Jonathan T. Lyon, PhD, Clayton State University
We are interested in understanding how different molecular compounds, such as small hydrocarbons, interact with transition metals in different catalytic processes, with hopes of improving the chemical reaction. Along the reaction pathway from reactants to desired products on a metal catalyst, we use computational chemistry techniques to predict the energetics and vibrational properties of the reaction intermediates with a single metal atom. We also perform experiments to synthesize and trap the reaction products between the metal atom and reactant molecules in an inert argon matrix. Comparing the experimental and theoretical results allows for accurate structural assignments of the reaction complex. Lastly, we predict how each reaction would proceed when a transition metal cluster, which is used to model the active sites in a real world heterogeneous catalyst, is used instead of a single atom. One chemical reaction of particular interest is the conversion of small alkanes to fuel alcohols.
In the past year, the experimental setup (Figure 1) was moved to a newly remodeled research room. Laser ablated metal atoms produced with an Nd:YAG laser are co-deposited with a dilute mixture of a molecular compound in argon onto a cesium iodide window cooled to approximately 10 K. The reaction is carried out in a vacuum chamber at low pressures (~10-6 torr). After deposition, the matrix is annealed to various temperatures (15 45 K), exposed to UV photolysis, and infrared spectra are collected after each procedure. Product absorptions that act similarly during each operation are assigned to a single reaction product.
We have studied the interaction of Group 5 transition metals with small halogenated hydrocarbons. For the reaction between vanadium atoms and CH2F2, for example, three product absorptions are observed. These experimental frequencies agree well with the vibrational frequencies of the calculated lowest energy isomer, a CH2-VF2 complex, predicted with density functional theory calculations. Similar species are formed and identified with the heavier niobium and tantalum metals. During the remodeling of the experimental research room, effort was also made to theoretically study different transition metal clusters to extend this research project to larger systems. We have found the TPSSH hybrid method performs better than other density functional methods in predicting the properties of small transition metal clusters when compared with known experimental results from the literature. We have also discovered that the studied transition metal clusters are often spin contaminated for low spin multiplicities when unrestricted. In the past year, we have studied different sized clusters of iron and ruthenium atoms, which are more important elements in catalysis, at the TPSSH level.
Select preliminary results of this study were presented by the principle investigator and an undergraduate student at the 2012 Southeastern Regional Meeting of the American Chemical Society (SERMACS) in Raleigh, NC. This PRF grant has financially supported three undergraduate students in the past year. A research student supported by this research in the previous year is currently in a chemistry PhD program. The principle investigator and students are sincerely grateful for the financial support from the ACS Petroleum Research Fund which has aided in allowing us to establish a productive research program for undergraduate students at Clayton State University.