Palladium-Based Nanocrystals with Controlled Surfaces for Catalytic Applications

Palladium is a key player in the field of heterogeneous catalysis. However, due to the high demand of this noble metal in catalytic converters, its price has been continuously increasing in recent few years and now even exceeds those of both Au and Pt. As such, there is an urgent need to reduce the loading of this metal by maximizing the activity and selectivity of Pd-based catalysts. In this dissertation, I demonstrate several methods for controlling the surface structure of Pd and Pd-based nanocrystals to understand their structure-property relationship, and to improve their performances toward a variety of (electro)catalytic reactions. Firstly, a facile method was developed to remove the Br− ions adsorbed on the surface of Pd nanocubes by heating the sample in pure water at 95 oC. However, the desorption of Br− would result in the formation of an oxide layer on the surface of Pd nanocubes due to the exposure to O2 in the solution. The oxide layer developed into steps and terraces during electrochemical cycling, which decreased the current density at anodic peak. By introducing a trace amount of hydrazine—a reductant that generates N2 and H2 only—into the system, the Br− ions could be removed by heating without generating a thick oxide layer. The as-cleaned nanocubes showed greatly enhanced activity toward formic acid oxidation. Besides its use as a reducing agent/oxygen scavenger, it was found that an increased concentration of hydrazine could further induce the change in crystal structure of Pd nanocubes from Pd to PdH0.706. The degree of hydride formation could be controlled by adjusting the concentration of hydrazine in the treating solution. Formic acid oxidation tests showed that the activity of the nanocubes was highly dependent on the content of hydride in the nanocubes, with pure PdH0.706 performing the best among all samples. The experimental observation was further verified by density functional theory calculations and in-situ infrared spectroscopy measurements. In my third project, Pd nanocrystals with different shapes and sizes were applied as catalysts toward the decomposition of H2O2. A systematic study was carried to evaluate the effect from particle shape and surface strain on the kinetics of decomposition. Finally, Pd@Pt core-shell catalysts were applied to understand their surface dynamics under calcination conditions. Through catalytic reaction and spectroscopy probes, it was found that the Pd atoms went through an inside-out migration during the calcination process. The migration was attributed to the interdiffusion at an elevated temperature, as well as the oxidation of Pd atoms during the calcination process. Taken together, this dissertation offers a few strategies for modifying the surface structure of Pd and Pd-based nanocrystals, contributing to the development of cost-effective catalysts.

Date

2022-04-20

Resource Type

Text

Resource Subtype

Dissertation

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