W. Erley

HREELS investigation of the orientation and dehydrogenation of cyclohexane on Pt[111]

Abstract In this article, we report on the investigation of cyclohexane adsorbed on Pt[111] with high resolution electron energy loss spectroscopy (HREELS). Variation of the angle of incidence of the electron beam is used to determine the orientation of the adsorbed molecule. The symmetry of the adsorbate/surface complex is C3v, which indicates adsorption with the C3 axis of the molecule perpendicular to the surface, i.e., with the plane of the molecule parallel to the surface. Annealing of the surface to different temperatures followed by HREELS of the resultant species shows that dehydrogenation begins by 200 K, but that benzene is not formed until 300 K. Spectra between 200 and 300 K show very little variation, implying a single predominant intermediate. Angular profiles after annealing to 260 K indicates the intermediate/surface complex has low symmetry and lends insight to the possible identity.

Infrared spectroscopy of NO adsorbed on Ni(111) at 85 K

Abstract The adsorption of NO on Ni(111) at 85 K has been studied by Fourier transform infrared spectroscopy. At low coverage a molecular adsorption state characterized by a NO stretching band at 1460 cm −1 is observed. This state saturates at a coverage of 0.02 and is interpreted as being due to a NO molecule adsorbed near a surface defect site. At intermediate coverage two NO stretching modes are observed, typically at 1508 and 1543 cm −1 . The two states are occupied sequentially. This is in remarkable agreement with an earlier EELS study by Lehwald, Yates and Ibach in which the existence of a bent and an upright NO configuration was proposed although only a single NO stretching band in this coverage range could be resolved. The conclusion was based on the observation of a NO bending mode and its subsequent disappearance. Finally, heating a NO saturated Ni(111) surface to 320 K causes the appearance of a strong band at 1681 cm −1 . This band, which has not been observed by EELS, is interpreted as being due to a NO molecule adsorbed at an on-top site.

Adsorbate-induced surface stress and surface reconstruction: oxygen, sulfur and carbon on Ni(111)

The absorbate-induced surface stress of carbon, oxygen, and sulfur on Ni(111) was investigated at different substrate temperatures. All three adsorbates cause a compressive stress which increases non-linearly with coverage. The results are discussed in connection with the adsorbate-induced reconstructions.

Photochemical reactions of water adsorbed on Pt(111)

The photochemical reaction of water adsorbed on Pt(111) at 85 K has been investigated by high resolution electron energy loss spectroscopy (HREELS) and thermal desorption spectroscopy (TDS). Under the action of ultraviolet (UV) light, molecular desorption as well as dissociation of adsorbed water into OH and H has been observed. In addition to the known vibrational losses of adsorbed OH at 430 cm−1 (PtO stretching) and 1020 cm−1 (OH bending), two new losses are observed at 340 and 820 cm−1, which are assigned to a hindered translation and a second OH bending vibration, respectively. Using central force field calculations, a tilt angle of 15° of the OH bond towards the Pt surface could be derived from the difference of about 200 cm−1 between the two OH bending vibrations. From energetic considerations it is assumed that the photodissociation (threshold: 5.2 eV) occurs via a substrate excitation. Thermal desorption spectra taken after UV irradiation of adsorbed water exhibit two peaks, one at 175 K and one at 180 K, indicating both molecular desorption of water and recombination of OHad and Had, respectively.

The conversion of ethylene to ethylidyne on Pt(111): non-first order kinetics and ensemble effects

Abstract The conversion of ethylene to ethylidyne on Pt(111) has been studied in detail by laser induced thermal desorption (LITD) in conjunction with Fourier transform mass spectrometry (FTMS) and Fourier transform infrared reflection absorption spectroscopy (FT-IRAS). The formation of ethylidyne shows strictly first order kinetics over the entire coverage range. In contrast, this holds only for low initial coverages in case of the decomposition of ethylene, whereas at high coverages a pronounced non-first order behavior is observed. The latter observation can be explained by an ensemble effect which assumes an increased decomposition rate for ethylene molecules in the immediate vicinity of an ensemble of free Pt sites. Using a simple Monte Carlo calculation, not only the measured decomposition rates can be reproduced but, in addition it shows that the reaction proceeds in patches rather than uniformly. This result is in agreement with recent scanning tunnelling microscope (STM) observations.

