Decomposition of formic acid on Ru(001): An EELS search for a formic anhydride intermediate

High resolution electron energy loss spectroscopy has demonstrated the existence of a symmetrical bidentate bridging formate as a stable intermediate in the decomposition of formic acid on the Ru(001) surface. This formate decomposes with two pathways — via CH bond cleavage to give CO₂ and adsorbed hydrogen, and via CO bond cleavage to give adsorbed hydrogen, oxygen and CO. Thermal desorption demonstrates the evolution of H₂, H₂O, CO and CO₂. The observation of this product distribution from Ru(100), Ni(100) and Ni(110) had prompted the proposal of a formic anhydride intermediate. The spectroscopic results reported here bring the presence of this postulated, normally unstable species into question.     

Characteristic energy loss measurements of slow electrons on clean and adsorbate-covered Ru(001)

Characteristic energy losses of low energy electrons backscattered from Ru(001) have been measured under conditions of very low primary electron currents for the clean and the CO- or oxygen-covered surface. The main losses found for the clean and the CO-covered surface are similar to those observed as XPS core satellites which may mean that the influences of the core hole on the initial and the final states of the valence shake-up are about the same. A peak in the secondary electron spectrum of the clean surface is found at 11 eV which is changed by adsorption. The results are discussed in terms of the excitations of the metal and the adsorbates.     

The decomposition of formic acid on Ru(101̄0)

The decomposition of formic acid was studied on a clean Ru(101̄0) surface adsorption temperature between 100 and 460 K by means of flash thermal desorption. The decomposition products observed were H₂, CO₂, H₂O and CO. HCOOH itself was also desorbed, although at low exposures no formic acid was observed. The H₂ and CO₂ products were desorbed in identical first order peaks, with a peak temperature of 395 K. The H₂O product desorbed in a second order peak at 813 K, in contrast to H₂O desorption from low coverage H₂O adsorption which occurs in two peaks in the region of 220 and 265 K. The CO product desorbed in a first order peak at 488 K, identical to CO from CO adsorption. The dependence of the product peaks on adsorption temperature of the Ru surface was also studied. These results suggest a model involving the formation and decomposition of a surface intermediate species.     

Electron energy loss spectroscopy studies of nitrogen adsorption on W(100)

Vibrational excitations of nitrogen on W(100) are investigated over the 100–300 K temperature range using elastic and inelastic electron scattering. New vibrational modes of nitrogen are identified that require different mode assignments from previous work. Experimental evidence for a molecular precursor to the atomic β₂ phase of adsorbed nitrogen is presented. Coverage dependent studies of vibrational modes suggests conversion between two different molecular surface phases and between atomic and molecular phases. A new ordered nitrogen phase characterized by a (4 × 1) LEED pattern is observed. The new phase appears to consist of orthogonal domains of p(4 × 1) symmetry that contain atomic nitrogen at the four fold sites (the β₂ atomic phase) with additional bridge-bonded nitrogen atoms in the unit cell.     

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.     

Electron energy loss spectroscopy of sodium covered Ge(111) surfaces

The ELS spectra of sodium covered Ge(111) surfaces are studied. It was found that sodium coverage upto 1 ML leads to a extinction of the ELS features of the clean Ge surface. With increasing sodium coverage a metal induced peak at 3.4 eV and a peak at 31.6 eV due to a sodium 2p-core level transitions appear. ELS spectra of a heat-treated surface covered with sodium imply a formation of a surface Na-Ge “compound” as a result of Na-Ge interdiffusion.     

Identification of the intermediates in the dehydration of formic acid on Ni(110) by high resolution electron energy loss vibrational spectroscopy

High resolution electron energy loss vibrational spectroscopy was used to study the intermediate formed in the dehydration reaction for formic acid on Ni(110) and Ni(110)(4 × 5)C. On the carbided surface only the formate was observed. The frequencies of the asymmetric and symmetric OCO sketch indicated a monodentate configuration. On the clean surface a mixed adlayer of CO and HCOO formed. No losses expected for formic anhydride were observed. Lateral interactions between CO and HCOO appear to be responsible for the autocatalytic decomposition of the formate.     

Electron loss spectra from a W(001)surface

A high resolution energy dispersion analysis of the electron loss spectra (ELS) from a W(001) surface has been done with a specular reflexion geometry. The non-dispersive ELS correlates well with a combination of the bulk and surface loss functions determined from recent optical data whereas the dispersive ELS showed significant differences. Multiple plasmon losses were demonstrated while for the interband losses a momentum selection method has been demonstrated to separate scattering in the surface region from that in the bulk.     

Femtosecond surface vibrational spectroscopy of CO adsorbed on Ru(001) during desorption

Using time-resolved sum-frequency generation spectroscopy, the C- O stretch vibration of carbon monoxide adsorbed on a single-crystal Ru(001) surface is investigated during femtosecond near-IR laser excitation leading to desorption. A large transient redshift, a broadening of the resonance, and a strong decrease in intensity are observed. These originate from coupling of the C- O stretch to low-frequency modes, especially the frustrated rotation, that are highly excited in the desorption process.     

Adsorption of pyridine on RU(001): a study by high resolution electron energy loss spectroscopy

The adsorption states of pyridine on Ru(001) have been studied as a function of exposure and temperature using HREELS. An unusually stable parallel-bonded state I is formed at low coverage below 250 K; at large exposure below 150 K a tilted form II can be additionally formed. A dehydrogenated α-pyridyl state III can be generated by heating multilayers or dense monolayers of type II to 190 K, or by adsorbing at this temperature. Neither III nor II can be formed by heating I.     

