(169e) Synergistic Application of XPS and DFT to Investigate Metal Oxide Surface Catalysis

Density Functional Theory (DFT), and experimental surface characterization techniques like X-ray Photoelectron Spectroscopy (XPS) are often used independently by computational and experimental researchers, respectively, to provide insights into metal oxide catalysed reaction mechanisms, pathways, and energetics [1, 2]. It is vital to develop novel approaches that integrate these surface sensitive experimental techniques and theoretical tools to gain better understanding of transition metal oxide (TMO) catalysed surface reactions. However, determining an appropriate Hubbard U-correction term[3] is a challenge for the DFT community[4] and identifying realistic reaction intermediates and their corresponding XPS shifts is a challenge for experimental researchers. In this work, using CuO as a model TMO, we demonstrate an integrated experimental and theoretical approach to (i) determine the optimum U value range for TMOs, and, (ii) identify adsorbate/intermediate species on the TMO surface (and their XPS shifts), in a synergistic manner. The O 1s spectra of the as-synthesized CuO 2D nonoleaves is measured using XPS and show the presence of 4 different peaks with core level binding energies (CLBEs) of 529.7 eV, 531.4 eV, 533.2 eV, and 534.6 eV. DFT+U is used to calculate the CLBE shifts for the possible adsorbed moieties (atomic and molecular oxygen, molecular and split water, carbon dioxide, and carbonate), in various configurations, on both, clean and vacancy defect containing surfaces. We scan the theoretical CLBEs over a range of U values and compare the experimental and theoretical CLBEs of the aforementioned adsorbates, to provide an appropriate U value that predicts the experimental shifts correctly. Among the structures, the experimental shift of (i) +1.7 eV falls only in the core-level shift range for O₂ in the η1(O) configuration (+0.56 to +1.87 eV), (ii) +3.5 eV falls only in the core-level shift range for the adsorbed H₂O at the surface oxygen vacancy site (+2.87 to +3.79 eV), and (iii) +4.9 eV falls only in the core-level shift range for the adsorbed HCO₂ (resembling the structure of HCO₃, +4.81 to +5.02 eV). In the process of U value determination, we parallelly uncover the existing surface moieties, their adsorption configurations and their XPS shifts. The U value between 4-5 eV correctly establishes the experimental XPS spectral peaks to the respective adsorbates and their geometries. This U value differs significantly from the bulk properties fitted U value of 7 eV and is in excellent agreement with our recent DFT+U benchmarking study[5] based on the heat of adsorption and Fourier Transform Infrared Spectroscopy measurements. The integrated approach elucidated in this article, results in the identification of adsorbates/intermediates (and their CLBEs) for the experimental XPS spectral analysis, in addition to appropriate U value determination for DFT calculations, concurrently. The method demonstrated in this paper can be extended to other TMOs, as well as to advanced surface characterization methods like temperature programmed XPS and in situ XPS studies to gain mechanistic insights into complex reactions and to guide the design and development of efficient and novel TMO based catalysts.

REFERENCES

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