A single molecule spectroscopy approach towards understanding the structure of catalytically active sites

Abstract

The work presented in this thesis has been performed with the aim of increasing our understanding of relationships between the structure of an adsorption site and the extent of adsorbate activation, a key step in catalytic processes. Unravelling such structure-performance relationships is often hindered by the complex structures displayed by heterogeneous catalysts; they can be multiphasic and are typically dynamic in nature with respect to structure, composition and local electronic environment. A reactive surface can be comprised of many step edges, kinks and defect sites, but only a small portion of this rich structural diversity may be responsible for catalytic activity. The challenges associated with identifying the structure of active sites can be simplified through the interrogation of single crystal surfaces, for which structural complexity is greatly reduced. By using a low temperature, ultra-high vacuum scanning tunnelling microscope (LT-UHV-STM), individual surface sites of interest can be identified and investigated on an atomic scale. The application of STM-inelastic electron tunnelling spectroscopy (STM-IETS) and scanning tunnelling spectroscopy (STS), allows the bond strength, and therefore the extent of adsorbate activation to be studied at surface sites of differing geometric and electronic structure. The work described in this thesis has focussed on the development of STM-IETS and STS to investigate the interaction of carbon monoxide (CO) with Cu-based surfaces. CO is an ideal probe molecule given the sensitivity of its vibrational frequency to local structure and its use as a C₁ building block in chemical processes. On Cu(110), adsorbed CO forms one-dimensional structures where the nearest neighbour sites are occupied in the [001] direction. The vibrational frequency of adsorbed CO is highly dependent on the CO coverage. A CO molecule with one of its nearest neighbour sites occupied by CO (a CO adsorption dimer) exhibits an increased C-O bond strength (higher frequency) relative to a terrace monomer. This is assigned to dipole-dipole coupling effects between neighbouring molecules. Conversely, the CO bond is weakened compared to a terrace monomer, when two nearest neighbour sites of the adsorbate are also occupied (a CO adsorption trimer). This is attributed to chemical effects that cause a broadening of the CO 2π* orbital and its increased occupancy. Further investigations have explored the effects of the structure of adsorption site on the CO bond strength. CO adsorbed at the lower [001] aligned Cu(110) step edge sites, exhibits lower CO stretching frequency when compared to CO adsorbed at the upper edge sites. This can again be attributed to changes in the occupancy of the antibonding 2π* orbital that is driven in this case by a substantial difference in electron density associated with these adsorption sites. The aforementioned studies have been extended to investigations of Co, a metal upon which CO activation is of great interest. Given the complexities of preparing Co single crystals, a model surface was formed by depositing Co on Cu(110). When deposition is performed in the presence of CO, a novel CoCu(110) alloy is formed in which two-atom wide Co structures aligned in the [001] direction are embedded in the first layer of the Cu(110) surface. When CO is adsorbed between the linear arrangements of Co atoms, there is a far greater activation of the CO bond relative to the Cu(110) surface. The results demonstrate the broad application of the STM-IETS approach to studying complex surfaces and the ability to provide deeper insights into data that is traditionally generated using techniques that provide a macroscopic picture of a surface.