The decomposition of methanol on Ni(100)

The decomposition of methanol and deuterated methanol on Ni(100) has been investigated using mainly IR ellipsometric spectroscopy but also thermal desorption and LEED measurements. The IR data support the existence of two intermediate states, here assigned to methoxy and formal moieties respectively. Starting from saturation conditions at ～170 K and warming in 10 K steps, the initial chemisorbed state, believed to be a methoxy species, begins to decompose to a carbonyl-containing species, believed to be a formal species, at ～230 K. At ～260 K, the second intermediate begins to transform to adsorbed carbon monoxide with concurrent desorption of hydrogen. Totally deuterated methanol behaves similarly but shows substantial isotope effects.     

The decomposition of formic acid on Ni(100)

The decomposition of HCOOD was studied on Ni(100). Low temperature adsorption of HCOOD resulted in the desorption of D₂O, CO₂, CO, and H₂. The D₂O was evolved below room temperature. CO₂ and H₂ were evolved in coincident peaks at a temperature above that at which h₂ desorbed following H₂ adsorption and well above that for CO₂ desorption from CO₂ adsorption; CO desorbed primarily in a desorption limited step. The decomposition of formic acid on the clean surface was found to yield equal amounts of H₂, CO, and CO₂ within experimental error. The kinetics and mechanism of the decomposition of formic acid on Ni (110) and Ni(100) single crystal surfaces were compared. The reaction proceeded by the dehydration of formic acid to formic anhydride on both surfaces. The anhydride intermediate condensed into islands due to attractive dipole-dipole interactions. Within the islands the rate of the decomposition reaction to form CO₂ was given by:

$\text{Rate = 6 × 10}{}^{15}\text{exp{?[25,500 + ω(}\text{c}\text{c}{}_{\mathrm{sat}}\text{)]/RT} × c}$

, where c is the local surface concentration, c_sat is the saturation coverage for the particular crystal plane, and ω is the interaction potential. The interaction potential was determined to be 2.7 kcal/mole on Ni(110) and 1.4 kcal/mole on Ni(100); the difference observed was due to structural differences of the surfaces relating to the alignment of the dipole moments within the islands. These attractive interactions resulted in an autocatalytic reaction on Ni(110), whereas the interaction was not strong enough on Ni(100) to sustain the autocatalytic behavior. Formic acid decomposition oxidized the Ni(100) surface resulting in the formation of a stable surface oxide. The buildup of the oxide resulted in a change in the selectivity reducing the amount of CO formed. This trend indicated that on the oxide surface the decomposition proceeded via a formate intermediate as on Ni(110) O.     

Ab initio investigation of the laser induced desorption of iodine from KI(100)

Based on potential–energy curves that were derived from ab initio calculations within the framework of many-body perturbation theory, mixed quantum–classical simulations have been performed to understand the dynamics of the photodesorption process of iodine from KI(100). Dissipation and recoil processes were included by adding a surface oscillator to the ab initio potentials. Using this approach, we reproduce and explain qualitatively the desorption spectrum and the kinetic energy distribution of the desorbing iodine. PACS  68.43.Bc; 31.15.Qg; 31.50.Gh     

Characterization of the adsorption and decomposition of methanol on Ni(110)

The chemisorption, condensation, desorption, and decomposition of methanol, both CH₃OH and CH₃OD, on a clean Ni(110) surface have been characterized using high resolution electron energy loss spectroscopy, temperature programmed reaction spectroscopy, and low energy electron diffraction. The vibrational spectrum of the saturated chemisorbed layer, 7.4 × 10¹⁴ molecules cm^?2, is almost identical to the infrared spectrum of liquid or solid methanol. Condensation of multilayers of methanol is facile at 80 K. The only quasi-stable intermediate isolated during the decomposition is a methoxy species, CH₃O, which decomposes thermally to CO and H. The evolution of both CO and H₂ occurs in desorption limited processes.     

The adsorption and decomposition of methanol on the Rh(100) surface

The adsorption and decomposition of methanol on the Rh(100) surface have been studied using high-resolution electron energy loss spectroscopy and thermal desorption mass spectrometry. Below 200 K, methanol is molecularly adsorbed and bonds to the surface via the oxygen atom. At 200–220 K, a saturated methanol layer undergoes two competing reactions: desorption and OH bond cleavage to form an O-bonded methoxy species. The methoxy species is stable to approximately 250 K. Between 250 and 320 K, a fraction of the methoxy species decomposes to form coadsorbed CO and hydrogen adatoms while the remainder recombines with hydrogen adatoms to desorb as molecular methanol. The hydrogen adatoms remaining on the surface desorb as H₂ between 270 and 400 K, and the CO desorbs between 450 and 550 K. Following a saturation exposure, approximately 0.2 monolayers of methanol decompose to eventually yield CO and H₂ as desorption products. These results are compared to the chemistry of methanol on other metal surfaces.     

