Experimental Equipment

^{Our research is conducted through the use of Ultra High Vacuum (UHV) Chambers, which operate at a base pressure of 10e-10 Torr to probe reaction kinetics occurring on a molecular level. Our research focuses on the molecular study of late transition metal oxides through a variety of spectroscopy and microscopy techniques. Using multiple techniques allows us to accurately assess the surface morphology as well as reaction kinetics. We have three separate chambers, each with unique capabilities that allow us to gather a wide array of experimental data from our surfaces. All three chambers are equipped with a quadrupole mass spectrometer, which is used to capture reaction products as the temperature of the sample is linearly increased, which is known as temperature programmed desorption (TPD) or reaction spectroscopy (TPRS).  All chambers also have a LEED and AES optic. LEED, or Low Energy Electron Diffraction, is used to determine the surface crystallography via electron impingement onto the sample. AES, or Auger Electron Spectroscopy, provides an elemental fingerprint of the sample surface.}

 

^{STM Chamber}

^{In addition to the aforementioned experimental equipment, the first of the three chambers is also equipped with Scanning Tunnel Microscopy and a Fourier Transform Infared Spectrometer. Scanning Tunneling Microscopy (STM), uses a single atom tip which is rastered across the surface to image the surface on an atomic level. An image of the surface showing individual atoms can then be generated which provides information about the surface morphology as well as lattice structure. A Fourier Transform Infared Spectrometer (FTIR) is an absorption spectroscopy technique that provides information about the wavelength of molecular bonds on the surface of the crystal.  This chamber also has a metal evaporator which can be used to deposit metals onto the installed crystal as well as an atomic oxygen source, which generates an atomic oxygen plasma used to oxidize the installed metal crystal.}

 

^{XPS Chamber}

The second chamber is equipped with a metal evaporator as well as an atomic oxygen source, similar to the first chamber. However, rather than STM, this chamber instead includes XPS. XPS, or X-Ray Photoelectron Spectroscopy, uses an Al or Mg source to create photons which then impinge upon the sample. This excites a core electron out of the atom, which is then collected in a hemispherical analyzer. The analyzer measures the kinetic energy of the excited ejected electron which provides information about the atomic species’ local bonding environment as well as its oxidation state through it’s binding energy. This is a very powerful tool which provides oxidation information as well as chemical bonding states.

 

^{HP Chamber}

The third and final chamber has a custom separate high pressure (HP) cell. This high pressure cell is located directly beneath the main UHV chamber and is accessed through the use of sliding seals which separate the two chambers. The sliding seals create a volume that is differentially pumped when the sample is placed into the HP cell, ensuring that UHV pressures are maintained while higher pressures are achieved in the HP cell. This cell can be used to create oxides on late transition metals that are harder to oxidize in UHV using atomic oxygen by allowing a high pressure of oxygen to bombard the sample. The HP cell can also be used to perform reactivity studies nearer to industrial conditions, which are closer to atmospheric pressures. The HP chamber is also currently being fitted with a Infared Spectrometer (which is also in the STM chamber).

 

Research Background

Metal-oxide surfaces studied by our group feature under-coordinated metal and oxygen atoms as they are lacking a bond when compared to the coordination of the bulk atoms. This lack of coordination allows them to form strong bonds with adsorbates. Under-coordinated metal atoms can be referred to as coordinatively unsaturated or cus atoms while the under-coordinated oxygen atoms can be referred to as bridging oxygen atoms or O_br, which is due to their position at which they are bound to the underlying metal atoms. These are the reactive sites on the oxide surface which promote the dissociation and oxidation of alkanes.

It has been discovered that the initial C-H bond cleavage that occurs prior to complete dissociation of light alkanes on metal-oxide surfaces is facilitated by the activation of the C-H bond via the formation of a dative bond between under-coordinated surface (cus) metal atoms and the C-H ligand. This dative bond weakens the C-H ligand and allows for subsequent dissociation of the molecule to occur upon the oxide surface. The O_br atoms act as a hydrogen acceptor which results in oxidation of the adsorbed alkanes species to CO_X and H₂O products. However, due to the electronic structure of the metal atoms as well as the surface structure of specific oxides, some films allow for selective oxidation of the adsorbed alkane species. Our research focuses on exploring a multitude of different metal-oxides as well as different surface structures in order to ascertain the most favorable catalyst for the selective partial oxidation of adsorbed species.