint of CO at carbon-covered B5 sites. The C 1s binding energy for the C/CO-covered B5 sites, 283.0 eV, is typical for Co carbide species, 11,30 to the formation of the C/CO-covered step sites and nano-islands predicted in this study.…”

Improving the deconvolution and interpretation of XPS spectra from chars by ab initio calculations

“…While these results have given some clarification on the origins of the defect peaks, the effects of various potential structures on the C1s profile is difficult to determine due to the variety of possible chemical states [37]. To better determine the effects of structure and bonding, density functional theory (DFT) calculations have been used to analyze a wide variety of systems [38][39][40][41][42][43]. For these calculations binding energies are determined using several possible approximations, including; initial state, final state, and transition state approximations.…”

Computational and experimental evidence for a new TM–N3/C moiety family in non-PGM electrocatalysts

“…[6][7][8][9][10] While remarkable progress towards the implementation of highly active nitrogen doped transition metal electrocatalysts has been achieved in the past decade, [11][12][13][14] catalyst optimization is significantly hindered by the general lack of appropriate X-ray Photoelectron Spectroscopy reference spectra that allow to correlate defect abundance and ORR activity. 16,17,22 Chemical shifts can arise from a myriad of complex interactions, which often partially cancel, 23 such as electronegativity differences (charge transfer) and chemical valence. 10,[16][17][18][19][20][21] It has long been recognized that differences in binding energies (BE) arise due to a change in chemical environments of the species.…”

“…10,[16][17][18][19][20][21] It has long been recognized that differences in binding energies (BE) arise due to a change in chemical environments of the species. 23 Similarly, the experimental identification of Fe-N x defect motifs, particularly in highly active pyrolyzed ORR electrocatalysts are based on XPS observations (Fig. 16,17,22 Chemical shifts can arise from a myriad of complex interactions, which often partially cancel, 23 such as electronegativity differences (charge transfer) and chemical valence.…”