A temperature-programmed desorption (TPD) study of CO and NH₃ adsorption on MnO(100) with complimentary density functional theory (DFT) simulations was conducted. TPD reveals a primary CO desorption signal at 130 K from MnO(100) in the low coverage limit giving an adsorption energy of -35.6 ±2.1 kJ/mol on terrace sites. PBE+U gives a more reasonable structural result than PBE, and the adsorption energy obtained by PBE+U and DFT-D3 Becke-Johnson gives excellent agreement with the experimentally obtained ΔE<sub>ads</sub> for adsorption at Mn²⁺ terrace sites. The analysis of NH₃-TPD traces revealed that adsorption energy on MnO(100) is coverage-dependent. At the low-coverage limit, the adsorption energy on terraces is -58.7±1.0 kJ/mol. A doser results in the formation of a transient NH₃ multilayers that appears in TPD at around 110K. For a terrace site, PBE+U predicts a more realistic surface adsorbate geometry than PBE does, with PBE+U with Tkatchenko-Scheffler method with iterative Hirshfeld partitioning (TSHP) provides the best prediction.
DFT simulations of the dehydrogenation elementary step of the ethyl and methyl fragments on α-Cr2O₃(101̅2) were also conducted to complement previous TPD studies of these subjects. On the nearly-stoichiometric surface of α-Cr₂O₃(101̅2), CD₃₋ undergoes dehydrogenation to produce CD₂=CD₂ and CD₄. Previous TPD traces suggest that the α-hydrogen (α-H) elimination of methyl groups on α-Cr₂O₃(101̅2) is the rate-limiting step, and has an activation barrier of 135±2 kJ/mol. DFT simulations showed that PBE gives reasonable prediction of the adsorption sites for CH3- fragments in accordance with XPS spectra, while PBE+U did not. Both PBE and PBE+U failed to predict the correct adsorption sites for CH₂=. When the simulation is set in accordance with the experimentally observed adsorption sites for the carbon species, PBE gives very accurate prediction on the reaction barrier when an adjacent I adatom is present, while PBE+U failed spectacularly. When the simulation is set in accordance with the DFT-predicted adsorption sites, PBE is still able to accurately predict the reaction barrier (<1% to 8.7% error) while PBE+U is less accurate. DFT is also used to complement the previous study of the β-H elimination an ethyl group on the α-Cr₂O₃(101̅2) surface. The DFT simulation shows that absent surface Cl adatoms, PBE predicts an activation barrier of 92.6 kJ/mol, underpredicting the experimental activation barrier by 28.7%, while PBE+U predicts a barrier of 27.0 kJ/mol, under-predicting the experimental barrier by 79.2%. The addition of chlorine on the adjacent cation improved the prediction on barrier by PBE+U marginally, while worsened the prediction by PBE marginally.
Grant information: Financial support provided by the U.S. Department of Energy through grant DE-FG02 97ER14751. / Doctor of Philosophy / Nowadays, density functional theory (DFT), a computational approach to chemistry has become increasingly more popular due to it being less computationally expensive than other traditional computational approaches. One major shortcoming of DFT is its inability to explain the electronic interactions within transition metal oxides, where the electronic configuration within one cation is intimately linked to those on adjacent cations. To address this, DFT+U, a variant of DFT, has been developed to better account for these special electronic interactions. However, not enough experimental comparisons have been established to verify the accuracy of DFT and DFT+U.
Our lab focuses on providing high quality experimental benchmarks that can be readily compared to by the DFT community. To establish the experimental benchmarks, we use a technique called temperature-programmed desorption (TPD), which focuses on measuring the rate at which gas molecules leave a sample surface populated with a pre-determined amount of gas molecules as the temperature of the surface is raised at constant but slow temperature ramp rate. Through analysis of the results, the adsorption energy can be obtained for a desorption process, or an activation barrier if the desorption is the result of a surface reaction. Some simple calculations involving PBE, a popular functional used in the DFT community, and its variant PBE+U were conducted for comparison purposes. The transition metal oxide surfaces chosen in this study is MnO(100) and of α-Cr₂O3(101̅2), because they both possess the special electronic interactions between their own cations.
For adsorption studies, we determined adsorption energies of carbon monoxide (CO), and ammonia (NH₃) on MnO(100) single crystal surface. For CO, TPD study revealed that CO undergoes weak adsorption on the surface, with no dissociation of CO detected. PBE predicts an unreasonable surface adsorption geometry while PBE+U predicts a reasonable one. When coupled with a particular dispersion correction method named DFT-D3 Becke-Johnson, PBE+U predicts a very accurate adsorption energy of CO on MnO(100). TPD shows that NH₃ undergoes a stronger adsorption on MnO(100) with no dissociation of NH₃. Similarly, PBE+U predicted a more reasonable adsorption geometry while PBE did not. Coupled with a dispersion correction named Tkatchenko-Scheffler method with iterative Hirshfeld partitioning (TSHP), PBE+U provides an accurate prediction of adsorption energy. In comparison to previous experimental works based on TPD results, the simple decomposition reactions of an ethyl group and a methyl group were also studied on α-Cr₂O₃(101̅2) surface using DFT. Overall, PBE gave better prediction on the activation barrier than PBE+U did in comparison to experimentally observed barriers.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/103814 |
| Date | 11 June 2021 |
| Creators | Chen, Han |
| Contributors | Chemical Engineering, Cox, David F., Ducker, William A., Xin, Hongliang, Karim, Ayman M. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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