Doctor of Philosophy
This thesis investigates approaches to modifying the surface structure of noble metal-based nanocatalysts. Noble metal-based nanocatalysts, such as Pt and Pd, play a significant role in heterogeneous catalysis due to their capabilities in activating the cleavage or formation of chemical bonds, but still suffer from the high-cost issue and unsatisfied catalytic performance due to too strong or too weak adsorption of intermediates. Considering the surface specificity of heterogeneous catalysis, Bi, a cheap metal, was used to modify the surface of Pt- and Pd-based nanocatalysts. This thesis aims to unveil the role of Bi in improving their catalytic performance from both experimental and theoretical perspectives.
In methanol electrooxidation reaction, Pt-based electrocatalysts suffer from the CO-poisoning issue due to the intrinsic strong adsorption of CO at Pt active sites. To alleviate the CO-poisoning effect, Bi was used to modify the Pt catalysts by an electrochemical reconstruction strategy. It was found that the bismuth hydroxide species formed on the Pt surface can efficiently weaken the CO adsorption while strengthening the OH adsorption at Pt sites. Following, a PtBi model catalyst with a PtBi surface alloy and a Pt-rich core was contrived to study the role of Bi in improving the methanol electrooxidation on Pt. Combining electrochemistry and spectroscopy characterizations, it was confirmed that Bi-modified Pt catalysts can completely inhibit the CO-pathway while enhancing the formate-pathway, thereby circumventing the CO-poisoning effect. The key role of Bi is enriching OH adsorbates on the catalyst surface, and the competitive adsorption between COand OH adsorbates switches the intermediate from COto formate, which even overwhelms the electronic effect brought by alloying Bi with Pt. More importantly, we have successfully extended this concept to modify the commercial Pt/C catalyst and realize its facile and large-scale production by a microwave-assisted method. This work deepens the understanding of the CO-poisoning issue and offers new opportunities for the design and practical production of CO-tolerance electrocatalysts in an industrial orientation
In selective hydrogenation of propyne, the over-hydrogenation occurring on Pd catalysts is blamed for the poor selectivity toward propene. Herein, a PdBi surface alloy structural model, by tuning the deposition rate of Bi atoms relative to the atomic interdiffusion rate at the interface, realizes a continuous modulation of the electronic structure of Pd. Using advanced X-ray characterization techniques, we provide a precise depiction of the electronic structure of the PdBi surface alloy. As a result, the PdBi catalysts show enhanced propene selectivity compared with the pure Pd catalyst in the selective hydrogenation of propyne. The prevented formation of saturated β-hydrides in the subsurface layers and weakened propene adsorption on the surface contribute to the high selectivity. This work emphasizes the in-depth understanding of the electronic properties of surface alloy structure and underlies the study of the electronic structure-performance relationship in bimetallic catalysts.
Summary for Lay Audience
Noble metals (such as platinum and palladium) play great roles in present life, especially in the catalysis field. However, their widespread applications are still facing the challenges of high cost and unsatisfied catalytic performance. Alloying noble metals with a second cheap metal is a solution to address these issues. Given chemical reactions take place on the surface of catalysts, directly modifying the surface properties of catalysts is a promising strategy to enhance their catalytic performance. However, methods of direct surface modification are lacking, and mechanistic study is not enough to explain how the second metal works.
In this thesis, bismuth, a cheap metal, was used to modify the surface of platinum and palladium by various methods, including the electrochemical reconstruction method and the seed-growth method. Bismuth-modified platinum and palladium were tested in the methanol electrooxidation and selective hydrogenation of propyne reactions, respectively. Combining advanced synchrotron X-ray techniques and laboratory characterizations, Systematic studies were carried out to elucidate the role of bismuth in each case and build reliable a structure-performance relationship.
As a result, in the methanol electrooxidation reaction, bismuth induces the competitive adsorption of hydroxyl (OH) and poisoning intermediate (carbon monoxide, CO), resulting in the reaction switching from the CO-pathway to the formate-pathway. This concept has even been extended to modify commercial platinum catalysts with a microwave-assisted method, showing great potential in practical applications. In the selective hydrogenation of propyne, the electronic structure of palladium is continuously tuned by bismuth. The palladium-bismuth surface alloy provides a good structure model for building a reliable structure-performance relationship.
Wang, Xuchun, "Design and Surface Modification of Noble Metal-based Nanocatalysts" (2022). Electronic Thesis and Dissertation Repository. 8794.