First-principles Calculations On Pd-TiO₂ Single-atom Catalysis:Mechanism And Modulation

Using first-principles calculations, we have explored the underlying mechanism for O2 activation and CO catalytic oxidation on transition metal (TM) catalysts at single-atom regime, such as Pd and Ni monomers and dimers deposited on rutile TiO2(110) surface. We successfully figured out the key parameter in determining the distinct catalysis of Pd single atom and Pd dimer for CO oxidation observed experimentally. Consequently, the catalysis of the intrinsically inert Pd single atom is considerably optimized by a simple and economic n-p co-doping approach.In chapter 1, we firstly introduced both the experimental and theoretical advances in the field of low-dimensional transition metal structures in heterogeneous catalysis. Moreover, the new concept of single-atom catalyst (SAC) is introduced, including the experimental characterization, fabrication, as well as the practical application in industry. Furthermore, we proposed potential theoretical approach in optimizing the efficiency of the SAC.In chapter 2, we briefly described the computational methodologies used in this thesis. Our first-principles theoretical calculations are based on the framework of density of functional theory. In addition, the software and simulation code used are also introduced in this chapter.In chapter 3, we present systematic first-principles calculations on the activation of O2 on TMn(TM=Pd, Ni; n=1,2) deposited on rutile TiO2(110). It is identified that the contrasting oxidation catalytic activities between the monomer and dimer are not simply governed by the spin-selection rule, but essentially determined by the well-known frontier orbital theory or a generalized d-band model, which emphasizes the position of the HOMO of the deposited TM rather than the position of its d-band center proposed in the classic d-band theory. In the atomic-sized regime, a TM catalyst forms discrete, quantized d-orbitals rather than a continuous d-band, and therefore, the HOMO(TMn)and LUMO(O2) near Ef can be associated with their chemical reactions. Specifically, the large HOMO(TM)-LUMO(O2) gap in the Pd monomer case results in a weaker interaction between the TM monomer and O2 because of their minimal orbital hybridization. However, the significantly reduced HOMO(TM)-LUMO(O2) gap in the Pd dimer case increases the hybridization of two orbitals. Here, the size of HOMO(TMn)-LUMO(O2) gap affects the magnitude of hybridization between HOMO(TMn) and LUMO(02), thereby playing a crucial role in determining the catalytic activities of TM catalysts.In chapter 4, based on the above mentioned underlying mechanism, we attempt to modify the electronic structure and chemical properties of the support TiO2(110) surface by using n-p co-doping method, such as p-type codopant V-C, compensated co-dopant pairs V-N and Cr-C, and n-type coupling Cr-N, and thus exert great influence on the deposited Pd atom, therefore enhancing its ability of oxygen activation and substantially lower the reaction barrier of CO catalytic oxidation. Interestingly, we established a very simple relationship between the O2 adsorption energy, charge state of Pd atom, charge transfer at the interface and the reaction barrier, as a function of the metal Pd atom and support interactions (EMSI), i.e., the stronger the EMSI, the larger the hybridization between the Pd atom and the substrate, and the smaller the HOMO(Pd)-LUMO(O2) gap; correspondingly, the stronger the O2 activation and the higher the rate of CO oxidation.Chapter 5:conclusion and outlook. Through the investigation of the contrast catalytic properties of Pd1 and Pd2 on the TiO2(110) surface, we have figured out the underlying mechanism that determines the catalytic activity of the single Pd atom deposited. Furthermore, we have also proposed an effective n-p co-doping method that can effectively modulate the electronic structures and chemical properties of the Pd atom, leading to significant optimization of the catalysis of the essentially inert Pd single atom deposited on TiO2(110). We expect that present findings can shed new insight into highly efficient and economic SAC design.