Electron energy loss processes in X-ray photoelectron spectroscopy

A technique is established in X-ray photoelectron spectroscopy (XPS), using spectra emitted from successively evaporated metallic films, to distinguish between electron energy loss mechanisms identified as, respectively, extrinsic and intrinsic to the photoelectron excitation process. It is demonstrated that tailing on the high kinetic energy side of many XPS peaks is due to intrinsic processes, while the background emission at energies generally some 30 eV below the peaks arises from extrinsic processes. Plasmon energy-loss peaks are believed to contain contributions from both intrinsic and extrinsic processes.     

Electron spectroscopy (ESCA) studies of RuCu catalysts

Electron spectroscopy (ESCA) studies were conducted on a series of unsupported ruthenium-copper catalysts with copper contents ranging from 1.5 to 5 at%. Previous chemisorption and catalytic studies had indicated that the copper in these catalysts tends to cover the surface of the ruthenium. The ESCA data, as represented by the ratio of the intensity of the Cu $\text{2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ peak to that of the Ru $\text{3d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ peak, are consistent with this view. The ESCA data were obtained with and without in situ hydrogen pretreatment of the catalysts at elevated temperature. When data on a number of ruthenium-copper catalysts are considered, there is an inverse correlation between hydrogen chemisorption capacity and the Cu/Ru intensity ratio obtained in the ESCA measurements. Moreover, when catalytic activity for ethane hydrogenolysis is considered as a function of the Cu/Ru intensity ratio, a very pronounced inverse correlation is obtained. With regard to the electronic structure of ruthenium-copper catalysts, the ESCA data showed no evidence of shifts in core level binding energies for either copper or ruthenium.     

Direct observation of vibrational energy delocalization on surfaces: CO on Ru(001)

We report the experimental observation of the gradual transition from a local oscillator to a two-dimensional delocalized phonon, observed for the CO-stretch vibration of carbon monoxide adsorbed on a Ru(001) surface by means of broadband-infrared saturation sum-frequency spectroscopy. The data are theoretically reproduced by an exchange model with residence times of the excitation down to 2.5 ps.     

Electron energy loss spectroscopy of ethylene on Ag(110) precovered with oxygen: energy dependence of the cross section

The intensities of the CH out-of-plane bending (v₇) and CH stretch (v₁) vibrations of ethylene adsorbed on Ag(l 10) precovered with oxygen have been measured in EELS as a function of beam energy from 2 to 20 eV. The energy dependence of the v₇ vibration is satisfactorily described by the dipole approximation. For the v₁ vibration, which is purely dipole forbidden, an entirely different energy dependence is found, with a maximum intensity at a beam energy of 2 eV. Comparison of the EEL intensities observed experimentally with those predicted from the dipole moments of the free molecule confirms the conclusion that the molecular plane of chemisorbed ethylene is parallel to the surface.     

Electron energy loss spectroscopy and ultraviolet photoemission of TmSe

Intraionic Tm²⁺ and Tm³⁺ ? → d and p → d charge transfer transitions are identified in the energy loss spectra of 100eV electrons of TmSe by comparison with X-ray photoemission data. In contrast the ultraviolet photoemission spectra with photon energies up to 21.2 eV do not show any evidence for the intermediate valence character of TmSe due to matrix element effects.     

Electron energy loss spectroscopy measurement of surface vibration dispersion on clean and oxygen-covered Ni(100)

Inelastic electron scattering has been carried out at 300 K on Ni(100) and at 150 K on p(2 × 2) and c(2 × 2) oxygen overlayers adsorbed on Ni(100). Impact energies ranged from 4 to 300 eV in order to measure the dispersion curves of surface vibrations throughout the two-dimensional Brillouin zone in the [110] direction. The Rayleigh mode has been observed in all cases. On O-coved surfaces a surface resonance and two vibrations of different polarizations associated with oxygen motion have been detected. The polarizations of the detected modes and the origin of the resonances, arising from the folding of the Brillouin zone due to the adsorbate, have been analysed with the help of symmetry considerations and the EELS selection rules. The O-coverage dependence of the Rayleigh mode frequency suggests a continuous outwards expansion of the first Ni plane starting from a contracted clean surface. The O-dispersion data are consistent with an O layer distant from the first Ni plane by ～ 0.9 Å in both investigated overlayers.     

Sum-frequency generation and temperature-programed desorption studies of formic acid on MgO(001) surfaces

Adsorption and decomposition of formic acid on MgO(001) surfaces were studied by means of Sum-Frequency Generation (SFG), Temperature-Programed Desorption (TPD) and Atomic Force Microscopy (AFM). Formic acid was dissociatively adsorbed on MgO(001) surfaces at room temperature. From SFG and TPD measurements, the existence of several different types of formate species was deduced and all species mainly decomposed into CO and H₂O on raising the temperature of the substrate.Paper presented at the 129th WE-Heraeus-Seminar on Surface Studies by Nonlinear Laser Spectroscopies, Kassel, Germany, May 30 to June 1, 1994     

Core level binding energy shifts of K and Cs adlayers on Ru(001)

Using synchrotron radiation from the VUV ring at NSLS and vacuum ultraviolet radiation from a HeI resonance lamp, we have recorded high resolution photoemission spectra of K and Cs overlayers on Ru(001). It is found that for “thin” multilayer coverages ( 3 ML) the K3p and Cs5p core levels exhibit three sets of core levels which can be assigned to interface, “bulkrd and surface emission in increasing binding energy. The results are discussed in terms of the nature of electronic interaction and a thermodynamic model. The K3p core level spin-orbit splitting is also resolved in these measurements for K in the condensed phase, for the first time with photoemission spectroscopy.