Catalytic and chemical properties of surfaces
Variable Pressure /Temperature Adsorbate Structures and Reactions of Cyclohexene on Pt(111) Single Crystals Studied by STM
Thedevelopment of the high-pressure, high-temperature scanning tunneling microscope (STM) has enabled in situ examinations of surfaces under catalytic conditions in real time and space. Previous studies have shown that adsorbed molecules like NO can form new structures in the Torr pressure range that were undetected in low pressure experiments. Using an STM at high pressures and temperatures also enables us to study the surface as reactions occur with the help of a mass spectrometer (MS). Current reactions of interest include pressure dependent cyclohexene studies on Pt(111) and cyclohexene hydrogenation and dehydrogenation on Pt(111).
At room temperature cyclohexene is known to form different surface species depending on pressure. At low pressures (~1x10-4 Torr) cyclohexene partially dehydrogenates to form a C6H9 species, while at higher pressures (>1 Torr) this species converts to 1,4-cyclohexadiene. Many proposals have been made as to how each species bonds to the platinum surface. Our STM studies hope to elucidate this bonding mechanism as well as learn about the surface structures each species forms. Initial studies have shown that indeed cyclohexene does order on the surface and two distinct types of ordering are observed. (Fig 1 & 2) A more intensive pressure study will hopefully yield valuable information about which species form each type of structure.
In addition our research hopes to investigate hydrogenation/dehydrogenation reactions of cyclohexene on Pt (111). As the temperature of a Pt (111) crystal is increased above room temperature in the presence of hydrogen and cyclohexene, two separate reactions are known to occur. The hydrogenation of cyclohexene to cyclohexane begins at about 300K and peaks slightly below 400K. The dehydrogenation of cyclohexene to benzene however doesnft start taking place until about 350K, peaking at about 480K. This difference in reaction rates at different temperatures allows one to control which reaction is occurring on the surface simply by changing the temperature. Initial experiments show that a sample exposed to mTorr of hydrogen and cyclohexene and heated to 350K for 30 minutes yields at least two different structures. Figure 3 is easily identified as benzene from previous studies, but the structure in figure 4 has not been observed. Further studies should clarify the composition of this surface structure as well as give us a more complete understanding of catalytic cyclohexene hydrogenation and dehydrogenation.
POSTDOC: Feng Tao
STUDENTS:Max Montano (recently graduated), Derek Butcher
(Collaboration work with Prof. Gabor Somorjai, Dept. Chemistry, UC Berkeley)
The detail of the instrument can be found here
Ambient pressure oxidation of Pd(111)
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Figure 1 (left): 30A x 30A image of Pt (111) exposed to 1 Torr cyclohexene |
Figure 2 (right): 75A x 75A image of Pt (111) exposed to 20 mTorr cyclohexene |
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Figure 3(left): 200A x 200A image of Pt (111) exposed to 20 mTorr hydrogen and 20 mTorr cyclohexene and heated to 350K for 30 min. |
Figure 4 (right): 50A x 50A image of Pt (111) exposed to 20 mTorr hydrogen and 20 mTorr cyclohexene and heated to 350K for 30 min. |