Adsorbate-induced surface stress: CO on Ni(100) and Ni(111)

Abstract We have determined the surface stress induced by the room-temperature adsorption of carbon monoxide on the (100) and (111) surfaces of nickel by measuring the bending of a thin nickel crystal. While CO adsorption induces exclusively a compressive stress on Ni(111), the stress is tensile on Ni(100) at CO coverages below 0.2 ML. The tensile stress passes through a maximum of + 0.96 N/m at θ = 0.09 to become compressive for θ > 0.25 with a value of −0.54 N/m at θ = 0.5. The latter value is small compared with the stress produced by other, more strongly chemisorbed adsorbates on the same surface. As CO molecules predominantly occupy terminal binding sites on Ni(100) at 300 K, we must exclude a site conversion process in order to explain the stress-coverage dependence. The sign reversal of surface stress might be attributed to a coverage dependent variation in the net charge transfer between the metal surface and the adsorbate, involving an enhanced splitting of the bonding and antibonding 2π ∗ CO orbitals of neighboring molecules at higher adsorbate coverages.

A RAIRS and HREELS study of isopropyl alcohol on Pt(111)

RAIRS, HREELS and TDS were used to characterize isopropyl alcohol (IPA) adsorbed on Pt(111). Little or no decomposition occurs during adsorption at 85 K, but during thermal desorption of monolayer and submonolayer coverages, all the IPA decomposes to acetone and hydrogen below 200 K. No CO or CH4 was detected. TDS of multilayer coverages produce a strong IPA peak at 153 K and less intense peaks at 173 and 217 K along with acetone peaks between 190 and 230 K. The desorption energy of the multilayer species is 10.6 kcalmol. Preadsorbed hydrogen (0.75 ML) markedly reduces IPA decomposition and allows the monolayer species to desorb at 208 K, which corresponds to a desorption energy of 13.2 kcalmol. RAIRS and HREELS spectra gave sharp bands for multilayer IPA that are in excellent agreement with liquid-phase reference spectra. Vibrational bands associated with acetone appear after decomposition, and flashing to 200 K removes IPA but leaves strongly bound acetone with bands between 1510 and 1610 cm−1 which are similar to those associated with an η2 acetone species on this Pt surface.

Thermal decomposition of trimethylamine on Pt(111): spectroscopic identification of surface intermediates

Abstract The adsorption and decomposition of trimethylamine on a Pt(111) surface have been investigated, using high resolution electron energy loss spectroscopy, Fourier transform infrared reflection—absorption spectroscopy and temperature-programmed desorption. Trimethylamine adsorbs molecularly at 85 K through the nitrogen lone pair on the Pt(111) surface. Upon heating, four steps of decomposition are observed, starting with sequential CN bond cleavage. Three different intermediate surface species can be identified by vibrational spectroscopy. Heating to 280 K generates a dimethylamino species, (CH 3 ) 2 N. Further heating to 400 K causes the formation of a methylnitrene species, CH 3 N. This species dehydrogenates to a methylene amido species, CH 2 N around 450 K. Finally, complete dehydrogenation takes place at temperatures above 500 K.

Spectroscopic identification of an HCNH species on Pt(111)

Abstract The adsorption and decomposition of methylamine on a Pt(111) surface have been investigated, using infrared reflection-absorption spectroscopy (IRAS) and high resolution electron energy loss spectroscopy (EELS). At 85 K, methylamine is found to adsorb molecularly on Pt(111). After heating the methylamine covered surface to temperatures in the range from 350 to 390 K, new infrared bands at 3371, 1565 and 1325 cm −1 are observed, indicating the presence of a new surface species which is formed by the dehydrogenation of adsorbed CH 3 NH 2 . These bands which shift to 2544, 1440 and 1296 cm −1 when using CH 3 ND 2 , are assigned to the NH (ND) stretching, CN stretching and CH bending modes of an adsorbed HCNH (HCND) species, respectively. The CH stretching band which is not detectable by IRAS is clearly observed at 2965 cm −1 using off-specular EELS.

Site occupation of CO adsorbed on Ni(100) at high CO pressures

Abstract The adsorption of CO on Ni(100) has been investigated by infrared reflection-absorption spectroscopy in the temperature range of 85 to 300 K. At 300 K we achieved CO coverages up to 0.8 ML by applying CO pressures up to 1 mbar. The fraction of CO molecules adsorbed at terminal and bridge sites on Ni(100) at a fixed temperature depends strongly on the total CO coverage. The variation of the adsorption site occupancy with coverage is well reproduced by a lattice-gas model which takes into account the lateral interaction of the adsorbed molecules.