62nd Annual Report on Research 2017

1. Background

            The use of heavy fossil fuels for transportation is increasing. A challenge to using such resources is that they contain higher concentrations of sulfur-containing compounds. Basic research is therefore needed on the reaction pathways followed in catalytic processes for sulfur removal. In our research, we use surface science methods to gain fundamental insights into mechanisms related to oxidative desulfurization (ODS) of dibenzothiophene (DBT) over a V₂O₅ model catalyst.

            Dibenzothiophene was chosen because it contains an S atom in a position that makes its removal by typical hydrodesulfurization (HDS) catalysts particularly difficult. In general, it is found that dibenzothiophenes are the dominant sulfur-containing species that remain in fuels after they are subjected to the normal HDS process. In ODS, the organosulfur compounds are generally converted to polar sulfoxides or sulfones, which can be extracted from the pure hydrocarbons. To skip the expensive extraction step, it would be desirable to produce gaseous sulfur compounds. It is thermodynamically feasible to oxidize DBT to SO₂ and hydrocarbons at typical refinery temperatures. However, to make this route sufficiently efficient, new catalysts with high selectivity for sulfide oxidation must be identified. For this reason, DBT and related molecules have been the subject of numerous heterogeneous catalyst studies.  In contrast, powerful surface science methods of the sort being used in this project have not been extensively used in past work, yet such studies can provide important basic information on the catalytic surface chemistry associated with removal of the S atoms from these molecules. Although conducting the experiments under UHV conditions permits the use of surface sensitive probes, we are finding that such conditions do not favor oxidation of DBT.

2. Results

2.1. Preparation and Characterization of V₂O₅ films on Pd(111)

            We have successfully implemented a new method for depositing stoichiometric films of V₂O₅ onto a metal surface. This is done by directly evaporating V₂O₅ from a small Knudsen cell. The films generated in this way have been examined with X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and reflection absorption infrared spectroscopy (RAIRS). The vanadium XPS binding energies and O:V ratio confirms that this method produces  stoichiometric V₂O₅ thin films. Our XPS results demonstrate that although thinner films have the V₂O₅ stoichiometry, the influence of the Pd substrate shifts the V binding energies away from the values found in bulk V₂O₅.

            The stability of the V₂O₅ films as a function of temperature was also studied. There is a loss of oxygen and a shift of the V binding energies for temperatures above 620 K. After heating to 1000 K, there is very little vanadium oxide left on the surface, but what remains has V binding energies characteristic of the V⁺³ oxidation state. The RAIRS results reveal a very sharp peak at 1041 cm^-1 characteristic of a V=O double bond stretch. The spectra undergo changes as the initially deposited amorphous film is converted to a more crystalline form. This transformation is manifested by observation of a variety of ordered structures with LEED. After annealing to 620 K, 2×2 spots appear in the LEED pattern, which persists up to 680 K. Annealing to higher temperatures leads to clusters of spots around the 1×1 spots, indicating formation of a structure with a large lattice constant. In the RAIR spectra, the V=O stretch reaches its minimum width of 4.1 cm^-1 after annealing to 620 K, indicating that this is the temperature of maximum crystalline order in the V₂O₅ film. The results demonstrate that a stoichiometric and ordered V₂O₅ film can be formed on Pd(111) and that it is stable up to at least 600 K. This then implies that surface chemical reactions, such as ODS, over V₂O₅ films can be carried out over a wide temperature range. Our work on the deposition and characterization of V₂O₅ on Pd(111) was published earlier this year (Surface Science 664, 1-7 (2017))

2.2 Interactions of DBT and Thiophene with V₂O₅/Pd(111)

            We have used RAIRS, XPS, and temperature programmed desorption (TPD) to study the adsorption and reactions of DBT with the V₂O₅ film. To determine which properties of the adsorbed molecule can be specifically attributed to the DBT-V₂O₅ interaction, we have also acquired a parallel set of data for DBT adsorbed on Pd(111). In both cases, RAIRS indicates that the molecule adsorbs parallel to the surface as only the out-of-plane C-H bending vibrations are observed. As the surface is heated, the DBT appears to mainly desorb molecularly from both surfaces. This demonstrates the relative inertness of the molecule as small hydrocarbon molecules generally undergo dehydrogenation and decomposition on Pd(111). On the V₂O₅/Pd(111) surface, no oxidation products are observed to desorb with TPD, nor are any vibrational features attributable to oxidation products observed with RAIRS. However, XPS does reveal shifts in the V peaks that may indicate vanadium reduction, a reaction expected to accompany DBT oxidation. However, it is difficult to know whether V₂O₅ reduction is due simply to increasing the temperature, or if it is due to interaction with DBT. The data is currently being analyzed to determine if the results warrant publication. Experiments to find other conditions under which DBT oxidation occurs have not been successful so far. Additional experiments were conducted to determine if thiophene reaction on V₂O₅/Pd(111) could be achieved. Data from those experiments are currently being analyzed.

2.3 Impact on Careers of PI and Students

            This grant is allowing me, the PI, to explore a new area of research. I hope to use the results obtained in other proposals to continue work in this area. The funding has been used to partly support four graduate students, and to partially support a postdoctoral researcher. The postdoc, Dr. Xu Feng, has moved on to a permanent position at Virginia Tech University where he provides instrumentation support for the teaching and researcher laboratories. One of the students, Joel Krooswyk, has successfully completed his PhD degree and is currently employed as a research chemist in industry. The other students are benefiting from PRF support as they pursue their PhD research.