Michael Trenary

An infrared reflection-absorption study of CO chemisorbed on clean and sulfided Ni(111) — Evidence for local surface interactions

Abstract Infrared Reflection-Absorption Spectroscopy (IRAS) has been used to characterize CO chemisorption on a clean and sulfur-modified Ni(111) surface under both ultrahigh vacuum and high CO pressure conditions. On clean Ni(111) correlations between the observed LEED patterns and the infrared spectra show that for the c(4 × 2) structure with a CO coverage of θco = 0.50, all CO molecules occupy two-fold bridge sites as indicated by a single infrared band and 1910 cm−1. For θco = 0.57, a ( 7 2 × 7 2 )R19.1° structure forms with infrared bands at 2058 cm−1 corresponding to terminally bound CO and at 1925 cm−1 for the bridge-bonded CO. The change in the overlayer structures is accompanied by an abrupt change in the coverage dependence of the bridge-bonded CO frequency for adsorption at 80 K. In studies involving known sulfur coverages on Ni(111). a new sulfur-induced CO state is observed with an infrared band which shifts from 2109 cm −1 to 2103 cm−1 corresponding to increasing sulfur coverage. The sulfur-induced CO state is only populated at 300 K when a high ambient CO pressure is present. The infrared data clearly indicate that the CO and S interact on the Ni surface through a local short-range mechanism: this short range COS interaction model is supported by kinetic studies of CO desorption from the sulfided Ni(111) surface.

Temperature dependence of the vibrational lineshape of CO chemisorbed on the Ni(111) surface

Abstract The lineshape of the carbon-oxygen stretching vibration for CO chemisorbed at the two-fold bridge sites and on top sites of Ni(111) has been measured over the temperature range 80 to 300 K with infrared reflection absorption spectroscopy. The bridge bonded CO undergoes pronounced broadening at higher temperatures while the terminally bonded CO is only slightly broadened. The results are interpreted according to a recent vibrational dephasing model developed for condensed phase molecules. In this model the dephasing is brought about by rapid energy exchange between low frequency modes of the substrate and low frequency modes of the molecule which are anharmonically coupled to the high frequency band being studied.

Adsorbate ordering processes and infrared spectroscopy: An FT-IRAS study of N2 chemisorbed on the Ni(110) surface

A newly constructed apparatus for Fourier transform‐infrared reflection absorption spectroscopy (FT‐IRAS) studies has been used in a detailed study of the N–N stretching band of N2 weakly chemisorbed on the Ni (110) surface. The high resolution and the high signal‐to‐noise ratios of the spectra allow observation of subtle changes of the band shape which accompany changes in the overlayer structure. The results are discussed with respect to a detailed two‐dimensional phase diagram recently proposed for the N2/Ni (110) system. The IR data clearly reveal that for low coverages nonequilibrium adsorption occurs at 81 K, while equilibrium is attained at 125 K in agreement with the phase diagram. The present results indicate that the incommensurate overlayer structure which forms at the highest coverages is characterized by an intense sharp IR band at 2194 cm−1, a weak shoulder at 2204 cm−1, and a still weaker satellite peak at 2220 cm−1. The IR results for the incommensurate N2 overlayer are compared with model...

Nanoindentation and Raman spectroscopy studies of boron carbide single crystals

The measurements of hardness and elastic modulus have been conducted on the (0001) and (1011) faces of B4.3C single crystals using nanoindentation. The results are in good agreement with the corresponding values obtained using a conventional microhardness technique on polycrystalline ceramics. Raman microspectroscopy analysis of the nanoindentations shows the appearance of several bands which suggest dramatic structural changes in the indented material. Localized contact loading may lead to damage in boron carbide resulting in disorder or a pressure-induced solid state phase transformation in the region under the indenter, although the exact mechanism responsible for the observed Raman spectra could not be identified at this time. This may explain why little variation in mechanical properties was observed with respect to the crystallographic orientation.

Infrared vibration-rotation selection rules for chemisorbed molecules with free internal rotation: Results for ethylidyne on Pt(111)

We have studied the infrared spectrum of ethylidyne, CCH3, chemisorbed on the Pt(111) surface over the temperature range 82 to 350 K. We observe three infrared active fundamentals: the C–C stretch at 1118 cm−1, the symmetric CH3 bend at 1339 cm−1, and the symmetric C–H stretch at 2884 cm−1. The absence of three other fundamentals in our spectra confirms that the molecule has C3v symmetry on the surface with the C–C axis oriented along the surface normal as had been determined from other studies. Our IR spectra demonstrate the strict validity of the surface dipole selection rules. We also observe a weak band at 2795 cm−1 which we attribute to the first overtone of the asymmetric CH3 bend at 1410 cm−1. The intensity of the overtone is enhanced by a Fermi resonance with the symmetric C–H stretch. At 82 K the symmetric bend has an unusually narrow intrinsic width of only 1.4 cm−1. The narrowness of this band makes it a good choice for investigating the influence of free rotation about the single C–C bond on t...

