Preparation of Silver Catalysts by Atomic Layer Deposition and Studies of Atomic Layer Deposition Reactions Using Operando Surface-Enhanced Raman Spectroscopy

The development of novel metal nanoparticle catalysts that can efficiently transform raw materials into useful products is an ongoing endeavor in catalysis research. The catalytic performance of nanoparticles is highly dependent on their size, structure, and chemical composition. Atomic layer deposition (ALD) is a gas-phase synthesis method that is used to prepare metal nanoparticles and metal oxide supports for applications in heterogeneous catalysis. ALD provides atomic-level control over the size, size distributions, and chemical composition of supported metal nanoparticles. Understanding the surface chemistry that occurs during catalyst synthesis and catalytic reactions is crucial in order to elucidate structure-function relationships that can lead to the design of better catalysts. This requires the use of operando characterization techniques that can probe catalyst structure and surface adsorbates under practical reaction conditions. Surface-enhanced Raman spectroscopy (SERS) is a powerful surface-sensitive operando spectroscopic technique that can be used to elucidate the chemistry occurring at catalytically relevant interfaces. SERS provides vibrational information about the catalyst material itself and chemisorbed surface species during the active state of the catalyst.;In this dissertation, two ALD methods of preparing supported Ag nanoparticles are presented. Quartz crystal microbalance studies show that AB-type Ag ALD has an incubation period while ABC-type Ag ALD does not have an incubation period. Both methods produce Ag nanoparticles with an average size of ~2 nm after 20 cycles with very narrow size distributions. ALD Ag nanoparticles are catalytically active in oxidative dehydrogenation of cyclohexene at 350 °C. Operando SERS was used to probe the appearance and decay of Al-C and C-H stretches after trimethylaluminum and water exposures, respectively. This is a proof-of-concept demonstration that shows that SERS can provide rich molecular-level information about ALD reactions under realistic reaction conditions. A high-resolution operando SERS distance dependence study enabled by ALD demonstrated that SERS intensity decreased to ~20% and ~7% of the original intensity at a distance of ~0.7 nm and ~3 nm from the surface, respectively. The use of operando SERS opens the possibility of investigating the complicated surface chemistry occurring during ALD and catalytic reactions and will lead to the rational design of improved catalysts.