DFT Study On Intrinsic Effects Of Metal-Supports Interaction And Doped Ni Atom On The Activity Pt And Pd Catalysis For The ORR

Fuel cell as a clean energy transform device is one of effecient measurements for energy shortage and environment pollution caused by fossil fuel direct combustion. Electrocatalyst is the key material of Fuel Cell. Currently, Pt/C catalysts are in the dominant position for oxygen reduction reaction (ORR). However, high cost of Pt/C catalysts, restrains commercialization of fuel cell. In this work, the focus was given to Pt free catalysts, Pd/TiO₂, for the ORR. Based on the experimental phenomenon, on the carbon blacks, Pt exhibits better catalysis for the ORR than Pd. Oppositely, on the TiO₂, Pd shows better catalysis for the ORR than Pt. Then, this work tried in the light of quantum chemistry to answer why the activity of Pt and Pd for the ORR changes with the catalyst supports; what is the quantum chemistry explanation behind the difference in catalysis caused by the catalyst support; and whether the catalytic activity of Pd/TiO₂ for ORR can be further improved by introduction of doping Ni ions. The main results are as follows:1) The factors associated with the first electron transfer of the ORR, orbital matching, energy difference and orbital maximum overlap between the electron donor (catalysts) and the electron acceptor (oxygen molecule), as well as influence of electric field on the ORR, were systematically investigated. With respect of orbital symmetry, Pd/C HOMO and Pt/C HOMO matche O₂ LUMO. However, the spatial size of Pd/C HOMO is too small compared with O₂ LUMO. There is a very small overlap between Pd/C HOMO and O₂ LUMO. The theoretical computation discloses that Pd/C HOMO is mainly constituted by C p-orbital and a little fraction of Pd d-orbital, but Pt/C HOMO is almost evenly composed by Pt d-orbital and CÏ€-orbital. The spatial size of Pt/C HOMO and O₂ LUMO is in the same scale. Thus, it is reasonable to expect a larger overlap between Pt/C HOMO and O₂ LUMO. It should be easier on Pt/C than on Pd/C for the first electron transfer of the ORR in the view of the maximum overlap principle. Fortunately, the small spatial size of Pd/C HOMO can be radically improved with change of catalyst support from C to TiO₂. The spatial size of Pd/TiO₂ HOMO and O₂ LUMO is almost in the same scale. The problem, i.e., lack of maximum overlap, hindering electrons from transferring between Pd/C HOMO and O₂ LUMO, no longer exists for catalysts Pd/TiO₂. Thus, the first electron transfer of the ORR was enhanced.2) The change in adsorption strength of intermediates (atomic oxygen, O) on catalyst surface was also evaluated. It indicated that the adsorption of O on Pd/C is stronger than that on Pt/C. On contrary, the adsorption of O on Pt/TiO₂ is stronger than that on Pd/TiO₂. The analysis of deformation density and partial density of states showed that the interaction between the Pt and surface Ti of TiO₂ is quite strong. This leads to increase of Pt/TiO₂ d-band vacancy. The increased Pt /TiO₂ d-band vacancy further leads to a strong adsorption of intermediate O on Pt/TiO₂. The tardiness in removing the intermediates hinders the subsequent reaction steps. However, the interaction between the Pd and surface O of TiO₂ is quite strong. It causes a weak adsorption of intermediate O on Pd/TiO₂. It makes it easier for intermediate O to leave from Pd/TiO₂ and then the subsequent reaction steps of ORR are accelarated.3) Study on Ni doped catalysts PdxNi/TiO₂(x=1,2,3) showed: it makes no radical difference in the spatial size of surfacial atoms'HOMO between PdNi/TiO₂ and Pd/TiO₂. However, the orbital matching between surfacial Pd atom s'HOMO and O₂ LUMO is remarkablely improved in the case of Pd2Ni/TiO₂ and Pd3Ni/TiO₂. The HOMO energy of Pd3Ni/TiO₂ first reaches O₂ LUMO before Pd/TiO₂, PdNi/TiO₂and Pd2Ni/TiO₂ as the electrode potential shifting toward the negative direction. It means ORR can conduct more easily on Pd3Ni/TiO₂ than on others. Although the adsorption energy of the intermediate (Oads) on the studied catalysts indicated that the adsorption of atomic oxygen on Pd3Ni/TiO₂ is most strong of all catalysts. The adsorption of Oads mainly happens on Ni atoms rather than Pd atoms. Ni atoms liberate Pd atoms from Oads adsorption and thus makes Pd atom catalyzes the subsequent steps of the ORR. The Pd crystal lattice shrinks with introduction of Ni atoms. It causes a weak adsorption of Oads on catalyst thus accelerate the subsequent steps of ORR. This result satisfactorily explains the experimental phenomenon why catalyst Pd3Ni/TiO₂ shows the best catalysis to the ORR of all studied catalysts.