AbstractThe rational design of efficient catalysts plays a vital and challenging role in heterogeneous catalysis. With the development of computational chemistry techniques, it is realistic to design the desired catalysts by quantum chemical calculations. In this thesis, density functional theory (DFT) calculations were used to understand the microscopic chemical properties of catalytic systems and the mechanisms of the catalysis, while microkinetic modeling was used to obtain the macroscopic behaviors of the catalytic reactions, which embodied to be the reaction rates.
Regarding the rational catalyst design schemes, we first proposed an explicit formula, namely quantitative structure-energy equation (QSEE), to quantitatively account for the surface structures of alloys and the adsorption energies with three chemically meaningful parts: (i) the bonding effect, (ii) the geometric effect and (iii) the electronic effect. This simple equation has been carried out only using the adsorption energies of pure metals and can predict the adsorption energies of alloys with high accuracy. The feasibility of QSEE has been tested with three adsorbates, atomic oxygen, OH and NO species, on the flat closed-packed alloy surface. Taking the oxygen reduction reaction (ORR) as an example, the mechanisms were investigated in detail using two different pathways, O2 dissociation and OOH formation. The overall volcano surface of ORR was then constructed. Using the QSEE, we can inversely design the surface structures with the corresponding optimal adsorption energies around the theoretical highest activity point. Ten high-activity alloys were successfully designed, which can even reach the theoretically predicted highest activity in the volcano surface.
Due to several species co-adsorbing on the surface in the real environment, we noticed that the coverage effect would be a critical issue, doubting the reliability of our results. Thus, further on, we concentrated on the investigation of the coverage-dependent systems. In this thesis, both the self and cross adsorbate-adsorbate interactions were considered on the NO oxidation for an accurate adsorption energy. Meanwhile, regarding that the coverage effect would influence not only the adsorption state, but also the transition state (TS), the TS-adsorbate interactions were taken into account as well. These interactions were identified to be a two-line model. Combining these interactions, a rigorous coverage-dependent model was put forward, whose microkinetics results agreed well with the experimental data. Considering the extensive calculations and the consuming time, a one-line model was tested, which was a simplified approach but also gave rise to a good agreement with experimental results.
Having obtained the rational catalyst design approach and the coverage-dependent model, the rational catalyst design was first investigated with an explicit coverage effect. A simpler reaction, NO oxidation, was set as a case study. Using the coveragedependent model, the coverage-independent adsorption energies with the maximum theoretical activity of an alloy can be inversely derived from the volcano surface. Then, ten Pt-based alloys and two Rh-based alloys were designed using the QSEE and the framework mentioned above. Most of them could reach the theoretically predicted highest activity.
Due to several issues was found in the traditional BEP relation, we finally included the adsorbate-adsorbate interactions in the BEP relation for the purpose of overall coverage activity prediction. By systematically investigating O2 dissociation, CO dissociation, CO oxidation and NO oxidation, two coverage-dependent BEP relation models were put forward. The first one was the coverage-dependent BEP formula, which can accurately predict the coverage-dependent transition state energy at a specific coverage. The other one is the bi-BEP relation, which divided the traditional BEP relation into the low coverage BEP relation and the high coverage BEP relation. Furthermore, a coverage-dependent volcano curve was calculated by the bi-BEP relation and the microkinetic modeling, showing a shift of the highest activity point.
|Date of Award
|Chinese Scholarship Council (CSC) & Queen's University Belfast
|Peijun Hu (Supervisor)