Electroless Deposition and Galvanic Displacement for the Control of Supported Nanoparticle Size and Composition

Catalyst performances can be greatly affected by properties such as particle size, composition, geometry, and cleanliness of the active surface. To achieve optimal catalytic performance, appropriate synthesis methods are required to synthesize the desired structures. Supported metal particle catalysts are usually prepared by impregnation and precipitation methods in industry. Although these two methods are simple, they exhibit poor control over surface morphology, leading to a wide particle size distribution and uncontrolled distribution of different metal components. In this dissertation, three topics on controlling surface morphology are discussed. For structure sensitive reactions, reactivity and selectivity can be strongly affected by particle size. In Chapter 1, we develop a continuous electroless deposition (ED) bath to systematically grow Pd sizes with tight size distribution over a Pd/SiO2 base catalyst. We are able to control the amount of deposition. A slower deposition rate, which results in smaller particle sizes and tighter distribution was achieved by adding more stabilizer. In Chapter 2, we compare the chloride poisoning from precursor on silica supported Pd and Pt. Residual chloride on the metal surface can block active sites, which is detrimental to the catalyst's activity. This poisoning effect can be observed from the size discrepancy between chemisorption and XRD or STEM. We prepared silica supported Pd and Pt catalysts by strong electrostatic adsorption (SEA) with chloride containing and chloride free precursors. Our results illustrate that chloride poisoningvii occurs on silica supported Pd, but not on Pt. To achieve a clean surface, chloride free Pd precursors should be chosen for synthesis. In Chapter 3, highly dispersed Ag-Ni/Al2O3, Cu-Ni/SiO2, and Cu-Zn/Al2O3 bimetallic catalysts were synthesized by galvanic displacement (GD). This GD process is spontaneous. The amount of deposition can be adjusted by bath temperatures and precursor concentrations. The GD synthesized catalysts have true bimetallic surfaces with intimate contact of component metals confirmed by EDX mapping. Dehydrogenation of methylcyclohexane (MCH) to toluene (TOL) over GD synthesized Cu-Ni/SiO2 catalysts shows higher intrinsic activity than DI prepared catalysts with the same weight loading. The ED and GD methods are simple, scalable, and controllable.