First-principles studies of nanostructure assembly and heterogeneous catalysis on surfaces

Abstract

The interaction between adsorbates and substrates on surfaces leads to diverse surface processes, which has been employed in physics and chemistry to fabricate desired structures with extended dimensions or to transform one set of molecules into another as products. Understanding these surface processes at atomic level would enable material design more rationally, increasing the efficiency and reducing the cost as compared to the traditional trial and error approach. With the fast development of computation science, density functional theory (DFT) increasingly becomes a standard tool for treating surface systems and obtaining information of forces, energetics, and configurations of atoms at surfaces. Based on first-principles calculations at DFT level, this dissertation presents investigations on adsorbates-substrate interaction in three different systems targeting low-dimensional materials fabrication and heterogeneous catalysis. In the first chapter, the general theoretical foundation underlying the DFT are briefly introduced. In addition, the low energy electron diffraction (LEED) technique is introduced, which has been widely employed to determine surface structures. In the following chapter 2, I focus on the fabrication and identification of a two-dimensional gold-phosphorus network (AuPhoN) on Au(111) surface. It is demonstrated that by using molecular-beam epitaxy (MBE), one may isolate inorganic blocks to form the 2D porous AuPhoN, wherein blue phosphorene (blueP) subunits are linked by gold atoms. AuPhoN has a porous inorganic structure similar to the interesting organic networks whose tailorable architectures offer fresh applications in sensing, catalysis, gas storage and topological phenomena. Evidence are provided showing that such metal-phosphorus networks are tunable in their chemical functionalities and electronic properties by simply altering the linkers and subunits.

The epitaxial AuPhoN on Au(111) was mistakenly assigned in some previous works as the successful obtainment of monolayer blueP, a predicted rising allotrope of blackP that attracts extensive attentions. The present work described in chapter 2 elaborates that the blueP with extended dimension is not realized on Au(111). In chapter 3, I propose a potential method to access the so-far unrealized freestanding blueP through the transformation of blackP as induced by surface Br adsorption. Formation of the Br-P bonds disrupts the original sp3 configurations in blackP, generates unpaired pz electrons, and causes a structural transformation that results in blueP formation. The result of this study deepens an understanding of adsorbate-substrate interaction where adsorbates “catalyze” the behavior of substrate.

The transformation from blackP to blueP however is not a conventional catalysis process. In heterogeneous catalysis, substrate activates adsorbates and lower the reaction barrier, where the Sabatier conflict limits the reaction rate. In chapter 4, a new strategy for designing catalysts for the water-gas-shift reaction (CO + H2O → CO2 + H2) is presented to resolve the Sabatier conflict. We avoid the conflicting tasks that require *OH or *O from dissociated water to adsorb on and then desorb from the substrate of a catalyst. Instead, the CO on metal directly obtains OH from water that are connected by weak hydrogen bonds to the substrate. Experimental and theoretical results show that bifunctional catalysts with weakly reactive substrates have significantly higher CO conversion rates.