We describe here the effect of coadsorbed hydrogen on the initial step in the dehydrogenation of cyclohexane on Pt(111). Many industrial catalytic processes take place in the presence of large background pressures of hydrogen. 1 Methods for studying these reactions and surface intermediates at high pressures are somewhat limited. 1 Several researchers have explored the effect of coadsorbed hydrogen on various hydrocarbon adsorption and dehydrogenation reactions in an ultrahigh vacuum (UHV) environment, but most of these studies did not involve saturation amounts of surface hydrogen. [2][3][4][5][6][7][8][9] Cyclohexane adsorption on Pt-(111) has been well characterized using many of the available modern surface analytical techniques. 10 The C 6 H 12 molecule is known to adsorb intact in the chair conformation at low temperatures with the three cyclohexane axial hydrogens sitting over the 3-fold hollow sites of the Pt(111) surface. 11 This direct cyclohexane axial hydrogen/Pt interaction gives rise to a soft C-H stretch mode observed in vibrational spectra for submonolayers of cyclohexane on Pt(111) at temperatures below 170 K. 12 Dehydrogenation of cyclohexane on Pt(111) begins at a surface temperature of approximately 200 K with the initial formation of a C 6 H 11 species. 13,22 UHV surface techniques used in this work to characterize the H/C 6 H 12 /Pt(111) system include Auger electron spectroscopy (AES) to obtain elemental composition, high-resolution electron energy loss spectroscopy (HREELS) for vibrational studies, thermal desorption spectroscopy (TDS) to monitor desorption products, work function measurements (retarding field method 14 ), and laser induced thermal desorption-Fourier transform mass spectrometry (LITD-FTMS) to determine the molecular composition of the adsorbed species. Three separate ion-pumped vacuum chambers with base pressures of 1 × 10 -10 Torr were used to obtain the spectra. 15-17 Figure 1 shows the vibrational (HREELS) spectra of cyclohexane adsorption on Pt(111) at 135 K for a 0.5 L (1 L ) 1 × 10 -6 Torr‚s) exposure of cyclohexane ( Figure 1a) and a 200 L D 2 exposure followed by a 0.5 L exposure of cyclohexane ( Figure 1b). These spectra were obtained using an LK 2000 HREELS spectrometer. 15 For a 0.5 L cyclohexane exposure to clean Pt ( Figure 1a) a number of vibrational modes are observed in the vibrational "fingerprint" region below 1500 cm -1 . What is most interesting in this spectrum is the appearance of the broad soft C-H stretch mode centered at about 2600 cm -1 which accompanies the other expected hydrocarbon C-H stretch modes at 2920 cm -1 . In Figure 1b the major consequence of a 200 L D 2 exposure coadsorbed with a 0.5 L exposure of cyclohexane is the complete disappearance of the soft C-H stretch mode of cyclohexane in the spectrum. It is important in evaluating this spectrum to know that AES results show that preadsorbed deuterium has no effect on the sticking coefficient of cyclohexane.In other experiments, TDS data on Pt(111) following cyclohexane adsorption at...