Abstract
Graphite basal surface is inert, and decomposition of n-alkanes on the graphite surface has not been discovered. We here report the evidence of decomposition of n-octanes on highly oriented pyrolytic graphite (HOPG) surface, heat-treated up to 1200 °C under high vacuum (10−7 Pa), at near room temperatures. Using a temperature programmed desorption apparatus equipped with a quadrupole mass spectrometer showed the production of hydrogen molecules, methane, and ethane, suggesting that the protonation of n-octane takes place on graphite surface at near room temperature. It is known that acidic functional groups are terminated at edges on the air-cleaved HOPG surface and they increase their acidity via reactions with water. However, it is most unlikely that they protonate n-alkanes at near room temperature such as superacids. We anticipate that superacidic protons, which can protonate n-octanes, are produced on the graphite surface through a novel reaction mechanism.
Highlights
The results of TPD–quadrupole mass spectrometer (QMS) measurements showed that the thermal desorption of adsorbed n-alkanes on graphite surface occurs without any detectable decomposition or chemical reactions[9], the desorption of the adsorbed n-alkanes from the graphite surface occurs in a narrow temperature range near their melting points, considerably lower than room temperature
STM has been employed to study the form of n-alkanes adsorbed on Highly oriented pyrolytic graphite (HOPG) surface, but no STM image has been reported on the alkane molecules shorter than n-C16H34 adsorbed on the HOPG surface at room temperature[16]
It is known that oxygen- and hydrogen-containing functional groups exist on the air-cleaved HOPG surface[6], and these functional groups serve to hinder alkanes to be adsorbed on the graphite surface
Summary
TPD spectra of HOPG surface brought into contact with n-octanes. Figure 1a shows TPD spectra of m/z = 2–120 obtained for the HOPG sample at temperatures between 30 °C and 600 °C with a heating rate of 100 °C/min, where x-axis represents the temperature measured using a thermocouple placed in the sample stage of quartz. The relative intensities of m/z = 27, 28, 29, 41, 42, 43, 55, 56, 69 and 70 signals obtained from the NIST mass spectrum database of propene, 1-butene, 1-pentene, 3-hexene, 1-heptene, and 1-octene are shown, where the intensities of the base peaks of these alkenes are assigned an abundance of 100 From these facts mentioned above, it is concluded that most of the m/z 27, 28 and 29 signals are not due to fragments from C3-C8 alkenes. The superacidic protons can give rise to room temperature superconductivity[27]
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