PROF. MARK E. BUSSELL

Research Interests


Link to Bussell Group Webpage

My research interests are in the areas of surface and materials chemistry, with an emphasis the development of heterogeneous catalysts for use in environmentally important chemical processes. Current projects are focused hydrotreating catalysis, and the hydrodesulfurization (HDS) process in particular. Hydrodesulfurization is an industrial process in which sulfur is selectively removed from organosulfur compounds present in fossil fuels. The HDS reaction is shown below for thiophene (C₄H₄S), the test compound used in this laboratory to measure sulfur removal rates.

Ambitious goals have been set in the United States and Europe to reduce sulfur levels in transportation fuels well below current standards. The motivation for dramatic reduction of sulfur levels in fuels is two-fold, concerns for the environment and for human health Lower sulfur levels in transportation fuels would reduce the emissions of sulfur oxides into the air where they are precursors to acid rain formation. In addition, lower sulfur levels in gasoline would help to reduce tail-pipe emissions of NO_x gases and unburned hydrocarbons as sulfur poisons the catalysts in automobile catalytic converters. In terms of pollutant emissions, a reduction of the sulfur level in gasoline to 30 ppm would be comparable to removing 54 million cars from the road according to proponents of the new U.S. regulations (Chem. Eng. News 1999, 77, 6). Sulfur in diesel also has deleterious effects on emission control technologies, leading to greater emissions of NO_x gases and particulates. These particulates have been linked to cancer, asthma and other respiratory ailments (Federal Register 2000, 65, 35479).

Development of New Catalytic Materials for the HDS Process

For over fifty years, the commercial HDS process has utilized alumina-supported molybdenum or tungsten catalysts promoted by cobalt or nickel (e.g. Co-Mo/Al₂O₃). The active sites for thiophene HDS are thought to be located on the edge planes of molybdenum disulfide (MoS₂) crystallites; the structure of MoS₂ is shown to the left.

Recent research in my group has focused on the synthesis, characterization and evaluation of oxide-supported monometallic and bimetallic carbide and nitride catalysts for use in the HDS process. Studies in a number of laboratories have shown that these materials have strong potential to replace the sulfide-based catalysts currently used in industry. Molybdenum carbides and nitrides are interstitial materials in which carbon and nitrogen occupy interstitial sites within the molybdenum lattice. The structures of the beta phase of molybdenum carbide (beta-Mo₂C) and the eta phase of cobalt molybdenum carbide (eta-Co₃Mo₃C) are shown below at left and right, respectively.

Thiophene HDS activity measurements carried out in our laboratory have shown that both of these materials, when supported on gamma-Al₂O₃, are more active than conventional sulfide-based catalysts of similar metal composition.

Current research in my group focuses on the synthesis, characterization and evaluation of amorphous and crystalline metal borides and phosphides for use in hydrotreating catalysis. This research is supported by a grant from the National Science Foundation (CHE-0101690) as well as a Scholar/Fellow Award from the Camille & Henry Dreyfus Foundation (SF-01-026). Boron and phosphorus are often used as additives in hydrotreating catalysts yet their roles in modifying the properties of these catalysts are not well understood. By preparing and studying oxide-supported metal boride and phosphide phases, we hope to gain insight into the properties of boron and phosphorus additives as well as investigate the HDS properties of these promising catalytic materials. Early results indicate that silica-supported molybdenum phosphide catalysts (MoP/SiO₂) are substantially more active than sulfided Mo/SiO₂ catalysts with similar molybdenum loadings. The crystal structure of molybdenum phosphide (MoP) is shown at right.

Fundamental Studies of the Adsorption and Reactions of Organosulfur and Organonitrogen Compounds on Hydrotreating Catalysts

To complement the catalyst development studies, research in my group also examines the adsorption and reactions of organosulfur and organonitrogen compounds on the surfaces of hydrotreating catalysts. Infrared (IR) spectroscopy and temperature programmed desorption (TPD) are used to probe the chemistry of adsorbed species over wide ranges of temperature (140 - 700 K) and pressure (10^-9 - 10³ Torr).

Recent studies in our laboratory, for example, indicate that thiophene adsorbs to the surface of sulfided Mo/Al₂O₃ catalysts via its sulfur atom in a vertical or tilted geometry as shown at right. This assignment was made after analysis of the IR spectra of thiophene adsorbed on sulfided Mo/Al₂O₃ catalysts as well as a collaborative study in which vibrational spectroscopy and normal mode calculations were used to investigate organometallic complexes containing thiophene ligands. Current studies in this area utilize IR spectroscopy and TPD to investigate the surface chemistry of CO, thiophene, and pyridine on metal phosphide catalysts.

Research Opportunities

Students working in my laboratory gain experience in a number of different areas of materials and surface chemistry. Catalysts are synthesized in a flow synthesis apparatus and are characterized by the different techniques listed below, all of which are available at WWU.

• Powder X-ray diffraction (XRD)
• BET surface area measurements
• Pulsed chemisorption (CO, O₂) measurements
• Temperature programmed desorption (TPD)
• Transmission Fourier transform infrared (FTIR) spectroscopy

Additional catalyst characterization studies are carried out at the Environmental Molecular Sciences Laboratory (EMSL) user facility of the Pacific Northwest National Laboratory (PNNL) located in Richland, Washington. Techniques used in this characterization studies include the following

• X-ray photoelectron spectroscopy (XPS)
• Transmission electron microscopy (TEM)
• Scanning electron microscopy (SEM)
• Inductively coupled plasma - mass spectrometry (ICP-MS)

Hydrodesulfurization activities are measured in a flow reactor apparatus at WWU; a schematic diagram of this system is shown below. The reactor apparatus is fully automated with gas flows regulated by mass flow controllers and reactant and product gases analyzed by on-line gas chromatography.

Combined IR-TPD studies of adsorbed species (e.g. thiophene, pyridine, CO) on catalyst surfaces are carried out in an ultrahigh vacuum (UHV) system (see below).