Reversible Control of Hydrogenation of a Single Molecule

Low-temperature scanning tunneling microscopy was used to selectively break the N-H bond of a methylaminocarbyne (CNHCH3) molecule on a Pt(111) surface at 4.7 kelvin, leaving the C-H bonds intact, to form an adsorbed methylisocyanide molecule (CNCH3). The methylisocyanide product was identified through comparison of its vibrational spectrum with that of directly adsorbed methylisocyanide as measured with inelastic electron tunneling spectroscopy. The CNHCH3 could be regenerated in situ by exposure to hydrogen at room temperature. The combination of tip-induced dehydrogenation with thermodynamically driven hydrogenation allows a completely reversible chemical cycle to be established at the single-molecule level in this system. By tailoring the pulse conditions, irreversible dissociation entailing cleavage of both the C-H and N-H bonds can also be demonstrated.

The thermal decomposition of azomethane on Pt(111)

Abstract The thermal decomposition of azomethane, CH 3 N=NCH 3 , and methylamine, CH 3 NH 2 ,on Pt(111) has been investigated by reflection absorption infrared spectroscopy (RAIRS) and temperature programmed reaction spectroscopy (TPRS). Using the surface dipole selection rule of RAIRS and comparing with the IR and Raman spectra of solid trans -and cis -azomethane, we find that trans -azomethane converts to cis -azomethane upon adsorbing onto the Pt(111) surface at 84 K. Additionally, we propose that the cis -azomethane tautomerizes to formaldehyde methylhydrazone, CH 3 NHN=CH 2 , which subsequently undergoes N-N bond cleavage between 200 and 275 K. Above 300 K aminomethylidyne, CNH 2 , is formed. Aminomethylidyne is also formed on the methylamine dosed surface. Cyanogen, C 2 N 2 (g), formed from the combination of CN ads was also observed by TPRS as a minor reaction product. A comparison of the IR spectra of azomethane and methylamine dosed surfaces indicates the formation of chemisorbed methylamine as one intermediate in the decomposition of azomethane.

Carbon-carbon coupling of methyl groups on Pt(111)

Abstract Although methyl radicals are widely postulated to be important intermediates in many catalytic processes involving hydrocarbon species, relatively little is known about their reaction kinetics on metal surfaces. In order to more fully understand the surface chemistry of methyl groups we have developed a source of gas-phase methyl radicals based on the pyrolysis of azomethane. The surface chemistry of methyl groups adsorbed on Pt(111) has been studied using TPD and reflection-adsorption infrared spectroscopy (RAIRS). Methyl radicals can be dosed directly onto the surface at 150 K and are bound in a C3v geometry. Methyl groups become thermally activated above 230 K, reacting to produce a hydrogenation product (methane) and various dehydrogenation products. The nature of these dehydrogenation products depends upon the initial methyl coverage. At low coverages CHx (x 3) species are predominant while at higher methyl coverages is evidence of carbon-carbon coupling reactions to produce higher order hydrocarbons one of which has been identified as ethylidyne (CCH3) Ethylidyne is also observed as a reaction product on Pt(111), at 150 K, for large methyl radical exposures.

Electron spectroscopy study of SiC

A silicon carbide single crystal has been studied using x‐ray photoelectron spectroscopy and x‐ray and electron excited Auger spectroscopy. A procedure for producing an atomically clean SiC crystal surface has been perfected. The SiC exhibits prominent bulk plasmon loss features associated with Si and C photoelectrons. This loss, at 22.5 eV, agrees well with optical data establishing the bulk plasmon energy.

Formation of aminomethylidyne from hydrogen cyanide on Pt(111)

Abstract The thermal chemistry of HCN has been investigated from 85 to 450 K by Fourier transform-infrared absorption spectroscopy (FT-IRAS) and thermal desorption spectroscopy (TDS). The adsorption of HCN at 85 K yields an infrared spectrum consisting of an intense band at 3298 cm−1 due to the CH stretch and a weak band at 1311 cm−1 that is assigned to the overtone of the HCN bend. The close resemblance between the IR spectrum of HCN absorbed on Pt(111) at 85 K and the IR spectrum of HCN in the gas phase and trapped in an argon matrix suggests that HCN is adsorbed molecularly at 85 K and is only weakly perturbed by the surface. Upon warming to 300 K, the H12C14N is transformed to a new molecular species with bands at 3363, 1567, and 1323 cm−1. From the shifts of these bands following initial adsorption of the H13C14N and H12C15N isotopes, they are assigned to an NH stretch (3363 cm−1), NH2 bend (1567 cm−1), and CN stretch (1323 cm−1). Comparison with the IR spectrum of an Os3H(μ-CNH2)CO10 complex indicates that the 300 K species is aminomethylidyne, CNH2.