The Structures and Reactions of Linear and Cyclic C₆Hydrocarbons Adsorbed on the Pt(111) Crystal Surface Studied by Sum Frequency Generation Vibrational Spectroscopy:  Pressure, Temperature, and H₂Coadsorption Effects

Abstract

We studied the adsorption structures and reactions of C 6 hydrocarbon molecules (cyclohexene, cyclohexane, 1-methylcyclohexene, n-hexane, 2-and 3-methylpentanes, and 1-hexene) on Pt(111) using sum frequency generation (SFG) vibrational spectroscopy. The experiments were performed in the presence and absence of excess hydrogen and as a function of temperature. Upon cyclohexene adsorption on Pt(111) at 1.5 Torr, 1,3and 1,4-cyclohexadienes and π-allyl C 6 H 9 were observed in the presence of excess hydrogen. Cyclohexane adsorption at 1.5 Torr resulted in the formation of cyclohexyl on the surface in the presence of excess hydrogen but π-allyl C 6 H 9 in absence of excess hydrogen. 1-Methylcyclohexene formed methylcyclohexenyl on the surface only in the presence of excess hydrogen. n-Hexane and 3-methylpentane adsorbed molecularly on Pt(111) at 296 K in the presence of excess hydrogen. 2-Methylpentane and 1-hexene were readily dehydrogenated to form metallacyclobutane and hexylidyne even at 296 K, regardless of the presence of excess hydrogen. n-Hexane was dehydrogenated to form hexylidyne or metallacyclic species at high temperature in the presence of excess hydrogen. Hexylidyne and metallacyclic species were also main surface intermediates in dehydrogenation of 2-and 3-methylpentane. The absence of excess hydrogen induced dehydrocyclization of n-hexane to form π-allyl c-C 6 H 9 . On the basis of the SFG results, the mechanism of the n-hexane conversion to benzene is discussed.

Major Successes of Theory-and-Experiment-Combined Studies in Surface Chemistry and Heterogeneous Catalysis

“…Performing in situ spectroscopy techniques such as polarization-modulated reflection-absorption infrared spectroscopy (PM RAIRS) and SFG under reaction conditions usually results in complex spectra [81,[88][89][90][91]. The development of reliable theoretical methods for predicting the vibrational frequencies of surface intermediates will provide tremendous help in the spectrum interpretation and in determining the coverages of the reaction intermediates that may adsorbed simultaneously on the catalyst surface [92,93].…”

Molecular surface science of C–H bond activation and polymerization catalysis

“…Cyclohexene adsorption under UHV conditions has been extensively studied on the Pt(111) and Pt(100) surfaces using thermal desorption spectroscopy (TDS) [23][24][25], reflection adsorption infrared spectroscopy (RAIRS) [23], electron energy loss spectroscopy (EELS) [23], bismuth post dosing TDS (BPTDS) [5,25], laser-induced thermal desorption (LITD) [24], high-resolution electron energy loss spectroscopy (HREELS) [23,24], and SFG [26][27][28][29][30][31]. Briefly, cyclohexene exists in a di-r form on the Pt(100) and Pt(111) surfaces at 100 K. Di-r cyclohexene is transformed to p-allyl c-C 6 H 9 as the surface temperature is increased to 200 K. Dehydrogenation and benzene desorption results from further increasing the surface temperature [23].…”

“…Upon adsorption at low pressures, cyclohexene is know to form the C 6 H 9 p-allyl species [35]. An image of this structure is shown in figure 11.…”

The Nanoscience Revolution: Merging of Colloid Science, Catalysis and Nanoelectronics

“…Thus, dehydrocyclization of n-hexane to benzene is dominant over isomerization of n-hexane in the absence of excess hydrogen. The difference in the catalytic selectivity from the presence or absence of excess hydrogen possibly results from an increase in the disorder of surface species and decrease in the surface coverage of the p-C 6 H 9 intermediate due to the weakening of the hydrocarbon-Pt bond when excess hydrogen is present [37,42].…”