Surface diffusion of hydrogen on Ni(100) studied using laser-induced thermal desorption

The surface diffusion coefficient for hydrogen on Ni(100) at low coverage has been measured between 223 and 283 K. A pulsed laser is used to desorb hydrogen from a small, well defined, region on the surface without perturbing the ambient surface temperature. Hydrogen from nearby regions on the surface migrates into the vacant area and the time required to refill that area is determined by subsequent laser-induced-desorption measurements. The diffusion coefficient is obtained using an equation derived from Fick's Second law with non-stationary boundary conditions. The temperature dependence of the diffusion coefficients yields a value of 4.0 ± 0.5 kcal/mol for the energy barrier to diffusion. A value of roughly 3 × 10¹³ s^?1 for the site-to-site hopping frequency is derived when the pre-exponential for diffusion is fit to a random-walk mechanism.     

Polarization dependence in the UV laser desorption of NO from NiO(100)

- and -polarized radiation. When the yield is related to the incident photon flux, desorption cross sections of (1.9±0.3)×10^-17 cm² and (3.3±0.5)×10^-17 cm² for - and -polarized light, respectively, are deduced. In order to assess the importance of substrate excitation the optical constants of NiO have been determined for the photon energy employed. When the desorption yield is then related to the photon flux absorbed in the NiO film, much larger cross sections are obtained, and an even larger effectiveness of -polarized light. For a direct optical excitation of charge transfer states the implications for the symmetry character of the states involved are discussed. It is concluded that excitations to A^′′ states are preferred. Received: 21 October 1998     

Hydrogen adsorption on Ni (100)

The adsorption of hydrogen on the (100) plane of nickel at room temperature has been investigated using the technique of flash desorption spectroscopy. It is shown that no variation in the adsorption enthalpy of 23.1 kcal/mole occurs during the chemical cleaning of the surface by repeated oxidation and reduction. The number of adsorption sites does however increase to 3.3×10¹⁴/cm² during this process. Determination of the partition functions of the adsorbed species and of the activated complex indicates that the hydrogen atoms are localised on specific adsorption sites but that greater liberty exists in the activated complex. Finally the experimental desorption spectra may be described using a model with a repulsive interaction of 400 cal/mole between nearest neighbours.     

Chalcogen adsorption on Ni(100)

The successive stages of the oxidation of Ni(100) have been investigated by angle-resolved uv photoemission. The adsorption spectra are very similar to that measured previously for the same surface saturated with sulphur. Results are interpreted using recent theoretical calculations. It is found that this interpretation can be extended to the other chalcogens adsorbed on Ni(100). When increasing oxygen exposures, photoemission spectra have shown a continuity of the electronic character in the oxidation process.     

X-ray induced electron stimulated desorption versus photon stimulated desorption: NH3 on Ni(110)

The X-ray induced desorption of H⁺ ions from NH₃ layers adsorbed at T = 90 K on Ni(110) has been compared to the corresponding total electron yield (TY) in the photon energy range 390 to 900 eV. The H⁺ yield exhibits a jump at the N K-edge and the Ni L-edge which inversely varies with the NH₃ layer thickness. The H⁺ Ni L-edge jump is closely correlated to the TY jump. Both vanish for the saturated NH₃ multilayer, indicating that the observed Ni L-edge jump in the H⁺ yield is exclusively due to X-ray induced electron stimulated desorption (XESD). At the N K-edge, the near edge absorption fine structure of the H⁺ yield and TY of the saturated NH₃ multilayer are distinctly different. This is interpreted as the H ⁺ yield being the superposition of direct photon stimulated ion desorption (PSID) and XESD. Based on the observed variation of the H⁺ yield near edge fine structure with varying NH₃ layer thickness, a deconvolution of the PSID and XESD contributions is used to derive the relative contribution of PSID versus XESD to be 40% versus 60%, respectively. The relevance of this result for future PSID-SEXAFS studies is discussed. For monolayer NH₃ on Ni(110) the polarization dependence of the N K-edge fine structure in the N(KVV) Auger yield indicates that the symmetry axis of NH₃, is perpendicular to the surface.     

UPS studies of the adsorption and decomposition of methanol and formaldehyde on a tungsten(100) surface

In a study of the interaction of methanol and formaldehyde with an atomically clean W(100) surface at 300 K, ultra-violet photoelectron spectroscopy (U has been used to identify chemisorbed species and to investigate the nature of the chemical ical bonding in adsorbed layers of mixed composition. Tempe programmed thermal desorption has been used to assist in the interpretation of the data. Methanol and formaldehyde, when exposed to a clean W(100) surf decompose to give initially a mixture of adsorbed CO and H. After further exposure, increasing CO coverage causes loss of H. This is followed by the ad of molecular species producing photoelectron spectra and desorption products which are dependent on the coverage of pre-adsorbed CO. The nature of the molecular species, in relation to gas phase methanol and formaldehyde, is discussed.     

