Oxygen reduction reaction (ORR) plays a vital role in various energy conversion and storage systems (such as fuel cells and metal-air batteries), and understanding which electronic factors are involved in the oxygen reduction reaction is a top priority. With the advent of single-atom catalysts (SACs), using a single metal atom to promote chemical reactions has become a more efficient method. The interaction between the substrate (holding materials) and the active metal atoms may have a significant impact on the catalytic activity of the metal. On one hand, to obtain a stable contact, the adsorption energy of the metal atom on the substrate needs to be high. On the other hand, the influence of the substrate on the electronic structure of the metal needs to be taken into account. Hammer and Nørskov[1–3] proposed the d-band center theory, which claims that the adsorption energy of the molecule is mainly determined by the occupancy rate of the bonding and anti-bonding states formed by the hybridization of the d-state electrons wave function of the adsorbate and the substrate. If the anti-bonding state lies below the Fermi level (EF), the interaction between the adsorbate and the metal surface will become repulsive, resulting in weak chemical adsorption. This situation can be well characterized by the position of the d-band center of the metal. The downshift of the d-band center will lead to stronger chemisorption. In addition, Stamenkovic et al.  found that ORR activity is correlated with the d-band center of the platinum crystal. As the center of the d-band moves away from the EF, the observed ORR activity of Pt is higher. Lu et al.  also observed a strong correlation between the d-band center and oxygen adsorption energy on the surface of various single-crystal metals, which is consistent with the d-band center model.
TiO2 is considered to be an excellent substrate for loading noble metal atoms as catalysts. FB Li et al. used Pt/TiO2 catalyst to improve photodegradation efficiency. The TiO2/Pt catalysts in the ORR reaction of proton exchange membrane fuel cells ( PEMFCs ) have been studied by Mirshekari G R and Rice C A. In order to improve the utilization of noble metal atoms, scattered Pt/TiO2 catalyst and single-atom Pt/TiO2 catalyst are growing. For scattered Pt loaded on TiO2, Humphrey N presents a multi-scale modeling study of atomically dispersed Pt on the (110) surface of Rutile TiO2, probing the dynamic evolution of the catalytic surface at elevated temperatures. Tianyi Wang et al. calculated the different transition metals‘ d-band centers of single atom loaded on the perfect (001) surface and (101) surface of Anatase TiO2, including Pt single atom, and obtained the relationship between the different adsorption energies and the center of the d-band center, it is believed that the (001) and (101) faces of Anatase TiO2 are suitable for single-atom catalyst substrates. Therefore, the study of Anatase and Rutile TiO2 as a single-atom catalyst substrate is important.
Besides, the surface defects of the substrate usually have a great influence on the adsorption energy and the center of the d-band. Oxygen vacancies in metal are of great importance in the structural evolution of active center in single-atom catalysts (SACs) . Among the various defects in TiO2, oxygen vacancy is the most common one due to its low formation energy. However, the oxygen vacancies rich surface is more likely to affect the d-band center and further affect the ORR performance[11–13]. Therefore, it is necessary to explore the influence of oxygen vacancies of the TiO2 substrates on the d-band center.
In this study, we systematically calculated the adsorption energy of the single Pt adatom on the (001) and (101) surface of Anatase TiO2 and (100), (011), (110) surface of Rutile TiO2 in the presence of oxygen vacancy, and the d-band center of the Pt adatoms by Density functional theory(DFT) method. The calculation results show that the (110) surface of Rutile TiO2 with oxygen vacancies has a better balance between the adsorption energy and the center of the d-band among the five common surfaces calculated, and is more suitable for SACs.