Abstract
Core-level X-ray Photoelectron Spectroscopy (XPS) is often used to study the surfaces of heterogeneous copper-based catalysts, but the interpretation of measured spectra, in particular the assignment of peaks to adsorbed species, can be extremely challenging. In this study we present a computational scheme which combines the use of slab models of the surface for geometry optimization with cluster models for core electron binding energy calculation. We demonstrate that by following this modelling strategy first principles calculations can be used to guide the analysis of experimental core level spectra of complex surfaces relevant to heterogeneous catalysis. The all-electron ΔSCF method is used for the binding energy calculations. Specifically, we calculate core-level binding energy shifts for a series of adsorbates on Cu(111) and show that the resulting C1s and O1s binding energy shifts for adsorbed CO, CO2, C2H4, HCOO, CH3O, H2O, OH, and a surface oxide on Cu(111) are in good overall agreement with the experimental literature.
Highlights
Metallic copper and copper nanoparticles play an important role in industrially relevant catalytic processes, such as the lowtemperature water gas shift reaction[1,2,3] and the synthesis of methanol from CO2 and H2.4–6 Considerable efforts have been directed towards understanding the mechanisms that operate in these systems and X-ray photoemission spectroscopy (XPS) has been the tool of choice in many experimental studies.[7,8,9,10,11,12,13,14,15,16,17]
In order to assess the accuracy of our calculations, additional C1s binding energy calculations were carried out for the free molecules CH4, C2H6, CO, CO2, CCl4, and CF4, and the O1s binding energy was calculated for H2O, CO, CO2, CH3OH, and HCOOH
It is possible to compare the absolute values of the theoretical binding energies to the experimental data for the free molecules, and for the C1s and O1s core levels considered in this work, we find that the values agree to within B0.3% for M06 and B0.5% for PBE
Summary
Metallic copper and copper nanoparticles play an important role in industrially relevant catalytic processes, such as the lowtemperature water gas shift reaction[1,2,3] and the synthesis of methanol from CO2 and H2.4–6 Considerable efforts have been directed towards understanding the mechanisms that operate in these systems and X-ray photoemission spectroscopy (XPS) has been the tool of choice in many experimental studies.[7,8,9,10,11,12,13,14,15,16,17] XPS is attractive for the characterization of surfaces because it provides information about the elemental composition of the surface as well as the chemical states of the elements. Benchmark calculations on molecular systems indicate that DSCF calculations based on DFT yield binding energies shifts within 0.3 eV of the experimental values.[41,42,43]. This accuracy is significantly higher than reported binding energy ranges for many adsorbates on Cu surfaces and insights from theoretical calculations should be very useful for the interpretation of experimental core level spectra. We compare our calculated C1s and O1s binding energy shifts for CO, CO2, ethene, formate, methoxy, water, OH, and a surface oxide on Cu(111) in detail with the available experimental literature and discuss the implications of our theoretical results on the interpretation of experimental spectra
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