Low temperature decomposition of ethylene over Ni(100): Evidence for vinyl formation

Thermal programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), and time-resolved laser-induced desorption (LID) have been used to study the chemisorption and decomposition of ethylene over Ni(100). Ethylene adsorbs molecularly on this surface at temperatures below 150 K. The molecule is π bonded in this state, showing very little rehybridization. At coverages below half saturation, decomposition to vinyl plus a hydrogen atom occurs unimolecularly with a rate constant of (8.0 ± 2.0) × 10⁻² s⁻¹ at 170 K. A strong kinetic isotope effect was observed; vinyl formation from C₂D₄ does not occur until about 200 K. The proposal of vinyl as the intermediate is supported by studies with C₂H₄, 1,1− and 1,2−C₂D₂H₂, and C₂D₄. The reaction is slower at saturation coverages, where molecular desorption is still seen above 200 K. Vinyl decomposes further at 230 K to form an acetylenic fragment.     

Fast leed intensity measurements from Ni(100)c(2 × 2)CO

The Ni(100)c(2 × 2)CO surface structure has been investigated by very fast LEED intensity measurements using a computer controlled television method. It turns out that the intensity spectra are strongly influenced by intolerably long measuring times during which the primary electron beam impinges onto the surface. The spectra have been taken within 16 sec at 100 K immediately after termination of the adsorption process for all beams simultaneously. They are compared with other measurements and with Pendrys model calculations for a CO molecule bonded linearly on top of a Ni atom with straight molecular axis normal to the surface. Using the r-factor formalism for theory-experiment comparison the bond length results to be 1.15 ± 0.1 Å for CO and 1.80 ± 0.1 Å for NiC. This is in agreement with the results of other methods and removes some discrepancies with those of earlier LEED experiments.     

Hydrogen desorption from Ni sites on (110) CuNi surfaces

Hydrogen adsorption on Ni-rich (110) CuNi alloy surfaces has been studied by means of thermal desorption spectroscopy. After adsorption near room temperature the hydrogen desorption spectra exhibit a coverage dependence similar to that known from pure (110)Ni. Besides a slightly composition dependent desorption energy the alloy surfaces behave like a (110)Ni surface diluted by practically inert Cu. These results are compared to those reported by Yu Ling and Spicer.     

Electronic structure of graphene on Ni(111) and Ni(100) surfaces

Physics of the Solid State - A comparative investigation of graphene prepared by cracking of propylene (C3H6) on nickel surfaces with different orientations, Ni(111) and Ni(100), has been carried...     

Structure of CO adsorbed on Ni (100)

The c(2 × 2) configuration of CO chemisorbed on Ni(100) has been examined by the dynamical LEED method of surface structure analysis. Experimental LEED intensity spectra of the (00), ($\text{1}\text{2}\text{1}\text{2}$) (10) and (11) LEED beams measured at 175 K are compared with the corresponding calculated spectra for two different CO potential constructions and a number of trial structures. The best agreement was found for a structure where the CO molecules sit directly above the Ni atoms with vertical spacings between the Ni and C and the C and O layers of 1.80 ± 0.10 A and 0.95 ± 0.10 Å respectively. It is proposed that the CO molecule is tipped over at an angle of 34° ± 10° with respect to the surface normal so that the actual carbon-oxygen bond length is close to the figure 1.15 Å found in Ni(CO)₄.     

Phase diagram of oxygen on Ni(100)

We have studied the coverage-temperature phase diagram of chemisorbed oxygen on Ni(100) with LEED and AES. We find that the oxygen p(2 × 2) structure undergoes a nearly reversible order- disorder phase transition for coverages between 0.15 and 0.30 monolayer (T_c ≈ 560 K for θ = 0.25). For these coverages, it is also found that the chemisorbed oxygen begins dissolving into the bulk nickel as the overlayer disorders. A reversible order-disorder transition is not seen for the oxygen c(2 × 2) structure, but there is evidence that bulk dissolution here also occurs coincident with the overlayer disordering.     

Time resolved Nomarski interferometery of laser produced plasma plumes

We report results from optical interferometric probing of a laser generated Zn plasma plume. The experiment was performed in a vacuum and O₂ rich environments where the background pressure of O₂ was maintained at 1000 Pa and the results from both regimes are compared. The focus of our work is very much on the early stages in the life of the plasma plume which remains, to date, a largely unexplored area of study, at least in the pulsed laser deposition research domain. It was found that the electron density profile normal to the target is different in the background gas at early times (∼30 ns) compared to that of the vacuum case. At later times (∼80 ns) both profiles have a very similar shape. We also observe the formation of a shock wave at the plasma-gas interface shortly after plasma breakdown (<15 ns).